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American |laturc ^ctics

Group III. The Inunctions of Nature

THE LIVING PLANT

A DESCRIPTION AND INTERPRETATION OF ITS
FUNCTIONS AND STRUCTURE

BY

WILLIAM F. GANONG, Ph.D.

PROFESSOR OF BOTANY IN SMITH COLLEGE

NEW YORK

HENRY HOLT AND COMPANY

1913

Copyright, 1913,

BY

HENRY HOLT AND COMPANY

Published April, 1913.

riiUss OK T. MOIiEY & SON,
(iUKKNI IKLI), M V^'^.. 1". S. A.

Of the scholars of Salomon's House, — "lastly, we have
three that raise the former discoveries by experiments,
into greater observations, axioms, and aphorisms. These
we call Interpreters of Nature."

Francis Bacon, The New Atlantis

PREFACE

The very first words I would write in this book are addressed
to my botanical colleagues, whom I wish to inform that the work
is not intended for them. In this statement I am by no means
invoking immunity from scientific criticism, but emphasizing the
aim of the book. It is not designed as a digest of our present
scientific knowledge of plant physiology for the use of experts in
that subject, but, in conformity with the aim of the series of which
it is a part, it seeks to present to all who have interest to learn an
accurate and vivid conception of the principal things in plant life.
I was once myself such a learner, and I have tried to write such a
book as I would then have delighted to read. It is, in a word, an
attempt at that literature of interpretation which was fore-
shadowed by Francis Bacon in the fine passage that stands on its
dedicatory page.

This aim will explain peculiarities of the work not otherwise
obvious. Thus, I have been at more pains to be clear than to be
brief, assuming on the part of my reader no great knowledge of
the subject, but a large willingness to take trouble to learn; and as
I have tried to discuss every process with fulness enough to eluci-
date its nature, my book has wandered through a leisurely course
to a length quite shockingly great. But I comfort myself with the
reflection that the plan and the subject hardly permit other treat-
ment ; for a royal road to a real understanding of plant phenomena
does neither exist nor can it be built. Perhaps, indeed, the very
portliness of the volume will act as a deterrent to any attempt at
a desultory reading in the hammock, and will rather suggest the
study table, and the principal feature of an evening's business,

vi Preface

and sternly-preserved leisure for reflective concentration on the
matters it considers. At least, any value it may have for the
reader will be realized best through this mode of approach.

As to the method of treatment in particular, I have sought
especially to interpret those phenomena of plant life which come
within ordinary observation and experience, penetrating just
deeply enough into each to make clear the principle of its opera-
tion, — ''the theory of the thing" in popular phrase; — and some-
times that has taken me far and sometimes it has not. Thus is
explained the absence of some matters of high technical interest,
which lie, however, outside the experience of the general observer.
Where explanations are concerned, I have given the known ones
when there are any, and when these are lacking I have not
hesitated to supply suggestions of my own, though in a way
designed to show their hypothetical character. As to statements
of fact, I have meant to present only those which have acquired
the impersonal validity of science, for which reason I have omitted
a good many of the newest ideas, even at the risk of seeming not
to know them; for I have noticed that he who is too closely up to
date in science has later a good deal to unlearn.

This deliberate conservatism is not, however, the inspiration
of my advocacy of Darwinian adaptation, for that is based upon
conviction as to its essential correctness. I am very well aware
that some eminently respectable people now consider adaptation,
except as an accident, an antiquated idea. I have myself expe-
rienced periods of this belief, but have always found myself back
to causative adaptation as the most rational explanation we
possess of the relations of living beings to their environment.
But while holding to the reality of adaptation as an historical and
causative process, I do not by any means suppose that all plant
phenomena are explainable on this basis; and in this book I have
tried to sort out the numerous influences at work, and to show
which phenomena are best explained by adaptation, which by
mechanical causation, and which by others of the possible forma-

Preface vii

live influences. But adaptation seems to me to guide the course
of a mightier current upon which mechanical causation and other
influences are ripples or eddies, or at least no more than the waves
whose only lasting influence is occasionally to open new directions
for the current to move in. With this belief in adaptation, I have
naturally not hesitated to use the corresponding language of
purpose, — not a mystical, supernatural, forethoughtful purpose,
but a physical, natural, experiential purpose, which does not
presuppose any forethought, but only the preservation and
accumulation of the results of past experiences wherein each step
in advance was purely chanceful, and survived only because it
happened to fit.

There is one other matter of this kind I would mention, and
that will be all. Throughout the book I have made great use of
diagrams, generalizations, and conventionalizations; and this may
seem inconsistent with the vitalistic rather than mechanistic tone
of the work. The scientific and educational status of this practice
are sufficiently explained in Chapter I, but I would like also to
say that I think our advance in plant physiology is measured
exactly by our ability to represent each detail in a mechanical
diagram, a physical formula, or a chemical equation. For the
evidence certainly indicates that every individual process of
plants is purely mechanical, physical, or chemical. What cannot
thus be explained, and what we have made as yet little progress
towards explaining, is the nature of the influence which establishes
and holds these processes in orderly sequences repeated in wonder-
fully complicated cycles generation after generation. When we
have explained the operation of each gun, and dynamo, and
powder-hoist on a battleship, have we thereby explained the
rationale of the operation of a battleship? Here is where the real
difference lies today between mechanism and vitalism. And this
is the vitalism of this book, — not a supernatural vitalism of the
theological type, and certainly not designed for theological needs,
but a perfectly natural vitahsm based on the superior interpreti\'e

viii Preface

power of an hypothesis assuming the existence in Nature of an
X-entity, additional to matter and energy but of the same cosmic
rank as they, and manifesting itself to our senses only through
its power to keep a certain quantity of matter and energy in the
continuous orderly ferment we call life. If those complicated and
regularly-recurring cycles of material and energy changes which
constitute the visible phenomena of life were mechanistically
self-originating, self-controlling, and self-surviving, then Nature
should be full of scattered fragments of such cycles, whereas she
is not. For everything in Nature has either all of the characteris-
tics of life, or else it has none of them; it is either alive, or it is not.
And there you have the chief argument of vitalism against
mechanism.

Having thus explained, the best that I can, the spirit and scope
of this book, I turn to make my grateful acknowledgement to
those who have rendered kind aid in its preparation. For the
illustrations, in particular, I am indebted to many persons. For
the privilege of using the two dozen or more fine pictures from
Gray's Structural Botany and the Chicago Textbook, as acknowl-
edged with the cuts, I am indebted to the publishers of those
works, the American Book Company; and I have also been per-
mitted by the Doubleday Page Company to use figure 8, and by
the Bullard Company to use figure 15, from publications of theirs.
Further, a ready consent has been given by Professor G. F. Atkin-
son to my use of figure 118, and by Dr. C. C. Curtis, to my use of
figures 67 and 73, from books of theirs published by IMessrs, Henry
Holt and Compan}^ In addition, I have copied a number of
figures from various foreign works, notably those of Sachs,
Kerner, Strasburger and Kny, taking pains, however, to acknowl-
edge the sources with the cuts themselves. Further, I have made
use without special acknowledgement of a good many pictures
which have been copied so often as to have become a kind of
common property (viz., figures 17, 35, 94, 147, 149 to 161, 164,
166-7, 169-171), although these, together with certain others

Preface

IX

whose source is acknowledged (viz., figures 81, 85, 107, 168, 175),
have been re-drawn for this work by one of my students, ]Miss
Bertha Bodwell, now Mrs. Richard Potter. The remainder of the
pictures, somewhat over one-half of those in the book, are new.
Several have been made by students of mine: — figures 18 to 23,
with 76 and 84 by :\liss Bodwell: figures 27, 56, 57, 132, illustrat-
ing physiological apparatus, with 126-7-8, showing phases of
growth, by ]Miss jMargaret Sargent: figures 103, 104, parts of a
series representing the development of representative plants, by
Miss Ruth Huntington, now Mrs. Max Brodel: figure 87 by Miss
Stella Streeter: figure 133 by Miss Hope Sherman: while the fine
graphs of figures 70 and 123 were worked out from the original
materials as well as drawn by Miss Marion Pleasants. The photo-
graph of figure 26 was given me by another student. Miss Anne
Barrows, now Mrs. Walter Seelye. The elaborate and exact
drawing of root tissues forming figure 53 was made by my col-
league. Dr. F. Grace Smith, Associate Professor of Botany in
Smith College, while the markedly original and very satisfactory
series of generalized drawings in illustration of the principal
physiological processes, embodied on the colored Plate I, and in
the multiple figures 54, 66, 139, together with the figures 30 and
99, were specially drawn for this book by another of my associates,
Miss Helen A. Choate, Instructor in Botany in Smith College.
To all of these willing and efficient collaborators I desire here to
express my indebtedness, and my grateful thanks. The remainder
of the illustrations, including the new photographs and diagrams,
are productions of my own.

But the greatest of my obligations is to Miss Choate, who has
read both manuscript and proofs in a critical spirit no less militant
because friendly. She has not been concerned so much with the
scientific aspects of the chapters as with their exposition, rep-
resenting in this the rights of the reader, for whose benefit she
has curbed much exuberance of expression, and eliminated many
an obscurity and inconsistency. That some of these faults re-

X Preface

main is not to be laid to her, since I have sometimes leaned back
on superior official authority and had my own way.

In the first announcement of the book it was said that keys,
similar in principle to those used in works on classification, would
be appended as aids to the reader in finding the explanations of
phenomena. These keys, however, have assumed such propor-
tions that it seems best to transfer them to a separate work. They
are now in process of elaboration in detail by another of my
associates, Aliss Julia Paton, Fellow in Botany in Smith College,
and will presently appear as a synoptical handbook.

Finally, I recall that in advising the reader to try as many
experiments as possible for himself, I said that practical guides
to experimentation would be suggested in the Preface. Un-
fortunately the one of these I consider the best, I am forbidden
by modesty to name, excepting that I may mention, as our friend
Mr. Dooley would put it in similar case, that it is entitled A
Laboratory Course in Plant Physiology, is published by Messrs.
Henry Holt and Company, and is written by myself.

The Author.
Smith College,

March 15, 1913.

CONTENTS

CHAPTER PAGE

I. The various ways in which plants appeal to the interests

AND MIND OF MAN. (Methods of Study in the Science of Botany) . . 1
II. The prevalence of green color in plants, and the reason why

IT EXISTS. (Chlorophyll and Photosynthesis) 16

III. The profound effect on the structure of plants produced

BY THE NEED FOR EXPOSURE TO LIGHT. (Morphology and Ecology

of Leaves and Stems) 47

IV. The kinds of work that are done by plants, and the source

OF THEIR POWER TO DO IT. (Respiration) 76

V. The various substances made by plants, and the uses thereof

TO them and to us. (Metabolism) 105

VI. The substance which is alive in plants, and its many remark-
able qualities. (Protoplasm) 138

VII. The ways in which plants draw into themselves the various

materials they need. (Absorption; Roots) 165

VIII. The ways in which substances are transported through
plants, and finally removed therefrom. (Transfer, Trans-
piration, Excretion) 198

IX. The peculiar power possessed by plants to adjust their
individual parts to their immediate surroundings. (Irri-
tability) 224

X. The various ways in which plants resist the hostile forces

AROUND THEM. (Protection.) 256

XI. The ways in which plants perpetu.-^te their kinds, and

MULTIPLY THEMSELVES IN NUMBER. (Reproduction) 278

XII. The MANY REMARKABLE ARRANGEMENTS BY WHICH PLANTS SECURE

UNION OF THE SEXES. (Cross-pollination; Floivers) 303

XIII. The ways in which plants increase in size, and form their

VARIOUS PARTS. (Growth; physiological) 327

XIV. The orderly cycles pursued in growth, and the remarkable

RESULTS OF DISTURBANCE THEREOF. (Groivth; Structural) 352

XV. The many remarkable arrangements by which plants secure

CHANGE OF LOCATION. (Dissemination; Fruits) 378

xi

31263

xii Contents

CHAPTER PAGE

XVI. The method of origin of new species and structures, and the

CAUSES of their FITNESS TO THE PLACES THEY LIVE IN. {Evolutiotl

and Adaptation) 403

XVII. The remarkable improvement made in plants by man, and the

WAY HE BRINGS IT ABOUT. {Plant breeding) 426

XVIII. The principal groups into which plants naturally fall,

WHETHER BY RELATIONSHIP OR HABIT. (Classification) 445

Index 467

A TABLE DESIGNED TO DIS

.A

The description and
interpretation of
the Living Plant
involves consid-
eration of, —

The interests and
capacity of the
human mind in
relation to the \
study of Plant
Life, discussed in
Chapter

The natiu-e and
properties of liv-
ing substance,
called Proto-
plasm, of plants,
which, however,
can be under-
stood better after
some study of
the physiological
processes, and
hence is discussed
in Chapter 6.
Protoplasm.

1

The physiological proc-
esses of plants, con-
cerned with, —

Maintenance of the
Individual, de-
pendent on, —

Nutritio

vision
physii
requii

tl

Preservation of the
Race, dependent
on, —

Fitness
roun
quirin

Replace]
indivif
quirin

The methods by
which plants be-
come altered in

structure, habits,
and identity, in-
cluding, —

The methods of alter-
ation

Attainm
size b
vidual
ing,-

In Natu
Under t

The results attained, considered i

,AY THE PLAN OF THIS BOOK

The acquisition of food, which is constructed by
plants inside their own tissues, as described in
Chapter

The development of photosynthetic structures,
to which is devoted Chapter

The release of energy, which supplies the power
indispensable for every kind of work, as shown
in Chapter

The transformation of food into special sub-
stances needed for particular functions, as de-
scribed in Chapter

[A suitable place for the chapter which is logi-
cally No. 2, as noted in column 2]

The absorption of substances into the plant,
with development of absorptive structures;
hence Chapter

The movement and removal of substances
through and out of plants, considered in

Chapter

[ The adjustment of individual parts to surroxmd-
ings, to which is devoted Chapter

The development of protective adaptations
against hostile external conditions, discussed
in Chapter

The formation and development of new indi-
viduals like those which produce them; hence
Chapter

The development of sex-uniting adaptations, se-
curing the cooperation of two parents in pro-
duction of offspring; Chapter

The formation of new parts and their increase
in size, to which is devoted Chapter

The development of structures through cycles,
both ontogenetic and climatic, — Chapter ....

The development of dispersive adaptations, se-
curing room for new individuals to grow, as

described in Chapter

to which is devoted Chapter

Chapters

1. Methods of

Study

2. Photosyn

thesis

3. Leaves and

Stems

4. Respiration

5. Metabolism

6. Protoplasm

re-

i

(

Absorption ;
Roots

Transfer and
Excretion

7.
8.
9. Irritability

10. Protection

11

12

Reproduc-
tion

Cross-pol
lination;
Flowers

13. Growth, phy

siological

14. Growth,

structural

15. Dissemina-

tion; Fruits

16. Evolution

land of man, to which is devoted Chapter 17. Plant Breed-
ing

:;hapter 18. Classification

THE LIVING PLANT

CHAPTER I

THE VARIOUS WAYS IN WHICH PLANTS APPEAL TO THE
INTERESTS AND MIND OF MAN

Methods of Study in the Science of Botany

ND he spake of trees, from the cedar tree that is in
Lebanon even unto the hyssop that springeth out of
the walL" Thus runs the record of the first botanical
teacher, reputed also the wisest of men, as writ in the
greatest of books. And from the days of King Solomon down
to our own, men never have ceased to speak and learn of plants,
until now the circle of knowledge has long been too vast for any
one mind to encompass. To us, plants embrace not alone the
cedar and the hyssop, but the fern, the moss, the hchen, the sea-
weed, the mushroom, the mold, the blight, the yeast, and the
germ of disease within the body of man. And it is not alone their
forms, their uses, and their habits which concern us, but as well
the minutest details of their internal construction: the mean-
ings of their resemblances and their differences : the ways of their
nutrition, increase, and adjustment to their surroundings: the
possibilities of their development to greater and yet undiscovered
utilities: and in truth no less than every fact which the intellect
of man can discover about them.

The field of botanical study is therefore not simply vast, it is
practically limitless, — in this respect transcending the natural
powers of man, which are small. Therefore, while every school-

2 The Living Plant

boy can grasp the sahent facts in that organized knowledge of
plants which we call the Science of Botany, no one person can
actually master any more than a limited portion thereof, es-
pecially if he have the ambition to know it sufficiently well to
aid in expanding the bounds of our knowledge. For the purpose
of specialized study, accordingly, there have been developed
within the science a number of divisions which are dependent
on the nature of the problems presented, and therefore on the
methods employed in their study. The divisions are these. First
is Classification (called also Systematic Botany, or Taxonomy), the
oldest and most fundamental of all, and doubtless the theme of
King Solomon's discourse. It establishes the relationships of
plants to one another, and arranges them accordingly, while
describing and naming them. It is studied through exact ob-
servation and comparison of the external parts of plants, which
can be kept preserved in a pressed and dried condition in col-
lections called Herbaria, while its results are embodied not
only in great monographs, but in handbooks, or Manuals, so
arranged as to enable any person to identify plants for himself.
Second is Morphology, which deals with the parts, or structures,
of plants, and establishes their relationships to one another while
describing and naming them. Morphology is very much the
same to the parts of plants that classification is to plants as a
whole. The name in the past has been associated most closely
with the comparative study of the large external structures, —
roots, stems, leaves, flowers, and fruits, — and their transforma-
tions into tendrils, spines, pitchers and the Uke, but is nowadays
given a far wider extension; while special names describe the
phases concerned with minute or internal parts, and needing the
use of such exact and delicate instruments as the microscope
and microtome, — Embryology or "life-history," for the develop-
ment of the structures in the individual plant. Anatomy, for
the cellular construction, and Cytology for the internal struc-
ture of the cells themselves. Third is Physiology, a word which

The Various Ways in Which Plants Appeal 3

has precisely the same meaning with plants as with animals,
comprehending the study of those functions or processes by which
they secure the maintenance of their daily lives and the per-
petuation of their kinds. It is studied chiefly through expermient
by aid of the exact methods and instruments of physics and
chemistry, though it reaches into realms which those sciences
do not touch. Fourth is Ecology, youngest of the divisions of the
science, and greater as yet in promise than performance, but
nevertheless of the very first interest to a great many people.
It explains the adaptations of plants and their parts, that is,
the ways in w^hich these are adjusted to the conditions of the
world around, involving the meanings of their forms, sizes,
colors and the like. This division has sometimes been called,
and still is by some Germans, Biology; but that word should be
kept for its legitimate use as meaning the study of life com-
prehensively, and therefore equivalent to Zoology and Botany
together. Fifth is Plant Industry (called also Economic Botany),
which is the study of the ways in which plants may be made to
yield the greatest service to man. The older phases thereof.
Agriculture, Horticulture, Pharmacology, and Forestry, originall}^
purely practical, are now scientifically studied, and to their
very great profit; while strictly scientific from their foundation
have been the newer phases of Pathology, or the study of diseases,
Bacteriology, or the study of germs and their effects, and Plant-
breeding, or the systematic development of better kinds of plants.
And to these divisions there is every promise that the near future
will add yet a sixth, Botanical Education, which will attempt not
only to train students much better in the science, but also to
interpret botanical progress to the world at large. An important
phase of this division will be the production of works, on the
Natural History of Plants, which will set forth, with a combination
of scientific accuracy and hterary charm, not only the technical
and economic aspects of plant life, but also those historical,
legendary, and imaginative aspects which give to a study its

4 The Living Plant

widest human interest. Indeed, the production of such works
may be viewed as the logical aim of all botanical study.

Such are the principal divisions of botanical science as we
know them at present. This book, concerned as it is with the
life of plants, deals chiefly with Physiology, but the divisions
are interlocked inextricably, and I must perforce make many
an excursion into the others. This science, and all science, is a
unit, and subdivisions thereof are nothing other than a concession
to the limitations of the powers of man.

As the reader reflects on this matter of the various divisions of
botanical science, he cannot but notice how unequal they are in
apparent utility to man, and he may even inquire why we should
study at all the ones that seem useless. Two reasons at least
exist why we should, and do. First, some people take pleasure
therein, precisely as do others in art, music, and literature. No-
body thinks of asking what use these latter may be, the value of
pure pleasure being obvious enough; but the world has mostly
yet to learn to extend the same approbation to the seemingly use-
less sciences. Second, the history of human progress has shown
that the greatest applications of science to the useful arts have
sprung from purely scientific investigations of a non-useful type.
Nothing, doubtless, could have seemed more useless to cotem-
porary critics than the studies of those early naturalists who de-
lighted to apply the new-made microscope to the investigation of
the living atoms which swarm in slime; and yet from these very
studies has come our knowledge of Bacteria, and our power to
control the deadliest diseases that scourge mankind. Likewise
photography, all the applications of electricity, a vast range of
chemical arts, and indeed most others of the wonderful applica-
tions of science to utility, have developed incidentally from purely
abstract scientific researches made without any regard to useful
applications. Furthermore, it is quite impossible to predict at
what point upon the general surface of expanding knowledge the
next useful discovery will spring forth. In fact there is no natural

The Various Ways in Which Plants Appeal 5

boundary between useful and useless knowledge; they are one
and indivisible, and such boundary as may seem to exist is simply
a shadow that shifts over the surface, changing with times and
our customs. Accordingly, the only possible way in which human-
ity can obtain useful results from science, lies through the en-
couragement of the development of all of its phases; and this
may be done with the assurance that now and then some useful
applications will somewhere appear, and pay manyfold for it all.
And this is precisely the reason, moreover, why no good system of
education can confine itself to teaching useful knowledge alone.
It is unfortunately still true, as it was when Stephen Hales, the
founder of Plant Physiology, wrote nearly two centuries ago, that
pure science needs protection ''from the reproaches that the ig-
norant are apt unreasonably to cast on researches of this kind,
notwithstanding that they are the only solid and rational means
whereby we may ever hope to make any real advance in the
knowledge of Nature." When, therefore, the reader hears anyone
asldng what is the use of this or that phase of knowledge, or when
he sees practical men showing mipatience with the impractica-
bility of great scholars and contempt for the uselessness of their
knowledge, he may well state these facts by way of courteous
reproof. And he may even add, as to such knowledge, that
those w^ho pursue it, in the absence of the material rewards
reaped in full measure by practical men, deserve no less tribute
of respect and approbation than is accorded by common consent
to those whose efforts bring them personal wealth. Both in fact,
though in different ways, are contributing to the welfare and
progress of humanity.

I have spoken, just now, of the pleasures of the study of Botany,
and over this theme I would hnger a little. It is true of all science
that the pleasures of its study lie deep, and one must reach far
before he can grasp them. It is not as with literature, for ex-
ample, which makes appeal to the feelings, that lie near the surface
and are easy to touch ; for science appeals chiefly to reason, which

6 The Living Plant

lies deeper and is slower of action. This is why literature is en-
joyed by nearly all people and science by only a few, and why
literary reputations can be made in youth while those of science
are mostly attained much later in life. Yet, when grasped, the
pleasures of science are no less keen than those derived from any
other field of intellectual endeavor, and I have even fancied that
they jdeld an especially deep and lasting satisfaction, though in
this perhaps I am wrong. There can be, I believe, no pleasure
in life any greater than that which comes to the scientific man with
the moment in which some truth heretofore not known to man-
kind first dawns upon him; and it is in the hope of such moments
of exaltation that he is willing: to undergo toil, poverty, hardship,
and even peril of life itself. The charm that there is in this pur-
suit of truth receives many illustrations from the biographies of
eminent scientific investigators, and especially from their familiar
letters, in which can be seen more clearly than elsewhere the
actual workings of the scientific spirit.* But though felt to the

* A characteristic example is furnished by the following letter written by Charles
Darwin to Asa Gray, — the eminent American Botanist.

Down, August 9 [1862].

My dear Gray, — It is late at night, and I am going to write briefly, and of course
to beg a favour.

The Mitchella very good, but pollen apparently equal-sized. I have just examined
Hottonia, grand difference in pollen. Echium vulgare, a humbug, merely a case like
Thymus. But I am almost stark staring mad over Lythrum; if I can prove what I
fully believe; it is a grand case of Trimorphism, with three different pollens and three
stigmas; I have castrated and fertilized above ninety flowers, trying all the eighteen
distinct crosses which are possible within the limits of this one species! I cannot ex-
plain, but I feel sure you would think it a grand case. I have been writing to Botan-
ists to see if I can possibly get L. hyssopifolia, and it has just flashed on me that you
might have Lythrum in North America, and I have looked to your Manual. For
the love of heaven have a look at some of your species, and if you can get me seed,
do; I want much to try species with few stamens, if they are dimorphic; Nes(p,a verl-
icillata I should expect to be trimorphic. Seed! Seed! Seed! I should rather like
seed of Mitchella. But oh, Lythrum!

Your utterly mad friend,

C. Darwin.

[Life and Letters of Charles Darwin, New York, 1888, II, 475.J

The Various Ways in Which Plants Appeal 7

fullest only by those who fare the farthest, the pleasures of science
are by no means unknown even to youthful students; and I have
myself experienced in the past and have since noticed in others,
a keen enjoyment in the use of exact scientific methods and tools,
a great satisfaction in the acquisition of knowledge that one feels
to be solidly grounded, and a lasting pleasure in an understand-
ing of the workings of the greater natural phenomena. But while
the personal and sesthetic elements are certainly by no means
absent from scientific study, as indeed the accompanying picture
will bear witness, the student must realize that the deepest
pleasures of science are of stern and sj^artan sort, somewhat
like those felt by the strong man when he rejoiceth to run a
race.

We must return for a moment to the matter of the unity of
botanical science in order to consider yet another concession,
besides its artificial divisions, to human Imiitations. This unity
of the science is of course but a reflection of the unity of Nature,
where all of the vast number of facts and phenomena intergrade
and interlock without any real boundaries. Yet the mind of
man is so made that it can grasp onlj^ definite conceptions, and
not many of these ; and it can no more form a definite miage of the
infinite intergradation of phenomena than it can of the infinite
largeness of space or the infinite smallness of the sub-constitution
of matter. Hence it is necessary, for purposes of education and
exposition, to create definite images out of indefinite material.
Take, as an example, the subject of leaves. Leaves are so many,
so diverse, so intergradient, that no learner can grasp any con-
siderable proportion of the facts about leaves as they actually
are. The substitute therefor, to which eveiy teacher and author
is obliged to resort, is a subjective conception of a generalized
or average leaf, built up for the learner from observation of a
number of actual leaves; or, better, it is a composite conception
of a leaf built up in the receptive mind of the learner from many
observations of actual leaves, much as composite photographs

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The Various Ways in Which Plants Appeal 9

of human faces are built up from exposures of many actual faces
upon the sensitive photographic plate. This is precisely what our-
Text-books are doing when they devote chapters to ''The Leaf,"
"The Stem," and the like. These titles do not represent things,
but ideas; there are leaves in Nature but no such thing as the leaf.
But the analogy of these composite conceptions to composite
photographs goes yet a step farther, for, just as a real face is oc-
casionally seen which resembles the composite face of the photo-
graph, so an actual structure or phenomenon is sometimes found
which is like our mental composite of its kind. Such a real thing is
then said to be typical, and that is what is actually meant by this
word in science. When, however, no typical representative of the
composite is a^^ailable, we are still not without resources; for it is
possible to give exact and clear definition to the dim and elusive
outlines of the composite itself by drawing firm sw^eeping lines
through its more prominent places, — a process which constitutes
generalization, or conventionalization. When the data concerned
are expressed in figures, then the result is a round-number aver-
age, or conventional constant; when they are expressed in pictures,
the results are generalized drawings, or, if simplified to mere struc-
tural aids to the imagination, diagrams; when they are expressed
in words, the results are generalizations, or verities, the "aphor-
isms" of Bacon. Throughout this book, in accordance with its
aim to interpret plant life in the large, I have made great use
of composite conceptions, typical things, conventional constants,
generalized drawings, diagrams and verities, — to a degree which
will meet with much disapprobation from my scientific colleagues.
But I maintain that such generalized knowledge of plants is not
only infinitely better than no knowledge at all, but is actually
the most useful kind, as it is the onl}^ practicable kind, for the
non-technical learner, whose knowledge in other departments
of learning, — in geography, history, and so forth, — is largely of
this character. And I further mamtain that if only we would
make greater use of it, along with its logically-correlated methods,

lo The Living Plant

in our educational system, we should have less cause to complain
of the comparatively empty condition of our elective science
classrooms. It is not of course representative of the methods
whereby scientific investigation is successfully pursued ; but where
else in human affairs do we insist upon teaching all people the
technical methods or none? In large measure, Science, in order
to be advanced, must be dehumanized; but in order to be used,
it must be humanized.

The fact is, the human mind is a very poor instrument for
scientific research, for which it was never developed. Unless all
of our knowledge is at fault, the mind of man was evolved under
stress of use as his chief weapon in the struggle for physical ex-
istence; naturally, therefore, all of its stronger traits are fitted
to that very concrete activity rather than to uses of an abstract
intellectual sort. Its power of concentration upon a single aim,
with determination to achieve it by any means: its instinctive
and partizan exaltation of its own case and minimization of its
opponent's: its tendency to warp all testmiony to its own credit:
its quick defense of its own caste or clan, right or wrong, with its
ready submission to the conventions thereof and contempt for
everything outside: its preference for keeping to beaten and safe
paths and for shunning the unknown, which it peoples with
mysteries and evil designs : its liking for following the most assert-
ive leaders and for leaning back upon their authorit}^; — all of
these are invaluable traits in the struggle of the individuals of a
social community for existence, but they form a very bad basis
for scientific investigation, which requires the opposite qualities
of disinterestedness, impartiality, and the judicial weighing of
evidence for the determination of the exact truth without any
regard to its effects upon persons, interests or dogmas. All men
have the primitive self-centering qualities highly developed; and
the scientific research of mankind is done upon a small residue
of the opposite qualities which a few of them happen to possess,
and which even in them are not so much natural as assiduously

The Various Ways in Which Plants Appeal ii

cultivated. Is it any wonder, then, that scientific progress is so
slow, so laborious, and so expensive?

There remains one other phase of the relation existing between
Science and the mind of Man, which is so fundamental to the
subject of this book that we must give it some special attention.
It concerns the apparent purposefulness of many biological
phenomena, as expressed especially in adaptation. What, then,
is this adaptation, with which the writings of Darwin have made
us so familiar? It is any feature, whether of structure or action,
which brings a life process into harmonious relation with the ex-
ternal conditions that affect it. The flatness of a leaf is an adapta-
tion to the need for a very wide spread of green tissue to light,
as is to be fully explained in the following chapter. The colors,
shapes, sizes and peculiarities of form in flowers are chiefly adapta-
tions to the utilization of insects in the transfer of pollen, which
is an indispensable prerequisite to cross fertilization, as will
also be demonstrated in the suitable place. And other cases
are known without number, involving not only single features,
but often the cooperation of several. Now the question is this, —
in what way has this remarkable fitness of form to function, of
structure to use, of parts to environments arisen? It was form-
erly supposed that these adaptations were the direct work of the
Creator, — the eternal, immeasurable, omniscient, and om-
nipotent, — as Linnaeus grandly characterizes him in the Systema
Naturoe. But Darwin gave evidence, in The Origin of Species,
greatest of all secular books, tending to show that they arose
by a gradual process of evolution, developing in causative touch
at every step with the conditions which they fit; and this view
has long appealed as satisfactory to most biologists. But in
our own day it is becoming somewhat customary to attribute
adaptations rather to various adventitious origins, and to explain
their persistence merely by the negative supposition that they
are not out of harmony with the conditions concerned. In a book
of this kind it is needful to take a definite position on this subject,

12 The Living Plant

if for no other reason than this, — that the language one may use
is concerned. My position in general is the Darwinian one, —
that adaptation in the main has arisen as a gradual causative
accompanhnent of evolution. Indeed, such a causative, or histor-
ical development of adaptation appears to me an inseparable
corollary of the very idea of evolution, and wholly independent
of its method, — whether it proceed by many imperceptibly small
steps as Darwin beheved, or by fewer and perceptible ones, as
newer evidence seems to be showing. And the point about use
of language is this, that if adaptation is a causative process, —
the feature developing in causal touch with the conditions con-
cerned, — then it is quite suitable and correct to say that the adap-
tation exists for such-and-such a purpose; and I do not hesitate to
use such expressions in this book. In so doing I am in the very
best of company, for Darwdn himself continually uses the language
of purpose, or teleology; and both Huxley and Asa Gray, Darwin's
devoted friends and co-believers, point out in their writings that
evolution on the basis of Natural Selection places teleology on a
scientific basis.* This fact is overlooked in our day by many,
who think it scientific to avoid teleological or purposeful language
as though it were a plague. Science, indeed, hath her fashions
and her dogmas no less than other fields of human endeavor.

A chief reason for the occasional denials of the causative origin
of adaptation arises from reaction against the over-importance,
and over-perfection, so often attributed to it. Adaptation has
often been clamied on the scantiest evidence without any attempt
at proof. At its best, however, adaptation can never be perfect,
but is rather a general or generic affair, very much hke our own
adaptations to the trades or professions we follow. This is be-
cause no feature of structure or function is free to respond to one
adaptive need alone, but has to compromise with other consider-

* An example of Darwin's teleological language is found in the passage from one
of his books cited on page 234 of this volume. As to his establishment of teleology
as a scientific principle, compare his Life and Letters, New York, 1888, 11, 430.

The Various Ways in Which Plants Appeal 13

ations which often have more influence than adaptation itself.
Thus, in addition to the principal adaptation, (such for example
as the flatness of a leaf in adaptation to the need for spreading
much surface to the light), there are secondary adaptive needs,
such as for protection against dryness or other hostile influences.
Further, a prominent feature may not be adaptive, but incidental
to some other process, as in autumn coloration of foliage, or the
mathematically-arranged origins of leaves: or it may be merely
a mechanical effect, like the drooping of old branches of evergreen
trees: or it may represent an individual adjustment to one feature
of the surroundings, like the bent-over leaf-stalks of house plants
in windows : or it may be inherited from the past without present
significance, as in the compound early leaves of the Boston Ivy:
or it may represent a spontaneous new variation, or mutation,
or sport, such as originate new garden varieties of flowers, leavfis,
or fruits; or it may have yet other meanings of minor sort. These
cases and illustrations will all be further explained in the following
pages, and I merely cite them to show that not all features of
plants are adaptations, while all adaptations are interwoven more
or less with these other considerations, the actual structure being
the resultant of the interaction of them all. The matter can be
expressed in this way, that adaptation can never fit a condition
as an old glove fits the hand, but rather as a cloak fits the body.
One should therefore neither expect too much of it on the one
hand, nor reject it altogether on the other. The real problem is
not so much to find adaptations as to separate out and define
the various factors that enter into the combinations of which
adaptation is only a part.

One other important phase of the relations existing between the
human mind and the w^orkings of organic nature, concerns the
question as to whether there is anything in living beings except
physics and chemistry, — in other words whether they are mechan-
ism only, or w^hether the mechanism is inspired by vitalism. The
evidence seems to be showing clearly enough that all of the in-

14 The Living Plant

dividual processes of plants and animals are purely physical or
chemical, with no trace of a vital force in the old sense. Further-
more, the orderly sequence and cooperation of these processes
is largely explained by their linking up through the medium
of stimuli, as will later be explained in the suitable places in this
book. But it does not seem to me probable that the processes
only happen to be thus linked up, or that these particular link-
ings are merely the accidental survivors of innumerable ones that
happened in the past. Indeed, the most reasonable explanation
of the phenomena of organic nature in the large seems to me this,
that all of the life processes are subordinate to some influence
which is using living matter as a seat for its operations. Thus
there would exist in nature not tw^o, but three working entities,
matter, energy, and this X-influence. Perhaps the living matter
is the home which the principle of intelligence in Nature has
built for its residence. This is something more than vitalism,
or even the neo-vitalism of some philosophers; it is a super-
vitalism. But its acceptance harmonizes some of the greatest
difficulties in the interpretation of Nature, as the following pages
will illustrate in the suitable places.

Finally there remains one matter which I wish to add at this
place. It may seem to the reader, as it will to some of my col-
leagues, that in laying so much stress as I do upon causative
adaptation, and a number of things of that sort, I am reading
into Nature a principle closely akin to intelligence. If I seem
to do this it is because that is my intention. I believe that the
evidence now accumulating is sufficient to show that the same
principle which actuates intelligence also actuates all the work-
ings of Nature ; or, as I have expressed the matter on a later page
of this book, all living matter thinks, though only the portion
thereof which enters into the brain of man is aware that it thinks.
Our intelligence is a kind of epitomized expression of the prin-
ciples underlying the operations of nature, very much as mathe-
matics is an epitomized expression of the relations of number.

The Various Ways in Which Plants Appeal 15

or as the daily newspaper is an epitomized expression of the doings
of civilization. And this I mean not as a metaphor, but as a
serious scientific hypothesis.

This discussion of adaptation and kindred matters, and per-
haps some others of the matters contained in this chapter, will
have little meaning, I know, to the reader who may be making
his first acquaintance with plant life through this book. But I
venture to hope that the case will be different after he has made
some study of the pages which follow. Perhaps I should earlier
have advised him to read this chapter the last; and at least I do
now suggest that he read it again after he has finished the rest
of the book.

CHAPTER II

THE PREVALENCE OF GREEN COLOR IN PLANTS, AND
THE REASON WHY IT EXISTS

Chlorophyll and Photosynthesis

manifold are the works displayed in the world of
living plants, that to one who seeks some tie to bind
them all into a single natural group they seem at first to
present only an endless diversity. They do in fact
exhibit every possible gradation and variation; in size, from the
stately Sequoia of the Sierras, or the giant Eucalyptus of Aus-
tralia, towering high above all other living things and mighty in
girth, down to the humblest weed of the wayside ; in form, from
the graceful tree with its spray of twigs and myriad leaves to the
simplest sea-born plant whose life is wholly encompassed within a
miniature globe : in color, from the quiet green of the forest to the
brilliant hues of flowers, sea-mosses, or mushrooms: in texture,
from the ivory-hard seeds of palms to the jelly-soft fronds of
some seaweeds ; in habit, from the independent life of the mightiest
trees in the woods to the parasitic existence of a deadly germ of
disease within the body of man. Nowhere among these features,
nor j'et among any others that we know, can we find a single
one which applies to all plants. What is it then which binds all
of this heterogeneous assemblage into a single natural group?

Failing to find any one feature conmion to all kinds of plants,
a scientifically-minded inquirer would next turn to ask what
feature prevails most widely among them. If one marshals
before his mental vision all of the great groups, from the flowering
trees to the microscopical germs, and centers observation upon

i6

The Prevalence of Green Color in Plants 17

one after another, it gradually becomes plain that one feature,
and only one, does prevail very widely, — and that is the possession
of green color. Moreover, a deeper study by aid of microscope and
experiment shows that this truth is more nearly universal than
appears at first sight, for a good many plants that display other
colors, — e. g., the red foliage plants of the gardens and the brown
and red seaweeds, — prove to be green in reality, though that
color is masked by the presence of the others.

But although the green color, which is that of a definite sub-
stance called chlorophyll, is thus very wide spread among plants,
there are some, nevertheless, which really do not have it. Such
are the mushrooms, molds, mildews, yeasts and germs, as like-
wise the Ghost Plant (or Indian Pipe), of the woods, the twining
Dodder of the fields, and a few others. These plants are mostly
white to brown, though they often exhibit very brilliant hues of
red, yellow, and even a kind of a green, which, however, is very
different in shade and nature from chlorophyll. All of these
brighter colors are easil}^ removable by chemical means; and when
that is done, the tissues are left either white or brown, with never
a trace of the chlorophyll.

There are, accordingly, plants which really are green and
plants which really are not. And the reader's first natural
thought, that so striking a difference in one feature is probably
linked with differences in others, is correct. In the first place,
observation at once shows a very fundamental difference between
the two lands in habit, for all of those lacking the chlorophyll
are dependent for their food upon other beings, either upon liv-
ing plants or animals, (in which case they are called parasites),
or else upon their decaying remains, (when they are called
saprophytes). In sharp contradistinction stand the green plants,
practically all of which subsist without aid from other living
things, thriving upon materials which they take from the air,
the soil and the waters. A second great difference consists in
this, that all of the non-green plants are small and of humble

1 8 The Living Plant

habit, as the hst above given will testify, contenting themselves
with the odd and obscure places of nature, while the green plants
grow grandly in stature and number, possessing the earth. And
still a third difference exists, less likely to be thought of but no
less important for our present inquiry, namely, the study of
classification has shown that the non-green plants, for the most
part at least, are descended in the course of a long evolution
from green ancestors, and therefore have been green in the past.
Hence we are brought to a generalization of the greatest impor-
tance, the first indeed of the great botanical verities, — the pos-
session of chlorophyll is a ivell-nigh universal characteristic of
plants, and their most distinctive feature.

Such is the notable fact concerning the occurrence of chloro-
phjdl in nature. Obviously so wide-spread a substance must
play some very great part in the life processes of plants, and it is
our manifest duty to determine what it is. In any such study
the first resort of the biologist, — his first aid, as it were, to his
ignorance, — is observation, exact and interrogative observation,
of so much as the eye can discover. If, now, the reader will look
over, from this point of view, any collection of plants in garden or
greenhouse, drawing meanwhile on his memory for additional
facts from his own experience, he will find these things to be true ; —
that chlorophyll is not omnipresent in those plants which pos-
sess it, being absent from their roots and interior parts not reached
by the light: that even in lighted parts it is not uniformly dis-
tributed, being denser in the better-lighted places, as well ex-
emplified in the deeper green of the upper as contrasted with the
lower faces of leaves: that it does not develop at all in leaves
which are grown out of the light, as witness the colorless sprouts
of potatoes started in the darkness of cellars, or the grass of lawns
accidentally left covered in spring: that it vanishes from green
parts kept away some time from the light, as shown in the blanch-
ing of celery when banked up with earth: and that most green
parts turn over towards light when this comes rather strongly

PLATE I

Single chlorophyll
grain from C.

v_y

The Prevalence of Green Color in Plants 19

from one side, as all plants kept in house windows attest. All
of these facts unite to imply an extremely close relation between
the meaning of chlorophyll to the plant and the action of light,
even suggesting, indeed, that the chlorophyll is inserted, as it
were, between the hght and the use thereof by the plant. To
this subject we shall later return, for we are dealing at present
with the distribution of chlorophyll in the individual plant, a
matter which can further be illustrated, in purely diagrammatic
or conventional fashion, by the picture which forms figure A of
Plate I of this book.

So important is chlorophyll, that the reader ought really to
make its closer acquaintance through actual experiment ; for here,
as everywhere else in science, an actual personal contact with
facts or phenomena makes all the difference in the world in the
clearness of one's understanding of them. It is possible to ex-
tract the chlorophyll very easily from leaves. If one takes two
or three soft thin green leaves, places them in any glass dish which
is uninjured by heat, covers them with alcohol (of any of the com-
mon kinds), and lowers the dish into hot water, then the chloro-
phyll will come out into the alcohol before one's very eyes. Its
most striking characteristic is the beautiful green color of the clear
solution, together with a remarkable and beautiful red fluorescence
which appears when the solution is held in some lights, and es-
pecially when sunlight is focussed upon it with a lens. And the

* This picture is meant to represent that which one would see on a surface ex-
posed by a lengthwise cut through the center of such a reduced conventionahzed
plant. Such sections, called optical sections, are very much used in biological works.
Thus, on the very same plate, (Plate I), appear optical sections of a piece of a leaf,
a single cell, and a chlorophyll grain; and a good many others occur elsewhere in
this book. In every case an optical section is supposed to be typical, that is, taken
through the part most illustrative of the structure in question; and, ^\here only one
section of an object is given, it means that the object is substantially alike all around
the axis that is represented. Such sections, therefore, always stand for solid objects,
and the reader should learn, as quickly as possible, to construct the solid in his mind
from the section on the paper. This intellectual visualization, of course, requires
imagination, but that is a quality which, despite the popular bdief to the contrary,
is highly essential to success in science.

20 The Living Plant

reader should experiment also upon its instability in sunlight, a fact
of importance as will later be proven; this he may do by dividing
his solution into two portions, of which he puts one in bright
sunlight and awaits its changes of color, while he places the
other in darkness for comparison. Incidentally, too, this experi-
ment will show an important fact about the color of leaves apart
from their coloring matters, for, when the action of the alcohol
is complete, the leaves appear a soft creamy white. This, in
fact, is the natural color of all living plant tissues when no special
coloring material is present.

We must, however, pursue a bit farther the study of the chloro-
phyll substance, partly because of its importance, and partly
because the study will lead the reader to an acquaintance with
other matters which he should learn very early in his botanical
studies. To the naked e^^e alone, no matter how closely applied,
the chlorophyll seems to color uniformly the whole of the leaf,
which, except for the veins, looks homogeneous in texture. But if
we call to aid that wonderful instrument by which the range of the
eye into the minute is increased a full thousandfold, — that first
and greatest tool of the biologist, the microscope, — and place
under its lenses a very thin section or slice cut right through some
green leaf from surface to surface, then a very different idea of
leaf structure is presented to the observer, as the accompanying
picture attests (figure 2). And with this picture of an actual leaf,
the reader should compare the generalized or conventionalized
section represented in figure B on Plate I. Clearly, the interior
of the leaf is not homogenous, but partitioned into a great many
little compartments, with empty spaces here and there inter-
spersed. These compartments are called cells, a word of vast
importance in Biology, because not only the leaf, but all parts
of all plants, and all parts of all animals, are composed of them.
These cells differ greatly in details of structure according to their
function, but are always compartments of some sort; and the
reader should as promptly as possible incorporate this idea of

The Prevalence of Green Color in Plants

21

universal cellular structure into his visual conception of plants.
In our picture (figure 2), carefully drawn from an actual leaf, and as
well in the conventionalized leaf {B on Plate I), the reader can
see for himself the cells of the upper and lower skin (or epidermis),
those of the vein (the clearer mass lacking chlorophyll), and
finally those of the green tissue, distinguished by the large black
or green spots which represent the chlorophyll grains. For the

Fig. 2. A thin slice, or section, cut across a typical leaf (the European Beech), and highly
magnified. From a wall-chart by L. Kny. In the original, the numerous black discs
are green, as in the living leaf.

chlorophyll really is contained in definite grains, and is not a dye
spread all through the leaf. These cells are roughly spherical,
cylindrical, or polygonal in shape, though the open clear air-
spaces between them are most irregular in form. Each cell has
its outer thin transparent wall (little more than a line in figure 2),
within which comes a complete lining of a thin gelatinous sub-
stance (shown in Plate I, B, by the faint grayish or dotted

2 2 The Living Plant

shading), so nearly transparent as to be almost invisible. But
though so insignificant in appearance, this grayish material
is nevertheless the most important of all substances, for it is Proto-
plasm, the exclusive seat and sole physical basis of all the phe-
nomena of life, as I shall show in a later chapter devoted to that
subject. Within this living substance, close up to the wall, lie
the chlorophyll grains, each of which has a definite shape, some-
thing like that of a disc or a lens, and consists of denser proto-
plasm deeply stained by a green liquid which is the chlorophyll
substance proper. Finally, it should be added, in order to com-
plete the reader's conception of the cell, that all of the remainder
of its interior is filled with the sap, which is simply water contain-
ing many kinds of substances in solution. As to the spaces be-
tween the cells, they contain as a rule notliing but air, which is
in connection with the atmosphere outside of the plant through
tiny little openings, called stomata, between the cells of the
epidermis. We shall return, and that often, to this subject of
cellular structure, and the reader will then recognize the ad-
vantage of having thus made some preliminary acquaintance
therewith.

We must now return to the problem involved in the observa-
tion that a close connection exists between the distribution of
chlorophyll and the presence of light. Observation alone, how-
ever, cannot lead any farther, and we must resort to the second
of the biologist's methods, — experiment. In such a situation
the scientific mind would reason somewhat like this, — if, as
seems implied by the facts, the chlorophyll has in the plant a
function dependent on the action of light, then some difference
should develop between leaves kept for a time in darkness and
others kept equally long in light. Accordingly the experimenter
would darken certain leaves on a plant, in a way that would not
injure their health, and then, after a day or two, would examine
a darkened and lighted leaf side by side. The result is always
disappointing to the naked eye, by which no differences at all

The Prevalence of Green Color in Plants 23

are discernible, but a very different story is told by the micro-
scope. That indispensable instrument shows in the lighted leaves
the presence of tiny white grains (figure D, Plate I), which are
absent from the leaves that were darkened, while chemical tests
prove these grains to consist of a definite and familiar chemical
substance, — starch.

This fact that starch makes appearance in ordinary green
leaves when exposed to the light but not in those kept in the dark,
is so important in plant physiology that the reader should make
some further and practical acquaintance with the matter. If he
selects some one of the commoner house plants, (e. g.. Fuchsia,
Garden Nasturtium, Horseshoe Geranium), covers some of the
leaves from the light by a box, exposes the plant for a day or
two to light, removes the darkened and lighted leaves at the
close of the second day, dips them for a moment into boiling water,
blanches them of chlorophyll by aid of warm alcohol, immerses
them in water a minute to neutralize the brittleness the alcohol
causes, spreads them out in a white saucer, and covers them with
a solution of iodine diluted from the tincture he may buy from
a druggist, he will be rewarded by seeing a very remarkable
difference develop between the lighted and darkened leaves, for
immediately the former will all turn a very dark blue, while the
latter will remain of their natural cream color. Now iodine, as
anyone may prove by a touch to some part of his starched hnen,
though brown of itself turns starch a dark blue; and thus our
experiment proves that the leaves form starch in the light but
not in the dark. So exact, indeed, is this relation that if a famil-
iar sharp pattern be cut in opaque material and applied during
the experiment to the upper face of a leaf, that pattern is found
reproduced in equivalent sharpness when the iodine test is ap-
plied; and not only this, but if a photographic negative be used
instead of the pattern, the picture will be printed very accurately
in starch in the leaf, and may be ''developed" in remarkable
fashion by the addition of iodine. For full success in these two

24 The Living Plant

latter experiments, however, special appliances and methods
are necessary; and these are fully described in the various works
devoted to experimental plant physiology, and mentioned in the
preface to this book.

If the reader should experiment at all widely upon this matter
of starch formation in leaves, he will sooner or later come upon
kinds which exhibit no starch whatsoever, even under perfect
conditions of light. Chemical analysis, however, always shows
this fact, — that such leaves contain an equivalent amount of
some sugar. Moreover, and this is a matter of consequence,
analysis shows also that even the starch-forming leaves contain
a sugar, and that, furthermore, it is from this same sugar the
starch is made. We come therefore to a generalization of the
greatest phj^siological consequence, the second, in fact, of the
great botanical verities, and one which the reader should fix deep
in his memory and incorporate with his visualized image of the
working green plant, that plants containing chlorophyll make in
the light a sugar which is commonly transformed into starch. The
process being one of formation, or synthesis, under action of
light, is called scientifically photosynthesis, while the substance
made is the photosynthate.

It will sooner or later occur to the reader to ask, especially
if he has tried these experiments for himself, whether this photo-
synthetic sugar is simply a transformation of something already
existent in the plant, or a new substance that has been added
thereto. This can be settled by the conclusive test of compara-
tive weights; for, obviously, if it is a transformation, photo-
synthesis would not be accompanied by increase in weight while
if a new substance it would. It is with difficulty that I resist
the temptation to describe to the reader the simple but highly
satisfactory methods and instruments by which this important
matter is experimentally determined; but my book has limits,
and besides I am well aware that any attempt to exhaust my sub-
ject is likely to produce a similar effect on my reader. So I must

The Prevalence of Green Color in Plants 25

simply state that the result of the test is perfectly conclusive, — it
shows that leaves, apart from varying amounts of water they con-
tain, always gain weight in the light but not in the dark. They
are always heavier in the evening than they were in the morn-
ing. As to what becomes of the starch and sugar which disappear

This square is Jo of a meter (a decimeter) on a side, and fj^ of a meter
in area.

An area of leaf exactly equal to this square would make iJo of a gram
of grape sugar in an hour, or ^ of a gram in a day, or 1 gram in
10 days, or 15 grams (which is h of an ounce) in a summer.

This amount of grape sugar made in a summer, viz. 15 grams, would
form a cube 2.15 centimeters on a side, the size of the small
square in the lower right hand corner of this square. Or, it
would form a layer over this entire square 1 millimeter (^V of
an inch) thick, the thickness shown by the space between the
larger and smaller squares.

Fig. 3. — Diagram to illustrate the quantity of photosynthate made per unit area

of leaf.

from the leaf, that will later be shown, though we may here note
in passing that there is a continuous movement of the sugar from
the leaves into the stem. Furthermore, this same method en-
ables us to establish the amount of the increase in weight. This
varies greatly, of course, with different plants and under different

26 The Living Plant

conditions of light; but calculations have shown that for many
plants collectively out of doors it approximates under average
summer conditions to one gram for each square meter of leaf
area per hour (scientifically expressed 1 gm'li), or one twenty-
fifth of an ounce per square yard per hour, and is about half that

Fig. 4. — These cubes, which are two-fifths the original size, show the amount of solid
crystalline grape sugar made bj' a square meter (or yard) of leaf in an hour, a day, and
a summer.

amount in greenhouse plants in the winter. This figure con-
stitutes one of those useful conventional constants which the
reader should store in his mind, and keep ready for use. Ex-
pressed in a different way, a leaf forms in a summer enough
photosynthetic sugar to cover itself with a solid layer a millimeter

The Prevalence of Green Color in Plants 27

(one twenty-fifth of an inch) thick. The same quantities are
also expressed in a graphic way in the accompanying figure 3, and
still more expressively, perhaps, in figure 4.

We must now examine more closely the photosynthetic sugar
and starch which appear in lighted green leaves. The microscope
does not show much about them, for the sugar is always dis-
solved in the sap of the cells, and the starch, although solid, is in
grains too small to be seen very clearly. Their chemistr}', how-
ever, is well-known and important. The sugar is of more than
one kind, but the commonest is that known as grape sugar,
or dextrose, which has the chemical composition, CgHjoOg, and
which is intermixed with some fruit sugar or fructrose having an
identical formula. This formula, I need hardly say to the reader
of this book, means that this sugar is composed of 6 parts of
carbon, 12 of hydrogen and 6 of oxygen, though why this particu-
lar combination of these three diverse elements should give a
substance with the properties distinctive of grape sugar, nobod}'
yet knows. IMuch less abundant in leaves is cane sugar, which
has the composition C12H22O11. Starch has for its formula
(06Hio05)/i, the n meaning a multiple, though for our purposes
we may treat it simply as CgH^o^s- Now it is immediately
obvious that these three substances, so closely associated in the
leaves of plants, are also very closely related in their chemical
composition, for they differ from one another only in their relative
proportions of hydrogen and oxygen. Thus, —

CeHi^Oe - H2O = CgHioOs
grape sugar water starch

CisHooOu + H2O = 2 parts CgHisOe

cane sugar water grape sugar and fruit sugar.

CeH.oO^ + H2O = C6H12O6

starch water grape sugar

2 parts C6H12O6 — H2O = Ci2H220n

grape sugar water cane sugar

These three important substances thus differ, so far as their

28 The Living Plant

composition is concerned, simply in the proportions of the in-
corporated water, though this tells by no means all of the story;
but it helps to explain why thej^ are so easily transformable by
the plant one into the other. Taken together the facts suggest
the probability that one of the three is a first-formed or basal
substance from which the others are transformed. In a general
way chemical research sustains this hypothesis, and points to
grape sugar as the usual basal substance first formed in the light
in green leaves. For all of our purposes, therefore, we may accept
grape sugar as the conventional basal photosynthate, and its
formula (Cg H^g Oq) should be fixed by the reader in his memory
as another of the valuable conventional constants.

It may seem to the reader just here that in treating this sugar so
fully, I dwell overlong on a point of only subordinate value. But
in this my critic would err, for, as a later chapter on the subject
will show in detail, this photosynthetic grape sugar is the material
from which, with certain transformations and some additions,
plants make all of their substance and special materials, includ-
ing their protoplasm, and derive all of their energy for work; in
other words, it is their food. And since animals all take their
sustenance, whether directly or indirectly, from plants, it is the
basis of their food also. These facts may conveniently be brought
together, even though somewhat in advance of all of the evidence,
in this generalization, which constitutes another of the great
botanical verities, — that the photosynlheiic grape sugar formed
in green leaves in the light is the basal food of both ylants, and ani-
mals. This sugar is therefore one of the three most important
substances in organic nature, chlorophyll and protoplasm being
the other two.

Our next task is sufficiently obvious; we must find the source of
supply of the materials entering into the composition of the sugar,
which, the reader will remember, is an addition • to the plant.
Now a scrutiny, from this point of view, of its formula, viz.,
CgHiaOg, at once reveals the suggestive fact that the H and the O

The Prevalence of Green Color in Plants 29

are present in exactly the proportions they exhibit in water,
(H2O) ; this suggests that they may be derived from the water
which, absorbed from the soil, always saturates the tissues of the
living plant, and this hypothesis is confu'med by experiment. As
to the carbon, a supply thereof exists both in mineral compounds
in the soil, and also in the carbon dioxide, commonly called car-
bonic acid gas, in the atmosphere. But experiment easily de-
cides between these two sources, for when plants are grown in a
soil or in water from which every trace of carbon is excluded,
the plants make their photosynthate as readily as ever, thus ap-
parently proving that the carbon must come from the air. At
first sight it may seem an objection that this gas exists in the
atmosphere in such an extreme of dilution, for it comprises only
3 parts in 10,000, that is .03 (or j^) of 1 per cent. This amount
is very small, it is true, though we must remember that the bulk
of the whole atmosphere is vast in proportion to the bulk of all
plants. However, suppositions cut small figure in comparison
with facts; and it is easy to prove by sunple expermients that
leaves, or even small parts thereof, exposed to an atmosphere
from which the carbon dioxide has been removed, can make no
starch at all, although neighboring leaves or parts, exposed in
the ordinary atmosphere, form it abundantly. Indeed, innumer-
able facts unite to prove that the carbon used by leaves in the
making of sugar is derived from the carbon dioxide (the carbonic
acid gas), of the atmosphere. This, as the reader well knows,
is the very same gas which is poured out by animals in breath-
ing, by organic substances in decaying, and by fires in burning.
The fact that leaves absorb this gas in making their sugar ex-
plains in part the scientific basis of a widely known and very
important phenomenon, — that plants purify the air which is
vitiated by animals.

All chemical processes can be expressed in equations of the
formulae of the substances concerned, and therefore we proceed
to set down together the formulsG of the carbon dioxide (viz.,

30 The Living Plant

CO2), and water, with the formula of the grape sugar they form,
thus, —

In photosynthesis CO2 and H2O form CgHigOe

carbon dioxide water grape sugar

Obviously now the proportions of the two former must be in-
creased in order to yield the latter, thus, — -

6 CO2 + 6 H2O are needed to form CgHiaOe

But a chemical equation must balance exactly on the two sides,
and this in the present case can occur only thus, —

6 CO2 + 6 H2O = C^HioOo + 6 O2

But such a balance of the equation miplies that, in the making
of sugar from carbon dioxide and water, oxygen is set free, and
not only so, but in a volume exactly equal to that of the carbon
dioxide absorbed. So striking a conclusion based upon purely
theoretical evidence demands rigid test through observation or
experiment. That a gas of some kind is released from green
plants in the light is easily seen in submerged water plants which,
if kept in an aquarium, give off tiny bubbles when lighted, though
not in the dark; and everybody has seen those large gas bubbles
which are caught in the felted green scum-plants floating on ponds.
Analysis shows that the bubbles, in both cases, consist mainly of
oxygen. But the matter can be tested much better by experiments.
In a word, it is only necessary to place a green plant or a leaf in
a suitable tight glass chamber, give it a known quantity of carbon
dioxide (it has plenty of water), expose it for some time to the
light, and then make a chemical analysis of the air in the chamber.
The experiment yields an invariable result. A certain amount
of the carbon dioxide has disappeared, and in its place there is
present an exactly equivalent amount of pure oxygen. As to the
significance thereof, it seems plain that the oxygen is a by-
product formed incidentally in the chemical transformations, and
useless in the main process.

The Prevalence of Green Color in Plants 31

Thus is our equation triumphantly vindicated, and we shall
know it henceforth as the photosynthetic equation. Its importance
and meaning may thus be expressed as another of our botanical
verities, — that the 'photosynthetic sugar made in green leaves in
light is constructed from water drawn from the soil, and carbon di-
oxide derived from the atmosphere, with an incidental release of pure
oxygen, according to the jjhotosynthetic equation 6 COo + 6 H/) =

It may interest the reader now to know what quantities of
these gases are necessary in the making of the sugar. For one
gram thereof there are required 750 cubic centimeters (about
f of a quart) of pure carbon dioxide, which is all that is con-
tained in 2 cubic meters of atmosphere, and there is released the
same quantity of pure oxygen. This, therefore, is the amount
of those gases absorbed and released by a square meter (or yard)
of green leaf each hour on a bright sununer day. This release
of oxygen, by the way, explains the remainder of the fact earlier
mentioned, that plants purify the air which animals vitiate, for
the plants not only remove the poisonous carbon dioxide from the
air, but replace it by pure oxygen. And it may interest the reader
to know how this balance of purification and vitiation works out
between green leaves and men. Calculations have shown, in
brief, that about 25 square meters (or yards) of green leaf are re-
quired to balance the respiration of a man on an ordinary sum-
mer day. But as the release of oxygen stops at night, it takes
about 60 square meters of leaf working for a day to balance the
man's respiration for 24 hours, and about 150 square meters work-
ing through the summer to balance his respiration for a year.

In composing the foregoing paragraphs I have given much care
to the form of their presentation, for the reason that this particu-
lar topic illustrates exceptionally well the principal method of
scientific procedure in the acquisition of new knowledge. First,
in the given problem, to observe all the facts that the militant
eye can discover : next to compare and marshal the data thus won

32 The Living Plant

with a view to finding an explanatory principle : then to express the
most probable conclusion in tentative form as an hypothesis:
and finally to devise experiments whereby the truth or falsity
of the hypothesis may be tested; these are the constituents of
that scientific method through which all of our great scientific
triumphs have been won. Hypothesis is a kind of a scout which
Science sends on ahead to spy out the way for a further advance.*
For the completion of our subject of photosynthesis, there re-
mains but one matter of consequence, and that is the explanation
of the association of light and chlorophyll with the process. We
have seen earlier that the chlorophyll occupies a position between
the light and the new-made starch or sugar, which fact implies
that it forms a necessary link between the two. This in turn
would suggest that the chlorophyll perhaps acts on the light in a
way to make it available for the photosynthetic process. Tak-
ing this hypothesis for guidance, we turn to investigate the effect
that chlorophyll exerts upon the light which penetrates into it.
Now the sunlight, as everybody knows, is a composite mixture
of vari-colored lights, which, taken together, give the impression
of whiteness. If this sunlight, however, be passed through
chlorophyll, whether a living leaf or a solution in alcohol, there
issues, as the reader will recall, only a clear green, or yellowish-
green, light; and this fact seems to imply that all of the colors

* That this is in practice, as it is in theory, the method of scientific men in their
researches is illustrated by the following passage from the wTitings of the great
German physiologist, Sachs. In connection with this very subject of starch forma-
tion, he tells of his preliminary observations, on the basis of which, he says, — " I
came to the conclusion in 1862 that the enclosed starch, which had already been ob-
served in the chlorophyll-corpuscles by Naegeli and Mohl, is to be regarded as the
first evident product of assimilation [i. e., photosynthesis] formed by the decom-
position of carbon dioxide. I said to myself, if this view is right, the formation of
starch in the chlorophyll-corpuscles must cease on the exclusion of light, since the
decomposition of carbon dioxide can then no longer take place ; and that in like man-
ner renewed access of light to the chlorophyll-corpuscles must also bring about a
renewal of the formation of starch in them. These and similar deductions were con-
firmed by appropriate investigations." {Lectures on the Physiology of Plants,
Oxford, 1887, p. 307.)

The Prevalence of Green Color in Plants

33

in sunlight have been stopped by the chlorophyll excepting only
the green. But the human eye is far too crude an analyzer of
color to be scientifically trustworthy, and we turn for aid to an
instrument which science has devised for the exact analysis of
light, — the spectroscope. I confess, it is only with reluctance that
I refrain from explaining to the reader the principle of this beauti-

FiG. 5 — Diagrams to illustrate analysis of light by the spectro-
scope, a. Spectrum of pure sunlight, b. Spectrum of sun-
light passed through chlorophyll.

ful instrument, one of the most delicate and exact, but withal one
of the smiplest in theory, of all that have yet been evolved
in the progress of science. It must suffice to say that the spectro-
scope takes any mixture of colored lights, no matter in what
complication, and, through the mediation of a prism, spreads
them out in a band (called a spectrum), each color by itself. So,
when a ray of white sunlight is sent into this instrument, it is
made to fringe out into its red, orange, yellow, green, blue, in-
digo and violet constituents, all beautifully clear and distinct, as
shown diagrammatically in our accompanying figure 5, a. Now

34 The Living Plant

if, while one is observing this spectrum, a solution of chlorophyll
is inserted into the path of the light, a remarkable phenomenon
follows, for the green liquid blots out from the spectrum most of
the red and nearly all of the blue-indigo-violet, making those
parts of the spectrum quite black, while all of the rest of the colors
are left practically unaffected, as represented in our diagram
(figure 5, b). Chlorophyll, therefore, has power to absorb red
and blue rays out of the sunlight, ignoring the others, — in ob-
serving which fact the active scientific mind would jump straight
to the conclusion that these red and blue rays are probably the
ones which are useful in photosynthesis. This hypothesis also
is easily tested by experiment, for, obviously, if the red and blue
rays really are those used in photosynthesis, while the others are
not, then starch ought to be made under red light and blue light,
but not under any others of the colors of the spectrum. It is
possible to supply the different colored lights singly to the green
leaf, either by use of colored glasses or liquids or by throwing a
solar spectrum directly upon a leaf. The result of the experiment
is conclusive; a leaf can form starch very readily under red light
or blue light; but it can form none at all under the yellow, orange,
or green. It seems a safe inference, therefore, that chlorophyll is
a substance which picks out of white sunlight and applies to
photosynthetic work, just those rays which can be utilized in the
photosynthetic process, while rejecting the others; and all evidence
attests the correctness of this conclusion.

This conclusion, however, raises a correlated question, which is
this, — for what particular purpose is light needed in photosyn-
thesis? Light, of course, is a form of energy, like heat and elec-
tricity; and energy is the source of power which underlies every
kind of work. Light, so physicists teach, consists of wave-
motions in a space-pervading medium called the luminiferous
ether; and the motion of these ether waves forms a source of
power that can accomplish work, just as surely as can the billows
of the ocean. Our problem, then, resolves itself into this, — is

The Prevalence of Green Color in Plants 35

there in photosynthesis any step requiring the doing of work,
and therefore the expenditure of energy? Our photosynthetic
equation supplies the answer, for it shows that the oxygen set
free has to be torn away from either the carbon or the hydrogen
of the carbon dioxide or water, as a necessary preliminary to the
union of the carbon with the remaining elements to form sugar;
and other evidence shows that the carbon dioxide at least
is thus dissociated. Now carbon dioxide is among the most
stable of natural compounds, which means that its constituent
atoms have an extremely strong affinity for one another, which
means in turn that ample power must be exerted to tear them
apairt. Most people know that in our laboratories water can be
separated into its constituent hydrogen and oxygen only through
action of an electric current (electrolysis), or of intense heat; but
carbon dioxide is even more difficult of dissociation. Here then,
in the preliminary dissociation of this very refractory substance
is that need for energy which we seek; and all the results of re-
search confirm this conclusion. Why it should be the red and
blue rays and no others which can do this work, we do not yet
know, nor yet precisely the way in which the chlorophyll applies
them to the task; but there is no question as to the facts. That is,
chlorophyll is a transformer of light energy into photosynthetic
work; and there j^ou have the explanation of its function in
plants, and the reason for its overwhelming prevalence in
vegetation.

We can now summarize this part of our subject as another of
our botanical ver.ities, — the formation of photosynthetic sugar in
leaves requires first the dissociation of the refractory carbon dioxide,
which is effected by the energy of the red and blue rays of the sunlight,
applied to that work by the chlorophyll.

It will perhaps contribute further to clearness if we summarize
the whole process of photosynthesis from another, and very human
point of view. The formation of the photosynthetic sugar, the
end of the whole process, is, after all, a manufacturing process

36 The Living Plant

comparable directly with those carried on by men, as the fol-
lowing table well shows.

The Factory The Leaf, or other green structure.

Rooms therein The cells.

The power Sunlight, the red and blue rays.

The machinery Chlorophyll.

The raw materials Carbon dioxide and water.

The manufactured product Grape Sugar.

By-products Oxygen.

The photosynthetic machinery can not only be apprehended,
but also represented in a mechanical plan, as our accompanying
diagram illustrates (figure 6). It represents the parts concerned
in the process, (shown simplified in figure B on Plate I,) reduced
each to a single one, and given a regular shape, though otherwise
constructed and related as in the plant. Later we shall consider
exactly the forces which keep the gases and liquids in motion
in the suitable directions.

The reader should now be able to visualize, or see vividly in
imagination, this process in progress. Streaming in through the
stomata and along the air passages is a steady current of the tiny
particles, or molecules, of carbon dioxide, which reach the cell
walls, pass in solution through these and the protoplasm into the
chlorophyll grains, where they meet with water supplied in a
continuous stream by the ducts. Here in the grain the chloro-
phyll is stopping the red and blue light, and turning their vi-
brating waves against the molecules of the carbon dioxide in a
way to shatter that substance into its constituent atoms. The
carbon thus forced apart from its own oxygen is uniting with
the elements of the water into sugar, which is streaming into the
sap cavity and then away through the sieve tubes, while the dis-
carded oxygen is passing out from the grains through protoplasm
and wall to the air space, along which it is streaming to the
stomata and the outside world. And this is what is occurring
inside of all leaves through all the bright days of the summer.

The Prevalence of Green Color in Plants

37

So striking and far-reaching are the conclusions already reached
in this chapter that anything added thereto must come as a kind
of anti-climax; and therefore I wish we could stop just here.
Moreover the chapter is al-
ready over-long, though no
longer, I maintain, than the
relative importance of its
subject sufficiently justifies,
especially as it seemed to me
best to make this first treat-
ment of very important top-
ics illustrate the methods
through which our scientific
knowledge has been gained.
Yet several closely related
matters, especially concern-
ing the colors of plants,
should have our attention
before we depart from this
subject, though I venture to
suggest to the reader that he
should not attempt to read
all of this chapter at one
sitting, but reserve the fol- ^ ^ , ,. , ^u ^ . ^u ^■

°' Fig. 6. — A diagram of the photosynthetic ma-

lowing part for a time by chinery, showing the parts reduced to the low-
est possible terms, viz., a single living cell, with
a single chlorophyll grain, a water-carrying
duct (on the left) and a sugar-carrying sieve-
tube (on the right) ; shading is protoplasm.
be dismissed very briefly. The circles are water; the squares are carbon

*^ dioxide; the triangles are oxygen; the crosses

Is it quite clear to the reader are grape sugar; the arrows show the direc-

^ . tions of movement.

why chlorophyll looks green Cells magnified about 200 and molecules about

to the eye? This, indeed, ---n-- times.

is told very plainly by the spectroscope, when it shows that
chlorophyll, in stopping the useful red and blue rays from
the light of the sun, allows the other and useless rays to

itself.

One of these matters may

38 The Living Plant

pass through as waste; and these of course are the ones which
come to our eyes. Now these waste rays include the entire green
light, which gives the principal color, together with all of the yel-
low, which, mixing with the green, gives thereto the characteristic
yellowish tinge which chlorophyll always shows. As to the re-
maining rays, they happen to form complementary pairs; thus
the bit of red and bit of green-blue form one pair, while the orange
and unabsorbed blue form another; and as complementary colors
(with lights) always give white or gray, these minor rays thus
neutralize one another so far as color is concerned, and do not at
all affect the positive yellow-green. If it had happened that, in-
stead of red and blue, the red and green had been the useful rays,
then chlorophyll, and all vegetation, would have looked blue; and
had green and blue been the useful kinds, then all vegetation
would have looked red. The greenness of vegetation is simplj^
the wastage of that part of the white light of the sun which is not
needed in photosynthesis.

In the early part of this chapter it was mentioned that many
leaves of a red color really possess chlorophyll, which becomes
visible when the red is removed by suitable solvents. This is
true of the seaweeds, the red and brown colors of which are due
to special pigments in the same grains with the chlorophyll; and
there is good reason for believing that these colors bear a relation
to the light conditions under which those seaweeds live, aiding
the chlorophyll to utilize the sunlight as altered by its filtration
through water. The case in the more familiar red plants of garden
and field, however, is different. The colors in the foliage plants
(Coleus, Copper Beeches, Japanese Maples) as well as in some
vegetables (Beet, Red Cabbage), is a product of enormous in-
tensification under cultivation ; but in all cases the wild ancestors
of these plants possessed some red color to begin with. This red,
indeed, is fairly common in wild plants, where it shows especially
in veins, petioles, nodes, or the under sides of leaves, and in the
stigmas of many wind-pollinated flowers. It reaches, however,

The Prevalence of Green Color in Plants

39

its most striking, though a temporary, development in the young
shoots of a good many plants (e. g.. Maples, Oaks, and many
herbs), which it flushes with translucent rose red as they push
from the buds in the spring, though later it fades away with the
increasing rapidity and vigor of growth. In all of these cases
the color resides in a particular substance, named erythrophyll
(or anthocyan), dissolved in the sap of the cells, from which it
can usually be extracted by hot water. It is typically a beautiful
deep rose red material, varying much, however, in tint according
to the conditions surrounding its formation, and the substances
with which it is associated. Its identity, therefore, is plain enough,
but concerning its significance to the plant there is very much
doubt. One explanation argues thus; erythrophyll, as its color
implies, permits the red rays of sunlight to pass unaltered, but
cuts off, or at least weakens, the blue-violet ones. Now it is
known that the red rays, while the most useful in photosynthesis,
are harmless to the living protoplasm, but the blue-violet rays,
though also useful in photosynthesis, are injurious, when un-
tempered, to the living protoplasm and detrimental to some of
the physiological processes; therefore (runs the argument), the
erythrophyll probably acts as a protective screen, especially in
the case of the early spring vegetation, admitting the beneficial
red rays and tempering the noxious blue-violet rays until the
formation of the chlorophyll, which, while developed for a differ-
ent purpose, incidentally acts as a protection to the protoplasm, —
a subject to which, by the way, we shall return for fuller discussion
in the later chapter on Protection. A second explanation is based
upon the fact that erythrophyll has been found to possess a not-
able power of transforming light into heat; it must therefore
serve, this argument holds, to warm the tissues which possess
it, and thus, during the bright but cool days of the spring, must
facilitate those processes, such as nutrition, translocation of food,
and growth, which are promoted by warmth. More recently a
third explanation has been offered, based upon the fact that when-

4o' The Living Plant

ever bright light, a relatively low temperature, much sugar, and
some tannin happen to come together in a living cell, then the
substance erythrophyll, of which the composition-color happens
to be red, is formed incidentally as a purely passive chemical
result. On this view the red color may be purely accidental,
and may have no utility whatever to the plants which possess
it, though the possibility is not thereby excluded that the plant
may bring those conditions together, adaptively, in a cell where
it has need for the red color to appear. The substance of the whole
matter in reality is this, — that we do not yet know surely the
significance of erythrophyll in the plant; and herein lies another of
the problems which make science so ever alluring.

Connected with chlorophyll in a different way is one of the
most striking and beautiful of all the phenomena of nature, the
transition in the foliage each season from the uniform green of
summer to the brilliant colors of autumn. Strangely enough, for
a subject so important, our knowledge thereof is still very im-
perfect, and there is even a difference of opinion as to the very
significance of the colors to the plant. A basal fact, however,
upon which there is agreement, is this, — that the autumn color-
ation results from changes connected with the death and fall of
the leaf. We know that in late summer our trees are preparing
for the annual leaf fall, in anticipation of which they are gradually
bringing the activities of the leaves to a close, ceasing to make new
chlorophyll, withdrawing certain precious materials into the stem,
and building right across the bases of the leaves those corky layers
which both cut them away from the stem, and also heal in ad-
vance the wound that is thus to be made. Now chlorophyll, as
the reader's own experiments will have shown, is soon destroyed
by bright light ; this destruction, indeed, is continually in progress
throughout the summer in the living green leaves, where the color
is maintained only through virtue of its constant renewal. It
was formerly believed (and I mention the matter because the
statement persists even yet in some writings), that this chloro-

The Prevalence of Green Color in Plants 41

phyll left in the leaf when the new supply ceases to form, breaks
down in the light to other substances, which either themselves
are highly colored, especially red, or else unite chemically with
other materials in the cells to form colored compounds, the autumn
colors being supposed, on this view, to be simply an incidental
product of chlorophjdl decay. But later research has shown this
supposition to be wrong, for chlorophyll, in breaking down, does
not form colors, but fades away to transparency in the leaf pre-
cisely as it does in the alcoholic solution which the reader has
placed in the sun. Now, sooner or later in the autumn the waning
activity of the leaf reaches a point where no more chlorophyll
is made, after which all of that substance already present fades
away, with this notable result, — that its disappearance renders
visible any other colors which may have been present in the
leaf, but masked by the greater brilliance of the green; and this
fact constitutes the basal step in the explanation of autumn color-
ation. As a matter of fact leaves do contain other coloring mat-
ters, especially a bright yellow material, called xanthophyll,
occurring in tiny grains associated with the chlorophyll. It is
the exposure of this xanthophyll by the fading away of the chloro-
phyll w^hich gives the yellow, most conmion of the autumn colors,
to autmiin leaves. If the reader desires, he can himself extract
this xanthophyll, and very easily, in a beautiful clear yellow solu-
tion, by treating yellow autumn leaves precisely as he did the
green leaves for extraction of chlorophyll, but using much leaf
in proportion to the quantity of alcohol. Indeed the reader has
seen the xanthophyll already, for, as he will recall, w^hen he placed
his solution of chlorophyll in the sun it faded away not to a trans-
parent whiteness but to a clear yellow; this was xanthophyll,
which itself fades away extremely slowly to whiteness. The
whole situation must now be quite clear. Chlorophyll and xan-
thophyll exist together in leaves, from which indeed they can be
extracted and separated in beautiful solutions well known to
all students in physiological laboratories; but xanthophyll is

42 The Living Plant

ordinarily completely masked by the far greater brightness of the
chlorophjdl, though it has influence enough to give the living
leaf its yellow-green rather than a pure-green color. But xan-
thophyll is vastly more resistent to the action of light than is
chlorophyll, which explains its persistence in both leaves and
solutions. The precise function of the xanthophyll, by the way,
is not known, although it seems probable that this is to be found
in some incidental chemical connection with the chlorophyll, in
which case its persistence in autumn leaves is purely incidental
and of no service to them.

Second in abundance, though first in brilliance, among autumn
colors is red, which has a very different origin. It is due to the
presence of that same erythrophyll which we have already con-
sidered in connection with foliage plants and the spring coloration.
This erythrophyll, also, the reader can extract for study in a beau-
tiful clear rose-red solution by aid of the method he used for the
chlorophyll, excepting that water must be used instead of alcohol,
and the material should be abundant and consist of the very
brightest red leaves he can find. Unlike the xanthophyll the
erythrophyll is not present in the leaves before the chlorophyll
fades away, at least not in appreciable amount; but it forms as
the disappearance of the chlorophyll admits the light to the in-
terior of the leaf cells. That the presence of bright light is es-
sential to its formation is easily proven by experiment, and by
the readily observable fact that in cases where one red autumn
leaf overlaps another closely enough to shield it largely from light,
the darkened portion is yellow not red ; and this same fact further
proves that red autumn leaves are actually yellow underneath the
red. The brilliancy of the red, indeed, is proportional in general
to the brightness of the light. But light alone is not sufficient
to produce a formation of erythrophyll without the presence of
the chemical substances requisite to its formation, which include
certainly sugar and probably tannin; and it is only those leaves
which happen to contain a sufficiency of these materials that can

PLATE III

The Prevalence of Green Color in Plants 43

turn red at all, the others being restricted to yellow. The Maples
and the Oaks are trees well-known for their richness in sugar or
tannin, which helps to explain why those particular trees are
more brilliantly red than most others. It happens, furthermore,
that erythrophyll foimation, contrary to the usual rule with
chemical processes, is promoted by lower temperature; and this
explains why it is that a cool season promotes the l^rilliance of
color, which indeed reaches its highest perfection in seasons or
places where the skies are very bright and the frosts come
early.

Thus much for the facts as to the yellow and red autumn color-
ation. We have now to take notice that two conflicting views
exist as to its significance to plants. Many botanists believe that
since erythrophyll seems to have definite functions in spring
vegetation (as we have seen a few pages earlier), it has also an
identical function in the leaves of the autumn, acting usefully
as a selective light screen. The argument runs thus: — chloro-
phyll fades away in the leaf before the protoplasm has wholly
ceased its activity: full exposure to bright sunlight, especially
the untempered blue-violet rays, would injure this protoplasm,
and act unfavorably on the translocation of the valuable materials
from the leaf into the stem : an erythrophyll screen must temper
the blue-violet rays while permitting the passage of the red rays
which are not simply harmless but, being warm rays, would actually
aid the final vital processes of the leaf during the cooling days of
autumn. And those who hold this view assmiie that xanthophyll
must have something of the same action, though inferior in degree
to erythrophyll. On this view autumn colors are beheved to be
useful if not indispensable to the plants which possess them, and
inferentially, have been developed adaptively to such use.

Sharply contrasting, however, with this utility explanation of
autumn coloration is the view that it is merely incidental. While
the utility theory has certainly some facts in its favor, the most
of the evidence seems to me heavily against it. Thus the utility

44 The Living Plant

theory, that of the protective and heating screen, requires in
autumn leaves certain features which the spring coloration does
in fact to some extent exhibit, — viz., a prevalence of red rather
than yellow, a fairly uniform coloration over all the parts to be
protected or warmed, an especially deep coloration in the conduct-
ing parts, and a fairly constant development of the color year after
year without much regard to the details of the weather. As a
matter of fact the phenomena of autumn coloration are differ-
ent at almost every point — red is less common than yellow; the
colors are very uneven in distribution, forming spots, blotches,
and streaks; the color shows no particular tendency to cover the
conducting veins: and its intensity varies greatly in different
years, even almost to suppression of red in certain kinds of leaves
in some seasons. The utility theory of autumn coloration re-
ceives, therefore, no support from comparison with spring color-
ation, even granting, as is not at all certain, that the latter is
useful. The facts, therefore, taken all together seem to favor
the incidental theory, which may thus be expressed ; — that autumn
coloration, for the most part at least, is a purely incidental result
of the chemical and physical conditions which happen to prevail
in ripening leaves and around them, and has in it no more element
of utility than has the red of a sunset or the blue of the firmament.
The yellow and red in the autumn coloration are so much more
common and striking than any other colors that they naturally
attract the most of our attention. Yet other colors occur, as
everybody knows well, and as appears very clearly on the accom-
panying plates (Plates II, III), which represent a selection from
New England autumn vegetation, photographed in the natural
colors. In fact, however, the great variegation thus displayed
results from permutations and combinations of a very few colors.
In addition to the red and yellow, there is only one other pigment
at all common in autumn leaves, and that is an occasional brown,
the mode of formation of which is uncertain. Most of the brown
color in such leaves, however, belongs to the cell-walls, which are

The Prevalence of Green Color in Plants 45

white-transparent when alive, but turn brown on their death and
decay. In fact the conditions prevailing in the ripening and dying
leaf are most complex, for not only are different chemical sub-
stances and physical forces interacting in large number, but their
interrelations are constantly changing as the death of the proto-
plasm weakens its regulatory control upon them. This combina-
tion of complexity and changeability produces a state of unstable
equilibrium, which permits even very minor external influences
to exert relatively great effects, — and thus is explained the differ-
ences in the coloration of the same plants in different seasons or
different places. In general, however, the effects of the weather
upon the intensity of coloration are clear. Thus a bright autumn
(and, equally, a sunny climate) intensifies the coloration, at least
for the red, while dull weather is accompanied by dull coloration.
Early frost helps somewhat to intensify color, partly by hastening
the death of the leaf, and partly by aiding the chemical formation
of the erythrophyll ; though frost is not, as many suppose, a cause
of the coloration itself. Furthermore, the coloration can be
brought on much earlier in the season than usual by any injury,
— a break in the bark, a split in the trunk, some damage to the
roots, — which weakens the vitality of the tree and hence pro-
motes the waning of life in the leaves; and this is the explana-
tion of the occasional reddening of a single branch, or even
whole tree, which one finds turning sometime ahead of its
neighbors.

The reader will feel, I am sure, that this is an unsatisfying answer
to his natural wish for a definite knowledge of the causes of
autmim coloration, but it is all that the present state of our
knowledge permits. The subject has been studied heretofore
by botanists from their side, and by chemists from theirs; but
its problems will not be solved until some competent investiga-
tor takes autumn coloration as his unit, and attacks it by any and
all methods,— chemical, physical, physiological, observational,
experimental, or any others essential for attaining his ends.

46 The Living Plant

Some day this will be done, and then we shall know the meaning
of autumn coloration just as surely as we now know the causes
of the colors of chlorophyll, of fruits, and of flowers. Meantime,
it is not the least of the pleasures of science that everywhere about
us lie problems of moment, whose progress towards solution we
may constantly watch, and the trimnph of whose conquest we
may perhaps even share.

CHAPTER III

THE PROFOUND EFFECT ON THE STRUCTURE OF PLANTS
PRODUCED BY THE NEED FOR EXPOSURE TO LIGHT

Morphology and Ecology of Leaves and Stems

N the foregoing chapter we have considered photo-
synthesis solely as a physiological process operating
within the body of the plant, and have taken no thought
for any relations it may have with the world outside.
Yet the internal process is dependent on the external world in
this very fundamental particular, that the supply of the indis-
pensable light, carbon dioxide and water has to come from out-
side. Furthermore, and this is a point of importance, the en-
vironment rarely offers these essentials in precisely the riglit
quantities, but sometimes too abundantly, oftener too sparsely,
and sometimes in ways involving grave dangers. Their photo-
synthetic needs plants cannot help, and their environmental
conditions they cannot change, but there is one thing that is al-
terable, and that is their own structure, with its large poten-
tialities of adaptive development. Accordingly, in the course of
long ages of slow evolution, plants have become so molded in
form and in structure as to bring the photosynthetic process
into advantageous or adaptive relation with the conditions of
supply of the photosynthetic essentials outside, and in such man-
ner, moreover, as to permit of particular adjustment to special
peculiarities of the surroundings. Plants are like housekeepers
who possess certain needs, and a desire for having the best, but
who have no control over the purse-strings ; under the circumstances
there is nothing for them to do but adjust the scale and style of

47

48 The Living Plant

the establishment to the exigencies of a fixed income. This is
the real meaning of the photosynthetic adaptations, which it is
now om" business to consider. Each one of the physiological
processes of plants produces, of course, in like manner its
effect upon their structure ; but the one process of photosynthesis
far surpasses all others, indeed all others put together, in
the profundity of its influence in making plants what they
actually are. The evidence thereof will appear in the following
pages.

The photosynthetic essentials for which plants are dependent
upon the environment are in reality four, because, in addition to
light, carbon dioxide, and water, plants need also, for reasons that
will later appear, certain minerals, which are, however, for the
most part very widely distributed in soils. Now in showing the
way in which these four are supplied by the environment to plants,
I must recall to the reader some very familiar and commonplace
facts. But I remind him that there is nothing in the world so
difficult to see in its real significance as the commonplace; more-
over let him remember the truth expressed by a brilliant writer
in the saying that little minds are interested in the extraordinary,
but great minds in the commonplace.

The crucial facts about the mode of supply of the four photo-
synthetic essentials are these.

First. They all exist widely even if not abundantly distributed in
nature, and moreover are incessantly in movement or circulation, —
the light with the swing of the sun through the heavens, the car-
bon dioxide with every breeze that stirs the still air, the water
in the form of the mists and the rain, and the minerals in solution
in the water which soaks and drains through the soil. Therefore
plants have no need to go in search of these essentials, as animals
must for their food, but are able to stay fixed in one place and
allow the essentials to be brought them by the general circula-
tion of nature. This method renders needless any self-motive
power, with the accompanying muscular system and jointed skele-

The Profound Effect on the Structure of Plants

49

ton such as animals must have, and permits a simply continuous
structure. This is why plants are sedentary beings, rooted for life
in one spot.

Second. The four essentials circulate in no definite paths or
directions, hut come to the plant from every point of the compass.
This is true even of sunlight, despite the regular path of the sun
through the heavens, for so uniform is the diffusion of the light
through the sky that plants really receive it from every direction.
And as to the wind, does it not blow where it listeth, and the
waters, do they not cover the earth? Therefore plants have no
need to face their parts in any particular direction, as animals
must do in connection with their movements in search of their
food, but face evenly outward in every direction, thus requiring a
symmetrical distribution of their parts around a central vertical
axis. This is why plants are radially built, presenting the same
face to all points of the compass.

Third. The four essentials are not evenly commingled, but seg-
regated into two strata, — the light and carbon dioxide in the at-
mosphere above, and the water and minerals in the soil under-
neath. Therefore plants must needs have two parts to their
structure adapted to life in these two very different situations.
This is why plants exhibit their primary division of structure into
the green shoot (leaf and stem), and colorless root.

Fourth. The four essentials exist rarely in abundance and then
never for much of the time, and most commonly are sparser than
plants can make use of. Frequently the light, always the carbon
dioxide, often the water, and sometimes the minerals are accessi-
ble only in dilution. Therefore the plant must needs reach out
extensiveh^ to come into contact with a sufficiency, — a condition in
great contrast to that prevailing in animals with their concen-
trated food and consequent compactness of body. This is why
plants are branched so profusely and slenderly.

Fifth. One of the four essentials, — viz., light, is of such nature
that it cannot be transmitted far into the plant, and therefore must be

50 The Living Plant

used at the surface. Hence plants have had to distribute the green
tissues of the shoot in a manner ensuring the exposure of a great
spread of surface to light, and this involves a flattening of most
of the tissues of the shoot to the thinnest practicable structures.
This is why leaves exist, and why the green plant consists of them so
largely.

Sixth, One of the essentials, the sunlight, falls upon plants from
every direction in the aerial hemisphere. Not only does it come from
a source which forever is changing its position in the skies, but,
furthermore, this light is so strongly diffused through the atmos-
phere that it falls upon plants from every direction in an in-
tensity which for most of the time is as great as leaves can make
use of; for it is a physiological fact that plants cannot use all the
energy contained in full sunlight, and strong diffused light is
enough for their needs. Hence it comes to pass that plants
receive light in amount and direction sufficient to illuminate a
great many leaves if only these are carried to various heights and
spaced well apart, in a general distribution answering to that
of the incident light. This necessitates the specialization of a
part of the shoot for carrying the leaves upwards and outwards.
This is the reason why stems exist and branch in such manner as
typically to carry the leaves to a hemisphere of foliage.

Thus it is evident that the most distinctive features of struc-
ture and form displayed by plants of the highest development,
the features indeed which are most closely associated with our
very idea of plants, — the sedentary habit, the radial symmetry,
the diffuse-slender branching, the primary division into shoot
and root, and of the shoot into flat leaves and supporting stems, —
all exist as adaptations which adjust the photosynthetic process
to the conditions under which the photosynthetic essentials are
supplied by the external world. It is therefore a fact that the
photosynthetic process determines the ground form and primary
structure of plants just as truly as it determines their ground
color.

The Profound Effect on the Structure of Plants 51

It is worth while to try to express the sum of these features
in diagraniniatic form, and my suggestion thereof is contained
in figure 7. The purely photosynthetic plant would exhibit
a system of equal rigid branches
springing as radii from a central
trunk, and forking regularly
outward to a vast number of
young twigs which would turn
up near the tips to spread the I^
leaves horizontally in a hollow
hemisphere of foliage. This
theoretical form, of course, is

modified in practice by other Fig. 7.— The form, as seen in vertical

• J ,• .11 ,1 section, which a plant would display

considerations, especially the (theoretically) if free to adapt itself to

exigencies of mechanical sup- photosynthesis alone. Further particu-
^ ^ lars in the text.

port, as we shall later consider;

but nevertheless it comes appreciably close to realization in the
most typical of the great trees, when these are free to develop
without interference, as was the case with the Oak of the ac-
companying picture (figure 8).

We turn now to a particular study of those two most distinctive
plant structures, the leaf and the stem. A first view over leaves
in general gives only the unpression of bewildering multiformity;
but continued observation gradually sorts out the miportant
from the trivial, and l^uilds one of those visualized composites of
which I have spoken in the first chapter. As the reader should
review and confirm for himself by inspection of a number of
kinds brought together for the purpose, the principal part of an
ordinary leaf is the spreading thin blade, which exhibits two con-
stituents, — first, the soft, seemingly-homogeneous, chlorophyllous
tissue, denser in green on the uppermost surface, and seat of the
food-making process, and second, the slender white veins, spring-
ing out from the leaf-stalk and variously branching and inter-
lacing while ever attenuizing towards the margin and tip of the

52

The Living Plant

blade. The tiniest veins are embedded within the green tissue,
where they end in polygonal areas, as one can see with a lens in
some leaves by holding them up to the light (for example in Rose,
Cabbage, and Wild Ginger), and as shown in the accompanying
cut (figure 9) ; but the larger veins stand out from the surface,
though always from the undermost side where they are out of
the way of the light. The veins have a double function, — the
conduction of water from the stem to the green tissue, and the

Fig. 8. — An oak tree, showing an approximation to the theoretical form of figure 7.
(Copied from Blanchan's American Garden.)

conduction of the photosynthetic sugar back to the stem; and
they have also a secondary use in helping a little' to support the
soft tissue, though the rigid but elastic stiffness of the healthy
green leaf is due for the most part to osmotic turgescence, of
which I shall speak in the suitable place. In addition to the blade,
most leaves possess a leaf -stalk, or petiole, stem-like in appear-
ance and function and varied in length, which carries the blade
out into the light and aids to adjust it therein, as we shall later

The Profound Effect on the Structure of Plants

53

consider more fully under light-adjustment, or phototropism.
Finally, some leaves exhibit, just where the petiole joins the stem,
a pair of little leaf-like bodies called stipules, whose most remark-
able feature is the diversity of their somewhat insignificant
functions and forms. All of the parts of a typical leaf, — blade,
petiole and stipules, — are well shown and in typical form, in the
accompanying picture (figure 10).

Fig. 9. — A fragment of the vein system of a leaf, highly magnified, showing the typical
mode of ultimate branching and ending of the vcinlets. (From Sachs' Lectures,
reduced.)

Fig. 10. — A typical leaf , — the Quince. (From Gray's Text-books).

The most striking of the features of leaves is perhaps the re-
markable variety of their shapes, which seem in their multiform-
ity to defy explanation or classification. Yet in reality the matter
is simple, for there exist only three primary forms of which all
the others are modifications and combinations, as the following
analysis will show.

First, the ideal condition for the best working of a leaf is ob-
viously that in which it can have full exposure to all the light that

Fig. 11. — Leaves selected to illustrate the typical shapes; a photograph of living specimens,

one-third the natural size.

54

The Profound Effect on the Structure of Plants 55

there is, without any shading by its neighbors. This ideal ex-
posure allows the development of the ideal type of construction,
i. e., the shape that encompasses the most green tissue within
the least outline, and a venation ensuring the shortest paths for
conduction of water and the photosynthate. Such a leaf must be
round, with its veins radiating from a central petiole. It is well-
nigh realized in the leaf of the Common Garden Nasturtium
(figure 11, c), a low-stemmed plant whose long petioles permit
a full exposure of each leaf to light (figure 12) ; and it is shown con-
ventionalized in figure 13, a. Furthermore, this association of
round-radiate (or, in the current terminology, round-palmate) ,
shape with full exposure to light is actually found in most plants
which grow in such manner that their leaves do not shade one
another, as for example in the floating leaves of Water Lilies
(figure 11, a), Ground Ivy (figure 11, h), Wild Ginger, and others
which trail or creep on the ground, and in low-growing long-
petioled herbs like Geranium, Cyclamen, and Pelargonium, and
partially in Ivies. Most of these leaves show a slit from the
petiole to margin, but that does not alter the principle of the
central-standing petiole, for the slit is merely a relic of the evolu-
tion of these leaves from kinds in which the petiole stood on the
margin; indeed all intermediate gradations exist in heart-shaped,
arrow-shaped, and ''auriculate" leaves, where a part of the blade
bulges backward on each side of the petiole.

Second, the opposite extreme of habit is found where leaves
are compelled to grow crowded together, as they are in most plants
living in especially dry or light places. In this case the best shape
and arrangement would be necessarily the exact opposite of those
found in the round type, that is, the leaves would be slender or
linear, without distinction of petiole and blade, and with the veins:
running parallel; while they would take such positions as would
admit the light most deeply and evenly among them, — viz., they
would point at the light and therefore stand parallel or radiating
with respect to one another. Such a position for the leaves is in

56

The Living Plant

fact not at all bad for illumination, since diffused light can pen-
etrate rather deeply among them, while the sun, in its daily swing
through the heavens, slants its beams at times to the innermost
parts of them all. The typical linear shape is actually realized
in a great many leaves, of which our figure 1 1 shows a few (/, g, h) ;
and it is shown conventionalized in figure 13, b. The association
of linear shape with a crowding of leaves into dense-radiating
heads is found typically developed in a good many plants, such as

Fig. 12. — The three types of plant form with which are associated the three fundamental
types of leaf shape. On the left is the trailing Garden Nasturtium, in the middle,
the half-desert Cordyline, on the right the typical woods-plant Ficus religiosus.

Spanish Bayonets, and the remarkable Tree Yucca of the deserts,
in Century Plants, the ornamental Cordylines (figure 12), and
some of the Bunch Grasses. The association of the linear form
with parallel-standing leaves is realized in the Flags and Cat-
tails of stream margins, and especially in the Grasses of the
meadows, which thus crowd a vast number of leaves into a lim-
ited area. And another phase of the very same thing is presented
by some of our evergreen trees, with their linear or needle-shaped

The Profound Effect on the Structure of Plants 57

leaves. These symmetri-
cal cone-shaped trees may
be viewed, indeed, as a se-
ries of superposed meadows,
spaced well apart in stories
so arranged that each is
smaller than the dne next
beneath it, thus avoiding
injurious shading thereof,
while the leaves point out-
ward as well as upward to-
wards the strongest light.
This condition is repre-
sented diagrammatically in
figure 14, and it comes very
close to actual realization
in some of our Spruces and
Firs when these are free to
develop as they will (figure
15). -^ This is the principal
factor, I believe, in the ex- b
planation of the conical
form of the evergreens.

Fig. 13. — Conventionalizations of the

Third, the conditions to
which are adjusted the round
and the linear shapes of
leaves are uncommon in
comparison with that in
which numerous leaves are
spaced at different heights
along ascending stems, —
for this latter is the prevail-
ing mode in vegetation,
(figure 12, right). Since this
condition is intermediate
between the other two, we
anticipate an intermediate
shape of leaf, which would
therefore be elliptical in out-

three fundamental types of leaf form.

58

The Living Plant

\iii. ^:j^

AV!^

—/////"

\NS\\\~?ii\

SN.N\\\\S\\N\ ,_

///////U^'

//'yy////'yyy

^yx^yy^yyy^y^.

-^'^^^S!

W^ms

line with the petiole at one end and the veins branching off pin-
nately from an axial mid-rib. This shape and venation are
actually realized in the leaves of some trees, very typically in
Chestnut (figure 11, d), Elm, Rubber-plant, and Banana. Much

oftener, however, this outline is
modified by a condensation of
the green tissue towards the
base of the leaf, which ensures
a shorter path of conduction
for water and the photosyn-
thate, while lessening simul-
taneously the weight and lev-
erage on the petiole. Such
leaves are necessarily of ovate
outline, and these ovate-pinnate
leaves are very common in na-
_,,.,, . ture. The shape is well typi-

FiG. 14. — The theoretical form, seen in ver-
tical section, of an evergreen tree. Further fied in the Catalpa, for CX-
particulars in the text. i / r> -< i \ i •

ample, (ngure 11, e), and is
represented in conventionalized form in our figure 13, c. In
some plants the condensation goes so far as to make the leaf al-
most round, as for example in the Red-bud (figure 11, i), when
the venation makes some approach to the palmate type and the
petiole is apt to be notably long. Such leaves often show a bulge
of the tissue downward each side of the petiole, thus displaying
a transition to the typical round shape with which we began.

It is thus evident that three fundamentally-distinct condi-
tions of leaf exposure exist, with three corresponding types
of leaf shape, — the round-radiate, the linear-parallel, and the
ovate-pinnate. But innumerable intermediate conditions of leaf-
habit exist, and therefore innumerable intermediate leaf shapes
occur. These shapes have a large practical importance in the
classification and description of plants, and accordingly have been
named for this purpose with very great accuracy; and it is inter-

The Profound Effect on the Structure of Plants 59

esting to note that while some of the shapes have been named for
their resemblance to familiar mathematical forms or common
objects (e. g., ovate, lanceolate), the majority have to be desig-
nated by combinations of these terms (as ovate-lanceolate, etc.).

For completion of our subject
of leaf shape, one matter of im-
portance remains, and that con-
cerns the curious emarginations,
lobings, and compoundings which
so many of the kinds exhibit.
The margin of a leaf is typically
smooth or entire, and many leaves
actually exhibit this character;
but others again are more or
less waved, toothed, or incised,
through the sagging, as it were,
of the green tissue between the
ends of the veins, or, occasionally,
its swelling out beyond them.
When this lobing becomes deep,
it influences greatly the form of
the leaf, especially as it follows
the type of the veining. Thus,
a deep lobing between palmate
veins results in a shape like that
of the Ivies, and the ]\Iaples
(figure 11, i), while if it goes clear down to the leaf-stalk (in which
case the separated segments usually develop little stalks of their
own), it results in a leaf that is palmately compounded, like
the Woodbine (figure 11, A-). A similar deep lobing in pinnately-
veined leaves leads through forms like those of the Oaks to
pinnately-compound leaves, like those of the Locust (figure
11, I) and many Ferns, which latter, indeed, are often again lobed
and compounded, and re-compounded again. In a general way,

Fig. 15. — Engelmann's Spruce, showing
an approximation to the theoretical
form of figure 14. (Copied from Kirke-
gaard's Practical Handbook of Trees,
etc.)

6o The Living Plant

as will later appear, there is a probable adaptational advantage
in the compounding of leaves, since it aids them to resist the
tearing action of strong winds, and there is a possible adaptive
explanation of the deep lobing of leaves like Ivies and Maples
in the opportunity thus afforded for an interlocking of the leaves
and consequent utilization of every ray of the incident light.
But nobody, so far as I can find, has yet been able to give a reason-
able explanation of the significance of the emarginations of leaves,
for the suggestion that the points thus resulting serve to collect
atmospheric electricity for some use by the leaf can hardly be
seriously entertained. Emargination, lobing and compounding
are evidently three degrees of the same thing, but it is by no means
necessary to believe that because compounding is adaptively
useful, therefore emargination must be useful likewise. On the
contrary, it is not only possible that the emargination of leaves
originates non-adaptively in some manner purely incidental
or accidental, and is later intensified adaptively to lobing and
compounding, but the method embodied in this supposition affords
the most reasonable explanation we yet possess of the origin of
adaptations.

While adaptation to the mode of exposure to light is the chief fac-
tor in determining the shape of the leaf, other adaptations and influ-
ences, very different in different cases, exert also their effects,
making the shape of any given leaf a resultant of the cooperation of
many influences. This fact the reader must remember when he
tries to apply the principles of the preceding pages to the ex-
planation of leaf shapes he may find in his walks abroad in the
country. At first he will find so many exceptions and contra-
dictions that he may incline to dismiss my explanations as ground-
less; but if he will continue his observations with patience, he
will gradually find the exceptions disappearing and the essentials
standing out in those composite conceptions of which I have
spoken in the first chapter; and then, I believe, he will agree
with the conclusions here expressed.

The Profound Effect on the Structure of Plants 6r

From the leaf we turn to the associated and well-nigh equally
distinctive part, the stem, of which, however, the structure is
comparatively simple and uniform. Since its principal function
consists in raising and spreading a great many leaves to the light,
it must of course be adapted to provide a firm mechanical support
in conjunction with much branching; and in fact it consists of
a cylindrical-tapering, rigid-continuous, regularly-ramifying struc-
ture familiar in the stems of the majority of plants. Although older
stems become strongly thickened and woody, and protectively
enwrapped in laj^ers of bark, the young growth is soft and green
like the leaf, and likewise consists of veins and soft tissue, though
the relative importance of the two is reversed in the stem as com-
pared with the leaf. The veins can be seen by the eye in young
stems that are translucent (e. g., Balsam), when these are held
to the light; and they can also be made visible through the tissue
in some others if these are stood with their cut ends in a deeply-
colored liquid. And they can always be seen in thin sections cut
crosswise of the stem, as well illustrated in some later figures
(73, 139, B) which accompany a fuller discussion of the stem
in another connection. The veins form a ring in most kinds
of young stems, though in some they are scattered about; and
wherever they branch to run out to the leaves the stem is commonly
swollen a httle, and oftentimes lighter in color, giving origin to
the so-called nodes separated by spaces called internodes, which
are by no means "joints," as sometimes described. Outside
the ring of the veins, as the later figures 73 and 141 show very
clearly, the soft tissue holds chlorophyll, and thus aids the leaves
in their photosynthetic function. The amount of such work
that stems can do must in fact be little; but the plant takes ad-
vantage, as it were, of every bit of its surface exposed to the light
and not needed for other uses, even including such parts as the
stamens and pistil of the flower, to spread out additional chloro-
phyll for the invaluable photosynthesis.

Stems, as a rule, grow continuously from buds at their tips,

62 The Living Plant

and new branches from buds in the angles between stems and
leaves, — a position which has the advantage of nearness to the
manufactories of food. This brings us to consider the causes which
determine the arrangement of leaves on the stem, a curious matter,
scientifically called phyllotaxy, and once discussed more commonly
than now in botanical books. Leaves do not originate on the stem
at hap-hazard, as may seem the case on some slender branches,
but in quite definite and even mathematical order, as rosette-
like plants, cones, and some other very compact structures sug-
gest. Two primary systems of leaf-arrangement are possible,
and occur. The simplest is the opposite (or whorled) system, in
which two leaves stand at the same node exactly opposite one
another, as occurs for example in the Mints, (figure 16, A), in
which case the next pairs above and below stand at right angles
and thus cover the space left by the first set, producing four vertical
rows often in remarkable symmetry, as our common cultivated
Coleus illustrates. This, with the other arrangements, is shown
diagrammatically in figure 16, where the reader is supposed to
look down from above on the stem, which is imagined to be tel-
escoped, so to speak, Chinese lantern fashion, to a single flat plane,
as indeed the stems actually are in the buds. In some kinds,
three instead of two leaves stand at a node, or four or five, or
more, producing a regular whorl, but in all such cases, illustrated
for instance by large Lilies (figure 16, B), the leaves in a whorl
are evenly spaced and cover the breaks in the whorls above and
below. This is the system prevalent in flowers, for, as everyone
will recall, the whorl of sepals covers the breaks in the whorl of
petals, with a similar arrangement in stamens and carpels. Thus
much for the opposite or whorled system; the other is the spiral,
in which only one leaf ever stands at a node, while the one on the
node next above or below stands part way around the stem,
the successive leaves falling always into a regularly-ascending
spiral. Now this space around the stem from one leaf to another is
a definite fraction of the circumference ; — in some plants it is V2,

The Profound Effect on the Structure of Plants 63

Fig. 16. — Diagrams to illustrate the principal systems of leaf-arrangement, as they would
appear from above if the stems were telescoped to one plane. The rings are nodes,
and the small heavy circles are leaf bases. Further particulars in the text.

64 The Living Plant

as in the Elm and Grasses, in which case one must pass once
round the stem and cover two spaces to reach a leaf over the
first (figure 16, C). In others, (e. g., the Sedges), the fraction is
\^3, and a spiral drawn through the bases of the leaves passes
once round the stem and across three spaces to reach a leaf over the
first (figure 16, 2)). In others, (e. g., the Apple) it is "/s, when the
spiral must pass twice around the stem and cross five spaces to
come to a leaf over the first (figure 16, E), an arrangement which
is, perhaps, the commonest of all. In others the fraction is ' /§
(in Holly and Plantain figure 16, F), or "^/js, as in cones of White
Pine, while %i, ^"^/34, and even some higher fractions are said to
have been traced in special places where the leaves are greatly-
condensed together in rosettes. And a curious thing is this, that
while these fractions occur, the various possible intermediate
ones do not. In these fractions, which primarily express the
amount of circumference between two successive leaves, the
numerator also expresses the number of turns that must be made
around the stem to reach a leaf over the first, while the denomina-
tor expresses the number of spaces that must be passed over for
this purpose, and also the number of vertical ranks into which the
leaves fall. Moreover, these fractions bear to- one another a very
curious relationship, for when they are arranged in a series, — viz.,

V2, V3, 2/5, 3/8, 5/^3^ 8/21, 13/34

it is found that each numerator is the sum of the two numerators
preceding, and each denominator likewise the sum of its two pre-
decessors, and moreover each numerator is the same as the de-
nominator next before the preceding. This curious series, known
in mathematics as the Fibonacci series, is said to find expression
in other phenomena of nature, including the arrangement of the
planets, and is therefore not peculiar to the phyllotaxy of plants.
The question of present importance, however, is this, — what is its
meaning in connection with leaf-arrangement? Of course one's
first natural thought is, — adaptation, which appears reasonable
enough with the opposite system and the whorls, and even with

The Profound Effect on the Structure of Plants 65

the lower fractions of the spiral system, where one can see the
advantage of a spacing which may give to the leaves the best
aggregate exposure to light. But this interpretation meets in-
creasing difficulties with the higher fractions, and even has trouble
with the lower when one notices how freely the leaf-blades, the
very parts which need the exposure to light, are swung by their
slender petioles into positions of advantageous individual exposure
in callous disregard of the orderly arrangement in which they start
from the stem. There is, however, another and very different
explanation of the systems of phyllotaxy advanced by some in-
vestigators, viz., that they are wholly determined by the positions
in which the young leaves originate inside of the growing bud,
which positions in turn are determined by mechanical principles
connected with the easiest mode of origin of new swelling parts
in buds of a certain size and shape. In other words the fractions
of phyllotaxy are merely an incidental result of mechanical
conditions present in growing buds, and have only a secondary,
if any, reference to adaptation. This explanation I believe to be
substantially correct. It is of course not an explanation of
phyllotaxy, but merely a transference of the problem into an-
other field, as most of our explanations are. But I dwell upon the
subject at this length because phyllotaxy seems to me to offer a
fairly clear case in which a conspicuous feature of plant structure
has merely an incidental and not an adaptive origin.

There is one other feature of leaf and stem structure to which I
have not yet made any particular reference, and that concerns
their sizes, which are wonderfully diverse in different plants.
Leaves are measured in terms of feet in Bananas and Palms, but
need the assistance of lenses to show them at all in some of the
kinds that grow in the deserts; they are merely of tissue thinness
in some kinds of Ferns, but cyUndrically-thick and stem-like in
Aloes and Century Plants. Stems display a thousand feet of
length in the Rattan Palm, but are invisible supports to tufts
of leaves in the Houseleek; nearly as thin as a hair in some Ferns,

66 The Living Plant

but quite as thick as a house in the larger species of Redwood;
branched to a spray in a Mango Tree, but an unbranched shaft
in the Royal Palm. Thus it is evident that leaves and stems ex-
hibit well-nigh as remarkable a diversity in size as in shape, and
we nmst conceive of our generalized or composite leaf and stem
as well-nigh indefinitely modifiable, possessing, as it were, a
kind of a super-elasticity in both of these features. As to the
causes determining size in these parts, that is reserved for dis-
cussion in the chapter on Protection, where it will be shown that
the size actually displayed by any leaf or stem represents in the
main a compromise or truce between the conflicting tendencies
of the plant to make its leaves larger for photosynthetic advantage
on the one hand, and smaller for better resistance to hostile ex-
ternal conditions on the other.

In this chapter thus far but little has been said concerning the
root. This is because the consideration of that organ is more
convenient and natural in the chapter that deals with its function
of Absorption; and there its description will be foimd in detail.
It is enough for our immediate purpose to say that roots, the
principal organs for the absorption of water and minerals, and
the third of the primary plant parts, grow out from stems, which
they closely resemble in structure, having much the same internal
cellular construction as well as the same long-tapering, freely-
branching forms. Though not without diversity in form, size,
and structure, they are yet far less varied in these respects than
are leaves and stems, and for a sufficient and obvious reason, —
namely, they grow under far more uniform conditions; for life
in the soil is much the same thing all the world over, however
varied it may be upon the surface.

Thus far w^e have considered only those diversities which leaves
and stems exhibit w^hile still retaining their typical function of
photosynthesis. But their remarkable plasticity does not exhaust
itself here, for these parts can even perform entirely different
functions, becoming adaptively modified therefor to such a de-

The Profound Effect on the Structure of Plants 67

gree that their original nature would hardly be suspected were it
not for the existence of intermediate stages. And not only that,
but conversely, substantially all of the structures performing
remarkable or unusual functions and displaying remarkable forms,
are simply transformations of the three primary parts, leaf, stem
and root. This subject of the formation of all the special organs of
plants out of leaf, stem, and root, (a typical example, by the way,
of morphological study,) we must now proceed to consider.

The particular structures performing definite functions in typical
plants, other than ordinary leaf, stem, and root, are the following :

Bud coverings, or scales, give needed protection to living buds
over winter. Adaptively to this function, they are small, con-
caved, thick, corky, brown, and often resinous, as the large winter
buds of any cormnon trees will illustrate.
Bud scales are transformed leaves, usually
leaf -blades, but in some plants (e. g., the
Horse Chestnut) are petioles, the blades
being suppressed, while in others they are
stipules, as shows very beautifully in the
Tulip Tree (figiu'e 17.)

Tendrils, or similar parts, enable slender
plants to cling to a support and thus mount
upward tow^ards the light. Adaptively to
this function they are slender, tough, cy-
lindrical, or cord-like structures, endowed
with remarkable powers (to be later con-
sidered in the chapter on Irritability), of
reaching out for a support, taking a firm hold thereon, and sub-
sequently shortening and toughening their structure (figure 85).
The best tendrils, like those of the Passion Vine or the Grape, are
transformed stems, issuing from buds precisely as branches do.
Others are transformed leaf-blades, as in the curious Lathyrus
Aphaca (figure 18), or a part thereof, as in Vetches, or Bignonia;
or are stipules, as in the Wild Smilax, or merely the petiole

Fig. 17. — The stipular bud
coverings of the Tulip Tree;
one-third natural size.

68

The Living Plant

which makes a turn around some object, as in the Clematis, or
a cylindrical part between two portions of blades as in those
Pitcher plants called Nepenthes (figure 20). In some tropical

plants, e. g., climbing Aroids, the aerial
roots clasp horizontally around a support.
In some others, and notably those having
the habit of the Ivies, and growing against
stonework, the tips of the tendrils do not
twine around a support, but end in discs
which are firmly appressed to the stones,
as in the Woodbine, though more com-
monly the disc-holding structures are aerial
roots, as the English Ivy illustrates.
,,, ^ , ., , Spines project repellingly from some

Fiu. 18. — Tendrils trans- -^ i j i .

formed from leaf-blades, kiuds of plauts as if they might form a

with stipular foliage, of , , • • i ,^ , , i ft

Lathyrus Aphaca; one-half protection agaiust the attacks of large
natura size. plaut-eatiug beasts. They possess a stiff,

hard, conical structure, and a firm attachment to the skeleton,
consistent with that use. In some plants they are no more than
prickles, erupted, so to speak, from the surface, as in the Rose;
in other cases they are the sharp-
ened ends of the veins, as in the
Holly; in others they are the leaf-
blades, as in the Barberry and
the Cactus; in others they are
stipules as in the most spiny
of the Euphorbias (figure 19),

though in some other kinds the Fig. 19.-The stipular spines of Euphorbia
spines are the persistent and in- ^plendens; one-half natural size.

durated floral branches; in others, such as the Locusts, they are
transformed branches coming from ordinary axillary buds; in
some Palms they are roots ; and cases are known where they are
petioles.

Food Reservoirs store up for later use the food-material made

The Profound Effect on the Structure of Plants 69

in the leaves of herbaceous perennial plants, and, adaptively
to this function, are greatly-swollen, soft-bodied, large-cellular
structures. They are leaves in the bulb scales of Lilies and Hya-
cinths, stems in the common Potato (the eyes being axillarj^
buds), and roots in the Sweet Potato.

Insect Traps effect the capture and digestion of insects, and
thus enable some plants to augment the scanty supply of nitrog-
enous compounds available where they
grow. Adaptively thereto these traps have
highly special forms and accessory features
contributing to the attraction and capture
of insects, as will later be noted in a par-
ticular description of these plants. The
trap is a pitcher formed by a special cup-
like-upgrowth of the leaf-blade, as in the
various Pitcher Plants (figure 20), or else
a hinged or inrolling blade, as in the Venus
Fly-trap and Sundew.

Flower parts contribute in various ways
to the efficiency of reproduction, as will
later appear in a discussion of that subject.
The parts are transformed leaves, and dis-
play features adaptive to their functions, —
the green leaf-like sepals which protect
the other parts while in bud, the brightly-
colored petals which exhibit the position of
the flower to the visiting insect, and (though
with a reservation) the stamens and pistil fig oo_An insect-trap-
concerned with the actual pollination. In ,tp*„£t;;,"„f1h:

some kinds of flowers the petals are miss- leaf tip in Nepenthes; one-
third natural size.

ing, but theu' function is performed by

brilliantly-colored leaves close under the flowers, as shown so

strildngly in the Poinsettia.

Miscellaneous. There are, furthermore, a great many special

70

The Living Plant

structures with particular functions not belonging in any of the
definite categories above mentioned. Thus, the bladdery air-
filled floats which keep the Water Hyacinth resting so lightly
on the water are petioles; the wing which ensures the carriage
of the Linden seeds is a leaf-blade (figure 157) ; the indurated hooks
by which some tropical vines do their climbing are stipules; while
the reduced or rudimentary leaves which we call bracts often
also possess functions of a minor sort.

Substitution foliage. Finally, we must take notice of another curi-
ous transformation in function and structure found in all parts
other than the leaf-blade, namely, they may be-
come transformed into foliage, either in aid of the
blade, or its replacement. Thus, in some kinds,
the blade is greatly reduced or missing, and the
petiole is flattened and thin and acts as the foliage,
e. g. in the Australian Acacias (figure 21), and
some kinds of Oxalis. In a good many plants
the stipules are sufficiently big to render appreci-
able aid to the leaf-blade. In Lathyrus Aphaca
(figure 18) they form all of the foliage there is,
while in the common Bedstraw or Galium, they
are as large as the leaves and so like them as
Tened petiole serv- commouly to be thought additional leaves helping
ing as foliage ^q niake up a whorl. In a great many plants,

(the blades being /^ _ ^ J i )

insignificant), in and especially those found in dry places, the leaves

an Australian , ,, , i ii c j-

Acacia; one-half Dccome Very Small or are absent, and the function
natural size. ^£ foliage is performed by the stem, which either

remains smooth and round, or becomes fluted by the presence of
vertical green ribs, or becomes flattened in various degrees, all
three conditions of which are found in the family of Cactuses. In
some cases the stem is flattened as thin as a leaf, while still dis-
playing the nodes distinctive of the stem, as in the Muehlenbeckia
of our greenhouses (figure 22) ; but in other cases no nodes appear,
and the stem assumes a form and general aspect so leaf -like that

The Profound Effect on the Structure of Plants 71

the botanical teacher has often much ado to convince his students
that it is anything else, even when he shows them the actual
leaves, reduced to scaly bracts, out of whose axils the leaf-like
branches clearly spring. Such is the case with the Butcher's
Broom of Europe, (figure 23), our common Asparagus, and the
cultivated Smilax of the florists. Finally there is even a case

Fig. 22. — The leaf-like stem, with some small leaves, of Muehlenbeckia; one-half

natural size.
Fig. 23. — The leaf-like branches of Butcher's Broom; one-half natural size.

in a tropical Orchid, Taeniophyllum by name, where the roots
serve as foliage, becoming suitably flattened and otherwise ap-
propriately constructed.

We cannot take space to follow any farther this most interest-
ing subject, but if the reader desires another and much fuller
discussion thereof, he will find it in the appropriate places in Asa
Gray's Structural Botany, where it is treated in a manner that in
my opinion cannot be surpassed. The subject, moreover, is one
which offers attractive opportunity for concentrated field study

\\\^ \^no\ocu< cN^ vlv\\\Vin\a VvoaA\s

Fig. 24. — A collection of specimens, pressed and dried, and arranged to illustrate a
morphological topic ; photographed one-third the original size.

72

The Profound Effect on the Structure of Plants 77,

in the discovery, identification, collection and arrangement of the
various special structures of plants, which can then be preserved
in some such manner as our picture illustrates (figure 24).

Thus it is evident that, on the one hand, the three primary-
plant parts, — leaf, stem and root, — though developed with a
structure adaptive to the very particular function of photo-
synthesis or food-making, have in many cases become trans-
formed into other parts of very different ecological significance
and structure; while, on the other hand, and correlatively, all
of the great number of highly specialized parts performing other
functions can be traced back to an origin morphologically in the
three prunary plant parts. This interlocking relationship of
morphological origin with ecological meaning, — of morphology
with ecology, — can perhaps be made clearer by use of a diagram
such as is given herewith (figure 25).

Although I ought now to end this long chapter, I will continue
far enough to answer two questions which I am sure have arisen
in the mind of the reader. Thus, he \sdll surely be wondering
why it is that some plants make their tendrils, for instance, from
leaf-blades, others from petioles, others from stipules, others from
stems, and others even from roots. The most reasonable answer
appears to be this, that when a plant, owing to a change of habit
forced on it by a change of environment, develops a need for a new
organ, that organ is made by a transformation of the part which
happens to be most available for the purpose, often some part
which the change of habit has happened to set free from its
former use; and sometimes that most available part will be one
thing and sometimes another. In the second place the reader
will wonder why some plants should abandon their leaf-blades
as fohage, and then proceed to replace them by petioles, stipules,
stems, or even roots, which are for the purpose converted phj'si-
ologicahy and structurally into leaves. In answer it may be said
that the abandonment of the leaf-blade, as will be shown in the
chapter on Protection, usually accompanies exposure to very dry

74

The Living Plant

Leaf-blades

Petioles

Stipules

Stems

Roots

Flower parts

Foliage

Insect traps

Bud covers

Tendrils

Spines

Miscellane-
ous

Support to
foliage

Storage

Absorption

Fig. 25. — Diagram to illustrate the interrelations of morphological origins with ecological

uses in the parts of the higher plants.

climate, in which case the function of foliage is taken over by
some other part, usually the stem. Now it is conceivable that
when, by another change of habit, the plant finds itself in need
of a much larger spread of chlorophyll surface, this may be more
easily obtained by further enlarging and flattening the already

The Profound Effect on the Structure of Plants 75

leaf-like stem than by re-developing the lost leaves. It is probable
that some pecuharity of this kind in the past history of the plant
will explain in each case such curious features, the course of devel-
opment being always that which offers the least resistance at the
moment.

The reader will now be prepared, I think, to admit that of all
the influences concerned in the determination of plant form, —
indeed in making plants what they are, — the most important by
far is the physiological process of food-making, or photosynthesis,
and that the feature of this process having the most profound
effect is the need for exposure to hght.

CHAPTER IV

THE KINDS OF WORK THAT ARE DONE BY PLANTS, AND
THE SOURCE OF THEIR POWER TO DO IT

Respiration

I HEN first I had written this chapter, and made it the
best that I could, it assumed that the fact of plant
work was already well-known to the reader. A later
experience, however, made me see very clearly that
most people do not know that plants work at all. Accordingly
I shall make it my first endeavor to show beyond question that
plants do work; then we can pass with better understanding to
the study of the very remarkable source from which they derive
their power to do it.

The principal reason why the majority of people do not as-
sociate with plants the idea of work is found in the slowness of
most plant actions. Our conception of work is almost entirely
subjective, and because plants are placid of mien, and do not hurry
and fret and strain, we think they are doing no work. When the
Master said of the Lilies, that they toil not neither do they spin,
his words expressed the popular fancy but not the physical fact.
Work is none the less real because it is slow, and the matter of
slowness is entirely relative and subjective. Even the very swift-
est actions performed by any of us must seem slowness person-
ified to the lightning, or to a dynamite charge which can finish
its work before you can think, or to the forces of collision which
reduce a railway train to a heap of tangled scraps within the
space of an instant. Probably the lightning, the dynamite, or

the collision forces, if interviewed on the subject, would say that

76

The Kinds of Work That Are Done by Plants 77

mankind does not work. But if plant actions could be magnified
immensely in speed they would impress one ver}^ differently in
this particular. For then the observer would see the tip of every
growing plant-structure nodding and moving energetically about,
so that a meadow, a copse, or a forest would seem all of a vigor-
ous tremble as if straining at some hidden leash : he would see the
buds of some flowers open and close with a straining yawn or
a sudden snap, and others burst into bloom like a rocket when it
breaks to a spray of mani-colored lights: roots in their efforts
to penetrate the earth turning and twisting like angleworms im-
paled on the fisherman's hook: seedlings in their struggle to break
through the ground heaving and straining at their burden of
superincumbent soil, like a powerful man at some load which
has fallen upon him: seed pods pushing into the earth on a twist-
ing or hard-thrust stalk : tendrils swooping in curves through the
air, gripping the first thing they meet, and jerking their plants
towards the support. As matter of fact, there does exist a way
in which we can readily behold these actions thus magnified,
for if the structure in question be photographed at regular inter-
vals, say of fifteen minutes to half an hour, and then these photo-
graphs are run at high speed through a moving-picture machine, —
the thing is done. Such studies have actually been made in the
case of twisting roots, moving fruits, and opening flowers; and all
of those who have seen them agree in the impression of vigorous
work thus presented.

Furthermore, if we could magnify in like manner the interior
parts of the plant we should witness as remarkable actions pro-
ceeding with equivalent vigor. In some plants the living proto-
plasm would be seen flowing in thick turbid streams round and
round within the encasing cell- wall; in certain cells those re-
markable structures called chromosomes would be seen perform-
ing their curious manoeuvres, — arranging themselves into groups,
collecting in pairs, passing backward and forward in a manner
suggestive of the measures of the dancers in a quadrille; else-

78 The Living Plant

where new cells would be seen in process of birth, and engaged in
forcing the older apart to make room for themselves ; while minor
actions without number, mechanical, physical, and chemical,
would appear in vigorous progress in various parts of the organ-
ism. Truly if one could see these actions under the conditions
here imagined, he would have no trouble at all in connecting
with plants the idea of real work.

We are not, however, dependent solely on imagination, or
the moving-picture machine, for a conception of the reality of
plant work. The rapid closing of the leaf of a Venus Fly-trap
upon a captured insect, or the sudden collapse of the Sensitive
Plant when touched, suggest some such idea. Everj^body has
noticed that the great granite curbstones along streets where
shade trees are grown, become heaved from the regular lines in
which they are laid, while the pavements themselves are often-
times thrown into irregular swells; this is all brought about by
the growth of the roots of the trees, which thus exhibit a work as
real as that of a jack-screw or derrick. If the reader has not al-
ready observed these phenomena, let him do so when next he
walks through a shaded street. In a similar manner young roots,
insinuated between the stones of buildings, tombs, or walls,
force the masonry apart in their growth, and finally accomplish
the destruction of the edifice. Occasionally asphalt pavements
are burst upwards by the growth of some kinds of plants, including
even soft-bodied Fungi, as the accompanying photograph well
proves (figure 26). And the technical literature of plant physi-
ology tells of the thousands of pounds pressure exerted by large
gourds, like Squash, when suitably harnessed to recording machin-
ery. And, finally, experiment proves that every operation of
plant life, even the least of them all, involves some movement,
and therefore real work; so that animals and plants are working,
and often right hard from the physical point of view, when they
merely are keeping alive, — a conclusion from which the reader
is welcome to draw any comfort that he can.

The Kinds of Work That Are Done by Plants 79

At this point, perhaps, some one will rise and declare I am wrong
in my statement that work is as real when slow as when swift.
But note that I sa}'' as real, not as hard. When a weight of a ton
is lifted a foot, no matter bj'^ what means, the work is the same
whether done in a day or a minute, although it is over a thousand
times harder to do, (to be exact, the power required, is 1440 times
greater) in the latter case than the former. But the fact of im-

FiG. 26. — An asphalt pavement burst upward by the growth of soft-bodied mushrooms,
whose conical heads are visible over the wreckage.

mediate importance is this, that the work is as real in one case
as the other.

We come now to the bond of connection between this matter
of plant work and the principal theme of this chapter, viz., — it
is a fact of physics, which the reader must long since have learned,
that every bit of work of every kind done anyrv^here whatsoever
in nature, whether in a plant, or an engine, or the skies, or the
thinking brain of a man, requires for its accomplishment the
presence and expenditure of energy, which is the source of all
power. The reader, of course, knows what energy is, — the en-
tity in Nature, and the only one, that produces motion by which

So The Living Plant

work is accomplished. Energy is most familiar as heat or elec-
tricity, though manifest also in light and in chemical reactions.
Without energy there is no motion, no power, no work; and with-
out it a plant or an animal stops as dead as an engine when no fire
burns under its boiler. Plant work, therefore, requires and im-
plies a supply of energy. And with this conclusion it will be well
to gather the foregoing matters into a generahzation, another
of our botanical verities; — all plants, like all anivials, are i7ices-
santly at work while alive, as truly as any moving machine, not only in
the performance of their active and visible movements, but also in the
bare maintenance of their existence; and this work requires a pro-
portional supply of energy.

It is now our business to find the source of the energy by which
plants do their work. We know the source of the energy in the
work of the engine just mentioned; it is the heat released from
the burning of coal in a grate. But what is the source of the energy
in the work of the plant, which has neither grates, nor boilers,
nor flaming of fuel?

When the student of science is faced by a problem like this,
his first resource is to look around for suggestions from some
analogous process. In this instance he would turn naturally
to animals, and his earlier studies on the physiology of man would
have taught him that the power of animals to do work is connected
in some way with their respiration, — that process in which they
give forth the gases carbon dioxide and water vapor to the air,
while absorbing the gas oxygen into their bodies. How inti-
mately this process is connected with work is easily realized
when we recall the familiar fact that respiration increases in pro-
portion as work becomes harder. Is it possible, then, that
plants also respire? That is, do plants in their work release car-
bon dioxide, and absorb oxygen? Obviously this matter is de-
terminable l^y experiment, and the following is a very good
method. In a bottle arranged as shown by the picture (figure
27), we place some plant parts which are actively working with-

The Kinds of Work That Are Done by Plants 8i

out the complications introduced by photosynthesis (e. g., ger-
minating seeds, such as Oats), then close the bottle air-tight by
means of the stoppers and clamp provided for the purpose, and
stand it for some hours in a warm
and dark place where growth can
take place. Obviously, any carbon
dioxide released by the seeds must
collect in the bottle, where its pres-
ence may be detected by its well-
known property of turning clear lime-
water milky. If, accordingly, clear
limewater is poured into the tall vessel
into which the dehvery tube leads,
the clamp is loosened, and water is
poured down the thistle tube, then
the gas will be forced from the bottle
and sent bubbUng up through the
limewater. The result is always de-
cisive. The limewater turns white-
milky proving the presence of car-
bon dioxide in abundance. And if
a bright person should here rise to
remark that the carbon dioxide al-
ways present in air is sufficient to ex-
plain the result, it is easy to prove it
is not; for, if an equal quantity of air
be forced from an empty bottle through limewater no milkiness
appears. And if, in the bottle, we place buds, or roots, or color-
less plants like Mushrooms, or even green leaves (in the dark) , the
result is always the same. Furthermore, it is also the same whether
the working parts are kept in the light or the dark, and it is still
the same, as the reader may be confounded to learn, even with
green leaves when kept in the light, though here the process is
obscured by the absorption of that gas in photosynthesis, as can

Fig. 27. — A Respiroscope, or ar-
rangement for demonstrating that
plants respire. Its operation is
explained in the text.

82

The Living Plant

be proven by experiments, too elaborate, however, for description
at this place. Furthermore, as we may conveniently note here,
all of these same working parts are simultaneously releasing water
as well. It is therefore true, as a general principle, that all working

parts of all plants are
giving off carbon dioxide
as well as water, pre-
cisely as animals are do-
ing.

But do plants exhibit
the other phenomenon
of animal respiration, —
absorption of oxygen?
It is very easy to prove
that plants must have
oxygen in order to live
and work, precisely as
animals must; for if two
sets of the same seeds are
placed in two similar
closed chambers, and
then the oxygen is re-
moved from one chamber
by a chemical absorbent
while it is left untouched
, , , . , . , in the other, the seeds in

Fig. 28. — Two similar tube-chambers in which were

placed similar sets of germinating oats kept wet the OXygeulcSS chamber
and in place by wads of moss, and treated pre- . 4- n

cisely alike except that those on the right were dc- Will UOt gCrnunate at all

prived of oxygen. ^^^ ^|j| ^^^^ ^-^^ ^j^-j^ • ^^

the other they will grow normally for a considerable time (figure
28). Furthermore, if the air of a closed chamber in which seeds
have been growing for some days be subjected to chemical
analysis, it is found that most of the oxygen has disappeared
from the chamber, and must therefore have been absorbed by

The Kinds of Work That Are Done by Plants 83

the seeds. And the same thing is true no matter what structures
we place in the chamber (saving only an apparent exception,
soon to be noted, in the case of lighted green leaves), and no
matter whether the chamber is exposed to the light or kept in
the dark. It is evident, therefore, that all parts of working,
(and that is to say, of living) plants, absorb oxygen and release
carbon dioxide precisely as animals do.

There is no one, I think, who can grasp fully the bearings of a
complicated subject after only a single presentation, no matter
how clear this may be. It is therefore quite likely that some reader
ere this has experienced a feeling of dazement, and been led to
exclaim, along with the much-puzzled German, ^'Jemand ist
verriickt, aber wer?"; and he may even inchne to imagine that
I am the ''wer." For have not I shown, in an earlier elaborate
chapter, that plants absorb carbon dioxide and release oxygen,
while now I have proven by evidence quite as conclusive that
they do exactly the opposite? But there is, nevertheless, no in-
consistency. For the reader will recall that it is only the green
tissues which absorb carbon dioxide and release oxygen, and then
only in light, and then only from the tiny little chlorophyll grains
embedded inside of the protoplasm. There should therefore be
no trouble in understanding how the protoplasm in which those
grains are embedded, like all other living parts of the plant, can
be respiring, while the chlorophyll grains alone are engaged in the
photosynthetic process. The case of the chlorophyll grains,
however, is not so simple as my statement implies, because,
since they are living protoplasm, there is every reason to think
that they also respire even m light, and that in them, — and in
them alone, — the two processes go on together. If, now, photo-
synthesis and respiration, with their exactly opposite gas ex-
changes, proceed together in leaves, why do they not neutralize
one another's results? The answer is easy. Experiment shows
that on the average the photosynthesis in green leaves in the
light is over twelve times as active as respiration (and it may rise

84

The Living Plant

very much higher), a preponderance that is obviously so great
as to over-balance not only the respiration of the leaves, but of all
the remainder of the plant besides, and not for daj^time alone,
but also for night. Therefore, day and night together, the green
plant absorbs much more carbon dioxide than it releases and re-
leases much more oxygen than it absorbs. It vitiates the air by
its respiration, but in the long run purifies it still more by its
photosynthesis.

Before leaving this part of our subject, we should look a little
more closely into the relations of the two processes within the

Fig. 29. — Diagrammatic sections across leaves, to illustrate the movements of gases in
and out of the same during, — a, light, c, darkness, and b, the balance period between.
The squares are carbon dioxide, the triangles are oxygen, and the arrows show the
direction of movement.

lighted green leaf, — a subject diagrammatically illustrated by the
accompanying figures (figure 29). At night all of the carbon
dioxide given off by the respiration of the living cells into the air
passages, makes its way along these and through the stomata
to the atmosphere outside, (figure 29, c). In the daytime any
carbon dioxide given off by the respiration of the protoplasm is
absorbed by the chlorophyll grains in the same cells, but as this
supply is wholly insufficient, a constant stream of that gas passes
in from the atmosphere through the stomata and along the pas-
sages to the different cells, where it is absorbed by the chlorophyll
grains; simultaneously a part of the oxygen given off by the
chlorophyll grains is absorbed by the protoplasm of the same cells
for their respiration, while the very large surplus is sent into the

The Kinds of Work That Are Done by Plants 85

air passages and along them and through the stomata to the at-
mosphere; and the reader should thus visualize these matters in his
imagination (figure 29, a). But here comes an interesting point.
Since photosynthesis is dependent upon light while respiration is
not, there must evidently exist a certain intensity of light at which
the two processes in a leaf exactly balance. At such times the
processes use one another's gases, and there is no movement of
carbon dioxide or of oxygen either into or out of the leaf (figure
29, 6) . Such a balance period must occur every day just after sun-
rise and before sunset, and on some very dark days it probably
lasts for considerable periods. It is of course by virtue of approx-
imation to such a balance that some kinds of plants such as Ferns,
if not given too much light, can thrive so well for long periods
of time in tightly-closed cases, or masses of red-berried vines
(Partridge-berry) can exist all winter in little closed globes on
dining-room tables.

We may now express the important facts of the past few pages
in another of our botanical verities, to this effect, — that plants,
like animals, respire, and in identical manner, absorbing oxygen
and releasing carbon dioxide, throughout all of their living parts.

In the preceding paragraph I have said that the gases enter
through stomata and pass along air passages, but I have given
no hint of the forces w^hich impel them. This matter will be taken
up fully in the chapter on Absorption, where it will be shown
that the gases move along diffusively under action of forces
internal to themselves. We need only note here that plants have
no system at all for absorbing and expelling large masses of air
as animals do by the use of their chest-muscles and lungs, — an
operation that is always called breathing. Accordingly, the matter
can be stated in this way, — that plants respire, but do not breathe.

It will be well, at this point, to turn aside for a moment from
our main subject to consider some phases of plant respiration
which have economic importance. The first is concerned with
aeration of soils. Roots, like all other living parts, must respire

86

The Living Plant

in order to grow, and, with the exception of a few which possess
long air passages connecting with the leaves, they take the in-
dispensable oxygen from air in the soil, by a method to be later
explained. A soil in the best condition for the respiration of roots
has the structure represented, under large magnification, in the
accompanying picture (figure 30). Soil is formed of particles

Fig. 30. — A generalized drawing of a section, highly magnified, through a well-conditioned
soil and a fragment of root. The soil particles are dotted, the water is concentrically-
lined, the air spaces are left blank; into the soil project the root-hairs from the root
on the left. (Improved from a picture in Sachs' Lectures.)

of rock, irregular in size and form. Around these particles and
in the angles between them is water, held in the capillary state,
while bubbles of air exist in the larger of the spaces among the soil
particles. When more water is added, then the air, being lighter,
is driven upwards and comes bubbling out of the ground; but
it returns again as the surplus water drains or evaporates away.
It is from this air in the soil that roots take their oxygen, and if
the air is kept out of the soil by excess of water, then the roots are
suffocated and die, precisely as air-breathing animals do when they

The Kinds of Work That Are Done by Plants 87

are kept under water. Roots, in fact, drown as truly and in ex-
actly the same physiological way as do animals, and with only
this difference, that roots can stand immersion for hours or dajs,
while animals can endure it only for minutes. This explains
the need for drainage of wet soils; it is not that these have too
much water, but too little air. It explains also why the soil of
flower pots needs to be carefully drained, and the cause of the
failure of so many persons in the care of their house plants, which
most people keep too constantly wet. The very best treatment
for most potted plants is to give to the soil an occasional soaking,
and allow it to dry out pretty well in between tmies; the roots do
not mind the absence of air for some of the time if they can have a
sufficiency at other times. IMoreover this method of watering has
another great advantage over that of adding a little water more
frequently, in the far greater effectiveness with which it drives
out the foul air and ensures a fresh supply.

Another economic phase of respiration is involved in the
popular belief that it is unhealthful to keep house plants in sleep-
ing rooms. It will now be plain to the reader that this belief is
correct. But in fact the danger is slight. The amount of carbon
dioxide given off in respiration by a square meter of leaf is only
about the three-hundredth part of that given off in the same time
by a person, and although buds and roots respire more actively,
it is hkely that a whole window-full of plants does not give off
one fiftieth of the amount that one person does. Or, it has been
stated thus, that all of the plants which could be crowded into
the windows of any ordinary sleeping room give off less carbon
dioxide to the air than would a tiny light kept burning over night ;
and nobody would consider this quantity injurious, especially
if the room were ventilated as it should be. Indeed, were the
respiration of the plants in a room not negligibly small, it would
obviously be unsafe for any person to camp out in a forest in
summer!

We must now come back to the more technical aspects of res-

88 The Living Plant

piration, and examine more closely the chemical and physical
aspects thereof. Since the plant, in this process, absorbs oxygen
only, but releases carbon dioxide, a question is raised as to the
source of the carbon. This must come, of course, from some of
the innumerable carbon-holding compounds inside of the plant,
but, for our present purpose it does not much matter from which,
since they all are derived by transformation from the basal
grape sugar manufactured in the leaves. This grape sugar, ac-
cordingly, is the ultimate, even though not the immediate source
of the respiratory carbon. Therefore we can state the end prod-
ucts of respiration in this wise : —

In respiration CgHioOg and O2 form CO2 and H2O
grape sugar oxygen carbon dioxide water

This general statement can be given a definite chemical form
by making the two sides sum up alike, which requires these pro-
portions : —

CeHi^Oe + 6 O2 = 6 CO2 + 6 H2O

Now although this equation is rarely if ever actually realized
in any particular case, (respiration being never so simple, but a
process highly comphcated in its details), it does represent the
facts as to the ultimate materials and products, the two extremes
of the process; and accordingly we may place it in our series of
conventional constants as the respiratory equation. And its
relations to the photosjmthetic equation will not escape the notice
of the observant reader. The two are the exact reciprocals of
one another, which fact is one of the most consequential in all
nature, as will presently appear.

And now we come to a matter which I wish to impress, the
strongest I can, on the mind of the reader. The phenomena we
have thus far considered, including the one which stands for
most people as the very embodiment of the process, viz., — the
remarkable exchange of the gases, — are by no means the ones of
greatest importance in respiration, but are secondary and in-
cidental to the central and crucial object of the process, which

The Kinds of Work That Are Done by Plants 89

is this, — the release of energy. This release takes place in a single
perfectly definite way, namely, as the result of the invariable
physical fact of Nature that at the instant carbon unites chemi-
cally with oxygen, it matters not in what place or under what
circumstances, energy is released. It is for the release of this
energy that the process of respiration exists ; and the plant no more
respires for the purpose of absorbing oxygen and releasing carbon
dioxide than we kindle a fire in the grate in order to make oxygen
rush into the furnace or carbon dioxide pour out of the chimney.
The object of respiration and of building the fire (i. e., of com-
bustion), are one and the same, — namely, to secure that energy
w^hich is always released at the moment of chemical union of
carbon with oxygen. Respiration and combustion are strictly
homologous terms, applying to phenomena which are also homol-
ogous. In the combustion of coal, which is carbon, in a grate,
the energy is released chiefly as heat (with some Hght) ; and by
causing that release to occur underneath a suitable arrangement
of boilers, pistons and wheels, the energy can be made to produce
motion and thus do work, as every steam engine is a visible wit-
ness. In the explosion (which is merely a rapid combustion), of
gasolene and oxygen inside the cylinder of an automobile engine,
we have exactly the same thing wdth a very much simpler machin-
ery. In respiration within the cells of an animal or a plant, the
machinery is simpler still, but the principle remains the same;
the energy is released at the moment of oxidation under such
conditions that it acts on the simple protoplasmic machinery
provided by the plant in a way to secure transformation into
motion and work. The source of the energy of the work done
by the engine and plant is identically the same; it is only the in-
termediate machinery which is different. The nature of this
machinery, it is true, is not at all understood in the plant, but we
know that something of the kind must exist. The machinery
must also differ somewhat for the different kinds of work that
plants and animals do; but in all cases it is driven by one and the

90 The Living Plant

same power, which depends on the energy released by the oxida-
tion of carbonaceous food. And it may interest the reader hav-
ing a turn for figures to know that the energy released by the
respiration of sugar is just about half of that released by the com-
bustion of an equal weight of the best coal.

These matters though clear on reflection, are hard to grasp in a
first presentation; and I suggest that we rest a little by consider-
ing an incidental matter of interest. In the foregoing paragraph
I impUed that the energy of respiration is not released as heat,
and thus differs from combustion. But the implication is not
strictly correct, as is easily proven. If one takes two handfuls
of seeds, soaks them, and starts them growing and therefore
respiring, kills one set by hot water, places them both in good
non-conducting chambers provided with thermometers, and leaves
them some hours, he will notice a remarkable result. The ther-
mometer in the living and respiring seeds will soon read several
degrees above that in the others, which are obviously similar in
all ways except that they cannot respire. And further experi-
ment shows that this release of heat by these respiring seeds is rep-
resentative of all respiring parts, and that the release of heat is a
constant accompaniment of respiration. Although usually small
in amount this heat sometimes becomes readily recognizable.
Thus the rapidly-opening flowers of Aroids (our Jack-in-the-Pulpit
and its relatives) often show by the thermometer a temperature
several degrees above that of the air; some alpine flowers can melt
their way up, by aid of this heat, through the snow; grain germi-
nating or fermenting in large masses becomes often noticeably
warm; the warmth of hot beds derived from fermenting manures
has the same origin, though here the respiration is that of bac-
teria or molds; and various cases of spontaneous combustion,
where correctly reported, must have the same origin. It does not
appear that this heat, in plants at least, secures any physiologi-
cal advantage but is rather an incidental result of the physical
forces at work, very much as incandescent electric lamps made

The Kinds of Work That Are Done by Plants 91

primarily to give only light incidentally give much heat as well.
But it is this very same heat developed and kept in regulation
which is the basis of the uniform warmth of the animal body.
A few pages earlier it was shown that the carbon in the carbon
dioxide released in respiration comes from inside the plant. This
being so, respiration ought always to entail a loss of weight in

Fig. 31. — Plants of Buckwheat grown from the same number and weight of seed in h'ght
and darkness respectively. The plants are in porous saucers supplied with water and
minerals from below.-

respiring plants or animals; which in fact is found by experiment
to be true. The loss must be compensated by new supplies of
food, else the phenomena of starvation, including emaciation,
ensue. The emaciation of a starved animal, indeed, is due much
more to the loss of substance through respiration than through
the ordinary excretions. In plants, however, it often happens
that those which have lost much weight by respiration without
opportunity to make it up by photosynthesis, look larger than

92 The Living Plant

others which have done the normal photosynthetic work, the ex-
tra bulk being nothing but water. Thus, the two sets of plants
in the accompanying picture (figure 31), were started by the water-
culture method, (later to be explained), from two sets of seeds of
exactly the same weight. But one set (that on the left) was grown
in the light and was able, therefore, to make up its loss by photo-
synthesis, while the other was grown in the dark and could not.
Yet the latter, owing to the habit of plants to spindle out greatly
in length in darkness, actually look larger than the former.
When, however, I weighed these two sets after all of the water
has been dried out, leaving only dry substance behind, the smaller
lighted plants weighed a good deal more than the larger ones
from the dark. It can always be accepted as true that respiration
entails loss of weight through the loss of carbon from the plant.

We can now gather up the facts set forth in the preceding pages
in another of our generalizations, or verities, — the energy indis-
pensable to the work of plants is principally provided by the oxida-
tion of carbonaceous food, and this is the essential feature of respira-
tion.

In the statement of the foregoing verity the reader will notice
that I have used the word ''principally," thus implying that
some other source of energy is available. In fact, while respiration
supplies by far the larger part of the energy used by organisms,
and especially by animals, they do derive some small part from
other sources, notably the heat of the surroundings. But this
part of the subject will all be elucidated later in this book.

We are now face to face with a question of a very fundamental
sort, — namely, what is the source of that energy which is thus
released from food in respiration? For everybody knows that
energy is not created upon the spot, but originates only by
transformation of pre-existing energy. In all science there is no
principle better established, or more important, than that of the
conservation of energy and matter, which teaches that the sum
total of both energy and matter in nature is constant, and that

The Kinds of Work That Are Done by Plants

93

none of either is ever created anew or obliterated, though they
may change their forms multifariously. Where, then, and in
what form was the energy in food before it was released by respir-
ation? The answer is easy, though its comprehension is not.
It was where the energy was in the coal before it was released
as heat in combustion: where the energy was in the storage bat-
tery before it turned the wheels of the electric automobile : where
the energy was in the coiled spring or the wound-up weight of
the clock before it turned the wheels to move the hands: where
the energy was in the full millpond before it drove the looms of the
water-power mill : where the energy was in the gunpowder before
it started the flying bullet. The fact of the matter is this, — that
energy exists in Nature in two different forms, not only in the
familiar active or kinetic form which produces motion and does
work, but also in a resting, latent, or potential form, when its
power to produce motion is held in suspension. Whenever, in
Nature, kinetic energy is exerted to force apart bodies whose
attractions, whether through gravitation, magnetism, cohesion,
or chemical affinity, tend to bring them together, the energy goes
into the potential form for so long as those bodies are kept apart,
and it becomes again manifest in kinetic form when the bodies
are allowed to re-unite. All unsatisfied attractions in Nature are
latent energy. WTien a small boy draws back the powerful elastic
of his favorite sling-shot, he is exerting kinetic energy against
the tension of the elastic; while he holds the elastic stretched to
take aim, that energy is latent as energy of tension; and when he
lets go of the string the energy becomes kinetic again as it drives
the stone in dehghtful swiftness of flight. So, kinetic energy can
raise a weight, go into the latent form as energ}^ of position while
it is suspended, and come out again in kinetic form, as it does
when it turns the wheels of an old-fashioned clock. Kinetic
energy can charge a storage battery, become latent for a time,
and come out once more as kinetic energy driving an electric
automobile. The storage battery, indeed, is typical of all cases

94 The Living Plant

where energy is potential in the form of unsatisfied chemical
affinity. The electric current forces apart the tightly-cohering
atoms of certain very stable chemical compounds; but these atom.s
nevertheless retain all their old attraction for one another, and
it is in the form of this unsatisfied attraction that the energy
is latent; and this energy is given out again in kinetic form at
the moment when the atoms are allowed once more to unite.
Now the very same thing is true of carbon dioxide, which is a
very stable substance of tightly-cohering atoms. To force apart
carbon dioxide into its constituents requires kinetic energy,
which then remains in the latent form, as energy of unsatisfied
chemical affinity, so long as the carbon and oxygen are held apart,
but becomes Idnetic again when the carbon and oxygen are al-
lowed to reunite to carbon dioxide. Does the reader see the ap-
plication? Surely he must. The kinetic energy of the sunlight
splits apart carbon dioxide in the green leaf, the oxygen going
out to the air and the carbon combining with the elements of
water into grape sugar; so long as this carbon and oxygen are kept
apart, that energy is latent in the form of unsatisfied chemical
affinity; and when the carbon of the sugar (or of any other sub-
stance into which the sugar is transformed) is allowed to unite
with the oxygen of the air, as it is in the process of respiration,
then kinetic energy is again given out and can be used for the work
of the plant. Such is the source of the energy of respiration, —
it is energy released from the latent state in food, where it was
placed (or ''stored") by the kinetic energy of the sunlight. Food,
therefore, is a storage battery charged by the sun, and discharged
by respiration.

The principal function of food must now be quite plain. As a
storage battery it has advantage over any that man has yet
made in the fact that it can be reduced to very small fragments,
or even to solution (by digestion), and thus transported to all
parts of plants and throughout the bodies of animals. Then, at
the spot w^here work needs to be done, just at the right instant,

The Kinds of Work That Are Done by Plants 95

under the suitable machinery, the carbon of the food is allowed
to unite with ox^^gen, and the energy is released to do the need-
ful work. And that is the way in which plants and animals ac-
complish their work; and the power to do this, — to absorb stored
energy, transfer it to all of their parts, hold it ready for use, and
release it when needed, — is the most distinctive feature of Uving
beings.

The reason is now evident also for the reciprocal character
of the photosynthetic and respiratory equations. In photosyn-
thesis carbon dioxide and water are made into sugar and oxygen
with storage of energy; the sugar is transported by plants or by
animals to places of need, undergoing chemical changes on the
way but ever retaining the store of unsatisfied carbon; then in
respiration oxygen is allowed to come into chemical contact with
the sugar, and the two are changed back to carbon dioxide and
water with release of energy. It is because substances exist which
thus permit of such storage and transportation of energy that
organisms as we know them are possible.

It may aid still more to a clear understanding of these two most
fundamental and important of all physiological processes if we
set their chief features in contrast in form of a table ; —

Photosynthesis Respiration

Occurs only in plants Occurs equally in plants and animals

Occurs only in chlorophyll grains Occurs in all living protoplasm

Occurs only in light Occurs equally in light and darkness

Manufactures food Destroys food

Increases weight Lessens weight

Absorbs carbon dioxide Releases carbon dioxide

Releases oxygen Absorbs oxygen

Forms CeHiiOe from CO2 and H2O Reduces CcHisOg to CO2 and H2O

Stores energy Releases energy

We can now gather up these latter facts in another of our
verities thus, — the energy released in respiration ivas previously
latent in the unsatisfied affinity of the carbon in the food for the

96 The Living Plant

oxygen outside, those two elements having originally been separated
by the kinetic energy of the sunlight in photosynthesis and kept
separate through all the subsequent transformations and trans-
portations of the food through the bodies of plants and animals; the
original source of respiratory energy is therefore the sunlight, and
food is primarily a storage battery, charged by the sun in green
leaves and discharged by respiration at the places of need.

It will doubtless ere this have occurred to some philosophic
reader to ask whether carbon dioxide and water are the sole
substances by which organisms can thus store and transport
energy, and whether, accordingly, life is dependent solely upon
them. There is, however, no chemical reason why organisms
might not use in the same way any other decomposable and
oxidizable substances, and indeed even in our common plants some
small quantity of energy is no doubt derived from the oxidation of
other elements, while certain Bacteria exist which can use the
energy derived from the oxidation of sulphur compounds. Plants
probably use carbon in photosynthesis and respiration chiefly
because its chemical transformations, which are very susceptible
to temperature, happen to be easily under control at the temper-
atures now prevaihng on the earth's surface. Under markedly
higher or lower temperatures carbon would be unavailable for this
purpose, but it is conceivable that life might still exist by the
similar use of other substances whose combinations would be
under control at those temperatures. It is only a step farther to
assume that life might even exist in this way in the flames of a
nebula, or the awful cold of interplanetary space, and hence
that its origin may be contemporaneous not only with the origin
of the earth, but even with the origin of matter itself. It is not
at all likely that hfe is something which results incidentally from
the properties of carbon; it is far more probable that it is some-
thing which uses the properties of carbon as the most convenient
tools for its own ends. This is a phase of the super- vitalism of
which I have spoken in the first chapter.

The Kinds of Work That Are Done by Plants 97

This chapter has ah-eady attained to a length so great that I
wish it were possible to end it right here. But certain additional
matters are connected wdth respiration so closely, and are be-
sides in themselves so important, that we must really keep on to
include them, though perhaps the reader will find it best to defer
a reading thereof for another occasion. These matters are fer-
mentation, decay, and disease.

Fermentation is a phenomenon familiar to all, and best known,
perhaps, in the ''working" of preserves, which become ''strong"
i. e. alcoholic, while giving off tiny
bubbles of gas. The most typical

kind of fermentation is that caused ""^^^ ' -^

by Yeast. Yeast, I venture to
remind the reader, is a very tiny >^ ^^

non-green plant which lives as a
saprophyte in sweet liquids. Mag- ( \

nified to a high degree by the mi-
croscope it looks much like our
picture (figure 32), though whiter, x- 00 v ^ i . u • , ,,

■^ ^ ^ ' ' ° Fig. 32. — i east plants, each a single cell

A Yeast plant is a single ovoid ^v■hich buds out from a parent cell; very

cell which buds out into others,

and these into others, in loose chains which fall easily apart, —
and so on, as long as the food supply lasts. And that is all,
except that when the liquid dries up, the cells produce very
thick-walled spores which float around in the air with the dust,
to start once more wiien they happen to fall into another sweet
liquid. It is by the growth of these cells that a sweet liquid is
"fermented" with a formation of alcohol and carbon dioxide.
This can be demonstrated very easily and clearly to the eye by
an interesting expermient. If one puts together in a glass flask
a solution of sugar and a cake of compressed (not dried) yeast,
and stands it in a warmish place, then within a very few
minutes tiny bubbles of gas begin to rise through the liquid,
producing a froth on its surface. If, now, the stopper of the flask

98 The Living Plant

be provided with an outlet tube bent over to end at the bottom
of a vessel of clear limewater, the gas will come bubbling up, and
will soon turn the limewater milky, thus proving its identity.
And when the fermentation is ended the liquid left in the flask
has always that "sourish" smell distinctive of the presence of al-
cohol, which, indeed, can be separated for testing by distilling
the liquid. As to its quantity, however, it is important to know
that even when all the conditions for fermentation are most
favorable and the sugar is present in plenty, the Yeast neverthe-
less does not form more than a limited quantity of alcohol, —
(about ten per cent of the hquid in round numbers), for then the
plant is rendered inactive and may finally be killed by the very
alcohol which it has produced.

Such is the process of fermentation, which, as everybody knows,
is vastly unportant in the arts. Sometimes it is used for the sake
of its carbon dioxide and sometimes for the sake of its alcohol.
The conspicuous case of the former is found in the making of
bread, where the carbon dioxide released from the growth of the
yeast cells throughout the mass of the dough, forms the cavities
by which it is lightened and raised. \Alien everything goes as it
should, the alcohol evaporates in the baking, but sometimes
it does not, and then the bread goes ''sour." Of course other
methods of raising bread are in, use, notably by aid of gases re-
leased in the dough from chemical action between the constit-
uents of suitable ''baking powders," or other substances, and
also by use of air blown into the dough; but yeast fermentation
is much the most used of the methods. But far more extensive
is the employment of fermentation for the making of the various
kinds of alcoholic liquids. When the sweet juice of the grape is
allowed to ferment (by action of yeast blown as spores through
the air to the fruits), the carbon dioxide escapes to the air, and
the remaining admixture of alcohol, water, and flavors we call
wine. AATien the sweet pulp of the germinating grains of barley
is allowed to ferment (by Yeast which is added for the purpose),

The Kinds of Work That Are Done by riants

99

we give the name beer, "lager beer," to the hquid resuUing.
And innumerable other sweet juices and saps are fermentable,
with resulting formation of alcoholic beverages, which are so
many and diverse in kind that most nations have each some
favorite one of its own, the differences between them being due
in the main to various flavoring materials originally present with
the sugar. None of these fermented liquids, however, are ever
stronger in alcohol than the ten per cent, or thereabouts, which
the Yeast can yield before it is killed. The stronger liquors are
obtained by an additional and very different kind of operation, de-
pending on the fact that alcohol boils at a much lower temperature
than water (78°C, or 172°F as compared with 100°C or 212°F).
For this reason a fermented liquid, if heated above 78° but
under 100°, gives off its alcohol (though also with some water)
as vapor, which can be conducted away, cooled and collected
as a strongly alcoholic hquid. The process is called distillation,
and in this way are made the stronger alcoholic drinks, — brandy,
whisky, rum, gin, and all the remainder of this precious rogue's
gallery, — their peculiar flavors and colors being due to particular
substances, sometimes naturally present and sometimes purj^osel}^
added, in the juices from which the alcohol is fermented. It is by
repeated distillation of the fermented juice of germinating corn
that the strong alcohol of commerce is made, and this when mixed
with a little of the poisonous wood alcohol to make it undrinkable
becomes the ''denatured alcohol" of the household and the chaf-
ing dish.

We turn now to the chemistry of fermentation, which is simple.
It is grape sugar which is fermented, for other sugars or starches
are first changed to that form or its equivalent. Therefore we
have this expression.

In fermentation CgHiaOe forms COg and C2H6O

grape sugar carbon dioxide alcohol

This statement can be given an exact chemical form in this
way,— ^

loo The Living Plant

CqR,A = 2 CO, + 2 CsH.O

And this equation expresses exactly the known facts of the
process.

Wliat now is the meaning of fermentation, and why does the
Yeast do it? Nowhere in Nature, so far as I can find, excepting
in the case of humanity, is there even the least evidence that any
kind of organism ever does anything whatever for the sake of
service to any other kind. We should not expect to find, accord-
ingly, that the Yeast makes the carbon dioxide and alcohol for
any disinterested or philanthropic purposes, — not for providing
thrifty housewives with light bread or their shiftless husbands
with strong drink, — and we turn to seek some desirable object of
its own to which the use by mankind is purely incidental. But
of course, the reader has inferred the explanation before this, — ■
fermentation is simply the Yeast's respiration, the source of its
power for growth and other work that it does. And the explana-
tion of so peculiar a form of respiration is well known. Living im-
mersed in a liquid, the Yeast cannot obtain respiratory oxygen
from the air, and must take it from some other source. Only one
source is available. Locked up in the molecule of sugar is some
oxygen brought into it with the hydrogen, which holds it away
from the carbon, as the formula C0H12O6 suggests. But the Yeast
plant, absorbing the sugar into its body, shatters the molecules
(by means of a peculiar agency called an enzyme soon to be
described), and allows the carbon and oxygen in the fragments
to unite with one another; this produces the usual result, — a
copious release of energy which the Yeast at once utilizes for its
growth, while of course the resulting carbon dioxide is thrown off
into the liquid. This is the object, or meaning, of fermentation; —
to secure a union of carbon and oxj'^gen for the sake of the energy
which is always thus released. As to the alcohol, that is simply
the remains of the shattered molecule; it is a chemical fact that
the number of atoms of carbon, hydrogen and oxygen which hap-
pen to be left after the carbon dioxide is formed, fall naturally

The Kinds of Work That Are Done by Plants loi

into alcohol, and the Yeast plant cannot help it. That is why the
Yeast produces the poisonous alcohol, despite the suicidal char-
acter of the proceeding. The Yeast, however, can respire in no
other way, and with commendable philosophy, prefers a short
life, even at the risk of an alcoholic grave, to no life at all. Yet
in fact the case is not really so bad, for the alcohol is very volatile,
and in Nature commonly evaporates as rapidly as formed; and
even when not, the drying up of the liquid and spore-formation
allow the yeast to escape and renew its activity at another time
and place. If the Yeast plant had nothing to do but respire,
the sugar would all be converted to carbon dioxide and alcohol,
which are probably the sole products of its respiration. But the
Yeast must also make new substance, protoplasm and walls,
for which purpose it uses some of the sugar in a different way,
along with other substances, and thereby develops incidentally
a small percentage of by-products, — glycerin, acids, etc., the pur-
suit and capture of which affords a fine joy to the special student
of chemistry, especially if some student of biology has previously
told him that carbon dioxide and water are the "products of fer-
mentation."

Alcoholic fermentation caused by Yeast is the most typical
and familiar kind, but other sorts occur, caused by germs (Bac-
teria), or Molds. Thus the souring of milk, the rancification
of butter, the genesis of vinegar, and even the development of
distinctive flavors in ripening cheese, are products of fermenta-
tions, caused in their respiration by various organisms. As these
cases illustrate, the secondary products need by no means con-
sist only of alcohol, but can include substances of the most diverse
chemical natures. All that is requisite is that carbon and oxygen
shall be allowed to unite; the matter of the particular compounds
is secondary.

If any doubt could exist that fermentation is simply the respir-
ation of the Yeast plant, it would vanish before the remarkable
fact that an exactly intermediate step is known between the

I02 The Living Plant

respiration of the higher plants and typical fermentation. Ideally,
in the resphation of the higher plants, the oxygen absorbed and
carbon dioxide released are equal in volume, but often they are
not. Thus, some lands of seeds, like Peas, if shut away from
oxygen, can release plenty of carbon dioxide without absorbing
any oxygen at all; and analysis of the seeds then shows the pres-
ence of alcohol. In other words, these Peas, like the Yeast plant,
can cause fermentation (though in limited degree) of some of
their own substance; and there is no doubt that it represents the
form of respiration to which the seeds resort when no oxygen
from the ah is available. This form of fermentation is called
in the Peas, and the other plants which make use of it, anaerobic,
or intramolecular, respiration.

There remain two other forms of fermentation so important
as to require a separate treatment. One is decay, or putrefaction,
which is really the fermentation of dead plant and animal sub-
stances by Bacteria, or germs. Bacteria are plants even smaller
and simpler than Yeasts. The products of their respiration and
growth are most diverse, including not only carbon dioxide and
water but various other gases, some of which possess those very
vile odors distinctive of rotting organic matter. WTien the de-
caying substances are complex, e. g., flesh or other proteins, certain
Bacteria ferment them to simpler sorts, other kinds to simpler
still, and so on, until they are finally reduced, as in ordinary respir-
ation, to carbon dioxide and water, and such other elemental
substances, (e. g., nitrogen) as may also have entered into their
composition. All decay is simply a form of fermentation, that is
respiration, by Bacteria, or, in some cases, by sunple Molds.

Another phase of the same phenomenon is involved in those
deadly diseases which are caused by Bacteria, — Asiatic Cholera,
Tuberculosis, Diphtheria, Typhoid, Lockjaw, and a number of
others. It is a popular belief that Bacteria produce their effect
in disease by destroying the tissues, or, as a plain-spoken student
of mine once expressed it, they "chew you all up inside." That

The Kinds of Work That Are Done by Plants 103

belief is far from the truth, for what happens is this. The Bac-
teria, in order to obtain energy and material for their own pro-
cesses, act on the tissues or the blood in just the same way that
Yeast acts on the sugar, likewise forming incidentally in the act
various accessory substances. Now some of these substances,
bearing much the same relation to the Bacteria that alcohol
does to the Yeast, are those alkaloids or ptomaines which happen
to be violently poisonous to man, and it is these poisons, and not
the Bacteria directly, which are the cause of his death. At least
they are the cause of his death if they are formed more rapidly
than his system can antagonize them, for the body has a wonder-
ful power of forming antagonistic chemical substances, or anti-
bodies, which neutralize these poisons, — which antibodies, by the
WSLY, can be made to form in the body, or even can be injected
as antitoxins, ensuring immunity against some diseases. These
deadly diseases are therefore an incidental result of the respiration
and growth of Bacteria which are leading their own Uves in their
own way, as oblivious to any harm they may do as is the Yeast
to the benefit it confers.

It is riot only true that fermentation, decay, and some disease,
are caused by the activity of Yeasts, Molds, and Bacteria, but
the converse is equally well-known, — that those processes occur
through no other agency and can be prevented entirely by killing
these organisms. This can be done by heat, poisons, certain
strong solutions, or even, in some cases, bright light; and such is
the basis of the various sterilizing and antiseptic processes so
familiar in the household, the arts, and in medicine.

We can now express these later facts in another of our verities
as follows; — all fermentation and decay, and some phases of dis-
ease, are forms of the respiration of simple organisms which thereby
destroy organic matter by reduction back to the carbon dioxide, water,
and other elements, from which it was originally built up.

It is thus evident that all of the carbon dioxide and water
built into plant substance by photosynthesis, are ultimately re-

I04 The Living Plant

leased again by respiration or decay. A quantity, rather small,
of the earth's supply of carbon dioxide and water is therefore
always locked up in plant and animal substance; but though the
quantity is approximately constant the precise molecules are
constantly changing, and with the changes go those transfoi-ma-
tions of energy which are the principal manifestation of life. And
if the question be asked, why are not more of the carbon dioxide
and water of nature locked up in plant and animal substance,
that is, why are there not more and larger plants and animals on
earth, I think the answer is easy. There do aheady exist upon the
earth all of the plants and animals, and as big ones, as the physical
conditions permit. As to plants, every spot on the earth that
can maintain plant life at all is bearing all the plants it can sup-
port, and these plants are just as big as the physical conditions
permit them to grow. As to animals, they are dependent upon
plants for their food, and it is evident that there is available for
their use only the smplus of food produced by plants over that
which these need for themselves, — and animals are just as abun-
dant and big as that surplus can support.

Thus, these apparently verj^ complicated processes of photo-
synthesis and respiration, Uke many another and probably like all
of the physiological processes in plants and in animals, can be
reduced to a basis of piu-e physics and chemistry. And we shall
learn later, in our chapters on Irritabihty and on Growth, that
we have a good explanation of the orderh^ sequence and regular
comiection of the processes in their hnking up together through
their interactions as stimuli. Is there then, nothing in the plant
except the interactions of chemistry and physics? Let the remain-
ing pages of this book give their testimony before we attempt the
answer.

CHAPTER V

THE VARIOUS SUBSTANCES MADE BY PLANTS, AND THE
USES THEREOF TO THEM AND TO US

Metabolism

]N chapter two of this book it was shown that plants
manufacture grape sugar in their lighted green leaves;
and I said it would later be proven that this sugar rep-
resents a basal food substance out of which, with sundry
minor additions, plants build all of their other materials. The
time has now come for this demonstration, to which, as a sub-
ject possessing perhaps more importance than interest, I bespeak
the reader's somewhat spartan attention. Since all of the sub-
stances constructed by plants have a meaning in their vital
economy, I might also have entitled this chapter ^'on the various
uses that plants make of their food," in which case I should
have to commence with a review of respiration, for that is the
most important of the uses of food. The others here follow in
an order determined chiefly by the chemical nature of the sub-
stances concerned.

The number of substances constructed by plants is verily
legion, for the vast variety of foods and fabrics, drugs and dyes,
and other materials yielded by them to us is only a small pro-
portion of those which they actuall}^ make. Fortunately, how-
ever, for our limited comprehensions, those which are really
important are few, and moreover, they fall into somewhat defi-
nite classes. Since the subject is new to most persons, I will give
these classes in synopsis as a kind of table of contents to this
chapter. They are these: —

io6 The Living Plant

Class I. The Basal Food, or Photosynthetic Sugar; the substance first
formed in lighted green leaves; composition CeHiaOe.

Class II. The Foods, active and reserve, and the Skeleton; chemically
called Carbohydrates, with a composition identical with or
readily transformable from that of the photosynthate, viz.,
CeHi-iOo, or C10H22O11, or (CgHioOs)/!.

Class III. The Secretions; various non-nitrogenous substances, mostly of
special ecological functions, derivatives of Carbohydrates
and containing the same elements, but in markedly different
proportions, and hence collectively expressible only in the form

CiiHuOn-

Class IV. The Nitrogen-Assimilates, chemically called Amides; inconspic-
uous but important substances containing the elements of the
photosynthate with the addition of nitrogen, and forming the
transition from Class I to Class VI; collectively expressible
only as CnH„OnXn.

Class V. The Principal Poisons, chemically called Alkaloids ; containing
(as a rule) the elements of the Amides but in different pro-
portions, substances of uncertain meaning, and collectively
expressible as CnH„ (On) N,i.

Class VI. The Flesh-forjiers, chemically called Proteins, contributing
to the formation of protoplasm and consisting of the elements
of the Amides with the addition of sulphur and phosphorus,
and collectively expressible only as CnHnOnNnSn (Pn)-

Class VII. The Regulators of Metabolism, called Enzymes, substances
of unknowai composition, but supposed to be proteins, possess-
ing remarkable properties of causing chemical transformations
in other substances.

Class VIII. Living Protoplasm.

Class I. The Basal Food, or Photosynthetic Sugar

This substance needs no introduction to the reader of the earlier
parts of this book; but for others it may be characterized as a
sugar made abundanth^ in the lighted green leaves of plants from
carbon dioxide and water, and forming the foundation of all
organic substances. It belongs in a class by itself only because
of its unique mode of formation and function, for chemically
it belongs in the second class, being nothing other than a mixture
of the grape and fruit sugars next to be described.

The Various Substances Made by Plants 107

Class II. The Food and Skeletal Substances, or Carbohydrates
Grape Sugar. This substance is formed abundantly in green
leaves as the photosynthate, and is common in nearly all parts
of all plants. It is, however, much less known than its import-
ance would imply, because it has no prominent economic uses,
and exists in the plant only in solution in the sap of the cells,
which therefore display through its presence no more striking
appearance than that represented in the accompanying example
(figure 33). However, it sometimes ac-
cumulates considerably in fruits, which
it helps to make nutritious and attract-
ive to animals in connection with dis-
semination, a subject to be later dis-
cussed in a special chapter devoted to
that subject; and in grapes, especially,
it is so plenty that it crystallizes out
when they are dried, forming the soft
sugar abundant on some kinds of raisins.
Its many and easy transformations into
other substances will be traced in the
following pages. It has, however, a
second origin and significance in the
plant, for it is that into which many other
substances are converted in digestion, as we shall presenth^ learn,
and is the commonest form in which substances are translocated
through the plant. It is white in mass, looks amorphous and not
crystalline to the eye, is sweet to the taste, though much less
sweet than cane sugar, and is the easiest of all sugars for Yeast
to ferment. It is interesting to know that it has been made
artificially in the chemical laboratory. Chemically its correct
name is dextrose, though often also called glucose, and its formula
is C6H12O6.

Fruit Sugar. This substance is extremely like grape sugar, with
which until lately it was more or less confounded, and with which

Fig. 33. — Appearance in opti-
cal section, highly magnified,
of a cell in which sugar is
stored in the sap.

Inside the wall is a lining of liv-
ing protoplasm which encloses
the large sap cavity wherein
is water containing the dis-
solved sugar.

io8 The Living Plant

it occurs in the various roles above mentioned for grape sugar.
It is sweeter than grape sugar but ferments less easily. Chem-
ically it is called fructose, and has the formula CeHigOg, differing
from grape sugar not in the kind or number of atoms entering into
its composition, but in the arrangement of these within the mole-
cule, as best demonstrated by physical tests with polarized light.

Cane Sugar. This substance is perfectly familiar to everybody,
for it is the granulated sugar of the table. It is widely spread
through plants dissolved in the sap, and accumulates in some kinds
so abundantly as to form a reserve supply of food for them, and
a store upon which animals, inclusive of man, are accustomed to
draw for their needs. This accumulation occurs conspicuously in
the Sugar Cane and the Sugar Beet (both of which plants have
had their percentage of sugar immensely increased by cultivation),
in the Maple tree, and in a few other less conspicuous plants,
while it is conmion as well in ripening fruits. Chemically cane
sugar is called sucrose, and has the formula Ci2H220ii- It is
built up by living protoplasm from photosynthetic sugar through
this simple step, 2 C6H12O6-H2O (water) =Ci2H220n; and it falls
back by a reverse process to a molecule of grape sugar and one of
fruit sugar. This latter step actually occurs in the ripening of
fruits, in cooking, and in digestion; and it is, therefore, as grape
sugar or fruit sugar that cane sugar is finally incorporated into
both the plant and the animal body.

In addition to these sugars, there are others of rarer sort de-
scribed in the technical books, — all closely related and more or
less intertransformable into those we have mentioned. Such are,
for example, maltose, mannose, galactose, arabinose, xylose,
fucose. I am very well aware that these names will have no great
attraction for the reader, but I take somewhat the same satis-
faction in their recital that Homer derived from the roll of his
heroes, whom also he mentions but once.

Starch. This substance is perfectly familiar to everyone as
common laundry starch, and especially as flour, which is mostly

The Various Substances Made by Plants 109

composed of it. Occurring as a rule in tiny white grains scattered
widely through all kinds of tissues, it collects in some organs,
which swell very greatly for its reception. Such is the Potato,
which is simply a starch-storing underground stem: the Sweet
Potato, a starch-storing root: bulbs, which are masses of starch-
filled leaves: and most seeds, including all of the grains, which
contain copious starch either inside or around the embryo. In
all of these cases, starch presents a characteristic homogeneous
firm whitish appearance, contrasting markedly with the soft
translucent aspect of structures in which the food is stored up as
sugar, e. g., the Sugar Beet, Sugar Corn. It happens, however,
that its presence can be detected in a very conclusive way, namely
by the deep blue color it assumes when touched by a solution of
iodine, as the reader already has learned, and as he can easily
prove for himself by applying a little of the tincture of iodine to a
lump of starch from the laundry box, or to a disused cuff, or to
water in which some starch has been scraped, — and heated until
it forms a fine paste. The test is one of the most satisfactory
and important in all organic chemistry, and so delicate that, by
its use with the aid of the microscope, one can detect even the
minutest quantities of starch in the tissues of a plant, where it is
sometimes distributed with a curious and beautiful geometrical
exactness. It is necessary to warn the experimenter, that in
living tissues, however, the test often works rather badly, because
iodine penetrates active protoplasm very slowly.

Starch, when it accumulates in the plant, serves as a store of
reserve food upon which the plant can draw when it starts new
growth; and starch is by far the most connnon and abundant of
plant foods. Moreover, it serves equally well as a food for an-
imals, which, accordingly, rob the plants; and these are there-
fore obliged as a whole to make a huge surplus in order to keep
any at all for themselves. The importance of starch as food for
man is evident when one recalls that Wheat, Corn, Rice, Barley,
Rye, — grains, which constitute the principal food of the great

no The Living Plant

majority of the human race, — are composed almost wholly of
starch.

Chemically, starch has the fornmla (CgHjoOg)??. It is formed
apparently thus, — from dextrose, C6H12O0, water, H2O, is with-
drawn, leaving CgHioOs; this substance does not occur in this
form in the plant, but the molecules immediately aggregate them-
selves (chemically, polymerize), to a considerable but unknown
number, expressed by the letter n, into compound molecules.
Starch is made up in this w^ay from dextrose, and it is of interest
to note that a corresponding substance made from fructose occurs
as a reserve food dissolved in the sap of the swollen roots of some
Composite plants, where it is called inulin. The formation of
starch has never been effected artificially outside of plants, and
in them it takes place only inside of those living protoplasmic
bodies called plastids, which include chlorophyll grains and which
are to be described more fully in the next chapter. The re-
conversion of starch to dextrose is effected through the action of
diastase, — one of those remarkable chemical agents called en-
zymes, which we are presently to study; and this is exactly what
happens in the digestion of starch in both the plant and animal
body. Indeed, this digestion can be carried on experimentally
and very easily in a test-tube by action of diastase bought from
any chemical supply company, the disappearance of the starch
being proven by use of the iodine.

A fact of another kind about starch should be noticed at this
place. Even to the unaided eye it looks granular in texture, while
the microscope shows that it really is composed of definite grains,
which, moreover, display a remarkable structure. If a section
be cut from the interior of a potato, for instance, and magnified,
the cells are found to present an aspect well shown in the typical
example here pictured, (figure 34). Within each cell are numerous
solid grains, various in details of their shapes, but all possessing
in common a focal spot near the smaller end, around which are
excentrically-arranged layers (figures 34 and 35). Starches from

The Various Substances Made by Plants iii

other plants are of different aspect, as our plate so clearly illus-
trates (figure 35) ; but each kind exhibits characteristics peculiar
to itself, and in general it is true that no two species of plants
have grains exactly alike, while each
species has a kind distinctive of itself.
This fact has a practical value, because
experts with the microscope can thus
learn to recognize the starches of dif-
ferent plants at sight, and by this means
can detect adulterations in starchy foods
or drugs. Biologically, also, this indi-
viduality of the starches is of very great
interest, for it gives us a clear case in fig. 34.— a cell, highly magni-
which a well-developed specific character f^"' ^Z^i^^^ii;^,
exists without any regard to utility; for search grains embedded in

living protoplasm.

even the most radical adaptationist would

hardly consider the forms of the deeply-buried and invisible
starch grains as useful in adapting the species to its environment.
And if an internal specific character can be useless, what need to
try to explain every external specific character as necessarily
useful? I am very well aware that this little digression will seem
without point to most of my readers, but I pray them to have
patience a little, for I have a good object. I am calling their
attention when I can to certain data which will later be useful
when we come to consider the subject of evolution.

Cellulose. This substance is vastly abundant and prominent
in plants, for it is the material out of which they construct the
walls of their cells and therefore their entire firm skeletons.
The reader can obtain a good idea of pure cellulose by recalling
the fibers of cotton, the pith of woody stems, or some of the pur-
est unstarched paper, such as the filter-paper of the laboratories, —
all of which exhibit the distinctive cellulosian qualities of tough-
ness, elasticity and transparency. In some plants also, it is
stored up as a reserve food in the seed, when it appears as an im-

112

The Living Plant

mense thickening of the cell- wall (figure 36). A conspicuous
case is the Ivory Palm, which has seeds so hard as to constitute
a substitute for ivorj'- in the making of buttons and other bijou-
terie, while the seed of the Date owes likewise its stony hardness
to the same material. Though so hard, this cellulose is easily
digested to sugars by the action of suitable enzymes, and the pro-

FiG. 35. — Typical grains of a dozen different kinds of starches, highly magnified. The
kinds, in order of arrangement in this picture are; —

Potato Maranta Pea Hyacinth

Wheat Oats Sago Smilax

Canna Corn Bean Oxalis

cess is applied commercially to ordinary wood in the manufacture
of wood alcohol. Naturally, the very cells which make cellulose
have the power to digest it away once more where needful; and
this is why cell-walls, even when well grown, can become perfo-
rated, absorbed, split, or even re-adjusted in such a way that they
seem to have slid upon one another.

The Various Substances Made by Plants 113

Chemically, cellulose is related to grape sugar and formed there-
from in much the same way that starch is, its formula being the
same as that of starch, (CeHioO-)?!, with the n, however, represent-
ing a different but unknown value.

Although cell-walls when young consist only of cellulose, in
some structures they become penetrated later by other materials
which are probably formed by alteration of
the cellulose itself, and which give new proper-
ties to the walls. Thus, it is a stiffening sub-
stance called lignin, added to cellulose walls,
which converts them into wood, and also
forms other hard tissues, such as the shells of
nuts; while a very different substance, cutin
or suberin, makes the walls thoroughly water-
proof, as they are in all cork, and in the thin
waterproof epidermis which ensheaths the en-
tire plant. An alteration of the cellulose of
another kind produces the mucilaginous ma-
terial displayed when some seeds (e. g., those
of the Flax), are placed in water, or when
fallen leaves turn gunrniy on sidewalks in wet,
warm, autumn weather; and such also is the
origin of the mucilage or slime found in des-
ert plants on the one hand and water plants on the other, with
peculiar functions in those plants to be later considered.

There are highly consequential facts of another kind about
cellulose whether lignified or not. It burns readily in presence
of oxygen, being converted back in the process to carbon dioxide
and water, the very substances from which it was originally made.
When, however, it is subjected for a long period of time to pres-
sure and heat, gradually it undergoes definite chemical changes
through which its hydrogen and oxygen are removed, leaving
behind the solid and non-volatile carbon. This is exactly what
has happened in the case of the plants which grew of old time in

Fig. 36.— a cell with
parts of four others,
from the interior of
the nut of Ivory Palm,
showing the walls im-
mensely thickened by
deposition of layers
of cellulose, through
which run canals per-
mitting a continuity of
protoplasm from one
cell cavity to another.

114 The Living Plant

the swamps of the Coal Period; their walls, losing the oxygen and
hydrogen, have become proportionally richer in carbon and in-
cidentally darker in color, passing gradually through stages repre-
sented by peat, lignite, soft coal and anthracite, which latter is
almost entirely carbon. It is thus that our beds of coal have
been formed. Somewhat the same thing occurs, through the action
of heat, in the charring of wood, and a similar process produces
the black humus of good soils from roots and the like. When the
carbon of coal remains yet longer exposed to suitable conditions,
it becomes graphite or black lead, while if crystallized it forms
diamond, the end of the series. And it is interesting to note in
this connection that we do not yet know any natural way by which
pure carbon can be isolated from oxygen without photosynthesis
constituting a step in the process. If one were to burn the dia-
mond, he would form carbon dioxide again, and thus close the
chain of transformations through which the carbon has gone since
it was absorbed from the air by a living green plant long ages ago.
And as to this burning, it is interesting to reflect that the heat and
light released in the combustion of coal is energy that was rendered
latent by the photosynthetic dissociation of carbon dioxide when
the coal was first formed as a photosynthate; it has been kept
stored all this time in the unsatisfied affinity of its carbon for
oxygen; and when released in our midwinter fires, it is really the
heat and the light of the ancient carboniferous sun that is warm-
ing and cheering us.

Gums. These are solid but very elastic sweet substances, of
which gum arable, used in gumdrops and on postage stamps,
is the most familiar example; the gum of cherry trees is another,
and the substance of marsh mallows another, though the spruce
gum, of the woods and the schoolroom, is quite different as will
be noted below under resins. These gums are accumulated in
rifts of the tissues of some trees, but it is not at all clear why the
plant should make them, though apparently they serve at times
as reserve food. Chemically they have the formula (CbIIkjOs)?!,

The Various Substances Made by Plants 115

the same as that for cellulose and starch, but with n meaning
another figure; and they are formed no doubt from grape sugar,
(probably via the mucilaginous modification of cellulose men-
tioned in the preceding paragraph), to which they are readily
digested back by both plants and animals.

Fruit- Jellies. These substances are familiar to all housekeepers
as the jelly which forms when fruits or vegetables are cooked
(e. g., grape jelly, orange marmalade, pumpkin preserves), though
it must be remembered that gelatine, from which the jellies of the
tea-table are made, is an animal product. In the living plant
they are solid, being insoluble in cold water; but they are dissolved
by hot water, which explains why they appear after cooking.
They represent, it is believed, another form of reserve food. Chem-
ically they are known as pectins, and they have also the same
general formula as starch (CgHigOg)?!. They are formed without
doubt from grape sugar to which they are easily digested back.

In reading this account of these various carbohydrates, two
questions will inevitably arise in the mind of the reader. First,
he will ask how it is possible that substances with properties so
different as those of starch, cellulose, gums and jellies can have
the same chemical comxposition. The answer is this, that on the
one hand the letter n in these formulae represents without doubt
a difTerent number in each case, and hence the composition is not
really identical, while on the other, even an identical formula
can be associated with very different properties, because the
properties depend not only on the elements present, but upon
the way these elements are arranged in the molecule; and they
can be arranged in very different ways. The differences between
grape sugar and fruit sugar are wholly of this latter kind. The
second question the reader will wish answered is this, — why do
some plants store up their reserve food in the form of sugar, some
as starch, others as cellulose, and others as oil, soon to be men-
tioned. This question we cannot yet answer with certainty, but
probably the general explanation offered in Chapter III for the

ii6 The Living Plant

diverse ways in which plants develop the same organs, applies to
the present matter, also, — namely, the plant makes the form of
food easiest chemically for it to construct, provided of course
there is no ecological reason for making one kind rather than
another.

Class III. The Secretions, or Derivatives of Carbohydrates

So heterogeneous are these substances in composition, proper-
ties, and uses, that they are held in one class by hardly any
stronger bond than that, while including the elements of Class II,
they do not belong therein. Nor is the name which I give them a
good one, for they include some things which are not truly secre-
tions, while not all of the secretions are included in this class; but
I can think of no better general designation. The principal
members are the following.

Plant Oils. These are of two distinct kinds. First, are the fixed
oils, which are properly plant fats, familiar to us in the various
oils used in food or in medicine, notably olive oil, castor oil, cot-
ton seed oil. They occur rather widely scattered in plants, as
tiny isolated drops, scattered through the protoplasm; but they
accumulate in quantity in many kinds of seeds, including nuts,
to which they give a distinctive oily luster, and in which they act
very obviously as a reserve food for the use of the embryo in
germination. A reason why oil is stored in seeds more frequently
than elsewhere has been found in a linking of two facts; — first,
food value for food value, oil is a much lighter substance than any
other kind of food stored by plants; and second, the seeds storing
it are mostly disseminated by the wind and hence need to be kept
just as light in weight as possible. And with these oils as with
other substances, good food for plants is good food for animals
also, the food needs of both being closely alike. Chemically
these fats are rather complex, a typical formula being C57H110O6,
which shows that they are markedly poor in oxygen; and herein
Ues the reason why plenty of fresh air is needed for their assimila-

The Various Substances Made by Plants 117

tion by man. They are formed in hving cells from starch, and
therefore ultimately from grape sugar, to which they can be
changed back in germination and digestion by the action of suit-
able enzymes.

Related to the fats in some respects, though to the later-
described proteins in others, are the lecithins, widely distributed
in plants, and possessing a considerable interest as the probable
basis for the formation of the vastly-important and complicated
substance chlorophyll, the composition of which, aside from the
presence of carbon, hydrogen, oxygen and phosphorus, is still
rather uncertain.

The other kind of oils, — the ethereal, essential, or volatile
oils, are very different in composition and meaning. They are
familiar to us chiefly in the fragrant oil of lemon and oil of cloves,
and are the causes also of the odors, sometimes fragrant and
sometimes acrid, of many kinds of leaves (e. g.. Lemon Geranium)
when cut or crushed; and they cause likewise the fragrance of
flowers and fruits. Camphor, and some other aromatic materials
are related substances. They are not food products, as the fats
are, but serve mostly ecological uses, either in connection with
the protection of plants against insects or Fungi, or for the at-
traction of animals in connection with dissemination of seeds
and cross pollination of flowers, as we shall later consider in detail
along with those respective subjects. They are stored as a rule
in special receptacles or glands, often of considerable size (figure
37). Chemically they are most diverse, some of them consisting
only of carbon and hydrogen, approaching near to the formula
CioH^g. Little is known as to their exact mode of formation.

It is a non- volatile oil (called toxicodendrol), which is the
poisonous susbtance in the Poison Ivy; and the fact that it is a
non-evaporating oil explains why it is so very difficult to remove
from the skin, and why it persists in plants which are long dead
and dried.

Plmit Acids. These are agreeably familiar to us as the sub-

ii8

The Living Plant

stances which give the pleasant acid taste to fruits. Thus, mahc
acid gives the tart taste to apples and currants, citric acid to
lemons and oranges, tartaric acid (from which cream of tartar

is made) to grapes. In all of these
cases there is a reason, as our
chapter on Dissemination will
show, why these fruits should be
eaten by animals, to which the
acids certainly serve to render the
fruits more attractive. On the
other hand tannic acid, which oc-
curs in the bark of many plants,
(and from which man extracts it

Fig. 37.-A gland, highly magnified, ^^Y tanning leather), haS an as-
formed by a fusion of several cells tringent taste unpleasant to ani-

containmg a large drop of an ethereal ^ ^

oil, as seen in a cross-section of a leaf mals, against whicll, aCCOrdiugly,

of Dictamnus Fraxinella.

its presence has some tendency to
protect the plant tissues. These acids, which all occur in solu-
tion in the sap, have a comparatively simple composition, the
formula of malic acid, for example, being C4Hg05. Their mode
of formation is not entirely understood.

Plant Waxes. These occur chiefly on the surface of plants,
where they constitute the bloom, commonly of bluish color, wliich
is familiar upon plums and some leaves, On the dry berries of
the Bayberry, a common plant of the coast, a wax accumulates
in such quantity that in early days it was gathered and used for
the making of candles. In general the waxes seem to render
plants immune against wetting, after the manner of the oil on
the back of the proverbial duck, — the disadvantage of the wet-
ting being this, that the water would clog the stomata, and hence
prevent the passage of gases that are needed in photosynthesis.
If, now, the reader should ask me why, when the wax is thus of
advantage, so many plants do not have it, I would answer by
asking in turn why it is, that, if riches are such an advantage (or

The Various Substances Made by Plants 119

at least are commonly thus reckoned), so few men possess them.
The reason I take to be fundamentally the same in both cases ; —
some kinds never get the right start towards constructing them,
or else have not the capacity to manufacture them. Chemically
the waxes are very closely related to the oils, and no doubt are
built up in the same general way.

Resins. Under this name falls a variety of substances of which
typical examples are familiar in the balsam of the Fir and the
Pine; in spruce gum; and in rosin; myrrh and frankincense are
others; and much of the milky juice (or latex) of plants, from which
the rubber of commerce is made, is composed of resins or closely
related substances. Chemically the resins are most diverse, and
their mode of origin is as little understood as is their function in
the plant. They are usually accumulated in special passages,
from which they sometimes flow out at a break (e. g., in Pines),
in a way to suggest that they serve as a temporary salve, — a kind
of first aid to an injury. At times they appear to be utilized as
food, which is likely enough, since there is every reason to sup-
pose that plants, precisely like animals, when driven by hunger,
will resort to the use of materials which they would otherwise re-
ject with disdain.

Glucosides. These substances are more interesting than con-
spicuous, the most familiar being that called amygdalin, which
gives the bitter taste to seeds of almonds and apples; while the
peppery taste, so common in plants of the mustard family, is
also due to a glucoside. Their meaning in the plants is not known,
although they may find some incidental service in protecting
against annuals the parts which possess them. With the gluco-
sides belong also some of the brightest coloring matters produced
by plants, including the red dye madder and the blue dye indigo.
Here also comes erythrophyll (called also anthocyan), that red
color with which we have made pleasant acquaintance already as
giving brilliant hues to ripened fruits, and the glory to the fo-
liage of autumn. Chemically the glucosides owe their name to

I20 The Living Plant

the fact that they are compounds of glucose with some one or
more definite substances, into which they can again be broken
up. Some of them contain nitrogen, as for instance the amyg-
dalin above mentioned (its formula is C20H27O11N), which allies
them in some measure with the nitrogen-containing substances
next to be considered, especially the alkaloids.

Class IV. The Nitrogen-Assimilates, or Amides

These substances, dissolved in the sap of plants and having
no particular uses to us, are not commonly known; but they are
vastly important nevertheless, inasmuch as they constitute the
connecting step between the carbohydrates and the indispensable
proteins, soon to be considered. The commonest is asparagin,
dissolved in the sap of young asparagus plants, from which it can
easily be crystallized out. Its formula, typical of the group,
is C4H8O3N2, which shows the presence of the nitrogen along with
the elements of carbohydrates; and there is no doubt that the
ultimate source of the materials is the photosynthetic grape sugar
together with nitrogen from compounds absorbed with water by
the roots. The amides are not known to perform any special func-
tion of their own in the plant, and probably find their significance
simply as a necessary chemical step in the formation of proteins.

The incorporation of nitrogen with the elements of the car-
bohydrates is a step of the first biological magnitude, since the
nitrogen is the most essential and distinctive additional con-
stituent of the most important of all biological substances, —
living protoplasm. We have already considered, (in Chapter
II), the source of the plant's supply of carbon, oxygen, and hydro-
gen, and must now turn aside from our main theme to examine
the source of the nitrogen supply, a subject all the more important
because of the fundamental economic bearings it has. Nitrogen,
it should be needless to recall to the reader, is the colorless gas
which makes up very nearly four-fifths of the atmosphere; and
from such an abundance plants ought apparently to have no

The Various Substances Made by Plants 121

difficulty in drawing all that they need. As a matter of fact,
however, the typical plants take no nitrogen at all from the air,
even starving to death for want of a little while bathed in this
lavish abundance; and the reason they do not is that they cannot.
The most prominent characteristic of nitrogen is its chemical
inertness, or reluctance to enter into combination with any other
substances, — a circumstance, indeed, to which its abundance
in the atmosphere is due; and its union with oxygen or other
substances can be effected only by the agency of electric sparking
machines, or other methods involving the expenditure of high ten-
sion energy. Now our typical large plants have not in their struc-
ture any equivalent for sparking machines or other arrangements
releasing suitable energy, although, as will presently appear, the
lowly Bacteria seem better provided in this particular. Since they
cannot make use of the free nitrogen of the air, plants have had to
resort to the only other possible source of supply, viz., substances
in the soil containing it already combined, which substances,
moreover, must be soluble in water to admit of their absorption
by the roots. The compounds called nitrates best meet these
conditions, and they, accordingly, are the source of most of the
nitrogen which, with appropriate intermediate chemical steps,
is combined with the elements of the carbohydrates to form
amides.

If nitrates were as plenty in soils as plants could make use of,
then our digression in pursuit of this substance could end right
here. But in fact the nitrates in most soils are so scant that the
majority of plants live all the time in touch with nitrogen scarcity,
and this is one of the chief of the factors which limit the luxuriance
of their growth and expansion. It is, perhaps, worth noting in
passing, that especial scarcity of nitrogen in some situations
is correlated with an insectivorous habit in plants which reside
there, — the advantage of this habit consisting in the abundance
of combined nitrogen obtainable by digestion from the bodies of
insects. A chief reason for the scarcity of nitrates in the soil lies

122 The Living Plant

in that very solubihty which renders them absorbable by plants,
for it leads to their constant drainage away with the superfluous
water; and were it not for a constant renewal of the nitrate sup-
ply plant life would soon be starved to extinction. This renewal,
known as the nitrification of soils, is a matter of such biological
and economic consequence that we must now consider it with
some care.

The natural nitrification of soils takes place in four ways.
First, there is a constant return of combined nitrogen to the soil
from the excretions of animals, and the decay of plant and animal
bodies. Second, a small amount of combined nitrogen is added
to the soil with the rain which falls during thunder showers, for the
lightning acts as a kind of gigantic natural sparking machine which
forces the nitrogen and oxygen of the air into combination;
thus is formed the soluble nitrous acid, which is caught and taken
into the soil by the rain. Third, nitrates are constantly though
slowly added to the soil by the natural decay of the rocks which
contain them. In moist climates they must drain away about as
fast as they are formed, but in dry climates the drainage is slower
than their formation and they accumulate in the soil. This is
a reason for the richness of the finer soils of the deserts, which
blossom as the rose when water is added by aid of irrigation.
Fourth (and far the most important) of the natural methods of
soil nitrification is bacterial activity. Everybody knows that a
soil in order to be rich must contain a proportion of humus,
the material which is dark in color and supplies the open char-
acter. This humus consists chiefly of decaying vegetable matter,
which provides both the home and the nourishment for countless
numbers of tiny organisms, chiefly Molds and Bacteria. These
Bacteria, popularly know^n as Germs, are of several kinds, of
which some, in the course of their own processes, incidentally
work over the less valuable nitrogen compounds of the soil to
more valuable ones, while still others, and these the most impor-
tant, actually force the nitrogen and oxygen of the air to unite

The Various Substances Made by Plants 123

into the simple compounds which later are worked up to nitrates
by the others. It is not yet known how these Bacteria accomplish
this crucial first step of nitrification, but the source of the energy
is plain ; it is supplied by their intense respiratory power, in which
they surpass some hundred-fold the larger plants. This fact of
the nitrification of soils through the activity of Bacteria is one
of the most important in nature.

It may here occur to the practically-minded reader to ask
whether this power of Bacteria to add nitrogen compounds to
soils cannot be utilized artificiall}" for the enrichment of poor soils.
It can be, and to some extent, has been; and living Bacteria
of the suitable sorts have actually been multiplied and distributed
for trial by our own Department of Agriculture, and have been
offered for sale to farmers both in Europe and America, though
the process is not as yet a commercial success. Plowever, in the
utilization of the nitrifying Bacteria man was long anticipated
by at least one great group of Plants, the Pea Famil}^, or Legu-
minosse, the members of which have actually colonized the nitrify-
ing Bacteria upon their own roots, thus making sure that the en-
tire product of the Bacteria shall be available to themselves
without any loss through drainage or use by other plants. Most
people have seen upon the roots of Peas, Beans, and others of
this family, the wart-like or pea-like swellings, whose appearance
is well show^n in the accompanying photograph, (figure 38).
These nodules are residences mside the plants occupied by the
Bacteria. The connection is mutually beneficial, for the Bacteria
receive carbohydrates from the green plants which receive nitrog-
enous compounds from them. It is because of the efficiency of
this arrangement that the seeds of plants in the Pea Family are
richer in nitrogenous food substances than any others; and this
latter fact in its turn explains why Peas and Beans are the best
of all plant substitutes for meat, which is mostly protein. This
relative richness of Leguminosse in nitrogenous compounds ex-
plains also the reason underlying the ancient farming practice

124

The Living Plant

of green-manuring, that is, plowing in Clover and other legumi-
nous crops to enrich the soil. It is from these same nodules, also,
that the Bacteria have been taken and grown for the commercial
enrichment of the soil, as mentioned on the preceding page.
In our consideration of these four natural methods of soil

Fig. 'Sb. — The roots of several Bean plants, photographed about half the natural size,
showing the collections of wart-like nodules which contain the nitrifying Bacteria.

nitrification we must not forget the artificial aid of man, who,
for his own purposes, adds to the soil both chemical fertilizers
and barnyard manures, with their rich supplies of nitrogenous
and other compounds.

Finally it is important to note that the plants, for their part,
have a way of meeting the nitrogen scarcity of soils, — viz., they

The Various Substances Made by Plants 125

waste none. To this end they even go so far as to remove from
their leaves, before these are dropped, such of the nitrogenous
and other compounds as can be used economically again. Unlike
animals, they excrete no nitrogen, or extremely little, in either
solid, liquid, or gaseous form, but conserve it with care and use
it over and over again; so that it is only released in the end by
their decay after death.

Class V. The Principal Poisons, or Alkaloids

These substances are notorious as including the most violent
plant poisons. Thus strychnine (from the Strychnos bean),
nicotine (from the Tobacco leaves), morphine (from the milky
juice of Poppies) are alkaloids, as is the poison, muscarine, of the
deadliest jVIushrooms. Some alkaloids, while not poisonous,
have strong properties in other respects, such as quinine, obtained
from the bark of the Cinchona tree and efficacious in breaking up
fevers; caffeine, the stimulating substance in Tea leaves and Coffee
berries; cocaine from Cocoa seeds, the well knoTvn local anaesthetic
and fatally-alluring drug. Their meaning in the plant is uncer-
tain, and all the more puzzling since they mostly are poisonous
to the very plants which produce them if injected into other
parts of their tissues. Nor is it certain just how they produce
their poisonous effects. Alkaloids occur also in animal tissues
as a product of the processes of fermentation and decay; they are
called ptomaines, and are very deadly, being the real cause of
death in bacterial diseases. Chemically the alkaloids are related
to the amides, from which they are no doubt formed, not at all
as a step in the formation of proteins, but as a side group. A
typical formula is that of caffeine, CsH^qOoN^.

It has recently been discovered that the roots of our common
field crops appear to excrete into the soil minute quantities of
substances poisonous to the plants which produce them; and it is
probable that the presence of such substances, and not the ex-
haustion of the necessary mineral matters, is the real cause of

126 The Living Plant

the sterility of some soils, which are therefore "poisoned" rather
than "exhausted." The composition of these substances is not
known, except that they are complicated and perhaps nitrog-
enous, in which case they may be found to belong with this
group of the alkaloids.

The reader will recall that the active properties of the alka-
loids were somewhat foreshadowed in the nitrogenous glucosides,
and later he will also make acquaintance with remarkably active
properties of another kind which characterize not only the pro-
teins entering into living protoplasm, but also the enzymes, with
their very striking chemical powers. The common feature which
distinguishes all of these substances in contrast with the more
passive groups is the possession of nitrogen, which seems there-
fore to be associated with the most active properties in plant
substances. This fact is sufficiently curious in face of the chemical
inertness of nitrogen, and one can fancy this element as reluctant
to enter into combinations, restless, so to speak, while in them,
and making disturbance in its efforts to escape to its original
freedom.

Class VI. The Flesh-Formers, or Proteins

These are the most important substances made by plants,
entering as they do into the composition of living protoplasm.
They are more familiar in animals than in plants, for flesh is
made up of them; but they are distributed throughout the living
parts of all plants, either in the active protoplasm or stored as
reserve food, especially in seeds. They are vast in number, elab-
orate in composition, and only imperfectly known. Chemically
they are distinguished from all of the preceding groups by con-
taining not only the elements of the latter, but also sulphur,
while some of them possess phosphorus too, so that their com-
position may thus be expressed CnHnOnNnSj,(P„). Some of their
molecules are of very great complexity; thus, there is an albumin
with the formula C720H1134N218O24SS5; and there are other proteins

The Various Substances Made by Plants 127

A cell, liighly
magnified, from the
proteinaceous layer
just under the husk
of Corn, showing nu-
merous protein
grains interspersed
with a few starch
(larger) grains, all
embedded in living
protoplasm.

in which the elements, or atoms, of the molecule, must sum up to

more than 15,000, or even, in some cases, more than 30,000. Many

of these substances differ little from one another in properties,

and moreover are readily convertible one into

another; and the facts seem to indicate that

these elaborate forms are really multiples (or

polymers) of some simple protein molecule,

built up in the same manner as are starch and

cellulose from a simple carbohydrate molecule.

Nor is it to be supposed that all of these sub- fig. 39

stances have each a separate meaning in the

plant, though they may have; but many of

them no doubt are simply manifestations of

chemical individuality in the plant, as the

forms of starch grains are manifestations of

physical individuality.

Several different groups of Proteins are recognized by chemists,

of which I shall here mention, even though in little more than the

Homeric fashion, the more important. They
are, — Albumins, substances like white of egg,
thinly spread through many plants: Globulins,
wiiich form definite grains in some seeds like
Corn (figure 39), and beautiful crystals in Castor
Bean, Potato and some other plants (figure 40) :
Glutelins, typified by the familiar gluten of

Fig. 40.— A cell, highly ^^^j. ^\j[q\j gives the agglutiuosity to dough:

magmned, from the " ^" "^ ^

interior of a Castor Prolaviins especially distinctive of the seeds of

Bean, showing the • tit 7 • • • ^ ^

crystalline protein graius : Nucleo-protems, coutammg phosphorus,
turrJendered Some- ^^^d fomiing the chrouiosome substauce of the
what clearer by nuclcus of cells : PJiospJiovroteins (called also

treatment with re- . .

agents), embedded in albiuuinates) cheesc-like materials found in

living protoplasm. 1 7-> , 7 t^ ,

some seeds: Proteoses and reptones, very un-
portant because they are the soluble and diff usable proteins into
which the insoluble kinds are converted in digestion by the

OcA

128 The Living Plant

action of peptonizing enzymes: and there are others hkewise of
rarer sort and lesser consequence.

The mention of the presence of sulphur and phosphorus in
proteins will lead the reader to inquire for the source of supply
of those elements. The answer is ready. They are derived from
soluble sulphates and phosphates absorbed from the soil by the
roots, and are incorporated, through chemical reactions still
imperfectly known, with the elements contained in the amides.
All soils contain all of the sulphates that plants need, and usually
all of the phosphates, though at tim.es the latter are insufficient,
and must be added as fertilizers to ensure good crops.

Class VII. The Regulators of Metabolism, or Enzymes

It is safe to say that the enzymes (called also ferments) are
the most remarkable and least known, although among the most
important, of all substances produced by plants, — or by animals,
either. They are characterized by this remarkable power, — viz.,
they can cause chemical changes, each of one definite kind, in
other substances, without themselves entering into the reaction
or suffering any appreciable alteration. Because of this mode
of action very small quantities of enzymes can alter chemically
great quantities of material. Thus the enzyme diastase, which
occurs both in the saliva of man and also in the starch-storing
organs of plants, can convert (chemically, hydrolyze) great
quantities of the insoluble starch by two or three steps into grape
sugar, a soluble diffusable material; likewise the enzyme protease
(pepsin) occurring in both plants and animals, hydrolyzes in-
soluble indiffusable proteins into soluble diffusable peptones;
also the enzyme lipase converts insoluble fats into soluble fatty
acids and glycerine: cytase converts cellulose of Ivory palm and
Date into soluble sugars; and there are many others of lesser
prominence. It is these changes which constitute digestion,
whether in plants or in animals. By aid of the enzymes the plant

The Various Substances Made by Plants 129

can not only produce and control chemical changes within its
own body, but, by pouring them out in suitable places, can dissolve
extraneous materials and later absorb these again for its own use.
It is thus that insectivorous plants can digest the insects they
capture; parasites can penetrate into the tissues of a host; and
pollen tubes can digest their way down the solid tissues of the
style, absorbing the digested materials for use in their own
growth. But there are many other phases of enzyme action also ;
thus the unfermentable cane sugar is hydrolyzed (or inverted)
to fermentable grape sugar by invertase, and grape sugar is fer-
mented to alcohol and carbon dioxide by zymase, produced by the
Yeast plant. And there are other cases innumerable which we
cannot take space to consider.

Chemically and physically we know very little about the en-
zymes, because it has not yet been found possible to extract them
from the protoplasm in a pure state; and even their very existence
would not be recognized at all w^ere it not for their effects. It
is not even certain that they are related to the Proteins, although
there is indirect evidence pointing that way; nor are we sure that
they are liquids thinly saturating the protoplasm, though this
seems probable. Still less is it known how they produce their
remarkable effects, although a homologous power exists in those
inorganic substances called catalyzers. Each kind can produce
only one chemical change, and that as a rule but a slight one,
but the cooperation of several can cause a series of changes large
in the end; and it may be true that they cause most, if not indeed
all, of the chemical processes which the living protoplasm carries
on. They are the tools, so to speak, with which the protoplasm
effects the chemical results it requires. Indeed to some investi-
gators it has seemed likely that the enzymes are the principal
material bases of heredity, and that the chromosomes of the
nuclei, known to be conveyors of heredity, consist chiefly of col-
lections of enzymes. Truly the importance of the enzymes is
great, and their further study in the near future is likely to throw

130 The Living Plant

much light upon some of the most fundamental problems of Bi-
ology.

Class VIII. Living Protoplasm

This substance is of such importance and complexity as to re-
quire for its treatment, a separate chapter, which follows. It
need only be said in this connection that so far as chemical
analysis has been able to penetrate into the mysteries of living
protoplasm, it appears to be merely a very complicated mixture
of proteins with many simpler substances. Here for example
is a list of the substances which have been recognized in a chemical
analysis of the protoplasm of one of the lower plants; —

Water, Pepsin and Myosin, Vitellin, Plastin, Guanin, Xanthin,
Sarkin, Amnionic carbonate, Asparagin and other amides, Pepton
and Peptonoid, Lecithin, Glycogen, Aethalium sugar, Calcic com-
pounds of higher fatty acids. Calcic formate. Calcic acetate, Calcic
carbonate, Sodic chloride, Hydropotassic phosphate, Iron phosphate,
Ammonio-magnesic phosphate, Tricalcic phosphate. Calcic oxalate,
Cholesterin, Fatty acids extracted by ether, Resinous matter, Glycerin,
coloring matter, etc.. Undetermined matters.

In this list, which I give in order to illustrate the chemical
complexity of protoplasm, all of the constituents are well-known
substances, no one of which has any of the properties of life,
unless such a substance lies hidden in the trifling amount of "Un-
determined matters" ; nor has any chemist yet been able to identify
any distinctive living substance, — any of that protoplasm par
excellence which we are logically bound to believe must exist.
But the further consideration of this subject belongs with the
next chapter.

Such are the groups of substances which plants build upon the
foundation laid by the photosynthate. We may summarize their
relationship in a diagrammatic manner, after the analogy of a tree
of ascent, as shown herewith.

The Various Substances Made by Plants

131

Living Protoplasm

'Enzymes

Proteins

Amides

'Alkaloids

It may perhaps have occurred to the
reader ere this to inquire what proportion
of the original basal photosynthate is used in
the construction of each of these classes of
substances. The question is a fair one but
difficult to answer, partly because the pro-
portions would be so different with the vari-
ous kinds of plants, and partly because we
Oils-Resins have so few data for making calculations.
However, it is possible to make a generaliza-
tion for plants as a whole, and this has been
(Photosynthate) done in the table below, which, although lit-

tle more than a guess, has yet some value. For simplicity I have
reduced the table to the kinds of known and visible substances,
grouping together the others as "special substances"; and inci-
dentally I have added the ultimate fate of the various groups.

Carbohydrates

^

6^

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^

^

^

lO

lO

lO

o

lO

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r- (

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+

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+

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+

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C3 :/}

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\X2

The Various Substances Made by Plants

^33

Fig. 41. — A cell, highly magnified, from
a Begonia, showing a mass of crys-
tals composed of calcic oxalate, lying
within the cell-cavity around which
can be seen the living protoplasm.
(Copied from a wall-chart by L.
Kny).

This table brings out clearly once more that most fundamental
of facts about the physical constitution of living things, that their
substance is all derived originally from carbon dioxide and water,
with a few minor additions, and is
all returned in the end back to the
same source, undergoing en route
transformations of substance and
energy which constitute the princi-
pal visible phenomena of life. The
organism is made up of a little
of those substances temporarily
withdrawn from the general cir-
culation of nature and interacting
vigorously with one another under
the stimulus of external forces, —
principally the sun. Organisms
are, as it were, little whirlpools in
the general circulation of matter and energy. And I cannot for-
bear to attempt to illuminate this matter somewhat further by
aid of one of my favorite diagrams, which is presented herewith
(figure 42).

There is yet one other group of substances made by plants,
very different, however, in kind from those already described.
In the tissues of all plants the microscope reveals mineral matters,
sometimes in great abundance and crystallized in very beautiful
forms, of which our illustration (figure 41) gives some, though an
inadequate idea. A few are probably useless minerals absorbed by
the roots along with the useful kinds presently to be noted, but the
great majority are by-products of useful chemical reactions. Thus,
the commonest of the crystals is oxalate of lime, which is formed
from oxalic acid, probably a by-product in the manufacture of
proteins. These crystalline matters are obviously of no use, but
are waste materials. In the absence of a regular excretory system
such as animals possess, the plant has no resource except to store

Fig. 42. — A diagram illustrative of the relation of plant and animal life to the circulation

of the principal substances of nature.

134

The Various Substances Made by Plants 135

them up in out of the way places, though they may ultimately
be partially removed by the fall of the leaves and the bark.

There remains one other important phase of our subject. It
concerns the indispensability of certain elements for healthy
metabohsm, although they do not enter into the composition
of any of the substance manufactured. Everybody knows that

Lacks, — all potas- cal- nitro- phos- mag- iron nothing
sium cium gen phorus nesium

Fig. 43. — Illustration of the method and results of water culture. The plants are Corn,
all started at the same time. (Copied from a wall-chart by Errera and Laurent.)

potash (potassium) is thus indispensable, to such a degree that
it must often be added as a fertilizer to soils; but its symbol (K)
is not found in any of the formulae cited in this chapter. The same
is true of the elements calcium, magnesium and iron, and probably
sodium and chlorine, all of which are indispensable to the healthy
growth of most or all plants, but none of which enter into the
composition of the most miportant plant substances. Naturally a

136

The Living Plant

great many attempts have been made to determine the exact
function of each substance, and why it is essential. The reader
will be interested in the principal method used to this end. It
depends on the fact that there are plants, and many, which will

grow through their whole
cycle from seed to seed in
water, without any contact
with soil, if only the needful
minerals be contained in the
water. This method is called
water culture, and the prac-
tical arrangements therefor
are well shown in the accom-
panying figure (figure 43),
while a product of the
method, produced in my
own laboratory, is shown by
figure 44. Now, by growing
one plant in water contain-
ing all of the necessary min-
erals except one, side by side
with another plant grown in
water containing all of the

Fig. 44. — Corn plants growing by water culture needtul mmerals, it IS pOSSl-
in a common tumbler. The screen is ruled ^Jg ^q obsCrVe what cffcct
in centimeters.

the absence of this one sub-
stance produces, and hence to infer what its use to the plant
must be. The general results of an experiment of this kind
are well shown in figure 43. In this way we have found that
potassium is necessary to the formation of the photosynthate,
calcium to its transfer through the plant, and iron to the
formation of chlorophyll (into the composition of which, how-
ever, it possibly enters); but further than this, and as to the
other materials, our knowledge is most vague and unsatisfac-

The Various Substances Made by Plants 137

tory. It seems quite plain, however, that the role of these
elements lies in services incidentally necessary to the greater
processes, — such as aiding in chemical steps, neutralizing poison-
ous excretions, and so forth. They are like the servants at a
party; they are indispensable to its success, but their names do
not appear in the list of those present. But our ignorance on
these matters, and upon so many other phases of our subject of
metabolism, is only acting as a spur to the efforts of many de-
voted workers, who, in laboratories all over the world, are attack-
ing these problems wdth the full determination to solve them.
The methods of science are slow, but they are irresistible; and
the solution of the problems is only a matter of time.

CHAPTER VI

THE SUBSTANCE WHICH IS ALIVE IN PLANTS, AND ITS
MANY REMARKABLE QUALITIES

Protoplasm

ILREADY more than once in this book the reader has
met with a mention of protoplasm, — the living sub-
stance of plants. Besides, almost everyone has some
knowledge about it, or thinks that he has, though much
of the current information is a very long way from the truth.
There are even some persons who believe that protoplasm is an ab-
stract conception evolved by the mind of man to help explain phe-
nomena otherwise incomprehensible; while a few seem to cherish
the idea that it is one of the many inventions sought out by science
for undermining the faith. Yet protoplasm is not any of these
notions, but a real material which can be seen, handled, and sub-
jected to experiment. The reader will wish to know the facts
about this most important of substances, and here is the suitable
place to consider them.

It is nowadays an educational axiom that a good understanding
of any scientific subject is possible only through personal contact
and experience with the matter in question. A great many people
do not comprehend this necessity, and beUeve that well-written
and fully-illustrated books are a sufficient, if not actually a supe-
rior, substitute for the laborious and time-consuming methods
of the field or the laboratory. When the reader meets with this
error he can refute it effectually by asking the objector whether
he considers that guide-books, even the best written and most

138

The Substance Which Is Alive in Plants

139

profusely illustrated, are a satisfactory substitute for foreign
travel. The case is still stronger with scientific facts and phenom-
ena, for these are mostly of a sort even more foreign to the stu-
dent's previous experience than are the sights and impressions
of distant lands. All this is quite true of the subject before us,
and if the reader would really understand the substance Proto-
plasm he must take steps to see it for himself, even if he has to
trouble some friend, his physician, or the nearest botanical ex-
pert, for the use of a microscope.

Fig. 45. — Typical cells, in optical section highly magnified, of hairs from Spiderwort,
Gloxinia, and Squash, respectively, showing as accurately as the author can represent
it by pencil, the appearance of their gray-granular threads and lining of living pro-
toplasm.

If, now, the reader will carefully remove some of the younger
of the hairs which are so prominent in the flowers of the common
Spiderwort of the gardens, (or the closely-related Wandering Jew
of greenhouses) , or some of the hairs on the young leaves or stems
of Squash, or Gloxinia, (or even of "Geranium"), will place them
on a glass slide in a drop of water, cover them with a thin glass,
and then examine them with the microscope, he will see before him
living protoplasm, the most remarkable of all natural substances.
These particular objects display an appearance represented in the
accompanying pictures, (figure 45) ; and they have an advantage

I40 The Living Plant

over others which might be chosen in this, that while compara-
tively easy to obtain, their protoplasm exhibits a streaming
motion, which, though often slow and difficult at first to detect,
nevertheless when seen forms a valuable proof of its living condi-
tion. The rather inconspicuous grayish-granular, translucent,
semi-fluid appearance here presented inside of the cells is repre-
sentative of the aspect of protoplasm in general. The granular
look is due largely to the presence of food granules, which in
some cases are absent, leaving the protoplasm so nearly trans-
parent that it can hardly be seen at all unless stained by some dye,
while in other cases the granules are so plenty as to give the proto-
plasm an appearance of solidity. Moreover, as these granules
consist largely of protein which has a slight yellowish color, they
give to protoplasm in dense masses a distinctly yellowish or
brownish-yellow tinge; and this is the cause of the yellow color
which shows so plainly through the tips of white roots, and of
the brownish-yellow of the interior of young ovules. In the hairs
supposed to be lying before the reader, the protoplasm is obviously
soft enough to flow freely, though it is not wholly a fluid; and it
is known to possess about the consistency of a soft jelly. Indeed,
if one were to imagine an uncolored jelly, somewhat too soft to
retain the form of its mold and all clouded instead of quite clear,
— in other words just the kind of jelly that the thrifty house-
keeper doth most despise, he would have a very good idea of the
protoplasm of these hairs. In some plant tissues the substance
is still softer and almost a liquid; in others it is firmer, to such a
degree that in seeds it becomes tough and hard as horn, though
never approaching the hardness of ivory, as a prominent diction-
ary says that it does. The visible streaming of the protoplasm
in these hairs, however, is not typical, for while in some kinds
the streaming is even more active, generally it is very much slower,
and commonly is miperceptible; so that the reader must not
allow the motion to become too prominent a feature of his vis-
ualization of plant protoplasm. A white-granular, slow-moving

The Substance Which Is Alive in Plants 141

jelly; — that is what protoplasm looks like, and that is precisely
what it is.*

^Vhile protoplasm for the most part can be observed in plants
only by aid of the microscope, there are cases in wliich it occurs
in masses sufficiently large to be studied by the unaided eye, and
to be taken in the hand. Everybody has seen those soft, whitish,
slimy masses which are flattened against decaying wood in damp
dark places, such as the rotten underpinning of old buildings,
in cellars and dark greenhouses, or on old shaded tan-bark,—
whence they are known as "Flowers of Tan." These are called,
scientificall}", 81ime-molds, and they are practically pure naked
protoplasm, the accessibility of which has made these low plants
very favorite objects for protoplasmic studies.

Such is the appearance of living plant protoplasm as seen
by the eye or through an ordinary microscope; and try as one
will, he can see little more. The supreme miportance of proto-
plasm among earthly substances has of course acted as a stimulus
to the most thorough researches into its structure; and all the
highest powers of the microscope, and all the most refined de-
vices and methods known to microscopical science, have been
brought to bear upon it. Yet these efforts have yielded little
additional knowledge, and even that little has been left involved
in uncertainty and controversy. We do not even know what tex-
ture the protoplasmic substance possesses. Some investigators
have concluded that such protoplasm as the reader has seen
streaming in plant-hairs is a loose network of fine elastic fibers,

* The streaming of Protoplasm is thus vividly visualized, though with some ex-
aggeration natural at that time, by Huxley, — "Currents similar to those of the hairs
of the nettle have been observed in a great multitude of very different plants, and
weighty authorities have suggested that they probably occur, in more or less per-
fection, in all young vegetable cells. If such be the case, the wonderful noonday
silence of a tropical forest is, after all, due only to the dulness of our hearing; and
could our ears catch the murmur of these tiny Maelstroms, as they whirl in the in-
numerable myriads of living cells which constitute each tree, we should be stunned,
as with the roar of a great city." The Physical Basis of Life in his Collected Essays,
New York, I, 136.

142 The Living Plant

holding liquids in its meshes as a sponge might do, — a view more
prevalent formerly than now, even though it is sustained by the
appearance of the substance when killed and colored by dyes.
Others consider that protoplasm, aside from certain solid gran-
ules, is chiefly an emulsion of various liquids, which rest suspended
as tiny globes in a matrix of fluid ground substance, very much as
the tiny globules of oils remain suspended in water after violent
shaking of a mixture. And the advocates of this view, now in the
ascendant, have supported it by constructing, out of ordinary
chemicals, certain emulsions or foams, which show striking sim-
ilarities to living protoplasm not only in appearance, but in move-
ments, though they are, however, far enough removed from
protoplasm in all other respects. And a third view tries to har-
monize the two others by supposing that some protoplasm has
one structure and some the other. In one part only does proto-
plasm display a definite structure, and that is in the nucleus dur-
ing reproduction, a matter we shall presently consider.

It may seem to the reader remarkable that I do not attempt to
illustrate so important a subject more fully by pictures. But
protoplasm in fact, because of the lack of clear definition in its
structure, is most difficult to represent well in any kind of pic-
ture. Indeed, hardly any two persons represent it alike, as
follows naturally enough from the fact that hardly any two per-
sons see it alike. In various figures in this book, however, I have
tried incidentally to give some, even though rather a conventional,
idea of its appearance, and to these figures (figures 33, 34, 39, 40,
41, 45) the reader will now find it worth while to refer. And I shall
at this point, add one more, and one of the best, in which the great
botanist Sachs has tried to represent it as if projected against
a black background, (figure 46).

We come now to the important matter of the chemical com-
position of protoplasm, from which, in view of its many remark-
able powers, we naturally anticipate something of very unusual
interest. The most striking of the chemical facts about it, as the

The Substance Which Is AKve in Plants

143

chapter on JNIetabolism further illustrates, is this, — that proto-
plasm, despite its aspect of simplicity, is not a single substance,
but a very heterogeneous mixture of many different substances
of diverse grades of complexity, from the simplest of mineral
salts up to the most complicated of pro-
teins. None of these substances, how-
ever, are of themselves alive, nor has
chemical analysis yet succeeded in lo-
cating any distinctively living constit-
uent, — any protoplasm jmr excellence,
although we are logically bound to be-
lieve that some such substance must
exist as a seat for the distinctive prop-
erties of life. Protoplasm, therefore, is
probably composed chemically of two
classes of materials; — first, a very small
amount of a distinctively living constit-
uent, not yet identified, but consisting,
in the fibers, or else the ground substance
of its physical texture; and second, a very
large amount of various non-living sub-
stances, nutritive and other, which are
under the control of the living constit-
uent.

There are, however, some further
chemical facts about protoplasm which f.^, 46.-The protoplasm of a
go a little way towards explaining its
various powers. Thus, a part of its con-
stituents (in general the most compli-
cated) are very unstable, or, chemically stated, labile, and
change their composition under slight provocation whether from
without or within. Such changes are accompanied, like all
others of a chemical nature, by transformations of energy, either
release or absorption. And these in turn cause other changes,

hair cell of a Gourd, projected
against a black background.
(Reduced from Sachs' Lec-
tures.)

144 The Living Plant

and these yet others, in an almost endless succession. Thus liv-
ing protoplasm, complex and unstable in its constituents, and
acted upon constantly by diverse forces both from without and
within, is a constantly seething mass of energy-and-material
changes; — and it is such changes which constitute the visible
phenomena of life. But, — and here is the crux of the matter, — •
these changes are not hap-hazard and aimless, but on the contrary
proceed in a definite and orderly sequence, resulting in the forma-
tion of definite structures and the performance of definite actions
time after time and generation after generation; and it is this
orderliness, this definite procession of physical and chemical
processes, rather than anything in the processes themselves,
which is the most distinctive characteristic of life. The failure
of the regulatory power breaks the circuit of the processes, and
leaves the protoplasm a helpless mass of matter all ready for de-
cay; and this failure we name death. Life thus consists of two
elements, first, material and energy changes, that is, purely physi-
cal and chemical processes, whose general nature we can under-
stand, and which are seated in the various substances that chem-
ists have identified in the protoplasm, and second, a regulatory
power which directs and makes use of those processes but whose
nature and location is still quite unknown. Perhaps the nature
of this regulatory power is incomprehensible, or unknowable, in
our present philosophies, though as to that, science never admits
that anything is unknowable, but works ever under the assump-
tion that everything can be known if we but refine sufficiently our
methods of investigation.

There is one other feature of the chemistry of protoplasm
which may have some importance in explaining its powers. In a
general way it seems true that the protoplasm of the higher and
more elaborate plants and animals is more complicated chemi-
cally, or at all events produces a greater number of complicated
substances (proteins especially), than the lower. This suggests
that each of the special physiological features successively ac-

The Substance Which Is AUve in Plants 145

quired by plants and animals in the course of their evolution
has its seat in a special chemical constituent of the protoplasm.
On this view, evolution, physiologically considered, depends upon
chemical experunentation, so to speak, in the protoplasm, and
follows step by step on the successful formation of new chemical
compounds. But let the reader beware of accepting this sug-
gestion as knowledge; it is merely a speculation, but one of those
which, in science, it is legitimate to throw out ahead as a tem-
porary guide to further investigation.

In common with all other substances in Nature, protoplasm
thus possesses its physical and chemical properties. But in ad-
dition it possesses another set not found in other substance;
and thereupon depend its powers to do the remarkable things
that it does. These may be termed its physiological or vital
properties, which are as follows; — the property of metabolism, or
power of causing orderly chemical changes within itself, including
photosynthesis and respiration, and the other changes recorded
in our chapter devoted to that particular subject: the property
of conduction, or power to transport substances in definite paths
through itself, including absorption, transfer, and excretion: the
property of growth, or power to incorporate new material and to
increase in size at special places : the property of division, or power
to separate portions of its own substance, the basis of reproduc-
tion: the property of mobility, or power to cause definite move-
ments of its own substance, the basis of protoplasmic streaming
and locomotion: the property of irritability (sensitivity), or power
to respond advantageously to various stimuli. This enumeration
of the physiological properties of protoplasm reads like the table
of contents of a book on physiology, — and it ought to, because
physiology is nothing else than a study of the properties of proto-
plasm. And here is a point of importance. Just as the physical
properties of any substance are believed to reside in certain ulti-
mate structural units, which are the smallest portions into which
that substance can be divided and still retain those properties,

146 The Living Plant

and which units in this case are the molecules, and just as the
chemical properties are supposed to reside in their ultimate units,
in this case the atoms, — so the vital properties must be supposed
to reside in some kind of units distinctively their own. These
units, obviously, must be larger than the molecules and made up
of organized aggregates thereof. They hsive been called by \'arious
names, notably plasomen, (in the singular, plasom), and are
probably identical "v\dth the micellae of which we shall have much
to say in the chapter on Absorption. All substances are made up
of atoms and molecules; protoplasm alone is made up of atoms,
molecules and plasomen. And the reader will observe, by the
way, that the very conception of the plasom involves the idea of a
distinctive protoplasmic main substance, and constitutes indeed,
an additional reason for believing in the existence thereof.

As one views the various physical features of protoplasm, and
thinks of the remarkable things it can do, he cannot but wonder
at the discrepancy between its aspect and its accomplishments.
For protoplasm is one of the most insignificant in appearance
of all substances, yet secures the most wonderful of all results.
For has it not built the whole plant and animal world, culminating
in man with his powers of thought? Yet this discrepancy be-
tween promise and performance is not without parallel in our
human experience. If some stranger from far away space, where
all things are differently done, were to visit this earth and be
shown the multifarious works of man's hands, and were after-
wards to have man pointed out as their maker, he would doubtless
exclaim in astonishment; — "How can a creature so small build
these cloud-cleaving towers a hundred times loftier than himself,
or these huge leviathans of steamships ten thousand times bigger
than he: or how can a thing so weak raise pyramids so ponderously
colossal: or one so slow of foot drive such fleet-flying engines:
or one with hands so soft bore tunnels through miles of solid
rock?" Man gives no suggestion in his appearance of the nature
of the power whereby he does these things, for that lies not in

The Substance Which Is Alive in Plants 147

his visible body but his invisible mind, which enables him to
plan and make use of tools, and harness the restless forces of
nature. So, we can only suppose that the physically-insignificant
protoplasm accomplishes its results by some analogous power.
Indeed, I venture for my part to believe that all protoplasm can
think, — not mind-thought it is true, for that appears to belong
only to man, but body-thought of which the mind is unconscious.
Or the matter may better be stated in this way, that man's thought
is but the conscious form of a principle which exists unconsciously
through all living substance. All protoplasm thinks, but only
the portion thereof in man's brain is aware that it thinks. How-
ever this may be, there is one thing that is plain; — man's is not
the only protoplasm which makes use of tools, and compels the
forces of nature to do its work, in evidence whereof let the reader
observe, for example, what is said in this book about enzymes,
and the dissemination of seeds.

We must here turn back for a moment to the chemistry of
protoplasm in order to notice a matter important to an under-
standing of the relations of the substance to the external world.
The chemical complexity and instability of protoplasm render
it extremely sensitive to the effects of external influences, which
act upon it in three different ways. First, if strong enough, they
act upon it forcibly, precisely as upon any other substance of
comparable sort, and quite without reference to whether it is
living or not. Thus, heat burns it; pressure crushes it; and some
chemicals dissolve it. Second, the forces when too weak to exert
any forcible effects, can yet act inductively to promote, or to
check, some of the processes in progress in the complicated chemi-
cal laboratory which the living protoplasm actually is, and thereby
may produce a profound effect upon the behavior of the plant
as a whole. Thus heat, in a degree far too low to injure the
protoplasm, promotes the activity of those physical and chemical
reactions which underlie the streaming, nutrition, growth and
other activities of protoplasm; and this explains why protoplasm

148 The Living Plant

streams faster, and plants grow better, in warmth than in cold.
Light acts analogously on the cell-contents, and one of the results
is the brilliant redness of autumn coloration. In some cases the
external factors, especially some chemical substances, act repress-
ively on the processes, which explains the action of anaesthetics.
Third, the factors, when far too weak to exert even an inductive
effect can act in a far more remarkable and consequential manner,
for they can then serve as guides, or stimuli, in response to which
the protoplasm can send its parts into positions found by past
experience to be best for the performance of its functions or avoid-
ance of dangers. Thus, light far too weak to be directly useful
or injurious to the plant yet serves as a guide whereby stems can
grow towards it, leaves across it, and roots away from it, those
positions being the most advantageous for the performance of
their particular functions. And innumerable other cases of this
kind are known, of such interest and importance, however, that
they must receive a chapter all to themselves under their proper
physiological name of Irritability. It is enough for our purpose
at present to make clear the existence of the three-kind relation
between protoplasmic activity and the external world.

One does not go far with his studies upon protoplasm before
he begins to take thought of its origin. In one way the prob-
lem is simple enough, for all of the protoplasm famihar to us
originates obviously in only one way, — by growth and division
from other protoplasm through reproduction. It is not so long
since even scientific men held the contrary belief, still widely
persistent among uneducated folk, that low forms of Hfe could
originate anew in slime or other fermentable masses; but later
experimental studies, chiefly led by the great Frenchman Pasteur,
have shown that in all such cases living germs are present, while
if precautions are taken to kill all germs by heat or suitable
poisons, then no life appears. Every known case of apparent
spontaneous generation having thus been investigated and dis-
proved, we infer that probably it does not now occur in our

The Substance Which Is Alive in Plants 149

world, and that all protoplasm nowadays originates from pre-
existent protoplasm through reproduction. This much is easy.
But when we try to trace back the continuously-reproducing
chain to its very first origin in time, we come soon to the limits
of our knowledge. Some philosophers have suggested that the
germs of life w^ere first brought to the earth in meteorites from
other planets; but this merely sets back the difficulty one stage
and does not remove it. Another explanation, which seems to be
that most commonly assumed by scientific men, places its origin
in spontaneous generation at some time in the earth's history
when the favorable combination of material and energy happened
to occur. Obviously, such a combination ought to be repeatable
experimentally; and it is upon this assumption that many learned
men, from astrologers of old to physiologists now with us, have
sought, though in vain, to make protoplasm anew in the flasks
of their laboratories. There is, however, a third explanation which
I have already suggested in an earlier chapter, — namely, that the
protoplasm known to us did not originate in its present form,
but is evolved or descended from a simpler substance adapted
chemically to the higher (or lower) temperatures which formerly
prevailed on the earth, while that substance in turn was evolved
from a still simpler, and so on backwards to a beginning cotempo-
raneous with that of inorganic matter itself. This view I hold
to be the most reasonable and probable.

But, after all, the most impressive and important thing about
protoplasm is its power to build those great and elaborate struc-
tures which we call plants and animals. For, structurally con-
sidered, a plant or an animal is nothmg other than a mass of
soft protoplasm which climbs aloft and reaches outward into the
form of the plant or the animal, building itself meantime a skele-
ton for the support of its helplessly-weak substance. Now, in
building these organisms, the protoplasm never exhibits the char-
acter of a continuous and homogeneous mass, but always sepa-
rates partially into tiny structural units called cells, which are

150 The Living Plant

mostly too small to be seen by the naked eye, but which appear
prominently in every magnified view of any part of any animal
or plant, as witness, for example, figm'es 2, 53, 73, 141, in this book.
We must therefore consider with some care the construction of
these cells, — a subject of the foremost importance in Biology.
The hairs earlier studied are fairly typical cells except that

they are partially isolated from their
^^.......y--^,-.^^^---:^.^ neighbors instead of deeply em-

f^ \ ' 4 bedded among them, and are elon-

\ ' ' ' ^^^^ , gated rather than rounded. If one

i • r ■ '" cytoplasm '^

observes an example of these hairs,

e. g., that of the Squash (figure 47),

i ' t." .;. .: nucleolus he is Hkcly to notice first the clear-

>■ ' _ ♦---piastid cut containing wall, inside of which

— j.„_ sap-cavity comcs a Complete lining of soft gray-

__jj/ granular protoplasm, very likely in

^ slow streaming motion, with threads

An optical section, highly ^f ^j^g g^j^g extending across the cell

magnified, through a cell of the "

Squash, showing all the parts of a at VarioUS aUglcS. This Soft proto-

plasm is called cytoplasm. Some-
where within it, though not carried in its streaming, lies a denser,
rounded granular structure, also living protoplasm, the nucleus,
which often exhibits a round mass within itself, — the nucleolus.
In the cytoplasm lie also certain scattered granules (not especially
distinct, however, in these hairs), which are larger than food gran-
ules and otherwise unlike them; these too, are living protoplasm,
and are called plastids. Finally, within the cytoplasm appear
large open spaces, various in size and number but commonly
merged to a single very large one in old cells; though apparently
empty, they really are filled with a watery sap and therefore are
known as sap-cavities. These parts, wall, cytoplasm, nucleus,
plastids, sap-cavities, are the prominent parts of typical plant
cells, and the great majority of cells possess them all. We can
accordingly construct a conventionahzed cell showing these

The Substance Which Is AHve in Plants

151

parts in their natural relations and fully-developed condition;
and such a cell is represented herewith (figure 48) .

We should now examine a bit further these parts of the cell
and their meaning.

The wall is composed of a firm-elastic transparent substance
called cellulose, whose chemistry is treated in the chapter on
Metabolism. It is built by the cytoplasm, which, in suitable
places, is supposed to lay down within itself tiny masses (bricks,
as it were) called micellae, of cellulose, and continues to add to

,j;m^^m^^mm^ — wan

car '^^

^^%r plastid

mV- sap-cavity

nucleus
nucleolus
/'it?/ cytoplasm

Fig. 48. — An optical section through a conventionalized complete plant cell.

their number until they accumulate to nearly a solid mass. I
say "nearlj^," because apparently there always are left between
these micellae thin sheets of protoplasm, like the mortar between
bricks, so long as the cells are alive, though they are withdrawn
when the cell has reached full maturity. It is these thin sheets
of cytoplasm, too thin to be visible even to the strongest micro-
scope, which keep the wall alive, as it were, so that it can become
enlarged, spht, chemically changed, absorbed in places, and in
other ways altered, a good while after its formation. But except
for such subsequent alterations, the walls of contiguous cells re-

152 The Living Plant

main parts of one continuous mass. As to the function of the wall,
that is perfectly obvious, — it is the skeleton of the cell, the me-
chanical support for the gelatinous cytoplasm, which has not
enough firmness of texture to raise itself unaided an inch from the
ground. It is interesting to let the imagination picture what
would happen to the loftiest and stateliest tree, if, by some subtle
chemical magic the cell- walls could be suddenly re-converted back
to the gases from which they were made; the protoplasm would
simply collapse to the ground as a shower of slime.

The reader at this point will observe how different in principle
is the construction of the skeleton in plants as compared with
animals. In animals, in conformity with the much higher de-
gree of division of labor in their parts, certain cells are set aside
to build the skeleton for the entire individual, either a deeply-
buried bony skeleton as in man, or a surface skeleton of lime
or horn as in crabs and insects; while all of the remainder of their
cells are without hard walls and devoted to other functions.
In plants, however, every individual cell has a wall around it-
self, and the collective mass of these walls makes up the skeleton
of the plant. Such a mass of cell-walls, however, by no means
represents, though one might naturally think so, a lot of origi-
nally separate walls fused together. Observation of growing parts
always shows (figure 101) that the new walls formed between
dividing cells are thrown across the protoplasm as single solid
structures, which may or may not in time become split and
divided between the two cells. Thus the cell- wall system of a
plant is one single mass from the beginning, just as is the wall
mass of a building; and the protoplasm lives in cavities therein,
precisely as people live in the rooms of a house they have built.
The reason for the difference in the method of skeleton building
by animals and plants is plain enough upon reflection. The
method of animals permits jointing and muscular movement,
as it must in order to allow the most fundamental of all animal
activities, — locomotion in search of food; the method of plants

The Substance Which Is Alive in Plants 153

permits only a fixed position, which, however, is sufficient, since
the materials for making their food are brought to them in the
general circulation of nature. And these conclusions are all the
more confirmed by the seeming exceptions, for some plants swim
or creep freely about (e. g., swimming spores of Algae and Slime
Molds) in a very animal-hke manner; but in these cases they lack
the firm cellulose wall distinctive of plants. But although the
skeletons of animals and plants differ, their protoplasm does not,
for in all essentials the protoplasm of plants and animals is alike.
This brief account of the plant skeleton has touched incidentally
on a matter which must now receive some further attention. As
the student soon learns when he studies many cells with his
microscope, they differ immensely in shape and in the thickness
and composition of their walls, to such a degree indeed as to
make them apparently too complex for analysis. Yet here, as
elsewhere, further study gradually crystallizes out the essentials,
when it appears that after all only a few ground forms exist, and
then only in correlation with definite functions or influences;
while all of the others are simply variations and combinations of
these. As to the shapes of cells, the simplest of all, and the one
to which all others tend to revert, is the sphere, that being the
mathematical form in which the most contents can be comprised
within the least wall. This shape, with the wall a spherical shell,
is actually realized in those cells which float freely in water or
air, as do the spores of many Algse and Molds, and some pollen
grains ; and this shape may become elongated to ellipsoid and ovoid
forms under particular conditions (figure 49, 94, 108). \ATiere such
cells occur inside the tissues of plants, however, and hence are
hard pressed by their numerous neighbors, the spherical shape
becomes necessarily modified to many-sided (polyhedral) or
faceted; and this shape is approximately realized in many stor-
age tissues of plants, where it comes measurably near to that
twelve-faced shape which always results when equal-sized spheres
are forced together by pressure (figure 49, 72) . There is also some

154

The Living Plant

Fig. 49. — ^Generalized drawings of optical sections through the principal forms of plant
cells, all of which are derivable by differential growth from the spherical form in the
center.

approach to this shape in the green cells of leaves, (figure 2), al-
though here a modification is introduced by the need for con-
centrating the chlorophyll grains towards the best-lighted sur-
face, for which a cylindrical shape is the best (Plate I, B), or else

The Substance Which Is AHve in Plants 155

by the need for the presence of very large air spaces, for which a
branching, or stellate form is most suitable. The polyhedral
shape due to mutual pressures, in conjunction with the formation
of new walls as plates thrown across cells from one wall to the
other, results in the formation of cubical cells in growing points
(figures 49, 53, 139 CD), or elongations thereof to four-sided
prisms, as in the cambium cells, which form the growth zone
between the bark and the wood in most trees (figure 139 B). In
other cases the cells become flattened to tabular shapes, as in
epidermis and cork (figures 2, 49) ; where the function of those
cells as the protective skin of the plant obviously requires such a
shape. Again, the spherical or polyhedral shape becomes elon-
gated to a cylindrical or prismatic form where the function re-
quires much length, as it does in the conduction of liquids through
the plant; and it is a line of such cylindrical cells, thrown into
a tube by absorption of the intermediate walls, which constitutes
the water-carrying ducts, (figures 49, 53, 54 C, 72) while the food-
carrying sieve-tubes are made in analogous manner (figure 72),
Or, the elongation takes place at two opposite points, result-
ing in a spindle or fiber form, which is developed wherever tensile
strength for resistance to strains is required (figures 49, 50 d). Fi-
nally, through the intermediation of a more active growth at several
points, the spherical or polyhedral shape becomes modified to a
branching, or even a star-shaped form; and this occurs in the
spongy cells of green leaves as a means of providing generous
inter-cellular air spaces, (figure 2, and B of Plate I): in some
Rushes as a part of their very flexible pith (figure 49) ; and in
certain excretion cells of Water-plants as a means of providing
more wall for the deposition of waste crystals. Thus these few
ground forms, — the fundamental sphere, with its lines of modifi-
cation, shown by figure 49, — viz., ellipsoid-ovoid, polyhedral,
tabular, cylindrical-tubular, spindle-fibroid, and branched-stellate,
represent the mathematical possibilities upon which the cells can
play, but by which they are also bound in their adaptations to

156

The Living Plant

their various functions; and although innumerable forms occur
not directly referable to any of these types, they are never-
theless only modifications and combinations thereof.

The cell-wall, however, is modifiable not only in shape, but also
in thickness. Ordinarily very thin, it can become thickened to
any degree required by function, even to the almost total ob-
literation of the cell cavity, as happens in some fibers (figure 50, d),
where the need for additional strength is perfectly plain: in cells

F^

Mr

e

T

Fig. 50. — Various methods of adaptive thickening of cell-walls; further particulars in text.
(All copied from von Mohl's classical work on the Plant Cell, 18.51.)

devoted to the protection of something, notably in the shell of a
nut (figure 50, a) : and in cases where the formation of a thickened
wall is a means of storing additional food, as in the Ivory Palm and
Date (figure 36). A similar thickening is used also as a pro-
tection to the resting spores of Molds, Yeasts and disease germs,
which thereby are so completely protected against all hostile
outside influences that they can float uninjured for months in
the air, and germinate finally in the most unexpected and least
desired of places. In some cases the thickening is not at all uni-
form, but takes the form of rings and spirals, as in young ducts
which they help to keep open while the walls are still very flex-
ible (figure 50, b) ; or it makes an elaborate fretwork of strength-
ening ridges surrounding thin areas easily pervious to water,
as in older ducts (figure 50, c) ; or it occurs upon one wall only, as

The Substance Which Is Alive in Plants 157

is frequently the case with protective epidermal cells (figure 50, e) ;
or it affects only the angles, in some cells which combine water-
storage with strengthening (figure 50, /); and it takes various
other forms too many to mention.

Furthermore, the composition of the wall is alterable both
physically and chemically. Cellulose is a very elastic substance,
and where greater stiffness than it can afford must be had, the
wall becomes penetrated by the far stiff er substance Hgnin; and
lignified walls are wood. Both cellulose and lignin, however,
allow ready passage of water, and where that would be a danger,
as at surfaces of plants which grow in dry air, the wall is made
waterproof by the formation all through its texture of a water-
repelling substance, called cutin or suberin; and such is the case
with the epidermis and cork which form the skin of plants. In
other cases the wall softens to mucilage on the access of water,
as in Flax seeds, though the reason thereof is not perfectly clear;
and there are yet other such modifications of more special char-
acter and meaning.

It is thus plain that cell-walls are well-nigh indefinitely plastic
in shape, thickening, and composition, while, moreover, any and
all of these features can be combined in various ways and de-
grees in accordance with the particular needs or functions con-
cerned. Furthermore, the cells are rarely isolated; but commonly
cooperate in large masses of similar function called tissues.
Masses of tissues cooperating in function, and mutually adjusted
to perform their work to the common advantage, form organs,
and organs make up the plant.

There remains one other matter of importance about the wall.
Although, at first sight, it seems to shut off completely the proto-
plasm of each cell from that of its neighbors, minute observation
exhibits the presence of definite thin places perforated by very
fine pores which permit the passage of tiny threads of living proto-
plasm from one cell to another (figure 51). This continuity of
protoplasm from cell to cell has been found in every part of the

158 The Living Plant

plant, where it has been sought; and it seems clear that every

living cell is thus in communication with its neighbors, and

therefore with every other living cell of the plant. Thus the

protoplasm though partially, is not wholly, separated into cellu-

, . f:.-^ lar masses, and is, after all, for any

^£^ ^/^^^;; ;• iv- ., / individual plant a single great con-

^. -r^^^'^^i^fe^^ ■;• tinuous sheet. There is every reason

.'VM ! i'^-^J^i ; 'i^^ *J>ar?Sri' fi^ to believe that impulses of different

v";:'>"^A '■'■'•y^ •-■■■■' ^Sii'-'-.'y^^i^^'H

''-{^:1>':::^4.--V'^:''-^^c^':M'\%:\ kinds can be transmitted from cell

to cell through these threads, which,

. )■■ ''.;<'■s^ ■•.:.;,.'..■ V'-':;-'

:.U-;V.:

KHV

';l'/^-» '-lii'' ;vy -/^ •^ therefore, take the place in part of

':vM\5l;vF''^;'5vfj^-iV^^ the nerve system of animals. This

....:;nU]"?^^:;^/f^:J;^;^^.t^ helps us to understand how it is that

' ■ir^' i, '^^^it^i^cYv'?:;^ the plant can act as a physiological

.:C'^ ,^^^^}>-^^^<rS{r^^-- unit: how the difTerent parts of a

1 ■ '''•"= '■' ' plant can be kept in harmonious

Fig. 51.— An ordinary cell specially COOperatioU I and hoW Stimuli applied
treated to show the thin threads

of protoplasm extending through at oue part of a plant, cau produce

the wall to connect its protoplasm j,. xvx j. • ^ ii ^•

with that of its neighbors. (Copied their eflects at a considerable dis-

from Strasburger's Lchrbuch). fance

Thus much for the wall of the cell, to which, it may seem to the
reader, I have devoted a disproportionate space and attention.
Yet while vastly less important than the protoplasm, the solidity,
prominence, and relative permanence of the walls makes them far
more accessible to study, — to such a degree indeed that our con-
ceptions of cellular structure center much more largely around
the walls than the protoplasm.* This, however, is less unfortu-

* The inconspicuousness of the living protoplasm of plants in comparison with
the prominence of the walls it builds finds striking exemplification in the history of
their discovery; for the mass of the walls was well described, and their cavities were
named cells, by Robert Hooke as early as 1667, while they were elaborately described
and beautifully pictured only a few years later, 1672-1682, in the fine books of Grew
and Malpighi. But the protoplasm was not recognized at all as a constituent of
cells until over a century and a half later, and was only first adequately described
and named by von Mohl, in 1844.

Here is Hooke's sentence, of 1667, in which cells were first named. He is describ-

The Substance Which Is AUve in Plants 159

nate than it might seem, because the constitution of the walls
is so closely interlocked with the functions of the cells that from
the one we can infer much as to the other.

Passing now to the cytoplasm we can briefly dismiss it, for,
being the typical protoplasm, it has already been fully described
in the earlier part of this chapter. It is the working body of the
cell, concerned with its nutrition, construction, etc., and the
streaming movements are probably concerned with the trans-
portation of substances through the cell, a view sustained by the
fact that the streaming is most active in general in the cells which
are largest. The cytoplasm does not differ particularly in appear-
ance in different cells, excepting that it is more fluid in some and
more solid in others. One point of present interest about it, how-
ever, is this, that just at this time of writing, certain newly found
tiny bodies within it, called mitochondria, or chondriosomes
are attracting much attention, and may prove to be very im-
portant.

We come next to the nucleus of the cell. It consists of living

ing the appearance presented by a thin section of cork placed under his microscope.
"I could exceeding plainly perceive it to be all perforated and porous, much like a
Honey-comb, but that the pores of it were not regular; yet it was not unUke a Honey-
comb in these particulars.

First, in that it had a very little solid substance, in comparison of the empty cavity
that was contain'd between, ... for the Inlersiitia, or walls (as I may so call them)
or partitions of those pores were neer as thin in proportion to their pores, as those
thin films of Wax in a Honey-comb (which enclose and constitute the sexangular cells)
are to theirs.

Next, in that these pores, or cells, were not very deep, but consisted of a great many
little Boxes, separated out of one continued long pore, by certain Diaphragms, . . .

I no sooner discern'd these (which were indeed the first microscopical pores I ever
saw, and perhaps, that were ever seen, for I had not met with any Writer or Person,
that had made any mention of them before this)." . . . (Robert Hooke, Micro-
rraphia, 166.5, 113.)

Here is von Mohl's sentence, of 1844, in which protoplasm was first named:— "So
mag es wohl gerechtfertigt sein, wenn ich zur Bezeichnung dieser Substanz eine
auf diese physiologische Function sich beziehende Benennung in dem Worte Proto-
plasma vorschlage." {Botanische Zeitung, 1844, page 273); or, in translation, "Ac-
cordingly it may be justifiable if for designating this substance I propose an appellation
having reference to this physiological function, namely, the word Protoplasm."

i6o The Living Plant

protoplasm, denser than the cytoplasm, and different, somewhat,
chemically. It varies comparatively little in appearance in differ-
ent cells, and ordinarily exliibits no particular structure; but when
the cells are dividing or reproducing, then a definite number of
rod-shaped structures become differentiated and perform re-
markable manoeuvres which we shall later consider in the suit-
able place along with reproduction (figure 101) . These rods, called
chromosomes, are the seat of the controlling power of heredity,
and thus guide the constructive work of the cytoplasm in growth.
The nucleus, therefore, bears to the cytoplasm a relation sug-
gestive of that of the brain to the body. Indeed, the resemblance
may extend pretty far, since there are those who maintain that
heredity in the chromosomes is substantially the same thing as
memory in the brain. But I hope the reader will not therefore
call the nucleus the brain of the cell, for it isn't. As to the nu-
cleolus, that is irregular in its appearance, and probably repre-
sents a reserve of chemical substance for use in the growth of the
chromosomes.

The plastids, likewise, are living protoplasm, and are present in
all cells of the typical plants, though sometimes they are incon-
spicuous. Thus, it is the plastids which hold the green color
in leaf-cells, where they are already well known to the reader as
chlorophyll grains, called also chloroplastids. In other cells,
of some fruits, such as the familiar Jerusalem Cherry, they con-
tain yellow or orange colors (chromoplastids), thus aiding to
make the fruits conspicuous. And in other cells yet, especially
in the storage parts of plants, they remain colorless and are called
leucoplastids, but perform the remarkable and indispensable
function of converting sugar to starch. It is, indeed, a fact of
the greatest interest about these leucoplastids, that they and the
homologous chloroplastids comprise the only places in nature,
either within the plant or outside of it, where starch is known to be
made. Starch is one of the substances which the chemist has not
yet been able to make in his laboratory.

The Substance Which Is Alive in Plants i6i

The sap-eavities are as simple in structure as they look. In
very young cells they are absent, as a later picture illustrates
(figure 129) ; but in those that are older little rifts appear in the
cytoplasm and gradually grow larger until finally, in the fully
mature cell, they become merged into a single cavity of very large
size, as the figure of the conventionalized cell clearly shows
(figure 48). This cavity is filled with water in which sugar and
other useful substances are dissolved. It thus represents a store-
house of useful materials, but serves secondary functions, like-
wise, in pressing the cytoplasm against the wall, and in aiding
growth, by methods which will later be described in the suitable
chapters.

It is of interest to note that not only does new protoplasm in
general originate only from preexisting protoplasm, but new
cells originate only from cells, nuclei from nuclei, and plastids
from plastids; while the same thing has been claimed even
for cell-wall and sap-cavities, or rather for the part of the
cytoplasm which forms them, though here the evidence is not
conclusive.

The question is now appropriate, why does protoplasm sep-
arate into cells at all, and what makes them of such minute size
as they are? It is sometimes assumed that the plant-structure
becomes cut up into cells in order to provide structural units of
convenient size and form, after the manner of the bricks of the
builder; but the analogy is wholly misleading, since the skeleton
of plants is not built at all from originally separate units, as brick
buildings are, but rather from a continuous mass of cell-wall
substance comparable with the cement construction now coming
into use. Another explanation maintains that each nucleus
can control only a limited quantity of cytoplasm; and thus are
established certain administrative units between which, natu-
rally enough, the walls are built, — the resultant being cells. As
to the reasons why their sizes are so small as to require a mi-
croscope to show them at all, we have again a few guesses, but no

1 62 The Living Plant

exact knowledge. Possibly the chromosomes need a certain
size in order to perform their functions; this would establish the
size of the nucleus, and hence (on the explanation above noted)
of the cell. Another explanation rests on a mathematical basis.
We may assume that the typical cell is a sphere filled solidly
with protoplasm. When a sphere enlarges in size, its bulk in-
creases much faster than its surface, the bulk increasing as the
cube of its diameter and the surface as the square thereof. Ob-
viously it is through this surface that the spherical cell must
absorb the oxygen for the respiration of the entire bulk of its
protoplasm; it is therefore quite evident that there must be a
certain size of the cell in which the surface is just sufficient to
aerate the bulk of protoplasm within, — and that size would de-
termine the average cell size. If the cell were to grow larger its
surface would not suffice to aerate the bulk, while if smaller the
surface would be needlessly great. In a general way this con-
clusion is sustained by the fact that where conditions for respira-
tion are harder the cells are smaller, and vice versa. Moreover,
the very largest cells occur in places well situated for aeration,
and besides, possess accessory arrangements, — viz., the flattening
cf the protoplasm in a thin layer against the wall (figure 45), and
protoplasmic streaming, — which aid to that end. These features
prevail in the hair cells already observed by the reader, and in
consequence those cells become large enough to be visible to the
eye without the aid of a lens. In general, therefore, it does seem
true that the relation of bulk to surface in a solid as affecting res-
piration is one of the principal factors, if not indeed the principal
one, in making the size of cells what it is.

In comparing the functions of the cells of plants with those in
animals, it soon becomes obvious that plant cells exhibit a far
lower degree of division of labor; and this involves a remarkable
consequence. It seems to be a fact that when protoplasm con-
tinues to perform a single function for long periods of time, as it
does in the highly-specialized organs of the anim.al body, it grows

The Substance Which Is Alive in Plants 163

stronger and stronger and works better and better up to a certain
culminating level, beyond which it tends to decline, and finally
to cease work altogether. It is probable that the decline and
cessation of work is dependent upon purely physical causes, some-
what as a bar of metal when too often bent, becomes weakened
and broken at last; but in this peculiarity of protoplasm we find
an explanation for the cycle of youth, maturity, old age and death.
When, however, the protoplasm can periodically alter its location,
habits, or functions, — can re-melt itself, so to speak, — it renews
its youth thereby, and can continue its vigor without limit, thus
becoming potentially immortal. In this fact is found the ex-
planation of the benefit ^vrought by a change of scene or occupa-
tion, or a vacation, upon ourselves, though the effect is here
limited; and if a way could be found to affect our protoplasm
more profoundly, — to make it mix itself up periodically, even
within the limits of the same cell, — then, it seems likely, man
would have discovered the long-sought elixir of life and the
secret of perpetual youth. This in fact is the case in full degree
in simple plants like Bacteria. Each of these is made of one cell,
and when it reaches full size divides into two, each of which
grows up and divides again, and so on without limit, in perennial
change, vigor, and youth. A similar rejuvenation takes place in
sexual reproduction, when the protoplasm of two individuals
mingles together in fertilization. Now, the higher plants possess
no organs at all in which the protoplasm continues to work within
the same cells throughout the life of the individual, but, as our
chapter on growth will abundantly illustrate, the protoplasm is
continually moving outward and onward into newly forming
buds, leaves, roots, and stems; and this removal permits it to re-
new its youth perennially. Therefore plants should never grow
old from internal causes, in the way that animals do, — and in
fact they do not, the exception presented by annuals being only
apparent and not real. Even the greatest trees continue to form
new leaves and roots with unabated vigor until they are brought

164 The Living Plant

to their death by external causes, chiefly connected with the
large size they attain.

The manifestations of life, wherever we know them, are as-
sociated closely with constant changes of matter and energy,
especially with respiration. But there is a case in which all of
these processes seem suspended, for our most delicate methods of
research fail to demonstrate them, and that is in resting struc-
tures such as seeds. Resurrection plants, and some low animals.
Not only can dry seeds retain their vitality for a great many years,
but in that condition they can withstand without injury a tem-
perature above boiling point, or even two hundred degrees below
freezing point. The question is important whether the usual
changes are proceeding in these seeds, but too slowly to be meas-
ured, or whether all processes stop and the vitality is really sus-
pended. The truth is not as yet known, but it is to be noted that
there is no logical difficulty in supposing that all of the processes
may slow down to a stop without any derangement of machinery,
precisely as an engine is stopped for the night simply by with-
holding the steam, leaving it all ready to start once more in the
morning.

When this chapter was finished down to this point, it was
handed like all of the others to a critic for judgment. And this
is in substance the comment with which it came back. '^The
chapter is clear enough in its statements, and appears to cover
the subject, but somehow it leaves you with a very unsatisfied
feeling." This opinion I take for a very high compliment, since
it shows that my chapter reflects precisely the scientific situation
of the subject.

CHAPTER VII

THE WAYS IN WHICH PLANTS DRAW INTO THEMSELVES
THE VARIOUS MATERIALS THEY NEED

Absorption; Roots

N the preceding chapters we have traced pretty fully
the principal processes occurring within the bodies of
plants. But as yet we have taken no thought of the
ways in which plants absorb the various materials they
need from outside; and this is the inquiry which now lies before
us. I give the reader fair warning that the subject will lead us
perforce into distant, unfamiliar, and recondite matters; but their
study will have the advantage of illuminating a good many things
besides absorption by plants.

Of all the substances absorbed by plants, the foremost is water,
which not only makes up a great part of plant substance, but
is also indispensable for various physical and chemical uses.
This water is absorbed, as everybody knows, through the roots;
but the fact is less familiar that the absorption takes place ex-
clusively through the young white terminal parts. Upon these,
accordingly, we now center our attention. They can be studied
most easily, and without obscuration by adherent soil particles,
in young roots obtained by the germination of seeds in flower-pot
saucers kept shaded and wet. From such specimens it appears
that young roots as a whole are remarkably alike, and possess
several features in common, — viz., a slender white shaft with a
yellowish tip, and a diaphanous garment of delicate radiating
hairs, — features which are shown very well, except for the color,
in the seedling of Mustard pictured herewith (figure 52). If

165

1 66 The Living Plant

one centers observation more exactly on these hairs, he will see
that nearest the tip they are plainly just forming, while farther
back they are progressively longer, until a maximum is reached,
behind which they are obviously withering and dying. Evi-
dently the single hairs have each their little day
and pass, while the zone as a whole moves forward
in perpetual youth, pari passu with the advancing
tip of the root. These hairs are of first importance
to our immediate subject, for they are the active
water-absorbing parts of the roots.

Thus much can be seen with the eye and a

lens, but hardly anj'thing more. If, however, one

cuts a thin section through the apex and along the

central axis of a root, and magnifies this section

, with the microscope, he will have before him an

Fig. 52.— a seed- , ^ '

ling of mustard arrangement like that of our picture (figure 53),
saturated air; with whicli it will be dcsirable to compare the
natural size. generaUzed section of figure 139 C. At the tip is
the root-cap, a cluster of cells which, continually renewed from
behind, acts as a protection to the delicate tip in its passage
through the rough and abrading soil; just behind lies a prominent
focal center, the growing point, whose closely-packed cells are so
densely filled with protoplasm that the characteristic yellowish
color of that substance shows through to the outside; while radi-
ating back from the growing point run long lines of cells which
gradually merge into the differentiated tissues of the older root.
These latter tissues, so far as they concern our immediate subject
of absorption, are shown generalized in figure 54, D. Of the lines
of cells, a few in the center constitute the pith, outside of which lie
the long lines of water-tubes, or ducts, readily identified by their
distinctive spiral markings. These ducts contain water, but,
contrary to what one would expect, are otherwise empty tubes,
possessing no living protoplasm after once they are formed; and
they run in continuous strands from the tips of the roots all

Fig. 53. — Typical parts of a section drawn lengthwise, cell for cell, through a young
root of Corn. The entire section is not presented because its length would be
much too great for the page. The tip portion is magnified more than the others.

167

D. Longitudinal section through a portion of root at «.

K-yf

Fig. 54. — Generalized drawings illustrating the absorbing and conducting systems of the

plant.

1 68

How Plants Draw in Various Materials

M

through the stems to the leaves, as sho^vn very clearly in the con-
ventionalized plant of figure 54, A. Outside of the ducts lie some
rows of rounded-elongated cortical
cells, each of which retains its lining
of protoplasm and is shown by tests
to contain a solution of sugar. Finally,
outside of all lies the single thin line
of epidermal cells, which display a
very striking feature, viz., a great
many are prolonged into slender cy-
lindrical closed tubes, which are ob-
viously identical with the root hairs al-
ready observed in the young living
roots; and each hair is lined by living
protoplasm and, as shown by suitable
tests, contains a solution of sugar.
Now the structures important from
the view of water-absorption are the
ducts, the cortical cells, and the root
hairs; and these parts constitute the
water-absorbing machine. And this
machine, if reduced to a single cell of
each kind, would be constructed some-
what as suggested by figure 55.

We turn now to the forces con-
cerned in absorption. Most people,
if questioned, would doubtless express
the belief that roots suck up water in
much the same manner that a wick
sucks up oil, that is, by the power,
called in physics capillarity, — the same
which takes liquids up fine tubes. In-
deed this was once the belief of botanists themselves, as witnessed
by their former use of the term "spongiole, " that is "little

Fig. 55. — A diagram of the con-
struction of the water-absorbing
machine, as it would appear if
reduced to a single root hair,
cortical cell, and duct. Pro-
toplasm is shaded; circles are
water; crosses are sugar; the ar-
rows show direction of move-
ment. Magnification as in Fig. 6.

I TO

The Living Plant

sponge," for the tip of the root, which was supposed to soak up
water and pass it on to the ducts. Later it was found that the
water enters chiefly through the hairs. But in these the condi-
tions for capillarity are absent; for capillarity requires openings,
and the hair walls contain none that even the most powerful mi-
croscopes can detect. The problem is, therefore, to explain an
absorption of water through membranes that are imperforate, or
solid, and through protoplasm-lined and sugar-holding cells into
protoplasm-less and sugar-less ducts. But the very mention of
absorption into sugar solutions through imperforate membranes
immediately suggests a direction for our further inquiry, since it
recalls a m.ode of absorption very well known in physics, and as-
sociated with those very conditions, viz., Os7nosis.

So important is this subject of osmosis to an understanding not
only of absorption of water by plants, but of many other notable
phenomena as well, that the reader ought really to make its more
intimate personal acquaintance. This he can do by aid of the
following experiment, which is one familiar to all workers with
plant physiology. Over the end of a large glass tube is tied
firmly, by means of waxed thread, a piece of soaked parchment,
(preferably a cylindrical parchment cup made for the purpose)
which is a physical equivalent of the wall of the root hair; into the
tube is poured a solution of sugar, for wliich molasses, a solution
ready made and conveniently colored, is excellent ; then the tube
is supported with the parchment in pure water, the whole arrange-
ment being much as displayed in the accompanying , picture
(figure 56). A surprising result always follows, for, without the
operation of any visible machinery or forces, and in a manner
which to me, despite long familiarity therewith, looks always
anomalous and even somewhat uncanny, the liquid rises steadily
though slowly in the tube, lifting its own considerable weight,
until within two or three days it has reached a height of two or
three feet or more. Indeed the process can be demonstrated
even more strikingly than this, for, if the parchment cup be

How Plants Draw in Various Materials

171

made very large and the glass tube very small, as is readily ar-
ranged for purposes of demonstration, the liquid will mount stead-
ily up before the very eyes to a height of several feet. Obviously
there is only one possible explanation of the rise of the liquid
against gravitation, — viz., water must pass through the parch-

FiG. 56. — An osmoscope, using a parchment membrane; further particulars in text.

ment, and that not simply in a manner that is passive, but with
a force sufficient to overcome a considerable resistance. The same
result invariably follows, with a difference, howe\'er, in the rate
of the ascent, no matter what solution is put inside of the tube,
and follows, moreover, in case there is also a solution outside, if
only the inner solution is the stronger. This is a typical example
of osmosis under its sim.plest conditions, but it is representative of

172

The Living Plant

all conditions, inside of plants and animals, as well as outside of
them. It thus constitutes one of the great natural verities which
may be stated as follows; — when water and a solution, or two solu-
tions of different strengths, are separated by a suitable membrane,

there is always a forcible osmotic
movement of liquid through the mem-
brane from the weaker to the stronger
solution. This is one of those ele-
mental cosmical facts which the
reader should fix in his mind as one
of the pillars of his natural knowl-
edge.

If, at this point, it seems to the
reader that however interesting such
experiments with parchment and
tubes may be, they can have little
to do with the processes inside of a
living plant, let him take a leafy
potted Begonia, Fuchsia, or Mar-
guerite, cut everything away close
down to the roots, and connect the
stump with a plain glass tube like
that which was used in the foregoing
experiment. Then, I believe, he will
change his opinion, for water always
rises in the tube, though slowly, to
a height of two or three feet (figure
57). There are of course plenty of
differences in detail, but sugar-holding cup and live roots agree in
the central and crucial feature that they absorb water into a sugar
solution through imperforate membranes and force it up tubes
against gravitation. There is no question that the primary
forces are the same in both cases, and that the absorption of
water by roots is osmotic.

Fig. 57. — An arrangement in which
the parchment cup of figure 56 is
replaced by living roots.

How Plants Draw in Various Materials 173

We return for a moment to our osmoscope, for such is the name
of our osmosis-exhibiting instrument. As the liquid ascends in
the tube, a brown color appears in the water outside, showing
that some of the molasses comes out, though of course in much
smaller amount than the water which enters, else the liquid could
not rise in the tube. This suggests at once the inquiry, — does the
sugar in the sap of the root-hair cells also come out into the soil?
It does not, as ample evidence attests. And if we seek in parch-
ment cup and root hair for a structural difference to explain this
difference in osmotic action, we can easily find it; for the hairs
possess a complete lining film of living protoplasm to which
there is no equivalent in the parchment cup. It is easily shown by
experiment .that this protoplasm really does stop the passage of
sugar while permitting that of water; and this fact explains not
only why no sugar passes out of the hairs into the soil, but also
the equally striking phenomenon, that none passes out of the
cortical cells into the ducts, for in general it is only pure water
which ascends through the ducts to the leaves. Protoplasm,
however, is not the only membrane of this type (which, because
permeable to water but not to dissolved substance is called semi-
permeable, in distinction from the ordinary kind which are per-
meable to both) , for they can be constructed artificially from chem-
icals, and even laid down in a uniform film all over the interior
face of the parchment cup. In this case our osmoscope becomes a
very close physical duplicate of the living root hair, and likewise
permits the steady absorption of water without the escape of any
of the sugar w^hatsoever. IVIy students have often constructed
such arrangements, with results that were wholly satisfactory.

If, now, the reader will compare point by point, an osmoscope
containing a semipermeable membrane, and the absorbing mech-
anism of the living root (which is diagrammatically represented
in figure 55), he will agree that they match very closely in physical
construction and operation except for one very notable difference,
— namely, while the liquid which rises in the osmoscope is a mix-

174 The Living Plant

ture of molasses and water, that which rises in the ducts is prac-
tically pure water. This difference, obviously, is correlated with
a difference of sti-ucture, viz., in the plant the water has to pass
through intermediate cells, which are wanting in the osmo-
scope. We have already learned why it is that the sugar does
not pass with the water into the ducts (the protoplasm stops
it), and our problem resolves itself into this, — how is it that
the cortical cells send water into the ducts when all of the con-
ditions seem rather to invite the absorption of water from them,
exactly as the hairs absorb it from the soil? This question,
I am sorry to say, I cannot yet answer, for it remains one of the
unsolved problems of plant physiology, though one of the most
inviting of them all. It is true, some physiological books at-
tempt to explain it, but in all cases, so far as I have observed,
either their physics is bad, or else their explanations are worded
in a manner more lethal than logical. I suspect the explana-
tion will ultimately be found in some ordinary physical or chem-
ical processes working under control of some still unknown prop-
erty of the protoplasm.

In the three or four paragraphs which follow I purpose to
explain how it is that substances in solution can pass through
imperforate membranes, and what are the forces which drive
them. The subject involves a consideration of molecules, and
things of that sort, and will require hard work from the con-
structive imagination. So the reader is given fair warning and
may skip if he pleases, though I beg to remind him that this
book makes appeal to his reason, and is an attempt to help him
to share the spartan pleasures of understanding.

The most striking feature of osmotic absorption consists in
the remarkable rise of a large body of liquid against gravitation
without the operation of any visible forces whatsoever. Yet
forces there must be, if not visible, then invisible; and accord-
ingly we turn for the sources of the power deep within the con-
stitution of the bodies themselves. As everybody knows, mem-

How Plants Draw in Various Materials i

/ :>

branes, water, and dissolved substances are all of them composed,
according to the teaching of physics, of ultimate excessiv^ely
small units, called molecules. In the sohd or liquid state, the
molecules are held together by a force of mutual attraction,
called cohesion, analogous to the force which holds an armature
to a magnet. But when heat in sufficient amount is supplied,
there comes a point at which the cohesion of the molecules is
suddenly overcome and replaced by an opposite tendency to
spread or diffuse just as far apart as they can; and this is what
constitutes a gas. The power that actuates the diffusion is heat,
which, catching the tiny molecules in the swirl of the violently-
vibratory ethereal waves of which it consists, imparts to them
its own vigorous motion, whereby they are set swiftly darting
and dancing hither and yon, bounding and rebounding energet-
ically against one another, with a result that they work steadily
outward, very much as a cargo of corks would be spread from a
foundered vessel on the waves of a tempestuous sea. Familiar
examples of this diffusion of gases are many,^for instance, the
spread and ultimate disappearance of odors, and the penetration
of cigar smoke though the house; but all gases diffuse in this
manner. And here comes a curious and consequential fact about
diffusion, namely, that it occurs not only in gases, but also in
anything, whether solid, liquid or gaseous, when dissolved in a
liquid. Examples thereof are abundant, — the gradual spread of
a bit of sohd dye when dropped into water: the spread of sugar
through coffee or tea without stirring if only time be allowed:
the spread of fertihzers evenly through soil though added in large
lumps on the surface. By diffusion, also, the molasses reaches
the water outside of the tube of our osmoscope. Such diffusion
occurs, as it seems, because an adhesive attraction existing be-
tween the molecules of the substance and those of the dissolving
liquid separates the molecules of the substance from one another,
and thus brings them into a condition such that heat can exert
upon them the same action as it does upon the separated mole-

176 The Living Plant

cules of a gas (figure 58). And if the reader objects at this
point that diffusion in a solution takes place at a temperature
too low to permit this explanation, I remind him that days far
too cold for our comfort are yet hot from the physical point of

view, for there is heat in the air
at all temperatures above the ab-
solute zero, which lies no less than
four hundred and fifty-nine de-
grees below zero of our ordinary
thermometer. And the phenomena
of diffusion are precisely the same

Fig. 5S.— a diagram designed to illus- insidc of plauts and auimals aS
trate the diffusion of a substance in r i ttt

solution. The circles are water, and OUtSlde of them. We are UOW
the crosses are the dissolving and dif- j , • ]*£C„„:^^

fusing substance,-e. g., sugar. The prepared to Summarize diffusion
molecules of water are supposed to ^g another Verity of uature, thus,

have a stronger attraction for the "^

molecules of sugar than these have for — when suhstauces are anywJiere

one another. Magnified as in Fig. 6. . 777

brought into a state, whether by
conversion to a gas or by solution in a liquid, such that their mole-
cules are separated from one another, then those molecules, set into
energetic action, and thereby given a mutually-repulsive motion, by
heat derived from the surroundings, spread, or diffuse, forcibly out-
ward from places of greater to those of lesser concentration.

Thus much for diffusion; we turn next to the other condition
involved in osmosis, — the nature of the membrane. ^\Tiat can
be the constitution of a body which, possessing no discoverable
openings, will permit water and other substances to pass through
with a freedom well-nigh as uncanny as if a fourth dimension
were concerned? The membrane, of course, is composed of mole-
cules, but there is also good reason to believe that, in walls at least,
the membrane is composed of larger units, called micellae, which
are aggregates of molecules (or perhaps simply huge compound
molecules) that may be represented diagranmiatically as cubical
(figure 59). Now these micellae, although structurally separate,
are held closely together by virtue of a certain cohesive affinity

How Plants Draw in Various Materials

177

for one another, somewhat as a magnet and its armature are
held together by magnetism; and this explains why the mem-
brane, although composed wholly of separate units, holds to-

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Fig. 59. — A diagram illustrating the construction of membranes. The circles are water;
the smaller squares are molecules, and the larger are micellae, of wall substance, a, rep-
resents a dry membrane (which always contains some water) and b, a saturated mem-
brane, supposed to be seen in section, reduced to only a few micellae.

gether as a solid. At the same time the micellae possess a still
stronger affinity for something quite different, namely water,
which accordingly they can draw in as thin films among and

178 The Living Plant

around themselves, thus forcing themselves apart against the
resistance of their own cohesion. This explains how it is that
membranes, and all bodies of similar constitution, like wood, can
forcibly absorb water throughout all of their structure, and
swell up in the process, the requisite energy being supplied by
the adhesive attraction between water and wood. This inter-
micellar absorption of water is called imbibition, and is represented
in the accompanying diagram (figure 59). But why, by the way,
are the micellae not driven entirely apart by the water, thus
making the membrane completely soluble therein? The reason
is believed to be this, — that while the adhesive attraction of
micellae for water, and the cohesive attraction of the micellae for
one another, are, like the attraction of a magnet for its armature,
strongest when the parts are the closest and weaker with increas-
ing distance apart, the adhesion is supposed to weaken with
distance more rapidly than the cohesion; hence, although the
adhesion between micellae and water is at first stronger than the
cohesion of the micellae (thus drawing in some films of the water)
there soon comes a point at which the rapidly-lessening adhesion
between water and micellae exactly balances the slowly-lessening
cohesion between the micellae, and this point of equilibrium is
that where the membrane is saturated with water and swollen
its greatest, as supposed to be represented in figure 59, 6. In this
condition the intermicellar spaces will possess a certain definite
size, differing, of course, with the nature of the membrane; and
in these different sizes we find the simplest explanation of the
different behavior of the types of membranes, for the semi-
permeable w^ould be one with intermicellar spaces too small to
allow the sugar or other large molecules to pass, while giving free
transit to the much smaller water molecules, while the permeable
has large enough spaces to permit both kinds to pass. The case
in reality, however, is not quite so simple as this, for plenty of
facts show that adhesion or solution relations between dissolved
substance and the membrane play also a part. Moreover, the

How Plants Draw in Various Materials

179

condition of balance in a saturated membrane explains how it
is that water can pass so readilj^ through it ; for the last films
absorbed, those farthest from the micellae, are held so verj'-
lightly that only a slight force is required to draw them from the
membrane. \^^hat the nature of the
force may be which withdraws the
water from the inner face of the mem-
brane in osmosis we shall consider in
a moment.

Diffusion, imbibition, osmosis it-
self are typical examples of molec-
ular forces, those operating between
individual molecules, in contrast with
the more familiar molar forces which
act upon masses. There is also one
other molecular force of some im-
portance in the plant, — viz., capil-
larity, which we must now briefly
notice. Capillarity is the well-
known force by which water is raised
in small tubes, — or any small pas-
sages no matter how irregular, — and
the higher the finer the tubes, as our
diagram illustrates (figure 60). It
is the power by which a towel dries
water from the skin, a blotter takes up ink, a wick raises
oil, or any porous substance soaks up liquids. It is only with
difficult}^, and under suasion from my critic, that I forbear to
explain this interesting process in detail to the reader; and I
must regretfully confine my exposition to the following brief
synopsis. The capillary rise of water is due to forces residing
within the water itself. Because the attractions mutually exerted
between the molecules inside of the liquid are not balanced at
the surface by equivalent attractions towards the outside (fig-

FiG. 60. — A diagram to illustrate the
rise and depression of liquids in
capillary tubes, drawn to approxi-
mately true scale. The liquid on
the left is mercury, and on the right
is water.

i8o

The Living Plant

ure 61, b), the surface layers of molecules are drawn strongly
inward so that collectively they press on the liciuid as if they were
tightly stretched rubber, — a phenomenon known as surface
tension. Now surfaces that are fiat press inward with a definite
force, but those which are concave, being partially buried, as it
were (figure 01, c), within the body of the liquid, and therefore
having the inward attractions of the molecules a little com-

a b c

Fig. 61.- -A diagram to illustrate the operation of forces concerned in capillarity, repre-
senting sections through convex, flat, and concave water surfaces. The small circles,
open and solid, are water molecules, and the larger circles are the areas within which
given molecules, represented black, are cohesively attracted by others. Where these
areas lie wholly within the liquid, as shown in the lower part of b, the attractions
balance one another, and no effect is produced; but where the areas fall partly outside
of the liquid, the inward attractions are not resisted by equivalent outward ones,
though the exact degree thereof depends on the form of the surface.

pensated by partial attractions outward, press inwards with less
force, while those which are convex, projecting as it were outside
of the liquid, have their molecules drawn in with an even stronger
attraction than have those of a flat surface (figure 61, a). There-
fore it follows that the very mobile water will always be pressed
away from fiat or convex surfaces towards those which are con-
cave. Now it happens, furthermore, that water adheres both to
glass and to wood, and hence in a tube of either of these substances

How Plants Draw in Various Materials i8i

climbs up a bit on the wall, as our figure 61, c well illustrates, mak-
ing the surface concave to a degree that is greater the smaller the
tube. Hence the greater surface tension of the flat surface outside
pushes the mobile water up against the lesser pressure of the con-
cave surface inside, forcing it to rise against gravitation until equi-
librium is established, which will occur at a higher point the smaller
the tube. And the reverse process occurs wdth liquids which
will not adhere to glass or wood, e. g., mercury, or with walls
of such composition that water will not adhere thereto, as in
some air passages of plants; for in this case the surface in the
tube is convex, and presses the water down against the flat sur-
faces outside, so that the liquid stands below the outside level in
the tube (figure 60, on the left), or, if the tube is not deeply im-
mersed, will not enter at all.

Such is capiUarity, deriving its energy from internal molecular
tensions given release by peculiarities of external conditions, and,
like all molecular forces, strictly limited in amount and without
possibility of continuous action. Capillarity plays in the plant
some minor part in the ascent of sap, in prevention of the entrance
of water into some air passages, and in other processes later to be
noted. Moreover, some physicists see in imbibition nothing but
a refined capillarity, although as I think, the phenomena of im-
bibition of water vapor, presently to be noted under hygrosco-
picity, is hardly consonant with this explanation. Still another
possible connection cf a refined capillarity with osmotic absorp-
tion will be noticed in a moment.

We have now reached the place where the reader who may
have used my permission to skip for a httle must resmne his
grasp on this narrative if he is to understand the essentials of
osmotic phenomena.

In watching the ascent of a liquid in an osmoscope, like that
of figure 56, one sooner or later comes to wonder what would
happen in case an insuperable barrier, e. g., a tight stopper, were
interposed against the further rise of the liquid. The matter is

l82

The Living Plant

Fig. 62.— Pfeffcr's cell, as
pictured by himself in his
own book (but reduced
to ?^4 his size).

A semipermeable mem-
brane is formed all over
the inner face of the
porcelain cup, which is
shown, in half, at the
lower right of the figure.
The cup, and all the re-
mainder of the appara-
tus, is then filled with
the sugar solution,
which, absorbing water
when the cup is im-
mersed, presses the mer-
cury up against the air
in the gauge to a height
which balances, and
measures, the pressure.
The remaining mechani-
cal features are con-
nected with filling and
sealing the cup.

easy of experiment and the answer plain;
the cup becomes stretched or even pushed
from the tube, or sometimes (and always if
provided with a semipermeable membrane)
it bursts. This shows that osmotic ab-
sorption, if confined, develops osmotic
pressure. Of course the pressures have
been measured exactly, chiefly by aid of an
instrument invented by the great botanist
Pfeffer, and shown by the accompanying
picture (figure 62). When its porous cup,
lined with a semipermeable membrane, is
filled with a solution of sugar like that in
root hairs, and then is immersed in pure
water, the gauge actually exhibits a pres-
sure equal to that of three or four atmos-
pheres, or fifty to sixty pounds to the square
inch. Nor is this all, for when very strong
solutions are used, which require, of course,
an instrument of enormously greater
strength, pressures of surprisingly high
magnitude have been registered, even up to
twenty-four atmospheres, or 360 pounds to
the square inch, — a much higher pressure
indeed than ever is used in the steam boilers
of even the swiftest express locomotives;
while recentl}^ even higher ones have been
measured. Nor are such pressures of merely
academic interest to the botanist, because
others higher yet, above one hundred at-
mospheres, have been found to exist under
special conditions in plant cells.

Here follows another paragraph which the
reader may skip if such be his inclination.

How Plants Draw in Various Materials 183

since it merely concerns the explanation of osmotic pressure,
and is not essential to the integrity of our subject. Now a
very remarkable and important point about osmotic pressures
is this, that in general they are the same in amount as would
be given by the respective substances if converted into gases
at the same volume, temperature and pressure. This carries
the implication that osmotic pressures and gas pressures, be-
ing the same in amount, are the same in kind, the dis-
solved substance being practically a gas, and it, not the liquid,
exerting the pressure. But while this explanation is satisfactory
for most of the phenomena, it meets with the physical difficulty
that the closely packed water molecules must prevent that free-
dom of back and forth movement upon w^hich a gas pressure
depends. Accordingly a second explanation has been given,
really an old one revived, which finds the source of the pressure
in an adhesive attraction between the molecules of the dissolved
substance and those of the water, whereby the former draw all
of the latter around them, and take more from the membrane
(which easily recoups itself from the outside supply); and thus
the solution swells and the pressure is obviously exerted by the
substance and liquid in combination. Or, one can express the
same thing by imagining that the molecules of the dissoh^ed
substance act Uke the micellae of the membrane and absorb
water (from the latter) by imbibition, with only this difference
that the adhesion between substance and water is stronger for
all distances than the cohesion of the substance for itself. And
still a third explanation is possible, namely, that the spaces
between the suspended molecules of the dissolved substance act
like excessively fine passages along which the water passes forcibly
by an extremely refined capillarity, — in which case the water, and
not the substance, exerts the pressure. And if it seems that
the correspondence between osmotic pressures and gas pressures
must be conclusive for the first explanation against the others, it
is to be said that this is not necessarily true, for the properties

1 84

The Living Plant

of substances in solution and in the gaseous state are so closely
and regularl}^ interconnected, that the same mathematical rela-
tions apply to them all. And as to which of the explanations is
correct, the future must decide.

The reader has now a sufficiency of data for understanding
pretty fully the nature of osmotic absorption and pressure, which
we may summarize here by aid of the accompanying diagram

(figure 63). The dissolved sub-
stance inside of a membrane is
always tending to diffuse out-
ward by the energy of its own
diffusion pressure, which de-
pends ultimately upon heat;
and if the membrane be per-
meable, then the substance dif-
fuses into and beyond it, as it
did from our molasses-holding
osmoscope; but if semiperme-
able then not. Meantime,
whether because the interrupted

Fig 63.— Diagram to illustrate osmosis cfiffusioU-preSSUre actS fikc gaS-
through a permeable membrane; the ^ _

symbols as in figures 58, 59. In case, preSSUre tO SWcU the interior

however, the membrane is semiperme- ,. . , , r ii •

able the dissolved substance cannot es- UqUld, Or bCCaUSC Ot adheSlVC

cape through it. attraction between substance

and liquid, or because of capillary action between substance
and liquid, the substance draws on the water supplied by the
membrane, which yields it very easily so long as it can recoup
itself freely from the outside supply. Thus the solution swells
and exerts pressure until the power of the substance to with-
draw water from the membrane exactly balances the resistance
interposed to its expansion. And this is all true inside of the
plant or the animal as well as outside thereof, whence we may
now deduce another of our natural verities, to this effect, — that
wherever the conditions for osmotic absorption exist, the membrane

How Plants Draw in Various Materials 185

acts as a check, either partial or total, to the further diffusion of the
dissolved substance while allowing the liquid itself to pass freely, as
a result of ivhich the dissolved substance, whether by gas-like ex-
pansion or direct attraction, draws liquid through the membrane,
sivells, and exerts an osmotic pressure proportional to its strength.

After this lengthy but needful discussion of physical principles,
we turn to the actual osmotic phenomena displayed by plants,
and here the reader who is skipping the hard parts must resume
the narrative. The absorption of water by roots is the most
important of these phenomena, but there are others of little less
consequence. First among them is the maintenance of rigidity
in very soft parts such as leaves, young stems and flowers. These
parts consist mostly of water (fully 90 per cent), while the re-
siduum of solid matter (about 10 per cent) is too small and un-
substantial to supply rigid support. Even the moderately firm
veins, as everyone knows, are quite unable to keep a wilted
leaf from collapsing. But every young cell, soft and weak
though it is, can absorb water powerfully through its semi-
permeable protoplasmic membrane into its sugar-holding sap,
and thus swell to turgescence, stretching the walls until they are
tense, and the structure is stiff. Again, osmotic pressure supplies
the energy by which young cells can expand their walls in growth,
overcoming the resistance of older cells around them; by which
buds or flowers can swell and unfold; by which young roots can
force a way through hard soil and even destroy masonry and lift
curbstones; and by which soft-bodied fungi can burst pavements.
Osmotic pressure is the mechanical power used by those parts in
effecting their work.

Of minor osmotic phenomena in plants, some of them familiar
in the household, there are many. Thus, if one places dry sugar
on fresh strawberries, pretty soon it becomes a syrup, and the
berries look shrunken; evidently the sugar, moistened by con-
tact with the berry, makes a dense solution which draws water
from the cells. The collapse of berries from this cause is very

1 86 The Living Plant

evident when preserves are made with plenty of sugar, but
fruits retain their shape in some of the processes where little or
no sugar is used. Dry raisins and currants become plump when
soaked, for their cells contain sugar though their protoplasm is
dead; and the process is hastened by the heat of cooking. The
crisping of celery or cucumbers when placed in water is a case of
increased turgescence, the tense cells actually exploding, as it
were, when crushed by the teeth. The reason, by the way, why
the water must be cold for best crisping is this, that warmer
water tends to drive out and replace the air of the intercellular
passages, thus deadening the explosive action in which crisping
consists. Moreover, the bursting of hard-skinned berries, like
cranberries, when heated in water, though apparently an os-
motic phenomenon, is primarily due to the swelling of the air
confined by the skin, the same thing which occurs in apples when
baking. A genuine osmotic bursting does, however, occur some-
times in fruits, like plums and grapes, while still on the plant,
because of a great absorption of water from the ducts by the
sugar-ripe cells under action of heat on bright summer days;
and the calyx of carnations sometimes bursts from the same
cause when the temperature rises in the greenhouse. There is
in tomatoes an osmotic disease, called Qlldema, due to an over-
absorption of water by soft cells, and the consequent formation
of blistery swellings. The swelling of soaking seeds with a power
sufficiently great to result in the bursting of strong vessels, is
chiefly due to osmosis though it is partly imbibition, and the
same is true of the forcible swelling of dried apples. Sugar and
salt are common preservatives, the one of fruits and the other of
meats, though neither is really poisonous to the germs and molds
which cause decay, while the former is actually nutritive; but in
strong solutions they act germicidally, because they withdraw
so much water from the decay organisms as to render these
inactive. Moreover, either of these substances, when eaten in
more than moderate amount, causes thirst, which results from

How Plants Draw in Various Materials 187

their osmotic action in withdrawing water from the walls of the
stomach, whose dryness, from whatsoever cause, gives the thirst
sensation. And there are doubtless other familiar osmotic
phenomena which will occur to the ingenious reader, who can
now have the pleasure of undertaking their explanation upon an
osmotic basis.

To complete our discussion of water absorption by plants, we
must consider the case of dry tissues like wood. Dry wood, as
everyone knows, absorbs water eagerly and powerfully, swelling
considerably in the action. The conditions for osmosis are ab-
sent, and all evidence goes to show that the absorption is due to
imbibition into the solid cell-walls. This helps to explain a
common phenomenon in connection with wood, — its warping.
When water is placed on one side of a dry board, the board warps
away from the wet side, often with power enough to tear it from
firmly-fixed fastenings; but if the supply of water be continued,
the board later flattens out, and a measurement will show that the
saturated board is considerably larger than when dry, precisely
as a membrane is. Evidently, the water forcibly absorbed by
imbibition upon one side forces apart the micellae and swells
the wood on that side before it has time to reach the other, al-
though, after the lapse of enough time, it penetrates to the other
side, swells that, and thus straightens the board, as represented
diagrammatically by a combination of the figures 59 and 64. It
will here occur to the reader, incidentall}^ that boards often warp
without access to water, and simply from the one-sided action of
heat. The principle, nevertheless, is the same; even the dryest
boards contain some water, the drying of which from one side
allows the water remaining in the other to w^arp the board in the
usual manner. Furthermore, the reader may recall that a board
will warp crosswise but never lengthwise, which fact is correlated,
obviously, with another well-known fact about wood, — a fact
of very great importance in building and carpentry, — viz., that
wood does not lengthen or shrink lengthwise as it does so freely

1 88

The Living Plant

crosswise. The basis of this fact is not known, but I venture to
suggest as a possible explanation that the sides of the cubical
micellse facing towards the end of the wood (those towards and
away from the reader in the sections of figures 59 and 64) have
no attraction for water at all, and hence absorb none; and this
view I propose that we hold as an hj^pothesis until it is disproven

Fig. 64. — A diagram illustrating the molecular basis of the warping of wood. It belongs

between a and b of figure 59.

or a better is offered. The supposition that micellar surfaces can
exist without any attraction for water will help also to explain
how cell-walls can be waterproof, as they actually are in cork
and epidermis.

A special form of imbibition by dry tissues is the absorption
of water vapor from moist air, with its return thereto as the
^ir becomes dry, — a phenomenon called hygroscopicity. Fa-
miliar examples occur in the softening and sagging of paper in
damp weather, in the uncurling and curling of hair, in the move-
ments of the wood of old furniture, giving rise to snappings and
creakings which are oft of uncanny effect when heard in the
stillness of night. Now, in essence, hygroscopic movement is the
same thing as warping, the water being absorbed as a vapor
instead of as liquid. Furthermore, if the tissues are made \'ery

How Plants Draw in Various Materials 189

thin, this warping may be rapid enough to be seen by the eye,
and forcible enough to exert a considerable pressure; and ad-
vantage of these features is taken by plants, to produce, by aid
of suitable mechanical arrangements, adaptive movements of
various sorts. Of this nature are sundry hygroscopic movements
described elsewhere in this book, — the self-planting of some
seeds; the creeping of some fruits by the twisting movements
of hygroscopic awns; the opening and closing, with changes of
weather, of most spore-cases and anthers; and the forcible shoot-
ing of seeds by hygroscopically-bursting pods. Man has also
taken advantage of this principle to construct instruments,
called hygroscopes or hygrometers, for showing or measuring
the amount of moisture contained in the air. By suitable mechani-
cal arrangements the hygroscopically swelling or shrinking sub-
stance may be made to twist a pointer over a graduated scale,
to cause suitably-clad little persons to make their exits and en-
trances to and from tiny houses, or to produce other visible
results having appropriate significance.

So much for the absorption of water; we turn now to absorp-
tion of minerals, several kinds of wliich are needed for the various
processes of metabolism inside of the plant. But the subject is
comparatively simple. The plant can absorb only those minerals
which exist in solution in the water of the soil, dissolved therein
from the rocks or from various fertilizers added by man. And
the minerals enter the plant with the water. In Water-plants,
and the simpler sorts of the land, they enter mostly by diffusion
from the outside supply, traveling everywhere through the water
which saturates the plant. But in the higher plants they are
swept in with the current through the hairs, cortex and ducts,
from which they pass by diffusion to the places of use. It would
seem at first sight that their passage through hairs and cortex
would be forbidden by the semi-permeable protoplasmic mem-
branes. But semi-permeability is wholly relative, and a given
membrane which prevents the passage of the relatively large

IQO

The Living Plant

sugar molecules, may permit the passage of the much smaller
mineral molecules. But aside from this, the evidence shows that
in protoplasmic membranes another influence comes into play,
and that the dissolved substance, in order to pass through such
a membrane, must be soluble in the material composing it.

There remains to be considered the absorption of gases, a matter
of great importance because of the indispensable part played in the

plant's economy by both car-
bon dioxide and oxygen, the
great reservoir of which is
the air. The first requisite,
of course, to gas absorption
by the living cells, the most
of which lie deeply buried
within the body of the plant,
is some system whereby those
gases can be conveyed from
the atmosphere into their
presence; and such a sys-
tem, as the reader already
FiG^ 65.-A cluster of cells in a piece of pith, j^ learned iu Chapter II, is

showing the intercellular air passages (in ^ '

black). (Copied from a wall diagram by provided in the iuter-cellular

Frank and Tschirch.) .

air passages, which are shown
in a typical tissue, a bit of pith, in the accompanying picture (fig-
ure 65). These passages do not exist in young tissue where new
cells are in process of formation, as figures 53 and 139 C illustrate;
but as the young cubical cells grow larger, they tend to round off
into spherical form, splitting in their mid-walls, first at the
angles and then along the edges, until the final arrangement
tends to approximate to that of the spaces and passages existing
between balls in a pile. These passages once formed always
persist, no matter what shapes the (sells may assume; and there-
fore they form a continuous system ramifying everywhere
throughout the plant, as is represented diagrammatically in the

How Plants Draw in Various Materials 191

accompanying figure 66, .4. Here, for simplicity, the passages,
represented in black, are imagined to fall into one plane; and here
also, by the way, a partial interruption in the system due to
the presence of the longitudinally-running woody bundles is
shown by the blank spaces. In young green tissues, as shown by
the detailed diagram (figure 66, B), in which, as in C and D, the air
passages are partially reduced to one plane, the passages open
through the epidermis by the stomata, while on older stems,
where a corky bark has formed, they open through the lenticels
(figure 66, C), those corky wart-like excrescences prominent on all
young stems, and consisting simply of open gashes in the bark,
partially sealed in the winter by corky cortical cells. In young
roots, however (figure 60, D), neither stomata nor lenticels are
present, but the continuous epidermis and hairs are commonly
and normally covered with films of water, through which the
gases diffuse in solution from the air spaces in the soil to those in
the root, and vice versa.

Thus much for the aeration system, whereby every living cell
of the plant is brought into communication with the external
reservoir the air. But what is the power impelling the gases
along these passages, — which are often of great length, small size,
and extreme irregularity? Plants possess no mechanism for the
forcible indrawing and expulsion of the air en masse, such as
animals have developed in their muscular chest-and-lung breath-
ing arrangements. In some degree a movement of air through
the inter-cellular system is promoted by the swaying of parts in
the wind, and by the expansions and contractions of the air under
varying temperature and barometric pressure; but such effects
are insignificant. The primary cause of the gas movement is
found in diffusion, that process, already described, whereby the
molecules, driven by the energy of heat absorbed from the sur-
roundings, tend ever to move forward from places of greater to
places of lesser concentration, and therefore from places where
they are being formed or released to places where they are not.

D. Longitudinal section through a portion of root at *

KU

Fig. 66. — Generalized drawings illustrating the aeration system of the plant.

192

How Plants Draw in Various Materials 193

and from places where they occur to places where they are being
absorbed. ^Moreover, each kind of gas diffuses by itself, no mat-
ter what others may be present, so that a gas in process of ab-
sorption by a plant can move inward in a steady stream through
another which is not being absorbed, and even against the op-
posite stream of one in process of release. Thus, in photosynthe-
sis, for example, a constant current of carbon dioxide diffuses
into the leaf, through nitrogen which remains without move-
ment, against a current of oxygen which is diffusing outward. It
is a condition hard to imagine, it is true, but the facts declare it
is so. The gases thus mipelled along the passages by diffusion
finally reach the living cells, and, being soluble in water, are
dissolved by the moist surfaces, and then diffuse through walls
and protoplasm to the places of use. And here I may add a sug-
gestion, for the benefit of the reader versed in physics, that this
movement of different gases in contrary directions along the same
passages is explained much better by the old-fashioned idea that
diffusion and gas pressure are caused by a mutual repulsion
between the same kind of molecules than by the modern kinetic
theory, which makes those phenomena the result of vibratory
movements of the molecules; and moreover the very same con-
ception explains perfectly how osmotic pressures and gas pres-
sures can be identical in kind as well as in quantity.

A very special case of absorption occurs in those plants which
absorb organic food substances already made. Such plants, of
which parasites are a good example, have the power of excreting
from their absorbing parts those special enzymes, or ferments,
which render soluble the organic materials they touch. The
dissolved substance then enters the plant by diffusion from the
place of high concentration outside to the places of use and low
concentration inside, — the intermicellar spaces, of course, being
adjusted for the admission of these large molecules. The ab-
sorption by pollen-tubes of tissues through which they pass; of
hmnus by the fungi which live thereupon; and of the materials

194 The Living Plant

dissolved from the bodies of insects by the pitcher-plants or other
insectivora, is also of this character. In all of these cases the
materials are not drawn in, as by osmosis, but are driven in by
the energy of their own diffusion.

In reviewing absorption by plants, the reader must be struck
by the fact that the forces at work are chiefly molecular, and
therefore slow and gradual, even though powerful in their action.
Plants, as it were, arrange the conditions to permit the molec-
ular forces to work for them. In this respect they stand in
rather marked contrast to animals, which tend rather to make
use of those larger or molar forces which permit greater rapidity
and range of action. In this difference we have the explanation
of the persistent placidity of plants in comparison with the
abounding activity of animals.

This chapter is already so long that it is only with reluctance
that I add anything more; but there remain a few matters which
must receive some discussion in this immediate connection. First,
we must examine a little farther the arrangements for aeration in
plants, especially under unusual conditionSo Wherever particular
need exists, there the inter-cellular system may become much
larger, as occurs conspicuously in leaves, which, requiring a
carbon dioxide supply for photosynthesis ten tmies or more
greater than the oxygen supply they need for respiration, exhibit
a far larger aeration system than any part of the plant needing
only a respiration supply; and that is wh}^ leaves have the mark-
edly spongy texture they so commonly exliibit. Again, there are
plants of such habit that their roots (as in Marsh Plants), or
even huge rootstocks (as in Water Lihes), lie deep under water
and must be aerated in some way from the surface. In such cases
the inter-cellular system is immensel}^ developed, even to the
formation of elaborate passages, in the parts which lead from the
surface to the parts under water; and this is the reason for the
soft, open, spongy texture of the petioles of Water Plants, and
the pith of Rushes and Sedges, and it explains why some plants

How Plants Draw in Various Materials 195

can grow in a soil that has no aeration. And it is interesting to
note, by the way, that many interesting accessory adaptions
are displayed by these plants, of which one in particular is here
apposite, viz., the walls of the air passages in these Water Plants
are so modified chemically that water will not wet them, and
therefore will not enter them by capillarity, on the principle dis-
cussed earlier in this chapter. This is ob\'ioiisly an advanta-
geous adaptation against the obstruction of these slender passages
by water in case of sub-aqueous accident to the petioles or stems.
In some other cases accessory aeration structures are developed
which permit a shorter route from the air to the roots. Of this a
conspicuous case has been claimed to exist in the great knees of
the Bald Cypress of the Southern swamps, which rise above the
water surface and contain an aeration system in connection with
the roots; and other comparable cases are known. In some
Water Plants, however, the aeration is of a simpler sort, con-
sisting indeed of an absorption of air dissolved in the water, in
precisely the manner used by the Fishes. In some kinds, for
example some Eel-grasses, the leaves are so thin as to present
a relatively great surface in proportion to the bulk of tissue to
be aerated; while in others the leaves are cut to the finest divi-
sions, presenting indeed a condition directly comparable physio-
logically with the gills of the fish. This is the reason for the tissue-
thin and thread-fine structure of practically all plants which live
wholly under water.

Finally we must give some further attention to the particular
organs of absorption, the Roots. The structure of the young white
tips has already been described except for one point, viz., the
water-carrying ducts and the food-carrying sieve-tubes do not
stand in-and-out from one another as in young stems, but alter-
nately. In this arrangement hes an obvious adaptation, since
it removes the sieve-tubes out of the path of the water from
hair cells to ducts; and this conclusion receives some confirma-
tion from the further fact that the arrangement is not main-

196

The Living Plant

tairied in the older jiart of the root, where the entire anatomy is
closely like that of the stem. Roots, however, have no nodes,
nor regular places of origin of new roots, which, unlike branches,
originate deep in the tissues, budding out as it were, from the

fibro-vascular bundles (figure 67),
and breaking their way (partially
by the aid of enzymes) out
through the cortex, at places de-
termined by the stimulus of more
abundant air, water, minerals, or
space. This method of origin of
side roots, by the way, stands in
marked contrast with that of side
stems, or branches, which always
originate by a transformation of
the cells of the cortex, as indi-

FiG. 67. — A cross section of a typical i • <2 i ot t^i i-i,

root, showing the way in which a side Cated m HgUre 137. IhUS, the

fn:r:S-by^g:rka^t; root system of any plant is al-

Curtis- Nature and Development of ^^^yg eXCeSSively irregular, al-
Plants.) ^ ^ D 7^

though, on the other hand, differ-
ent kinds of roots present comparatively little variation in
structure or appearance, as indeed is to be expected from the
comparatively uniform conditions under which most of them live.
Typically, roots are much more slender than stems, and have
their strengthening tissues condensed nearer the center, in obvi-
ous correlation with the fact that they have no lateral strains
to withstand, but only pulling strains exerted upon them by the
stems for which they must provide a firm anchorage. Therefore,
while stems approximate to hollow columns in construction, roots
approximate rather to ropes or cables. Indeed, in many roots,
one can trace a distinction between features connected with
absorption and others connected with anchorage of the stems;
and the difference in some cases goes so far that one distinguishes
between absorbing roots and anchorage roots, which often occupy

How Plants Draw in Various Materials 197

different positions or directions in the soil, the former seeking
usually the dampest places, while the latter tend rather to pene-
trate radiately from the stem into the earth.

While absorption and anchorage are the typical functions of
roots, occasionally they perform others quite different, as we
have noticed already in the chapter on leaves and stems. Thus,
they become modified, with appropriate anatomical changes, to
swollen storage organs, in the Sweet Potato; to slender and
toughened climbing organs in English Ivy and many tropical
climbers; to tough pointed spines in some Palms; to slender
penetrating haustoria or sucking organs in some parasites; to
flat green photosynthetic organs in some tropical orchids; and to
yet other structures of minor account. Thus roots, like stems
and leaves, formed for one function can be modified greatly for
the performance of others, illustrating once more Nature's won-
derful capacity for ringing changes on her favorite ideas.

CHAPTER VHr

THE WAYS IN WHICH SUBSTANCES ARE TRANSPORTED
THROUGH PLANTS AND FINALLY REMOVED THERE-
FROM.

Transfer, Transpiration, Excretion

HE living plant, as the reader of the foregoing pages
will surely agree, can be viewed as a kind of central
station for the transformation of substance and energy,
both of which forever are streaming into, passing
through, and issuing forth from the plant, undergoing en route
quite definite changes in correlation with adaptive results. These
transformations we have already considered in our chapters upon
Photosynthesis, Respiration, and Metabolism, while their Absorp-
tion was the theme of the chapter just finished; but we still have to
consider their passage through the plant and their final removal
therefrom. These matters can be treated conveniently together
as they are in this chapter, although, for a practical reason which
will later appear, we may best reverse the natural order, and
treat first the subject that logically should be last.

The most abundant of the substances transferred and elimi-
nated as well as absorbed, by plants, is water. Most people are
aware in a general way that plants are forever giving off water
as vapor to the air, although they have little idea of its amount.
The fact can be demonstrated, by the way, very conclusively to
the eye by placing a potted plant, of which pot and soil have
first been enwrapped by a water-tight covering, in a glass case
or bell-jar, after which, within a few minutes, there will collect

on the glass a cloud of water-drops which can have come from

198

How Substances are Transported and Removed 199

no other possible source than as vapor from the leaves. This is
the source also of most of the moisture that collects upon win-
dows near which house plants are grown, and likewise of the
water-drops which gather, sometimes to annoying extent, on the
glass faces of ferneries, though such water is commonly assumed
to originate from evaporation out of the soil. This release of
vapor from leaves or other green parts is a practically universal
phenomenon in plants. It is called in physiology Transpiration;
and I wish to warn the reader at this point, out of the depths of
a considerable experience as a teacher, not to allow a mere re-
semblance in words to create any confusion in his mind between
this and the utterl}^ unrelated process of Respiration. Transpira-
tion is one of the great primal physiological facts about green
plants, and it has, like Photosynthesis, this further distinction,
that it is one of the very few processes of plants for which there
is no equivalent in animals, the animal process of perspiration
being utterly different both as to method and meaning. The
reader should therefore incorporate into the visualized picture
of the living plant now under construction in his imagination,
the idea of a tenuous cloud of vapor rising forever from all its
green parts.

But no student of science, and therefore I hope not the reader,
will rest content with the general fact that water is given off as
vapor by plants, but will insist upon knowing the quantity. The
most practicable and accurate of the several methods by which
transpiration quantities may be determined lies in the use of the
balance. If one takes an ordinary potted plant, — Fuchsia,
Hydrangea, Rubber Plant, or other, — encloses soil and pot in a
water-tight cover to prevent evaporation therefrom, then weighs
the plant at intervals on an accurate balance, the comparative
weights, aside from some minor, and largely self-compensating,
errors arising from photosynthesis and respiration, must obvi-
ously exhibit the exact transpiration from the leaves and the
stems. Such experiments are frequently tried in botanical

200 The Living Plant

laboratories, and never without exciting an interested attention
from all students, young or old. Some of the results are shown
vividly in the accompanying photograph (figure 68), wherein the
plant, with its pot and soil enclosed water-tight for this study,

Fig. 68. — A potted Sunflower prepared for transpiration studies as described in the text.
The measuring glasses show the number of cubic centimeters, and therefore of grams,
of water transpired in twenty-four hours and in a week. In three and a half days the
plant transpired a quantity of water equal to the capacity of the pot in which it is
growing.

is shown standing beside measuring glasses which display the vol-
ume of its transpiration for a day and a week. The quantity of
transpiration must necessarily depend on the size of the plant ; and
in order to compensate this variable, and at the same time to
permit a comparison between different plants, it is customary

How Substances are Transported and Removed 201

to express transpiration in standard units. For greenhouse
plants, which have been the most carefully studied from this
point of view, it has been found that while the transpiration in
one hour from one square meter (roughly a square yard) of leaf
ranges according to circumstances all the way from near nothing
up almost to 300 grams (11 ounces), the generalized average, or
conventional constant, is 50 grams per square meter (nearly
2 ounces per square yard) per hour, i. e., 50 gm'-h, by day and \ of
this quantity, 10 gm-h, by night, which equals 30 grams per square
meter, 30 gmVi (an ounce per square yard) per hour, day and
night together. Upon this basis, an average leaf during an ordi-
nary summer season transpires an amount of water equal to its
own area, and a centimeter (| of an inch) deep. These quantities
are well worth remembering.

The first sensation of the student as he really comprehends
these data, especially whenever they are yielded by experiments
of his own, is always one of surprise at the largeness of the quan-
tity. It is, indeed, this copiousness of transpiration, rather than
the existence of the process, which is the remarkable thing about
it; and it helps to explain a number of more or less familiar
phenomena. Thus, the rapidity with which leaves alwaj s wilt
when cut from their stems, and the quickness and completeness
with which plants can dry out the soil of their pots, are conse-
quences of transpiration. In this way some plants can serve as
good drainers of marshy soils. Thus Eucalyptus trees, especially
active transpirers, have been used for this purpose in the Roman
Campagna with such success that the marshes have become
freed from the former scourge of malaria-carrying mosquitoes,
and therefore habitable by man; while the malaria-repelling
virtue often ascribed in this country to Sunflowers, which are
sometimes planted around dwellings with this end in view, has
the same genuine scientific basis. It is also transpiration condi-
tions chiefly that determine which kinds of plants can be grown
in dwellings as house plants. House plants are by no means the

202 The Living Plant

most attractive kinds there are, but are the most attractive that
can withstand the dryness that prevails in our houses in winter, — •
a dryness that is due not so much to the heat of the house as to
the fact that the general atmosphere in the winter has a very low
content of water vapor. A house plant in fact is one whose
transpiration in that dry heat is no greater than can be com-
pletely compensated by the absorption and conduction of water
from the soil. And this relation of transpiration to conduction
explains another notable phenomenon in plant nature, namely
the limitation in the height of trees, which in general are just so
high as the water can be conducted in sufficient abundance to
supply the transpiration from the foliage. When that height is
reached the tree can still spread out laterally, which explains the
flat tops of the largest Elms, IMaples, Oaks and others, and of
many forest trees when seen from mountain tops. A transpira-
tion effect of a very different sort is displayed by a good many
plants in the early spring. It is a fact that roots absorb water
very slowly when chilled, and if they are kept for a time at a low
temperature, while leaves and stems are exposed to conditions
favorable for transpiration, as is effected quite easily by experi-
ment, the plants will wilt very rapidly. These very conditions
are often supplied naturally in the spring, for if the soil remains
frozen or very cold after warm bright days have forced out the
leaves, or if a cold spell that chills the soil is followed abruptly
by very warm bright windy days, then the young leaves transpire
so much faster than the water can be supplied by the roots, that
they become dry-blasted as if by a frost, to which latter cause,
indeed, this effect is commonly but mistakenly ascribed. This is
the explanation also of the fatal browning of the leaves of many
ornamental evergreens, whose leaves are awakened to active
transpiration before the roots can supply the water they need;
and it is, indeed, a chief cause of winter-killing generally. And
finally, as to transpiration effects, there is one more way in
which this process exerts a very remarkable influence upon

How Substances are Transported and Removed 203

plants; for the necessity that it be regulated and minimized in
places where water is habitually scanty, as occurs conspicuously
in deserts, has resulted in the development of protective adapta-
tions which, as the weird aspect of desert plants abundantly
attests, affect the forms, sizes, and other structural features of
plants more profoundly than does any other influence whatsoever

Fig. 69. — A transpirograph m action. The loss of a gram oi water from the plant permits
that end of the balance to rise and close an electric circuit ; this acts, through an electro-
magnet, to force a pen against a revolving time-drum (seen on the left of the stand),
and at the same time to drop a spherical gram weight from a cylindrical reservoir
into the box under the scale pan, which is thus depressed, again breaking the circuit.
Thus a record is made on the time-drum at each moment when the plant has lost a
gram of water.

excepting only Photosynthesis. But this subject belongs really
with a later chapter (on Protection), where it will be treated in
detail.

The results of all experiments on transpiration show remarkable
variations in its amount; but it soon becomes evident that such
variations are correlated closely with changes in external condi-
tions. This can be tested by weighing the plants while kept
under somewhat extreme conditions of heat or cold, humidity or
dryness, light or darkness; and the results are all the clearer if one
makes use of some form of self-recording instrument, one of

204 The Living Plant

which, called a Transpirograph (a little thing of my own, by the
way) is shown in operation in the accompanying photograph
(figure 69). By its use the plant is made to write, precisely and
continuously for days together, a record of its own transpiration.
Further, there also exist instruments, invented long ago for use
in meteorological stations, which write continuous records of
the very conditions that affect transpiration, viz., tempera-
ture and humidity, while light is recorded by a special method.
When the cotemporaneous graphs of transpiration and the
external conditions are plotted together upon the same sheet, as
in case of the accompanying graph (figure 70), the relation be-
tween process and influencing factors is displayed in a way
which leaves little to be desired in the direction of exact and ex-
pressive exhibition of the relation between this physiological
process and the external conditions. Indeed, I am accustomed
to use this study with my own students as an example of a well-
nigh ideal piece of physiological method, whereby Nature is
compelled not only to display, but even to write down, for the
edification of man, the tale of her own operations. I often recall
with delight the remark once made by an eminent literateur who
happened to visit my laboratory at a time when this experiment
was in progress. As soon as he had grasped the full scope of the
matter, he turned away with this comment, — "Well, I don't see
what there is left for Nature to do but lay down and holler." In
these words he expressed very well both the aim and the joy of
scientific investigation, which after all is a kind of great game
where one matches wits against Nature, and generally loses, but
now and then wins and gathers the stakes, which consist in a
share of her jealously-kept secrets.

But to return to our experiments on the effects of external
conditions upon transpiration, they show these results. Heat
increases, and cold lessens it. Heat, indeed, may hasten tran-
spiration to such a degree that water is lost from the leaves much
faster than the roots can absorb it or the stems conduct it, in

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2o6 The Living Plant

which case a wilting results even though water is plenty in the
soil; but plants thus wilted can quickly recover when the weather
grows cooler, for then the absorption and conduction catch up,
so to speak, and again fill the leaf. Light increases, and darkness
lessens it. This harmonizes with our transpiration constants,
which showed that in general the process is five times more acti^'e
in dajdight than at night; and it explains why plants that wilt in
the day recover at night. Dryness increases, and humidity lessens
it. This is the reason why most kinds of plants will not live in our
houses, the air of which is so dry that the leaves lose their water
much faster than roots and stems can supply it, no matter how
plenty in the soil. It explains, too, why leaves never wilt in the
weather called muggy, no matter how hot, and also why leaves
that are wilted recover when sprayed, even though experiment
proves that none of the spray is absorbed. As to other external
climatic conditions, their influence is slight, except in the case of
the wind, which always promotes it. Thus it is evident that in
general transpiration is promoted by the very same factors which
favor evaporation, though later studies have shown that the
parallel does not hold true in detail.

We must now consider the structural basis of transpiration,
with which, however, the reader already has incidentally made
some acquaintance. If he will recall his knowledge of the cellular
structure of the leaf, refreshing his memory, perhaps, by another
inspection of figure 2, Plate I, B, and figure 54, B, it will be clear
that every cell borders, for purposes of respiration and photosyn-
thesis, upon the inter-cellular air-system, which ramifies through-
out the leaf and opens to the outside world through the stomata,
— the little slit-like openings through the otherwise continuous
epidermis. Now these cells are all gorged with water, which
saturates their walls; and where these border on the air spaces
the water necessarily evaporates. The vapor thus formed satu-
rates the air inside of the leaf, and is then moved by the force of
its own diffusion along the passages and through the stomata to

How Substances are Transported and Removed 207

the relatively dry atmosphere outside. Such is the structural
and physical basis of transpiration, and it explains perfectly why
heat, which is an evaporation accelerator, and dryness and winds,
which are diffusion promotors, increase the process.

But though such is its basis, transpiration is really not so
simple as this, for it is influenced much by another condition,
and that is the number and size of the stomata. As to their num-
ber, that varies immensely with different kinds of plants, there
being none at all on the upper surface of a good many leaves,
while on lower surfaces they vary from a few up to near 500 to
every square millimeter (one-twenty-fifth of an inch), with a
conventional mean at 100; and this equals no less than 100 mil-
lions to the square meter (yard), which is another of our con-
ventional constants. And it is worth while to add that when all
of the stomata are open their wddest, about one-hundredth of the
whole area of the leaf is exposed. As to the size of the stomata,
that not only varies with, the kinds, but in each kind is highly
variable, since they open and close, from near a circle through a
narrowing oval to a slit and perhaps no passage at all, by the
movements of two bordering cells called guard cells. These
guard cells, as shown by the typical example pictured herewith
(figure 71), are of aspect distinctive and unmistakable, with
little resemblance to others of the epidermis. They are usually
somewhat kidney-shaped, forming together two halves of an
elongated oval, and they contain chlorophyll. Their construction
is such, as figure 71, lower, illustrates, that the natural spring of
their walls tends to bring them together and close up the stomatal
slit; but the development of osmotic turgescence in their cavities
rounds them out so that they separate, thus opening the slit.
Now this turgescence of the guard cells is influenced much by the
quantity of water contained in the leaf, rising and falhng there-
with, so that when water is plenty the stomata tend to be open,
but when it is scarce they tend to be closed. Thus it seems as if
the guard cells ought to act adaptively as regulators of transpira-

2o8

The Living Plant

tion, keeping it down to safe limits when water is scanty, but
allowing full play when water is plenty. The turgescence of the
guard cells, however, is influenced also in another way; for

they (and they only of epidermal
cells), contain chlorophyll, which
has to make sugar in light and
thus increase their turgescence
and cause them to open the sto-
mata. This arrangement would
explain to perfection why light
increases transpiration so greatly
quite apart from any accompany-
ing heat, while a definite ecologi-
cal advantage seems equally
clear, viz., it should ensure open-
ing of the stomata at those times
when the demand for carbon di-
oxide is the greatest, and allow
them to close with the lessening
of this need. From the structure
of the guard cells, therefore, we
should expect them to serve
as automatic valves, regulating
transpiration adaptively to the
external conditions; and thus

lower figure shows diagrammatically in they haVe USUally bcCU regarded
cross section the method by which the

turgescent rounding of their cavities by botauists. But this COUCCp-

opens the stoma, — the dotted walls .. ■, , ■, j. • j u

showing the closed, and the unshaded tiou has uot been sustamed by

walls the open position. (The upper j^^^^. g^udieS, wMch haVe shoWU
figures reduced from a wall-chart by '

L. Kny, and the lower from a much- go much irregularity, and cveu

copied diagram by Schwendener.) . , .

anomaly, in their action that we
have to remain in doubt until further researches shall give us
the truth. Meantime we can only consider that any regulatory
action they may have is clumsy at the best.

Fig. 71. — Typical guard cells, with a
stoma between them, highly magnified,
in surface view and cross section. The

How Substances are Transported and Removed 209

Such are the principal facts as to transpiration, and they bring
us to the problem of its physiological meaning, upon which also
there is uncertainty. The older explanation argued thus: —
plants need in all parts, and especially their leaves, certain
minerals from the soil: their only possible method, apparently,
of raising these minerals to the places of use consists in absorbing
and transferring them in water, and evaporating the latter to
leave them behind : some of the minerals are so scarce that plants
hardly ever can get as much as they need: the more copious the
transpiration the more minerals are raised; presumably, there-
fore, transpiration is the mineral-raising process and is the more
efficient the more copious it is. On this assumption, plants would
be expected to develop adaptations for promoting transpiration,
and a great many such have actually been claimed to exist, as
will presently appear. A second explanation argues thus: — the
stomata exist primarily for admission of carbon dioxide needed
in photosynthesis (they occur, in general, only in green tissues) :
when open for this purpose, evaporation and diffusion of water
will necessarily take place from the saturated cell-walls of the
interior of the leaf as a purely physical operation which the plant
has no power to prevent: presumably, therefore, transpiration is
merely an incidental physical accompaniment of photosynthesis,
a kind of necessary evil, as it were. Upon tliis explanation
adaptations would be expected for its prevention, especially of a
kind which would not interfere with photosynthesis; and of these
a good many have been described, as we shall note in the follow-
ing chapter. This explanation accounts best for most of the
phenomena, and is the one that is generally accepted at present.
A third explanation argues thus: — w^hen full sunlight falls on a
leaf, it beats thereon with an energy overwhelmingly greater
than the leaf can employ in its work (for it actually uses no more
than some three per cent) : this energy, both light and heat,
would work disaster to the Uving protoplasm unless dissipated
in some manner: evaporation is a highly effective method of

2IO The Living Plant

energy-dissipation: presumably, therefore, transpiration is an
adaption to protection against injury from the over-plentiful
energy of sunlight. Each of these explanations has its merits and
its difficulties, and no one alone is sufficient. Probably the truth
will be found to involve some participation of all three; transpira-
tion may be fundamentally a process which the plant cannot
prevent, but that is no reason why the plant cannot employ
it, and even develop it highly, as an easy method of raising its
requisite minerals, and a convenient means for the dissipation of
superfluous energy. But this question, too, is one of the many
whose solution lies with the future.

Transpiration, however, is not the sole method by which water
is removed from the plant. Everj^body has noticed the clear
shining drops which bejewel the margins of Grape leaves on
mornings that follow hot days and cool nights; these drops are
commonly thought to be dew but are not. They show very
strikingly also on young plants of Nasturtium and seedlings of
Grasses, where they can be made to appear whenever desired,
simply by covering the actively-transpiring plants for a few
minutes by a cooled, darkened, or dampened bell-jar. In a great
many other plants, too, the drops appear and are mistaken for
dew. The slender wet streaks often seen on the leaves of the
Cannas just after sundown, come from similar marginal drops;
and a tropical plant is said to exist from which water is projected
in a very fine jet. In all of these cases the water is known to
come from inside the plant, and the process, known physiologic-
ally as guttation, is a result of the following conditions. On very
warm days the vigorous transpiration is accompanied by an
equally energetic absorption and transfer, but the comparatively
sudden check to transpiration caused by the cool of the evening
does not at once affect the absorption; therefore water continues
to be forced into the stems and leaves to an extent which might
prove a serious detriment were it not for an avenue of escape pro-
vided by openings existing in the ends of the veins, for it is here

How Substances are Transported and Removed 211

that the water-drops always appear, Guttation, therefore, is a
kind of a safety-device for the plant even if transpiration is not.
Furthermore, it happens at times that roots keep their vitality
long after the stems have died, and continue to force up water
which can find an outlet only through rifts that it makes in the
withering stems. Besides, in cold weather all stems tend of
course to contract, thus squeezing from such rifts any over-
abundant water they may happen to contain. When water from
either of these sources is forced out in cold weather, it freezes in
lines, which soon become flat plates as more and more issues
from the stem, pushing the already formed ice before it ; and this
is the origin of the ice crystals or shells, often of great beauty and
commonly mistaken for ''frost," which are seen on the stems of
some plants in the early part of the winter.*

If I seem to have dwelt over-long on this matter of water-
removal from the plant, I claim in explanation that the process,
because of the profundity of its effect upon plant-structure and
habit, is worth all the space I have taken; and the later chapter
on Protection will help to support this conclusion. But now we
are ready to proceed to the topics remaining, of which the re-
moval or excretion of substances other than water comes naturally
next. These excretions belong to four different classes. First, of
course, are the gases, for oxygen is an excretion in photosynthesis,
and carbon dioxide in respiration. But the subject is simple, for
they pass off by diffusion, either through stomata and lenticels
of leaves and stems, or in solution through the wet epidermis of

* A conspicuous case occurs in Helianthemum canadense, commonly called Frost-
weed, which is described in Gray's Manual of Botany thus: "Late in autumn crystals
of ice shoot from the cracked bark at the base of this and the next species, whence
the popular name." Another, and even more striking, example is the Dittany
{Cunila Mariana, or origanoides) , in which the ice-forming habit has thus been de-
scribed: "Our Cunila has attached to the stem a shell-work of ice, of a pearly white-
ness, beautifully striated, sometimes, like a series of shells one in another — at others
curved round on either side of them like an open, polished, bivalve; then, in others,
again, curled over in every variety of form, like the petals of a tulip." (J. Stauffer,
quoted in the Botanical Gazette, XIX, 1894, 326.)

212 The Livine Plant

1*^

the roots. Second, are various minerals, which in part are useless
materials absorbed along with the useful kinds, and in part are
by-products of chemical changes inside of the plant. For their
removal plants have no regular excretory system as animals
have, though a partial substitute exists in the fall of the leaves
and the bark, which thus remove crystalline matters they con-
tain. Other minerals are left behind as crystals in the old dead
cells when the living protoplasm advances into the new ones it
forever is building (compare figure 41). Third, are the root-
poisons, little known to us yet and even by some experts not be-
lieved to exist. They appear to be highly complex organic sub-
stances, slow of diffusion and drainage, and poisonous to the roots
which produce them though not necessarily to different kinds;
and this fact gives a new explanation of the advantage of rotation
of crops and of letting a soil lie fallow. Fourth, is extra-floral
nectar, apparently identical in composition and mode of forma-
tion with the nectar of flowers, which performs the invaluable
service of attracting cross-pollinating insects, as later we shall
note in detail. The extra-floral nectaries are very tiny structures,
sometimes marked by blotches of color, occurring commonly at, or
near, the bases of leaves in young plants (e. g. in some Ferns,
Horse Beans, Castor Beans and others), or with the spines (in
Cactus), and elsewhere. They have been supposed to attract
small ants which may perform some ecological service; but the
evidence thereon is so unsatisfactory that it seems best to place
this nectar for the present among the excretions, though surely
it is a puzzling sort.

So, and by such means, are substances removed from plants.
The reader knows also in what ways they are absorbed. Between
absorption and removal they have to be transported, often for
very long distances; and this is the matter which next needs
attention.

The principal substance to be transported is water, of which
transpiration demands so great a supply that it has to be moved

How Substances are Transported and Removed 213

in a copious and continuous current through the plant. This
involves of course a highly efficient water-carrying mechanism,
which we should first consider. The principal feature thereof is
the ducts, which are tubes, beginning near the tips of the roots
(figure 53) and running in bundles throughout the length of the
stem to the leaves, as our earlier generalization of the system
so clearly illustrates (figure 54, A) ; and here they end in little areas
of green tissue, as we have noted already in the description of the
leaf. Structurally, the individual ducts are short, but the end of
each one lies against the end of another with only a thin partition
between; and therefore the practical effect is that of a continuous
tube with occasional thin cross partitions. When roots and stems
are young and flexible, the soft walls of the ducts are supported
inside by ringed or spiral thickenings, which keep the cavities
open when the young roots or stems become sharply bent back
by accident, and also against the turgescent pressure of neighbor-
ing cells. The ducts formed later, however, when the tissues are
thicker and harder, have not the spirals, but stiff bands or a fret
work, or even a uniform thickening, pierced by thin areas for
the escape of some water to the neighboring tissues. These dis-
tinctive features of ducts are very well shown in the picture
given herewith (figure 72; also 54, C).

We turn now to the study of the transfer of water through the
plant, or, as it may also be expressed, the forces impelling the
ascent of sap. Transpiration makes very great demands for a
water supply, especially in lofty and broad-leaved trees, and in
weather that is bright, dry, and windy. By what forces is so
weighty a volume of water raised so quickly to a height so great?
Recently I had occasion to calculate the work done in a day in
transferring the water from roots to leaves in one of the largest
kind of trees, and I found it was just about equal to that which
would be done by a man in carrying 500 large pailfuls of water
up a ten-foot flight of stairs within ten hours. This is nearly a
pailful a minute for ten hours without cessation, my figures being

214

The Living Plant

expressed in this form in order to bring the matter home to my
students. Now, strangely enough, the botanists are not yet
agreed either as to the source of the energy or the precise physical
method by which this considerable work is accomplished; and in
default of precise information I can only present to the reader a
synopsis of such data as we possess, along with some comments
on their probable bearing. And here follow the principal explana-
tions which have been offered for the physics of sap ascent.

Fig. 72. — A generalized drawing of the tissues of a typical stem, showing the water-
carrying ducts (the three larger tubes), and a food-carrying sieve-tube (the single
dot-lined tube), with the associated tissues. (Copied from Kerner's Pflanzenlcben.)

1. Root pressure. — In the preceding chapter it was shown that
roots absorb water osmotically and forcibly start it up the ducts.
But this pressure, which, in some greenhouse plants has been
found sufficient to raise water 40 to 50 feet, and in trees up to
80 feet, is wholly insufficient to explain the ascent when trees
reach 400 feet, as they do in some kinds of Australian Eucalyptus ;
and therefore this cannot be the explanation.

How Substances are Transported and Removed 215

2. Atmospheric pressure. — This will suffice, when the suitable
conditions are provided, as they are in a pump, to raise water
some 32 feet, but no more; in the plant, however, the requisite
conditions are wanting, while this height is obviously quite in-
adequate. Therefore this cannot be the explanation.

3. Capillarity. — This is the power, as the reader will recall,
by which water, driven by its own internal molecular energy,
rises in small tubes, the higher the smaller the tube. But even
the slenderest ducts known to occur in plants are not small
enough to raise the water more than a few feet even if all the other
conditions were most favorable, which indeed they are not.
Therefore this cannot be the explanation.

4. Imbibition. — This was the favorite theory of the great
botanist Sachs, who defended it to the end of his life. He con-
ceived of the wall-system of the plant as a kind of gigantic con-
tinuous membrane, extending all the way from the root hairs to
the cells of the leaf; into this membrane, by forces and method
already considered, water was absorbed by imbibition, and raised
by the same energy, to be finally removed by evaporation at
the leaf-cells. The theory is simple and plausible, but is shattered
by one fatal fact, — viz. it requires that the transpiration stream
shall move in the walls of the ducts, not their cavities (which
Sachs took simply for reservoirs), whereas experiment proves
beyond question that the water does move in the cavities. There-
fore this cannot be the explanation.

5. Propulsion. — This theory maintains that the water is
forced or propelled upwards by some action of the living cells
distributed along the course of the ducts, each hving cell being
supposed to draw water from a lower duct and force it out into a
higher. It really is an extension of root pressure to the whole
stem, the living cells passing water from one duct to another
precisely as the root hairs and cortex pass it from the soil into the
ducts, — and by the very same physical power and method, which
is still unknown in detail. It differs from the preceding explana-

2i6 The Living Plant

tions in this, that it involves the activity of cells which are alive;
and herein also it meets its greatest difficulty, because, accord-
ing to some experimenters, when the living cells are killed by
suitable methods, the water continues to ascend, at least for
some time. Therefore, they say, this cannot be the explanation.
But others are not convinced that the cells are really all killed in
these experiments, and hold that this explanation is substantially
correct.

6. Traction. — This, the most recent explanation, has been
worked out by a botanist, Dixon, and a physicist, Joly, working
in collaboration, and is often known by their name. It maintains,
in brief, that water in very thin threads holds together, by the
force of its own internal cohesion, with a tenacity sufficient to make
it as strong as a solid fiber or wire; wherefore the thin threads
of water in the ducts can actually sustain their own weight for
a length as great as the height of the tallest trees. These threads
being practically continuous from the tips of the roots to the
cells of the leaves, hang, as it were, from the leaf-cells, into which
they can be lifted by any power that can remove the water from
those cells. This power is supplied by the energy of evaporation
in transpiration, which latter process, therefore, lifts or drags
the water threads up the ducts much as a man on a roof would
pull up a rope from the ground. On this view the energy which
raises the water in the tree is the same which lifts it to the clouds.
This theory finds its chief difficulty in the lack of complete demon-
stration that the water can thus cling together in threads of such
great length, and it has not been universally accepted.

It sometimes appears as if the extent of our knowledge of any
subject were inversely proportional to its importance. At all
events we found this to be true of the structure of protoplasm,
and it also seems true of this subject of sap ascent. And at
present there is a pause in the advance of our knowledge thereof.
With this subject, as with others, we find out everything that
existent methods of investigation can yield, then turn for a time

How Substances are Transported and Removed 217

to other matters. Presently, however, somebody, working per-
haps in a quite different field, chances upon some new method
that happens to be applicable to this subject, to which students
then turn once more, and make another long step in advance.
The very fact that all knowledge thus grows by appreciable
stages makes it all the more interesting to follow; and the watch-
ing for such new knowledge, and the grasping it when it appears,
constitute the principal charm of the scientific life.

There remains but one other point in connection with the
transfer of water. The current must supply not only the tran-
spiration loss, but all the working needs, — chemical, osmotic and
other, — of the various tissues besides. This matter, however, is
simple, for all kinds of ducts possess plenty of thin places through
which the water can pass outward, after which, by imbibition
and osmosis, it gradually penetrates from cell to cell throughout
all of the tissues that need it. And with the water in this way go
the various minerals in solution, which explains their transporta-
tion, as well as their absorption, by the plant.

From the transport of water and minerals we turn to that of
the various food-substances made in the plant, — a subject known
in plant physiology as translocation. The subject is comparatively
simple. In the first place such substances travel invariably in
solution; and substances which are not soluble in water never
move from their places of formation. The very physical nature
of some substances, e. g. the sugars, makes them naturally soluble,
but others, viz. starches, oils, cellulose, and most proteins, are for
the same reason insoluble. In such cases solubihty is obtained,
for purposes of translocation, by their conversion (or hydrolysis)
into closely-related substances which are soluble, — thus starch
and cellulose into sugar, oils into fatty acids, insoluble proteins
into peptones. These changes are effected by those remarkable
substances called enzymes, whose method of action we have con-
sidered in the chapter on Metabolism. The enzymes are widely
scattered through plants, and some of them are identical with the

2i8 The Living Plant

digestive juices (diastase, pepsin) found in the aUmentary system
of animals; for the solution or hydrolysis of insoluble foods by
enzymes constitutes digestion in plants just as truly as in animals.
This digested material is then in suitable condition for transporta-
tion, which takes place in two ways. First, it may be carried
with an onward-moving water current, as happens with the sap
in the spring (witness the Sugar Maple), w^hen the food stored
for the winter in the roots or lower trunk of the tree diffuses from
the storage cells into the sap current and rises therewith. Sec-
ond, it may travel by diffusion alone, for a substance dissolved
in water is in perfect physical condition for diffusion, — that
is, has the power and the tendency to move outward and on-
ward, by its own diffusive energy, from places of greater to
places of lesser concentration until equilibrium is established.
When, furthermore, the substance is being produced at one place,
as occurs with sugar in the leaves during photosynthesis, and is
being removed in another, as occurs in places of storage where it
is converted into insoluble starch, then a steady diffusion current
is established between the place of production and the place of
use. And it is by such diffusion currents that most of the trans-
location of food-substances through the plant is effected, though
it is to be remembered that diffusion alone, from its very nature,
can never completely empty a part. This explains why some
sugar and other food materials remain in autumn leaves when
they fall.

This translocatory diffusion proceeds in part from cell to cell
through the walls, the protoplasmic linings thereof being adjusted
(by appropriate chemical modification or intermicellar spacing,
as noted earlier under Absorption) to permit the passage of the
molecules of the substance; and, given time enough, there is no
limit to the distance that substances may thus pass in solution.
Obviously, however, such translocation through long distances
must be greatly facilitated if long tubes replace the short cells;
and such a system is actually found in the elongated sieve-tubes

How Substances are Transported and Removed 219

which are very well illustrated in our figure 72. These sieve-tubes
accompany the ducts all through the plant from root-tips to
stem-tips and leaf-cells, as our generalized plant illustrates so
clearly (figure 54), thus forming a part of the same fibro-vascular
bundles. But sieve-tubes are more slender than ducts, and unlike
them have thin soft walls, and a continuous lining of protoplasm;
while the occasional cross partitions, thicker than the walls, are
perforated by openings in a way wliich has given these structures
their name (figure 54, C). The presence of this protoplasmic
lining in the sieve-tubes when diffusion alone does not require its
presence at all, suggests that it plays some part in helping to force
substances along the tubes, perhaps in a manner analogous to the
way in which the food is moved along the intestines of animals;
but no such action has been proven. Doubtless the movement is
aided materially by the swaying of branches in the wind, and,
when it is downwards, by gravitation; but these influences are
obviously both incidental and irregular, and diffusion is the only
motive force in translocation that we surely know. The reader,
therefore, must visualize this process as one of constant diffusion
along the sieve-tubes. It is not an onward movement of the
solution they contain, but a movement of the sugar and other dis-
solved substances through water that is standing still, a process
in great contrast with the onward rush of sugar-carrying sap in
the spring. The method of this diffusion, by the way, is illus-
trated diagrammatically in figure 6.

The sieve-tubes, in which translocation of food principally
proceeds, lie in the inner bark of woody plants, down through
which, accordingly, all summer long, there is a constant move-
ment of food-substances towards the roots or other underground
parts devoted to winter storage. That this is really the path is
easily proven by experiment, such for instance as removing a
narrow ring of the bark, or constricting it by a metal ring. This
often happens by accident in Botanical Gardens where the en-
circling wires wliich support the labels are left too tight. In all

2 20 The Living Plant

such cases the obstruction in the bark causes an accumulation
of the food just above, with a resultant swelling of the tissues
that often is very prominent. The same thing happens also
naturally where a twining stem, such as that of a Bittersweet,
tightly constricts a growing tree, in which cases the swelling stem
always shows a very much greater enlargement above than below
the vine.

Such is the method whereby food materials are transported
from their places of formation to the places of storage ?nd use.
The same general method explains the transport and accumula-
tion of all those special substances, usually of definite and adapt-
ive functions, which we call secretions, — the volatile oils, nectar,
some coloring matters, and others which have been considered
in the chapter on Metabolism.

This is really the place to bring this particular chapter to a
natural conclusion ; and it is truly a pity that it cannot be done.
For somewhere in the book we have to consider the prominent
subject of the cellular anatomy of stems, and this is the most
suitable place. However, the matter is not indispensable to a
clear understanding of the chapters that follow, and therefore
the reader may skip the remainder of this chapter if he wishes.
And if the said reader should ask why I do not skip it myself, I
would answer that the integrity of my subject requires its pres-
ence. For with regard to this book I feel with Nehemiah Grew,
who wrote more than two centuries ago in the dedication to his
great work on the Anatomy of Plants, — ''Not I, but Nature
speaketh these things."

If, accordingly, in pursuit of a knowledge of the anatomy of
stems, one cuts with a sharp knife a clean section across any
young stem, he can always discover the ends of the fiber-like veins
distributed in a uniform ground-work of tissue. And if, further-
more, he makes a thin section from a typical young stem, such as
Castor Bean, and magnifies it moderately, he will have before
him such an appearance as is pictured herewith (figure 73).

How Substances are Transported and Removed 221

while a typical stem is shown generahzed in our later figure 139 B.
Among the many cellular elements in the symmetrical, almost
geometrical structure thus displayed, it is easy to identify the
bundles of ducts from their relatively large size and their obvious
resemblance to the cut ends of
round tubes. Associated with
the ducts, and a little way re-
moved towards the outside of
the stem, lie clusters of smaller,
thinner-walled, and more angu-
lar cells, which are also the cut
ends of long tubes, the food-
carrying sieve-tubes] while be-
tween sieve-tubes and ducts lie
two or three layers of small
squarish cells presenting an

aspect which later the reader ^^^ 73.-Cross section of a young stem of

will learn to associate with t^e Castor Bean, magnified about twenty

times. (Copied, reduced, from a drawing
growth, for they are the cam- by H. O. Hanson, in Curtis' Xature and
7 . ,. 1 • 1 p 1 , Development of Playits.)

bium cells which form new ducts

and sieve-tubes as long as the plant lives. Ducts, sieve-tubes and
cambium, to which often are added strengthening fibers, grow
all or a part of them together in bundles, forming fibro-vascular
bundles which are identical with the veins,- — both the kind that
can be seen in young translucent stems, and also those familiar
in leaves. The bundles begin, as our generalized picture of the
conducting system illustrates (figure 54), near the ends of the
roots, where they consist of a few ducts and sieve-tubes only;
farther back they acquire cambium and fibers and enlarge greatly
in size; in the stem they branch at the nodes and run out to the
leaves, when they fringe away gradually to the veinlets, each of
which ends as a single duct and sieve-tube in the midst of one of
the ultimate areas of green tissue.

The fibro-vascular bundles have not only this definite com-

222 The Living Plant

position, but a definite arrangement in the stem, where they
lie in a ring, as our pictures illustrate (figures 73, 139 B). The
tissue in which they are embedded consists mostly of thin-walled
cells, of rounded or polyhedral shapes. The part thereof lying
inside of the ring of bundles makes up the pith, which is com-
monly utilized for storage; that between the bundles constitutes
the beginnings of structures later to be considered as the medul-
lary rays; while the tissue outside of the bundles forms the cortex,
which contains some chlorophyll, and aids in the photosynthetic
work. This cortex, by the way, is continuous and morphologically
identical with the green tissue of the leaf ; and one (3an form a very
useful and reasonably accurate conception of the anatomical
relations of stem and leaf by imagining that one of the fibro-
vascular bundles of the stem is snipped out from among its
neighbors, and, with its adherent cortex, bent outward at right
angles to the stem and then flattened and fringed out to a network
which the green tissue surrounds and fills in. But as to our
stem, outside of the tissues aforementioned comes the single layer
of epidermis, physiologically the plant's skin, with its distinctive
flat chlorophylless cells pierced here and there by the stomata.
Finally, sometimes in connection with the sieve-tubes, some-
times as a ring or as scattered islands in the cortex, or just under
the epidermis, occur masses of very thick-walled cells, showing
long and pointed when seen lengthwise, which are the important
fibers that give strength to the stem. Howsoever these fibers are
distributed, there is always one constant feature about their posi-
tions, that they tend to keep close towards the outside of the
stem. And the reason therefor is sufficiently plain, — ^it is a
fundamental principle of mechanics that any given amount of
strengthening material exerts its greatest supporting effect against
lateral strains if disposed in the form of a hollow cylinder or tube,
which is the reason why columns used in building construction
are hollow, not solid, why a bicycle frame is constructed of tubes,
not of rods, and why a great tree can stand as a mere shell of

How Substances are Transported and Removed 223

wood long after its center has rotted away. It is true, this prin-
ciple would require for greatest efficiency, that the fibers should
lie on the very outside, as indeed they do in some cases ; but such
an arrangement would prevent all access of light and therefore
the use of the surface for spreading of chlorophyll. It is easy to
understand how the plant could find it advantageous to sacrifice
a trifle of effectiveness in the strengthening system for the sake
of the marked advantage of spreading more chlorophyll; and in
this arrangement we see one of those innumerable compromises
with which plants, like mankind, are accustomed to meet the con-
flicting problems of existence.

Such is the primary or ground structure of stems, as typically
displayed in their earlier stages, and up to the time when they
cease to be flexible, green and soft. Then they begin to undergo
remarkable changes, connected adaptively with their continuous
growth into trees ; but these we can better postpone to our chapter
on Growth, where the reader will find them fully described.

It will interest the reader to know that the principal theme of
this chapter, — the transfer and transpiration of water, — will al-
ways be associated in the minds of plant physiologists with the
foundation of their science; for to it, of all the phases of plant
physiology, was first applied that exact scientific method of
measurement which is the only sure means for advancing natural
knowledge. Its founder was Stephen Hales, whose book Vegetable
Statics, though published in 1727, might have been written-
yesterday so far as its spirit is concerned. He will always be
considered the father of this science, and his book one of the
greatest of botanical classics.

CHAPTER IX

THE PECULIAR POWER POSSESSED BY PLANTS TO ADJUST
THEIR INDIVIDUAL PARTS TO THE IMMEDIATE SUR-
ROUNDINGS

Irritability

F the reader at this point will turn back to the Table
displaying the plan of this book, he will see that we
have now reached the end of our survey of the processes
concerned with the nutrition of plants. These proc-
esses are primarily internal, but they are all more or less depend-
ent, especially for their supply of material or power, upon some
one or the other of the external conditions. Now these external
conditions, — heat, light, water, minerals, and so forth, — are never
distributed quite uniformly around any individual plant, but are
more or less abundant in some spots or directions than others.
Obviously it would be a very great advantage to plants if each
separate one of their parts, — each leaf, stem, root, and so forth, —
could be adjusted or swung individually into the direction or
position that would enable it to work to the very best advantage
under the conditions presented by its own immediate surround-
ings. Such a power, and in high degree of efficiency, plants in
fact do possess, as we shall now proceed to consider. The reader
will be surprised, I predict, by the importance and interest of the
phenomena which belong under this head.

We may best begin our study of the subject by considera-
tion of its most familiar example. When a potted plant, like a
"Geranium," is grown in a greenhouse lighted evenly all around,

it assumes a symmetrical form, alike on all sides, as everybody

224

Power to Adjust Parts to Surroundings

225

knows; but when the same plant is grown in the window of a room,
where the hght is wholly one-sided, it turns all its parts in that
direction, even to the extent of seeming to reach out, as it were,
after the light (figure 74). The same thing occurs commonly in
nature, as may be noticed along the margin of shrubbery or close

■•«

r

^

H*-

V

wt^

'/

j^m^^L^yBrV/^^^ttSV^

t

jm p. '■ >:- ■ ^hJ

f

"■

1

r

Fig. 74. — Two "Geraniums" which for two or three days before their pictures were taken,
were kept, respectively, in a uniformly lighted greenhouse and a chamber lighted only
from the right hand side.

to high buildings or banks; and it can be demonstrated ver}^
prettily by experiment (figure 75).

A close observation of these cases shows always that stems and
leaves behave very differently in relation to the direction of the
light, for while stems point straight towards it, leaves set their
faces across it. This suggests the inquiry, — what, then of roots?

226

The Living Plant

And for answer we turn to experiment. If seeds of mustard or
radish are started in water-culture vessels, by methods described
in an earlier chapter (page 136), the young seedlings grow rigidly
upright in darkness; but if, when well started, they are given a

m \

Fig. 75. — Sets of Radishes grown side by side in a chamber lighted wholly from the right
hand side; but those on the instrument were kept continually revolving.

one-sided light, they turn always as shown in our figure, — the
stems to the light and the leaves across it as before, but the roots
distinctly away (figure 76). And such conduct is typical of or-
dinarv stems, leaves and roots.

This process of light-turning is called in physiology Photo-

Power to Adjust Parts to Surroundings

227

tropism (pronounced with the accent on the second syllable), or
Heliotropism. Parts that turn towards light are described as
positively phototropic (with the accent, despite the seeming
anomaly, on the third syllable), those that turn away as negatively
phototropic, and those that turn
across as transversely phototropic.
Phototropism is so thoroughly typi-
cal an example of the power of indi-
vidual plant parts to adjust them-
selves in relation to the immediate
external conditions that we can use
it as a basis for the analysis of the
nature of this power, which is known
physiologically, though not very
happily, as Irritability. Now the
elements entering into irritable re-
sponses are these : —

First, the reason why the parts do
it. — As to this, the explanation
must be amply obvious. The turn-
ing towards the window brings the
leaves into positions where they
secure the best exposure to light, —
the light which is indispensable to the photosynthetic func-
tion for which they exist. The best position for performance
of this function must of course be that which sets tliem at
right angles to the light; and this in turn requires that the
stem, whose function is simply to carry the leaves, shall point
or reach towards the light. As to the roots, not only does
their function (the absorption of water and minerals), require
no light, but their unprotected protoplasm is actually injured by
exposure thereto ; and this shows the advantage of their power to
retreat from light. The reason for the characteristic phototropism
of ordinary leaves, stems, and roots, respectively, is therefore to

Fig. 76. — A Mustard seedling germi-
nated by water culture in darkness
and then exposed to light falling
from the direction of the arrow.

228

The Living Plant

-i I »'

be found in an advantageous functional adjustment of those
parts in relation to the direction of light. And this principle of
advantageous individual adjustment of parts is characteristic of
irritable adjustments in general.

Second, the mechanical method whereby the turning is effected. —

The turning of the leaves, stems, and roots
into their respective new positions requires
both a considerable power and a definite
mechanism. Now it is quite evident that in
phototropism neither of these is supplied by
the light, for that has no power at all to lay
bodily hold on the parts and forcibly pull,
bend, or push them into their respective
positions, while it is easy to prove on the
contrary that the power is supplied by the
plant, and derived from its own respiration.
Thus, if oxygen be withdrawn from a cham-
ber in which a symmetrical plant is sub-
jected to one-sided light, not a trace of
phototropic response ever follows. The con-
nection will be clear to the reader: — The

Fia. 77.-Succes3ive stages ^-esponse requires energy, energy depends on
in the downward turning respiratiou, respiratiou demands oxygen ;

of a root, showing, by the

spread of the marks, that therefore uo oxygen, no response. And as

the apparent movement ..i -i • tjij • ,1,1

is effected by new differ- ^o the mechamsm oi the turnmg, that also
l?.ttiZl^rtLZl at i^ ^a^ily determined by experiment, for if
ready formed. The tri- stcms, pctioles, or roots are marked across

angular piece is a paper

index. (Copied from with evculy-spaced liues before the plant is

Sachs' Lectures.) t , -iii-iiii , ^ ^

exposed to one-sided light, then the marks
spread apart in a way to prove that the bending accompanies
growth in those parts, and is due to a more rapid growth on
one side than the other, — on just that side, indeed, where it is
requisite in order to swing the parts concerned into the advan-
tageous positions (figure 77). In phototropic adjustments,

Power to Adjust Parts to Surroundings 229

therefore, the already-existent tissues are not forcibly bent,
but the new tissues grow in such an unequal or differential
manner as to swing the parts into their new positions. In these
respects phototropic responses are typical of others, for in all
cases the power is supplied by the responding plant; and the
motor mechanism consists, as a rule, in such differential growth,
though occasionally it is of different sort, as we shall presently
note.

Third, the way the light operates in connection with the turning.—
Since it is not the light but the plant which accomplishes the
turning, we still have to seek the nature of the role that light
takes in the process. In l^rief, observation suggests and experi-
ment proves that in phototropic responses the plant parts, which
in general can grow quite as readily in one direction as another,
use the light simply and solely as a convenient guide or signal
(called scientifically, but not very fortunately, a stimulus) , indica-
tive of the most advantageous direction to take. It plays, indeed,
very much the same part for the plant that the compass does for
the sailor, establishing a definite Une of direction, towards,
across, or from which, according to circumstances, definite move-
ments may be made. This case is typical of the action of stimuli
in general; they never take any part in the mechanical accom-
plishment of the irritable adjustments, but serve merely as signals
for guiding, and sometimes for starting or stopping, the same.

Fourth, the way the light stimulus is perceived by the plant. —
The plant has no eyes for the light, as the sailor has for his com-
pass, yet it must possess some means of perception of the stimulus
else obviously it could not react. The details of the matter are
still much in doubt, but in general this much is certain, that the
light falling on the sensitive protoplasm of the plant part sets up
(probably by chemical means, since the blue rays are mainly
concerned) a condition of irritation or strain, which puts the side
towards the light in a condition different from the side away
from it, and thus establishes the line of light direction. This case

230 The Living Plant

is typical of all stimuli, which act by producing in the sensitive
protoplasm on which they impinge a condition of differential
irritation or strain which serves to impress a line of direction on
the part concerned. Then the part is swung by the motor mech-
anism into a position where this condition of strain is the same
all around, which position is kept in the subsequent growth. Ob-
viously only those agencies can act as stimuli at all which can
thus produce a differential state of the protoplasm, and con-
versely, any agency capable of producing such a condition can,
theoretically, act as a stimulus. And as to how strong a stimulus
must be to produce an effect, it is only essential that it have
enough power to produce the impression of differential strain
on the sensitive protoplasm; and above that degree its strength
does not much matter.

Fifth, how it is that a single uniformly-acting stimulus can evoke
different directions of turning. — The fact that in phototropism the
light neither pushes nor pulls the parts to their positions, but acts
simply as a guide to direction, involves the corollary which is
confirmed by experience, that it is exactly as easy for parts of the
plant to grow away from or across the light as towards it, pre-
cisely as the sailor, guided by his compass, which neither pushes
nor pulls him over the sea, can steer as easily to the south, east,
or west as to the north where it points; and the reader should
learn to think of all stimuli in this way. But if the parts of the
plant can turn as easily in one direction as another in relation to
light, what feature of their growth-mechanism is it which sends
stems so unerringly towards it, leaves across it, and roots from
it? Here again there is very great doubt as to particulars, but
hardly any as to principle, which can thus be illustrated. In a
locomotive, as most people understand, there is a certain lever,
which when set in one direction determines that the engine shall
move forward, and when set in another, that it shall move back-
ward, after the steam is turned on; and an engine is easily imagi-
nable in which, with the lever in yet a third position, the move-

Power to Adjust Parts to Surroundings 231

ment would be sideways. In all cases it is the same engine, the
same machinery, the same motive power; the difference consists
only in the way a small part of the machinery is set; and the
reader will please to observe that this set of the machinery is not
the cause of the movement of the engine, but merely determines
the direction thereof w^hen the power, which is steam, is applied.
Now something of analogous kind, it is most probable, deter-
mines the direction of turning of the plant organs. The structure
and motive power in all of these parts is substantially the same,
but in each some portion of the machinery is differently set, so
that the application of the power, which is growth, causes turn-
ing in the distinctive direction, — the stem towards Hght, leaf
across it, and root from it. Of course the machinery is not metallic
but protoplasmic, and in last analysis is probably of a chemical
nature, while, moreover, the set of the machinery is usualh' not
alterable at a touch, but is hereditarily fixed in each kind of
organ. And the subject may stand out yet more clearly if we
return for a moment to our sailor, who, in order to reach a cer-
tain eastern port, sets his steering gear to hold his good ship at one
angle to his compass, and in order to reach a western port holds
her at another. It is the same compass, ship, machinery, and
power; only the set of the steering gear is different. This is the
principle, I believe, which underUes the different kinds of responses
to any single uniformly-acting stimulus.

Sixth, how the advantageous direction of response has become fixed
in each part. — Or, in the simile of the preceding section, how did
the machinery become set so differently in leaf, stem, and root;
and especially, how did it become set in each of those organs in
the manner most advantageous for the performance of its particu-
lar function? Now it is perfectly plain that the power of a part
to respond advantageously to a stmiulus, that is to say, the set
of its responding machinery, is an hereditary and adaptive
feature, and must therefore have arisen in precisely the same
manner as any other adaptive features, including those of visible

232 The Living Plant

structure, — precisely, for example, as chlorophyll has been de-
veloped in the leaf, a fibro-vascular cylinder in the stem, and
hairs on the roots. Unless our whole philosophy of nature is
wrong, there was a time when these things were not: now they
are : at some time and in some way meantime they have arisen, and
by gradual stages in the course of evolution. Our problem of the
origin of the set of the machinery is therefore identical in kind
with that of the origin of any adaptation, and thereby is trans-
ferred into that separate field of inquiry which forms the subject
of our later chapter on Evolution and Adaptation.

The turning window-plant illustrates very clearly the nature
of typical sensitive responses in plants; and all of the more com-
plicated cases are identical in principle. Thus, not all stems
turn towards hght, for those of wall-climbing Ivies (e. g. the
Boston or Japanese Ivy) turn away from it, as manifest by the
way in which these plants grow into porches and windows. The
advantage, however, is evident on reflection; if these stems
turned towards light, like the ordinary sort, they would be car-
ried away from the wall and the possibility of clinging thereto;
but, turning away from the light, they are flattened up against
the wall where their holding discs can secure an attachment. This
example shows also that no necessary connection exists between
stemness, so to speak, and a set of the growth machinery towards
light, but that the set is developed in the organs in correlation
with their habits quite regardless of their morphological nature.
Again, not all leaves set themselves across the light, for a good
many kinds belonging in places very brilliantly lighted, like
sub-tropical plains, set their edges to the direction of maximum
brightness. In some this position is permanent, and may thus
bring the leaves to a vertical north-and-south position, as in the
Compass Plant of our prairies, which owes its name to this cir-
cumstance; or, the leaves may change their positions, rising
from horizontal to vertical at the time of maximum brightness,
as in sundry plants of the Pea family (figure 78). The advantage

Power to Adjust Parts to Surroundings

^2>i

of these vertical light-positions is believed to consist in a pro-
tection given to the living substance of leaves against the full
exposure to a brightness too intense for their good; for we know
on the one hand, that too bright a light does chemical damage to
protoplasm, even when partially screened
by the chlorophyll, while on the other hand,
leaves can make use of only a moderately
strong light, the extra brightness being
wasted upon them. It is this last-mentioned
circumstance, by the way, which explains a
problem that sooner or later will puzzle the
reader, viz., why all the vegetation in the
northern hemisphere does not have a turn
towards the south where the sun is. This
is no doubt because the diffused light falling
on the plants from the north is quite as
strong as they can use; and hence they have
no object, so to speak, in turning to the side
of the sun.

There remains one other phase of photot-
ropism in leaves which must here be consid-
ered, and that is their lateral shif tings out
from beneath one another's shade, a move-
ment chiefly accomplished by twisting and
lengthening of the petioles. The result is
often to bring them, especially in spread-out plants like the
vines, into a one-planed pattern where no leaf is overlapped
by another, — an arrangement commonly known as a leaf-mosaic
(figure 79); and there are even some botanists who believe that
the angular shapes of such leaves (e. g. in the English Ivy) are
partly determined by the advantage of interlocking to use all
the space.

Such lateral shiftings imply that the whole upper surface of
the leaf is equally receptive to the light stimulus; and a very

Fig. 78. — A plant of Melilo-
tus, showing the position
assumed in the bright
sun by the leaflets which
in weaker light are hori-
zontal. (Copied from a
paper by W. P. Wilson.)

234 The Living Plant

ingenious and higlily probable theory has been advanced in
explanation, viz., that the epidermal cells, focussing the light in
a special manner, are light-sensitive organs, and that the leaf
keeps turning and shifting until all of these cells receive their
full quota of light at the most desirable angle. In some other
cases, however, the reception of the light stimulus is known to
take place in a specialized spot, as for example in the seedlings
of Grasses, which are light-sensitive only in the tip of the first
sheathing leaf. The same thing is true, for several stimuli, of
the growing-point of the root, and other cases are known. Evi-
dently some such structures advance pretty far in the direction
of the special sense organs of animals, such as eyes.*

Thus much for the phototropism of stems, leaves, and roots:
what now of flowers and fruits? As to flowers, they turn their

* The localized reception of stimuli by the growing points of the roots is strikingly
expressed by Darwin in the closing paragraph of his great book, The Power of Move-
ment in Plants; and this passage illustrates so well a number of other phases of
irritable responses that it is here reprinted in full.

"We believe that there is no structure in plants more wonderful, as far as its
"functions are concerned, than the tip of the radicle. If the tip be hghtty pressed
"or burnt or cut, it transmits an influence to the upper adjoining part, causing it
"to bend away from the affected side; and, what is more surprising, the tip can
"distinguish between a slightly harder and softer object, by which it is simultane-
"ously pressed on opposite sides. If, however, the radicle is pressed by a similar
"object a little above the tip, the pressed part does not transmit any influence to
"the more distant parts, but bends abruptly towards the object. If the tip per-
"ceives the air to be moister on one side than on the other, it likewise transmits an
"influence to the upper adjoining part, which bends towards the source of moisture.
"When the tip is excited by light (though in the case of radicles this was ascertained
"in only a single instance) the adjoining part bends from the light; but when excited
"by gravitation the same part bends towards the center of gravity. In almost every
"case we can clearly perceive the final purpose or advantage of the several move-
" ments. Two, or perhaps more, of the exciting causes often act simultaneously on the
"tip, and one conquers the other, no doubt in accordance with its importance for the
"life of the plant. The course pursued by the radicle in penetrating the ground
"must be determined by the tip; hence it has acquired such diverse kinds of sensi-
"tiveness. It is hardly an exaggeration to say that the tip of the radicle thus en-
"dowed, and having the power of directing the movements of the adjoining parts,
"acts like the brain of one of the lower animals; the brain being seated within the
"anterior end of the body, receiving impressions from the sense-organs, and directing
"the several movements."

Power to Adjust Parts to Surroundings

235

faces, as a rule, directly to the light like the leaves, as anyone
can observe in our house plants, or in those that happen to grow
close to a building (e. g. a border of Nasturtiums), or against
walls (e. g. Trumpet Creeper), or otherwise in one-sided light
(figure 80). In a few flowers (e. g. Sunflowers), the phototropism
even extends to the following of the sun through the day, though
the adjustment is only moderately effective. Perhaps at first
thought it will not be evident why flowers are phototropic at all,

Fig. 79.— The adjustment of Ivy leaves (of English Ivy) into one plane, affording the best
aggregate exposure to light. (Copied, reduced, from Kernel's Pflanzenlehen.)

because, unhke the leaves, there is nothing in the function of the
flower requiring the action of light. But on further contempla-
tion of the use of the flower (a subject to be fully explained in
the chapter upon Cross-pollination) , and especially of the function
of the showy corolla as an advertisement to show insects its
position, the matter becomes evident; because obviously this
function of conspicuousness requires that the corolla must stand
out where the light can strike on it most fully. As to fruits, they
are as a rule indifferent to light, though responsive to some
other kinds of stimuli, as will later appear. One special case,
however, deserves mention because illustrative of an additional
fact about stimuli. There grows in Europe a little cliff-dwelling
vine, Linaria Cymbalaria (figure 81), which turns its flowers as
usual to the sun, but its ripening seed-capsules away therefrom.

236

The Living Plant

In consequence these seed capsules are brought into contact with
the cliff, and, moving about more or less, are reasonably sure to
push into some crevice where the seeds can be dropped in posi-
tion for starting the new plant
in its favorite habitat, instead
of at the foot of the cliffs.
There are two good reasons
why I cite this example. In the
first place it shows that the
phototropism of a part may
change — the lever may be
thrown — during its own life,
though this is not common.
In the second place, the seed
capsule has obviously no need
to get away from the light as
such, but simply to get back
against the cliff. Since, how-
ever, there exists no cliff-ward
stimulus, the light, which hap-
pens to act in the suitable di-
rection, is used for the purpose.
Light in this case acts as a
foster-stimulus as it were, and
may thus be described, in con-
trast with the direct stimuli of
the examples earlier described.
There remains one other class

Fig. 80. — A cut shoot of Bellflower, kept for r t i j. j^i 11 i

two days in a chamber lighted wholly ot hght respOUSCS,— the SO-Callcd

Inrnn?'''^f*;v,^fl^'''''^'*^'^°''*''^'P^°' slccp moveuients. It is very

totropism of the flowers. ^ "^

well known that some leaves
droop at night, as in Clovers, Wood-sorrels, Beans, and many
other members of the Pea family (figure 82); and most people
have seen, at some time or other, the remarkably tight-shut ap-

Power to Adjust Parts to Surroundings

'^2>1

pearance presented by those plants at night. The same plants,
moreover, can be put to sleep very easily, even at midday, by
simply covering them up from the light. Now the exact meaning
of the sleep movement is somewhat in doubt, as our chapter on
Protection will show; but there is no question at all that light is
the stimulus concerned. This response has, however, an interest
in another direction, for the motor-mechanism is not growth, but
a simple hydraulic contrivance contained in the clear little
swellings at the bases of the sleeping leaflets. In the daytime,

Fig. 81. — The cliff-dwelling plant, Linaria Cymbalaria, showing the positive phototropism
of it.s flowers and the negative phototropism of its seed capsules, which thus are
brought ^into advantageous positions for the deposition of the seeds. (Copied, sim-
plified, from Kerner's Pfianzenleben.)

under stimulus of light, their cells become strongly turgescent
and hold the leaves stiffly expanded ; but at night the turgescence
is lessened, and the spring of the tissues, aided more or less by
gravitation, causes them to droop. It is perhaps simply a high
degree of development of sleep movement which gives us the
remarkably-balanced leaf mechanism of the Sensitive Plant,
later to be considered.

In viewing these sensitive responses, and others of similar
sort, one soon comes to wonder what the limits may be to the
changes they can cause in the construction of the plant. This,
like most of our problems, is amenable to experiment. If the
most favorable possible conditions for one-sided stimulation are

238

The Living Plant

supplied to a plant, it will turn to that side to a considerable
degree; but the turning is never without limit, for, generally
speaking, the farther it turns the more reluctant, so to speak,
it is to turn any farther. If, on the other hand, the plant is so
grown that it does not receive a one-sided stimulation, which is

Fig. 82.— a typical example of the sleep of plants. Both are Acacias, identical in kind and
age, but the one on the right has been covered for an hour from light.

easiest accomplished by keeping the plant in continual rotation
by aid of an instrument (called a clinostat) designed for the
purpose (figure 83), then it always develops wdth remarkable
symmetry, determined, very obviously, by internal and hereditary
causes. The plant, accordingly, is born with an internal tendency
to symmetrical form, but likewise with a considerable though not
unlimited margin of possible deviation therefrom; and it is
within this margin that the irritable responses take place. But

Power to Adjust Parts to Surroundings

239

this margin has a greater interest than this, for it is characteristic
of animals also, including ourselves, where it offers the basis for
improvement of the body
through exercise, and of the
mind through education,
while it is the field, as well,
within which plays such free-
dom as is possessed by the
will.

Phototropism has received
this generous measure of
attention because it is so
thoroughly typical of irri-
table responses in general.
Accordingly the remaining
forms of irritability can be
treated much more briefly.

Hydrotropism. — If one pre-
pares a porous clay germina-
tor of the cylindrical form
represented in our picture
(figure 84) : fills it with water :
hangs it horizontally : fastens
small seeds along its sides:
and places it in a chamber
with a vapor-saturated at-
mosphere, then the stems
and the roots will grow stiff-
ly up and down as shown
by the first of the figures.
But if the surrounding air
be partially dry, then the roots will chng close to the porous
and water-soaked germinator, though the stems will act precisely
as before. In the first case the moisture is the same all around;

Fig. 83. — The clinostat, an instrumfiit which
allows the effect of one-sided stimuli to be
neutralized through the continual slow rota-
tion of the plant. Note the resultant sym-
metrj' of the Nasturtium which has been
grown from seed on the instrument.

240

The Living Plant

in the second it is most ahundant on the side towards the ger-
minator. The experiment, therefore, shows that roots turn in the
direction where moisture is most plenty; — that is, they possess a
definite hydrotropism, another typical form of irritable response.
The advantage of hydrotropism is perfectly evident when one
recalls that the very first function of roots is the absorption of

Fig. 84. — Porous water-filled cylinders, to which seeds of Mustard were attached. That
on the left was then kept in a saturated, and that on the right in a drier, atmosphere.

water. The stimulus acts in this waj^; the water, absorbed
more rapidly on the side of its greatest abundance, doubtless
causes an osmotic swelling and tension stronger on that side
than on the other; and this difference is ample to establish a line
of direction towards which the roots turn in their growth. It is
equally easy to see why stems and leaves display no hydrotro-
pism at all, for, as they do not absorb any water under normal

Power to Adjust Parts to Surroundings 241

conditions, its one-sided abundance is a matter of indifference
to them. This fact illustrates anew the adaptive character of
these responses; for it is a general rule that plant parts are in-
different to stimuli to which there is no profit in responding.

The hydrotropism of roots involves matters of some practical
consequence. It is said that when trees develop in a uniform
soil, the root tips tend to collect in a circle just under the outer
drip of the foliage, which is obviously the place where the water
is usually most plenty. But in case the soil is moister on one
side than another, the roots grow more freely in that direction,
and may even extend to a distance several times the diameter of
the tree. In their progress thus towards the most copious wet-
ness, they sometimes are led to a drain, and, insinuating them-
selves through some crevice left in the tiles, find therein a com-
bination of water, air, and mineral substances so agreeable that
they grow very profusely, even to so great a degree that they
sometimes choke the drain quite completely.

Chemotropism. — But roots have also other irritable responses,
notably to some chemical substances. Thus they turn, though
rather feebly, towards a source of supply of some of the minerals
they absorb, and this is typical chemotropism, with a very ob-
vious advantage. But they turn much more strongly towards
air (a special phase of chemotropism called aerotropism), — of
course for the oxygen it contains, which they need for their
respiration. It is easy to see in these cases how the stimulus is
received by the root, for the chemical substance, especially the
oxygen, must react with some of the materials found in the
complicated protoplasm with which it first comes into contact,
thus originating a differential chemical disturbance wliich would
establish the line of direction.

But other structures besides roots are markedly chemotropic.
Thus pollen-tubes in their growth turn towards the substances
secreted by stigmas and styles. In the fertilization of Ferns, an
egg-cell at the bottom of a protective flask-like archegonium is

242 The Living Plant

fertilized by a male antherozoid which swims through the water
(figure 104). Now when this egg-cell is ready for fertilization, a
weak solution of malic acid pours out of the archegonium into the
water, and diffuses steadily outwards. As soon as some wandering
antherozoid perceives the presence of the acid, it turns and swims
directly towards the source of supply, and hence to the egg-cell,
which otherwise it would have no means to discover. And there
is reason to think that such a secretion of special chemicals at the
time when the egg-cells are ripe is very wide spread through the
plant and animal kingdoms, providing the method whereby the
swimming or growing male cells are enabled to find the female
cells. This function is obviously not simply advantageous but
indispensable.

There are many important phases of chemotropism, but I
have the space to mention only one more. Water-plants, which
have floating leaves, alter the lengths of the petioles in accord-
ance with the depth of the water, a matter which can be shown
very beautifully by experiment. Now it is found that this regula-
tion is chemotropic, or, more exactly, aerotropic, for, as ex-
periment proves, petioles continue to grow until the leaves
reach a supply of free oxygen, when they stop. This case illus-
trates an additional fact about stimuli, viz. that they can serve
as signals to stop a process as well as to guide it; and other cases
are known in which they act to start a process. Such stimuH
are probably very important in controlling the various processes
of growth, as our later chapter on that subject will demonstrate.

Thigmotropism. — This name is applied to those turnings and
movements made in response to a touch as a stimulus. The
most typical case is exhibited by tendrils, which, as the reader
will recall, are those long slender structures sent reaching out for
a support by a good many kinds of climbing plants. These
tendrils sweep in long slow courses through the air until they
touch some hard object, such as a stem, or a wire, around which
they then curl in three or four turns (figure 85), thus obtaining

Power to Adjust Parts to Surroundings

243

a grip which holds the vine firmly and permits a still farther
ascent. Now it is easy to prove by experiment that it is really
the contact with the support which constitutes the stimulus
producing the bending, for anyone, by rubbing one side of a
tendril with a pencil, can call out
the turning, and watch all of the
steps in its progress. Even a mo-
mentary contact is followed by
a turning within a few minutes,
though the tendril will straighten
again in case the contact is not
maintained; but if the contact be
continuous the tendril will wind
completely around the pencil. The
advantage, the motor-mechanism
(which is growth), and the mode of
reception of the stimulus, in this
form of thigmotropism, are all suf-
ficiently obvious.

Most persons who have knowl-
edge of plants would doubtless put
forward a different case as a type
of thigmotropism, viz., the well-
knowTi Sensitive Plant, which droops
promptly and completely at a touch
(figure 86). But I think this move- fig. 85.
ment is only accidentally thigmo-
tropic. Nobody has yet found,
even after study of the plant in
its native home, any satisfactory reason why the plant should
droop for a touch, while, on the other hand, it responds in
the same manner to other kinds of stimuli, — a scorch of flame,
a strongly-focussed light, a trifle of acid — -to which there can
be no question of adjustment. The leaves have, however,

nr^

-Four successive stuges in
the thigmotropic curling of a tendril
around a support. (Copied, sim-
plified, from a wall-chart by Lau-
rent and Errera.)

244

The Living Plant

yet one other marked response, and that most important, be-
cause, as is probable, it explains the original adaptation, —
viz. a marked sleep movement just like those which we have
noticed already under phototropism. The motor-mechanism
underlying the droop of the leaves of the Sensitive Plant is a

Fig. 86. — Two Sensitive Plants, of which the one on the right was struck a sharp blow

just before the photograph was taken.

particularly efficient example of the hydraulic type already men-
tioned; and probably it is so highly perfected and delicately-
balanced that although developed originally in connection with
sleep movements, it can now be set ofT, so to speak, by various
other stimuli, such as touch, — ^precisely, for example, as a cannon
can be fired by a lighted match, an electric current, some chemi-
cals, or a sharp blow. The sensitiveness of the Sensitive Plant to
touch is upon this explanation accidental ; and there are probably
yet other examples of such accidental stimulation in other phases

Power to Adjust Parts to Surroundings

245

of irritability. Indeed, in the very highly complicated and un-
stable organization of the plant, it must often happen that the
motor or growth mechanisms are set olT, quite accidentally,
by various wholly unrelated stimuli. Such is undoubtedly the
nature of many of the ''mechanical responses," which by some
recent writers have been made the basis of all plant activities,
development and evolution, quite regardless of the innumerable
other elements and conditions entering into the constitution of
organisms.

A good many additional cases of thigmotropic irritability are
known. Thus, the leaves of some Insectivorous Plants close upon

Fig. 87. — Corn seedlings, showing the uniformity of position assumed by the growing roots
and stems, respectively, from very diversely placed seeds.

flies that alight upon them, — quickly in the Venus Fly-trap, and
slowly in Sundew. Some stamens when touched by insects, move
up in such a way as to dust those visitors thoroughly with pollen,
thus aiding in the utilization of insects for cross-pollination of
flowers, of which the importance will later become apparent to
the reader. In these and some analogous cases, the advantage,
mechanism, and method of stimulation are all more or less well
understood.

Geotropism. — When seeds fall to earth, or are placed in the
ground by a gardener, they come to rest in the most diverse
positions, with their embryonic roots and stems pointing at any
and all angles. Nevertheless, as they germinate, the young
roots, with a singular unanimity, turn downwards and the stems
upwards. The same thing can be shown very clearly by ex-

246

The Living Plant

periment, for if a number of large seeds, such as Windsor Beans,
or Corn, be fixed in the most diverse possible positions (figure 87),
the new stems and roots will grow themselves round into the
up-and-down directions respectively. Furthermore, the side

roots as they come out, and side
V n*^ / branches as well, assume and hold

for a time a definite angle to the
same up-and-down line. That the
positions of these parts are taken
with reference to the up-and-down
line, and not simply in relation
to the main root and stem, is
proven by a very conclusive ex-
periment; for if the young plants,
when their parts are well formed,
are tipped over at an angle, or up-

FiG. 88.— This Bean seedling was grown side down aS shoWU by OUr figure
for a time in this position; then it was ,„ oo\ji ii rj^i ^

inverted, and the new growth is reprc- (figure 88), then all ot the parts

?redr„itLt\°:siiiriL:rd; grow as quicUy as they can into

Still further growth. The direction of their former dlrectious. A case

growth is obviously geotropic, not

relative to the main root. (Copied, of aUalogOUS SOrt is found alsO

reduced, from Sachs' Lectures.) • tvt i i ^

m Nature, where evergreen trees
that grow on irregular steep hillsides show no relation what-
ever to the slope of the ground, but grow as stiffly upright,
and with branches as truly horizontal, as if the ground were
quite level. These simple illustrations are typical of a well-
nigh universal fact about plants, — that they send their first
roots down and their first stems up, and their side roots and
side stems out at definite angles to the up-and-down direc-
tion, regardless of the conditions under which they originate.
This fact is fundamental in the economy of vegetation, for it
helps to explain the way in which large plants can guide their
growth into upright positions, and hold themselves therein,
and how they can spread out their branches at such definite

Power to Adjust Parts to Surroundings 247

angles as to give to these plants their characteristic outlines.
Furthermore it also explains how stems can so readily recover
their natural positions when the plants are over-turned, whether
by accident, or by intention in experiment.

We must next turn attention to this crucial matter of the
up-and-down line. Now there is in this world only a single
determinant thereof, and that is the attraction of gravitation,
which forever is drawing all objects towards the center of the
earth. Gravitation, therefore, would seem to be the stimulus
used by the plant in assuming the positions we are considering.
In other words, the parts of the plant are geotropic; — and all evi-
dence confirms this conclusion.

The wide use of gravitation as a stimulus raises at once the
question as to the physiological value of gravitation to the plant.
In itself, however, it has no value, so far as anyone has been able
to discover. The plant has no object at all in sending roots
downward and shoots upward merely to have them down and
up; but it happens that down is the direction of moisture and
minerals, which roots need, and up is the direction of light, which
shoots need. No doubt those parts could be guided in the need-
ful directions by their hydrotropism and phototropism respect-
ively, but gravitation has this advantage over moisture and
light as a stimulus, that, while happening to act in the suitable
direction, it is present unvaryingly at all times, whereas light
and moisture are most variable in quantity, and sometimes-
absent altogether. This is especially true of light, which is missing
at night when gro^\i;h is most active and the guiding stimulus
most needed. Gravitation, therefore, is neither a direct, nor a
foster stimulus, like those we have already considered, but a
substitute stimulus, adopted by the plant in place of other
stimuli because it acts better than they. The use of the compass
has just the same advantage over observation of the sun and the
stars, which would also take the sailor to his port ; for the compass
is constant in its action, while the sun and the stars not only

248 The Living Plant

vary in direction all through the twenty-four hours, but often-
times are obscured altogether. Moreover, this principle of sub-
stitution stimuli is often important in connection with the de-
velopment of structures, for it helps to explain how an organ or
other feature can form in advance of perception of the stimulus
to which it is later to react, — e. g. the formation of the eye before
birth in animals, and of chlorophyll in the embryos of plants.

The way in which the gravitation stimulus is perceived by the
plant seems clear. Gravitation draws the heavier contents of
the cells, especially the starch grains, down to the bottom of the
cell, where their weight presses hard on the sensitive protoplasm
and produces a condition of strain different from anything in the
upper part of the cell; and this difference establishes the line of
direction. Then the responding mechanism is so set that main
roots are sent growing towards this pressure, main stems away
from it, and side parts across it, precisely as in other typical
responses. Geotropism, by the way, is a perfect illustration of
the fact that a stimulus acts merely as a guide, and not as a
physical aid, to responses; for while gravitation might be sup-
posed to help pull roots downward, obviously it cannot be
imagined to help push stems upward or to drive side parts out
crossways.

Thus much for the geotropism of stems and roots; what of
leaves, flowers and fruits? As to leaves, their geotropism is
usually disguised by their stronger phototropism ; but that they are
geotropic is shown by the vertical or horizontal positions they
assume when kept in dark rooms. We see another illustration
thereof, as I take it, in Nature, in some of the broad-leaved
shrubs which grow in the shade of the forest; here the diffused
light is so evenly distributed that it exerts no one-sided stimulus,
and the leaves are left free to assume their geotropic position,
which is strildngly horizontal. As to flowers, they also, for the
most part, are definitely geotropic. Thus, if one selects a long
terminal cluster of unopened irregular flowers, such as Larkspur

Power to Adjust Parts to Surroundings

249

or Snapdragon, bends it over and fastens it down at the tip, as
shown by our figure (figure 89), then each of the blossoms, as it
opens, turns over individually to the very same position it would

Fiu. 89. — Flower shoots of Larkspur, the curved one of which was bent over and fastened
a few days before this picture was taken. Note the uniform geotropic positions as-
sumed by both buds and flowers.

have had in the vertical cluster. The position of each separate
flower is here established geotropically, and for a very good rea-
son, — viz., these irregular flowers, as our later chapter on the

250 The Living Plant

subject will show, are specialized for cross-pollination in a way
which makes the lowermost petals alighting places for insects;
and therefore these petals must be kept horizontal. For the
same reason the long tubes of Daffodils are geotropically hori-
zontal, as one can prove by fastening the young flower-stems in
horizontal positions; and there are other cases without number.
As to fruits, they are mostly indifferent geotropically, but a few,
e. g. Cyclamens and Pea-nuts, use gravitation as a guide as they
bury their seeds in the earth.

So many and interesting are the manifestations of geotropism
in special cases that I must take room for a few more examples.
Trailing vines, whose main stems rest flat on the ground, like the
Periwinkle, Twin-flower, and Ground Pine, and perennials with
horizontal stems just beneath it, like Solomon's Seal, keep these
positions by virtue of the fact that their main stems have not the
usual main-stem geotropism, which is upright, but the trans-
verse kind characteristic of side-branches; twining plants are
kept encircling a vertical support under guidance of a lateral
geotropism, and this is what prevents them from twining around
horizontal branches or supports which would not take them up
towards the light; the aerial roots of many tropical climbers, and
most tendrils, have likewise this lateral geotropism, which keeps
them swinging horizontally until they meet with a support; and
there are many other cases of which some may be identified by
the reader himself if he keeps observationally alert in his walks
abroad in field, garden, or forest.

Of all of the stimuli made use of by plants for guiding their
parts to positions of greatest advantage, gravitation is much the
most important. Plants are born with an hereditary tendency
to put forth their parts in a symmetrical manner, as can be
demonstrated experimentally by aid of the clinostat; but they
depend upon geotropism to guide those parts into the suitable
positions, and thus to realize the ultimate shape of the plant.
And this is the case no matter what the form of the plant may be,

Power to Adjust Parts to Surroundings 251

whether a symmetrical cone of horizontally-spread branches ra-
diating from a central main stem, as in the Firs or the Spruces,
or a great urn of up-and-outcurved branches, as in the Maples
and Elms, or in any of the intermediate shapes; and the reader
should learn to visualize all of the main trunks and branches as
thus developing in touch with gravitation and largely under its
guidance. This applies, however, only to the main structures;
the smaller branches and most of the minor parts are more or
less controlled by other kinds of stimuli which determine the
final details of form; and this is especially the case with roots.
The fact that geotropism is thus ever tending to hold the plant
to a certain upright symmetrical form explains why any one-
sided turning in response to other stimuli, is of Imiited amount,
and why the plant always tends to recover its former upright and
symmetrical position in case it is disturbed.

Some minor tropisms. — These include, — thermotropism, a turn-
ing towards warmth, rather rare: traumatropism, the turning
of roots away from an external irritation or injury: rheotropism,
a turning against a water current, which, however, has been
shown to be only a special phase of thigmotropism : electro-
tropism, a certain adjustment to mild electric currents; and some
others of lesser importance. The case of rheotropism, by the
way, illustrates a confusion of stimuli, the root apparently mistak-
ing the pressure of the flowing water for that of some hard ob-
ject in the soil. The case of electrotropism, involving response
to an influence to which the plant is never subjected in nature
and to which it cannot have become adaptively sensitive, illus-
trates the same thing, or else, perhaps, an accidental release of the
motor-mechanism after the manner already described for the
Sensitive Plant. And the occasional responses found in plants
to other stimuli new to them (e. g. to X-rays, radium emana-
tions), are hkewise due without doubt to confusion of stimuli, or
accidental release.

Thus far we have considered for the most part only cases in

252 The Living Plant

which the stimuH act from a single direction, and therefore evoke
only one-sided responses. But some of the very same stimuli
may act in a diffused or all-around manner, becoming impressed
on the sensitive protoplasm of the plant through a change in
intensity; and in such cases the responses are all-sided or sym-
metrical. Thus the sleep movements of leaves, already con-
sidered, are of this nature, being a response to the change in in-
tensity of the circumambient light; and the same thing occurs
with some flowers, which close at night or in very dark weather.
Other flowers, e. g Tulips, are affected in like manner by changes
of temperature, opening as the weather grows warmer, and clos-
ing as it becomes cooler; and some evergreen leaves, notably of
Rhododendrons, rise and fall in this way even in winter. Such
responses are distinguished from the ordinary sort in scientific
terminology by the termination, nasty (photonasty, thermonasty,
etc.) ; and we may note by the way, that the responses due to a
free-swimming movement, as in the case of the antherozoids of
Ferns already described, are distinguished by the termination
taxis (chemotaxis, phototaxis, etc.).

There are, furthermore, several other types of responses to
stimuli, some of them vastly important in connection with the
growth and development of plants. Thus, it has been claimed
that the strains set up by the swaying of stems back and forth,
whether in nature by winds, or in the laboratory under experi-
ment, serve as stimuli to the larger development of strengthen-
ing tissues in the places where the strains are most felt, thus pro-
ducing a needed enlargement at those places. It is perfectly
clear that the great knees which rise from the roots of the Bald
Cypress of the Southern Swamps and which probably are aerating
structures, are formed in response to the presence of water, for
they do not form at all when these trees grow in soil that is well-
drained. Other cases are known where the thickness of cell-walls,
the arrangement of tissues, the sizes of parts, and other structural
features are regulated by responses to well-known stinmli from

Power to Adjust Parts to Surroundings 253

the environment. Again, the chmbing roots of some Ivies, and
the sucking roots of some parasites, grow out at those places
where the stimulus of contact is felt, and therefore exactly at the
places where they can best serve their uses; and the places of
origin of even ordinary roots are largely controlled by the stimulus
of especially abundant moisture or minerals, which explains why
roots branch so profusely upon entering drains. Then there are
stimuli which start particular stages of growth. Thus it is a
stimulus given by some phase of fertilization which starts the
formation of the fruit in the higher plants. The advantage is
clear, since the fruit would be wasted, and its formation a useless
drain on the plant, if no fertile seed were produced; for the dis-
persal of the seed is the function for which the fruit exists. Stimuli
can also serve as signals to produce a cessation of growth, as in
the case of the leaves of the water-plants already considered;
and there are plenty of other cases w^here stimuli regulate growth
and development in various ways, the further consideration of
which we may postpone to the chapters which deal with those
subjects, where also we may consider the correlation and Unking
of stimuli, with their very important consequences.

There is one other phase of responsiveness to stimuli which we
must consider at this place. It is a familiar fact about organisms
that they have a certain power of adjusting themselves, or be-
coming toned, as it were, so as to work their best under the pre-
vailing conditions to which they are exposed; and when they are
thus working in full harmony with those conditions they are said
to be in tone. We have a familiar illustration thereof in our
human affairs in the way we become accustomed to certain pe-
culiarities of food, temperature, fresh air, occupations, etc., to
such a degree that we become uneasy when exposed to any others,
and hasten back with reUef to the congenial conditions. Thus,
most of us work our best at about 70° Fahrenheit and become
very uncomfortable when the temperature rises to above 90°,
though this is still much less than the natural heat of our bodies.

2 54 The Living Plant

Moreover this condition of tone is more or less alterable under
continuous action of new conditions, and such tonic adjustment
to new conditions is commonly called acclimatization. We do not
yet know much as to the nature of the process, but there seems
little doubt that it is chemical in its nature, and represents a
process of chemical adjustment to the external conditions acting
as stimuli. An important phase of the same process is found in
the formation in the animal body of those special chemical sub-
stances called collectively ''antibodies," which neutralize chemi-
cally the injurious substances formed in disease. Probably the
acquisition of tone and acclimatization are fundamentally similar
in principle, consisting in chemical alterations in the protoplasm
of such character that substances or features less efficient under
the prevailing conditions are replaced by others more efficient.
At least such seems to be the principle, though as to the details,
they are still with the future.

As one views the various adjustive structures produced in
response to external stimuli (such as the knees of the Bald Cypress
just mentioned, the thicker epidermis of plants in dry places,
and so forth), one cannot but ask how these may be distinguished
from adaptive structures produced in the course of evolution;
and whether, after all, the two may not be fundamentally the
same thing. As to the first point, one cannot distinguish adjustive
from adaptive structures by any evidence except the test of
heredity, for adjustive structures are produced anew in each
generation only in response to certain stimuli and are absent
when the stimuli are lacking, while adaptive structures are pro-
duced regularly every generation quite regardless of the presence
or absence of the given stimulus. The only thing that is hereditary
in irritable adjustments is the capacity to make them. We have
an analogy in the different methods whereby republics and
monarchies are provided with rulers, for while the president of a
republic is often indistinguishable in mode of life and other
characteristics from a monarch, and may even surpass one in

Power to Adjust Parts to Surroundings 255

power, he is chosen quite anew at regular intervals in adjustment
to the popular demands of the moment, only the method of elect-
ing him being permanent, or, so to speak hereditary; while the
monarch holds his office by heredity quite regardless of the
fluctuations of politics. As to our second question, whether in
the last analysis, the two may not be fundamentally the same,
adaptive structures being only permanently-fixed irritable ad-
justments, the view is attractive but as yet unproven, as we shall
further consider in the chapter on Evolution.

There remains one other important matter to mention in con-
nection with stimuli. The response to a stimulus, w^hile highly
efficient, is blindly invariable, and not alterable for particular
conditions. For example, if a wdnd-blown seed of an ordinary
plant were to lodge in a cleft of an overhanging ledge, it would
be an advantage for this plant to be able to reverse the usual
positions of roots and stem; yet we know it would send its stem
up, though only to die in the earth, and its root down, only to
perish in the air. In this invariability of particular responses,
and in many respects besides, these irritable responses of plants
agree with the reflex actions familiar in animals; and it is now
very clear that they are essentially the same. Furthermore, if
two or more stimuli act upon the same part of the plant at the
same tune, the result is simply the product of the effort of the
part to respond to them all. There is no sign of an attempt on
the side of the plant to correlate these stimuli, so to speak, and
to respond in a manner which would be best in the face of this
particular combination. In this respect animals have gone far
ahead of plants, for they have acquired that last-mentioned
power. Herein we have the chief feature which distinguishes
the higher animals from the higher plants, and also, I believe,
the origin of consciousness. Thus, out of one and the same
origin, plants ha^'e developed irritability, while animals have
developed reflex action, consciousness, and ultimately reason.

CHAPTER X

THE VARIOUS WAYS IN WHICH PLANTS RESIST THE
HOSTILE FORCES AROUND THEM

Protection

]Y the methods considered in the preceding chapters,
plants provide most effectively for their nutritive
needs, and also for advantageous adjustment to the
external conditions affecting the same. But they have
not thereby solved the whole problem of daily existence, for thej^
still have to reckon with the presence of a great many hostile
external conditions. Thus the winds, which in moderation do
no damage to plants and even may work them some benefit,
occasionally swell to great tempests possessing a power well-
nigh too great for resistance. Again, water, which is indispensa-
ble to plants in considerable quantity, becomes sometimes,
through drought, quite dangerously scant, or through floods
quite as dangerously plenty; while various parts and places of
the earth, — deserts on the one hand and swamps on the other —
though perfectly habitable by plants in all other respects, remain
permanently in one or the other of these undesirable conditions.
Further, light, which is likewise essential to plants, is in some
times and places too weak for efficiency, and in others so intense
that unprotected protoplasm can by no means endure it. And
again, the food supply manufactured by plants, while ordinarily
ample for both themselves and their hereditary dependents the
animals, is in some parts indispensable to the continuance of the
plants' activity, so that its destruction by animals would consti-
tute a serious menace. Finally, while endowed with indefinitely

256

How Plants Resist Hostile Forces Around Them 257

great powers of reproduction and growth, plants live in a world
already quite filled, and are therefore exposed to a competitive
struggle with one another, of which natural selection is the re-
morseless arbiter, and a survival of the fittest the inevitable
outcome. In a word, plants live in a world that is generally
friendly, but sometimes is hostile even to a mortal degree.
Against the hostile features of the environment they have had
to develop protective adaptations, some of which are extremely
conspicuous and play a large part in the determination of the
habits and aspects of plants. These protective adaptations, of
course, must co-exist and compromise with those physiological
adaptations in leaf, stem, and root, which we have already con-
sidered. The identification, separation, and definition of the
structures and features of plants which are protective is the task
that now lies before us.

To begin with, the protoplasm of plants is physically weak,
but secures an efficient first line of defense by the most obvious
of all methods, viz., through encasing each one of the soft-bodied
cells in a separate coating of armor, — the cell-wall. As the
reader will recall, from the description in the chapter on Proto-
plasm, the plant skeleton is constructed from the united wall-
mass of the cells; and it thus combines both support and seclu-
sion for the protoplasm in its cavities, very much as the walls of
our many-storied houses do for us. Such a combination of skele-
ton and protecting wall is permitted only by the sedentary habits
of plants, and stands in very great contrast with animals, w^hose
locomotive habit requires a jointed skeleton, moved by masses
of contractile, and therefore naked (muscular) cells.

Turning now in detail to the various hostile influences against
w^hich plants need protective adaptations, the most obvious is
that of the winds, which, however, become a danger only as they
rise into gales. Then, as all will agree who have seen a great
tree tossed in the grasp of a tempest, protection is found in the
slenderness and elasticity of the branches, which yield in great

258 The Living Plant

curves that permit the smaller to stream with the wind in the
lee of the larger, where they can tug at their anchorage in safety.
Doubtless in a windless world the plant skeleton would be rigid
and brittle, probably to such a degree that an ordinary one of
our storms would shatter it to fragments, much as at times they
do now with the ice of a silver thaw. As to older stems, we have
learned already how it is with them; their hollow-column princi-
ple of construction holds them up against great lateral strains.
Furthermore, a good many kinds of stems exhibit a special
strengthening arrangement at the place of maximum weakness,
which lies at the contact of stem and root, where the leverage
exerted by wind on the top is most felt. Thus, some kinds of
plants, like the Corn, develop prop roots that extend from the stem
above ground diagonally down to the earth, while many tall
trees possess buttress-like thickenings between the stem and the
principal roots, as appears very well in some of our Elm trees,
and especially in some of the tropical giants, where they attain a
good many feet of height and breadth, though only a few inches
of thickness. As to leaves, whose broad faces would present
much exposure to wind, their slender-elastic petioles permit them
to yield, and to swing like so many weather vanes, presenting
only edges to the blast, w^hile they can also sway accommoda-
tingly to every irregular gust. In this adaptation, indeed, we
find one of the principal functions of the petiole, as follows from
a discovery made by one of my own students, — w^ho found that
the petioles from the exposed part of a tree average longer than
those from more sheltered situations, although the leaves are
smaller in the former locations than the latter.

But it is not alone on the individual tree that the sizes of leaves
are inversely proportional to the degree of their exposure to
winds, for it is true in general of plants as a whole. Do not the
largest leaves that are known to the reader grow in the shelter
of undergrowth? And if at first sight it appears that the gigantic
fronds of Palms and Tree Ferns contradict this view, a second

How Plants Resist Hostile Forces Around Them 259

thought is enough to confirm it; for, although morphologic-
ally single leaves, they are cleft to a great many small leaflets,
each of which acts physiologically as a single leaf. This division,
or ''compounding" (as it is called scientifically), of leaves in such
plants appears clearly to constitute a protective adaptation
against the tearing action of winds; and I believe the same factor
is the principal one in determining the compounding of leaves in
general, though sometmies the compound condition, as in our
undergrowth Ferns, means rather a persistence of an ancestral
condition than anything of immediate importance. Nor is one's
natural thought at this point, that the sizes of leaves are de-
pendent on their thickness, correct. The thickness of leaves is
determined by the depth to which sunlight can penetrate green
tissues without losing all of its photosynthetic power; and hence
it is approximately the same in all leaves exposed to the sun in
the same climate, with a trend towards more thickness in extra-
bright places, and thinness in shade. Undoubtedly the whole
tendency of wind action is to produce an adaptive lessening in
size, which is directly antagonistic to the tendency of photo-
synthesis to produce a larger spread of surface; and the resultant
between the action of these two factors, modified it is true by
certain other minor influences, makes leaves the sizes they are.
This explains why our common deciduous trees of similar habit,
our Oaks, Elms, Maples, and Chestnuts, possess leaves of much
the same size, or at least of the same order of magnitude. That
size represents the equilibrium between the contesting photo-
synthetic and wind factors acting on leaves of standard thick-
ness growing in similar situations.

Another kind of strain to which plants are exposed is the
weight of the winter's snow and ice. This danger is greater, of
course, for evergreen than deciduous trees, but against it the
conical shape characteristic of evergreens provides a manifest
protection. This follows from the fact that only the ends of the
branches are exposed to the falling load, while their slender forms

260 The Living Plant

and horizontal positions permit them to yield greatly without
damage, and thereby even to shed their burdens (figures 14, 15).
No doubt the protective adaptation involved in the conical shape
has operated along with the photosynthetic considerations
earlier mentioned (page 56) to fix this form for evergreen trees,
which in general are commonest in the snowiest regions; while,
correlatively, the danger involved in the accumulation of snow
upon the leaves borne by upwardly springing branches, like those
of most of our deciduous trees, is doubtless one factor in making
such trees drop their leaves in the winter.

This mention of the shapes of trees makes this a suitable place
to consider their modes of resistance to certain other strains.
The stems of trees have not only to carry great masses of foliage
high up in the air, but also to support it out laterally for con-
siderable distances, and all in opposition to a heavy downward
strain from gravitation. In some trees, conspicuously those of
the cone-shaped evergreen type (figures 14, 15), the branches
spread horizontally from a central upright trunk; but this ar-
rangement, however advantageous from other points of view,
is mechanically the worst for resistance to gravitational strains,
and is only possible with comparatively slender branches and
special methods of strengthening the same. Thus, bracket-like
swellings often occur in the angles between such branches and
stems, while extra material is commonly placed all along the
under side of the branch, making it excentric in cross section. In
such cases the extra material acts much like a long stiff spring
bent upward just enough to counterbalance the weight of the
branch, whose horizontal position is maintained by the counter-
action of the two forces, as is shown quite conclusively by the
very great bending of such branches when spring and weight are
allowed to act together by the inversion of the tree (figure 90).
But a cone-shape of trees is uncommon in comparison with that
in which great branches, often well-nigh as large as the trunk, rise
up therefrom at sharp angles, swing gradually outward to near

How Plants Resist Hostile Forces Around Them 261

the young parts, and then curve vertically upwards again to
bear the new leaves, — the whole stem melting away, as it were,
to a spray of such branches. This is the form prevailing in most
of our deciduous trees, as the reader can see for himself by
examining the tracery of
Oaks, Maples, Elms, or
Chestnuts when projected
against the winter sky.
Such a sigmoid form of
the branches affords them
the best possible anchor-
age in the trunk with the ^r^ on t^ • r u . u r .u

_ _ riG. 90. — Tracings from photographs of the same

minimum of leverage on Balsam Fir, in the natural position and inverted,
. . , . illustrating a point explained in the text.

their heaviest parts, while

providing enough spread and a vertical tip for support of the
foliage. If, now, we apply this sigmoid mechanical modification
to the theoretical form of our photosynthetic tree represented in
figure 7, we obtain the form illustrated herewith (figure 91).
This theoretical form, modified by some minor and largely acci-
dental circumstances, is very nearly realized in the noble Oak
shown in figure 8, and by many of our common deciduous trees.
The chief difference consists only in this, that whereas the
theoretical tree is hemispherical, the actual kinds are often
ovoid, cylindrical, or top-shaped, — in obvious adaptation, as I
think, to a diminution of the excessive gravitational leverage
that accompanies too extensive a spread.

We pass now to a second of the greater environmental in-
fluences hostile to plants, namely excessive light. The reader
does not need to be told that light, and in large quantity, is in-
dispensable to plants for their photosynthetic work; but it is an
important physical fact that the amount they can thus use has a
limit, above which any increase is not only useless but positively
harmful. And that limit is often surpassed in the open sunlight
of summer. However, not all of the mani-colored rays that make

262 The Living Plant

up the white Hght are thus injurious, but only the blue-violet,
and then only when received in great force; for these very same
rays, like some of the red, are the ones that are useful in photo-
synthesis. They produce their bad effects, as it seems, through

their peculiar power of promoting
chemical changes, whereby they in-
duce in the complicated living
protoplasm illegitimate reactions,
as it were, which interrupt the or-
derly series of chemical processes
in which the very life of the
protoplasm consists. However,
whether this, be the correct ex-

FiG. 91. — The theoretical form of a de- ■• ,• j. -i. • iU 1 „

ciduous tree, consisting of the photo- plauation or uot, it IS nevertheless

synthetic groundwork shown by fig- ^ f^^^ ^J^^^ strOUg UnSCreeUCd light,
ure 7 modified in adaptation to the '^ o /

mechanical support of the weight of because of its blue rays, is always

the foliage. ... , i- • , 1 rr-u-

injurious to nving protoplasm. 1 ms
is the reason why bright light is fatal to disease germs, or Bacteria,
and explains the basis of the hygienic value of sunlight in the
home; while blue light is used with success for the very same reason
in the cure of some diseases of the skin. Now because the red
rays of the sunlight are not only harmless but also useful, even in
fullest intensity, while the blue rays are harmful only when in-
tense, but otherwise useful, the problem of adaptive protection
against too intense light resolves itself into one of tempering the
blue rays without affecting the others. This can be perfectly
accomplished through use of a screen which permits red rays to
pass while checking the blue, and such a screen is of necessity
red. It is upon precisely this principle that photographers use a
ruby glass screen in developing their plates, for this color cuts
off the blue rays, which are those that took the picture originally
and therefore would spoil it in development, while admitting the
red rays which are not only harmless to the plate but useful in
showing the photographer what he is doing; — only the photog-

How Plants Resist Hostile Forces Around Them 263

rapher needs a total exclusion of blue rays and therefore a screen
of much deeper color than the plant requires for only a partial
exclusion of those rays. Such is most likely the adaptive signifi-
cance of that charming red blush which mantles the face of the
fresh vegetation of spring, for, without some such protection,
the young leaves and stems that push out of the buds before
the formation of the chlorophyll, which constitutes later a suffi-
cient though incidental protection, would expose their unshielded
protoplasm to the full force of the bright hght then pre^•ailing.
And there are some students who find a similar function in the
redness of leaves in the autumn, believing that it shields the
protoplasm after the chlorophyll has faded away; though here, as
I believe and have argued in the second chapter, there is little
warrant in the evidence. Certain it is that there are cases, e. g.,
the red under sides of leaves of some tropical undergrowth
plants, where the explanation must be totally different. But
the light-screen function explains very well the reddish or brown-
ish colors of spores which must float long-time in the ah exposed
to the brightest of light, and perhaps it explains also the red
color assumed by roots and underground stems when these be-
come exposed to the light, though here the color may represent
simply a chemical incident.

A second method of light protection may consist in those hairy
or woolly coatings, or even in the waxy or resinous layers, which
overspread a good many plants of open bright places, resulting
in a distinctive aspect of grayness found especially often in plants
of the deserts. Such covers must act to reflect and refract the
light, without, of course, any distinction of rays, to an extent suf-
ficient to weaken very greatly its power to penetrate the tissues.

The third of the methods of light protection, bound up, how-
ever, with protection against excessive transpiration soon to be
noted, is more important. It consists in the assumption by the
green tissues of a vertical position, whereby they present only a
thin edge, or at least a low angle of incidence, to the mid-day

264 The Living Plant

brightness of the sun, with the full exposure to its less intense
action at morning and evening. Such a vertical position of the
green surface is coimnon in plants of open bright places,— in
some, notably certain clover-like kinds, as a temporary and
irritably-adjustable position of the leaflets (figure 78), but in
others as a permanently vertical arrangement of the leaves. In
the most perfect of the latter cases, all the leaf-blades present
their faces to the east and the west, thus bringing their edges
north and south; and such is the real meaning, and the reason
for the name, of the Compass Plants, of which the most perfect
and famous example occurs on our own western prairies. In
some kinds, instead of the leaf-blade it is the petiole which is
flattened and set vertically, the blade being suppressed, as in
most of the Australian Acacias (figure 21). In others, advantage
is taken of the naturally vertical position of the stem, the function
of foliage being transferred thereto from the leaves which are
simultaneously reduced or abandoned. This is the case with the
Cactuses and innumerable other plants of the deserts, which
sometimes acquire additional vertical green surface by the de-
velopment of longitudinal ribs. The readiness with which the
green tissue can be developed in one part of the plant as well as
another helps, by the way, to explain some of the curious mor-
phological overturnings represented by plants like the Butcher's
Broom (figure 23), or, still better, the familiar Smilax of the
florists, in which the apparent leaves are in reality branches,
while the actual leaves are no more than tiny scales just beneath
them. It is easy to understand that if plants of the desert have
once transferred their chlorophyll to their stems, simultaneously
suppressing or abandoning their leaves, and then a change of
clmiate, or migration to a moister region, should require a larger
spread of green surface, this would more easily and naturally be
secured through a further flattening of the stems or branches than
through a restoration of the lost leaves; and with time such
branches would become more and more leaf-like even to the

How Plants Resist Hostile Forces Around Them 265

extreme degree represented by the Smilax. This very over-
turning does actually occur in the Cactus family, in which,
happily, all of the steps without exception are represented by still
living forms. It is the relics, indeed, of such devious windings in
the past history of plants which give us our principal morpho-
logical puzzles.

This consideration of light naturally suggests the question as
to heat. This, likewise, is indispensable to plants, since it supplies
a condition requisite for some chemical reactions and physical
movements, notably diffusion. Heat also, like light, is more
and more useful up to a certain intensity (about that of blood
heat in ourselves) , beyond which any increase is not only without
benefit, but soon becomes an injury. Thus, plants in the fields in
summer by no means thrive better the hotter it gets. It is doubt-
ful, however, whether the natural heat of the sun ever attains
an intensity dangerous to plants, and even if it does, the same
structural adaptations, especially refractive coverings and a
vertical position of green tissues, protective against light, would
be equally effective against heat. And there is perhaps yet
another method of protection against both, but especially heat,
namely transpiration, which dissipates through evaporation the
too intens