woe

ALBERT RK... Nas
LIBRARY
AT
CORNELL UNIVERSITY

QK

wl

The life of the plant,

19

Cornell University

The original of this book is in
the Cornell University Library.

There are no known copyright restrictions in
the United States on the use of the text.

http://www.archive.org/details/cu31924002064107

THE LIFE OF THE PLANT

THE LIFE
OF THE PLANT

BY G Ar TIMIRIAZERP YE

PROFESSOR EMERITUS MOSCOW UNIVERSITY
CORRESPONDING MEMBER OF THE ACADEMY OF SCIENCE, ST. PETERSBURG
LL.D. GLASGOW ; SC.D. CAMBRIDGE
D. ES, SC. GENEVA
F.MLR.S.

Translated from the

Revised and corrected Seventh Russian Edition by

MISS ANNA CHEREMETEFF

WITH ILLUSTRATIONS

LONGMANS, GREEN, AND CO.
39 PATERNOSTER ROW, LONDON
NEW YORK, BOMBAY, AND CALCUTTA
IQ1t2

on

PREFACE TO THE ENGLISH TRANSLATION

A GLANCE at the preface to the first Russian edition
will, I hope, convince the English reader that I was fully
aware of the exceptional difficulties of the task I had
undertaken. Seven editions in the course of thirty-five
years have in a certain degree contributed to dispel my
fears, but on being asked to give my assent to this
English translation I experienced afresh the same feeling
of diffidence at the prospect of addressing a new audi-
ence. Just at that moment I came across that admirable
article by Professor Armstrong on The Future of Science
in our Schools.1 1 was glad to see that not only in its
general tendency, but even in the choice of matter and
in the order of exposition, my book seemed to answer
the present requirements of English schools as formu-
lated by so eminent an authority as Professor Armstrong.
The inspection of the table of contents of this book will
suffice to show that even in details it agrees with the
short programme proposed by Professor Armstrong
(l. c. p. 438, 439) ; both begin with the analysis of flour
and culminate in an exposition of Darwin’s theory.

‘ The main thing we ought to teach our youth is to see
something.’ This maxim of John Ruskin, chosen by
Professor Armstrong as a heading to his article, has ever
been present to the author of this book. A pair of healthy
eyes and occasionally a good lens is all that is required

1 Presidential Address to the Association of Public School Science
Masters, delivered January 13th, 1910. Science Progress, January 1910,

Pp. 417.

¥

vi THE LIFE OF THE PLANT

to see the external forms of our common plants. But
how different is the case when we are expected to show
even the commonest phenomena of plant life, for the
most part invisible, and in so many respects quite dif-
ferent from the familiar manifestations of animal life!—
think only of respiration without inspiring and expiring,
or of feeding on air. At every step we require more or
less complicated, or, what is highly desirable but not
so easily attainable, the simplest possible apparatus.?
Moreover, all the results obtained must be considered
from the general point of view of those two sister (or
rather mother) sciences—physics and chemistry. In
this respect I have consistently complied with Professor
Armstrong’s precept, to which I readily subscribe:
‘ Whatever we teach in our schools, chemistry must not
be neglected ; it is the science of life, life being but a
succession of chemical changes : it is therefore the basis
of physiology.’

I fully expect that not a few of my botanical colleagues
may consider some passages of chapter vii. out of date ;
but I must frankly confess I consider a return in a certain
sense to the sound notions of Andrew Knight or
A. P. De Candolle, of Dutrochet or Hofmeister may
prove to be a desirable corrective to the alarming spread
of the ‘ Reizphysiologie’ with its morbid outgrowth
of ‘ Neovitalism’ and ‘ Phyto-psychology,’ and _ their
natural corollary, anti-Darwinism. Nowadays in our
pursuit after the quasi-nervous stimuli we have nearly
lost out of sight the object stimulated and the mode of
action of the external agents. No less an authority
than Sir Joseph Thomson has recently warned us that

1 I may perhaps be allowed to add that I believe I was the first to
introduce lecture experiments into my annual courses on plant physiology,
which began in 1870. At least, at a much later date, Professor Julius
Sachs, the head of the German school of physiologists, as I was told in
1877 by one of his assistants, never introduced any ‘ Vorlesungsversuche’
into his lectures.

PREFACE Vii

even in the higher realms of science ‘something more
grossly mechanical, a model, is felt by many to be more
suggestive and manageable, and for them a more
powerful instrument of research.’ ... I really think
that some such models as those formerly proposed by
De Candolle for the heliotropic effect or by Hofmeister
for the elucidation of geotropism, adapted of course to
the growing exigencies of the time, might bring back
the study of the mechanism of growth to a more promis-
ing field of research.

That the ideas I venture to advocate are not so
utterly out of date may be inferred from the fact that
similar ideas have been recently advanced by a repre-
sentative of a much younger generation of botanists,
by the regretted Professor Barnes. For my part, I
am as firmly convinced as I was forty years ago that the
‘mechanistic conception’ and Darwinism have been
bequeathed by the ‘wonderful century’ to the still
infant science of plant physiology as the two sure guides
for its further evolution, and I may adduce in support
of this opinion the eloquent testimony of the late
Professor Boltzmann: ‘If I were asked, how will
our century be called by the coming generations—the
century of iron, of steam, or of electricity >—I would reply,
in all earnest, it will be called the century of the mechani-
cal interpretation of nature, the century of Darwin.’ ?

It is impossible for me to bring to a close this pre-
fatory notice without expressing my best thanks to

1 «Tn fact there is an inclination after endowing protoplasm with such
properties as ‘‘irritability,”’ ‘automaticity,’ and “self-regulation,” to be
satisfied with these words and there make an end.’—‘I propose only to
present some suggestions on the matter of these phenomena as a con-
tribution towards a mechanistic conception of plant.’ . . The Nature of
Physiological Response. The Botanical Gazette, New York, 1910, pp. 322-
323.

2 Das zweite Hauptgesetz der mechanischen Warmetheorie, 1886. Populare
Schriften, von Professor Ludwig Boltzmann, 1905, p. 28.

Vili THE LIFE OF THE PLANT

Miss Chéréméteff for having undertaken and success-
fully completed this translation.

As a foreigner I am, of course, not entitled to judge
of the literary merits of the translation, but on the
other hand, having carefully read through the whole of
the proofs of this volume, I am bound to bear witness
to the many and considerable difficulties overcome by
the translator.

My warmest thanks are also due to my colleagues, Pro-
fessor Seward of Cambridge and Professor Vinogradoff
of Oxford (lately of Moscow), for their friendly help
with regard to the publication of the book.

C. TIMIRIAZEFF.

Moscow, January 1912.

PREFACE TO THE FIRST RUSSIAN EDITION

For about a quarter of a century there has been a great
gap in the botanical literature of the west of Europe,
as also of Russia, since there has been no book that
might inform the public in a popular way of the present
state of vegetable physiology. I decide to publish
these lectures in the hope, were it only in slight measure,
of meeting thisend. In submitting this book to the judg-
ment of the public, I fully realise the difficulties of the
undertaking. Every popular exposition, precisely be-
cause of its popular nature, deprives the author of the
possibility of expressing the whole truth, i.e. of criticising
from all sides the facts he brings forward ; and, more-
over, it obliges him not to say anything but the truth, a
requirement that can scarcely be complied with in a
science which is far even yet from being firmly estab-
lished. Hence it is clear that a popular exposition of
such a science as the physiology of plants presents many
more difficulties than a similar exposition, for instance,
of chemistry or physics.

The second requirement for such a book is that the
author should give up for a while his usual point of
view, that of a specialist; and should, so to speak, step
back a little in order to see what science looks like at a
distance. The main condition for success consists in
the selection of such a point of view as will be close
enough to allow of the observation of main details, and
yet not too close to spoil by detail the impression of the
whole. It is not for me to decide whether I have been
fortunate enough to find such a point or not.

ix

4 THE LIFE OF THE PLANT

The position of an author of a popular book differs
also from that of an author of a special treatise, in that
he is deprived of any opportunity for self-justification or
defence. He surrenders himself defenceless into the
hands of his judges. The reader appears as his first
and last court of appeal. A specialist may consider
his exposition to be conscientious, to have overcome
considerable difficulties; but if his work so much as
displeases the reader, it will fail of its aim and be
therefore doomed.

I hope that I may find as kindly critics among my
readers as I had the privilege of finding in my audiences.
They have appreciated the difficulty of my task, and have
indulgently criticised its fulfilment.

Moscow, 30¢h March 1878.
1 These lectures were delivered during the winter of 1876 in Moscow.

‘The Plant as a Source of Energy,’ placed in the appendix, was
delivered at St. Petersburg in the spring of 1875.

TRANSLATOR’S PREFACE

IN presenting this book to the English public my
sincere thanks are due to my friend Miss E. I. M.
Boyd, M.A., who kindly undertook the revision
of the MS., and has shown the closest interest in
the translation and its publication.

I should like also to acknowledge a debt of
gratitude to Professor Seward of Cambridge, and
Professor Vinogradoff of Oxford for their kind
help in regard to the publication of the book.

My best thanks are due to Mr. D. Thoday,
of Trinity College, Cambridge, Lecturer in Plant
Physiology in the University of Manchester, for
his valuable assistance in the matter of scientific

revision and the correction of proofs.

A. CHEREMETEFF.

Sr. PETERSBURG,
November 191.

CONTENTS

I

SCIENCE AND SOCIETY. EXTERNAL AND INTERNAL
STRUCTURE OF THE PLANT

PAGE
The general public’s meagre knowledge of botany. Two old-
fashioned types of botanists. The contemporary trend of
science. Morphology and physiology; form and life. Two
reasons for the comparative backwardness of botany: the
logical and the practical reason. Art and science. Agriculture
and the physiology of the plant. Science and the general

public in mutual relationship.

Survey of the external organs of a flowering plant. Meta-
morphosis. Spore-bearing plants—of earlier date and simpler
in structure than seed-plants. A spore—a cell. The cell—
the foundation and beginning of every organism. These facts
in relation to the problem of the origin of organisms. Treat-

ment of subject ; i : if

II

THE CELL

Law of the conservation of matter. Origin of plant-substance
—in the external environment. Elements and compounds
entering into the composition of plants. Three fundamental
groups of chemical compounds: albuminoids, carbohydrates,
fats. Chemical and microscopic investigation of the plant.
Absorption of nutrient substances by the plant. General
conception of the diffusion of matter. Diffusion of gases and
liquids. Colloids and crystalloids. Transformation of substances
in the cell explains their absorption. Fundamental mechanism
of the nutrition of the cell . 3 : : ‘ 35

III

THE SEED

Structure of the seed and external phenomena of germination.
Three conditions of germination: water, air, heat. Mechanical
function of water. Chemical function of water. Ferments.
Diastase. Pepsin. Insectivorous plants. Independence of
the parts of the embryo. Artificial nutrition of the embryo.
Mechanism for the translocation of the nutrient substances in
the plant.

The seed in relation to air; evolution of carbonic acid,
Xx

xiv THE LIFE OF THE PLANT

absorption of oxygen—respiration. Loss in weight and rise of
temperature as a result of respiration. Importance of the
surrounding temperature. Temperatures: maxima, minima, and
optima. Effect of the age of the seed on its germination.
Longevity of seeds. General characteristics of the period of
germination. Division of labour between different organs of
the plant, already apparent in the lowest plants é 59

PAGE

IV

THE ROOT

Function of the root. Composition of the soil. Method for
defining the necessary nutrient substances. Artificial cultures.
Cultures without organic matter. Watercultures. Importance
of nitrogen, potassium, iron, silicon. The necessary nutrient
substances absorbed by the root. Nutrient substances in the
soil for immediate use and in reserve. Absorbent properties
of the soil. Importance of saltpetre in the soil. Assimilation
of nitrogen by leguminous plants. Form in which nutrient
substances are found in the soil. ;

Structure of the root. Its striking elongation and the
purpose of this character. The root in relation to liquid and
solid substances. General mechanism for the absorption of
nutrient substances by the root . a : . 88

Vv

THE LEAF

Function of the leaf. The nutrient substance assimilated by the
leaf. The leaf in relation to carbonic acid. Structure of the
leaf. Evolution of oxygen. Decomposition of carbonic acid
in water. Obviousness of the experiment. Decomposition of
carbonic acid in an artificial mixture of gases and in the
atmospheric air. Formation of a carbohydrate (starch) in the
chloroplast.

The decomposition of carbonic acid from the point of view
of the transformation of energy. Nutrition of the plant at the
expense of organic matter. Fungi and parasites. Physio-
logical functions of the leaf 7 * . g . «m9

VI

THE STEM

Function of the stem, secondary as a medium between the leaf and
the root. Forms of stems. Internal structure. Cell, fibre
and vessel. Three types of tissue: nutritive, mechanical, and
conducting. Connective tissue and bundles. Structure of
stems in monocotyledonous and dicotyledonous plants. Wood
and bark.

Ascending currentof water. Its course and destination.
Participation of the root—its water-raising power. Participa-

CONTENTS XV

PACE
tion of the leaves—evaporation of water. Function of the

stomata. Function of the vessels. Function of bordered pits.
Velocity of the sap. Purpose of the cork tissue. Movement
of nutrient substances formed by the leaf. Course of this move-
ment. Function of the sieve and latex-tubes. Causes of this
movement. Formation of stores of nutrient substances . 149

VII

GROWTH

Nutrition and growth. Direction of growth in the root and
stem. Attraction by the earth. Turgidity of tissues. Mode
of action of gravity. Influence of light. Heliotropism.
Methods of measuring growth. Influence of temperature.
Thermotropism.

Growth and multiplication of cells. Division of the nucleus.
The proximate effect of light on the growth of the cell-walls.
Effect of pressure on the form of cells. Growth mechanism of
cells. Possibility of hearing plants vegetate. The art of

experiment 182

VII

THE FLOWER

Sexual and asexual reproduction of plants. The flower. Essential
parts of the flower—ovule and pollen. Fertilisation. Fertilisa-
tion in the lowest plants. Adaptations securing the fertilisation
of flowering plants.

Function of the so-called non-essential parts of a flower. Self-
fertilisation and cross-fertilisation. Co-operation of wind and
insects. Parts of the flower attracting insects. Special forms
of flowers adapted to cross-fertilisation by insects. The part
played by art in the production of cultivated varieties. Purpose
of selection. Insufficiency of physiological knowledge of the
nature of the sexual process :

225

IX

THE PLANT AND THE ANIMAL

Current ideas as to the difference between plants and animals.
Capacity for movement in a plant. Microscopic movements :
of protoplasm, zoospores, and antherozoids. Movements of
organs in the highest plants under the influence of external
conditions (heat, light). Sensitive organs. Mechanism of these
movements. Spontaneously moving organs. Utility of various
movements.

Similarity between the internal processes of movement in
plants and animals. Similarity in the processes of nutrition.
Similarity in the process of respiration. Respiration and fer-
mentation. Similarity between the phenomena of stimulation

XV1 THE LIFE OF THE PLANT

and anaesthesis in plants and animals. Is a plant capable of
consciousness ? The difference between plants and animals
is not that of quality but of. quantity—not in kind, but in degree.
The sum-total of experimental physiology does not exhaust the
problems of the science y é :

PAGE

252

x

ORIGIN OF ORGANIC FORMS

The adaptive character of organic forms can be explained only by
the historical process of their development. Palaeontology,
morphology, and embryology together testify to the genetic
connexion between organisms. This conclusion conflicts with
the once prevalent conviction as to the permanency of species.
Are species really invariable ? Logical fallacy underlying this
opinion.

Why does the historical process lead to perfection ? Darwin’s
theory. The struggle for existence and natural selection.
Explanation of the absence of transitional forms. What we
have to be content with in explaining particular cases of
adaptation. Analytical and synthetical paths followed by
the reader. General conclusion and aim of the course . 289

APPENDIX

THE PLANT AS A SOURCE OF ENERGY

Twofold significance of food for the animal organism—as a building
material and as a source of energy. Conception of work and
energy, actual and potential. Law of the conservation of
energy. Mechanical theory of heat. Chemical affinity. The
animal organism, considered as a mechanism. Combustion
and respiration.

Necessity for the existence in nature of a process the inverse
of combustion and respiration. Priestley’s discovery. Decom-
position of carbonic acid by the plant. This process considered
from the point of view of the theory of the conservation of
energy. Robert Mayer. Production of organic matter by the
plant. Chlorophyll, its optical properties, and the explanation
they afford of its function in nature. Economic value of the
process taking place in the green organs of plants. Theoretical
limit to the productiveness of the earth Generalinference . 324

CHAPTER I

SCIENCE AND SOCIETY. EXTERNAL AND INTERNAL
STRUCTURE OF THE PLANT

It is not, I think, much beside the mark to say that the
word ‘botanist’ still calls up in the minds of many
even well educated people not conversant with science
one of two pictures. Either they expect in the botanist
a tedious pedant with an inexhaustible vocabulary
of double-barrelled Latin names, sometimes most
barbarous, who is able to name at a glance any kind
of plant, and also ready on occasion, it may be, to
describe (quite incorrectly) their medicinal properties
—the type of botanist who bores one to death and is
certainly incapable of exciting any interest in his
subject : or, on the other hand, ‘ botanist’ depicts the
somewhat less sombre figure of the passionate lover of
flowers, who flits like a butterfly from one bloom to
another, admiring their bright colouring, inhaling their
perfume, singing the praises of the proud rose and the
modest violet—in other words, the elegant adept of
the amabilis scientia, as botany was called in olden
times. These are the two extreme types associated
in the minds of so many people with the word
‘botany,’ and I am afraid I know it by personal experi-
ence! A botanist is either a pedantic nomenclator or an
amateur horticulturist, an apothecary or an aesthete;
but in no sense is he a man of science. The real man of
science seems to stand screened behind these types,
if such a person as a scientific botanist exists at all.
And, after all, what kind of science is botany ? What
are its aims ? What are the ideas which control it,
A

2 THE LIFE OF THE PLANT

if it is indeed working out any ideas at all? If the
public seems ignorant on these points, the fault lies
partly with botanists themselves, and partly with the
historical development of science. Let us consider
these conditions.

Living organic Nature meets us under a twofold guise.
We find her in bodily forms, 7.e. in plants and animals,
and we observe her in phenomena, 7.e. in life itself.
We call living beings organisms, because they are made
up of organs or instruments. Every organ, every
instrument has a certain function peculiar to itself, and
bears at the same time a certain relation to the general
life of the whole organism. It is impossible to study
organs apart from their function, or organisms detached
from their life—almost as impossible as to study a piece
of mechanism and its parts without regard to their
function. Who would have the patience to study the
description of the parts of a mechanism, say of a clock,
without any explanation of their function? Such a
study would be not only tedious but fruitless. Likewise
it is obviously impossible to become acquainted with
the working of a machine without knowing its con-
struction. It follows that the independent study of
an organism from the two arbitrary points of view
mentioned above, 7.e. in relation to its form and its
functions, is artificial and even illogical. These artificial
points of view, however, and a corresponding division
of the subject, long ago became established in science.
Biology, the science of living beings, was split into
two branches : (1) the study of forms, called anatomy
or, more generally, morphology, and (2) the study of
phenomena; of life, called physiology. This division
was caused partly by the necessity for applying the
principle of the division of labour to the manipula-
tion of such large numbers of accumulated facts, partly
by differences in the methods of investigation, and also
partly by difference of aim in the two branches of this

SCIENCE AND SOCIETY © 4

particular science. The one observes and describes, the
other experiments and explains. The impossibility of
carrying such a division of the subject to a logical issue
proves how artificial it is. In fact it can never be
strictly applied. The morphologist is bound to describe
the function of an organ and the physiologist its struc-
ture. Nevertheless, this division of the science of
botany, and particularly the narrow specialisation of
scientific activity, threaten to become a serious danger
for the future, a confusion of tongues as at Babel:
for surely the morphologist will cease to understand
the physiologist, and the physiologist will cease to
take interest in the work of the morphologist: every
specialist will shut himself up in his narrow province,
without troubling himself as to what takes place out-
side of it. The existence of these two provinces is,
nevertheless, an inevitable fact, owing to a necessity
against which it is entirely futile to demur.

It is nevertheless clear that these two provinces are
capable in very different degree of attracting general
attention, the attention of people not conversant with
science and only interested in its supreme.achievements.
A simple description or enumeration of the plants
and animals about us cannot excite any general interest,
although the number of people who find pleasure in an
acquaintance with the native flora and fauna does prove
a certain degree of scientific development in the public.
The fragmentary description of remarkable plants and
animals arouses but little interest, being too hackneyed,
and suitable only for children’s books, or for occasional
illustrated publications for grown-up people. General
attention may perhaps be attracted by some marvel,
such as a carnivorous plant devouring living people,
an absurdity which appeared some time ago in many
foreign papers as well as in our own dailies, and even
slipped into more specialised publications.

The situation is different with regard to the explana-

4 THE LIFE OF THE PLANT

tion of phenomena common to all the organisms of both
kingdoms, the study of the fundamental laws of life.
This can and must attract the attention of all thinking
men who wish to understand what is going on around
them. The same holds true in the inorganic world.
Mineralogy, which is a simple description of matter
that forms the crust of the earth, certainly cannot
excite the same interest as chemistry, which explains
phenomena taking place as the result of the reaction of
substances, or as geology, which recites the history of
our planet.

There is no doubt therefore that physiology rather
than morphology, function rather than structure, and
life rather than form, may be expected to attract general
attention. Let us now see which of the two tendencies
has been the more fully worked out in botany—is it
the one which deals with life or the other which con-
fines itself to lifeless forms ?

The history of science shows that botanists have spent
nearly all their energies upon the latter kind of work.
Men of science have devoted themselves entirely to
that extreme of the subject, forgetting the life of which
the body is but the vehicle. At no very distant period
the great majority of botanists belonged to the first of
the types described above, and even to-day not a few
may be found ready to repeat the words of a French
zoologist who, in the course of an exciting debate in
the Paris Academy, prided himself upon the fact that
during the whole of his scientific career he had not
expressed a single idea, but had only defined and
described, described and defined. If we turn from the
exponents of such old-fashioned ideas to our con-
temporary scholars, we shall find many who may criticise
their predecessors and recognise the superiority of the
physiological tendency of the present day, but who
nevertheless work along the same exclusively morpho-
logical lines. According to these modern scientists, a

SCIENCE AND SOCIETY 5

botanist is a man who spends his life over a microscope,
z.€. a man who goes on examining and describing
microscopically minute organisms, or else microscopic-
ally minute details of large organisms. Although
apparently different, the activity of both is essentially
analogous: the only difference between them lies in
the scale of their operations. While the one observes
with the naked eye, the other uses the lens or the
microscope; but both do no more than observe and
describe, and the description of a fungus or of a water-
weed does not differ from the description of a grass or
of a tree. The one and the other forget that the chief
object of the scientist is not to describe but to explain
and command Nature; his method must not be that of
a passive observer, but rather that of an active experi-
menter; he must engage in strife with Nature, and
by the power of his mind extort from her answers
to his questions, so that he may master and sub-
ordinate her at will, provoke or arrest the phenomena
of life, direct or vary them. Of course, among the
representatives of the exclusively morphological, or
descriptive, tendency there have been powerful minds,
who have thrown light upon the mass of accumu-
lated material and made it live—a little further on we
shall even study an illustration of this—but on the
whole their energies have been spent upon conceptions
inaccessible to the uninitiated, and therefore they have
not been able to excite any general interest. The fine
simplicity of some morphological laws, the harmony of
natural systems of classification which stand as wonder-
ful memorials to the power of the human mind, all this
is lost to those who are without the knowledge of the
details necessary to the understanding of it.

It is therefore evident that up to the present time
botany has been developing mostly along the lines
which least interest the public. As we have already
seen, the reason lies partly in the historical course of

6 THE LIFE OF THE PLANT

the development of science and partly with botanists
themselves. The historical development of every science
requires that the more complicated be preceded by the
more elementary, and it is obvious that the problems
of physiology are much more complicated than those of
morphology, and presuppose a greater store of informa-
tion. The description of organic forms does not necessi-
tate any preliminary knowledge. In order to explain
the phenomena of life, on the other hand, 7.e. to resolve
them into the simplest physical and chemical phenomena
—which is, asa matter of fact, the object of physiology—
it is necessary to start with some knowledge of these
phenomena. A morphologist need be but a morpho-
logist, whereas a physiologist must to a certain extent
be at once a physicist, a chemist, and a morphologist.
It was in fact inevitable that the physiological tendency
should develop later in the history of science, i.e. only
after physics and chemistry had reached a certain point
of development. That the backwardness of physiology
as a science was nevertheless due in large measure to
the onesidedness of botanists themselves is proved by
the fact that while the latter were still engaged ex-
clusively in the study of form, chemists and physicists
were penetrating into the attractive province of the life of
the plant and founding the science of plant physiology.
The fundamental principles of physiology were therefore
formulated by chemists and physicists and not by
botanists. The backwardness of botanists in this
direction is even more striking when we compare what
has been done in the sphere of the physiology of plants
with that which has been done in animal physiology.
This may seem somewhat paradoxical: the problem of
the physiology of plants is far simpler than that of the
physiology of animals. The life of plants is far less.
complicated than the life of animals, and yet our know-
ledge of the latter is much fuller and more definite.
However, there are perhaps some extenuating circum-

SCIENCE AND SOCIETY 7

stances which may be advanced in the defence of
botanists. The progress of the science of animal
physiology can be explained by causes lying outside the
province of science, by considerations of a practical kind.

To develop and prosper, every science requires the
moral and material support of society; but, on the
other hand, society takes practical interest only in
things which it considers useful. Society has already
been convinced of the usefulness of animal physiology,
while the idea of the usefulness of the physiology of
plants has only just dawned. Almost every science
owes its origin to an art of some sort, just as every art in
its turn is the outcome of some need in man. This
appears to be the inevitable course of the development
of human knowledge. To begin with, man appreciates
knowledge merely as a means towards obtaining the
fullest possible amount of material enjoyment; only in
a later stage does knowledge become to him in itself a
source of enjoyment. Intellectual aspirations are then
as exacting as material wants. Knowledge considered
as a means to an end is art ; knowledge considered as an
end in itself is science. Medicine is the art under whose
wing the physiology of animals developed. After many
unsuccessful efforts to solve its own problems by means
of rough empiricism or abstract thought medicine came
to the conclusion that it must go further back to study
the laws of animal life and join hands with science ;
thus it was that the science of animal physiology arose
and developed in the medical schools. But, together
with the necessity for preserving physical health to
which medicine answers, man has other needs; he
requires food, clothing, a roof over his head, and means
of locomotion. He obtains the majority of these
commodities directly or indirectly from plants which
he cultivates and tends. It is only after studying the
laws of their existence, after learning by observation or
experiment from the plant itself the means by which it

8 THE LIFE OF THE PLANT

accomplishes its aims, that we are able to direct its en-
ergies to our advantage and oblige it to yield us the best
and most abundant fruit. Obviously the physiology
of plants must be made the foundation of agriculture.
Agriculture, like medicine, rambled on for a long time
in the sterile provinces of empiricism and speculation
before it came to this conclusion. The same thing has
happened there as happened in medicine so many years
previously.

Rational agriculture is a much younger science than
rational medicine; consequently the necessity for a
knowledge of the physiology of the plant, and a demand
for such knowledge, arose also later. But the necessity
having once arisen, it cannot remain without influence
upon the fate of the physiology of plants. The physi-
ology of plants will develop in the schools of agriculture
in the same way as the physiology of animals developed
in the schools of medicine. A whole network of ‘ experi-
mental stations’ has already spread over Germany and
America; the Government in France, private individuals
and societies in England, are working towards the same
end ; even poor Italy, overburdened with debt though
she be, is making an effort to pursue the same course.

In all such ‘ stations,’ as well as in other agricultural
institutions, experimental physiology has established
-itself beside agriculture, and is setting to work to further
its progress, and gaining at the same time the advantage
of the precious experience it has accumulated during so
many centuries. So must it be on the analogy of other
sciences, and so doubtless it will be. Meanwhile,
‘however, a comparison of these modest experimental
stations and the still more modest botanical laboratories
of Europe with the luxurious palaces in which medicine
dwells, and especially a comparison of the insignificant
number of botanists engaged in physiological research
with the thousands of doctors who are and have been
engaged all over Europe in the study of the physiology

SCIENCE AND SOCIETY 9

of animals, make patent to every one the fact that this
extraordinary number of workers accounts for the
appearance of such men as Helmholtz, Claude Bernard,
Du Bois-Reymond, and others, beside whom botanical
physiologists can as yet cite not a single name. This
wealth of equipment, and especially the wealth of mental
energy which has been expended upon the subject, has
conditioned the success of animal physiology asa science,
and may be regarded as an extenuating circumstance for
the backwardness of the physiology of plants.

Happily, however, during recent years a fresh aspect
of botany has been discovered : life has begun to attract
attention which hitherto was exclusively devoted to
form. The public has realised at the same time that
the physiology of plants tends to an end not merely
useful, but even necessary, to society; that it is served
by this science in the same way as by other sciences,
which have already gained their civil rights.

I must explain myself. I do not wish it to be under-
stood from what I have said that I expect science to
aspire exclusively to utilitarian ends, as if I found its
highest sanction in its practical tendency. On the
contrary this practical tendency, which characterises
the infancy of a science, cannot and must not be its aim.
Throughout the development of a pure science its
results find application spontaneously. The develop-
ment of a science can be determined only by the logical
sequence of its achievements, never by the external
pressure of necessity. Scientific thought, like every
other form of mental activity, can work only under
conditions of absolute liberty. Oppressed by the weight
of utilitarian demands, science can produce but pitiable
artificial work, after the same kind as any meagre and
mechanical work of art fashioned under similar circum-
stances. We may ransack the archives of any science
and yet find scarcely one daring idea, one brilliant
generalisation which owed its origin to its application ;

10 THE LIFE OF THE PLANT

and, vice verséd, history is full of examples of discoveries, ©
which, though unassociated with any practical purpose,
have become the source of innumerable practical issues.

Now I must summarise this rather lengthy introduc-
tion. Comparatively speaking, botany meets with no
great amount of sympathy from the public, which
interprets it wrongly on the ground of its having
pursued objects and been engaged with ideas which
could interest but the small class of the initiated.
This tendency, caused in the first instance by the
inevitable historical development of the sciences, was
fostered and is fostered still by the attitude of most
botanical scholars. Recently, however, a new and
refreshing trend of thought has been observable gradually
forcing its way to the front, viz. the trend of thought of
experimental physiology. The new awakening of
interest is being followed by the realisation of the utility
of this science. Agriculture is beginning to demand a
knowledge of the physiology of the plant, and in this
way the solidarity of interests between science and
society is being established. Whereas, however, this
community of interests does not on the one hand
authorise society to dictate to science its modes of
action or the method of its further development, neither
has science on the other hand any right to retire as it
were into a sanctuary, to conceal itself from the public
gaze, expecting its utility to be taken on trust. If the
votaries of science wish it to attract the sympathy and
support of the public, they must remember that they
are the servants of the same public, that occasionally
they must step forward as trustees and duly render
their account. This is what we have accomplished,
they must say ; this is what we are accomplishing, and
this is what we are going to accomplish : judge how far
our activity has been fruitful, and consequently what you
may expect in the future.

Personally I think this is a problem for what may be

SCIENCE AND SOCIETY II

called popular scientific literature, for popular lectures—
a problem often lost sight of because those who set out
to treat scientific subjects in a popular way generally
devote their attention to but one side of their aim,
namely, how they may teach in the easiest and most
amusing way.

I have said, that in order to understand the life of a
plant it is necessary to study its form; in order to
understand the working of a machine a study of its
construction is needed. Let us glance at the external,
formal manifestations of the life of a plant, the observa-
tion of which does not require any preliminary study,
nor any technical method of investigation.

Let us begin our sketch with the awakening of the
plant’s life after the winter’s slumber. In what state
will the spring find it? Where is hidden the origin of
this new life ? It lies concealed in the seed which has
maintained its vitality under the shelter of the soil
and the thick cover of snow. It is maintained in buds,
which have endured the misery of the cold under the
protection of their scales. By the action of the warm
spring sun every bare piece of ground produces green
shoots; on every tree or bush buds swell, burst, and
lose their unsightly and already useless scales. The
seed and the bud—those are the two organs to which
daily experience attributes the origin of the plant’s
life. It is therefore with an investigation of them that
we shall begin our study.

First, what is a seed and what are its component
parts? Let us investigate the well-known seed of a
bean. If soaked in water it will swell and become
detached from its skin, or coat. Under the seed-coat
we shall find it split into two fleshy or rather hard and
cartilaginous parts. In between these will be found
inserted a small body connecting them together.

12 THE LIFE OF THE PLANT

With the naked eye, or, better still, with the help of a
lens, a small germ plant, a young shoot, consisting of a
tiny stem with leaves and rootlet,

is easily recognised (fig. 1). This

shoot binds together the two

halves of the seed, which are

called the cotyledons. These,

though much larger than the

Rie. t. shoot itself, are nothing but two
appendages of it. But what is the

nature of these cotyledons? Botanists say they are leaves.
Those colourless, round, fleshy bodies, which remain
underground are called leaves not without reason, as we
shall immediately see. We have only to pass from a
bean to its nearest relative—say the haricot—to find
cotyledons appearing above the soil and becoming green
like ordinary leaves (fig. 2). In the maple and the ash
the cotyledons are still more like a common leaf, and
the lime actually has small thin green leaves with well-
marked veins and crenate outlines. Therefore the
cotyledons of a bean, though they grow underground
and are far from reminding us of actual leaves by their
colouring or appearance, must be nevertheless regarded
as such. Following upon those first organs, so unlike
leaves, there appear, as the stem elongates, real leaves,
though not yet of the shape we are accustomed to
meet on a grown-up plant. Here is, for instance, a
young ash plant. Everybody knows the shape of its
leaf. Several pairs of leaflets are distributed on a
common stalk with one leaflet more at the top. In this
way a whole leaf consists of seven, nine, or more leaflets.
This is called a compound leaf. What, then, do we
notice here ? (fig. 3). The two fleshy, tongue-shaped
cotyledons are followed by two toothed leaves with
prominent venation, which are simple, not compound,
leaves. If we look further up the stem we shall notice
other leaves composed of three leaflets, higher up

STRUCTURE OF THE PLANT 13,

others of five, and lastly of seven or nine leaflets; z.e. here
commence leaves like those of which the foliage of a
grown-up tree is generally composed. This passage
from the cotyledon to the true leaf has happened
gradually ; it includes a whole series of intermediate
forms. We receive involuntarily from the series the im-

Fic. 2. Fic. 3.

pression that one of these organs is formed from the
other, and that these are the intermediate stages
through which a leaf has to pass.

Let us now consider the bud of a tree, say of a maple,
of a horse-chestnut, or of any bush, like that of the
currant. We find peculiar organs on the outside of
them : dark brown, thin, tough, sometimes sticky and
resinous scales. If we pull the bud to pieces or let it
open by itself, then tear off its parts one by one and
spread them out in a row, we notice the following facts.

14 THE LIFE OF THE PLANT

First in the series are several scales darkly coloured,
short, obtuse, almost round in shape (fig. 4). Then
this shape becomes
more and more
elongated and the
colouring passes in-
to green; we notice
on the top of one
of these scales an
indefinite rather
crumpled protuber-
ance, which further
on increases in size
and opens out.
This protuberance
is a real slightly
wrinkled little leaf.
The deeper within
the bud the more
clearly this pro-
tuberance reveals
itself as the part
of the leaf which
is called the /amina,
Fic. 4. while the distended
part of the first
scales becomes narrower and more elongated, taking
the true stem-like form of a petiole (fig. 4, horse-
chestnut, and fig. 5, currant bush). This is therefore
the same phenomenon as in the young ash: there the
cotyledon and here the scale passes into a leaf, through
a graded series of intermediate forms. And again the
suspicion arises that these are one and the same
organ, only modified in appearance according to their
special functions.
Having thus started with a seed or with a bud, we
have arrived at the typical leaf which makes up all the

STRUCTURE OF THE PLANT 15

green foliage of plants. Having produced such a leaf
the plant seems to have reached the beaten track and
produces one leaf after another, modelling them as it
were according to the same pattern, casting them, so
to speak, in the same mould. But the leaves are not
the only product of a growing plant ; at a certain age
it produces other organs such as flowers and fruit.
As a rule the transformation of leaves into quite distinct
flower organs hap-
pens suddenly; but
cases are frequent
in which the ap-
pearance of the
flower is anticipated
by changes revealed
in the upper leaves.
Let us study the
well-known garden
peony. Everybody
knows its leaves (fig. 6). Starting from the lowest and
passing up the stem towards the flower we notice that
the shape of the leaf changes until it becomes at last
almost unrecognisable. At first the whole leaf consists
of eleven or nine leaflets distributed in threes. At a
certain point we have only three leaflets ; in the interval
between these two kinds of leaves we are also likely to
find such as have seven and five leaflets. In the end
the whole leaf consists of only a single leaflet (fig. 7, left).
The process is the converse of that noticed in the ash.
There the shape of the leaf became gradually more
complicated, whereas here it becomes less so, passing
through the same stages but in the reverse order.
So far the simple leaflet has entirely resembled the upper
part of the whole leaf, but gradually it also changes its
appearance: its short petiole broadens into a flat
scale, while the lamina continually decreases until it
becomes a small, green, tongue-shaped object on the top

Fic. 5.

16 THE LIFE OF THE PLANT

of the scale (fig. 7) ; later still it appears like a small
bristle in the topmost hollow of the scale, and at last
disappears altogether (fig. 8). We are left with a thin
yellowish-green scale, reddish at the edge. Our leaf

Fic. 6.

has gone through its entire transformation before our
very eyes, so to speak. Its lamina has disappeared,
while its petiole has changed into an organ, similar in
origin and purpose to the scale studied in the bud of
the chestnut. The one as well as the other represent
a petiole, developed like a lamina. As the one protects
the young leaves of the bud, so the other protects the

STRUCTURE OF THE PLANT 1c

inner delicate parts of the flower. This organ is called
a sepal, and the whole whorl of such leaves the calyx.
Thus a sepal is nothing but a modified leaf. In many
cases this fact is obvious—as, for instance, in the sepal of
the rose, which keeps its thin lamina. Very few flowers
give us the same opportunity as the peony of following
this gradual transformation.

The sepals in a flower.are followed by a number of
leaves coloured white or some other bright shade with a
satin or velvet surface, so vainly imitated in artificial

flowers ; these are petals, forming together
the corolla. This seems a great leap; the
sepal and the petal of a rose have no

Fic. 7.

Fia. 8.

similarity. But let us put aside the rose and pass to

other flowers. Even in the peony some connection

between a sepal and a petal can be traced in the red

border of the former and in the notch of the upper part

of the latter (fig. 8), which is similar to that in the

sepal (fig. 7, right hand). In the Camellia, however, we
B

18 THE LIFE OF THE PLANT

are thoroughly perplexed as to where the sepals end and
the petals begin, so gradual and unnoticeable is the
passage from the hard green sepal to the delicate white
or red petal. So a petal is nothing but a modified sepal,
which in its turn is a modified leaf. It follows that a
petal is nothing but a leaf.

Let us now peep into the inside of a flower, and choose
for our purpose one of the larger flowers, say a lily.
From the centre of the flower several organs project,
composed of a thin stalk, on the top of which are
inserted crosswise two yellow oblong sacks split longi-
tudinally. The slit discloses a dry dust, orange in
colour, the pollen. These organs are called the stamens ;
the receptacles containing the pollen are the anthers, and
the stalk bearing them the filament. One would think
that a stamen and a petal have no connection whatever.
But let us look for a suitable illustration before jumping
to a conclusion. Probably every one is familiar with
the white water-lily, so common in our streams and
ponds, with its large almost round leaves and its flowers
floating on the surface of the water. Let us pull one
of these white flowers to pieces and spread out its
several parts, as we did with the bud of the chestnut,
beginning with the outermost, 7.e. the external white
petals, and ending with the part nearest the centre of
the flower, the organ, composed of the yellow receptacles
filled with pollen and a filament rather-flat in form, in
which we easily recognise a stamen (fig. 9). We notice
once again the same imperceptible transformation : here
is a typical white petal; on the top of it appear two
yellow spots, which increase in size as the base of the
petal becomes narrower ; two oblong receptacles become
clearly marked, and the base of the petal transforms
itself into a narrow filament. Here at last is a real
stamen, the anthers of which split longitudinally and
shed the pollen. The petal has passed into a stamen.
The possibility of such a transformation is proved

STRUCTURE OF THE PLANT I9

by horticulturists who produce reverse transforma-
tions, changing stamens into petals. Such staminate
flowers changed into petaloid are called double.1 Take,
for example, the common peony. It has five petals
and many stamens, but the double peony has many
petals and correspondingly féw stamens. On closer
observation we shall become convinced that the inner
petals are the transitional form of stamens : on the edge
of the bright red, slightly wrinkled petal are situated

Fic. y.

yellow anthers more or less well developed. In the
dog-rose, which is the prototype of our rose, we notice
only five petals and a great number of stamens; in
the rose some of the stamens have been transformed
into petals: this is why their number is greater than
five. Double flowers are also of interest from the
physiological standpoint, because they can be produced
artificially. The outer scale-leaves of the bud can also
be artificially transformed into real leaves. We there-

1 In Nature as a rule the different parts of flowers probably appeared
in the same way as in the case of the double flowers just described,
t.e. the stamens were transformed into petals, and not the petals into
stamens.

20 THE LIFE OF THE PLANT

fore reach the conclusion that the transformation of
one kind of leaf into another can be demonstrated not
only by observation, but also by means of experiment,
generally by far the more convincing method.

Proceeding with our study we reach the very heart of
the flower. After the stamens we meet the last organ of
a flower—I say
the last because
it forms its ceh-
tral part and
thus terminates
its growth and
consequently the
growth of the
part of the stem
which ends in
the flower itself.
This organ is called the carpel or pistil on account
of its form, which, with its swelled base (ovary),
elongated neck (style), and rounded top (stigma) is very
like a pestle.

There may be one or many carpels in a flower. The
lowest part of a carpel, the ovary, is hollow inside, so
that the whole organ in this illustration (fig. 10, flower
of cherry) is like a small bottle. This cavity contains
one, several, or even many bodies, round and white,
called ovules. We meet this organ again with distrust.
This time there seems to be not a trace of likeness
to a leaf, but another successful choice of illustration
will prove that this organ also is derived from one or
many little leaves. Some abnormal flowers will give us
the necessary clue. For instance in the double flowers
of the cherry the carpel often transforms itself from a
bottle-shaped organ into tiny leaves, one or two in
number (fig. 11).1_ In many cases it is even unnecessary

Fic. to.

1 A—Pistil partly transformed into a leaf. B—The same pistil in a
transverse section. C—Pistil transformed into two leaves.

STRUCTURE OF THE PLANT 21

to refer to abnormal plants to see the leaf-like character
of the carpel and the resultant fruit. It is enough to
glance at the fruit of a legu-
minous plant, such as a bean,
or still better the fruit of the
peony, to be convinced that
it is nothing but a leaf, the
edges of which have curved
over and grown together, thus
forming an organ with a longi-
tudinal join (suture) with a
hollow space inside. In other
cases the ripe fruit in bursting
shows quite clearly thatitcon- 4 B c
sists of several little leaves Fic. rr.

grown together at their edges.

The carpel, then, has been derived from one or more
little leaves modified in form. But not in all ab-
normal flowers are the carpels transformed into real
leaves as we see it in the cherry. In other cases the
carpel transforms itself into organs more closely related
to it, such as stamens and petals. The transforma-
tion of a pistil into a stamen can sometimes be
studied in the flower of a willow. Occasionally bright
red petals can be found in the centre of double
peonies with white, shiny ovules on their edges. These
are surely carpels which have become transformed into
petals, but which have kept their ovules. It follows
that a pistil can transform itself into all the preceding
organs, 7.e. into stamens, petals, and real green leaves.
Does not this prove that all these organs are of one and
the same origin ?

In our analysis of the plant we have reached its
topmost organ—the carpel; we cannot proceed any
further—we can only go deeper into the interior of the
carpel, the cavity of the ovary. We shall find there
ovules, as has already been said. What are these ovules ?

22 THE LIFE OF THE PLANT

In the flowers where carpels have changed into green
leaves we notice small green leaflets or whole leaf-buds
on their edges at places where we should expect ovules.
Therefore ovules and parts of ovules are nothing but
leaflets or parts of leaflets. Thus we conclude that all
the parts of a flower are nothing but modified leaves,
and the whole flower is nothing but a transformed
leaf-bud. This opinion is supported by the not un-
common cases of flowers from the centres of which
grow shoots covered with leaves. Such twigs have
also been known to grow out of the cavity of the
ovary ; when cut off and planted they have occasionally
taken root.

But what becomes of the ovule—not the abnormal
one, which grows into a green leaf, but the ordinary.
normal one? After a plant has flowered and the
petals have fallen off, after the stamens have died and
the ovary has changed into the fruit, the ovules will
become transformed into seeds, containing the embryos
of new plants. Here evidently our description of the
external features of a plant ends. I have unrolled before
you the whole picture of the outward manifestations of
the life of the plant. We started with the seed and we
have returned to it, and have thus completed the full
cycle of a plant’s life. This cycle will be followed by
another, and so on through the infinite succession of
generations. I have tried to enliven the tedium of this
enumeration of organs, which is indispensable for my
subsequent exposition, by linking them together by the
one leading idea of transformaticn or the metamorphosis
of organs, an idea for which science is mainly indebted
to the scientist and poet Goethe. Examined from this
point of view the life of a plant is like a phantasmagoria,
a successive series of changing magic-lantern pictures.
An organ has only time to assume before you a definite
shape, when it already loses its configuration, becomes
unrecognisable, changes into something indefinite, and

STRUCTURE OF THE PLANT 23

then gradually becomes again more distinct, appearing
this time in another form, as another organ, and so on:
the one replaces the other, the one passes imperceptibly
into the other, until the whole cycle of development is
closed and the primary and original organ reappears.
So far we have had only the leaf organs in view, but
beside them the body of the plant reveals two other
organs, the beginnings of which are to be found already
in the seed: these are the stem and the root, the structures
which support the leaves. These two organs, apparently
so different and growing in different environments, are
in some rare cases, however, capable of transforming
themselves into each other: the stem sinks into the soil
and assumes the character of the root, or the root grows
up into the air, covers itself with leaves, and assumes
the character of the stem. Hence the stem and the
root, forming the axis of which they are the two modified
forms adapted to different conditions of existence, and
the appendage of the axis—the leaf—with its manifold
variations (scales, petals, stamens, and so forth) are
the fundamental external organs produced by a normally
developed plant during its life-time.

In accordance with the general conception of the life
of a plant we have thus far taken it for granted that it
begins and ends with the seed. Doubts, however, arise
as to our right to attribute the origin, the real starting-
point of the life of a plant to the seed. May we not
perhaps go further back and find out its ultimate origin ?
For the seed we have been describing is still a very
complicated body; we find in its embryo a com-
plete little plant with practically all its parts already
developed.

In order to discover this simplest starting-point of
plant life we must turn to plants which are exceptions to
the general rule of the typical plant with seeds and
flowers, which we have just been considering.

24 THE LIFE OF THE PLANT

Suppose by an effort of imagination you can detach
yourself for a moment from your present environment
and transport yourself in thought to one of the pictur-
esque landscapes of Russia, say the neighbourhood of
Moscow, and suppose you try to recall your impressions
of a walk down into the ravine of Kunzevo. As you

Fic. 12.

descend into the green thicket with its damp atmosphere,
impregnated with many exhalations, you will notice
quite a singular kind of vegetation. At every step the
waving fronds of ferns grow from the floor or the slopes
of the ravine, like bunches of green ostrich feathers, or
the crowns of palms stuck into the soil (fig. 12). Lower
down along the swampy bank of the stream, in the water
itself, or in some marshy pool, you will see a brush-like
mass of horse-tails crowded together here and there with

STRUCTURE OF THE PLANT 25

little black cones still surviving on their tips (fig. 13).
Such a scene always strikes us as strange and uncommon.
Involuntarily one feels that this vegetation is totally
different from that left behind at the top of the ravine.
This subconscious
impression is no
illusion. This
world of ferns and
horse- tails is in
very truth a singu-
lar world; it isa
sampleof the vege-
table world which
used to cover
our planet in by-
gone geological
epochs. Those
ferns and_horse-
tails, and other
plants closely re-
lated to them and
very common in
our woods, like
these dry, moss-
like, creeping
plants, with their
yellowish cones
occasionally up-
raised, called club-
moss (fig. 14),
all these plants,
I say, or rather
forms related to
them, used to
be the prevalent
vegetation on our planet in the period when our
coal-beds were formed. Coal contains the remains of

26 THE LIFE OF THE PLANT

whole trunks which belonged to them and the impres-
sions of their leaves and fruit. These remains enable
us to reproduce with the help of a certain amount
of imagination the aspect of the former vegetation of
our planet, the landscapes that no human eye ever
looked upon. The forests of that remote period
contained tree-like ferns which exist to-day only in
certain moist tropical countries and in hot-houses.
Our short, creeping club-moss existed then as a stately,
scaly tree, Lepidodendron, whereas our humble horse-
tail, which reaches the height of some dozen feet only in
a few places in South America, was represented by the
similar but tree-like Calamites, Equisetites, and others.

I have just used an expression which needs explana-
tion, and which will naturally take us back to the main
thread of our argument. I have said that the club-moss
is related to ferns and horse-tails, and that all the existing
forms of these plants are related to fossils. Wherein
consists that relationship and wherein do these ferns,
horse-tails, and club-mosses differ from coniferous and
broad-leaved trees ?

Some peculiarities in the life of ferns long ago
attracted the attention even of unscientific people.
There is a poetic fancy in Russia that ferns flower on
St. John’s eve. This legend is based on the notice-
able fact that ferns never bloom, never have
flowers like other plants. The same is true of horse-
tails and club-mosses. All these plants are known
by the name of flowerless plants. But if they are
without flowers they must be also devoid of seeds,
which are usually formed from the ovules of flowers.
How do they then reproduce themselves ? If we look
at the under side of a fern-leaf, at the black cones
of the horse-tail and the yellow cones of club-moss,
we shall notice that towards maturity they all present
the following general characteristic : if you shake them
over a sheet of white paper you get some very fine

STRUCTURE OF THE PLANT 27

powder, brown or yellow in colour. This powder is
composed of very minute bodies, visible only through a
microscope, and so small in size that a row of them one
inch in length would contain something like one thou-

Fic. 14.

sand of them. Every such grain of powder can produce
anew plant. Here is the so-called Lycopodium powder,
yellow, soft to the touch, which falls from the cones of
the club-moss (fig. 14), and is used by chemists for
powdering pills. I throw a handful of this powder into
the flame of a candle and the cloud of dust is illuminated

28 THE LIFE OF THE PLANT

with lightning-like flashes, an effect used in former days
to represent lightning on the stage. In this explosion
have perished in their embryonic state millions of future
plants. These microscopic bodies are called spores by
botanists, and all the plants derived from them and
devoid of flowers and seeds are called spore-bearing
plants. Beside the plants already mentioned this class
comprises mosses, water-weeds called green-slime in
everyday language, and also fungi, a group which in-
cludes moulds as well as mushrooms.

Thus we notice that a spore-plant, whether micro-
scopic mould or tree-fern, owes its origin to an invisible
grain of dust—a spore. What is this spore? Is it not
the simplest starting-point of plant life, for which we
have been seeking and which we could not think that
we had found in the seed ?

As a matter of fact microscopic investigation shows
that the spore consists of a bladder with a solid exterior,
containing within it liquid and semi-liquid matter.
This is the so-called cell, and it is to the cell that we
must look for the simplest origin of every organism ;
we are unable to split it into parts capable of independent
existence ; it marks the limit of morphological analysis ;
it is the organic unit. This being the case a question
at once occurs to us: could we not also trace a seed
back to a single cell, for surely it does not arise straight
away with its root, stem, and cotyledons? We shall
have an opportunity in a subsequent lecture of proving
that every seedling also starts from a single cell. We
shall discover this cell in the ovule when we come to
know its structure better. Hence it follows that every
seed-plant or spore-plant starts its existence as a single
cell. The only difference between them consists in the
fact that in the case of the spore-bearing plant the cell
becomes separated from the plant which has produced
it; whereas in the seed-plant the cell develops and
grows into a complicated organ, a seed, and only in

STRUCTURE OF THE PLANT 29

that form separates itself from the maternal plant.
All that lives, be it the simplest plant or man, starts
from a single cell. Some microscopic plants and even
some that are visible to the naked eye preserve their
unicellular condition throughout their life-time; whereas
others as they develop become more complicated in
their structure and form two, several, millions of cells
out of the original one. t

Thus every plant not only springs from a cell, but
consists of cells in all its parts. Cells are, so to speak.
the bricks out of which the body of the plant is built.

This can easily be proved by very simple means.
Examine, for instance, a thin slice of a ripe water-
melon, and you will see that it consists of bubbles
very loosely joined together and having the appear-
ance of glass beads. These are cells, which generally
lose their mutual coherence in the flesh of a ripe fruit
and become detached. In other cases this coher-
ence is not broken naturally, but can be broken
up artificially. For instance, a slice of raw potato
presents a compact body in which it is difficult to per-
ceive a definite structure of any kind, without the help
of a microscope; but if you look closely at a boiled
potato you will see quite clearly, even with the naked
eye, that it consists of separate cells. Boiling water, or
rather the action of steam during the process of boiling,
has destroyed the coherence between the cells and
liberated them. It is somewhat more difficult to
separate the cells in more compact organs. But there
is no organ too hard to render such a process impossible,
were it even a piece of wood, a cherry stone, or the seed
of a palm, such as Phytelephas macrocarpa, which is as
hard as ivory, and is sometimes used by turners instead
of it. To break up the cohesion of cells in such compact
bodies we must necessarily seek the help of chemical
reagents.

It is not even necessary, however, to destroy the

30 THE LIFE OF THE PLANT

cohesion of cells to be convinced of the fact that vegetable
matter is composed of them: if we cut very thin and
transparent slices with a razor from any part of a plant
we can soon satisfy ourselves with the help of a micro-
scope that these are composed of cells, closely compacted
together, forming what is known as cellular tissue.

It is clear from what has been said that it is im-
possible to become acquainted with the structure and
life of plant organs without an acquaintance with the
cell. As in chemistry we start the study of substances
with the elements and then proceed to their combina-
tions, so in botany the study of the organs of plants
must be preceded by that of their elementary organ—the
cell.

We have now collected enough facts to be able to
make a general plan for these lectures. During its life-
history the plant produces a series of organs, the
external aspect of which, together with their relation
towards their environment, makes it evident that
they serve very different purposes and perform very
different functions. It is clear that the function of
the root which sinks into the soil is different from
that of the green leaf which grows up into the air
towards the light ; that the function of the cotyledon is
different from that of the petal ; that the function of the
stamen with its pollen so easily disseminated in the air
is not the same as that of the ovule buried deep in the
ovary. The physiologist first of all must discover the
purpose of every organ, 7.e. its function. Hence a
twofold problem confronts him from the outset: given
an organ, to find its function ; and given a function, to
find the organ. Evidently he has first to study the
function of the elementary organ, the cell, in its general
and special manifestations. Later on, when he becomes
convinced of the perfect way in which the organs fulfil
their purpose and are adapted to their environment,

STRUCTURE OF THE PLANT 31

when he learns how necessary and well balanced is
their mutual interaction, resulting as it does in the
general life of the organism, he then begins to realise
that his problem is not yet solved, that from behind
all the particular questions there emerges the most
general of problems, the question of all questions.
How have all these wonderful organs combined ? how
have all the organisms themselves arrived at that degree
of perfection which strikes us so forcibly when we study
living Nature?

By thus including this general question among those
which confront physiology, it is evident that we take
our stand among those students of Nature who consider
the sotution of this question feasible and timely. It is
notorious that there have been two schools working in
the province of natural science, two parties engaged
in warfare. The extremists of the one school saw
in living Nature nothing but a collection, a kind of
museum, of immutable living things, cast in definite
fixed forms. According to them the work of the student
of natural science resolved itself into an endeavour to
make a general catalogue of those forms, label them and
arrange them in a collection. The other school looked
upon organic Nature as a vast whole which is ever
changing and transforming itself. To-day the organic
world is different from what it was yesterday, and
to-morrow will be different from what it is to-day.
The forms of life at present on our planet have derived
greater perfection from less perfect ancestors by means
of gradual modifications. This school has Darwin as
its head, Darwin who harmonised the whole mass of
accumulated evidence and gave strictly definite direction
to its hitherto indefinite trend. Obviously the question
as to how organs and organisms have originated and
perfected themselves cannot exist for exponents of the
first-mentioned theory. According to their point of
view these organisms have never formed nor developed ;

32 THE LIFE OF THE PLANT

they arose perfectly formed ; they were created in the
same perfect form as we see them now. Only those
who are convinced of the fact that organic beings are
by nature capable of transformation, that they developed
the one from the other, becoming more complicated or
more simple as the case may be, but always improving,
only those can raise the question as to how organic
forms have developed and why they are so well adapted
to their functions and environment. I will do my best
in my final lecture to investigate the answers that
science at its present stage of development is able to
give to these questions; nevertheless I should be sorry
to miss this opportune occasion for demonstrating the
superiority of the modern theory, if not conclusively,
at least so far as to show how facts, otherwise incompre-
hensible, are thereby elucidated.

_ In choosing and comparing certain striking examples
I have tried to explain the cycle of the life-history of the
plant from the point of view of the theory of meta-
morphosis. Let us consider some of the facts above
stated. If plants were created in final, perfectly
definite forms, what purpose is to be attributed to all the
transitional organs, such as petals and non-petals,
stamens and non-stamens (as in the water-lily), or to
those appendages at the top of the sepals of the peony ?
Taken independently these transitional organs are
quite useless, since they fulfil neither the purpose of the
organ from which they have developed, nor of the organ
into which they are about to change (this is why they
have survived only in a few exceptional cases). They
are utterly incomprehensible from the point of view of
individual acts of creation. But they will acquire a
very definite meaning as soon as we admit the other
explanation, as soon as we accept the theory that all
the numberless organic forms in Nature have not been
created finally nor in isolation, but have gradually
developed the one from the other, becoming more or

STRUCTURE OF THE PLANT 33

less complicated as the case may be, but always improv-
ing, 7.e. adapting themselves to the conditions of their
existence. Then we see in those transitional forms real
stages of development, gradual steps towards perfec-
tion, towards the improvement of the organ necessary
to the plant. Only then will the theory of meta-
morphosis, admitted by the exponents of the opposite
theory, however obscure and metaphysical it may be
from their point of view, acquire perfectly real and
definite meaning. ‘This metamorphosis is the expression
in space of what has taken place in time. Those thick,
colourless cotyledons as well as these bright perfumed
petals have been derived from the origin of the common
leaf, and have gradually adapted themselves to their new
functions ; and those intermediate, transitional forms are
nothing- but the surviving formal evidences of the
process of transformation. They are memorials which
enable us to build up the history of the vegetable world.
This is the reason of their being so precious to science.
But are we entitled to affirm that the vegetable world
has a history? Geology answers in the affirmative, and
we have just studied an illustration of the fact. We have
seen that our ferns, horse-tails, and club-mosses are only
degenerate descendants of former mighty masters of the
soil; degenerate forms, forced nowadays to hide them-
selves in the depths of forests, or at the bottom of
ravines, to escape from the aggressive denizens of the
vegetable world of to-day. This means that the earth
used to be inhabited by other plants, and that these
belonged to the simpler spore-plants, which have
receded before our more perfect seed-plants. Hence
the fact of metamorphosis, as well as many other
similar facts which we shall consider later on, on the
one hand, and geology on the other, prove that the
plant world has a history of its own, and therefore that
our question as to the origin of vegetable forms is

perfectly legitimate.
Cc

34 THE LIFE OF THE PLANT

The physiologist’s horizon thus becomes wider and
wider. After studying the life of separate organs, be-
ginning with the elementary organ from which all others
are formed, i.e. the cell; after studying the general
effect of the interaction of these organs, i.e. the life-
history of the plant as a whole, he tries to grasp, in so
far as it is accessible to him, the life of the plant world
as a whole, and thus attempts to shed light on the
greatest and most mysterious problem—the problem of
the origin of the plant and the reason of its perfection,
in other words, the problem of the harmony of the plant
world.

Before we step forward, however, on this gradually
rising synthetic path, we must go a little deeper in our
analysis. We have dissected the plant into organs
and the organs into cells, but so far we have only
examined the external structure of the cell. We must
peep into its interior, into the microscopic laboratory,
where the innumerable substances produced by the
plant are formed. We must study them and disin-
tegrate them into their elements. For this purpose
balance and chemical reagents will come to the assist-
ance of our microscope. This study will form the
subject of the next chapter.

CHAPTER II
THE CELL

THE most remarkable fact in the life of the plant is its
growth. When we analyse the phenomenon of growth
we realise that it consists in the multiplication of cells.
If we examine it still more closely we realise that it
involves the appearance and accumulation of matter in
places where it was before absent. We put an acorn
into the ground and an oak appears; we drop an
imperceptible grain of dust, a spore, and a tree-like fern
springs up. The question naturally arises: whence
came this substance? Evidently this question pre-
supposes the conviction that matter cannot be newly
created, nor disappear. This law of the non-disappear-
ance, or the conservation, of matter underlies all
scientific conceptions of Nature. The ancients ad-
mitted that ex nihilo nil fit, but they would certainly have
been in a sore quandary had they been asked, for
instance, to prove that burnt matter has not ceased to
exist, or to decide whence comes the substance of the
plant. Only by long-continued and laborious experi-
menting could the law of the conservation of matter
as applied to the phenomena of plant life be demon-
strated. Even in these days people unfamiliar with the
results of science still believe that the growing substance
of the plant is derived from the soil, whereas the error
of this theory was proved more than three hundred
years ago. Van-Helmont, one of the forerunners of the
scientific epoch of Natural Science, one of those clear and
fearless minds who steered the way for positive science

notwithstanding the hampering snares of scholastic
35

36 THE LIFE OF THE PLANT

metaphysics, at once a mystic and an ingenious experi-
menter—Van-Helmont, I say, made the first exact
experiment, which tended towards the solution of the
problem of the origin of the substance of the plant.
This experiment is remarkable not only because it is
the first exact experiment in the province of plant
physiology, but also because it was among the first
cases in which a balance was used as a means for solving
a problem in chemistry. It is well known that chemistry
owes to Van-Helmont the original application of this
instrument, which later on, in the hands of Lavoisier,
revolutionised that science. Let us describe Van-
Helmont’s experiment in his own words. ‘I placed,’
he says, ‘two hundred pounds of earth, previously
dried in an oven, in an earthenware pot and planted a
willow slip in it, weighing five pounds. Within five
years the willow slip weighed one hundred and sixty-nine
pounds, three ounces. The pot was regularly watered
with rain and distilled water. The pot was large, and
buried in the soil; and, that it might be protected from
dust, it was covered with perforated tin foil. I did not:
weigh the leaves shed by the plant during the four
successive autumns. At the end of the five years I
redried the earth and found that it weighed the same
amount of two hundred pounds minus two ounces,
which meant that water alone had been sufficient for
the production of one hundred and sixty-four pounds of
wood, bark, and roots’ (Ortus medicinae, p. 109). This
experiment proved beyond doubt that earth or rather
soil cannot be considered the exclusive or even the
chief source of vegetable matter. Van-Helmont saw it
in the water he used for watering the plant ; we know,
however, that the plant derives its substance not only
from earth and water but also from the air. Neverthe-
less, Van-Helmont’s inference was perfectly correct as
far as he could go. In his day science had no definite
conception of the third, z.e. the gaseous, form of matter.

THE CELL 37

It is to him that science owes the first idea of gases, and
even the very introduction of the word gas. Not
before the end of last century and the development of
the chemistry of gases, could the origin of the sub-
stance of the plant be fully explained. This explanation
followed as a result of the investigations of the three men
of science : Priestley, Ingenhouss, and Senebier.

In order to find out which of the components of this
threefold medium—earth, water, and air—participate
in the formation of the plant, we must know the com-
position of the plant itself. Since Lavoisier, chemistry
has taught us that matter not only cannot be created,
but in a certain sense does not even change ; that there
exist a certain number of so-called simple substances
or elements, incapable of transformation one into the
other. Therefore, when we find some element present
in a plant, we look for it in the surrounding medium,
knowing that it must have penetrated thence and
could not have been created in the plant, nor produced
within it from some other element.

By no means all the chemical elements are to be
found in plants, and even of those which do occur, we
shall mention only the principal ones, i.e. those which
play a prominent part in the life of the plant. In order
to get an idea of the chemical composition of a plant,
we submit it to the action of a high temperature. Water
evaporates first, and at a temperature a little above
100° C. we obtain the so-called dry matter of the plant.
This is the first step in our analysis. It shows that
different parts of a plant contain water in very different
proportions (see table on p. 43). At a higher tempera-
ture we notice that the dry vegetable matter turns
brown and black, and then becomes charred, until it
begins to glow and burn with a flame, leaving in the
end a heap of ashes, very small in comparison with
the quantity of substance with which we started.
Most of this substance must therefore have burned

38 THE LIFE OF THE PLANT

away and volatilised. If we carry out this combustion
with certain precautions and collect the volatile gases,
we discover that the part of the vegetable matter which
burns away consists of four elements: solid carbon
and three gases—oxygen, hydrogen, and nitrogen.
This combustible part, which always contains carbon,
as is shown by the fact that it chars before it burns, is
called the organic substance of the plant. It is called
organic because it enters into the composition of all
organisms. At first people thought that organic matter
could be formed only in living bodies, in organisms, and
that only less complicated substances, which make up
dead or inorganic nature, could be produced artificially
in laboratories. But this opinion has been shaken by
recent progress in organic chemistry. Chemists can
now produce a great number of bodies, the forma-
tion of which used to be considered a mystery of
the living organism. All organic substances do not
necessarily consist of all four elements ; some of them
are composed of three only, carbon, hydrogen, and
oxygen ; or only of two, carbon and hydrogen. More-
over, these same elements are combined in different
proportions in different substances, so that obviously
in different plants, or in different parts of the same
plant, the elements will be present in different pro-
portions. Nevertheless, by taking the mean of a
number of analyses of various plants and of their
component parts, we can form an estimate of the
average elementary composition of a plant. One
hundred parts of dry vegetable matter contain on an
average :—
45°0 per cent. of carbon.

65 Cy, of hydrogen.
Si yy of nitrogen.

42°0 5 of oxygen.
5'0 3 of ash.

This table gives a clear idea of the ratios in which the

THE CELL 39

solid and gaseous elements must combine in order to
produce a certain amount of vegetable matter. When
we pass from the combustible organic part of a plant
to study the ash, we find that a greater number of
elements enter into the composition of the latter.
We shall here enumerate only the principal ones, having
to return to the closer study of them in our fourth
lecture.

ELEMENTS
IN THE ORGANIC MATTER. IN THE ASH.
Carbon. Sulphur. Potassium.
Hydrogen. Phosphorus. Magnesium.
Oxygen. Chlorine. Calcium.
Nitrogen. Silicon. Tron.

The first four elements of the ash form acids, which
with the four metals mentioned in the second column
form salts.

When once we know of which elements a plant is
composed, and knowing also that elements are incapable
of transformation one into the other, we can say before-
hand what are the sources from which these sub-
stances have been derived.

In the air, in the atmosphere, a plant comes into
touch with free oxygen and nitrogen, and with small
quantities of carbonic acid—a gas composed of carbon
and oxygen—and also with very small quantities of
nitrogen combined with oxygen and hydrogen. In
the soil, besides the substances just mentioned, the
plant comes into touch with others, which, owing to
their non-volatility, cannot exist in the air; these are
salts which contain the other elements found in the
plant. Some of these salts are dissolved in the water
of the soil, and so form part of the liquid environment
of the plant ; others exist in solid form.

So far we have only disentangled the chemical
elements of which the body of a plant is composed;

40 THE LIFE OF THE PLANT

or, rather, we have discovered the elements into which
the substance of the plant can be broken up: for this
purpose we had to destroy the plant itself, to burn it
down. This elementary analysis does not, however,
give us any information as to the substances or com-
pounds which enter into the composition of a living.
plant. For this purpose another course must be fol-
lowed ; and, first of all, as has been already said, we must
peep into the cell, the microscopic laboratory where
all kinds of matter, produced by the plant, are formed.

It is not difficult to see a cell, every part
of a plant consists of them ; but to see it
alive, uninjured, is easy only in such parts
as consist of single cells or of single rows of
cells ; such, for instance, as hairs. Many
people will know by sight, if not by name,
a plant very generally grown indoors and in hot-houses
with long, narrow leaves and violet-coloured flowers
with three petals—I mean Tradescantia virginica
(Spiderwort). The stamens of this flower are made

THE CELL 4I

conspicuous by a great number of violet hairs (fig. 15,
B), each of which consists of round or oval cells,
arranged in a row, like a rosary. If you detach one
of these threads with a needle and place it under the
microscope you will notice younger cells at the tip of
it which are nearly round, whereas at the bottom the
cells are older and oblong (fig. 15, C).

To begin with, we distinguish in such a cell between
its thin and perfectly transparent wall and the actual
contents of the cell. At first the cavity of the cell is
filled by a uniform, semi-fluid mass called protoplasm,
with a round body called a nucleus embedded in it, which
we shall study later on. Subsequently little spots
appear in the semi-fluid protoplasm, like cheese eyes,
so to speak, filled with liquid. Thus the contents of
the cell become separated into two parts, the proto-
plasm and the liquid cell-sap, becoming more and more
frothy. Later still the proportion of sap to protoplasm
increases; the volume of the protoplasm diminishes
relatively as that of the cell augments. In the end
almost the entire cavity of the cell becomes filled with
the watery sap, and the protoplasm remains only as a
thin layer, lining the inner wall of the cell, or stretching
from one wall to the other in little strands. In Trades-
cantia such a differentiation of the contents of the cell
is particularly well marked, because the cell-sap is violet
in colour while the protoplasm is colourless. Besides
these two substances, protoplasm and cell-sap, we also
frequently notice in the cavity of the cell something of
a different kind—small, shining drops with an oily
appearance, or round, colourless little grains, the char-
acteristics of which will be studied later. At a later
stage the contents of the cell sometimes disappear, and
the cavity fills with air. Such a skeleton of a cell must
be considered dead. The dry, sapless part of a tree,
for instance, may be considered as formed of such dead
cells. Thus in a living, active cell the microscope

42 THE LIFE OF THE PLANT

reveals the following substances : the wall, the proto-
plasm, the sap, and occasionally other bodies such as
drops or grains.

So much for the microscope. Now let us return to
chemistry with its balance and reagents; but this time
let us stop a little earlier in our analysis without reducing
the plant right down to its elements. We shall try to
separate out the different substances which enter into
the composition of the plant without destroying them,
dealing with them as they actually exist in the plant.
In a word let us study the proximate constituents of a
plant—I say proximate in contradistinction to the
ultimate constituents, which are the elements.

Evidently it is impossible to study here all the
various substances which the vegetable world produces
—everything we find at our grocers’ and chemists’
shops, at the carpenters’ and the confectioners’, in-
spinning factories and at dyeworks. We shall limit
ourselves to the commonest bodies, or rather groups of
bodies, without a study of which it is impossible to
understand vegetable life.

Let us choose for an illustration some vegetable
organ, say grains of corn. Let us take them in a
powdered form, as flour. As we shall see in a moment,
flour represents a heterogeneous mixture of substances.
To separate them let us prepare a small lump of dough,
and wash it a long while with water, working and knead-
ing it with. ourhands. At first the water runs off milky-
white in colour, but gradually it becomes quite clear.
We have now instead of dough a lump of something,
greyish-white in colour, sticky, and flexible like india-
rubber or leather. This is called gluten, and is that
constituent part of flour which makes dough sticky.
If, on the other hand, we let the water stand which ran
off during the washing we observe that it becomes quite
clear, while a very thin white sediment, quite soft to
the touch, forms at the bottom of the glass. This is

THE CELL 43

starch, the well-known substance which is used for
dressing linen and also in the kitchen. Thus we have
separated the flour, simply by washing it, into two of
its components: gluten and starch. If we had mixed
the flour with ether and let it stand, then poured off
the ether and let it evaporate in an open dish,
we should have obtained an oily residue. Thus flour
or grains of corn consist chiefly of three substances :
gluten, starch, and oil.

The methods of separating these substances which
have just been described may serve as a rough but
obvious example of a so-called proximate analysis.
In such an analysis we try if possible to extract sub-
stances, without altering them, by taking advantage of
their properties of dissolving or not dissolving, of
volatilising, crystallising, and so on.

These three bodies, starch, gluten and fat, may be
taken as representatives of the three principal and
most widely diffused groups of vegetable substances.
These groups are known as carbohydrates, albumin-
oids, and fats. Other substances are generally met with
either in comparatively small quantities or else in
exceptional organs or plants, and consequently do not
affect the general phenomena of vegetable life. Here
is a table giving the proportions in which these proximate
constituents are present in various widely differing
vegetable products. These analyses fully endorse what
has just been said about the large mass of the plant
consisting of the three classes of compounds which have
been enumerated.

IN 100 PARTS. CLOVER WHEAT LUPINE FLAX
plant. flour. seeds. seeds

(¢.e. linseed).
Carbohydrates . 16°6 748 45°5 62°2
Albuminoids . - 37 11'8 34°5 20°5
Oils i ‘ . 0S 1'2 6'0 37°0
Ash ; . gl ay o'7 3°5 570

Water. . . 780 12°6 14°5 12°3

44 THE LIFE OF THE PLANT

The carbohydrates are so called because hydrogen and
oxygen are combined in them in the same ratio as they
are found in water; since they also contain carbon,
they seem to be composed of carbon and water. The
following substances belong to this group of carbo-
hydrates: common cane sugar, beetroot sugar, and
grape sugar, or glucose, which is found in old raisins ;
gums, such as the gum which oozes out of the stems of
cherry trees; starch; and, lastly, cellulose, the sub-
stance which forms the solid skeleton of the plant, its
cell-walls, and which is used in our cotton and linen
cloths, and in paper. The carbohydrate group is some-
times spoken of also as the sugars, because some of the
members of the group, as we have just seen, are actual
sugars, while others can be easily changed into sugar.
For instance, by treating starch with dilute sulphuric
acid starch sugar is obtained. Cellulose can also be
changed into sugar if treated with the same acid. The
same method will transform old rags into sugar. The
carbohydrates we have mentioned seem to fall into a
series : cane-sugar and glucose are easily soluble in
water and capable of crystallisation ; gums, like cherry
gum for instance, are soluble in water, forming a thick
viscous liquid, but are incapable of crystallisation ;
starch does not dissolve in cold water, but swells in
hot water, forming a sort of paste; lastly, cellulose
neither dissolves nor swells in cold or hot water.

Now let us see how we can detect the presence at
least of the chief of these substances. They are all
colourless, but we possess means of producing in them
certain characteristic colour changes. The colourless
liquid in this glass is a solution of grape sugar, the other
glass contains a bright blue liquid. I pour the colour-
less liquid from the first glass into the blue liquid
in the second, and slightly heat the mixture. It
becomes turbid, then turns a dirty green colour, and
finally forms a yellow precipitate which turns brown,

THE CELL 45

then bright red, and sinks to the bottom of the glass,
leaving the liquid colourless. Therefore grape sugar
produces a red precipitate in our blue liquid ; or, in other
words, this blue liquid, otherwise called Fehling’s
solution, by changing colour reveals the presence of
grape sugar. This reaction is so delicate that it will
betray in a liquid the presence of the most minute
quantity of this sugar. Thus we have in Fehling’s
solution a valuable reagent for detecting the presence of
very small quantities of grape sugar. In iodine we have
a similar reagent for detecting the presence of starch.
I take a large beaker of water, add to it a few drops of
starch solution and stir. I have thus in the liquid
minute traces of starch. I add to it a few drops of
iodine solution, yellow in colour, and the liquid at once
turns blue. In the same way if I drop iodine solution
on a lump of dough or a piece of bread, I get a dark blue,
almost black spot, because starch is contained in both
substances; but if I drop some iodine solution on a
piece of gluten, I do not get any black spot, because the
starch has been previously washed out with water.
So iodine stains the colourless starch blue, and therefore
serves as a reagent for detecting starch. We have now
to find means for a similar detection of cellulose. Iodine
by itself does not stain it blue, but iodine and zinc
chloride will. We have only to drop this solution on a
sheet of white paper, which, as we know, is cellulose,
to produce on it a blue spot. Such are our reagents,
our means for detecting the most widely diffused
carbohydrates, grape sugar, starch and cellulose.

Now let us pass to another group, that of albuminoids.
These are found either in solution, as in the juice of the
cabbage, or in a solid form, e.g. the gluten we have just
obtained from our wheat grain. As soon, however, as
we heat cabbage juice, we see it turn into flakes: the
albumen has ‘set’ or coagulated in the same way as
an egg ‘sets’ when it is boiled. Chemistry presents a

46 THE LIFE OF THE PLANT

whole series of reagents by which we can detect the
presence of albumen. Let us experiment with one of
these reagents, the most obvious if not the most certain.
I have in a glass a certain quantity of the white of an
egg in water. I add to it some ordinary syrup of sugar,
together with concentrated sulphuric acid. A pre-
cipitate forms which dissolves again, and all the liquid
gradually turns a splendid red colour. In this way
albuminoids can be detected by means of sulphuric acid
and sugar.

There remains a third group, that of oils and fats.
We have no clear and simple reagents to produce in them
such characteristic changes of colour; but instead, as
we have already noticed, we have only to treat a sub-
stance in which the presence of oil or fat is suspected
with ether, and the ether will dissolve them. Then if
we expose this solution to the air, and let the ether
volatilise, we get oil or fat with its characteristic
properties.

Now we can reproduce in cells under a microscope all
the reactions we have mentioned. Suppose we add
sugar and sulphuric acid to water in which a cell is being
observed. We shall notice the protoplasm turning
pink, which proves that it consists chiefly of albuminous
substances. Let us use Fehling’s solution, and if the
cell-sap contains any trace of grape sugar we shall get
a red precipitate. We add a drop of iodine solution, and
notice that the small colourless grains in the cavity of
the cell turn blue: this indicates the presence of starch.
We take next iodine dissolved in zinc chloride solution,
and the whole cell-wall turns blue, which means that it
consists of cellulose. Finally we add ether, and notice
that the drops which had attracted our attention by
their oily appearance have disappeared, have dissolved,
which proves that they were drops of oil. Such is the
way that chemical analysis and microscopic investiga-
tion work hand in hand, mutually supplementing each

THE CELL 47

other. Analysis shows (see table on p. 43) that the
substances most abundant in the plant are carbo-
hydrates, and the microscope confirms this fact, showing
that carbohydrates form the cell-wall, appear in the
shape of grains of starch, or are dissolved in the cell-sap
in the form of sugar. Analysis shows that in relative
abundance albuminoids take the second place, and also
that the younger parts of a plant are comparatively
richer in nitrogenous substances than older parts; the
microscope demonstrates that protoplasm consists chiefly
of albuminous substances, containing nitrogen, and that
this protoplasm is the predominating constituent in
young cells. Lastly, both microscope and analysis
point to the presence of fatty substances in the plant
and in the cell.

We have now made acquaintance with the principal
substances contained in a vegetable cell. Already we
had come to the conclusion that the cell builds up
all these substances from gases, salts, etc., which
surround it. In other words it must feed from the
outside. Every cell must draw its food from the soil,
from the air, or from some neighbouring cell. A
question naturally arises here: in what way can this cell,
this little bladder without any opening, or any mouth
or jaw, attract and absorb surrounding substances ?

To explain this first phase in the nutrition of the
vegetable cell we must turn aside from it for a while,
we must turn aside even from botany itself, and study
some purely physical phenomena; we must study
certain general properties of matter manifested in dead
as well as in living nature. We shall often use this
method in the future. It is the only sure method
whenever we wish to find the explanation of vital
phenomena ; for, in the language of physiologists, to
explain means to reduce complicated vital processes to
more simple physico-chemical phenomena.

48 THE LIFE OF THE PLANT

Physics teaches us that particles of matter are
endowed with motion, that we do not know any matter
without motion. This motion is most clearly manifested
in fluids, and more especially in the gaseous state of
matter. Particles of gaseous matter are endowed with
rapid motion: they tend to disperse until they fill up
all spaces unoccupied by them ; this goes on until they
are equally distributed everywhere throughout the region
accessible to them.

This capacity, this tendency of matter to spread in
space, is called diffusion. It is a simple matter to prove
the existence of the phenomena of diffusion, especially
in respect of gaseous and volatile substances. We have
only to sprinkle a few drops of ether to smell it in an
instant not only in the immediate neighbourhood but
also in the remotest corners of the room. The ether
has changed into vapour, and that vapour has
distributed itself throughout the whole room. The
diffusion of liquids is also easily demonstrated. I only
need to remind you of the probably well-known experi-
ment with water and wine. We gently pour some
claret on to the surface of water, and notice that the

liquids form two distinct layers ;

a 6 but little by little the sharp
<= < boundary between them disappears,
k=! the wine permeates the water and
the water the wine, so that both
liquids mingle together. We can
perform here a similar but still more
striking experiment (fig. 16). Here
are two almost colourless liquids
which, when poured into each other,
<< ~<a produce a blood-red liquid. We

a pour the denser of the two liquids
into this long, narrow beaker, then

with care the lighter one on the top of it. A narrow
layer of red solution appears between them; but in time

THE CELL 49

this narrow, hardly visible red line will broaden, and at
the end of this lecture will be several inches in breadth ;
while in several hours, or it may be days, the whole liquid
will be a uniform red colour. Apparently both liquids
interpenetrate each other. This depends on motion
—peculiar to their particles, and therefore invisible—
on their tendency to spread in space; otherwise we
cannot explain how, in spite of the force of gravity,
the lighter particles sink to the bottom while the
heavier rise to the surface.

Different substances are endowed in a different degree
with this property of diffusion—in
other words, particles of different
substances move with different
velocities. This is best demon-
strated with gases. This vessel
(fig. 17) made of very porous clay
(a) is joined below to a glass tube
(b) immersed at its lowest end in
water coloured red. Both vessel
and tube contain air. The object
of this apparatus is to demonstrate
the slightest change in the volume
of air, contained both in the vessel
and the tube. If by any chance
the volume increases, air will begin
- to escape in bubbles through the
coloured liquid. On the other hand
if the volume of air in the apparatus
decreases, the coloured liquid will
rise in the tube. In the meantime
neither happens because the air in-
side the vessel is just like that
outside. But if we surround the Fie, 1
vessel with another kind of air,
with another gas, it is clear that a mutual interchange
of gases will take place through the porous wall,

D

50 THE LIFE OF THE PLANT

which can be penetrated by them; each of them will
strive to permeate the other. Obviously if the two
gases tend to spread, and their particles move at
different velocities, a temporary change of volume will
result in the apparatus; the volume will increase or
decrease according as the gas without enters more
quickly or more slowly than the gas within escapes.
A similar phenomenon will take place in a few minutes
on the threshold of this hall. Let us suppose that
there are three hundred persons at present in the
hall; and, further, that one hundred of them are
bored with this lecture (all too prolonged), and are
impatient to hear the end of it and to leave the hall ;
while another hundred persons are standing outside
waiting to enter for the next lecture. If the former
leave the hall in the same hurry as the latter enter it
the number of people in the hall will remain the same.
But if those outside, not feeling so weary after an hour’s
mental effort, should prove more energetic, the number
of people in the hall in the first instance will increase,
the hall will fill up, and only later, when those wishing
to leave actually do so, will the number remaining
decrease to the original three hundred. The same thing
happens here. If I surround this porous vessel with gas,
the particles of which enter more quickly than the
particles of air contained in it pass out, the vessel will
for a short time contain more particles of gas than it can
actually hold, and the superfluity of gas will escape in
bubbles from the end of the tube. I take a glass bell
full of hydrogen. Since this gas is lighter than air, it
can be kept for a certain time in a vessel with its opening
turned down. I lower the bell (c) over the porous
vessel (a). The inside of the vessel contains ordinary
air ; outside, under the bell, is hydrogen. If particles
of hydrogen are endowed with more rapid motion than
particles of air, the inner volume of gas must increase,
and you hear and see the bubbles of gas bubbling through

THE CELL 51

the coloured liquid in the beaker. I lift up the bell now ;
the conditions are altogether reversed ; hydrogen is
now inside the vessel, air outside it ; hydrogen moves
towards the outside, air passes in; but particles of
hydrogen move more quickly than those of air, so the
volume inside the apparatus decreases, and you notice
the red liquid rising quickly in the glass tube (6).
Therefore gases, even more than liquids, are capable
of diffusion, 7.e. are capable of permeating all spaces as
yet unoccupied by them. The hydrogen rushed into

oh ag y/

Z=
aS , 2
<
C
A
eS

Fic. 18.

the vessel only because the latter contained no hydrogen,
and later rushed out of the vessel only because none
was present in the air of this hall. Likewise all gaseous
matter and also matter dissolved in liquids tends to
occupy the whole space accessible to it, and to spread
uniformly through it.

Now let us see in what relation the phenomena of the
diffusion of gases and liquids stand to our question
concerning the nutrition of the cell. Here is an appar-
atus reminding us pretty closely of a cell (fig. 18). It
is a thin bladder moistened with water and transparent
as glass, made out of a substance like cellulose or
rather actually made of cellulose itself, only a little

52 THE LIFE OF THE PLANT

modified chemically. This is nothing but collodion,
such as is used by photographers. The bladder (A) is
joined to a horizontal glass tube (B) which contains a
drop of a coloured liquid (a2). We can judge whether
the volume of air in the bladder increases or decreases
according to the movement of the drop towards or from
the bladder. I let the bladder down into the broad
and empty vessel (C) and pour into it some carbonic
acid. You cannot see it because carbonic acid is a gas
as colourless as air. But I am correct in saying that I
pour in the carbonic acid because it is heavier than air,
and so can be poured from one vessel into another
while remaining totally invisible. Carbonic acid can
be kept for a certain length of time in a vessel open at
the top in the same way as hydrogen can be kept for a
short time in a bell open at the bottom. After having
introduced carbonic acid into the vessel surrounding the
bladder, the drop of coloured liquid trembles and then
runs along in the direction of the arrow, thus demonstrat-
ing that carbonic acid has begun to penetrate through
the moist wall of the bladder, which is our artificial cell,
and moreover that it is doing so more quickly than the
air is escaping from the bladder. In the same way a
vegetable cell has no need to attract or imbibe gases,
such as carbonic acid, which, owing to its property of
diffusion, will penetrate independently any cell devoid
of it.

Now let us observe the behaviour of vegetable cells
towards substances dissolved in the water of the soil.
Let us take some oblong bladders made of the same
collodion and fix them to the extremities of lamp
glasses (fig. 19). Suppose these collodion bladders
represent root-cells, by means of which the plant comes
into contact with the nutrient substances contained in
the soil. A plant, as we know from its chemical com-
position, needs among other things iron salts. We
choose these for our illustration, because they give

THE CELL 53

very obvious reactions by which it is easy to detect
slight traces of them ina solution. I have, for instance,
some water in this glass. J add to it a few drops of an
iron salt and then some of another liquid (a solution of
tannin), and the solution previously as colourless as
water turns as black as ink; in fact it is not quite
correct to say as ink, because it actually is ink. We
put into the vessel which contains water a bladder of
collodion also filled with water (1). Then we pour some

Fic. 19.

iron salt into the vessel and some tannin into the
bladder. Almost immediately a greyish tint appears
near the inner wall of the bladder, and in a few minutes
all the liquid in the bladder is turned into ink (2). We
notice therefore that the iron salt spontaneously
penetrates into our cell ; and we know that this process
will continue as long as the solution of the salt in the
cell is weaker than that in the vessel outside it, for only
then will as many particles enter the cell as pass out of
it—in a word, until equilibrium is established. But
here a question arises : can an equilibrium of that kind

54 THE LIFE OF THE PLANT

be reached in our illustration ? Apparently not; as
soon as our iron salt penetrates into our cell, it enters
into combination with the tannin, forming what for the
sake of brevity we shall call ink. Therefore the cell
will contain ink, but no more iron salt, and if it does not
contain any iron salt a fresh supply of the salt will
enter from the vessel outside ; this in turn will become
ink, and so on. If the bladder contains a sufficient
quantity of tannin, the equilibrium will never be reached,
and the iron salt will diffuse into our cell in a continual
stream. Thus we have only to take the collodion
bladder containing the solution of tannin, and put it
down into the vessel containing the solution of iron
salt, to withdraw from this solution the whole of its
salt and transfer it into the bladder. Let us put
aside this apparatus for a few hours or days, and we
shall find that there is no more iron salt in the outer
vessel : our artificial cell will consume it, will absorb it
completely.

Apparently we are approaching the simple physical
explanation of the way nutrient substances enter into
the vegetable cell. We have seen that a gaseous or
soluble substance penetrates into a cell spontaneously,
and goes on penetrating into it until it finds itself
present equally on both sides of the cell-wall. We have
noticed further that this equilibrium will never be
established if the substance which penetrates into the
cell is transformed there and enters into a new combina-
tion. In that case it will rush into the cell in a per-
petual and continuous stream, and become precipitated
therein. We perceive here one of the reasons why the
mass of a plant increases, i.e. why matter becomes
accumulated in it; but to complete our explanation we
require one more link in the chain. The process of the
accumulation of matter in a cell will become quite clear
only in so far as we admit that substances enter freely
from outside into the cell, and that those into which

THE CELL 55

these are transformed inside the cell, z.e. substances
within the cell, do not again escape from it. The
experiment we have just performed fully confirms this.
In fact the liquid blackens only inside the collodion
bladder ; outside, it is as colourless as water. This
would not have happened could tannin, or its compound
with the iron salt, ink, pass through the cell-wall. To
verify this let.us perform the converse experiment.
Let us pour some iron salt into the cell and some tannin
into the outer vessel. In a few moments black streams
will be noticed in the outer vessel, and in the end the
whole liquid in it will become so black that the bladder
will be invisible (3, fig. 19). Let us take it out of the
vessel—the solution inside of it is as colourless as it
was at first. Without doubt it is only the iron salt
which passes freely through the membrane, with equal
ease in either direction; but neither tannin, nor its
compound with iron, can pass through it. It follows that
two kinds of substances exist : some of them are capable
of passing through the membrane of the cell, others are
not; the iron salt serves as an illustration of the
former kind and tannin of the latter.

Indeed these two substances may serve as types of
two great classes of chemical bodies. Those of the one
class pass easily through vegetable or animal membranes;
those of the other pass with difficulty. We have noticed
in speaking of the diffusion of liquids that some diffuse
more quickly, others more slowly; some are more
mobile, others less. We may now add that those
substances which diffuse slowly are precisely those that
pass still more slowly through membranes. Chemists
call substances of the former class crystalloids, since they
are all capable of crystallisation; substances of the latter
class they call colloids, i.e. gum-like substances ; these
are all incapable of crystallisation.

We have here at once an explanation of our experi-
ment, and a general key to the phenomena which take

56 THE LIFE OF THE PLANT

place in the nutrition of a vegetable cell. It is the iron
salt which moves toward the tannin, not the tannin
towards the iron, because the iron salt is a crystalloid,
whereas tannin is a colloid. Going back to the nutrition
of the cell we meet, roughly speaking, the same pheno-
menon. What, in fact, are the substances a cell finds
in its environment ? Gases, water, and salts dissolved
in water ; that is, crystalloid substances—which means,
generally speaking, extremely mobile substances which
easily pass through the cell membrane. What sub-
stances does such a cell contain, into what does it
transform the substances absorbed from the outside ?
It transforms them chiefly into albuminoids, oils, gums,
starch, or cellulose—in other words, into colloids,
scarcely mobile substances which will not pass through
membranes or into other substances totally insoluble.
This may be easily grasped with the aid of the following
table -—

MAIN SUBSTANCES
|

Vegetable Substances. Their Sources.
Cellulose. Carbonic acid.
Starch. Water.
Albuminoids. Salts.
Oils.

Insoluble bodies and colloids. Gases and crystalloids.

Throughout its life the cell is continually surrounded
by substances which easily diffuse into it. Thus, for
instance, the carbonic acid of the air rushes constantly
into every cell with which it comes into contact. But if
this carbonic acid remained in the cell unchanged, only
a very little of it would penetrate into the cell ; however,
as we shall see later on, once in the cell it is changed :
carbonic acid and water form a carbohydrate, and this
transformation leaves room for the entry of fresh

THE CELL 57

quantities of carbonic acid. Therefore, the two phases
of nutrition: the diffusion of nutrient substances and
their transformation into the very substance of the cell,
their assimilation, are closely related to each other.
One process is conditioned by the other: did assimila-
tion not take place there would be no more diffusion ;
did diffusion not take place there would be no matter
for assimilation. Moreover, since by means of such
assimilation the substance is transformed into a hardly
mobile or even totally immobile form, it does not diffuse
away again, but accumulates in the cell.

When we examine the nutrition of the plant from
such a general, physical point of view, we get a concep-
tion of it quite different from the usual current ideas
upon the subject. It is not the plant, nor the cell,
which attracts or imbibes nutrient substances ; on the
contrary it is the nutrient substance itself which rushes
into the cell owing to its inherent mobility. A cell
is simply a microscopic centre, where the equilibrium of
the surrounding substances is constantly disturbed, a
kind of whirlpool, into which these very mobile sub-
stances rush in a continual stream, and within which
they lose their mobility, are transformed, and become
deposited. A vegetable cell is a trap which lets things
pass easily one way, but does not let them out again.
In this way we come to understand the fundamental
feature of vegetable life : increase of mass, accumulation
of matter.

As we shall soon see, these general ideas as to the
nutrition of the cell will prove to be essential at almost
every step in our study of the phenomena of the nutrition
of the whole plant. The nutrition of the root by sub-
stances in the soil, the aerial nutrition of leaves by the
atmosphere, or the nutrition of one organ at the expense
of another adjacent to it—to arrive at the explanation
of any of these phenomena we shall have recourse to
the same fundamental causes: (1) diffusion, i.e. the

58 THE LIFE OF THE PLANT

property by which substances spread, rush from where
they are present to where they are absent, and (2) trans-
formation, i.e. the passage of substances from very
mobile to less mobile or quite immobile forms.

In this way the study of the fundamental phenomena
underlying the nutrition of a vegetable cell brings us to
the conclusion that they are really phenomena of
diffusion, not essentially peculiar to living organisms,
but following rather from the general properties of
matter. We come to the conclusion that so funda-
mental a process as nutrition is conditioned by laws
common to both animate and inanimate nature.

CHAPTER III
THE SEED

Let us begin our survey of the living functions of a
plant with the awakening of the seed after its long
winter rest under the snow, or at the moment when it
is cast into the earth in spring. Probably no other
phenomenon in the life of a plant has attracted so much
attention as this, its first manifestation. Scientists,
philosophers, and poets have alike meditated upon it ;
a mystic and poetic veil hangs over it ; we find in it the
personification of life itself, the symbol of awakening
from dreams and death. There is something indeed
attractive, something that stimulates thought in this
sudden awakening of activity in an object hitherto
apparently indistinguishable from the rest of inanimate
nature. In fact there is something enigmatical about
this hidden and arrested life which suddenly bursts
forth again. Without indulging in the poetical fancies
with which imagination loves to enshroud this pheno-
menon, let us try to submit it to strict scientific analysis;
let us try to reduce it, complicated as it is, to its lowest
terms and explain the difference between a resting
seed and an active one. We may thus discover wherein
the very impulse consists which provokes this activity.
Outwardly the renewed activity of a seed is manifested
by its swelling and by the consequent rupture of its seed-
coat, followed by the appearance first of the root and
then of the plumule, z.e. of the stem with its first leaves.
These organs develop and increase in size every day.
It is obvious that this development must proceed at the

* expense of some substance, which serves as food for the

59

60 THE LIFE OF THE PLANT

growing parts. Yet, notwithstanding its rapid growth,
it is just at this period of germination that the plant is
practically independent of the soil. As a rule germina-
tion takes place in the soil ; but here is a brush-like mass
of green cress grown on felt, and there are seeds of
maize and beans grown on thin gauze net and, therefore,
surrounded merely by air and having their root-tips only
in distilled water.

On examining germinating seeds such as beans more
closely, we notice that while the root and the stem
with its young leaves in-
crease in size, the first pair
of leaves, the cotyledons, be-
come wrinkled, are gradually
absorbed, and become smaller
\ (fig. 20). This observation
may serve as an indication
of the fact that the develop-
ment of some parts of a shoot
takes place at the expense of
others.

Other seeds, such as grasses,
present a somewhat more com-
plicated structure than the
seeds of beans. If we split a
grain of wheat longitudinally,
we find under the seed-coat
two perfectly distinct parts
(see fig. 21: b, whole grain; c, d,
the separate parts). At the

Bre 20: base of the figure, a little to

one side, there is a small body

which is simply a seedling, an embryonic plant, such as

can easily be seen in any germinating seed (see fig. 21 8,

d,e). Wenotice in it a leaf-bud and the beginning of a

root. The remaining larger portion of the seed is filled
with a white, uniform, mealy mass, called the endosperm +

THE SEED 61

(fig. 21 ¢; 6). The part of the embryo lying close to
the endosperm i is called the scutellum (fig. 21 6, d; sc.=
scutellum). It is a kind of modified leaf and represents
the cotyledon of the embryo. In this case we find only
one cotyledon instead of two.

The nature as well as the position of the endosperm
may differ in different seeds. In grasses, for instance,
it is mealy; it is such endosperm which forms the
essential part of flour,
the embryo being com-
paratively small. The
embryo lies to one side,
and comes into contact 4
with the endosperm @M.-J
only by means of its t
scutellum. In the poppy,
on the other hand, the
embryo is surrounded by
the endosperm, and em-
bedded in it; and the
endosperm is not mealy,
but is fatty and oily (see
fig. 21 a; b=endosperm, S¢, .
em.=embryo). Lastly, in
coffee beans the greater
part of the seed consists
of hard, horny endos-
perm, and the very small
embryo is on one side Fic or:
enveloped by the endo-
sperm. A curious experiment will reveal the presence
of the embryo. Coffee beans are known to have already
lost their germinating power when they reach us; in fact
they possess that capacity only for a few days after being
gathered ; but if soaked in boiling water, or, still better,
in a solution of caustic alkali, we notice what appears to
be the germination of a seed which is certainly dead.

62 THE LIFE OF THE PLANT

In an hour, or even less, a small snow-white rootlet
protrudes from a slit in the testa ; sometimes the entire
little embryo is pushed out afterwards. What happens
is this: the endosperm of the coffee bean, although
as hard as horn, becomes soft and very elastic through
the action of the boiling water or of the alkali, and in
swelling compresses the embryo and squeezes it from
its place.

We see therefore that seeds may be of two kinds:
in some we find very well developed fleshy cotyledons ;
in others mealy, oily, or hard and horny endosperm.
Just as cotyledons shrink and decrease in size during
germination, so also the endosperm disappears little
by little, being apparently absorbed. This arouses the
suspicion as to whether the decrease of substance in the
cotyledon or endosperm is not connected in some way
with the increase in size of the shoot, z.e. whether the
development of the young plant is not achieved at the
expense of the food-substances stored up in the cotyledon
or endosperm. But all these substances are also
present in a resting seed ; why then is their displacement
manifested only during germination ? The answer to
this question will be apparent if we recall what we
learnt in our last lecture. The nutrient substances in
the endosperm or cotyledons exist in a solid and,
generally speaking, insoluble form. You remember
our analysis of flour, 7.e. of powdered seeds. We
detected there insoluble starch, insoluble gluten, and
oil. All these substances are immobile and incapable
of diffusion from one cell to another, a property which
is quite essential to them as reserve foods, since other-
wise they would not remain stored up.

Hence we have in the seed an embryo, and in a certain
part of the embryo the cotyledons; or in its immediate
neighbourhood, in the endosperm, we have stores of
nutrient substances in an immobile form, and on that
account inaccessible to the embryo. We now inquire

THE SEED 63

what conditions must be fulfilled in order that the
embryo may make use of these stores, may bring into
circulation this sunk capital.

These conditions are well known. Water is needed
—for the seed does not germinate in dry soil; heat is
needed—for a seed sown during a cold spring does not
show any sign of development until the sun warms it ;
lastly, air is needed—for a seed buried deeply in the soil
may remain very long without germinating.

Thus water, heat, and air are the three essential
conditions which awaken the seed to life. Let us in-
vestigate them in turn.

First of all water. Seeds generally contain very little
water (see table on p. 43). This is one of their essential
peculiarities. A seed which is not dry loses its most
important property—the power of concealing life, of
living through winters, years, and even centuries in a
dormant condition. If the seed is not dry it cannot be
preserved ; we cannot get good seed in a wet autumn—
grain then germinates in the sheaves or even before the
corn is cut. For a seed to remain in the resting state
the principal condition is thus the absence of water.
As soon as the seed is brought into contact with water
we notice an immediate awakening to life. The seed
swells and breaks the seed-coat which protected it.

This absorption of water is generally accompanied by
a considerable manifestation of energy. An English
scientist named Hales studied this phenomenon as early
as the beginning of the eighteenth century. He filled
a small iron pot with moistened beans, and covered
them with a lid upon which he placed a weight. He
proved in this way that bean seeds as they swell can
lift nearly two hundred pounds. Hofmeister has
demonstrated that seeds swelling under similar circum-
stances exercise on the walls of the vessel containing

64 THE LIFE OF THE PLANT

them a pressure equal to that of several atmospheres.
Anatomists make use of this property of seeds when
they wish to separate the bones of the skull: the
cavity of the skull is usually filled with beans, which are
then moistened. The bones of the skull separate at
the sutures owing to the strong and uniform pressure
all over the inner surface of the skull.

Such is the mechanical effect of water upon seeds;
it enables them to shed their coats which they no longer
need, and to overcome the resistance of the surrounding
particles of soil. But the chemical action of water is still
more important: without it the solution and consequently
the transfer of the stores of nutrient substances cannot
take place. Water, however, is not alone sufficient for
the purpose, because all these substances, as we have
seen, are insoluble in water ; in order to become available
for nutrition they must first of all be changed into other
substances. Starch, for instance, could be dissolved
in water if previously changed into the sugar, glucose.
Such a transformation is possible—the preparation of
starch sugar is based upon it, and it can easily be proved
that such a transformation actually takes place. We have
only to taste a raw barley grain and then a malted grain,
i.e. a germinating barley grain, to realise that the former
is tasteless whereas the latter issweet. But perhaps the
taste has deceived us ; in that case we can avail ourselves
of a test I described in our last lecture. We have seen
how the blue liquid—Fehling’s solution—gives with
glucose a bright red precipitate. We take malt mixed
with water, add to it Fehling’s solution, and get a red
precipitate. We cut off a thin slice of a germinating
seed, place it under the microscope, add a drop of the
same Fehling’s solution, and get a red colouration in the
cells. Therefore the taste as well as the more con-
clusive chemical reaction prove that sugar appears in
the germinating seed. But is it true that this sugar is
formed from starch ? Both quantitative analysis and

THE SEED 65

microscopic investigation answer this question, The
former shows that throughout the germination of the
seed the quantity of starch contained in it decreases.
The latter reveals a change in the starch grains: they
lose their characteristic form, appear as if corroded, and
sometimes break into pieces like ice that has been
thawed. ‘They do, in fact, dissolve away.

Now let us try and explain the reason for such a
transformation of starch into sugar. We can produce
this transformation artificially by using sulphuric acid ;
but seeds cannot obtain any free sulphuric acid. A
special substance called diastase appears instead of it
during the period of germination, producing on starch
quite a similar effect. Diastase may serve as a repre-
sentative of a whole group of substances generally called
ferments. The word ‘ferment’ usually denotes a
substance which when used in a minute quantity is
able to produce the chemical transformation of other sub-
stances. There are many such ferments. Bitter almonds,
for instance, in themselves have no characteristic taste
or scent ; these qualities are produced in them by means
of a ferment, which begins to act as soon as the seed is
brought into contact with water. Mustard seeds, too,
would not have their pungent odour and taste unless
they contained the ferment, myrosine, which with water
decomposes a substance contained in them (the salt of
the so-called myronic acid) and liberates the pungent
mustard-oil. A very curious experiment demonstrates
this phenomenon. Chemists sell mustard plasters which
consist of two sheets of paper, to be laid one upon the
other and then moistened with water. Neither leaf by
itself constitutes the mustard plaster, but the character-
istic pungent odour of mustard is produced as soon as
they are brought in contact with each other. This is
because one of the sheets is smeared with the ferment
and the other with the substance upon which the
ferment acts; the effect of the ferment only manifests

E

66 THE LIFE OF THE PLANT

itself when the sheets are moistened. These examples
are quite sufficient to illustrate the action of vegetable
ferments. A similar effect is also produced by diastase,
which is easily obtained from malt liquor, z.e. liquid
obtained from germinating grain. One part of this
diastase dissolved in water is sufficient to turn into sugar
more than a thousand parts of starch ; the warmer the
liquid the more quickly does this transformation take
place.

Thus the nutrition of the embryo of the seed by means
of the starch stored in its endosperm or cotyledons
becomes quite comprehensible. It is curious that this
process is exactly similar to that which takes place
during the nutrition of an animal organism. The saliva,
and the gastric and other juices secreted by the alimen-
tary canal, contain ferments which like diastase change
starch into sugar. It is quite easy to realise this:
if we suck a piece of bread a little longer than usual, we
notice a sweet taste. Thus both animals and plant-
embryos can make use of insoluble starch by changing
it into soluble sugar.

A similar change must also take place in seeds, like
coffee beans or date seeds, with hard and horny endo-
sperms. The character of the endosperm is in such seeds
due to the very hard cellulose walls of the cell. During
germination this cellulose dissolves and serves to feed
the embryo. This dissolution aroused the suspicion
that a special ferment was concerned, and the existence
of such a ferment has since been proved.

Let us pass to the second group of foods stored up in
the plant, to the albuminoids. In wheat grains and
flour, as we have already seen, they are present in the
insoluble and therefore immobile form of gluten; but
even soluble albumens, such as the white of a hen’s egg,
or the soluble albumen that occurs in vegetable endo-
sperms, are immobile, because they are colloids, i.e.
substances which do not pass through membranes,

THE SEED 67

In order to pass from one cell to another, and thus to
serve as food to plants, albuminoids must go through a
transformation similar to the transformation of starch
into glucose.

The study of the nutrition of an animal organism
will give us once more the key to the explanation of the
phenomena which take place in a germinating seed.
Gastric juice contains a ferment called pepsine, which
with a few drops of an acid has the property of turning
insoluble albuminoids into soluble ones; for instance, the
white of a boiled egg, or the albumen of cooked meat, into
soluble substances, called pepiones. These are not only
soluble in water, but are also capable of passing through
animal and vegetable membranes. For a long time
nothing of the kind was ever observed in the vegetable
world, and so long the translocation of albuminoids
remained unexplained; but at last almost simultaneously
discoveries were made in totally different directions
which demonstrated the transformation of albuminoids
in vegetable organisms.

Even as early as the eighteenth century a plant called
the ‘ Catch-fly ’ had been observed to seize, by means of
its irritable leaves, insects which came into contact with
them in their flight and then use them as food. This
fact, however, had not been sufficiently appreciated :
the sceptical even cast doubts upon the existence of the
phenomenon, and it might eventually have been quite
overlooked had not Darwin paid attention to it. Dar-
win added considerably to the list of such carnivorous
plants, and acquainted botanists with curious details as
to their functions. We shall postpone to a future
lecture the description of the mechanical side of such
phenomena, and here consider them only in so far as they
illustrate the fact of a plant’s capacity for using as food
an insoluble albuminoid. These phenomena of digestion
in plants were studied by Darwin more especially in the
sun-dew, a plant fairly common in marshes. The

68 THE LIFE OF THE PLANT

mucilage secreted by the hairs with which the leaves
of this plant are covered and which seize upon the insects,
contains a substance apparently similar to pepsine. This
substance, in the presence of an acid produced by the
hairs of the sun-dew when irritated, like gastric juice
has the power of rendering albuminoids soluble. Insects
which, in the natural course of things, fall on these leaves
and fragments of meat or white of egg placed upon the
leaves, as in Darwin’s experiments, alike become dissolved
and assimilated by the plant. These experiments which
proved that it is possible to feed a plant with insoluble
albuminoids led scientists to look for ferments like
pepsine in germinating seeds. Their discovery was not
long delayed. Such ferments were found first in
leguminous plants, and then in others such as hemp
and flax, and, lastly, in malted barley. A ferment
very similar to gastric juice in its effects has also been
discovered in the latex of Carica papaya. The nutrition
of the embryo at the expense of the stores of albuminoids
is now comprehensible : the pepsine-like ferment which
develops during germination acts upon the albuminoid,
transforming it into a soluble, diffusible form. In addi-
tion a certain quantity of albuminoid matter undergoes
a still greater transformation during germination, into
bodies capable of crystallisation—i.e. into crystalloids—
which diffuse still more readily.

Thus the embryo of a grass seed, for instance, not only
feeds upon the same starch or gluten that we use in
eating bread, but also digests them in the same way as
we do; treats them with similar ferments, and changes
them into glucose and peptones. We know less about
nutrition at the expense of the stores of fatty substances,
though we have some indications in this direction also.
Oils as such are generally unable to pass through cell-
walls moistened with water. They consist, however,
of so-called fatty acids in combination with glycerine,
a substance easily soluble in water; and certain facts

THE SEED 69

suggest that during germination oil is decomposed into
its constituents, acid and glycerine, likewise by means of
a ferment. Moreover, it is well known that fatty acid
when set free furthers the breaking-up of oil in water
into very fine drops with the formation of so-called
emulsions, such as the white oily liquids we call ‘ milk’—
cow’s milk, coconut-milk, and so on. This formation
of emulsions plays a great part in the nutrition of an
animal’s organism; very likely it also plays a certain
part in the nutrition of the embryos of oily seeds.
The first stage in the nutrition of a young seedling has
now been explained. By the action of water and
ferments the immobile material stored up within the
seed is brought into circulation, and becomes available
to the seedling. We can easily prove that the develop-
ment of the embryo takes place at the expense of the
stored material. We have only to cut off the cotyledons
of a leguminous plant to stop the further development
of the embryo, even though its root and stem have
already attained to some degree of development. The
cessation of further growth in the embryo cannot be
explained by the fact of its having been wounded ;
on the contrary, experiments prove that it has still
considerable vitality. We can cut it in pieces, in
various ways, and each segment will develop if only a
connection with the cotyledons containing the food-store
be maintained. In fact, if we cut off the rootlet, leaving
the plumule connected with the cotyledons, the stem
will develop even more quickly than if it had been
‘attached to an uninjured embryo; and, vice versd, if
we cut off the plumule, leaving the rootlet connected with
the cotyledons, the rootlet will develop more strongly
than if it had been attached to an uninjured embryo.
In these cases one of the two organs evidently uses the
food stored up for both. In seeds with endosperm
(albuminous seeds) the embryo is not organically
connected with the source of its food supply ; it may

70 THE LIFE OF THE PLANT

be closely attached to the endosperm, or even surrounded
by it, but in either case it can be separated from it
without being injured; this is why albuminous seeds
are the best specimens for the study of the nutritive
phenomena of the embryo. In grasses the endosperm,
originally dry and farinaceous, becomes less dense on
germination, till it resembles gruel or milk. Meanwhile,
the outer cells of the scutellum, the part of the embryo
adjacent to the endosperm (wm in fig. 21, 0, d, e), grow
out as papillae into the softened endosperm, and absorb
the nutrient solution from it. The embryos of buck-
wheat and of many other plants find themselves under
still more favourable conditions. They simply swim
in the semi-fluid mass of the endosperm, and absorb the
nutrient substances with their entire surface. If in
such seeds we remove the embryo from the endosperm,
it will stop growing; but its development can be
maintained artificially if, after removing it from the
endosperm, we enclose it within a lump of dough made
of flour or starch. That the embryo obtains its food
from the dough is shown not only by its successful
development, but also by the signs of decomposition in
the grains of starch in the immediate neighbourhood of
the embryo.

We have several times spoken of the embryo absorbing
the nutrient substances from the cotyledons or the
endosperm, but evidently this is only a metaphorical
expression, and the translocation of the nutrient sub-
stances into the embryo has to be explained on the
basis of the general phenomena of diffusion, which were
studied in our last lecture. We have seen that during
germination every nutrient substance passes into a
soluble form; and these solutions, according to the laws
of diffusion, have to distribute themselves equally in all
parts of the seed, including the embryo. The part played
by diffusion ends, however, with this equal distribution
of substances, and the attainment of equilibrium.

THE SEED 71

What is it, then, that disturbs this equilibrium, and, so
to speak, transfers the centre of gravity from the
endosperm to the embryo ? How can we explain this
transfer of substance from the endosperm to the embryo?
We can do so in the same way as in our last lecture we
explained the passage of the iron salt from the outer
vessel into our artificial cell, by the reconversion of the
diffusing substances into insoluble compounds. The
substances in solution which penetrate into the embryo
are used up in the development of new organs. Glucose,
a soluble carbohydrate, is thus converted into the
insoluble carbohydrate cellulose, of which the walls of
the new cells are built up. The soluble and diffusible
albuminoids are transformed into the insoluble and non-
diffusible protoplasm of these cells. This transforma-
tion, as we already know, will cause the diffusion of
fresh quantities of glucose into the embryo, and so on.
This dissolution and precipitation of substances in a
seedling, this drift of matter from endosperm to embryo,
will continue as long as they are in contact with one
another. Let us imagine that two persons have agreed
to share their movable belongings from time to time in
equal parts; then let us suppose that one of the two
persons is so imprudent as to gradually exchange his
immovable property into movable, whereas, on the
contrary, the other exchanges part of his movable
property for immovable. In the end all the property
of the former will have passed into the hands of the
latter. This is precisely how an embryo acquires its
food from the endosperm and the cotyledons. It
absorbs the food because of its growth, and it grows
because of the food it absorbs—here cause and effect are
mutually connected very intimately, as they are in any
vital function.

We therefore see that underlying the nutritive
phenomena of the embryo there are the same general
phenomena of diffusion and transformation, by means of

72 THE LIFE OF THE PLANT

which the nutrition of the cell was explained in our last
lecture. This parallel might, indeed, have been ex-
pected, since the life of the embryo is the sum of the
life of the cells which compose it.

We have now proved that during the process of
germination matter is only translocated from one organ
to another within the seed. Notwithstanding the
apparent increase in size and the growth of the young
plant, we can prove by weighing the seed and the seedling
that no increase in substance really takes place during
this period. Simple weighing would, however, be
insufficient for the purpose; if we record the weight
first of the seed and then of the seedling developed from
it, we certainly do notice that the latter is heavier than
the former ; but this is easily explained. We have seen
that different parts of a plant contain very different
quantities of water: seeds scarcely contain any water,
while the plant as a whole contains a considerable
quantity... During germination water is absorbed first
by the whole seed and eventually by the rootlet, a fact
which explains the addition in weight. If, on the other
hand, we had dried both the seed and the seedling at a
temperature of 100°, and determined their weight in a
dry state, we should have found that the plant has lost
in dry weight during the process of germination, although
it has increased in size. The question arises: what
has become of the lost substance ? Asa rule we do not
notice any excretion by plants of dry or liquid matter
such as takes place in animals, and even if there were
any such excretion, having taken it into consideration,
we should still find that the whole of the loss in weight
was not by any means accounted for. We can only
conclude that the seed loses some of its substance in
the form of gaseous products which disappear into the

alr.
1 See table of analysis on p. 43.

THE SEED 73

This supposition brings us to the consideration of the
second of the three conditions of germination, defined
a little while ago, namely, the importance of air. Air,
as we know, consists of oxygen and nitrogen. Experi-
ments point to the fact that seeds require oxygen. A
seed buried deeply in the soil, or remaining under water
which is never changed, does not germinate; but it
likewise does not germinate, or if already begun, the
process stops short, if the seed be surrounded with air
deprived of oxygen. It needs oxygen, undoubtedly.
Wherein, however, does the function of the oxygen
consist ?

It is easy to prove that oxygen is absorbed by the
seed. Now oxygen maintains combustion; in its
absence burning bodies become extinguished. There-
fore, if germinating seeds absorb oxygen and we leave
them for a certain length of time in a limited volume of
air, we shall deprive that air of oxygen and so rob it of
the property of maintaining combustion. Ten hours
ago we laid some germinating seeds at the bottom of
this wide-mouthed vessel, tightly closed with a glass
stopper. I open it now and introduce into it a burning
taper ; it is at once extinguished. Evidently the air in
this vessel does not contain oxygen any longer. The
oxygen has been absorbed by the seeds.

Having observed before how close is the parallel between
the nutrition of seeds and animals, we may legitimately
raise the question whether also seeds do not use oxygen
for the same purpose as animals? May they not use
it for respiration ? Respiration, as we all know, is in
its essence combustion. We inhale oxygen; it is
carried by the blood all through the body, and oxidises
or burns up part of its carbon and hydrogen, giving
them off in carbonic acid and water. We can see this
in the following simple experiment, which proves that
the gases we inhale and exhale are different in kind,
and that the gas exhaled contains carbonic acid. The

74 THE LIFE OF THE PLANT

two following tests serve to distinguish carbonic acid
from other gases. If carbonic acid is passed through
lime water, z.e. through water in which quicklime has
been dissolved, this clear solution becomes clouded with
a milky precipitate of chalk, z.e. calcium carbonate, a
compound of lime with carbonic acid. I take a flask
(fig. 22 A) into which two bent tubes are introduced
through the cork ; one of them goes right down into the
lime water, while the other, the shorter one, ends above
the surface of the liquid. I start by taking the shorter

Fic. 22.

tube (a) into my mouth and inhaling air through it.
The external air enters through the other long tube,
and passes in bubbles through the liquid, which remains
transparent. I turn the vessel, take the end of the long
tube (0) into my mouth, and exhale the air; the air
repasses in bubbles through the liquid which immediately
becomes turbid. In order to prove that the white
precipitate at the bottom is really chalk, and that it
contains carbonic acid, I add a few drops of vinegar—
the precipitate dissolves with effervescence, and these
effervescing bubbles of gas are nothing but the carbonic
acid which I have just exhaled.

THE SEED 75

Another test for carbonic acid is as follows. All
caustic alkalies readily absorb carbonic acid. I take a
glass tube with some carbonic acid in it, close its open
end with my finger, and sink it into a vessel containing
an alkaline solution. When I take away my finger,
which has been blocking the mouth of the tube, the liquid
rushes into it and fills it. The carbonic acid contained
in it has disappeared, 7.e. it has been absorbed by the
liquid.

With these means at our disposal for detecting the
presence of carbonic acid, we can now return to the
question: do germinating seeds breathe? We have
answered already one part of the question ; we have seen
that germinating seeds cannot get on without oxygen,
and that they absorb it ; we have now to show that they
exhale carbonic acid, in exchange for the oxygen they
inhale. For the sake of clearness I will give another
form to the experiment. A stream of common air is
driven into this intermediate vessel through an aperture
indicated by an arrow (fig. 22 B). (How this is done
does not concern us here, as it is only a technical detail
having nothing to do with the main point of the experi-
ment.) The air passes in bubbles through the solution
of caustic alkali, depositing in it those traces of carbonic
acid which are always present in the air, especially in
a room where so many people are breathing. From this
vessel the stream of air, now deprived of carbonic acid,
divides into two parts, passes (as indicated by arrows
on the figure) through two vessels (c and c’), and,
emerging from each, bubbles through the lime water
in the funnels (b and 6’) at the top of them. The
vessels are identical, a stream of the same air is driven
into both of them ; but there is this difference, that the
one marked c’ contains a layer of living germinating
hemp or bean seeds, while the other contains similar
seeds previously poisoned with corrosive sublimate.
The air in passing through the apparatus continually

76 THE LIFE OF THE PLANT

plays over the surface of the seeds, and then passes
through the liquid in the funnels b and 6’. You notice
already the difference which manifests itself : while the
liquid maintains its transparency in the left funnel, it
becomes turbid and turns milky white in the right one,
and within a short time a considerable layer of chalk is
precipitated. Evidently air after passing over a layer of
living, germinating seeds contains carbonic acid. Seeds,
therefore, absorb oxygen and give off carbonic acid.
We have now to show that these two processes are
correlated, z.e. that carbonic acid is given off in place of
oxygen which has been absorbed.
This can be demonstrated by the
following experiment, which at
the same time allows us to judge
of the energy with which this
process of respiration proceeds.
A narrow glass bell a (fig. 23)
is divided into two parts by
wire gauze; some germinating
hemp seeds are scattered in the
upper part, while in the lower,
which is closed with an india-
rubber stopper, a small beaker
is placed containing a solution
of caustic alkali. The upper
aperture of the bell is likewise
corked with an _ india-rubber
stopper through which passes a
bent manometric tube containing
a column of coloured liquid and
provided at b with a stop-cock.
This stop-cock is open for the pur-
pose of keeping the air inside and outside the apparatus
in equilibrium. Whenever this stop-cock is closed the
column of coloured liquid rises in the left limb of the
manometer and falls in the right one, so that its level will

Fic. 23.

THE SEED 77

very soon reach ¢ on one side and c’ on the other. The
meaning of this experiment is obvious : the seeds in the
upper part of the vessel give off carbonic acid, which
as we know is greedily absorbed by caustic alkali, some
of which is contained in the beaker below them; conse-
quently, there is a decrease in the volume of air in the
whole apparatus, manifested by the rising of the column
of liquid in the left limb of the manometer. This experi-
ment proves that carbonic acid appears in exchange for
another gas, absorbed by the seeds ; because, if carbonic
acid were only added to the air enclosed in the apparatus,
one of two things would happen: either the volume of
air in the apparatus would increase, or else it would
remain unaltered (on the supposition that the whole of
the carbonic acid produced is absorbed by the alkali).
The decrease in volume depends on the absorption of
oxygen by the seeds; in place of this oxygen an equal
volume of carbonic acid is generally given off, and this
is absorbed by the caustic alkali. Hence, this decrease
of volume serves both to measure the quantity of oxygen
inhaled and of carbonic acid exhaled. The column of
liquid rises so quickly that I shall have to open the
stop-cock (b) several times during the lecture in order
to keep the coloured liquid from running over into
the bell. This continual motion of the liquid in the
manometer demonstrates without further explanation
the invisible, inaudible, and yet fairly energetic breath-
ing of the seeds.

The latest investigations prove that the formation of
diastase, a ferment already familiar to us, is apparently
closely connected with respiration. When seeds, already
swollen in water, were enclosed in a vessel filled with
hydrogen instead of air, they never developed, nor was
any diastase to be discovered in them ; whereas, when
seeds of the same kind were left in contact with air
they sent out shoots containing diastase. Thus it is
that we come to an understanding of one of the im-

78 THE LIFE OF THE PLANT

mediate results of the respiration which awakens plants
to life.

Respiration as such explains the continual loss of
dry matter, the fact which called our attention to the
relation of the seed to the air. Respiration is a slow
and continual combustion of the carbon and hydrogen
of an organic substance, and as a matter of fact if
we compare the analysis of a seed with that of the seed-
ling which has grown out of it, the decrease in dry
weight has to be put down precisely to those elements,
whereas the quantity of nitrogen shows no change.

Having proved that a process of respiration takes
place in the germinating seed similar to that in the
animal organism, we may now go a step further and ask :
is not this process followed in the vegetable organism
by the same results as in the case of the animal organism ?
Respiration, being as a matter of fact slow combustion,
keeps up the temperature of the animal, warms it;
will it not warm in the same way a young germinating
plant, providing it with the heat necessary for its
development ? This question brings us to the considera-
tion of the third of the three conditions on which
germination depends, 7.e. to the consideration of the
effects of heat.

Even without exact experiment we notice by general
observation that seeds become perceptibly warmer
during germination, evidently owing to respiration.
Long ago it was noticed that during the malting process
the heaps of germinating barley grain get so warm that
the rise in temperature can be detected without a
thermometer, simply by plunging one’s hand into them.
It has been noticed also that rotten seeds spontaneously
burn, although here to normal vital processes there are
added processes of decay which are due to the activity
of certain micro-organisms. In more exact experiments,

THE SEED 79

in which micro-organisms were excluded as far as
possible, a rise of forty, fifty, and even more degrees
above the surrounding temperature has been observed.
Apparently this heating process is of advantage to
growing seedlings, because numerous observations made
by farmers and also more exact experiments by botanists
have proved that the rapidity of germination, 7.e. of the
first appearance of the rootlet and the further growth
of the embryo, depends directly upon the temperature ;
and, moreover, that for different plants there are
different limits at which germination is arrested. For
a great number of plants we can determine the lowest
and the highest temperatures at which germination will
begin ; between these two limits the rapidity of growth
increases up to a certain temperature, beyond which it
begins to decrease. In this way we distinguish three
temperatures : the minimum and the maximum, forming
the two limits of possible germination, and then the
optimum, at which the process is most successful, 7.¢. the
most rapid. Our cereals, for instance, generally begin
to germinate at about 35°-40° Fahrenheit, and the higher
the temperature the quicker the development; but
beyond 66°-68° Fahrenheit the process becomes slower
again, and almost stops at 100°. For a long time it was
supposed that at 32° F., z.e. at the freezing-point of water,
all active life, and consequently germination also, were
impossible ; but recently the interesting discovery was
made that seeds can germinate even in ice. The experi-
ment was as follows: a small cavity was hollowed in a
piece of ice, seeds were placed in the hollow and covered
with another piece of ice; the whole block was then
placed in a box surrounded with a layer of ice two feet
thick. Two batches of seeds were treated in this
way, one in January and another in March, and kept in
a cellar. Two months afterwards, z.e. in March and
May respectively, seeds of the most various plants,
such as wheat, rye, beans, cabbage, mustard, were

80 THE LIFE OF THE PLANT

found germinating ; their tiny rootlets were piercing
through the ice. This strange, unexpected but quite
authentic experiment, as well as similar facts concerning
the blossoming of some alpine plants, which flower even
in the snow, are probably to be explained by the fact
that heat is generated by the respiration of plants and
is capable of melting ice in the immediate neighbourhood
of the plants in question. Life was formerly considered
impossible at 32° F., among other reasons, because
water should freeze at that temperature ; but this is a
mistake, for it is known that water may not freeze even
at a temperature below 32°; for instance, it does not
freeze even at 10° F. in very narrow capillary tubes.

We see, then, that germination (and, generally
speaking, this is true of all vital processes) can take
place only within the narrow range of temperature
between the limits 32° and 104° F. These limits do not,
however, apply to resting seeds. Owing to their dryness
they are able to bear without injury much greater
extremes of temperature. When thoroughly dried
seeds can be exposed on the one hand to such high
temperatures as 250°-320° F., and on the other hand to
very low temperatures, obtained by means of liquid
air, without losing their capacity for germination.
A resting seed is thus distinguished by possessing to a
remarkable degree the power of enduring great extremes
of temperature, and this endurance constitutes one of its
important attributes.

Such is the significance of heat, the third factor by
which germination is conditioned. We must not,
however, suppose that the accelerating effect of heat,
the repressing influence of cold, and the fact of limits of
temperature, constitute a peculiarity of living organisms
as such. On the contrary, we know that the majority
of the chemical and physical processes at work in it
depend on the temperature. With an increase of
temperature, diffusion and the conduction of liquids are

THE SEED 81

accelerated in narrow capillary vessels ; with an increase
of temperature, too, diastase acts more rapidly upon
starch. But, we might ask, if physical and chemical
processes are accelerated by a rise in temperature why
is it that a further rise in temperature depresses the vital
activity of the seed ? Why is it that an intermediate,
most favourable temperature exists at all? Is this
really a peculiarity of the living organism ? In the
meantime no such supposition is necessary. We know
that if heat furthers some chemical processes, which
contribute to the acceleration of vital processes, it also
brings about other effects with which life is incompatible.
Albuminoids, for instance, such as enter into the com-
position of protoplasm, the foundation of every living cell,
coagulate like the white of an egg at about 144° F.,
and probably begin to change even before that tem-
perature. Obviously, if a rise in temperature causes at
the same time processes both favourable and unfavour-
able to the activity of the seed; that activity will be
most energetic at a certain mean temperature when the
salutary and the noxious effects of temperature present
the most advantageous combination. Thus we see
nothing in the effects of temperature on the germinat-
ing seed that would induce us to give up the physico-
chemical explanation of phenomena taking place under
its influence.

There is one more aspect of the life of seeds, an aspect
as yet unfortunately far from being explained, and that
is the difference of degree to which different seeds
preserve their vitality, i.e. their power to germinate.
Some seeds preserve it for years, or even centuries.
Others can germinate only within a few days after being
separated from the parent plant, and then lose the
capacity very quickly ; such are, for instance, coffee
beans and willow seeds. Others, again, can germinate
only after a considerable lapse of time. In this last
group we find most of the stone fruits. Here also closer

F

82 THE LIFE OF THE PLANT

investigation will probably enable us to discover the
immediate cause of these phenomena. As a matter of
fact the property of conserving during many years the
capacity for germination should not strike us as strange.
If a seed does not contain the necessary water, or is
isolated from atmospheric influences by its membranes,
or in some other way one or other of the conditions for
chemical changes is therefore not fulfilled, it is difficult
to imagine what influence time can have, once the possi-
bility of mechanical injury is removed. There are
indubitable facts which prove that seeds taken out of a
herbarium, in which they had lain for more than a
hundred years, germinated very successfully. Another
illustration frequently cited is the so-called ‘mummy
wheat’ which remained in Egyptian tombs for more
than a thousand years; but this case is not authenti-
cated. The capacity for remaining in a dormant state
during so many years without losing the power of
reviving is not the exclusive property of seeds alone.
Many of the lowest, microscopic animals, when in the
condition of a dry powder, can be kept for years in that
state, and revive again as soon as they are moistened
with water. The other extreme, the loss within a few
days of the capacity for germination, is harder to
explain. The facts here seem rather to bespeak some
special vitality in the seed which is gradually lost.
Yet such facts are not beyond explanation nor without
analogy elsewhere. Coffee beans, for instance, have
their nutrient material stored up in great part as
cellulose, which makes their endosperm hard and horny.
Very probably the solubility of this endosperm varies
greatly in course of time, since cellulose is able to
undergo such changes even outside the organism.
Freshly precipitated cellulose, or cellulose kept in a
moist condition, easily dissolves in a certain reagent,
but the same cellulose when dried and forming dense and
horny masses becomes almost insoluble. Possibly some-

THE SEED 83

thing of the kind happens in coffee beans, so that only
the fresh beans contain cellulose capable of passing into
a soluble state. As to the last category of seeds, those
that sometimes require several years to germinate, such
as the seeds of stone fruits, the mere mechanical obstacles
they have to overcome may be regarded as one cause
of such a delay. Indeed germination can frequently be
accelerated by making incisions in the hard wall of the
seed.

Let us now sum up all we have found out about the
phenomenon of germination, and let us try to define
the general characteristics of this unique period in the
life of the plant.

The period of germination is unique because of the
fact that during its course the plant does not require
any external supply of food; it subsists on the food
stored up in the endosperm or in the cotyledons. Three
factors are indispensable to arouse the seed to activity ;
these are water, the oxygen of the air, and heat. Water
acts in a twofold way : mechanically and chemically.
Mechanically, it causes the seed to swell, tears off its
membranes, and supplies the energy necessary for over-
coming the resistance of the surrounding particles of the
soil ; chemically, it first dissolves the various ferments,
and then with the help of these the insoluble substances
stored up in reserve. These solutions flow into the
embryo and are spent in its growth, in which they are
once more transformed into an insoluble or hardly
diffusible form.

Respiration, apparently a function of every organism,
every living thing on the earth, with very few excep-
tions which will be mentioned later, manifests itself in
the germinating seed. At the same time it account
for the loss in dry weight and the rise in temperature,
noticed in germinating seeds. In this way the seedling,
in spite of its increase in bulk, not only shows no

84 THE LIFE OF THE PLANT

increase in substance during that period, but on the
contrary expends its substance. During germination
it is only transformation which takes place, and not
assimilation. Consequently, we have arrived at the
conclusion that the phenomena of the assimilation of
nutrient substances and the phenomena of growth do
not always take place simultaneously ; and the period of
germination may be best characterised by saying that
it is a period of growth without assimilation.

This period in the life of the plant may be more or
less compared to the period of education and develop-
ment in man. Both plant and man are incapable
of independent and productive activity before they
reach the close of that period. They exist at the
expense of stores carefully accumulated by the preceding
generation, though man can scarcely be said to gain by
the comparison ; on the contrary, it is the plant that
sets an example worthy to be followed. On the one
hand plant-parents, after having accumulated a modest
inheritance, do not seek to secure an idle and careless
existence for their children; they simply endow them
with what is strictly necessary for their development
and well-being ; and, on the other hand, plant-children
do not dissipate their inheritance: they spend it on their
proper development and the generation of energy indis-
pensable for the first struggle for life that awaits them.

At the close of this period we find the plant already
provided with organs well formed and quite ready for
use. It is very curious that certain plants pass the
whole period of germination without separating from
the parent plant. Such is Rhizophora Mangle, which
grows on the shores of the tropical seas, on strips of land
generally covered at high tide. The seeds of this plant
germinate in the fruit and form long, heavy, and sharp-
pointed roots while still growing upon the maternal
plant. After having reached a certain stage of develop-
ment they break away, and falling vertically stick by

THE SEED 85

means of their roots into the mud, and continue their
existence without any interruption whatever.
At the end of the period of germination the physio-

Fic. 24.

logical division of labour manifests itself in the plant.
From the general physiological point of view the plant
represents two more or less well-developed surfaces—

86 THE LIFE OF THE PLANT

the surface of the root and that of the leaf—adapted to
its corresponding twofold environment; both these
surfaces are connected through the intermediate organ,
the stem. Such is the general physiological scheme of
the plant, a scheme that manifests itself very early,
even at the lowest stages of plant life. There are weeds,
for instance, which consist of single cells, but which
have parts analogous to leaf, root, and stem. Here isa
weed (Botrydium), not uncommonly found on flooded
plains, consisting of a green, round head and a colour-
less branching base, something like a root (fig. 24 to
the left), by which it is fixed to the soil. This is only a
simple bladder, but in it we notice already two physio-
logically differentiated parts. Another more striking
example is a weed found in the seas of warm countries
(fig. 24). Aspecimen of this sea-weed (Caulerpa) picked
up at Puzzuoli, in the bay of Naples, when straightened
out on a sheet of paper, covers an area as large as the
palm of the hand, and sometimes even larger. This
giant cell, one of the largest in the entire organic world,
presents expanded parts remarkably similar to green
leaves, stems, and colourless rootlets. The stems
spread along the bottom of the sea, the leaf-like laminae
grow up vertically, while the rootlets bury themselves
in the ooze; and this is nevertheless one and the same
cell, having one continuous cavity.

Such dissimilar organs as the root, leaf, and stem
must evidently fulfil absolutely different functions ;
and therefore in our subsequent study of the life
of the plant, we must study separately the life of
these organs.

There is, however, one more question to be settled :
when does the period of germination come to an end,
and the independent life of a plant begin ? Asa matter
of fact the period of germination ends the moment the

1 Fig. 24 represents two species of Caulerpa—the upper one the
common Caulerpa prolifera, found in the Mediterranean.

THE SEED 87

stores of food become exhausted ; the independent life
of the plant begins when the activity of the leaves
becomes apparent ; but in order that this may happen,
the leaves must be exposed to the action of light, in the
absence of which they will never become green, but will
remain yellow and unhealthy. Thus we have finally
one more peculiarity of the period of germination ;
during its course the plant is completely independent
of light, and does not require it ; this is why that period
can be passed in total darkness, underneath the soil.
But the independent life of the plant starts with the first
ray of light which falls on its leaf; it then begins to
develop new organs at the expense of surrounding
inorganic compounds instead of from food stored up
within itself. The decrease in weight diminishes and
soon passes into an increase. Assimilation of matter
begins.

CHAPTER IV
THE ROOT

WE saw at the end of our last lecture that during the
period of germination the young plant does not exhibit
the most characteristic feature of vegetable life, increase
in mass. On the contrary, in spite of a visible increase
in volume, it loses continually in dry weight, burning
away part of its substance in respiration.

It is only when the organs become individualised and
begin to fulfil their natural functions, the root sinking
into the soil and the stem with the leaves stretching
into the air towards the light, it is only then that the
full and independent life of the plant manifests itself in
the real assimilation of matter from the external
environment.

We know already what kind of matter is thus assimi-
lated: it consists of the twelve elements already
enumerated. We have now to settle a further question :
From what part of the environment does the plant obtain
these various substances—from the soil, water, or air ;
and what are the means by which they enter the plant?
We shall learn at the same time which of the two organs,
the root or the leaf, is to be considered the nutritive
organ of the plant, or whether they both serve that
purpose, each in its own way.

Let us begin with the root, since the problem seems
simpler in connection with this organ. Scarcely any one
has ever doubted the fact that the root serves the
purpose of nourishing the plant, and it is fairly easy to
prove that at least one form of nutrient substances
cannot penetrate into the plant otherwise than through

88

THE ROOT 89

the root. Thus, for instance, substances that enter
into the composition of ash cannot exist under normal
conditions in a gaseous state ; it is for this reason that
after complete combustion they remain in the ash and
do not volatilise with the rest of the vegetable matter.
Consequently, all that is found in ash must be looked
for in the soil; and therefore we conclude that such
substances enter the plant through its root. As to the
other elements, however, they may be in the soil as
well as in the air. We remain as yet uncertain with
regard to them: we cannot tell, until we get clear
evidence from an experiment, whether they are obtained
from the soil or from the air, and so whether they pene-
trate into the plant through its root or through its
leaves.

Let us turn our attention first of all to the root;
let us see what it absorbs from the soil, how it absorbs it,
and why it absorbs precisely that which is indispensable
to the plant.

But before we begin the study of the root and its
functions, we must become acquainted with the environ-
ment in which it manifests its activity, and cast a
passing glance at the soil itself and its composition.

Every kind of soil which is covered with vegetation
has two classes of components, which are sharply
differentiated ; these are the combustible and non-
combustible, the organic and inorganic substances.
The organic part, which is merely the residue of decayed
plants, gives the soil its black colour. This black
matter, which is burnt away when soil is calcined, is
called humus. Even the blackest soils, however, the
real moulds, contain comparatively small quantities of
humus, rarely more than Io percent. Soil which has been
thoroughly calcined and therefore deprived of decaying
organic matter no longer appears black, but yellow or
red. The mineral parts of the soil which remain after
calcining, and are in bulk its main constituents, can be

90 THE LIFE OF THE PLANT

divided into three groups according to their degree of
solubility. One part of them, the smallest, dissolves
in water; the second, larger part does not dissolve in
water, but dissolves in acids; and, lastly, the third and
by far the largest part does not dissolve either in water
orinacids. These three degrees of solubility correspond
roughly to the three degrees of accessibility of those
substances to the plant. The substances of the first
group which dissolve in the water of the soil are appar-
ently easily accessible to the plant; the substances of
the second group are less accessible ; while the sub-
stances of the last group are totally inaccessible to the
plant, unless in the course of many years they are partly
transformed into substances belonging to one of the
first two groups.

Thus at any given moment we find among the mineral
constituents of the soil, first, a basis, useless at present,
but containing stores of food which will be available in
the remote future; next, stores of food within com-
paratively easy reach of the plant; and, lastly, a very
small quantity of matter which serves the plant for
immediate use. The truth of this statement can be
easily proved. We have only to take the most fertile
soil, to calcine it, and to treat it with an acid, in order
to get an almost white residue which will prove totally
sterile.

It follows that we may look upon the greater part of
the soil at any given time as a substratum which merely
serves to hold plants firmly, without taking any im-
mediate part in their nutrition. The food of the plant
must be sought in the remaining component parts of the
soil, z.e. in the humus, and in those parts which dissolve
in water and acids. Let us see how the twelve elements
discovered in the plant are distributed in these parts.
The organic humus contains four elements: carbon,
hydrogen, nitrogen, and oxygen. Substances soluble
in water and acids consist of salts containing all the

THE ROOT gI

elements found in the ash of the plant, together with
two other compounds containing nitrogen, namely
saltpetre, a salt of nitric acid, and ammonia, a com-
pound of nitrogen and hydrogen. Hence, the four
elements of organic matter, the elements of ash and the
two compounds of nitrogen, nitric acid and ammonia,
are the substances to which the analysis of the soil
points as the possible sources of food for the roots of the
plant. Let us see which of these possible sources will
prove to be the actual indispensable sources. In order
to find this out we must interrogate the plant itself,
by placing it under such circumstances as to oblige it
to give us an answer to the question.

How otherwise, indeed, can we determine what
substances are really necessary for the nutrition of the
plant ? At first sight it would seem quite sufficient to
analyse the plant and determine which substances
enter into its composition, and to declare these the
necessary substances. But doubts at once arise as to
the soundness of such a conclusion. Many of the
substances found in the plant might clearly be, as it
were, luxuries ; their presence might even be accidental
and useless, following merely as the result of their
existence in the environment. We can only recognise as
necessary, substances in the absence of which the very
existence and development of the plant are impossible.
This can be determined only by means of an exact
experiment like the one which proved the sterility of the
insoluble mineral substratum of the soil. The essential
conditions for such an experiment are as follows:
we provide one plant with all the substances discovered
by analysis in the plant itself, or in the soil on which it
successfully grows ; another exactly similar plant beside
it we supply with these same substances, omitting one,
and observe the consequences. If we do not notice any
apparent difference in the development of the two
plants, we may conclude that the eliminated substance is

92 THE LIFE OF THE PLANT

not important for the nutrition of the plant; but if
under exactly the same conditions we get a weaker
plant in the second case, we shall be justified in attribut-
ing the difference to the difference in the conditions,
i.e. to the absence of the eliminated substance. This
chapter in the physiology of the plant affords us a
series of good, simple illustrations of a strict and
repeated application of the second canon of inductive
reasoning. ‘If an instance in which the phenomenon
under investigation occurs, and an instance in which it
does not occur, have every circumstance in common
save one, that one occurring only in the former, the
circumstance in which alone the two instances differ
is the cause, or an indispensable part of the cause, of
the phenomenon.’ ?

Thus by eliminating one after another all the sub-
stances discovered in the plant and in the soil, we
discover which of them is absolutely indispensable as
food for the plant. Let us study the principal results of
these experiments.

First of all our attention centres upon the organic
substances, the products of decay. Everyday experi-
ence proves that dark soils are more fertile than light
ones. It would seem, then, that the black humus
must form the main food of the plant. Yet exact
experiment tells quite a different tale. We may
thoroughly calcine black mould, burning away the whole
of the organic matter in it, and yet it remains a soil in
which a plant will normally develop. We can grow on
such a white soil a plant that could not be distinguished
from any grown on the best of moulds. Therefore, it is
not from the humus that the plant obtains its food.
It can grow without it. We have already seen that the
largest part of the mineral matter in the soil, the part
we have called the insoluble substratum, is useless as
food ; so the circle of substances which can be looked

1 J.S. Mill, A System of Inductive Logic, etc., p. 256.

THE ROOT 93

upon as food for the plant becomes still narrower :
it is reduced to those substances which are soluble in
water and acids.

It would be futile, however, to attempt to grow
a plant in artificial soil composed exclusively of nu-
trient substances; for instance, in plant ash. Such
soil would be totally unfit for the purpose; the
nutrient substances would be in much too concentrated
a form, and the plant would certainly perish. In
order to serve the purposes of nutrition, these substances
must be mixed and diluted with other inactive matter,
such as is found in the insoluble mineral substances of
the soil. But if such be the meaning of the latter we
can surely substitute for it other substances, less
complicated in composition. Indeed, experiment shows
that artificial soils can be prepared out of sand, crushed
pumice-stone, glass beads, and similar materials. By
adding to such foundations the necessary nutrient
substances we can obtain very fertile soils.

We are now within a step of the method for reducing
artificial cultures to the simplest form imaginable. Ifa
considerable part of the natural soil as well as the artificial
soils just enumerated serve merely for purposes of
uniformly distributing and, so to speak, diluting the
nutrient substances, would it not be possible to use
instead distilled water in which all the nutrient sub-
stances necessary to the plant have been dissolved ?
Experiments extending over many years, accompanied
by many failures, were crowned at last with complete
success. To-day, with certain precautions, we can
substitute for soil an entirely transparent medium, and
grow the most diverse plants in a watery solution, rearing
them to the same normal size which they reach in the
most fertile soils.

We take for the purpose a glass jar (fig. 25) containing
three or four pounds of distilled water, and dissolve in it
something like six or seven grains of a mixture of different

94

THE LIFE OF THE

PLANT

salts; itisimportant
that the amount of
salts in the solution
should not exceed
two parts in one
thousand, otherwise
the solution will be
too concentrated.
We fix a seedling of
any plant whatever
to the lid of the jar
in such a way that
only its rootlets will
be immersed in the
water. Then we
shall be able to ob-
serve the develop-
ment of the aerial
part of the plant,
and also that of the
root, which is now
entirely exposed to
view.

The photographic
plates reproduced a
little later on (figs.
27, 28, 31) record
the results of ex-
periments made in
1896 at the Nijny
exhibition. I
attribute a_parti-
cular significance to
them, because it is

: doubtful whether

such experiments in
all their stages and

THE ROOT 95

details were ever before performed in the same way in
the presence of so many thousands of spectators. With
great pleasure do I recall one sceptic, a local inhabitant
of Nijny, who acknowledged that he used to observe our
water cultures day after
day with the sinister in-
tention of convicting us
of quackery ; but in the
end he became enthusi-
astic and thoroughly con-
vinced.

Our problem is now
reduced to this: of all
the mass of black mould
surrounding the plant,
it is only an insignifi-
cant pinch of a mixture
of certain salts that is
at any given moment
indispensable for its nu-
trition. Now let us find
out which of the chemi-
cal elements entering
into the composition of
these salts are essential.
It has been necessary for
this purpose to make a
series of experiments
either with white sterile
soil into which necessary
salts had been introduced, or with culture solutions as
described above.

Here are experiments showing the necessity of
nitrogen for plants (fig. 26). We take two flower-pots
filled with a soil calcined and washed with an acid, and
consequently white and sterile. To one of them the
ash of plants has been added, containing all the mineral

96 THE LIFE OF ‘THE PLANT

substances that exist in a plant; to the other the same
ash has been added and also nitrogen in the form of a
nitrate, namely saltpetre. Sunflower seeds exactly
similar in weight were planted in both pots, two in
each. They have come up, but a distinct difference is
evident at the end of the experiment: the first flower-
pot contains two miserable, unhealthy plants, scarcely
rising above the soil; the second contains. two healthy
specimens with flowers and seeds, leaves and stem being
as well developed as those of sunflowers grown simul-
taneously in the best garden soils.1 Yet the only
difference between the two experiments consists in the
fact that to the second flower-pot some saltpetre, i.e.
nitrogen, was added. Similar results might have been
reached had nitrogen in the form of an ammonium salt,
instead of saltpetre, been used. The inference is that
plants need nitrogen.

Here is another experiment. We take several jars
containing nutrient solutions (fig. 25) ; some of the jars
contain all the necessary salts, others the same salts
minus that of potassium. We place exactly similar
buck-wheat seeds in every jar. After a certain time
we notice that the former jars contain healthy plants
which flower and produce ripe seeds, while in the rest,
instead of developing well, the plants are weak or
have perished. We may repeat these experiments
many times over, and always get the same results.
The inference is that the plant needs potassium, that
it cannot exist without potassium.

You see on this table the results of similar experi-
ments also made with buck-wheat seeds (fig. 27).
The first, third, and fifth row received a complete
nutrient solution, the second received no nitrogen,

1 Fig. 26 represents on the left-hand side plants grown with saltpetre
(for the sake of comparison a leaf of a garden specimen is given), and on
the right plants grown without saltpetre. This is the classical experi-
ment of Boussingault.

THE ROOT 97

the fourth no potassium nor phosphoric acid. The
results speak for themselves.
Here is one more experiment, the most obvious and

FIG. 27.

striking of all in its results. Among the salts found to

be indispensable for the nutrition of the plant, there

stands that of iron; it enters into the composition of

the ash of the plant in very minute quantities. This

salt cannot be used as a solution in water cultures,
G

98 THE LIFE OF THE PLANT

because it forms with another substance equally indis-
pensable for the nutrition of the plant, phosphoric acid,
a precipitate insoluble in water. We shake up this white
insoluble precipitate in the liquid, so as to bring it into
contact with the surface of the roots. We take several
jars : some with entirely clear solutions, which means
that they do not contain any iron salt; others with a
certain degree of turbidity, owing to the presence of
the iron salt. Suppose we grow a plant such as maize
in each of these solutions. At the end of two or three
weeks we already notice a sharp difference between
them. While the full nutrient solution produces a
normal plant that will flower and produce ripe seeds,
the other will produce a plant with only a few narrow
and unhealthy leaves that will soon die altogether
(fig. 28). These leaves, moreover, will show a remark-
able peculiarity : the first two or three of them will be
of the usual green colour, but the rest will be white.
It is clear that the absence of iron has stopped the
development of the plant, and has resulted in a peculiar
disease, a ‘pallid sickness,’ called chlorosis. The
following simple experiment will attest the accuracy of
this inference ; we have only to add some of this iron
salt to the solution hitherto without it to see the
sickly condition coming to an end, the plant becoming
green and growing ; moreover, we have only to moisten
one part of a totally white and sick leaf with an iron
salt to see a green spot appearing soon after on that very
place.t Our attention has already been drawn more
than once to the similarity between the vital functions
of vegetable and animal organisms; the action of the
iron salts presents a striking illustration of this point.
Such unfortunate cases as the following may have been

1 In the middle of Fig. 28 we see a vigorous specimen of maize still in
flower reaching the top of the green house; on either side are two
specimens of a smaller variety (Cinquantino) already bearing full-grown
cobs; in between are two specimens grown without any iron salt.

THE ROOT 99

happening quite lately : somebody of your acquaintance
feels ill, the illness being accompanied, among other
symptoms, by an unusual pallor. A doctor is sum-

Fic. 28.

moned. He begins at once by examining the gums,
and then prescribes pills or medicine. The patient takes
the medicine and after a time regains his healthy
appearance. The remedy contains iron. The same

100 THE LIFE OF THE PLANT

iron that brings back healthy colour to a faded cheek
brings back the natural green colour to a white leaf.

Results similar to those just described in connection
with nitrogen, potassium and iron, can be obtained in
the same way in respect of phosphorus, sulphur, chlorine,
lime, and magnesium. All these substances have proved
to be indispensable for the nutrition of the plant, which
sooner or later perishes without them.

But among the constituents of the ash of the plant
we find silicon. Silicon, together with oxygen, forms
silica, which in a pure form occurs in nature as rock
crystal, and a little less pure as quartz, white sand,
etc. The same silica forms the main constituent of
glass. This silica is also found in many plants, in
their cell-walls, making them brittle like glass;
if we burn such a cell we are left with a glassy
skeleton, which under the microscope preserves the
outward form of the living cell in its minutest
details. By very unpleasant experience every one has
had opportunities of learning the existence of such
glassy cells. The stinging hair of the nettle is simply
a long-pointed cell, the walls of which, especially at the
top, are as brittle as glass, because they are full of silica ;
this is why they pierce the skin so easily, break in the
wound, and inject their poisonous sap. Large quantities
of silica are contained in the straw of cereals and in the
stem of the horse-tails. The latter are so hard that
carpenters use them for polishing wood.

Silica, then, is found very generally among plants, and
we might suppose it to be indispensable to the plant. An
idea has grown up to the effect that it not only adds
hardness to the external tissue of cereals, but even gives
solidity and firmness to the whole body of the straw ; it
has been supposed that by increasing the supply of silica
in our cultivated cereals, these would be less readily laid
by wind and heavy rain, which do so much damage to
corn. But direct experiment has put an end to all these

THE ROOT IOI

suppositions, which before seemed so probable. Cultures
in artificial soils as well as in solutions deprived of silica
have proved that quite normal specimens of cereals
develop even in the entire absence of silica; a plant
must therefore be able to exist without silica. Further,
experiments on a larger scale have been made actually
in the open fields, the soil being manured with silicates ;
but these experiments gave a negative result. Plants
in a silicated soil were laid worse than those in untreated
soil. It might have been suggested that the manure
had not reached the plant, but analysis proved that the
plants had really become richer in silica. This in-
comprehensible result was to a certain extent explained
when, after the general analysis of the plant, par-
ticular analyses were made of its several parts. It
appeared then that it was the leaves and not the stem
or the straw which became richer in silica ; and therefore
the increase of silica might work rather to the detriment
of the plant, making it top-heavy and more liable to fall,
instead of contributing to its steadiness. Eventually
it transpired that a plant can exist without silica, and
that its presence has nothing to do with the firmness of
the straw, as was formerly believed. We shall see in
one of our subsequent lectures that the laying of crops
is to be explained by other causes, and therefore can be
averted by other means.

So, then, in striking silica out of the list of the elements
present in the ash given in the second chapter, and by
substituting for it the indispensable nitrogen, we get
eight elements that exhaust the list of substances which
must necessarily be supplied to the root in order to
nourish the plant. Four of them—nitrogen, phosphorus,
sulphur, and chlorine—form acids. These acids by
combining in pairs with the four metals, potassium,
calcium, magnesium, and iron, produce four salts.
These four salts satisfy all the requirements of the root ;
they furnish all the nutrient solutions used for the

102 THE LIFE OF THE PLANT

experiments we have made. The most sterile soils,
when watered with such a solution, become fertile in
the sense of becoming quite fit for the purpose of feeding
the plant.

Such are the results, so brilliant in their simplicity,
to which the study of the physiology of the root has
brought us. Let us, however, remember that this
simplicity is the result of many years’ stubborn labour
of scores of scientific investigators.

A question naturally arises here: would it be right
to conclude that all the other substances which form
the main bulk of the soil are quite useless to the plant ?
Evidently not. Some of the substances, while they do
not serve as food at any given moment, may be of use
as food at some future time; others, without taking
any direct part in nutrition, may indirectly contribute
towards it. For instance, besides saltpetre and
ammonia, the soil contains considerably larger quantities
of nitrogen in the form of organic matter. Yet this
nitrogen is of no immediate use for the purposes of
nutrition ; soil, which contains nitrogen only in that
form, is almost sterile ; but this nitrogen may gradually
change into ammonia and nitric acid, and then serve
as food. This is an example of a substance useless at
any given moment, yet serving as a future supply of
food. Substances in the soil may be useful to the
plant in still other ways. They are of value to it by
reason of their capacity for retaining moisture, and
for absorbing heat, and they help to hold the nutrient
substances and distribute them uniformly.

With regard to this last property the capacity of the
soil for absorption is remarkable. If we fill a funnel
with soil and water it with some nutrient solution, and
then collect the water after it has passed through the
soil, we find that it contains very little of the nutrient
substances. Ammonia, phosphoric acid, and potassium
are especially absorbed ; all these, as we have seen, being

THE ROOT 103

elements necessary to the plant. This remarkable
property of the soil is of great importance in the
economy of nature. Substances necessary to the
plant, z.e. those that exist in the soil in very limited
quantities, are thereby prevented from being washed
away by the rain, and are kept in the soil, which only
gradually gives them up to the water circulating between
its hard particles.

Nitric acid (as saltpetre) forms an exception to this
rule, as it is rather easily washed from the soil ; yet,
as we have seen, it supplies the plant with nitrogen,
the most important of nutrient elements. The investi-
gations of scientific agriculturists are drawing the:
attention of farmers more and more urgently to the
necessity of utilising this substance as fully as possible
by means of cultivated plants. It is in this very rela-
tion of the plant to the nitric acid in the soil that an
explanation has been sought for the part played by
leguminous plants in the rotation of crops. Until quite
lately their rédle seemed very mysterious. Leguminous
plants contain more nitrogen than cereals, and yet
nitrogenous manures affect them less than cereals.
Moreover, when leguminous plants are cultivated in
alternation with cereals in an unmanured soil, cereal
crops are gathered as heavy as any succeeding the bare
fallow. This seemed to show that leguminous plants,
instead of exhausting the soil, even enrich it, an opinion
which would have been strictly justified could it only
have been proved that leguminous plants absorb

-nitrogen from the air instead of from the soil. Yet
this was for a long time contradicted by exact experi-
ments.

The only other possible explanation of the relation
of the leguminous plants to nitrogen lay in the fact
that, developing a network of roots which sank very
deeply into the soil, and growing in the soil for a longer
time, leguminous plants absorbed more completely the

104 TE Tike OR THE PLANT

stores of nitric acid, which under other circumstances
were washed away by the rain and therefore lost
to the farmer. Such a utilisation of the nitric acid in

KPS

box |

the soil would
to a certain
degree explain
why it is that
crops of legu-
minous plants
produce more
nitrogen than
other plants;
and, moreover,
leave an excess
of the nitrogen
in a form in
which it can-
not be washed
away by the
rain, namely,
in the remains
of their roots,
for the benefit
of the succeed-
ing plants.
Neverthe-
less, this ex-
planation was
unsatisfactory,
and the ques-
tion remained
unanswered
until it sud-
denly received

an explanation about the end of the eighties. This
discovery is one of the most brilliant contributions of
recent years to the theory of the nutrition of the

THE ROOT 105

plant, and we may therefore dwell on it at some length.
As has already been said, saltpetre manures, while
they exercise such an essential influence upon cereals,
sometimes remain entirely without effect upon legu-
minous plants. One such experiment! is represented in
fig. 29 (lower half). Two pots of oats (marked by the
letters KP.) received all
the necessary mineral man-
ures with the exception of
saltpetre; two other pots
(marked by the letters
KPS.) received the same
mineral manures with the
addition of saltpetre: the
result speaks for itself. A
similar experiment was
made with peas (at the
top of the figure), and the
result was negative; the
presence of saltpetre was
not manifested in any way
whatever. This means that
peas can procure nitrogen
for themselves, even if it is
not present in the soil.
Evidently beans can ob-
tain nitrogen from the air ;
but under what circum-
stances? In raising this
question investigators recalled the fact that certain
small swellings were noticed even by the ancients
on the roots of leguminous plants. These nodules
(fig. 30) appear in consequence of the roots becom-
ing infected by certain bacteria, apparently widely
distributed in the soil. This can be proved very simply.
A leguminous plant is grown in an aqueous solution in

1 One of Professor P. Wagner’s remarkable experiments.

Fic. 30.

106 THE LIFE OF THE PLANT

such a way that some of the roots are in one vessel,
while the rest are in another. The solution in one of
the vessels is previously boiled, whereas a small quantity
of water, in which soil containing bacteria has been
standing, is added to the other. Nodules do not
appear on the roots immersed in the boiled and sterilised
solution. The fact that the assimilation of nitrogen

Fic. 31.

from the air depends upon the presence of such bacteria
in the soil can be proved in the following way (fig. 31).
A row of glass vessels containing peas are filled with
soil deprived of nitrogen, but infected with soil-washings
containing bacteria; while another row are filled with
soil sterilised by means of heat, and watered with the
same soil-washings previously boiled, and therefore also
sterilised. The result is marvellous : it is only plants

THE ROOT 107

(fig. 31, odd numbers) grown in the soil that contained
the bacteria which have formed nodules, and have
developed normally; the others (fig. 31, even num-
bers) have perished. This proves that the property
which differentiates peas and likewise all leguminous
plants from cereals, namely the property of assimilat-
ing free nitrogen from the air, is bound up with the
capacity of their roots for becoming infected by certain
soil-bacteria. How and where this assimilation of
nitrogen takes place has not yet, however, been fully
explained.

In view of the fact mentioned above that the soil-
water is such an extremely weak solution of nutrient
salts, it may well be asked whether it is really sufficient
for the nutrition of plants? We can answer this
question by reference to the following calculations.
We know the quantity of rain that falls on a certain
area of land; we also know the quantity of nutrient
substances that this rain water can extract from the
soil; on the other hand we know the quantity of ash
contained in a whole crop, gathered from the same
area of land. These data are sufficient to provide an
answer to our question whether this liquid food is
sufficient. It is only the most fertile soils for which
the answer is in the affirmative, in all other cases
the answer is negative. In general the liquid food
alone is not sufficient for the plant. Evidently it must
also use substances insoluble in the soil-water. But
in that case the root must, so to speak, seek. out its
own food, searching round all the neighbouring particles
of the soil in order to find among the mass of sterile
matter minute particles of the nutrient substances so
sparsely scattered in it. This brings us directly to
the consideration of the second question we have
raised. Having ascertained wherein the food of the
root consists, let us now try to find out how it gets it.

108 THE LIFE OF THE PLANT

In order to discover how the root fulfils its function,
we must begin by studying its structure.

In external appearance a root belongs to one or other of
twoclearly differentiated types. Iteither grows vertically
downward as a single continuous trunk which gradually
tapers to a fine thread, e.g. the beet, the carrot, flax ;
or else it branches at once, almost at the surface of the
soil, into a tuft of thread-like fibres, such as those of
our cereals—rye, wheat, and soon. Roots of the former
kind are called tap-roots; of the latter, fibrous-roots.
These two extreme types have many modifications,
but they underlie all the variety of forms presented by
this organ.

Every root, whether tap-root, single fibre, or lateral
root, grows in the same way as the stem, by elongating
and expanding at the apex. But we notice a great
difference when we compare the apex of the stem with
that of the root. If we remove all the leaves of a bud,
thus laying bare the apex of the stem, the so-called
‘ growing point,’ we find that it is the youngest and also
the most tender part of the stem, consisting of minute,
undeveloped cells. If we examine at the same time
the apex of the root, which is generally bare already
since there are no leaves to remove, we sometimes see
with the naked eye, but better with a lens, and better
still with a microscope, that it presents an untidy and
ragged appearance. It seems to be covered with a cap
consisting of several rows of cells, the outer of which
have already lost coherence, and only stick together by
means of a kind of mucilage. This cap is nothing but
the external dying layers of tissue, which cover and
protect the young and tender tissue of the growing
point lying under them (fig. 32). Sometimes this cap
can be pulled off the root-tip like a glove. The physio-
logical purpose of this organ is easily understood ; it

1 Fig. 32 shows under a low power the growing point of the root with
its cap.

THE ROOT 109

serves as a shield, under the cover of which the delicate
growing point of the root pushes its way into the soil.
If the growing point of the root ended with the unpro-
tected youngest cells, it would evidently not be able to
fulfil its function ; it is only by pushing forward the
cap that it is able to force its way without serious harm
through the hard, rough, and sharp
particles of the soil.

At a short distance from the
very tip, protected by the cap, the
whole external surface of the root
is covered with long thin hairs
(fig. 33).1_ Every such hair is simply
a very much elongated cell of the
surface layer. Further from the
tip this belt of hairs comes to an
end; there the surface of the root
is protected by a hardened outer
layer (fig. 33) which has lost its
hairs. Still further up this layer
becomes torn, and is replaced by
another protective tissue like that
which covers the stems of trees,
and which botanists generally call
cork; like all cork it is impervious
to water. Thus the root is differ-
entiated into three zones: the cap at the very tip of
the root, then a belt of hairs, and, lastly, the oldest
part with a dried skin and a corky tissue. This
oldest portion cannot absorb water and nutrient
substances; the very tip is also unable to absorb,
or at any rate absorbs inadequately, a fact which can
be proved experimentally. The absorbing surface of

Fic. 32.

' Fig. 33 shows a young root (A) covered with hairs; a similar root
with particles of soil sticking to the hairs (B); an older branched root
with old parts already without hairs (C) ; and part of the transverse section
of a root under the microscope, showing the structure of the hairs and
their adhesion to the particles of the soil (D).

110 THE LIFE OF THE PLANT

the root is in fact confined to the belt of hairs, which are
in the highest degree permeable to water, far more so
than the tissues composing the aerial parts of the plant.

The root is especially interesting to us as an organ
of absorption, and it is important from this point of
view to form some idea of its length, and also of the
extent of its absorbing surface. A mere glance at the
root of any plant which has been thoroughly washed
and freed from particles of soil shows us how consider-
able its full length must be if all its numberless branches
and fibres were joined end to end. But the boldest
conjecture falls far short of the reality. A German
scientist undertook the following laborious task : provid-
ing himself with a pair of forceps, a measuring scale, a
pair of compasses, and an almost inexhaustible stock of
patience, he actually measured the length of a wheat
root down to its minutest ramifications. The result
obtained was astounding. It turned out that the total
length of the root was 520 metres. However consider-
able that figure may be it does not even yet represent
the whole length of the absorbing surface of the root.
As a matter of fact it is the hairs which present the
actual absorbing surface. Let us see how many such
hairs the wheat plant possesses—no difficult matter in
round numbers. Determining under the microscope
how many occur on one square millimetre and then
multiplying this by the number of square millimetres in
the total surface of the root, we get approximately ten
million. If we multiply this number by the average
length of the hairs we get the enormous length of twenty
kilometres (twelve and a half miles). Such is the path
traversed by the wheat root together with all its hairs
in the volume of soil contained in a common flower-pot.
I said the path traversed by the root together with all
its hairs, for in point of fact this figure does not represent
their length at any one moment in the life of the plant.
All the root hairs do not work simultaneously. Thus

THE ROOT III

for instance in fig. 33 C only the lower parts of the root
are active; there are no hairs higher up, and there is
no need for them there; in that region they have
already done their work, and have exhausted the nutrient
substances from the hard particles of the soil. If we
now calculate the total surface of all the hairs, together

with the fibres that bear them, on the root of a wheat
plant during its life-time, we find that this surface is
almost a hundred times larger than the area of land
allotted in the fields to each wheat plant. If, on the
other hand, we calculate the volume of these hairs
which cover a length of nearly thirteen miles, we find
that they could all be packed into a vessel of the size
of a thimble (about 1°5 cubic centimetres).

Thus in the root we have an organ which, especially

112 THE LIFE OF THE PLANT

inits hairs, presents a large surface, although its volume
is insignificant, owing to the fact that this volume is
stretched out to so extraordinary a length. Here Nature
has had recourse to a trick similar to the one attributed
by poetic legend to Dido, the foundress of Carthage.
Dido obtained for the asking as much land as could
be measured by a single bullock’s hide. As it turned out,
that same hide was made to enclose the whole future site
of Carthage, for Dido cut the hide into very narrow
strips. But the strips of Dido are not to be compared
with root hairs, which are considerably finer than a
human hair.

It is most difficult adequately to realise the great
physiological importance of this prevalent development
in length. The root is thereby able at the smallest
possible expenditure of building material to encompass
the greatest possible number of particles of the soil, and
to come into the closest possible contact with them.
The data we possess are sufficient to calculate approxi-
mately the distance of the particles of the soil from the
surface of the root hairs of our wheat plant.

For this purpose we must turn to the statistical
method, the procedure adopted by statisticians ; they,
neglecting the individuality of separate persons, speak
only of the average man, the average scale of living,
and so on, and describe phenomena in terms of average
quantities. Let us follow their example, and try to
depict in the most obvious way the volume of soil
occupied by an average root fibre of wheat. We know
the average area in a field occupied by each plant ;
we know the depth of soil occupied by the roots, and
hence know the volume of soil occupied by each plant.
This glass jar contains that measured quantity. Sup-
pose that we wished to transfer this soil to a vessel or
rather a tube one-third of a mile in length, z.e. of the
length of our root, what would the diameter of such a
tube be ? The answer works out to three millimetres.

THE ROOT 113

‘If the root fibres with their hairs were introduced into
a glass tube with this internal diameter, the hairs
would touch the wall of the tube.1_ Therefore if all the
fibres of the root were distributed quite uniformly in the
soil allotted to them, every fibre would occupy just a
cylinder of soil, pierced through in all directions by its
hairs; and, consequently, the greatest distance of a
particle of the soil from a root hair would be equal to
half the distance between the neighbouring hairs of the
same root ; this would be something like one-fifteenth of
a millimetre. This calculation gives us therefore the
greatest distance from which our average fibre must
obtain its food, and it gives us an idea of the close
contact into which a root comes with the hard particles
of the soil. Obviously it is not every root fibre that finds
itself in such favourable conditions. I repeat, this is
only a statistical average giving us a clear illustration
of the perfect absorbing powers of the root. This
adaptation is the more perfect in that the root becomes
specially developed in those parts of the soil where it
finds the most nutrient substances. This fact has been
proved in the following way. A plant was grown in a
flower-pot filled with alternate layers of fertile and
sterile soil. Roots developed very luxuriantly in the
fertile soil, but only poor and meagre fibres were pro-
duced in the sterile soil. This fact together with the
great length of the hairs seems to indicate that roots
must themselves go in search of their food, and that the
liquid food furnished to them by means of water is
generally inadequate. This supposition apparently
finds confirmation in the fact that roots of plants grown
in solutions or in soil submerged in water have very few
hairs, if any at all; and yet the plants do not seem to
suffer. It is, therefore, sufficiently evident that nutrient
substances in a liquid medium penetrate the root very

1 During the lecture this fact was illustrated by means of a lamp glass
and its brush.

H

TI4 THE LIFE OF THE PLANT

readily, so that it has no need of a particularly large
surface.

We have said more than once that a root probably
obtains food also from the hard particles of the soil ;
but how is this to be explained ? The surface layer of
the root with its hairs consists of small cells, which
have, of course, no apertures in their walls. The
particles of the soil can come into very close contact
with the root hairs, as is shown in fig. 33 D, but never
pierce their walls. How can we reconcile these con-
tradictory facts, that solid bodies serve as food to the
root and yet do not pass through its cell-walls? In
order to explain this apparent contradiction, we have
recourse to the following experiment. A glass jar is
filled up to the very top with water, and then covered
with a bladder. The external surface of the bladder is
carefully wiped with blotting-paper, so that it appears
to be quite dry. We scatter some chalk powder on the
surface. Chalk is a solid body; the bladder does not
contain any apertures; and yet we soon find that the
chalk disappears from the surface of the bladder, passes
through it, and appears in the solution inside the jar.
We need not wait until all the chalk disappears from the
surface of the bladder, for we possess very delicate
reagents for detecting the presence of lime in water.
This colourless liquid (ammonium oxalate), for example,
has the property of forming a white precipitate with
soluble lime salts. I pour some of it into the water
taken out of the jar before the experiment; there is
no precipitate. I take some of the water out of the jar
some time after the chalk has been lying on the bladder ;
I pour into it some of the reagent, and get an abundant
white precipitate, which means that the water already
contains lime; that is, part of the chalk has passed
through the bladder. This experiment, which rather
surprises us at first, can be very simply explained.
However carefully we wipe the bladder with the blotting-

THE ROOT 115

paper, it only seems to be dry; in reality it is always
suffused with the liquid that washes its inner surface,
and this liquid was not simply water, but water slightly
acidified with acetic acid. Therefore the bladder is
moistened with an acid, and acids, as we know, dissolve
chalk. The chalk dissolves at every point of contact
with the moistened bladder, and this solution passes
through the bladder into the jar. All this happens
imperceptibly, and so it appears as if a dry solid body
passes in some incomprehensible way through a dry
bladder. We gather from this experiment that if only
cell- walls be moistened with an acid, solid bodies
dissolving in that acid can easily pass through them.

Does not something of this kind happen in roots ?
In order to prove the possibility of such a phenomenon
it is necessary to show that the surface of roots gives
an acid reaction. For this purpose we have only to
apply to the root a piece of litmus paper, which chemists
use for detecting the presence of acids. Under the action
of acids the blue colour of this paper changes to red.
The root-tips do actually leave a red trace on the blue
paper. There are indications which point to the fact
that sometimes this acid is actually acetic acid which
we used in our last experiment. Moreover, the root,
like every other part of the plant, is continually breathing
and giving off carbonic acid. This can be proved by
an experiment similar to the one demonstrating the
respiration of germinating seeds. Now carbonic acid
dissolves many substances which are insoluble in water.
Here is, for instance, water into which phosphate of
lime in the form of a very fine powder is stirred ; two
very important nutrient substances, calcium and phos-
phorus, have thus been mixed with the water. I pass
through this water a stream of carbonic acid, and in a
short time the turbidity disappears—the salt has
dissolved.

So, then, roots have an acid reaction and, moreover,

116 THE LIFE OF THE PLANT

give off carbonic acid; and these acids must act as
solvents on the surrounding particles of the soil, the
more so because the hairs, as we have seen, come into
the very closest contact with, and almost grow into, these
particles (see fig. 33 D). But perhaps instead of all
these indirect considerations it would be well to prove
by experiment that roots produce such a solvent effect
on the solid particles of the soil. Let us take for this
purpose a piece of white marble carefully polished—
marble in its chemical composition is practically chalk—
and let us bury it at the bottom of a flat flower-pot.
Then let us plant something in the pot, say a bean.
The roots of the bean plant will soon reach the piece of
marble, and will spread over it, closely adhering to its
pelished surface. If in a few days we take out the piece
of marble, wash it, dry it, and then examine it, holding
it to the light, we shall notice on its smooth surface,
which reflects the light, some dull worm-like traces.
These are the imprints of the roots, which in adhering
to the polished marble have eaten out their image
upon it by the action of their acid surface. These
impressions are certainly not very deep, but they are
nevertheless perfectly clear.1

There can be no further doubt, after what has been
said, that a plant is able to obtain its food from the
solid particles of the soil as well as from solutions.
This fact can be made doubly sure by the following
curious experiment. The root of a plant of suitable
age was carefully washed and divided into two tufts,
one of which was sunk in water, while the other was
buried in soil, and given no water. Nevertheless, the
plant has continued to develop. It has absorbed
water with the one tuft of roots, while with the other
it has taken from the solid particles of the soil the sub-
stances necessary for its nutrition.

1 These imprints can be made clearer by rubbing them with powdered
graphite.

THE ROOT 117

Lastly, there are such plants as lichens, for instance,
which settle like froth and scum upon the inhospitable
surface of rocks and stones, and even, it is said, attach
themselves to the surface of polished glass, destroying
these bodies in drawing out of them the mineral food
they require. It is a remarkable fact that these plants
are distinguished by the abundance of acids in them,
especially oxalic acid.

We have now to answer the last of the three questions
raised at the beginning of the lecture. Why is it that,
among the various substances with which the roots
come into contact in the soil, they attract especially
those that are necessary to the plant? Let us study
the fact itself more closely before we answer this ques-
tion. If a plant is grown in a solution of two salts,
say saltpetre and common table-salt, it is soon evident
that the root entirely absorbs one of these salts, namely
the saltpetre, while it scarcely draws at all upon the
common salt, which it does not require. Such facts
were formerly disconcerting to scientists ; it looked as
if roots could discriminate between the different sub-
stances and choose their own food, accepting one sub-
stance and refusing another. How, indeed, can such
discrimination be explained ? Surely we cannot admit
that a root is endowed with will-power or instinct ?
The explanation is very simple, and we came across it
some time ago. You remember our artificial cell and
the behaviour of iron salt towards it (see chapter ii.).
Similarly, both saltpetre and common salt easily diffuse,
and therefore both will penetrate into the cells of the
root, and hence into the rest of the plant. But the
subsequent fate of the two salts inside the plant will
be totally different. The saltpetre will there be decom-
posed, and its nitrogen will serve to form albuminoids

118 THE LIFE OF THE PLANT

and other complicated nitrogenous compounds : 3 as the
result of this transformation fresh quantities of salt-
petre will enter the plant, will again be transformed
into the substance of the plant, and this process will
continue indefinitely. It is different in the case of the
common salt. According to the laws of diffusion it
will pass into the plant until a solution of equal strength
be formed within and without the plant; then its
further absorption will cease. If there should by any
chance happen to be more of it inside the plant than
outside, according to the same laws of diffusion, the
superfluity will pass back again from within the plant
to the solution. Now it is clear why just those sub-
stances which are transformed and assimilated by the
plant, being necessary to it (as is saltpetre in our
experiment), are extracted from the solution, whereas
those which are useless to the plant (as is the common
salt in our experiment) remain untouched in the solu-
tion, or, to be more accurate, almost untouched.

It is therefore quite unnecessary to presuppose any
rational will, habits, tastes or instinct in order to
explain the discriminating property of the root—simple
laws of physics are sufficient for the purpose.

We must leave the root here and pass in our next
chapter to another organ, the leaf. It is obviously
impossible to exhaust such a rich subject in one short
chapter; but I think that what we have now learned
is sufficient to give us a general idea of the life of the
root, which in such a limited space covers a course of
many miles, and sucks, eats into, and breaks up the soil
with its million hairs, and absorbs from it the soluble
mineral substances so sparsely scattered in it—nitrogen
and the elements of its ash, those eight elements without
which the very existence of the plant is impossible.

1 We are entitled to make this assertion because we can grow a plant
by furnishing it with saltpetre as the sole source of nitrogen.

CHAPTER V
THE LEAF

In the present lecture let us undertake the study in
general outline of the life of the leaf. This task will
be a little more difficult and complex than that under-
taken in our last lecture, since there appears to be more
confusion, among people not conversant with science,
about the leaf than about any other plant organ. No
other vegetable organ has suffered so much injustice.
For centuries, even down to the close of the eighteenth
century, people refused to see any direct utility in the
leaf. From time immemorial they had admitted as indis-
putable the utility of the root as an organ of nutrition,
and of the flower and seed as organs of reproduction ;
but the leaf continued to enjoy the superficial reputation
of a showy but useless ornament: the utmost that
people would consent to see in it was an organ for the
excretion of noxious vapours. Yet, as we soon shall
find, the leaf is as necessary as the root in the nutri-
tion of the plant; moreover, it is the leaf that obtains
for the plant what is quantitatively and qualitatively
its principal food. It may, indeed, be said that the
leaf embodies the very essence of a plant’s life.

The erroneous ideas which prevailed for such a long
time with regard to the leaf and its significance are
fully accounted for by the peculiarity of the processes
of nutrition which go on in this organ. These are
totally different, both as regards the nature of the
food absorbed and the mode of its absorption, from
the nutritive processes of the animal organism, which

involuntarily recur to our mind whenever we speak
119

120 THE LIFE OF THE PLANT

of nutrition. It is owing to this marked difference that
these processes form the most characteristic and
essential feature of vegetable life.

We already know in part what substances are absorbed
as food by the leaf. They must evidently be those
substances which enter into the composition of the
plant, but are not absorbed by the root.

We have seen that among the eleven elements
enumerated in our last chapter (the twelfth, silicon, was
proved to be useless to the plant), the seven elements
of the ash, phosphorus, sulphur, chlorine, potassium,
calcium, magnesium, and iron, together with nitrogen,
enter by way of the root; and that water also, con-
sisting of hydrogen and oxygen, enters the plant by
the same path. There still remains carbon—the foun-
dation of all organic substance. So far we have not
troubled about it in our artificial cultures of plants,
though these contain a thousand times, even many
thousand times, as much carbon as the seeds we took
for our experiment. Quantitatively carbon forms the
most important constituent of the plant (something
like 45 per cent.), yet we did not supply it to the root,
but even systematically removed it from the surrounding
soil. This means that a plant can live without absorb-
ing carbon by its root.

These experiments are not, however, sufficient to
tell us the way the plant obtains carbon in the natural
conditions of life. To say that a plant can live with-
out absorbing carbon through its roots is obviously
different from saying that a plant is unable to absorb
carbon by its roots, although this mistake is often made.
It has not yet been proved that a plant is unable to
obtain its carbon from the organic substance of the soil.
The discussion of this point would, however, carry us too
far afield ; moreover, it is of little interest, because it is
easy to show that if carbon obtained in this way takes
any part at all in the life of the plant, that part is in-

THE LEAF 121

significant, and therefore scarcely worth our attention.
If plants extracted their carbon exclusively or even chiefly
from the organic substance of the soil, then soil covered
with vegetation, the products of which are periodically
removed in one way or another, must in course of time
become poorer in humus; on the contrary, everyday
experience shows that soil becomes richer in humus
when it is under cultivation, as in corn-land, pasture,
or wood. In cultivating our fields we extract from
them every year in the form of crops more organic
substances than we introduce into them in the form
of manures; and yet soil carefully manured becomes
richer in humus. It is clear that ultimately plants
do not reduce the amount of organic substances present
in the soil, but even increase it; and therefore they can-
not find at all events their principal source of carbon
in the soil. But if not in the soil it must be in the air ;
and if so, it is probably absorbed by an organ pre-
eminently aérial—by the leaf. Let us see what is the
source of the carbonaceous food in the air and how it
is obtained by the plant.

Together with nitrogen and oxygen, atmospheric air
also contains a very small quantity, about three ten-
thousandth parts, of carbonic acid. This gas, although
colourless and in appearance indistinguishable from
air, is a compound of carbon and oxygen. No one will,
I am sure, doubt the accuracy of this statement, yet
we must see the proof of it, as of every other state-
ment, as far as possible with our own eyes; and it is
possible in the present case. In order to detect the
presence of carbon in carbonic acid, we must remove
the oxygen. This can be done by causing the oxygen
to combine with a body having still stronger affinity for
it. Such is, for instance, the metal magnesium, the
wire of which burns with a dazzling light. I light this
piece of wire and sink it into a glass jar containing
common air; the wire burns down and a perfectly

122 THE LIFE OF THE PLANT

white ash drops to the bottom of the jar: this is
magnesia, a compound of the metal magnesium and
oxygen. I repeat the experiment, sinking the wire this
time into a jar of carbonic acid ; it must now procure
oxygen for itself by separating it from the carbon, which
will then be set free. As a matter of fact the wire does
not burn so quietly this time, but crackles as if so many
weak explosions were taking place, and black soot is
deposited on the walls of the glass vessel. This is the
free carbon.

Great stores of carbon are therefore always present
in the atmosphere though in an invisible form.

All water in contact with the air contains carbonic
acid, and plants which grow submerged in water are
thus able to obtain it. It is by experimenting with the
leaves of submerged plants that the interchange of gases
between a leaf and its environment is most readily
demonstrated. Here are several experiments which
can easily be made on any clear sunny morning.

Let us gather some leaves and place them while
fresh under a glass bell, filled up to the top with water
and then inverted in another glass
vessel (fig. 34). We fill them ina
pail of water in which it is easy
to sink both bell and vessel. If
we use common water, or still better
water through which carbonic acid
has been passed, and then set the
apparatus in the light, we shall
soon notice the lower surface of the
leaves becoming covered with a
silvery layer of bubbles. A little
later a considerable amount of gas
will be accumulated in the upper part of the bell, while
the surplus water will be displaced into the outer
vessel, as is shown in fig. 34.

Let us repeat this experiment with boiled water, or

THE LEAF 123

with water deprived of carbonic acid. We shall not see
any bubbles of gas. We infer that leaves give off gas,
but only in water containing carbonic acid.

We are astonished at first to notice that bubbles are
given off only on the lower surface of the leaf ; but this
phenomenon will be explained as soon as we become
acquainted with the microscopic structure of the leaf.
We notice in every leaf, or rather in its lamina, two
different parts, which are readily distinguishable :
the nerves or veins, and in between them the ground
tissue of the leaf. On closer examination we find that
the upper and lower surfaces are covered with a layer of
tissue, the epidermis, which is easily peeled off; and
if we prepare the leaf, by macerating it well in water, we
can separate it into three layers, the upper epidermis,
the middle part, and the lower epidermis. From this
middle part, consisting of the ground tissue and the veins,
we can remove the former by carefully beating the leaf
with a soft brush and get a beautiful thin transparent
network of ribs and veins like a cobweb. We shall
consider later the significance of this part of the leaf ;
meanwhile let us confine our attention to the ground
tissue and the epidermis. The epidermis consists of
one layer of cells, distributed in one plane ; the ground
tissue is porous and spongy, with spaces of considerable
size filled with air. Because of these air spaces leaves
float in water; but if we pump all the air out of them
under the water, they will sink, becoming at the same
time darker in colour and more transparent; this is
because water has taken the place of air in between the
cells. After these preliminary explanations we shall
now understand this diagram which shows in a magnified
and somewhat diagrammatic form a little square of
leafy tissue cut out somewhere between the veins
(fig. 35). Here sections in both the transverse and the
longitudinal planes are shown, as well as the lower
surface of the leaf. The ground tissue consists of two

124 THE LIFE OF THE PLANT

kinds of cells: in the upper part of the leaf the cells
are cylindrical in shape and arranged like stakes in a
palisade, vertical to the surface of the leaf. In the rest
of the leaf the cells are of varied and irregular form,
with considerable spaces between them. All the cells
of the ground tissue, especially those of the palisade
layer, contain minute green granules ; we shall return

Fic. 35.

to them presently, only noting here by the way that the
leaf, like all green parts, is in itself colourless, and owes
its colour to those granules. The epidermis which we
see here in surface view and in section consists of flat,
oblong, almost tabular cells. Between them on the
lower surface of the leaf there are scattered organs of a
special form—one of them happens to be cut in half at the
edge of the section. We notice that it consists of two cells,
each curved into an arch, forming between them an oblong
slit. These are apertures, like ventilators, in the lower

THE LEAF 125

epidermis, leading into the leaf from the outside air ;
they are called stomata. Their number is enormous.
One lime leaf contained more than a million of them;
and this number need excite no dismay, because the
method of reckoning the stomata is very simple and
precise. The restriction of these organs mainly to the
lower surface of leaves explains why it is that, in experi-
ments such as the one described above, gas is observed
to be given off in most cases on the lower surface of the
leaves. We shall return later on to these stomata.

Let us see what kind of gas is given off by a leaf when
sunlight acts upon it. For this purpose we must wait
until a sufficient quantity of gas is accumulated under
the glass bell (fig. 34). Then we take out the cork and
introduce a slightly smouldering splinter into the neck of
the bell. It instantly glows and bursts into flame,
scattering sparks in all directions. This is the regular
test for oxygen. Therefore the air given off by leaves
is oxygen, or a gas very rich in oxygen. We have
already seen that this experiment is successful only when
carbonic acid is dissolved in the water. Is there not
some connection then between the presence of carbonic
acid and the formation of oxygen? Evidently our
experiment does not give a sufficient answer to the
question. In order to find out what happens to the
carbonic acid, we make another experiment. Let us
take an apparatus consisting of a tube in the shape of
a horse-shoe (fig. 36), with one side closed and the
other with a stopper. We pour some water into the
tube and introduce carbonic acid in such a way as to
fill the space in the left-hand closed end of the tube up
to the movable pointer on the stand (as is shown on
the figure). We lower the long leaf of a cereal into the
right-hand open end of the tube, and having filled the
tube up to the very top with water close it with the

1 Known as Hofmann’s apparatus, very much used in the practical
teaching of chemistry.

126 THE LIFE OF THE PLANT

stopper, taking care not to leave air bubbles below it.
Then we place the apparatus in the light. As before,
the leaf becomes covered with minute
bubbles, and these, after reaching a
certain size, rise to the upper part of
the tube; here there accumulates a per-
ceptible and ever-increasing quantity
of gas. While the volume of gas
constantly increases in the right-hand
side of the tube the volume of car-
bonic acid diminishes in the left. At
the moment when the level of the
water in the right-hand side reaches
a’ the level in the left-hand side will
also beat a. Evidently the gas in the
right-hand side is oxygen; but to be
quite sure, we can withdraw the
stopper and test the gas with a
splinter. Once having proved that
it is indeed oxygen, we refill the tube
with water and repeat the experiment.
Again we get a certain quantity
of oxygen while a corresponding
quantity of carbonic acid disappears
from the other side of the tube. We
know it is carbonic acid because we introduced it our-
selves, but for greater certainty, after several similar
experiments, we fill up the right-hand side of the tube
with water, replace the stopper and, turning the whole
tube upside down, transfer the remaining gas from the
left into the right-hand side. If we test this gas we
find not only that a smouldering splinter does not glow
in it but that even a burning one will be extinguished.
This means that the gas was and remains carbonic acid.
What happens in this experiment is easily understood :
carbonic acid continually dissolves in water in the left-
hand side of the tube ; but the solution in the right arm

Fic. 36.

THE LEAF 127

is decomposed under the influence of the leaf and oxygen
is given off. Consequently a fresh quantity of carbonic
acid dissolves and so on. This experiment proves
very clearly, though not very exactly, that in passing

Fic. 37.

by the leaf from one part of the tube into the other,
carbonic acid changes into oxygen. During this process
of decomposition the volume of carbonic acid which
disappears at one end of the tube and the volume of
oxygen which appears at the other end are approxi-

128 THE LIFE OF THE PLANT

mately equal. We learn from chemistry that when
carbon burns in oxygen and carbonic acid is formed a
given volume of oxygen forms an equal volume of car-
bonic acid. In our experiment, therefore, the carbonic
acid is entirely decomposed ; all its oxygen is given off,
while all its carbon remains in the plant.

We can perform this experiment still more satisfac-
torily by using some large floating leaves of the water-
lily. Water plants have the peculiarity that their
stomata are scattered over the upper surface of the
lamina which is in contact with the air ; its air cavities
are continuous with similar cavities in the petiole.
We sink the lamina in water under a piece of glass,
which is shown in the figure (fig. 37) behind the
vessel, and introduce the stalk into the tube filled
with water. Let us commence by using boiled water,
i.e. water without any trace of carbonic acid. We
do not see anything taking place. We add some
mineral water containing carbonic acid, and cover the
vessel with a piece of cardboard ; and again we do not
observe anything. But the moment we remove the
cardboard and expose the apparatus to sunlight, a
current of bubbles rushes from the cut end of the petiole.
We collect the gas and measure its volume: from what
we already know we decide that this gas, which is given
off exclusively in presence of carbonic acid and in
sunlight, contains oxygen. Let us gently draw the
petiole out of the tube, close the latter with a finger,
reverse it, and introduce into it a glowing splinter. It
burns with a bright flame. We remove the splinter,
extinguish the flame, and repeat the experiment
ten times over with the same result, and conclude
that this is actually a gas rich in oxygen. A leaf
exposed to sunlight transforms carbonic acid into
oxygen.

We have so far been experimenting in the presence
of sunlight, but this phenomenon of the breaking up of

THE LEAF 129

carbonic acid by the plant can be demonstrated any
time to a large audience by means of an improved
magic lantern much in vogue nowadays—which throws

on the screen a
magnified image
of the plant and
the tube in which
the gas given off
by the plant is
_ investigated.

- Here is one of the
most convenient
forms of this ex-
periment (fig. 38).
A small glass
cell made of a
glass tube bent
into the shape of
a horse-shoe (e)
and two glass
plates (d) forms
a kind of aqua-
rium in which
common water
plants are grown.
If we arrange fora
sufficiently strong
source of light,
either sunlight,
electric, or lime-
light, we can
throw upon the
screen an image

fil il ; my " TATTTT

ll | th i io

Cl ul Th

IMUM PLU

it CLAN §

TAN AUa

UFR TTTTTOCREMTATOTIOR

Fic. 38.

seven or more feet across of this minute aquarium
(diminished by one half in fig. 38), and we observe
at all places where the stems or the petioles of the
leaves are cut across that a curious phenomenon is

I

130 THE LIFE OF THE PLANT

exhibited by the plant: it gives off bubbles of oxygen
in place of carbonic acid it has decomposed.?

In order that this may happen we must have a strong
light, and the water must contain carbonic acid ; in the
absence of either of these two conditions the giving off
of bubbles will not be observed ; but, on the other hand,
if the sunlight or the electric light is strong enough the
bubbles rise in a continuous stream, like a string of beads.
We have now to prove that this gas is oxygen, or rather
that it is rich in oxygen, since it generally contains
an admixture of other gases that are present in solu-
tion in the water. For this purpose we pass the ends
of several branches under the widened end of the
graduated tube (a), which is full of water like that in
the basin, and collect the gas which comes off. This
tube is tightly closed at its narrow end with a stopper
in the form of a glass rod (c) which passes right through
its wider part (b). When a sufficient quantity of gas
is thus collected we proceed to test it.2 It might be
oxygen given off by the plant, or atmospheric air, or
carbonic acid dissolved in the water and _ therefore
capable of penetrating into the cavities of the plant. We
pour some solution of caustic alkali into the wide funnel-
like upper part of the tube and gently raise the glass
rod so that the alkali may pass into the lower part which
is marked with divisions. Alkali, as we already know,
absorbs carbonic acid.? At the beginning the tube
showed, say, fifty divisions of gas; there will be only
forty-eight left after the absorption of the carbonic acid.
Then we pour another substance into the funnel, a
solution of so-called pyrogallic acid, which has the

1 Water plants have no stomata on their submerged parts but are
provided instead with internal air cavities, into which oxygen diffuses,
coming off later in bubbles through any chance apertures.

* The circles on Fig. 37 represent the circles of light thrown on the
screen by the magic lantern ; in order to get a larger image we look first at
one part of the apparatus and then at the other.

3 See the third chapter on the respiration of the seed.

THE LEAF 131

property of absorbing oxygen, becoming dark brown in
colour during the process. We lift up the rod, and as
soon as the first drops of this liquid penetrate into the
tube and come into contact with the gas contained in it,
the liquid becomes coloured and the volume of the gas
rapidly decreases. In the end we shall have something
like fifteen divisions of gas instead of forty-eight. This
remaining gas is nitrogen, and this means that we had
thirty-three parts of oxygen. Not more than five parts
of oxygen will have penetrated with the fifteen parts of
nitrogen in the form of atmospheric air, so that twenty-
eight out of thirty-three divisions represent oxygen
given off by the plant owing to the decomposition of
carbonic acid.

The apparatus we have just described, together with
electric light, makes it possible for us to-day to demon-
strate before an audience on a dark winter evening a
phenomenon that generally takes place in nature only
in the daytime during the warm season of the year ; we
can throw it upon the screen as easily as an ordinary
lantern slide. Of course the apparatus can also be
used without the lantern as a simple and convenient.
method of investigation.

So far we have been studying the decomposition of
carbonic acid by plants submerged in water. This
form of experiment is the most convenient for a pre-
liminary study of this phenomenon, because it clearly
shows the giving off of gases by the plant. We must
now ascertain whether the same kind of decomposition
also takes place as the result of contact of the leaf
with air which contains carbonic acid.

Here is a very rough and simple form of experiment,
by which the phenomenon was demonstrated for the first
time a hundred years ago by the great chemist Priestley.
We take a glass jar (as in fig. 34), pour into it a small
quantity of water and fix at the bottom of the jar a
lighted bit of candle, of course large enough to stick up

132 THE LIFE OF THE PLANT

out of the water. We cover this bit of candle with a
glass bell (similar to the one in fig. 34) in such a way
that its edges dip into the water at the bottom of the
jar. The air in the jar will be thus enclosed and isolated
by a layer of water from the external atmosphere. The
bit of candle will go on burning under the glass bell for
a while and then die out. This means that the oxygen
necessary for the process of combustion has already been
exhausted from the air under the glass bell, and has been
replaced by carbonic acid, as the result of the com-
bustion. If at this moment we were to introduce
(through the neck of the bell) a burning splinter, it
would certainly die out in the same way as the candle.
But if we carefully introduce some leaves or a green
branch into the glass bell through the water, and place
the whole apparatus for a long time out in the light, we
shall find eventually that the splinter will continue
to burn under the bell—which will mean that oxygen
has reappeared. Apparently the plant has transformed
into oxygen the carbonic acid formed by the burning
candle. We might have given another form to the
experiment : we might have put a mouse under the
bell instead of the candle and taken its death as a proof
of the fact that the air under the bell no longer con-
tained a sufficient quantity of oxygen for respiration.
By then introducing a green branch under it and
exposing it to the sun, we could have restored to the air
its power of supporting respiration by giving back to it
oxygen.

Hitherto in all our experiments we have studied only
qualitatively the transformation of carbonic acid into
oxygen under the influence of the plant, or rather we
have estimated the relation between the disappearance of
carbonic acid and the appearance of oxygen only approxi-
mately. For the quantitative study of this phenomenon
science is provided with other methods incomparably
more precise, but the description of them would be

THE LEAF 133

out of place here on account of the technical details
involved. I will only say that in those methods we
make use of the property of carbonic acid, already
familiar to us, of being absorbed by caustic alkali.
A definite amount of carbonic acid is supplied to a
plant, or to a single leaf, placed in a graduated glass
tube closed at its upper end (as in fig. 37), which is
then exposed to the light. The experiment being over,
we determine, by means of an alkali, the quantity of
carbonic acid remaining in the tube. Knowing how
much gas was introduced, and how much is left, we can
tell how much has disappeared.

This method has helped to solve many interest-
ing problems; for instance, that of determining the
percentage of carbonic acid in the air most favourable
for the plant. The experiment proves it to be some-
thing like eight per cent.; a greater percentage is
evidently noxious to the plant.

Another question dealing with the same point
deserves our attention. We have proved experimentally
that plants decompose carbonic acid supplied to them
in our apparatus; but it is questionable whether we .
are entitled to infer from these experiments that a
plant in its natural condition is also able to decom-
pose the carbonic acid of the air. You remember that
we supplied in our experiments comparatively large
quantities of carbonic acid to the plant, as a rule in
the proportion of several parts in a hundred, whereas
the atmosphere contains only a few ten-thousandth
parts of it. It may seem unlikely that a plant should
be able to discover and assimilate the particles of car-
bonic acid so sparingly diffused in the air. In order
to settle this question, the renowned French chemist,
Boussingault, made the experiment which we will
now demonstrate. We take a large glass globe
with three apertures (fig. 39). A vine branch with
leaves is introduced through the lower opening into

134 THE LIFE OF THE PLANT

the globe, while still connected with the vine, and
therefore under perfectly natural conditions. With
the help of an aspirator a continual stream of fresh
external air, as is shown by the arrows on the figure, is
slowly passed, first through the glass bell and then
through the apparatus A connected with it. We mark
on the aspirator the volume of air passed through
the globe during the experiment ; we also analyse the
air of the place where the experiment is made, and

FIG. 39.

determine the proportion of carbonic acid contained
in it. When once we know the amount of air which has
been passed through the globe containing the plant, and
the proportion of carbonic acid contained in the air, we
can easily determine the quantity of carbonic acid that
has entered it. We have now only to determine the
quantity of carbonic acid that has come out of the globe
in order to find out how much of it has been decom-
posed by the leaves. The apparatus A serves this
purpose. I will briefly describe its significance, again
leaving out all technical details, since my point is merely
to explain the fundamental idea of the experiment and

THE LEAF 135

not the way it is carried out. The main part of this
apparatus consists of two bent glass tubes (a) through
which the stream of aspirated air passes, and which
are meant to absorb the carbonic acid. For this purpose
one of them contains small pieces of caustic alkali.
Caustic alkali will become heavier in absorbing carbonic
acid ; and therefore what we have to do is to unfasten
the part of the apparatus marked a and to weigh it
before and after the experiment. The increase in
weight will indicate the quantity of carbonic acid
remaining in the air after it passes out of the globe. It
happened that under favourable conditions of illumina-
tion the air came out of the ball almost deprived of
carbonic acid. Consequently, in passing over the green
surface of the illuminated plant, the air has left behind
it almost all its carbonic acid ; in spite of the fact that.
its particles are so scantily diffused in the atmosphere,
lost, so to speak, in the mass of its other components.
This result will become more comprehensible if we
recall the diffusion of carbonic acid into our artificial
cell.1 Then carbonic acid pressed into the cell only
because the cell contained none of this gas; but in the
leaf also, since it is continually decomposed, it dis-
appears, without, so to speak, leaving any traces ; and
therefore, according to the laws of diffusion, it must
continually be replaced by fresh quantities from the
atmosphere.

Boussingault’s classical experiment was made more
than half a century ago, and has hardly ever been
repeated since. Lately a distinguished English chemist,
Horace Brown, undertook a whole series of similar ex-
periments of an improved form, by which he succeeded
in removing any remaining doubts upon the subject.
The insignificance of the total area of the minute
openings afforded by the stomata made it still seem
incomprehensible, however, that a plant should succeed

1 See chapter ii.

136 THE LIFE OF THE PLANT

in extracting from the atmosphere the meagre propor-
tion of carbonic acid contained in it, until it was dis-
covered that, owing to peculiarities in the diffusion
of gases, demonstrated for the first time in Brown’s
experiments, carbonic acid enters the leaf through
these stomatal apertures almost in the same quantity as
it would diffuse had the whole surface of the leaf taken
part in the process of absorption. Horace Brown wittily
remarked, in commenting upon this remarkable dis-
covery, that a plant evidently knows more about physics
than we are inclined to admit! His experiments also
showed that if the percentage of carbonic acid in the air
were increased five times, from the usual proportion of
aoio0 tO row, the quantity decomposed by the plant
also increased to about five times as much; a result
which justifies us in concluding with greater certainty
than before, how successfully the plant can use up
the atmospheric carbonic acid, seemingly available
in such minute quantities.

On coming into contact with a green plant in sun-
light carbonic acid is decomposed : its oxygen is set free,
while its carbon is deposited in the plant. Let us try
to trace the further fate of this carbon inside the plant.
Let us again turn to the microscope. Almost without
exception every observation of this phenomenon, and
every experiment, point to the conclusion that this
process takes place only in the green parts of the plant.
We can really say with certainty that if an organ is
not green, it does not decompose carbonic acid ; while
if it does decompose it, the green colouring matter when
not directly visible is hidden by other colouring sub-
stances. These green granules, which contain this green
colouring matter, called chlorophyll, serve as organs in
which the decomposition of carbonic acid takes place.

THE LEAF 137

Apart from chlorophyll there is no assimilation of carbon
in the plant.

This chlorophyll is found in cells in bodies of varied
form, in small granules, or in disc-shaped (fig. 40) 3
or band-shaped bodies (fig. 58). These bodies are
called chloroplasts. If we keep a plant in the dark
for a time and then examine these chloroplasts under
a microscope their structure appears quite uniform
(fig. 40 a on the left) ; but if we then set the plant in
the light, we find after a certain length of time,
occasionally after even a few
minutes, that tiny granules
have appeared in them (fig.
40 a on the right). In some
plants these granules increase
in size, in time protrude them-
selves and continue to grow on
the side in contact with the
chloroplast (fig. 40 6). They
then have a characteristic
stratified appearance, and we
recognise them as grains of
starch. We need not, how-
ever, wait until they develop ; Fie. 40.
we can detect starch in a
granule as small as a pin point by colouring it blue
with iodine, a reaction with which we are already
familiar.

Starch grains, then, are formed in the chloroplasts,
and continue to grow where they are in contact with
chlorophyll. We can easily prove that the formation of
starch is connected with the decomposition of carbonic
acid. To begin with, no starch is formed in the chloro-
plasts when the plant is not supplied with carbonic acid ;
nor, in the second place, is there any formation of starch

1 Fig 40 a. On the left, chloroplasts without starch; on the right—
with starch grains inside them. 0 Single large starch grains.

138 THE LIFE OF THE PLANT

in the dark. Thus starch is formed and carbonic acid
decomposed only in presence of chlorophyll; both
processes are conditioned by the presence of light, and
only when carbonic acid is present and being decomposed
does any starch appear.

It therefore seems more and more evident that starch
is the very substance we are looking for, which is
formed out of the carbon in carbonic acid. Its com-
position supports the conclusion: like other carbo-
hydrates it can be regarded, as the name suggests, as
made up of carbon and water. Cells always contain
water ; so we may explain the origin of a carbohydrate
by supposing water and carbonic acid to combine,
and at the same time all the oxygen of the latter to be
withdrawn. Such is the course of events as far as they
are known to us; but we must remember that our in-
formation on this subject is as yet far from being
complete. We know that a cell receives carbonic acid
and water, gives off oxygen and forms a carbohydrate ;
we know that these processes must have a causal
connection, that they take place in the same chloro-
plast and follow upon each other with striking
rapidity. As to how it all happens, where the oxygen
comes from, whether it is produced entirely from
carbonic acid or also partly from water (which is more
probable), and whether other simpler or perhaps more
complicated combinations precede the formation of
starch—up to the present we do not know anything
about these questions, and it would certainly be out of
place to enter here into speculative comment upon
phenomena as yet unexplained by science.

It is far more important to remember that, in observ-
ing these processes of carbonic acid decomposition and
starch formation, we are witnessing one of the most
important phenomena of life, one on which depends not
only the life of the leaf and of the plant, but the life of
the whole organic world. This transformation of the

THE LEAF 139

simple inorganic substances carbonic acid and water
into the organic substance starch represents the sole
natural process by which organic matter is formed upon
our planet. All organic substances, however diverse
they may be, and wherever they are found, whether in
plant, animal, or man, had their origin in the leaf,
have been formed from substances manufactured by the
leaf. Nature does not possess any other laboratory for
the formation of organic matter, except the leaf, or,
more strictly, the chloroplast. In every other organ
and organism, organic matter is merely transformed ;
only here does it arise anew from inorganic matter.

From starch, for instance, is formed soluble sugar
which reaches the furthest parts of the plant by passing
from one cell to another. From this sugar the hard
skeleton of the plant called cellulose is formed. And,
lastly, from this same sugar and the inorganic substance,
ammonia, the most complicated organic substances,
such as proteids, can be formed.

Thus the leaf assimilates carbon, and within itself
forms an organic compound with which not only the
plant itself but the whole animal kingdom is also
supplied.

We have at last arrived at the source of carbon in
the plant, and have explained how it penetrates into it.
We have thus explained the first stage of the phenomenon
of nutrition ; we now know whence and by what means
all the elements that enter into the composition of the
plant are obtained, carbon being the Jast of the series.

So far we have been examining the activity of the
leaf and of the plant in general, exclusively from the
chemical point of view—from the point of view of the
transformation of matter. Starting from the funda-

140 THE LIFE OF THE PLANT

mental law of chemistry, that matter is neither created
nor destroyed, we tried to discover the sources of the
matter composing the plant, the way in which it
penetrates into the plant, and the changes it undergoes
during the process.

But the vegetable body is a storehouse, so to speak,
of energy as well as matter—heat-energy for in-
stance. Burning a single seed of a birch-tree will not
warm our frozen hands for a moment, whereas a birch-
tree a hundred years old will serve to heat our stove
for many a day. A birch-tree therefore accumulates
heat during its life-time, which we use as such, or else
as a source of mechanical force.

Where does this heat, this energy, come from ? We
raised a similar question with regard to matter. Just
as we admit that matter neither disappears nor is
created, so do we also assume that neither does energy
disappear, nor is it created. In fact just as the
chemists of last century came to the conclusion that
matter is indestructible, so also the physicists of the
present day have come to the conclusion that energy
is never destroyed. The different sources of energy
may suffer endless change, passing from one form into
another, or become concealed ina state of tension ; but
they are never destroyed, never created anew.

What is this latent energy, this heat concealed in our
fuel, and whence does it come, since it could not arise
spontaneously ? In order to explain this we must
glance again at the chemical phenomena already
familiar to us that take place in the leaf, but this time
from the purely physical point of view of the trans-
formation of energy involved.

All chemical phenomena can be divided into two
categories : those in which heat, light, electricity—in a
word energy—appears, is given off ; and those in which
energy disappears, 1s absorbed. Phenomena of the former
category take place spontaneously, or require but an

THE LEAF 141

insignificant impulse to start them ; those of the latter
kind on the contrary require an external supply of energy,
which can be absorbed in the process.

Most of the phenomena of chemical combination fall
under the former of the two categories; most of the
phenomena of chemical decomposition fall under the
latter. Burning, t.e. combining with oxygen, may serve
as the simplest example of chemical combination. The
converse phenomenon of unburning, the withdrawal of
oxygen from a body, which is called reduction, may
serve as the simplest example of decomposition.

We can try to demonstrate what takes place during
chemical combination and decomposition, z.e. during
combustion and reduction, by means of the following
simple model consisting of two little balls of lead at
the end of thin threads (fig. 41).

The cause of any chemical combination lies in the
fact that chemical substances are endowed with a
tendency, a kind of gravitation, towards each other.
We call that tendency chemical affinity. Particles of
carbon and oxygen tend towards each other in the way
these balls a and 0 do if I
move them apart and leave
them to themselves. But
we know that the impact
of one body against another
produces heat and some-
times light. Heat and
light developing from this
impact, z.e. from invisible
collisions between particles Fic. 41.
of carbon and hydrogen
and particles of oxygen, are indeed the heat and light
that we observe in a burning flame.

We thus perceive the cause of spontaneous chemical
combinatjon, and the reason why it is accompanied
by the development of heat. In combining, chemical

142 THE LIFE OF THE PLANT

elements are only yielding to a mutual attraction, just
as these balls are, but having knocked against each
other they are warmed, and give off heat.

The phenomena of decomposition are quite differ-
ent. In order to decompose a chemical compound we
must expend a certain amount of energy. In our
illustration the same quantity of energy is required to
drive the balls away from each other as will be mani-
fested at the impact of the bodies against each other,
when I withdraw my hands. This equality in the
quantity of energy employed in decomposition and
liberated during combination is easily illustrated by
our mechanical contrivance. In order to move one
ball away from the other I must raise it, must over-
come the force of gravity. I estimate the quantity
of energy expended in this instance by my work, and
this work is measured by the product of the weight of
the ball multiplied by the height to which it is raised.
But just at the moment when the balls hit each other,
the falling ball is endowed with energy sufficient to
raise to the same height a ball of the same weight.
We draw this conclusion from the fact that if it had
not come into contact with the other ball it would
have swung like a pendulum to the other side, and
raised itself to the same height, 7.e. it would have raised
its own weight to the same height from which it has
just fallen. In the same way, in order to break up a
compound, to overcome the affinity of two chemical
bodies and separate them, it is necessary to employ
the same amount of energy as was liberated at the
moment of their combination. If a certain amount
of carbon burning in oxygen gives off, say, one thousand
units of heat, then in order to set free this carbon from
the carbonic acid which is formed, and overcome its
affinity for the oxygen, we must of necessity employ
the same one thousand units of heat. We can, indeed,
decompose carbonic acid, 7.e. liberate its carbon, as we

THE LEAF 143

have seen, only by exposing the compound to the high
temperature of burning magnesium.

However, this case of the decomposition of carbonic
acid by means of magnesium cannot serve as an example
of a simple process of decomposition, because a com-
bination of magnesium with oxygen takes place at the
same time. For a long time chemists thought that the
decomposition of such stable compounds as carbonic acid
and water could not take place without the co-operation
of a third body, possessing a still stronger affinity for
oxygen, but they have comparatively recently come
to the conclusion that the action of heat alone is
sufficient to cause the decomposition, or the dissocia-
tion of carbonic acid and water. According to modern
physics heat is a kind of motion—a rapid, invisible,
but palpable vibration of the particles of a body. In
heating any compound body to a very high tempera-
ture, we bring its particles into such a state of vibration,
and loosen them to such an extent, that their mutual
coherence finally breaks down, causing decomposition.
To give an example: at a very high temperature water
vapour does not exist any longer as such, but as a
mixture of hydrogen and oxygen.

During such a process of decomposition energy is
absorbed : but what happens to this energy ? It can-
not disappear—that would contradict the law of the
conservation of energy. It passes during the process
into a Jatent condition of tension. Every one is familiar
with instances of energy stored in a latent state among
everyday mechanical phenomena: a_ sledge-hammer
ready to fall upon a stake which is being driven into
the soil ; a bow bent ready to shoot an arrow—these are
simple cases of energy stored in the form of tension.
This expression sounds curious, however, when applied
to light or heat. Can such forms of energy as light or
heat be stored ? Could I, for instance, get hold of a
certain amount of light or heat, such as that evolved

144 THE LIFE OF THE PLANT

during the combustion of magnesium wire, and keep it,
say, until to-morrow ? I not only can, but I have already
done so. When I dipped a burning wire into a vessel
of carbonic acid, I employed a certain amount of this
energy in the decomposition of carbonic acid with the
liberation of carbon. I can burn this carbon to-morrow ;
or I can bequeath it to remote posterity, and they by
burning it will enjoy the light and heat that we store
to-day, in using them to decompose carbonic acid.

It follows that carbon, like any other combustible
material (wood in the stove, food in our body), although
naturally free, always tends to combine with oxygen,
and therefore represents a store of energy; and, in
every chemical process in which a non-combustible body
changes into a combustible body, there is a storing up
of energy.

In the end we come to the conclusions: (1) that the
decomposition of carbonic acid and the setting free of
carbon can only take place on condition that some
external source of energy is employed in the process ;
and (2) that the energy so employed passes into a
reserve form.

Having these facts in view let us return to our leaf.
A precisely similar process takes place there. Out of
non-combustible carbonic acid combustible starch,
wood, etc., are formed. It is clear that this process
cannot take place without the co-operation of some
external source of energy. Indeed, as has already been
said more than once, the decomposition of carbonic acid
takes place only under the influence of light; the
activity of the leaf does not begin until a ray of light
falls upon its surface. This ray of light is indeed the
energy by which the decomposition of carbonic acid is
brought about, and which is absorbed and stored in the
process.

To make matters still clearer, let us compare the
phenomena of light with those of heat. We have seen

THE LEAF 145

that heat is motion, which by loosening the particles of
a body causes its decomposition. But light is also motion,
a regular, undulatory form of motion. The following
rough comparison will help us to explain the decom-
posing effect of light. Suppose two light bodies, say
two wooden balls, float side by side on a smooth surface
of water. We throw a stone into the water near them.
Circles will radiate from the stone, and every time a
new wave passes under the floating balls it will separate
them, will break the connection between them, driving
one of them up to its crest, and plunging the other
into its hollow. To this stone, which produces the
circles in the water, we may compare the sun, with
the waves of light continually running from it and
diverging to infinity; with the sole difference that
these waves travel about 190,000 miles a_ second,
and are so fine, and follow upon one another so rapidly,
that on the average 50,000 of them are included within
a single inch.

These waves, following upon one another at an
almost inconceivable rate, come into contact in the
leaf with still smaller atoms of carbon and oxygen,
combined in carbonic acid, loosen them and break up
their combination. The oxygen is set free while the
carbon immediately enters into combinations of another
kind. The first of these new compounds that we
recognise under the microscope is starch.

We have just seen how the heat and light of the
burning magnesium can be stored. The same is true of
the rays of the sun. We cannot simply seize and shut
up a ray of sunlight; but we grow plants for the
purpose, and these not only extract carbon from the
air by means of their leaves, but together with the
carbon absorb and store, concealed in it, the sun’s rays.
It is the rays of the summer sun which warm us in our
wood fires; it is again the same rays that give us light
in our candles for our long winter evenings.

K

146 THE LIFE OF THE PLANT

The form of the leaf finds its physiological signifi-
cance in this function of light absorption. For this
the flat form is more efficient than any other. The area
of the light-absorbing leaf-surface is in some plants
eighty times as large as the area of the soil they cover.

It is only now that we can fully appreciate the
significance of the processes that take place in the
leaf. In the first place there is the assimilation of
one of the most important elements that enter into
the composition of the plant—carbon—and at the
same time a transformation of inorganic into organic
matter. As we have already said, all organic matter
in plants and animals comes directly or indirectly
from the leaf; while the process of its manufacture
in the leaf links up the whole organic world to the sun.
The leaf serves as intermediary between any mani-
festation of energy in the organic world and the sun,
the universal source of energy. It is not the plant
alone, but also the whole of the animal world, man
included, that profits by the energy of the sun stored
up by the plant. We have seen that a seed gets warm
during germination. This heat is derived from respira-
tion, the combustion of some part of the organic matter
bequeathed to the seed by the parent plant. In the
formation of this organic matter, the sun’s energy has
been employed. The seed, therefore, germinating in
the soil, profits by the heat of the rays of the sun
absorbed by the parent plant. Similarly, when we use
organic matter as food, we also take in the rays of the
sun latent in it, and use them to warm our bodies-or
set them in motion.

This means that the leaf, the only natural laboratory
where organic matter for both animal and vegetable
kingdoms is prepared anew, in this assimilation of car-
bon stores up the energy of the sun’s rays, thus becom-
ing the source also of energy, the retailer of heat and
light to the whole organic world.

THE LEAF 147

We have spoken in general terms till now of car-
bonic acid being decomposed in the leaf, of the rays
of the sun being absorbed by the leaf, and so on; but
we can express ourselves much more definitely. In
speaking of the assimilation of carbon in contradistinc-
tion to the assimilation of other nutrient substances,
we can determine quite clearly the microscopic seat of
the process. It is the green chloroplast. We can show
that certain of the sun’s rays are really absorbed by
chlorophyll, and that it is just those rays which are
absorbed which bring about the decomposition of car-
bonic acid, the initial stage in the assimilation of
carbon, and also the formation of starch, its final stage.
The green colour, which depends upon the peculiar
absorption of light by the chlorophyll in the chloroplast,
is thus not an accidental property of the plant, but is
closely bound up with the most essential process of its
nutrition. It is not the leaf as a whole, but the
chloroplast that colours it green, which serves as a
connecting link between the sun and all things living
upon the earth.

We have now studied the function of the green leaf.
Plants without green organs are unable to manufacture
organic matter for themselves out of carbonic acid, but
are obliged to live at the expense of organic matter made
by other plants. Fungi, for instance, the plants we
generally call mushrooms and the microscopic moulds,

1 The necessity for maintaining in due proportion the different parts of
this course prevents me from working out as fully as it deserves this
most interesting chapter in the physiology of the plant. Those who wish
to study more closely this side of the life of the plant will find a more
detailed exposition of the subject in a chapter appended to the course
entitled The Plant as a Source of Energy, which in its turn presents a
popular exposition of the principal results of my special work On the
Assimilation of Light by the Plant, and of my further researches in the
same direction. These facts are set forth in still greater detail in a lecture
entitled The Plant and the Energy of the Sun (in my Lectures and
Addresses, Moscow, 1888), and in my Croonian lecture The Cosmical
Function of the Green Plant.—-Proceedings of the Royal Society, 1903.

148 THE LIFE OF THE PLANT

can exist only on soil already containing organic matter :
every attempt to grow them in a medium devoid of
such matter proves fruitless. Plants feeding on other
plants by attaching themselves to their stems or roots
belong to the same category: e.g. the broom-rape,
which grows on the roots of hemp ; the dodder (Cuscuta),
which twists round the stems of hop, flax, and clover,
and clings to them until it finally exhausts them alto-
gether. All these plants have either ugly scales of
some other colour than green in place of leaves, or else
do not possess leaves at all. They are therefore incap-
able of independent existence, but suck the sap of other
plants. Such plants are called parasites. All of them,
and especially the minute fungi, which cause various
diseases in plants, give farmers much trouble and
frequently rob them of entire crops.

CHAPTER VI
THE STEM

IF we consider the plant exclusively from the point of
view of nutrition, we are entitled, as has already been
said, to see in it simply two strongly developed surfaces
adapted to the twofold medium in which the plant
lives. These are the root-surface and the leaf-surface :
the former, being adapted to the solid medium, the soil,
is developed specially in length, because the root must
come into contact with the greatest possible number of
particles of the soil; the latter, being adapted to the
absorption of atmospheric particles and, especially, to
the absorption of the light that falls upon it, is developed
especially in one plane. Owing to such an arrangement,
under favourable conditions scarcely a particle of the
soil can escape the root, nor a single ray of sunshine be
lost to the plant.

The substances absorbed by the root and the leaf
are totally different, but at the same time are equally
necessary to the plant. Evidently the existence of each
of these organs, the very existence indeed of the whole
plant, requires that there should be constant intercourse
between them.

The organ joining the two surfaces, which bears
the leaves and serves as intermediary between them and
the root, is the stem. As an intermediary this stem is
not an organ so essentially necessary to the plant as the
root or the leaf, and it is therefore sometimes very
poorly developed ; but where, on the contrary, it is well
developed it plays the most prominent part in determin-
ing the general aspect of the plant, and, in fact, the

149

150 THE LIFE OF THE PLANT

aspect of the whole vegetation of a locality. Everybody
knows, for instance, the meadow plantain, consisting of a
bundle of leaves gathered into a rosette and lying almost
flat upon the ground. In this case the stem is scarcely
developed at all, and this is why the leaves are so closely
brought together. Something of the kind, only on a
larger scale, is illustrated by the American Agave, which
grows out of doors in southern Europe, and also in our
greenhouses here in the north. That plant consists
simply of a bundle of very large, fleshy leaves almost
seven feet in length, which once in ten years throw out
a flowering stem like a huge candelabra about twenty-
eight feet high. We find a stem very slightly developed
also in a certain extremely curious African plant.
Picture to yourselves a vast expanse of barren steppe,
on some parts of which are what appear to be stumps or
logs scarcely rising above the soil, and slightly hollowed
like funnels with little furrows across them. On both
sides of the stump, from the furrows at its edges, there
stretch two broad strap-like objects from four to seven
feet in length—coarse and leathery, at first greenish in
colour but turning brown at the edges, and torn into
narrow strips—looking, in fact, quite tattered. Here and
there at the edges of the stumps there grow small
branches with minute cones like those of the fir-tree.
This is Welwitschia, surnamed mirabilis, wonderful,
on account of all its remarkable characters. The
significance of its various parts is as follows: the
stump, always half-buried in the soil and merging
gradually into the root, is the trunk of this tree; it is
seldom more than two feet in height, although the
plant itself may live to be a hundred years old. The
two tattered shreds described above are a pair of leaves
which the plant keeps during the whole of its existence ;
dying at the edges they gradually grow from the base,
and reach a very great age.

Let us pass from these stunted almost stemless

THE STEM I51

plants to the tall graceful palms which Endlicher called
Principes, 1.e. the princes of the vegetable world. Their
trunks grow upwards as straight vertical pillars with a
crown of leaves at the top like the very columns for
which it is believed they served as model. But the
trunks of the palms represent only a one-sided develop-
ment—development in length; they are very tall and
graceful, but they do not usually branch or increase
in thickness. A totally different aspect as well as the
greatest development in size is seen in the trunks of our
broad-leaved trees and our fir-trees. They, throughout
their existence, increase in thickness and throw out
branches, and may thus reach very great dimensions.
Thus, for instance, within the circle of the bark of a
Californian Wellingtonia, there would even be room for
dances ; a small chapel has been fitted up within the
hollow of a huge chestnut tree on Mount Etna; while
travellers tell us that whole caravans find shelter under
the green shade of the baobab. Although such giants
do not exist in Russia even in the forests, we can still
find hoary denizens of the past like the oak of Kunzewo.
Its mighty trunk, of the thickness of four horse-girths,
rises from the bottom of a deep ravine, while its summit
towers above the lime and aspen trees that are crowded
all along the edges of the ravine.

Such are the dimensions that can be attained by a
stem in the fulfilment of its destiny, bearing a canopy of
foliage, the large leafy surface for the absorption of
the rays of the sun; and one cannot fail to see how
well it is adapted to this purpose. We have only to
remember the partial obscurity which reigns in a pine
wood, even on a sunny day, to realise that the needles
must be distributed on the stem in the most advan-
tageous way if, in spite of their insignificant breadth, they
are to arrest as many rays of light as possible. Indeed,
although the distribution of the leaves on a stem seems
at a first glance entirely hap-hazard, a closer investiga-

152 THE LIFE OF THE PLANT

tion reveals remarkable regularity in their arrangement.
The first.person who called attention to this fact was,
I believe, the famous Leonardo da Vinci; but it was
not until the nineteenth century that the phenomenon
was studied in any detail by botanists. This regularity
of distribution is seen mainly in the way the leaves are
distributed on the stem, so that as far as possible they
neither screen nor shade one another, and at the same
time leave no free spaces through which the rays of the
sun may pass to no purpose. This statement can be
verified by a mere glance at the rosette of leaves on the
plantain. They alternate in such a way that only the
ninth leaf covers the first (i.e. the very lowest). Cer-
tainly the more leaves are separated the one from the
other the less they shade one another; but a great
development of the leaf system is only possible when
the stem has reached ‘certain dimensions. In most
cases this can be attained only at great expenditure of
building material, because in order to bear a great
number of leaves the stem must be very steady and firm.

There are plants, however, which do produce a great
number of leaves and reach a great height, while at
the same time economising their building material.
These are the climbing plants, the thin delicate stems
of which select other plants or inanimate objects as
supports. Twisting and turning round these they climb
to a considerable height, and produce a large mass
of foliage which they could not support unaided. Such
are, for instance, the hop, the bindweed, the ivy, and
many other plants growing in tropical forests and known
under the general name of lianes.

In general the stem has a twofold function : it must
bear leaves and conduct the nutrient sap from the root
to the leaf and from the leaf to the root. For this
purpose it must evidently be equipped with something
that will give it solidity, firmness, elasticity, and other
mechanical properties; but at the same time it must

THE STEM 153

have some system of canals, or some other kind of
passage for the conduction of saps. In order to
understand how the stem proceeds to serve these
purposes, we must study its structure and, first of
all, the structure of the actual cells, the bricks, as we
called them in our first lecture, out of which the plant
is built up.

If we make a very thin transverse section of wood,
we at once notice that it is perforated with very minute
holes. These are the cavities of cells. We have already
studied the inside of the cell, and its chemical contents.
In the present instance, we are mainly interested in it as
building material from the mechanical point of view ;
and in this connection the main part is played by its
solid skeleton, its wall, upon which the entire form of
the plant depends.

When isolated the cell is for the most part spherical
in form ; when connected with other cells, as is the case
in tissues, this spherical form passes into the polygonal
(as is represented on fig. 42, 1). A polygonal form shows
that throughout its life-time the cell has uniformly
developed in all directions, whereas if it develops mostly
in two directions, 7.e. along two axes, a flat tabular form
will result. Such tabular cells are formed mostly at
the surface of organs; they constitute the epidermis
of the plant (fig. 42, 2). Lastly, the cell may develop
almost exclusively in one direction, along one of its
axes only. Then instead of a polygonal or a flat,
tabular cell, a very narrow and elongated fibre appears.
(as is represented on fig. 42, 5 and 6). Such long fibres
form most of the wood of trees; but even they are not
the very longest, and the fibre-like cells of flax, for
instance, which are used in spinning, are sometimes a
thousand times longer than their own diameter, so that
we could only represent them here on their true scale
by a single line. Cells not only vary in general
outline and external appearance, but the very wall of

154 THE LIFE OF THE PLANT

the cell as such may vary widely in structure; it is
either uniform and thin (as in fig. 42, 1 and 2), or
thickened everywhere in concentric layers (fig. 42, 3 and
5); or, lastly, these inner layers may not be uniformly
deposited on the cell-wall, but only at certain places,

2882 6 SSD

Fic. 42.

thus forming the most curious patterns. For instance,
if only small parts of the cell-wall remain unthickened,
the cell as a whole instead of being uniformly transparent
will appear spotted, and in transverse section will be
found canals perforating the thickness of the cell-wall
and corresponding to these spots (fig. 42, I and 3). Itis

THE STEM 155

at the same time curious that the canals of neighbouring
cells generally correspond, so that these spots, pores, or
pits, as they are called, are simply points where adjoining
cells are divided only by the very thin primary membrane
through which the sap can easily diffuse. Sometimes
even this extremely thin cell-wall disappears, and the
cavities of neighbouring cells are brought into open
communication with each other. On the other hand,
if most of the cell-wall remains unthickened, the thicken-
ings appearing on the inner side will present the most
varied forms of nets, rings, spirally-twisted bands, etc.
(fig. 42, 4 and 7). Such varieties of form in thickenings
and pores are generally described and enumerated with
special care by micrographers. To physiologists, on
the other hand, form as such, however curious it may
be in itself, does not present any interest as long as its
significance, the part it plays in the life of the plant, is
unrecognised ; and this is precisely the case with
regard to the different forms of thickening. It is only
comparatively recently that attempts have been made
to explain these structures from the physiological point
of view. We shall briefly return to this matter a little
further on.

These two considerations, viz. the general outline of
the cell and the structure of its walls, do not, however,
exhaust the ways in which cell structure may vary.
Cells can also become fused into more complicated
organs, generally known as vessels or tubes. These are
generally formed by the perforation or the total dis-
appearance by absorption of the transverse partitions
between vertical rows of cells. For instance, if a row of
cells with spiral thickenings lose the transverse partitions
which divide them from each other (fig. 43, I, on the left),
a continuous tube called a spiral vessel will appear
(fig. 43, I, on right). Sometimes, however, as we have
said, vertical rows of cells, in transforming themselves
into tubes, do not entirely lose their transverse partitions,

156 THE LIFE OF THE PLANT

but communicate with each other by means of pores,
larger or smaller as the case may be. One form of such
tubes is very curious: the transverse walls of the com-
ponent cells are perforated
with minute pores and form
a sort of sieve, and the cells
themselves are called szeve-
tubes. One such cell fused
with two others into a single
tube is represented in fig. 43,
II. The contents of these
cells can communicate
through their pores. Very
minute grains of starch have
been observed in these pores,
and we shall soon see how
important is their physio-
logical significance. As well
Fic. 43. as thevessels andsieve-tubes,
which are long and straight
tubes, we come across others which branch and inter-
weave and form a whole complicated network of com-
municating canals. Such tubes generally contain white
or sometimes yellow sap; hence their name of latex
tubes. Plants containing latex are known more or less
to everybody ; e.g. the dandelion and the poppy which
exude a white sap when slightly wounded ; the blood-
wort, probably known to us all since our childhood,
from the injured stems and leaf veins of which a yellow
sap escapes ; lastly, the common Ficus, which is grown
indoors with us, and which, like some other tropical
plants, secretes an abundant latex, known under the
name of india-rubber when it is dried. These different
kinds of latex are contained in a complicated system of
branching and interwoven tubes to be found all over the
plant, but especially in its rind and leaves.
All these elementary organs of the plant can be divided

THE STEM 157

into three groups, to which different functions are
mostly, if not exclusively, natural. These three groups
or categories are the following: cells proper, fibres, and
tubes. In the cells the processes of nutrition take place,
i.e. the formation and transformation of the nutrient
substances. They contain the chlorophyll ; in them the
stores of albuminous matter are deposited, also starch,
sugar, crystals of mineral salts, and so on. The cells are
the laboratories and warehouses of the plant. Fibres
serve mainly for mechanical purposes, their contents
being of no value whatever. Their chief end is served
by their elongated form and by their walls, which are
sometimes so much thickened as to entirely fill the cavity
(as is shown in fig. 42, 5). Recent investigations have
shown that the material out of which these mechanical
elements are formed, as well as their structure and
especially their distribution in the stems, render them
wonderfully well adapted to their function, which is to
impart to the various parts of the plant the necessary
firmness and elasticity with all possible economy of
building material. Investigation has proved, for instance,
the remarkable fact that the material out of which
these fibres are formed is almost as strong as iron in
some respects, and that the fibres are distributed accord-
ing to the laws of engineering. The tubes, belonging
to the third category, serve mainly as sap conductors.
Let us now see how these elements, so various in
structure and function, are distributed in the plant.
Cells proper form a connective or fundamental
tissue, 7.e. the foundation of all organs connecting
all their various parts ‘together, while fibres and
tubes (or vessels) are grouped together in strands,
called vascular bundles, which pass through this funda-
mental tissue. We see this best of all in leaves. The
middle part of the leaf between the upper and under
epidermis is occupied, as we already know, by the
fundamental tissue, through which run veins or nerves.

158 THE LIFE OF THE PLANT

These are the bundles, and they are either long and
straight, running parallel to one another, or they inter-
lace in a complicated fashion, forming a network of which
only a faint idea is obtained on simply glancing at the
leaf. In order to realise the thinness and delicacy of
this network, we have only to let the leaf rot in water
for some time ; then we shall be able without difficulty
to remove with a soft brush both the epidermis and the
fundamental tissue of the leaf, and separate this network
of nerves, to which no lace can be compared for delicacy.?
The name ‘nerves’ is not very happily chosen, since
these organs have almost nothing in common with the
nerves of animals. If any analogy is necessary it would
be more appropriate to compare them to a skeleton and
a vascular svstem combined, since they form the hard
skeleton of the leaf, and also a system of canals for the
translocation of nutrient substances. I have expressed
myself rather carefully in saying that they have almost
nothing in common with the nerves of animals; because,
as we shall see, there is a theory that they are the paths
by which irritation is transmitted in the plant. If this
theory be confirmed we shall evidently have to recognise
in them a certain, though remote, resemblance to the
nerves of animals.

The same nerves or veins so apparent in the leaf
extend also into the stem, where they are less sharply
defined, and do not strike the eye in the same way.
Different plants, however, present a very different
structure in this respect. Let us examine two very
common examples. In the monocotyledonous? class
of plants, to which for instance our cereals, asparagus,
and palms belong, these bundles are scattered in the
fundamental tissue as is shown in the transverse section
of fig. 45, I. A longitudinal and transverse section of

1 Other parts of the plant can be treated in the same way, and then

semi-transparent ‘phantom’ bouquets, so to speak, can be made of them.
2 So called on account of their possessing only one cotyledon.

THE STEM 159

one of these bundles with its surrounding fundamental
tissue, from the stem of maize, is shown very much
magnified in fig. 44.1 Here it is clearly seen that the
bundle consists of different vessels, with spiral, net-
like, or ring-shaped thickenings, etc., sieve-tubes, and
also fibres ; whereas the surrounding tissue consists of
ordinary cells.

This structure of the monocotyledonous stem is

dil

ij

LTE

a

Uh

Uh

=
{T

fit

more clearly seen if a section of it is placed for a time
in a suitable stain, say a solution of fuchsin. Then,
as in fig. 45, I., the bundles in the transverse section
will appear as red spots on the colourless ground of the
fundamental tissue.

1 A transverse and longitudinal section has been taken of the stem of
maize, showing a single vascular bundle together with the surrounding
fundamental tissue. The large aperture, surrounded by small ones, is
the opening of a very large vessel. A number of smaller vessels are cut

longitudinally.

160 THE LIFE OF THE PLANT

Quite a different structure is presented by the stems
of conifers and dicotyledons,! to which all our forest

Fic. 45.

trees belong, such as the
oak, lime, maple, etc. In
order to understand the
structure of these stems, it
is necessary to enter into
some anatomical details,
without which further ex-
position of the subject would
be useless.

Botanists, as well as non-
botanists, differentiate three
parts in the transverse
sections of the trunk of a
tree : the bark or rind, the
wood—showing a series of
concentric rings—and the
pith (fig. 45, III.). But
botanists go further and
distinguish also between the
fundamental tissues and the
fibrousand vascular bundles,
the same distinction as has
been already clearly seen in
the stems of the mono-
cotyledons. Let us try and
make this point clear. We

_ see the predominance of the

fundamental tissue in the
stem of a monocotyledon;
bundles are scattered in it

indiscriminately, and are also surrounded by it. But

suppose these bundles were distributed regularly in a

circle, and, moreover, were so much developed that

only comparatively narrow layers of fundamental
1 I.e. flowering plants possessing two cotyledons,

THE STEM 161

tissue were between them, we should then have such
a stem as is shown in fig. 45, II.1. Such in reality is
the structure of a very young stem of any of our forest
trees in its first year. The fundamental tissue in it is
found in the middle of the ring of vascular bundles;
this is the pith or medulla. We also see it in the form
of narrow rays diverging from the pith in between
the bundles; these are the so-called ‘medullary rays’ :
lastly, we see on the outside of this ring the so-called
primary cortex, generally green and sappy, and com-
posed of small cells. Thus every vascular bundle en-
closed between two medullary rays has in transverse
section the form of a triangle with its apex turned
towards the centre.

This triangular form of the bundles is also preserved
inanoldstem. The dark diverging rays seen in fig. 45,
III. represent medullary rays, and the lighter triangles
between them are vascular bundles. Thus the bundles
form the predominant part in the perennial trunk of
a tree; the fundamental tissue is found in between the
bundles in the form of narrow medullary rays, some-
times almost invisible : hence it is clear also that the
difference between the bundles and the fundamental
tissue is not so well defined as in the case of mono-
cotyledons, and is only seen under the microscope.?

The principal part of the stem of our forest trees
consists, therefore, of vascular bundles; this is not,
however, their most striking peculiarity. They differ
from monocotyledons, such as palms, in that they in-
crease in bulk during the whole of their existence—
which is quite impossible to the monocotyledons—
owing to the following anatomical arrangement. We
all know that the rind of our forest trees is sharply

1 Fig. 451. The stem of a palm or of asparagus. IT. Structure of the
stem of an annual dicotyledon. III. Stem of a dicotyledonous tree. All
three are shown in transverse section.

2 See fig. 62, showing a small section of wood cut across, with a

medullary ray.
L

162 THE LIFE OF THE PLANT

differentiated from the wood ; in spring when the plant
is full of sap it even peels off easily. Non-botanists
presume, and formerly even botanists also presumed,
that there is a space in between the rind and the
wood, which is filled, especially in spring, with a thick
liquid, out of which the new parts of the plant can
be formed. Exact microscopic investigation has proved
that there is no such space, but at that part of
the stem there is a ring of exceedingly delicate juicy
tissue capable of continually forming new cells—
hence its name of formative tissue, otherwise cambium.
In fig. 45, II, the cambium is shown as a dark ring
which cuts across the vascular bundles as well as the
medullary rays, the whole stem being divided by it
into two parts—the wood lying inside the ring, and the
rind lying outside it. Owing to the presence of this
continuous circular formative layer, which is absent
from the monocotyledons because their vascular bundles
are scattered instead of being distributed in a regular
circle, the stems of dicotyledons and conifers are capable
of long continued growth in thickness. Every year
this formative tissue deposits new rows of cells towards
both the wood and the bark ; only the wood increases
more rapidly (fig. 45, III.), and the rows of its cells are
deposited more evenly; this is why it presents the
regular alternation of annual rings that we notice on
every transverse section of a tree. Let us now see
what is the anatomical structure of these two kinds of
vascular tissue on either side of the formative tissue,
z.e. in the wood and in the rind. On the wood side
we find almost exclusively fibres called wood fibres
(fig. 42, 6) and various vessels, pitted, reticulate,
spiral, and so on, but no sieve-tubes. On the bark side
we find very elongated fibres with very thick walls (fig.
42, 5), similar to the fibres of the plants used for spinning
that we mentioned before, and the only tubes we meet
are the sieve-tubes mentioned above (fig. 43, II.). The

THE STEM 163

fibres form the part of the rind generally called bast; it
is highly developed in lime trees for instance, and is used
for splints, bast-strings, and so on. It is from this bast
that all fibres having the structure we have described,
wherever they are found, have received the name of
bast. The transverse section of an old tree will, there-
fore, present the following parts: on the outside the
part which we have called the primary cortex ; here, as we
shall soon see, a special tissue serving as a protection
to the tree is formed at a later period ; then under the
primary cortex there is a layer | of vascular tissue
consisting mostly of bast and sieve-tubes ; we shall
call this part the secondary cortex to differentiate it from
the primary ; then comes a ring of formative tissue ;
nearer still to the centre is the wood, and, lastly, in the
very centre the pith. We shall content ourselves with
these particulars ; they may have seemed rather tedious,
especially when presented in such a necessarily terse
form, yet they are indispensable for the understanding
of the ‘physiological activity of the stem. Now we
enter upon the investigation of the question: what
are the paths by which the sap of the plant moves, i.e.
by means of which the natural interchange is effected
between the substances absorbed by the root and
elaborated by the leaves ?

Let us start with the former, as being the simpler
case, that is with the passage of substances from the root
towards the aerial parts of the plant, the so-called
ascending current. This current furnishes all parts of
the plant with the water they require, and in addition
with the salts dissolved in it. It is quite easy to dis-
cover the path of the moving water, because the want
of the necessary quantity of it is generally betrayed
by the plant fading. Therefore by making trans-
verse incisions at different places on the stem of a living
plant, and observing where and when it begins to fade,

164 THE LIFE OF THE PLANT

we can easily see whether we have cut across the path
of the ascending current of water or not. Experiment
has proved that we can cut through the whole of the
rind, and even peel it right off, without making the
plant wither, which means that its aerial parts go on
receiving water from the soil. We can also cut through
the pith, which by the way often dies naturally in old
trees, leaving hollows at the heart of them; in course
of time decay spreads also to the inner layers of old
wood, and yet the tree does not seem to suffer for a
long time. Apparently the ascending current of water
must flow through the wood, and moreover through
the young wood. This inference is also proved by
another experiment which has already been mentioned,
by the experiment in which the vascular bundles are
stained by coloured solutions. This experiment gives
specially clear results with leaves which are variegated,
or wholly white. In a short time the whole network
of veins stands out in colour on the white ground.
Microscopic investigations show that the wood vessels
are the first to become stained, which means that wood
is to be considered as the course for the ascending
current of water in the stem.

And how can we explain the reason of this ascent of
water, sometimes to a height of three hundred feet ?
The reason of this movement must lie in the stem, as
well as in the root; in the stem, because stems and
branches when cut off a tree continue to absorb water,
carrying it to the leaves; in the root, because if the
stem be cut off close to the ground, and even more so if
the upper part of the root be cut, water will exude from
the cut surface of the part remaining in the soil. Let us
first study this phenomenon of the exudation of water
from the upper cut end of the root, which is apparently
the primary cause of the penetration of water into the
stem. It was long ago noticed that sap flowed in abun-
dance from some wounded or cut stems ; to this pheno-

THE STEM 165

menon the name of ‘bleeding’ has been given. It was
thought to be the exclusive property of certain woody
stems, and to take place only at certain seasons of the
year; this bleeding is specially profuse in the vine in
spring. Comparatively recent investigations have

ta.

Fic. 46.

shown, however, that this phenomenon is common to
all plants, herbs as well as trees ; and that it takes place
all the year round, although certainly with very varying
intensity. In order to see this bleeding, and to measure
its force, the following method is used: a stem is cut
across not far from the ground, and we attach to the
stump, with the help of an india-rubber tube, a small

166 THE LIFE OF THE PLANT

bent glass tube, if we only want to gather and measure
the quantity of liquid exuded; whereas, if we want to
find out the pressure under which the sap is driven out
through the section we use another tube of the form
shown in fig. 46 on the left. This doubly bent tube,
filled partly with water and partly with mercury, is
simply a manometer, serving to measure the pressure
under which the sap of the plant is exuded. The sap
drives the mercury before it as it flows into the tube ;
it is by the rise of the column of mercury in the open
bent part of the tube that we estimate the pressure.
Experiment has shown that this pressure can be equal
to that of a column of water thirty-six feet high, z.e.
water is driven out of the section so vigorously that it
might still come out even if a column of water thirty-six
feet high had been made to press upon the cut surface.
How can we explain this property of the root to. raise
water to such a height ? The following experiment will
give us the answer. Let us take a small glass bell
(fig. 46 on the right, and 0), close its lower opening with
a bladder and introduce a cork with a glass tube into
its neck, and sink the whole as shown in the figure into
a basin of water. If the glass bell also contained water,
there would be no interchange between the water of the
outer and inner vessels, so long as the level is the same
in both vessels, at z, because otherwise water would
soak through the bladder under its own pressure from
the vessel where its level is higher into the one where
it is lower. But let us suppose that a solution is
introduced into the inner vessel instead of water, a
solution of some substance found in the cells of plants,
say sugar, which, as we know, is found in abund-
ance in the roots of the beetroot, for instance. Then
a phenomenon rather perplexing at first sight will be
observed, even rather contradictory to what has just
been said about the tendency of water to reach a common
level in two vessels communicating with one another

THE STEM 167

through a membrane. The solution of sugar—which can
be coloured to make it more conspicuous—will quickly
rise in the glass tube and soon reach a considerable height
(f). The explanation of this fact is as follows : accord-
ing to the laws of diffusion the water and the solution of
sugar tend to mix, the one moving towards the inner
vessel, the other towards the outside of it. But the
particles of water move more quickly than those of
sugar; therefore the water will pass into the sugar
with greater rapidity than the sugar into the water ;
moreover, water passes through a bladder far more
easily than sugar; therefore, by the joint working of
these two causes, the current of water into the inner
vessel will be far more rapid than the opposite movement
of sugar, and hence the rise of the solution in the tube
which at first seemed incomprehensible because it
contradicts the laws of hydrostatics. We should get
the same effect, though less clearly, if instead of sugar
we took albumen, gum, or some other substance gener-
ally found in vegetable cells. Therefore here also the
phenomenon resolves itself into diffusion, complicated
by the presence of the membrane. Phenomena of this
kind have been called osmotic. The rate of this diffusion,
all other conditions being equal, will also depend on the
area of contact between the two liquids, in our case on
the size of the opening closed by the bladder. Suppose
we grant that our apparatus presents a certain resem-
blance to a root-cell, a root-hair, and remember how large
is the area of contact between such root-hairs and the
water of the soil: we shall then soon realise what
the result must be if such an apparatus is multiplied a
million times, however microscopic it may be. Every
cell greedily absorbs water and squeezes it through its
presumably thinner inner wall into vessels which send
it up the root into the stem.

Such is the explanation we can give of the water-raising
capacity of the root or root pressure. Alone it is probably

168 THE LIFE OF THE PLANT

insufficient to explain the raising of water to the summit
of the highest trees ; and we also know that cut stems
sunk into water are themselves able to absorb it. But
in order to explain the reason of this absorption of water
by the stem, we must first of all begin by studying the
part played in this process by the leaves. The best
way to do this, is to perform the following experiment.
We cut off a small branch, say of a birch-tree, covered
with leaves and dip its cut end in water. On taking
the branch out of water we notice a drop suspended at
the cut part, but in half a minute, if not less, the drop
will be absorbed ; we dip the branch in again and notice
once more the rapid disappearance of the drop, showing
the greediness with which our branch absorbs the water
provided for it.

The root drives water into the stem; the stem
greedily absorbs it and drives it further on. What will
become of this water when all the parts of the plant are
saturated with it? Apparently, if it continually enters
at one end it must pass out at the other. This passing
out of water is very clearly observed under certain
special circumstances. If on a warm damp evening in
May or June we bend to the ground after the sun has
set and glance at the surface of a field of oats, we shall
observe round drops on the very tips of the upright
blades. If we patiently observe one leaf for some time,
we shall see the little drop growing larger and larger until
it rolls down; in its place, at the very edge of the leaf,
another drop will appear and so on. The same pheno-
menon will be observed if oats are sown on a plate
covered with a glass bell. Small drops of water will
continually appear at the tips of the leaves and dis-
appear whenever the glass bell is removed. This
phenomenon is seen in some plants much more clearly,
and they give off water in considerable quantities.
Anatomical investigation has even revealed special
apertures in these leaves at the points of exudation.

THE STEM 169

But such exudation of water in the form of drops is a
comparatively rare case. It generally happens under
the conditions just described, z.e. when the surrounding
air is saturated with water vapour; but, as a rule,
plants give off water in great quantities in the form of
invisible vapour.

We can realise the amount of water evaporated by
plants from the following approximate calculations: one

ES

Fig. 47.

acre of oats evaporates during the summer from 150
to 250 tons of water; an acre of mixed meadow grass
something like 750 tons. Wecan determine this amount
of evaporated water in many ways. Here is one of the
simplest and most exact methods. The pot in which a
plant is growing is placed in a glass or tin vessel and
covered with a glass or tin plate with a hole through it
for the stem (fig.47). In this way evaporation from the
surface of the soil and the pot is prevented, and if we
weigh the whole apparatus from time to time we know

170 THE LIFE OF THE PLANT

that the loss in weight is due to evaporation from the
plant. We can also take two glass bells of equal size, a
little smaller than the leaf the evap-

| oration of which we are going to
investigate, so that the leaf can be

! held tightly between them (carefully,
of course, so as not to crush it, but

Ww at the same time in such a way that

’ the greased edges of the bells are

tightly fixed to it). Under each bell

we place some substance which

greedily absorbs water vapour, such

as sulphuric acid for instance, so
h that it may absorb the water which
- evaporates from the leaf. By weigh-
ing the vessels containing the sul-
phuric acid we shall find out the
amount of water it has absorbed.
In this way we can solve many
interesting problems. We learn, for
instance, that it is the lower side of
the leaf, i.e. the one which, as we
have seen, bears the stomata,! that gives off water vapour
most actively. It appears that these stomata are to
be looked upon as the regulators of evaporation.
When the plant is saturated with water the slit-like
aperture of the stoma opens wide (fig. 48 6) and
evaporation increases; but as soon as the leaves begin
to fade, whether on account of too much evapora-
tion, or too little water, the stomata contract and
almost close (fig. 48 a) ; evaporation decreases and the
_plant revives. We also learn from similar experiments
that leaves with a bright shiny surface evaporate less
than leaves such as grass ; this is the reason why plants
with shiny leaves apparently stand dry, torrid climates
more easily. Lastly, such experiments teach us that

1 See chapter v.

Fic. 48.

THE STEM 171

young leaves evaporate more quickly than old ones of
the same plant, and this fact affords a partial explanation
of why the nutrient sap flows precisely to these young
growing organs.

Having learnt what a considerable quantity of water
is evaporated by the leaves, we can return to the in-
vestigation of the actual mechanism of this movement
in the stem.

This question has attracted special notice of late,
and yet it cannot be said to have been settled quite
satisfactorily. It is true that there are many explana-
tions of the phenomenon, but their very abundance
proves that none of them is completely satisfactory.
Let us then dwell only on facts easily verified by
experiment. To begin with, it was necessary to decide
what course the stream of water takes up the
stem: does it pass by way of the cavities, or within
the walls of the vessels? Contrary to the first
most natural supposition that the sap circulates in the
cavities of the vessels, since this would appear to be
their simplest course and since these cavities form
continuous channels through the plant, it has been
pointed out that vessels are not generally filled with
liquid, but contain bubbles of air alternating with
columns of liquid. Yet this very presence of air, which
at first was an objection to the theory that water
circulated through the vessels, is now taken as a key to
the explanation of the phenomenon. It happens that
this air generally exists in a very rarefied state, and
that owing to this circumstance every vessel acts as a
pump. We can demonstrate this fact by the follow-
ing simple experiment: a stem of any kind is bent
down into a vessel containing mercury until one part
of it is sunk under the mercury, and zt is then cut
across under the mercury. If we make longitudinal
sections of the same stem later on, we shall see that
mercury has penetrated into the cavities of the vessels

172 THE LIFE OF THE PLANT

in the form of the finest threads. This phenomenon
is seen best if the microscopic preparation is lighted
from above instead of in the usual way from below ;
then we clearly see in the vessels bright threads of
mercury like that seen in the capillary tube of a
thermometer (fig. 49, I).1 Let us remember that
mercury does not rise by itself as, for instance, water
does in a capillary tube, but that on the contrary
it can only be forced into

T such tubes by pressure—
the narrower the tubes the
greater must the pressure
L. be. But the diameter of
|} plant vessels is much nar-

rower than the diameter of
a | the capillary tubes with
which experiments in physics
are generally made. Hence
bs | we can measure approxi-
| mately the degree of its
H rarefaction that brings about

1 I this aspiration of mercury.
I I Two questions naturally arise:

Fie. 40. why is no equilibrium estab-

lished between the rarefied

gases in the vessels and the external atmosphere ?
and, what is the reason of that rarefaction ? The first
question is answered very simply: the air inside the
vessels is separated from the external parts of the
plant, containing air at the general atmospheric pres-
sure, by a layer of impenetrable tissue, which entirely
isolates it from the external atmosphere. But when-
ever the internal parts of an organism come into con-

' Microscopes for handing round to the audience at lectures are specially
convenient and have been much used of late years. They are provided
with concave metallic mirrors which concentrate the light upon the upper
surface of the object.

THE STEM 173

tact with the atmosphere by means of a transverse
section, the equilibrium between the internal and
external atmosphere is suddenly established. This is why
it is necessary to cut through the stem under mercury.
However quickly we dip a cut stem into mercury, we
shall always remain without any result whatever.
But if we leave the end of a cut stem sunk in mercury
for some time, we shall find that mercury will begin
to rise in its vessels. This experiment will give us an
answer to the second of the questions raised above—
how is the origin of this rarefied atmosphere to be
explained ? Its explanation is as follows. Leaves
evaporate water, and hence more concentrated solu-
tions of substances contained in them are formed in
their cells. These solutions, as we have already seen
(fig. 46 on the right), draw fresh quantities of water
from the neighbouring cells, and thus from cell to cell
absorb the water stored in the vessels. But if water
be drawn out of the vessels, the air of the bubbles
alternating with it takes its place, and increases in
volume, 7.e. becomes rarefied. As a result of this rare-
faction, a fresh quantity of water is absorbed by the
vessels from the cells of the root. The truth of this
inference can be proved by direct experiments. If we
make a section of the tip of a stem (bearing leaves)
transparent enough to be placed in a drop of water
under the microscope, we shall be able to learn the
following facts. If small particles of a powdered sub-
stance are suspended in the drop of water, we shall
notice them tending towards the apertures of the
vessels and flowing right through them. The bubbles
that we see in the vessels will either decrease in volume
with the decrease of evaporation from the leaves, or
else increase, 7.e. the air will become rarefied, with
the increase of evaporation from the leaves (fig. 49,
II, a and d).1

1 In Fig. 49 II, a and b, two consecutive stages of one and the same vessel

174 THE LIFE OF THE PLANT

Despite, therefore, the many doubts cast upon
the subject, the part played by the vessels, as the
aqueducts of the plant, can no longer be disputed.
Together with the explanation of the part played by
the vessels, the significance of one more peculiarity
of their structure which long puzzled the most cele-
brated anatomists has also been explained. This is
the so-called bordered pit found in vessels and conduct-
ing fibres (tracheides). They are easily observed in the
wood of our fir-trees, e.g. in a very thin section of almost
any match-stick. Jf we make a longitudinal section
in the plane of the axis and diameter of the trunk

Fig. 50.

(a radial section), we shall notice on the walls of the
vascular fibres numerous rings with double outlines (fig.
50, A, @ and 8). On closer observation we notice a
third less definite circle (c) lying between a and b. If
the longitudinal section does not lie in the plane of the
diameter of the trunk, but intersects it at a more or less
acute angle, the figure will change. Instead of the sur-
face view of the pit we shall see it at an angle, say of
45° (B), and ascertain that there generally are two
inner (small) rings. In order to understand the structure
of the pit more clearly, we must make a third section

are shown under the microscope during vigorous evaporation of water
from the leaves. By comparing them we notice that drops of water have
decreased at c, while air bubbles have correspondingly increased.

THE STEM 175

‘at right angles to section A. This will give us the profile
of the pit, z.e. its section (C, D, E), which will explain
everything to us. It happens that the common walls
of the two adjoining cells form here a cavity shaped
like a lens, the margin of which corresponds to the
outer ring of the pit (6).1 This cavity seems to be
formed, as it were, by two watch-glasses, perforated in
the middle with round apertures (a in fig. A). These
apertures lie one below the other, so that their out-
lines blend into a single inner ring (a). In fig. B
both circles are seen, owing to the fact that we are
looking at them obliquely. The lens-shaped cavity
is divided into halves by a very thin membrane
stretched across it, the centre of which is thickened
like a disc (C). The edges of this disc, generally visible
through the glass-like transparent cell-wall, give the
impression of the middle ring (cin A and B). Having
realised the arrangement of these pits, and knowing
that the air and water in them are under tension like
that produced by a suction pump, their significance
is easily understood. They are valves—most perfect
valves. When the suction pressure in the vessels is not
great, water passes through the thin membrane (as
shown by the arrows at C). The resistance of these
membranes to the movement of water is insignificant :
if we let fall a drop of water upon the upper cut end of
a long branch, the exudation of a similar drop from
the lower cut end follows almost instantaneously.
But these thin membranes might easily break under
pressure such as we have observed in vessels. The
membrane then curves, and the disc applies itself to
the one or the other aperture according to the direc-
tion of the pressure (as indicated by the arrows at
DandE). Thus the bordered pit is shown to be a very
conveniently arranged double valve, adapted to a variety

1 Dotted lines are drawn to show the corresponding parts of all the
figures.

176 THE LIFE OF THE PLANT

of pressures in vessels. These valves secure the uniform
distribution of water in plants.

Having seen that water moves through the vessels,
and also the reason for this movement, we must try
and discover its rate. We proceed as follows : we put
the cut end of a branch of the plant to be examined
into water which contains a small quantity of some
substance the presence of which is easily detected in a
plant; thus, if we cut the stem across at intervals after
some time, we can discover the height to which the
water had time to rise during the experiment. The
very greatest height ever reached in this way is approxi-
mately seven feet per hour.

Evaporation from the leaves, which constantly draws
water from the aerial parts of the plant, is therefore the
principal reason for the absorption of fresh quantities
of water from the root into the stem. But, one must
ask, why do we ascribe this function of evaporation to
the leaves and not to the stem? The anatomical
structure of the stem gives us the answer to this
question. It is only at a very young stage that the
stem has a skin or epidermis like that of the leaves ;
this very early dies, splits, and falls off, while under-
neath it, in the part of the stem called the primary
cortex, there is formed a corky tissue. It is called cork
because it is strongly developed in a certain species of
oak, where it forms the material out of which bottle
corks are manufactured. The structure as well as the
external appearance of this tissue may vary very much:
thus, for instance, in the cork oak it forms a con-
tinuous layer several inches thick ; while in the birch-
tree it is only a thin and scaly bark. In all cases,
however, it has the same general property, namely,
that it is impermeable to water, and thereby forms a
kind of impervious covering on the stem guarding
it from unnecessary or even harmful evaporation. It
is a curious fact that this cork tissue spontaneously

THE STEM 177

appears when a vegetable organ is wounded, that is,
just when it becomes exposed to abnormal evaporation,
and thus puts an end to this unhealthy condition.
Thus, for instance, we have only to wound any vege-
table organ and thereby lay it bare and leave its inner
tissues unprotected, and in a short time we shall see
the skin of cork tissue forming over the wound.

Thus the root drives water into the stem, the stem
carries it along to the leaves, the leaves evaporate it into
the air. It is only in the conjoint and uniform fulfil-
ment of all these functions that the activity of the plant
will be completely normal. The balance is upset when
the plant evaporates more than it absorbs—then it
withers ; the balance is also upset if the plant has no
time to evaporate all the water it absorbs—then it
begins to exude it in the form of drops, such as we notice
on the blades of grass on warm damp evenings when,
owing to the saturation of the atmosphere with water
vapour, evaporation from the leaves has almost ceased.

We pass now to the investigation of another movement
of the nutrient substances, tending not towards the leaf,
but from the leaf towards all parts of the plant, including
the root. The fact that there must be such a movement
is evident a priori, because the organic matter out of
which all the parts of the plant are built up is formed
in the leaf; the fact that it actually exists is clearly
proved by the following simple experiment. Let us
cut off a willow branch and place it in water. After a
few days or weeks a kind of excrescence appears round
the lower cut end of the branch, and from this excrescence
there spring little roots. Evidently these rootlets must
have been formed at the expense of matter obtained
from the leaf, or else of matter which was already on
the way from it, 7.e.in the stem. Let us try to show the
way by which it has come down to the newly formed

M

178 THE LIFE OF THE PLANT

roots, using for the purpose the same method as we
used in determining the course of the rising sap. Let
us make a circular cut in
the rind right down to
the cambium, as is shown
in fig. 51, and place our
branch in water for several
weeks. We shall notice
that the roots will appear
this time not at the lower
part of the stem, but at
the upper edge of the
circular cut; in cutting
across the rind we have
_ evidently barred the way
for the nutrient sub-
stances which moved
down the stem. It follows
that the circular cut in
the rind, which does not
hinder the ascent of the
Fic. 51. sap from the root, com-
pletely stops the sap
moving in the opposite direction. This means that
whereas the sap rising from the root passes by way of
the wood, the sap coming from the leaves passes by
way of the rind.

The truth of this inference is also proved by another
experiment. Let us choose a branch of some plant, on
which fruit has only just begun to set, and let us
cut a ring in the bark between the fruit and the near-
est leaves: the fruit will cease to develop. Thus
the circular cut in the rind, which separates an organ,
such as the root or the fruit, from the leaves it feeds
on, will deprive that organ of the very possibility of
development. We have therefore proved beyond doubt
that the nutrient substances serving to build up the

THE STEM 179

various organs of the plant move through the rind.
But the rind, as we have already seen, presents a
complicated structure; we differentiate in it the
primary and secondary rind; by way of which of
these two systems does the nutrient sap move?
Again let us repeat our girdling experiment; but
this time we carefully cut only through the external
part, the bark proper, being careful not to injure the
secondary rind, z.e. the bast of the vascular bundles.
We get results similar to those of the former experiment,
z.e. roots are formed at the base of the branch. This
proves that the movement takes place by way of the
secondary rind. Let us venture one step further, and
determine by which elements of the secondary rind this
sap moves. We know that they are mainly two in
number : bast fibres and sieve-tubes. A comparison of
the mere form of these two elements makes it probable
that the latter fulfil the functions we are considering,
because the fibres have very thick walls with almost
no cavities, whereas the sieve-tubes have broad canals,
communicating with each other by means of open pits,
through which not only liquid and semi-liquid substances
but even minute grains of starch can pass. This
probability changes into certainty after the following
experiment. We take an oleander branch and manipu-
late it in the same way as we manipulated the willow
branch in our second experiment, 7.e. we cut off a
complete ring of the bark right down to the cambium.
A wholly unexpected result follows. Roots are formed
not only at the edge of the cut, but also at the base of
the branch—it follows that the nutrient substances
pass otherwise than by way of the rind. This apparent
contradiction is fully explained when we learn that the
__stem of the oleander shows a deviation from the typical
structure of the stem above described. Besides the
sieve-tubes in the bark, bundles of these elements are
also found in the pith; and it is these which, in spite of

180 THE LIFE OF THE PLANT

the circular cut in the bark, convey the sap to the
lower part of the stem. Thus all these four obvious
experiments with willow and oleander branches gradu-
ally and systematically reduce the circle of possible
suggestions, and in the end point decidedly to the
sieve-tubes as the course along which the nutrient sub-
stances of the plant spread—the plastic substances, so
called because they serve the purpose of building up
new parts in the plant.

Recent investigations as to the distribution of the
latex-tubes in a leaf lendacertain colour to the suggestion
that they likewise serve as a very convenient course for
the movement of the nutrient sap. This suggestion is
based upon the fact that they are generally found in the
immediate neighbourhood of the green tissue of the leaf
where nutrient substances are manufactured. This
suggestion is supported by the observation that the loss
of latex exhausts some plants.

After having traced the course of the movement of
the sap from the leaves, we have still to find out the
causes which set it in motion. Again, for the last time,
the key to the puzzle is found in diffusion—the word
which like a constant refrain is repeated every time
there is any question as to the absorption or transloca-
tion of matter into the plant from the external medium,
or from one part of the plant to another. According to
the laws of diffusion, matter when dissolved apparently
flows especially to places where it is changed into an
insoluble form, either being deposited in store for the
future, or simply spent in the building up of the solid
parts of the plant. The nutrient substances are
deposited all along the system of vascular bundles.
Cells surrounding these bundles are generally very rich
in starch, and occasionally also in crystals and other
matter. We have observed the storage of nutrient
substances in the endosperm of the seed; similar

1 See chapters ii. and iii.

THE STEM 181

storage, only in much larger quantities, is also found in
other parts of the plant. It is deposited, for instance,
in the pith, in the medullary rays, in a word, in the
fundamental tissue of the stems. In the pith of the
sago-palm, starch is stored in quantities which can be
measured by hundreds of pounds ; potato-tubers also
store starch, the beetroot an abundance of sugar,
cabbage-heads or turnip roots the most varied nutrient
substances ; lastly, in the fleshy leaves of the afore-
mentioned Agave, sugar is stored up during many years.
In fact, there is scarcely any vegetable organ which
may not become the receptacle and store-house of
nutrient substances. These stores are either used up
the next year after they are deposited, as is the case
with the beetroot or cabbage, where the stores are
spent on the development of the stem and flower organs
in the second year of the plant’s existence ; or else they
are accumulated during many years, as is the case with
the sugar in the leaves of the Agave, which is eventually
spent in the formation of a huge branching inflorescence
bearing the flowers and fruit. In every case storage
is only a temporary, transitory destination of nutrient
substances: their final destination is reached only
when they are entirely used up in the formation of
new parts of the plant, of new organs, new cells, 7.e.
when they contribute to its growth. Thus after having
studied the phenomena of nutrition, in the sense of
absorption, digestion, and translocation of food, we can
pass in our next chapter to the study of the phenomena
of growth.

CHAPTER VII
GROWTH

In the folklore of some northern people the gods and
diviners are endowed with a faculty for not only seeing
but even hearing the grass grow. In the present
chapter we shall investigate the question whether the
eye and ear of a simple mortal can ever develop such
acuteness as to see and hear the growth ofa plant. Let
us begin by settling in what sense we are going to use this
term. By growth, in the narrow sense of the word, we shall
understand the increase in bulk of the plant, which takes
place as a result of the transformation of the assimilated
food-substances into the solid skeleton of its structure,
consisting mainly of cell-walls. Thus, although growth
necessarily presupposes nutrition, these two processes
are not bound to take place simultaneously. Growth
can also take place under conditions which make nutrition
for the time impossible, as in the absence of light. These
two processes may in fact be carried on in different
places and at different times. Growth is usually most
active in the youngest parts of the plant, which develop
at the expense of the activity of organs already developed
and serving mainly for the purposes of nutrition. These
two main functions of vegetable life, nutrition and
growth, are sharply separated in time, particularly in
those cases enumerated in our last lecture, where growth
takes place at the expense of abundant stores of food,
often the accumulation of many years. We have
already seen that during germination the increase in
bulk of the seedling does not depend on a correspond-

ing addition of matter, but is accompanied by a
182

GROWTH. 183

continual and important loss of substance, owing to
respiration.

Let us begin our survey of the phenomena of growth
with the moment the little root and stem emerge from
the germinating seed, when the one seeks, so to speak,
to escape from the light as quickly as possible and
buries itself in the ground, while the other stretches up
into the air to meet the light. The first question that
ought naturally to arise in our mind when we investigate
this phenomenon, but probably one which seldom occurs
to people, so accustomed are we to the fact, is : Why do
the root and stem grow in different directions, the one
into the soil, the other into the air ; the one downwards,
the other upwards ?

This point has perplexed scientists very much, and
even now it cannot be considered settled in all its details.
In seeking the cause of this phenomenon scientists very
naturally turned to light and the moisture in the soil.
It has been pointed out that stems grow towards the
light and roots away from it ; and consequently it has
been said that light must be looked upon as the external
force which conditions the direction of growth. But
this theory is pretty easily proved to be untenable.
The direction of the organs in question will be the same
in the absence of light; moreover, if seeds are sown
in a sieve suspended above a window so that they are
lighted only from beneath, after passing through the
layer of earth the roots will equally pass through the
holes of the sieve and continue to grow towards the
light, while the stems will grow upwards and therefore
away from the light. The other theory that the direc-
tion of the root is determined by the moisture of the
soil is disproved by an experiment, in which germinating
seeds are surrounded by moist earth or embedded in a
wet sponge. The degree of moisture in such cases is
uniform, and yet the direction of the root and stem will
be as before, perpendicular.

184 THE LIFE OF THE PLANT

Thus the main direction of growth of root and stem
bears no permanent relation to light or moisture ; it is
only their position with regard to the horizon that is per-
manent : the root grows always downwards, the stem up-
wards ; in other words and speaking more strictly, since
this phenomenon is observed all over the globe, at the an-
tipodes as well as here with us, the root is directed towards
the centre of the globe, and the stem away fromit. This
very constancy of direction points to the fact that the
force to which it is due must be gravity, 7.e. the attraction
of our planet. This can be proved by experiment. If this
direction of the parts of the plant depends upon the force
of gravity, then by eliminating its action, we shall arrest
the phenomenon itself ; by modifying the force we shall
modify the phenomenon also ; finally, by substituting
for this force another, which acts in a different direction,
we shall change the direction of the phenomenon
correspondingly. But how shall we actually do this ?
how can we remove any body on the surface of the
earth from the earth’s attraction? How can we
arrange matters so as to prevent the plant from having
a top and a bottom ? It is clear that we cannot do it
literally. We can, however, make gravity act at short
intervals in opposite directions and thus counterbalance
its own influence. For this purpose let us fix a germinat-
ing seed to the rim of a rotating wheel (e.g. a wheel set
in motion by electro-magnetic power). If the wheel
moves in a horizontal plane (as is shown in fig. 52, A 1)
it will certainly not prevent the root from growing down-
wards and the stem upwards. But if the wheel moves in
a vertical plane (fig. 52, B 1), or, what comes to the same
thing, if we fix the germinating seed to the minute hand
of a clock, it is clear that at every half-turn the position
of the root and stem will change ; in the enda top anda
bottom, a right hand and left hand side will cease to
exist for the seedling ; the continual action of the force
of gravity in any one direction will be prevented.

GROWTH 185

XL

Experiments performed under these conditions have
proved that the root and the stem may be made to
assume any desired position, and generally to keep
growing in the direction in which they have been fixed
(see fig. 52, B 1).

So far we have supposed the wheel to rotate slowly,
but yet quickly enough to prevent the seed from remain-
ing a long time in the same position with regard to the
horizon : now let us make it rotate more quickly. In
this case a centrifugal force will be set up, just as when
we quickly twirl in the air a heavy body of some kind at

A B

Fic. 52.

the end of arope. This force acts, as it were, from the
centre to the circumference, as is easily seen by the
following experiment. A ring is placed on a smooth
spoke near the axle of a wheel which rotates horizontally.
As soon as the wheel begins to rotate at a moderate rate
the ring begins to slide down the spoke until it touches
the rim of the wheel. Therefore when this centrifugal
force acts upon bodies it compels them to move in the
direction away from the centre towards the circumfer-
ence of the wheel. It is clear that this force cannot
remain without influence upon germinating seeds. In
fact if we make the wheel B rotate fairly rapidly we shall
observe that rootlets and stems will assume a certain

186 THE LIFE OF THE PLANT

definite position : the rootlets will grow in the direction
of the force, 7.e. away from the centre, while the stems
will stretch towards the centre of the wheel (fig. 52, B 2).

Now let us see what will be the effect of a wheel rotat-
ing quickly in a horizontal position. Obviously the con-
ditions will be different here from what they were in
the case of the vertical rotation. There the influence
of the force of gravity was completely neutralised and the
centrifugal force alone was directive. When the wheel
rotates horizontally, on the other hand, both forces act.
The force of gravity alone would have induced the root
to point in the direction indicated by the arrow n
(fig. 52, A 2). Centrifugal force alone would have
caused it to point in the direction indicated by the
arrow m. With both forces acting simultaneously it
must necessarily assume an intermediate position, as is
shown in the figure—a position that will be nearer
to the horizontal the stronger the action of the centri-
fugal force, z.e. the larger the wheel and the more
quickly it rotates. Experiment fully confirms this
hypothesis.

Thus the direction of the different parts of the plant
depends upon a force tending towards the centre of
the earth. By neutralising the action of that force
(as on a slowly rotating vertical wheel) we destroy its
influence. By the action of another force (as in the
experiment with the wheel rotating quickly in a hori-
zontal position) we modify the effect accordingly. But
only one force is known to us which corresponds to these
facts, and that is the force of gravity, z.e. the attraction
of our planet. Finally, we can cause similar phenomena
by substituting centrifugal force for the force of gravity
(as in the experiment with a wheel rotating quickly in a
vertical position). Weshall then see that the two organs
tend in the corresponding directions, z.e. the root in the
direction of the action of the force, the stem in the
opposite direction.

GROWTH 187

It follows that the attraction of the earth is the force
which determines the permanent direction of the growth
of the stem and root. But it is one thing to indicate
the force to which a phenomenon is due and quite a
different thing to explain just why and how this force
acts in that way.

As a matter of fact it would be quite easy to under-
stand that the force of gravity would make the root grow
towards the centre of the earth; but how are we to
understand that, under the influence of the very same
force of gravity, the stem tends on the contrary away
from the centre of the earth ? This is, nevertheless,
exactly what does happen. It is not only that a stem
placed vertically continues to grow in this direction,
but even a stem laid horizontally bends sharply upwards.
Here is a small seedling of cress, which some hours ago
was placed flat on a glass plate (a, 8, fig. 53). Its little
stem has turned up as you notice, has reached the posi-
tion of 2 from that of m. Here is some cress, grown up
ona piece of felt. First the felt lay horizontally, later on
I placed it on its edge, then successively upside down, on
the other edge, and horizontally again. In this way the
stems have changed

their position four

times with regard ia

to the horizon and =
after having de-

scribed a whole
Fic. 53.

b

circle, and twisted
themselves into a
knot they continue
to grow upwards.
It is clear that under the force of gravity the stem turns
away from the direction of its action. How is this to be
explained ? We must, of course, always keep in view
not the stem only, but also the root. Only such an ex-
planation can be considered satisfactory as will explain

188 THE LIFE OF THE PLANT

both the reason why the stem rises and at the same
time the reason why the same thing does not happen to
the root. The desired explanation must consist in a
difference of structure of some kind between the stem
and the root, because we cannot admit that one and
the same force will act on exactly similar bodies in a
different way.

Let us see what explanation we can find for the growth
upwards of stems. In order to do this we must begin
by studying a curious property of vegetable organs, a
phenomenon known as the tension of tissues. Let us
cut a long piece from the middle of a young growing
stem, as is shown in fig. 53, p, where the shaded part
represents the epidermis and cortex, and let us moisten
it with water to prevent it from drying up; then let us
split it longitudinally into two halves with a sharp knife.
Both parts will immediately curve as in fig. 53, 7. This
curvature can only be caused either by the outer side
of each part becoming shorter or the inner side longer
than before, or both at the same time. At all events we
come to the conclusion that in the undivided section
the external and internal parts are in a mutually
strained state ; the one stretches the other, and is itself
restrained in its tendency to elongate by the resistance
the other offers to extension. We can prove this fact
by making two slits instead of one, and separating
the external tissue in two parts and disengaging from
between them the middle tissues (fig. 53, s). We shall
now actually see that the middle part will stretch
and become longer than it was before at p, while the
external parts will shrink and become shorter than
they were at p. It is clear that the inner parts of
the stem tend to elongate, but being opposed in this
tendency by the external parts stretch these instead.
This mutual tension of tissues plays a very important
part in the life of a plant; it is to this property
that delicate and succulent stems owe their rigidity.

GROWTH 189

Tissues consisting of very thin cell-walls and liquids
would not by themselves be rigid. It is only when
cells become overfilled with liquid till their walls
distend, and the inner tissues press upon the outer ones
and themselves are compressed, that an organ becomes
turgid, and does not easily bend over or droop as do
fading stems in which, owing to an insufficiency of water,
the tension of the membranes in individual cells as
well as the mutual tension of the tissues is weakened.

Let us make a comparison, rather rough it is true,
but one which will give us a general idea of what takes
place in the plant during its growth. I hold a glove
in my hand. Its empty fingers hang down. I breathe
air into one of the fingers and seize it near its base,—
now it can preserve the vertical as well as the horizontal
position without drooping, or bending. This finger
filled with air represents to a certain extent a cell over-
filled with sap or a stem, the external parts of which
remain stiff under the outward pressure of its more
rapidly growing inner parts.

Let us now discover the relation between all that has
so far been explained and our original question : Why
is it that a stem laid in a horizontal position curves
upwards by itself ?

Whilst a stem remains in a vertical position, the force
of gravity acts uniformly on all its parts ; but as soon
as we place it in a horizontal position, the conditions
change. Owing either to a stronger current of the
nutrient substances or to other causes, the lower part
will grow and stretch more quickly than the upper.
We already know that the tendency of the inner
part of the stem to stretch is continually checked by
the resistance of the epidermis outside. But in the
horizontal stem the lower half of this inner part will
grow more quickly and will at the same time stretch
the epidermis unequally —stretching the lower and
nearer part more strongly than the upper and more

190 THE LIFE OF THE PLANT

distant part. Moreover, the lower epidermis itself
will grow more quickly than the upper, and consequently
will more easily yield to stretching. This explanation
is also supported by the fact that upward curvature
in a lying stem only takes place in the part which
grows most rapidly ; in parts where growth has already
stopped such a phenomenon is impossible. Therefore
by placing the stem in a horizontal position we cause
in it unequal and unsymmetrical growth; the lower
side gets ahead of the upper, the stem curves and rises.
But this method of argument may appear unconvincing ;
in that case it can
be enforced by a

a direct experiment.

é i j Let us take two
O —> 34 similar stems, leav-
a 4 ing the one _ to

grow vertically and

obliging the other to

grow horizontally by
pushing it through
a narrow glass tube
where it will not
be able to curve.
After a certain time
let us split the
@ latter stem into an
upper and a lower
half. The moment
FIG. 54. we do so the upper

part will shorten

and the lower elongate, and if we compare them with
the length of a vertical stem we shall see that the

1 It is quite clear that the resistance exerted by the upper half of the
epidermis against the tendency to stretch will be stronger because it acts
so to speak on the longer arm of the lever, whereas the lower acts on its
shorter arm (see fig. 54, IT., c’).

GROWTH 191

upper half of the horizontal stem is shorter and the
lower longer than the vertical stem, as was to be
expected. What is true of a horizontal stem can be
applied also to one in an inclined position: as soon as
a stem deviates from the perpendicular, the force of
gravity by causing increased growth of the lower side
of the stem brings it back to its vertical position.

Now we understand why it is that owing to the
force of gravity the stem curves in a direction con-
trary to the direction of this force. But a question
arises : Why is it that the same thing does not happen
to the root? You see now that our closer investigation
of the phenomenon has reversed the question. At first
we thought it quite natural and comprehensible that
the root should grow along the line of the force of
gravity, and incomprehensible that the stem should
grow in a contrary direction ; whereas now we under-
stand why the stem grows exactly as it does, and find
it difficult to understand why it is that the root grows
differently. Let us turn for an explanation of this
apparent contradiction to the following model. Let us
imagine two wooden discs (fig. 54, I., ¢ and d), joined
by means of springs to across bar (b). Two pliable india-
rubber rods (a, a) joined by a transverse handle (e) are
passed through holes in this bar. Their ends press against
the centres of the discs c and d. If we push these rods
in the direction of the arrow, we shall separate the coils
of the spirals and bring the two springs into a state of
tension. The india-rubber rods in our figure are meant
to represent the rapidly growing axial parts of organs,
while the compressed coils represent the slower growing
external tissues of these organs, distended by the growth
of the inner parts. This movement demonstrates sym-
metrical growth, and the tension of tissues resulting
from it. Let us now try to show by means of the same
figure unsymmetrical growth, such as is caused by the
action of the force of gravity, in which the lower part

192 THE LIFE OF THE PLANT

of an organ grows more quickly. We do this by mak-
ing the points of contact of the india-rubber rods with
the discs lie this time near their lower edge instead
of at their centres (fig. 54, II). On pushing in the
handle as before, we notice quite a different result ;
while the lower spiral elongates in a straight line, or
even hangs down a little (d’) under its own weight, the
upper one curves upwards (c’) more or less sharply.
This result is easily explained by the construction of
the model. The springs are chosen purposely of varying
elasticity : the upper one of much thicker wire exerts
a stronger resistance to the movement of the bar than
the lower one of thinner wire. We infer that the un-
equal, unsymmetrical pressure manifests itself in a visible
curve only when there is a certain degree of mutual
tension between the parts. Evidently the same can
be applied to growth. Unequal and unsymmetrical
growth will be followed by a sharp and perceptible
curvature of the organ only when this organ attains
a certain degree of turgidity owing to the mutual
tension of tissues. But does a young growing root
actually show such tension of tissues as we saw in the
stem ? A glance at such a root will convince us of
the absence of such tension inside it. If we hold a
stem horizontally it will not bend, nor droop, whereas
a root will frequently hang down like a stem already
withered. If we study the structure of the epidermis in
the stem and the root, we shall find a further difference
which has long attracted the attention of anatomists.
The epidermis of the stem consists of cells with thicker
walls, and moreover is covered with a special membrane
which it is difficult to moisten with water, and which
is very elastic ; on the other hand the epidermis of the
root consists of cells with thinner walls, it easily absorbs
water, and therefore is more easily stretched, and
less elastic than the epidermis of the stem. If now
we make a direct experiment similar to the one we

GROWTH 193

made with the stem, we shall see that there is not the
same tension in the root as in the stem. If we splita
length of root into two halves (like the stem in fig. 53, 7),
no curvature will be noticed in these halves; if it be
cut into three parts (53, s) no elongation of the middle
part, nor shortening of the outer part, will be noticed.
To sum up, the root has none of the tension of tissues
peculiar to the stem ; its external parts grow as quickly
as the inner ones. This is also evident from one more:
property of the root: a young root generally elongates
more quickly than a stem, and this is why it does not
show any tension, which is nothing but restrained
growth.

Thus if the force of gravity does not cause the
upward curvature of the growing root-tip, this is partly
explained by the absence of a mechanical condition
necessary for the purpose: it lacks the corresponding
tension of tissues. Our model shows how, given merely
a difference in the structure of two organs, we may get
entirely opposite results from a similar action of the
force of gravity. It is useful to remember this whenever
physiological facts are discussed. If one and the same
external factor causes different effects in different
organs, we must admit either a difference in the pro-
perties of the organs or a complexity in that same
factor. This second alternative is impossible with
regard to the force of gravity ; but the difference in
the tension of tissues is certainly not the only possible
difference between the properties of the stem and root.

Our explanation would be perfectly satisfactory could
we further prove that the lower half of a root lying
in a horizontal position grows more quickly than its
upper half, as is the case with the stem; and that
in spite of this, owing to its own weight, it passively
bends down like our spiral d’, II in fig. 54. Some
experiments seemed to prove this, but doubts have
arisen in connexion with them, because contradictory

N

194 THE LIFE OF THE PLANT

results have been subsequently obtained, so that the
question as to the mode of action of the force of gravity
upon the root must be considered as still open.1. Further
on we shall see that this question becomes considerably
complicated, and that in order to explain the phenomenon
we must take into account not only the structure of
entire organs, or of the tissues that form them, but also
the details of the structure of the cells forming these
tissues.

Now let us see what other external conditions influ-
ence the phenomena of growth. In looking for the
causes which determine the natural, vertical direction
of growth of both the stem and root, we have proved
that it does not depend on the light. Further, we have
seen that growth is possible even when light is com-
pletely absent : potatoes and turnips put out long shoots
in the total darkness of cellars. The same can be
proved by experimenting with any seed or shoot ; they
all will grow in the dark.

Are we entitled to infer from this that fete does not
influence growth ? Not in the least. A very simple
experiment will prove how considerable this influence is.
If we let cress seeds germinate in two pots filled with
exactly similar soil, placing one of them in the dark
and leaving the other in the light, the difference will
not be long in showing itself. Cress grown in the dark
will sometimes be ten times longer than that grown in
the light ; but its stems will be slender and unhealthy, and

1 For instance, the considerations generally brought forward by
botanists concerning the growth of the root-tip in mercury are far from
being convincing, two different phenomena having been confounded in
these experiments: growth (of the whole organ in length) and curvature
(depending only upon the difference in growth of the upper and lower
sides of the curving part). A root exerts its pressure first of all because
it grows, and it is quite obvious that this pressure caused by growth has
nothing to do with the weight of the root, any more than the weight of

the flexible rod (fig. 54, II d) when overcoming a certain resistance and
at the same time bending passively downwards,

GROWTH 195

many of them will droop. Cress grown in the light will
have short but healthy, thick and turgid stems. This
means that light is not without an influence upon the
growth or rather the elongation of stems; but this
action is not an accelerating or favouring of growth
in length, but on the contrary an inhibiting of it.
The influence of light is not limited to this retarda-
tion of growth. If we have a plant in a room so that
it receives light always from one side, we shall see
that its young growing stem will bend over and turn
to the light, as we generally say. Evidently we have
no right to attribute any attractive power to the rays of
the sun, and there is no need to have recourse to such a
futile hypothesis. By comparing the two experiments
just described we can infer what is the explanation of this
phenomenon—of the turning of stems towards the light.
Light retards the growth of stems, but a one-sided
illumination will not act with the same effect on both
sides of the stem—the front will receive all the light and
the back will always remain in the shade. The front will
consequently grow a little more slowly than the back,
and the result will be the turning towards the light. In
a word we have here a case contrary to the action of
gravity. The force of gravity accelerates growth on
the side turned towards the centre of the earth—the
stem withdraws from it. Light retards growth on the
side turned towards its source—the stem turns to it.
The name heliotropism has been given to this
phenomenon.

But if light retards the growth of stems, does it not
follow that plants must grow chiefly during the night ?
This question has been raised many a time, and has been
settled in various ways. These contradictions must not
puzzle us, because the point itself is a very complicated
one, and the observation of growth at such short
intervals requires rather delicate methods of investiga-
tion, which science has come to possess only recently.

196 THE LIFE OF THE PLANT

In fact, apart from some rare cases,! growth in length
during an interval of ten to twelve hours is not con-
siderable enough to be easily observed, if experimental
methods had not stepped in to help us where our sense
organs appear to fail. Let us see what are the methods
science possesses for demonstrating longitudinal increase
in growth, which, owing to its insignificance, escapes
immediate observation. We turn for the purpose to the
microscope, 7.e€. we can magnify the object of observation,
or use another method which will demonstrate in a
magnified form not the plant itself, but simply its motion
called by us growth. The most
convenient microscope for the
purpose is the so-called ‘solar
microscope,’ which makes it pos-
sible by means of sunlight, or
some other sufficiently strong
artificial source of light, to throw
the image of an object consider-
ably magnified upon a screen, as
we shall now proceed to do with
the root-tip of germinating cress.
Once on the screen we shall draw
a pencil line round the image
and leave the root growing (in
water), and return to it at the
end of the lecture to see that
it has succeeded in growing con-
siderably in the interval. Mean-
while, here is a figure showing (fig. 55) the successive
outlines of a wheat root, observed about every five
minutes for an hour.?

29 March

1 Such are, for instance, the shoots of the bamboo, and the inflorescences
of Agave mentioned in one of the previous chapters, which grow several
inches a day; such are also the spiral stalks of Vall/sneria, a plant known
to all lovers of indoor aquariums (see chapter viii.)

2 Fig. 55 shows the successive outlines of a wheat root, projected by
means of a microscope and a magic lantern.

GROWTH 197

The sensitiveness of the method as demonstrated in
this instance leaves nothing to be desired, but it would
be less convenient for investigating the growth of larger
organs or whole plants; in such a case we have to
use the second of the methods indicated, z.e. instead
of magnifying the growing organ itself, we magnify
only the motion of the growing parts. We use for
the purpose an apparatus, the main part of which
consists of an index fixed to the axis of a small pulley
(fig. 56, I, a). Over the pulley a silk thread is thrown to
one end of which a small weight (b) is attached, and to
the other a small hook (c) made of thin wire. Catching
the apex of any stem with the hook (a small wound
as the result of the prick will not do it any harm) we
let the weight hang freely on the other side of the pulley,
and pull down the silk thread. Let us now suppose that
our stem has grown a little; what will be the conse-
quence ? In growing, the stem will slightly loosen the
silk thread, and the weight will fall as far as the plant
has grown ; at the same time the silk thread lying close
to the pulley will oblige it to make a turn of correspond-
ingly insignificant magnitude owing to the friction it
exerts upon it. With the pulley the index will also turn ;
but its point will naturally trace a much wider course,
so that an imperceptible movement on the part of the
tip of the growing stem will cause an already very
considerable movement of the index point. This latter
movement will exceed the former as many times as the
length of the index exceeds half the diameter of the
block. In our apparatus half the diameter of the block
is equal to two millimetres, the length of the index is
twenty-centimetres, z.e. a hundred times as much;
therefore every increase in the length of the stem
will be shown by a movement of the point of the
index a hundred times as great. The advantage of
this instrument is obvious. We may fasten behind the
index a circular scale with divisions, like those on a clock,

198 THE LIFE OF THE PLANT

and read what division the index points to from time
to time. But we can do something better still: we can
turn this apparatus into an automatic one, and make the
plant itself record the progress of its own growth during
the different hours of the day. For this purpose let
us apply the point of the index to a cylinder, the
axis of which is set in motion by clock-work, so that it
makes a complete rotation in twenty-four hours in the
direction indicated by
the arrow. In order that
the index of the appara-
tus should trace a visible
line it is convenient to
blacken the surface of
the cylinder with a layer
of soot. Naturally, if
the cylinder moves very
rapidly in comparison
with the movement of
the index, the line traced
on it will be almost hori-
zontal (as isa...m in fig. 56, II). On the other hand,
if the arrow moves very rapidly in comparison with the
rotation of the cylinder, it will trace an almost vertical
line (a... m). Ifthe arrow moves at a moderate rate
the line will be oblique: the steeper its inclination the
quicker must the movement of the index have been ;
the more the line slants, the slower the movement of
the index. A glance at the line a, 8, c, d, e, f shows
that the stem grew rapidly from a to 8, slowly from 6 toc,
and soon. Knowing how long the cylinder was revolv-
ing, we can tell also to what hours of the day the rapid
growth and the slow growth respectively correspond,
and are able to trace the influences controlling such
acceleration and retardation. The plant, so to speak,
writes down its own impressions, as we have already
said.

m $

GROWTH 199

The sensitiveness of the indications of this apparatus
depends upon the length of the index. It is incon-
venient in many ways to use a very long index;
and therefore, when we wish to have a very sensitive
apparatus such as will enable us to observe growth
during exceedingly short intervals of time, during one
minute, for instance, or, as in the present case, to demon-

5a

40).

3g ae

_

10;

Fic. 57.

strate this phenomenon of growth to a large audience,
we have recourse to another method. We use some-
thing intangible instead of the index—a ray of light,
to which we can give any desired length without
technical inconvenience. For this purpose, instead
of the index we fix to the axis of the pulley a small
mirror (fig. 57, mm). Light coming from a lamp or a
candle placed in front of this mirror will be reflected
and form a bright spot somewhere on the wall. The

200 THE LIFE OF THE PLANT

little hook at the end of the silk thread is fixed as
before to the plant, and naturally every increase in
growth which results in even the minutest rotation
of the pulley, and the mirror connected with it, will
cause a considerable displacement of the spot of light.
If the index magnified the amount of growth a hundred
times, the apparatus with the mirror will magnify it
many thousand times, or, generally speaking, any de-
sired number of times, since the magnification only
depends on the distance of the mirror from the wall.
In order to be able to judge more conveniently, as
well as more exactly, of the displacement of the patch
of light, large divisions are marked on the wall. Let us
notice the mark at which it stands at present—it is
exactly on the figure 1o—and let us leave the plant
(which happens to be a shoot of asparagus) to grow in
peace, and then return to it at the end of our lecture.

Possessing such perfect means for observing growth,
botanists have been able to study a whole range of
questions concerning these phenomena. Thus for
instance, the reason for the above-mentioned fluctua-
tion in the rate of growth with night and day has been
investigated. In order to settle this question it was
necessary to realise that light is not the only condition
that influences growth ; it is affected also by the amount
of moisture, and especially by temperature.

By growing plants in the dark and a constant amount
of moisture, and varying only the temperature, we find
that they grow more quickly at a high and more slowly at
a low temperature. If we make the temperature rise
and fall alternately, we shall notice that the index of the
recording apparatus described above will draw upon the
surface of the cylinder a line similar to the line a, b,c, d, e, f
(fig. 56),where every more steeply inclined part of the line
will correspond to a warmer interval and every more
sloping part to a colder one. This means that heat
acts in an opposite way to light: while light retards

GROWTH 201

growth, heat on the contrary accelerates it; which as a
matter of fact was known very long ago to gardeners,
who base upon it their method of forcing plants, ac-
celerating or retarding their growth in order to make
them develop in readiness fora certain date. Evidently
the question as to the time when the plant grows most
is not as simple as it seemed to be. It is dark in the
night but also colder ; it is light in the day-time but
also warmer. It is difficult to say beforehand which
of the two influences will preponderate in any case ;
the only obvious thing is that growth must be most
energetic during a dark and warm night, and least
energetic on a bright and cold day.

We have brought forward the simplest explanation
of the phenomena of heliotropism, 7.e. of the inclination
of stems towards the source of light ; but many people
are not satisfied with it, because together with the
general phenomenon of bending towards light, there are
also cases, comparatively rare it is true, of bending
away from light; or, as it is sometimes stated, amid
the preponderating phenomena of positive heliotropism
there are some rare cases of negative heliotropism. This
objection, which has obliged many botanists to give up
the explanation just brought forward, may be easily re-
moved on the strength of the more recent discovery that
one-sided heating can cause phenomena similar to those
of heliotropism, and called thermotropic phenomena.
Evidently the result of thermotropism will be quite
the opposite. Heat accelerates growth—therefore the
heated part will grow more quickly and the organ will
bend away from the source of heat. But sunshine acts
at the same time both as light and as heat ; it is obvious,
therefore, that according as the one or the other influence
predominates the organ will bend either towards or
away from the light. We have just mentioned that
the production of different effects attributed to a single
factor may depend either upon a difference in the pro-

202 THE LIFE OF THE PLANT

perties of the organs or upon the complexity of the
apparently simple factor. Here we have, it appears,
an example of the latter case.

The way in which botanists conceived the dependence
of growth upon external influences became of necessity
considerably more complex after the brilliant and
always original investigations of Darwin. He showed
that the point affected by the external influence may
not coincide with the place where its effect is mani-
fested. Thus, for instance, the force of gravity appears
to act chiefly on the root-tip, although its effect is
manifested by geotropic curvature in the region of
most vigorous growth, lying some distance from the
tip. We come to this conclusion because rootlets, the
tips of which have been cut off, hardly ever curve until
a new root-tip is produced. Stems appear to behave
differently ; but on the other hand some shoots show a
similar behaviour in relation to light, a phenomenon
which is not observed in roots. Thus, for instance, the
tip of an oat seedling, and, especially, of the seedling
of canary grass, are remarkably sensitive to light. If
we cover these with caps made of. tinfoil, the helio-
tropic curvature generally observed in the lower parts
is considerably weakened.

These facts were sufficient to make some botanists
think that in the root-tip and at the tip of the seedlings
of cereals there existed special sense-organs, which
communicated their impressions to the growing parts
in some unknown way and caused them to curve. We
shall see presently that there is no reason whatever
for presupposing any such sense-organs or nerves in
plants; here we need only say in passing that there
is no reason whatever for such an explanation of the
facts just described until other simpler explanations
have been exhausted ; and this condition, for all scientific
explanation, is as yet far from being fulfilled in the
sphere we are now engaged in studying.

GROWTH 203

Having now grasped in general outline the influence
of the chief external agencies, light, heat, and the
attraction of the earth, let us try and study more closely
the very essence of the phenomena in question. So
far we have investigated the plant as a whole ; but the
life of the plant is the sum total of the life of innumer-
able cells. Let us see how the development of cells
is related to the general phenomena of growth. We
know that every cell in the course of its life-time increases
in size, changes its shape and the structure of its walls
—in a word, grows. We also know that whatever be
the size of the plant, it commences as a single cell and
eventually contains millions of them. It is evident
that the growth of a whole plant depends upon
two phenomena, the growth of single cells and their
reproduction.

It is now necessary for us to catch a glimpse of how
these phenomena of the growth and reproduction of
cells, which underlie the growth of the plant as a whole,
take place. We must choose for the purpose appropriate
material, an organ or organism in which we can study a
living cell without disturbing it. We find such appro-
priate material in the filamentous weeds that form the
main part of what is generally known as ‘ green slime.’
If we place one of these green filaments under a
microscope, we shall see that it consists of a single
straight row of cells. Fig. 58 shows at the top such a
cell with the green matter we have called chlorophyll,
which gives plants their green colour distributed in a
very characteristic way. Here it forms green bands
with toothed edges, twisted spirally round the inner
surface of the cell-wall: hence its Latin name of
Spirogyra. Apart from this peculiarity the cells of
Spirogyra do not differ in any way from the ordinary
type of a cell. The same wall of cellulose is found in

204 THE LIFE OF THE PLANT

them, and within it the same protoplasm and cell-sap;
while in the middle, like a spider in its web, we find a
nucleus connected with the wall by very fine strands
of protoplasm. We can observe such a filamentous
weed in a drop of water under the microscope for
several hours, and even days, and submit it during
that time to different temperatures, and to varying
illumination. In this way we see, for instance, that in
the absence of light cells grow or rather elongate more
quickly than in the presence of light. Light has a
similar retarding influence on the multiplication of
cells; at all events under normal conditions, this pro-
cess takes place mainly and sometimes even exclusively
at night. Formerly investigators had to arm them-
selves with much patience: they either had to sit up
part of the night to observe one and the same cell, or
else to lay aside, in spirits of wine, specimen filaments
every hour or so, and subsequently observe the succes-
sive steps of the process in these different cells.1_ Nowa-
days the same result may be reached in a far easier
way : when a vessel containing the weed is placed for
the night in a cool place like a cellar, the process of
multiplication is arrested till the next day, so that in
this way we can always take advantage of the more
pleasant hours of the day-time instead of the less pleasant
hours of night for observing the process in the same cell.

The process itself is very simple. It consists
in the division, the splitting into two of the contents of
a single cell. This happens in the following way :
when a cell has reached the stage of development at
which it divides, two outgrowths from the wall appear
projecting into the cavity of the cell (fig. 58, C, D).
The cells under investigation are cylindrical in form.
By making them rotate round a longitudinal axis by

1 Under continual artificial illumination division also takes place in the
presence of light. Evidently the light does not interfere directly with
division, but rather promotes the activity of the cell in another direction.

GROWTH 205

cautiously rolling them to and fro under the micro-
scope, we see that the outgrowths just mentioned retain
the same appearance whatever be the position of the
cell. This means that they are not simply a pair of
horns, as might have been supposed at first sight, but
a whole ring encircling the inside of the cell. If we

Fic. 58.

go on observing the same cell we shall notice that this
girdle will grow deeper and deeper into the cell, dividing
its contents in half. About this time two nuclei are
observed instead of the single central one. In the end
the girdle joins up completely in the middle, forming a
continuous partition across the cell. Two cells are
thus formed out of the one, each with its own nucleus
protoplasm and chlorophyll band, and separated by a

206 THE LIFE OF THE PLANT

partition of the same cellulose that forms the external
walls. A similar process of division takes place in each
of the newly-formed cells after it has reached maturity,
and soon. In the end a whole series, a whole filament,
of cells is formed out of a single cell.

Such is the process of division in the simplest cases
observed, but in most cases it takes place a little
differently. Scientists were struck by the fact that
during the growth of tissues in the highest plants, one
has never the chance of observing the above-mentioned
gradual ingrowth of a new partition into the cell
cavity. The partition seemed to appear almost in-
stantaneously, but on closer investigation they found
out that here also it is formed gradually, though in a
little different way. This became apparent when due
attention was paid to a part of the cell we have already
mentioned more than once, and upon which we shall
now dwell. This is the cell nucleus, to which whole
volumes have lately been devoted. In Spirogyra we do
notice that the process of cell division is preceded by
the division of the nucleus, but in most cases the con-
nexion between the two processes is closer still. The
division of the cells is preceded by a series of changes
which take place in the nucleus with invariable uni-
formity, and, what is very curious, are almost identical
in the cell-division of plants and of animals. Under
the microscope two component substances can be
easily differentiated in the nucleus; the one is easily
stained with different colouring matters, the other
is not. Some time before division takes place,
the easily stained substance—chromatin—has the ap-
pearance of a bundle of tangled threads (fig. 59, A).
Later on these threads break into short pieces, which
group themselves (as is shown in fig. 59, B on the right)
in the equatorial plane of the nucleus. Later still these
pieces, of which there are usually a definite number,
split into halves and are drawn towards the poles of the

GROWTH 207

nucleus, which by this time has assumed the configura-
tion of a spindle, with a striped appearance (these stripes
are not stained, hence the substance which forms them is
called achromatin, fig. 59,C). When the chromatin has
gathered at the poles and become bunched up together,
so that we have already two nuclei (fig. 59, D) and
division of the nucleus is complete, the division of the
cell begins. Again in the equatorial plane of the

FIG. 59.

spindle, there appear minute grains (fig. 60, 1) which
blend later on into a kind of plate consisting of the
same substance as the cell-wall, z.e. of cellulose (fig. 60, 2).
As it forms, this plate pushes itself against the cell-wall,
and we see a partition dividing the cell into two parts
(fig. 60, 3). Each of the newly-formed cells has a
nucleus of its own, and starts an independent existence,
grows, and on reaching a certain dimension divides
in its turn.

We had before a general explanation as to why the
plants elongate more quickly in the dark. This depends

208 THE LIFE OF THE PLANT

on the fact that both the elongation of the cells and their
division—the two phenomena which determine the
growth of organs as a whole—take place more energetic-
ally in the absence of light. But can we not find out the
approximate cause of this inhibitive effect of light ?
There are investigations which do to a certain extent
explain this phenomenon, and which at the same time
are connected with a question of great practical import-
ance. It was long ago known that stems grown in the
dark appear to be more watery and to have less tension
in their tissues. They are in general less turgid, as is
easily seen in cress grown in the light and in the

HIN
Me

Fic. 60.

dark. But a difference similar to that noticed in these
extreme cases (z.e. between stems grown in the light
and in total darkness) could be shown—to a smaller
degree, it is true—in plants grown respectively in the
shade and in a bright light. The suggestion has been
made that some such effect might explain the laying of
crops which, as we saw, people have vainly attempted
to explain by want of silicon (see chapter iv.). This
supposition might well have been based upon the fact
that for the most part it is cereals which are sown very
thickly that are badly laid, while this never happens
with thin crops, however strange it may seem at first
sight. The following method was used in order to
shade the plants of a crop artificially. One or more
plants were surrounded with a drain-pipe; as soon as

GROWTH 209

the straw rose above it, a second pipe was added on the
top of the first, and so on. It is clear that under such
circumstances the plant received light only from above,
being always shaded at the sides. As was expected, these
conditions produced very high, weak, and flexible
straw. On submitting this straw and normal healthy
straw to a comparative microscopic investigation, the
following difference was observed: the former had per-
ceptibly longer
cells, but the
walls were con-
siderably thinner
than those of the
latter, the cells
of which were
shorter and had
thicker walls. It
follows that light
does not, so to Fic. 61.

speak, retard

growth, but apparently only modifies its direction.
Instead of growing in all directions, the cell - walls
thicken. The same difference has been observed when
healthy straw is compared under the microscope with
laid straw, as may be seen by comparing the transverse
sections of two pieces of straw, one of each kind, in fig. 61.
All the cell-walls of the normal one (on the right-
hand side) are much thicker, and the lower rows (the
external layers of the straw) are even so thick that
the cavity is reduced almost to a point. (The lines
in fig. 61, joining the cavities of adjoining cells, are the
pits.) As well as having their walls thus thickened,
the cells of the straw of a normal specimen are con-
siderably shorter in longitudinal section. Therefore it
is in a too rapid elongation of the straw, together with
an insufficient thickening of the cell-walls caused by
mutual overshadowing in a thick crop, that we must

fe)

210 THE LIFE OF THE PLANT

look for the real reason of the laying of crops. This bad
effect can be prevented by sowing the seed more thinly,
or by drilling. Then every plant will get sufficient
light for its normal development.

These facts lead us to contradict one more current idea
about the growth of the plant. We have already seen
that growth does not always imply increase in matter—
during germination the increase in volume is accompanied
by a loss of substance. We now see that growth does
not always imply the elongation or expansion of an organ,
because sometimes growth can show itself in a different
way—in a thickening of the cell-walls. Strictly speak-
ing, we see that a plant grows only when its cell-walls
grow, whether in length, in breadth, or in thickness.

But it is not only light which may modify the form
of cells. Occasionally this is also possible as the result
of strictly mechanical causes. We have seen in our last
lecture that so-called annual rings are noticed on the
transverse section of the trunk of a tree (fig. 44, III).
These rings are most clearly marked in fir-trees—in a
pine, for instance. The reason for such rings appears
to be connected with the periodical cessation of the
vegetative processes during the winter. Neverthe-
less, if in spring, z.e. after the winter’s rest, new rows
of quite similar cells were added to the cells deposited
the previous autumn there would be no difference
whatever between the adjoining layers ; the boundary
between them would be imperceptible, and they would
all blend into one continuous mass. The layers are
clearly marked only because the wood formed in spring is
distinctly different from the wood formed the previous
autumn. Even with the naked eye we may see that
two parts can be differentiated in every annual ring:
the spring or early wood which lies nearer the pith, and
the autumn or late wood which lies nearer the bark ;
the former is looser and therefore lighter in colour,
the latter is denser and consequently darker. We notice

GROWTH ‘ 211

this alternation of dark and light layers in every splinter,
eveninamatch. The microscope betrays the immediate
reason for this difference. This figure (fig. 62) shows a
small piece of pine wood, the transverse section of a
match. Across the middle of it runs the boundary
between two annual layers.t In the lower part lie
some summer and all the autumn cells, say of last
year; in the upper the spring cells of the present

é X=) te
Sousa
= veyvo\2) AOf-

Fic. 62.

year. The sharp transition from the autumn to the
spring cells is easily perceived: the former have a
flat shape, thick walls, small and narrow cavities ; the
others are almost square, have thin walls and large
cavities. For a long time botanists could not account
for this change in the form of the cells laid down at
different seasons of the year, until they conceived the
idea that the fact must depend on the mutual pres-
sure, the mutual tension of the tissues. We made

1 The transverse line dividing the section into two parts—to the right
and to the left—is the medullary ray (see previous chapter).

212 THE LIFE OF THE PLANT

acquaintance a little while ago with phenomena of
longitudinal tension, 7.e. of tension depending on the
unequal elongation of different tissues in a stem.
Similar inequalities of growth can and do also take
place in the transverse direction. The bark is con-
tinually compressing the wood which grows more
quickly than itself, and in its turn it is continually
stretched and strained by the latter. This is proved by
the longitudinal fissures that generally appear in the
bark owing to the internal pressure of the wood. It is
not difficult to see that the smoothest bark is usually
in a taut condition, and therefore presses on the wood.
If we make a longitudinal cut with a knife, we shall
see that the wound will gape; or, better still, if we cut
off a ring of bark and replace it immediately (without
giving it time to dry) in the same place, we shall notice
that the edges will no longer fit and no effort will bring
them together again. This means that the bark presses
like an iron ring on the growing wood, and the more the
latter develops the stronger is the pressure it has to
overcome. Evidently this pressure will continually
increase, and in autumn reach its maximum. Owing to
this increasing pressure the cells of the wood will
become increasingly flat. The truth of these statements
is fully supported by experiment. If the pressure on
the wood is artificially increased at the beginning of
spring by an iron ring being girt round it, cells similar
to the autumn ones will appear at those places through-
out the year; whereas, on the contrary, if the pressure
of the bark is weakened in summer and autumn by
means of longitudinal cuts, we shall find in those places
throughout the year cells similar to those found in the
spring. Such, therefore, are the changes of form in the
cells, which can be caused simply by mechanical means.1

1 Another curious effect of a purely mechanical influence upon the
growth of tissues has been demonstrated. The movements of the trunks
of trees caused by the wind (as was shown at the beginning of the

GROWTH aio

Can we not go a step further now and explain the
very mechanism of growth—why it is that a cell
grows, and how it is that under the influence of
external agents growth takes this or that direction ?
The fundamental mechanism of the growth of cells is
explained by botanists in the following way. Owing
to chemical changes in the albuminous substances of
the protoplasm, there appear substances which absorb
water very greedily ; drops of watery fluid,’ the so-
called vacuoles, appear in the cell. These vacuoles blend

Growth
C C
Pr. u. Pre u
Vac.
> Vee.
Oe a
' Plasmolysis
Fic. 63.

into a general vacuole which drives the whole proto-
plasm back against the cell-wall (fig. 63, 1, Vac.), so that
the protoplasm assumes the form of a bladder, called
the primordial utricle (fig. 63,1, Pr.u.). Into this vacuole,
as into our apparatus in fig. 46 under the influence of
substances dissolved in it, there will pass osmotically
larger and larger quantities of water ; and since a cell
nineteenth century by a famous English scientist, Knight, who also
discovered the effect upon growth of the force of gravity), as well as the
subjection of the growing parts to artificial strain (as has been demon-
strated by some contemporary German botanists), lead to an increased
development of mechanical tissues. Thus mechanical tissues develop,

most strongly when they are most needed.
1 See chapter ii., fig. 15.

214 THE LIFE OF THE PLANT

is like a whole bladder, this increasing volume of cell-
sap will press on the wall and oblige it to stretch—to
grow. The protoplasm meanwhile will go on producing
on its external surface adjacent to the cell-wall fresh
quantities of cellulose.

That cellulose is formed by the protoplasm is proved
by the following neat experiment. We cut a living
cell under the microscope, and let part of the proto-
plasm out into water. This protoplasm, like any other
liquid in a free state, assumes a spherical form, and
in a short time produces cellulose on its surface,
transforming itself into a new cell. It is most curious
that only those portions of protoplasm which contain a
nucleus possess this property of forming new cells. There
is a similar close connexion between the growth of cells
and the nucleus. During the normal growth of cells
a close relation is observed between the protoplasm
and nucleus and the formation of cellulose. During
this process the walls are thickened either uniformly all
over their inner surface, or only at certain parts of it,
and this depends on the relative positions of the proto-
plasm and the nucleus.

On the strength of what has been said, the process of
growth comes to this. A mixture of substances, called
protoplasm, consisting mainly of albuminoids, in break-
ing down and undergoing chemical transformation, on
the one hand, gives rise to substances that dissolve in
the cell-sap and osmotically attract water—hence the
increase of the vacuole and the stretching of the cell-
wall; on the other hand, owing to the same breaking
down of the protoplasm cellulose is formed, 7.e. the
material with which the walls of a growing cell are built.
If our explanation is correct, on reversing the conditions
we shall get a phenomenon contrary to growth, i.e.
instead of an increase a decrease in the size of the cell, and
particularly of the vacuole and the protoplasmic bladder
distended by it. This conclusion is in fact supported

GROWTH 215

by the following simple experiment. If the increase in
the volume of cells and, therefore, of whole organs
depends on the supply of water attracted by the solution
in the vacuole, by surrounding the cell or the organ
instead with a solution that will draw the water out of
the vacuole we shall cause a decrease in its volume.
Let us actually observe under a microscope a living
cell in a solution of sugar or salt more concentrated
than the cell-sap. We shall notice that the volume
of the cell will decrease (fig. 63, 1 and 2), and when
the wall consisting of solid matter cannot reduce itself
any longer the protoplasm will loosen itself from the
cell-wall, and owing to its surface tension will follow
the further decrease of the vacuole (fig. 63, 2), and in the
end shrink into a regular little ball (fig. 63, 3). This
shrinking of the protoplasmic bladder is termed for the
sake of brevity plasmolysis. Evidently we may con-
clude that the fundamental mechanism of growth is
the converse of that of plasmolysis.1_ Even if we have
no microscope at hand we can easily prove the truth
of what has been said. Let us take the succulent
stem of any herbaceous plant, measure its length
exactly, and place it in a solution of common kitchen salt
containing five per cent. of salt. After a short time let
us measure its length again. We shall notice that the
stem has considerably shortened, which means that here
we have a phenomenon the reverse of growth. Suppose
‘we transfer the stem back into water. It will regain its
former size, and go on living and growing. Therefore
this experiment gives us an opportunity for verifying
our explanation of the mechanism of growth, without
injuring the stem in any way.

If the increased pressure of the sap on the cell-wall be

1 We have indicated this symbolically with arrows in fig. 63. As has
been already said, the wall, consisting of solid matter, is unable to
contract as completely as the primordial utricle. C means cellulose;
Pr. u.—primordial utricle; Vac.—vacuole.

216 THE LIFE OF THE PLANT

somehow arrested; for instance, if during the process
of absorbing water the cell should lose it by evaporation,
the cellulose formed from the protoplasm will go on
forming on the inner side of the undistended wall and
cause it to thicken. This thickened wall, in its turn,
yields still less to the pressure of the sap, and retards the
growth of the cell still further. Thus we may explain
the above-mentioned fact that checking the growth of
straw goes hand in hand with the thickening of its cell-
walls. We notice at the same time that this retardation
in the growth of organs and the simultaneous thickening
of their cell-walls must happen whenever there is a
deficiency of water. Probably the inhibitive influence
of light upon growth, which we have already studied,
depends upon the fact that plants evaporate more water
in the light; and therefore the pressure of sap upon the
cell-wall, which causes growth, will not be so great as
when there is an abundance of water, in the shade or in
the dark. If the phenomena of heliotropism may thus
be connected with the phenomena of the evaporation of
water, perhaps we may also explain in the same way the
particular case of the transmission of the effects of
heliotropism in Darwin’s experiments upon the seedlings
of cereals. You remember the energy with which these
organs exude water at the tips—from which we may
conclude that these same tips give off water vapour at a
similar rate. This loss of water must be made up by
the lower girdle of growth. Hence one-sided illumination
will cause one-sided transpiration and growth, as the re-
sult of which there will be a curvature towards the light.

We have already seen that the growth of tissues can
be retarded by direct mechanical pressure (as in the
growth of the wood) ; now we notice that it can be also
retarded by the weakening of the internal pressure, and
this retardation is accompanied in both cases by greater
thickening of the cell-walls. Under the influence of a
one-sided action of external factors, the growth of

’ GROWTH 217

whole tissues will be irregular, causing the curvature of
whole organs. But similar unequal growth may also be
manifested in different parts of one and the same cell,
according to the distribution of the protoplasm and
the nucleus, as we have already seen. It has, in fact,
been noticed that whenever single cells curve, accumula-
tions of protoplasm become noticeable on the concave
side. This side is probably more thickened, and offers
greater resistance to the osmotic pressure of the sap,
and gets less distended. We may add that it is not the
thickness of the cell-walls alone which figures in the
phenomena of growth: the chemical and physical pro-
perties of the cell-wall may vary, and make it more or
less resistant and elastic. It has been proved that a
certain ferment exists which softens the cellulose of the
cell-wall. The local appearance of this ferment can
influence the direction of the growth of the cell and its
external outlines. Perhaps these facts will furnish us in
their turn with a clue to the explanation of another of
Darwin’s observations which we mentioned before—
his observation concerning decapitated roots. Very
possibly during the cell’s earliest period, when it is
consequently lying close to the root-tip, there is an
irregular distribution of the protoplasm and the nucleus,
etc., which may result in a subsequent irregular growth
and a curvature of the whole organ.

Only now can we fully estimate the endless variety of
the effects of external conditions upon the growth of
organs, tissues, cells, or even parts of cells, and the com-
plicated combinations all these phenomena may present
when taken together ; and only now can we realise what
a mistake is made when, instead of trying first of all to
find out these possible explanations, people make up
their minds that in the phenomena of growth are to be
seen the results of some psychical, almost conscious,
activity of the plant.

218 THE LIFE OF THE PLANT

But we must now return to our cress and asparagus.
You may have noticed, while I have been talking, how
the patch of light has been steadily creeping up the
wall; it is no longer at the tenth division, but some-
where near the fortieth. This is because the mirror has
passed from the position mn to that of rs; so we have
seen for ourselves how the plant grows. At the same
time the root-tip of the cress has long ago moved
beyond the circle traced round it with the pencil, and
has considerably elongated. This means that we can
observe not only the result of this process, not only its
immediate cause, 7.e. the growth and division of cells,
but that we can also grasp the very process, 7.e. the very
movement that we call growth.

We have thus performed the first part of the task
we set ourselves at the beginning of this lecture. But
what of the second part ? Can we hear how the plant
vegetates? Can we, for instance, make the plant tell
us by means of sounds of some kind how it thrives;
whether it is hungry or satisfied ? The following ex-
periment will show us that we can. A plant is being
grown in an artificial soil? under a glass bell, with
its flange carefully ground (fig. 64, A). We know,
however, that one of the most important sources of
a plant’s life is the carbonic acid of the atmosphere.
How can we guarantee a continual source of car-

1 In order to ascertain how much the plant has really grown, we have
only to measure the distance between the axis of the pulley and the wall.
The real increase in growth, as has been already said, will be as many
times smaller than the visible transposition of the image of light as is this
distance greater than half the diameter of the block. Evidently, if people
at the back of this large audience can be shown with the help of this
apparatus the growth of a stem during the interval of an hour or even
half an hour, the observer standing nearer will be able to notice the
displacement of the beam of light during the interval of one minute. In
fact this method permits of our observing, minute by minute, the increase
in length of the stem; and of seeing the movement, so to speak, as if it,
were the movement of a minute hand on a clock.

2 See chapter iv. :

GROWTH 219

bonic acid to the plant under the glass bell; and,
still more, how can we learn that the plant actually
uses it ?

We know that the activity of plants and animals is
diametrically opposed with regard to the atmospheric
gases. Plants absorb carbonic acid and_ give off

on ee

Fic. 64.

oxygen ; animals absorb oxygen and give off carbonic
acid. Hence, if we placed an animal under the glass
bell along with the plant both would thrive together.
But we can substitute an apparatus for the animal
(fig. 64, B), which, so far as the exchange of gases goes,
will act precisely in the same way as an animal would.
It will be in a certain sense an artificial animal. This
is how the apparatus is arranged. A liquid greedily
absorbing the oxygen of the air is poured into a glass

220 THE LIFE OF THE PLANT

basin. A glass jar is placed in the middle of the basin ;
through the cork of the jar the tube of a glass funnel
is passed right down to the bottom, and also another
tube twice bent. An acid is poured into the jar, while
a ball of marble or chalk is placed in the funnel. Let
us see what happens to the apparatus when it is her-
metically covered by the bell A. There is air under the
bell ; therefore there is also oxygen. This oxygen will
be absorbed by the liquid in the basin; as a result
the volume of air (a) under the bell will decrease ; the
pressure of air under the bell will diminish; and, if it
diminishes, the small volume of air (6) contained in
the jar, above the acid, will begin to expand, and,
pressing upon the acid, will oblige it to rise in the tube
and to appear in the funnel (z.e. the level of the acid in the
beginning at m will be now at 2, fig. 64, B). But here
it will meet the ball of marble or chalk, and will oblige
it to give off its carbonic acid.2. This carbonic acid
will replace the oxygen absorbed, and will go on being
given off until the former pressure is established under
the bell A; then the air in the jar (0) will fall back to
its original volume ; the acid at the same time will go
down from a again to m, and everything settle down
until the liquid in the glass basin, after having absorbed
a fresh quantity of oxygen, upsets the balance again
between the air under the bell (a) and the air of the
jar (6). Was I not right in calling this apparatus an
artificial animal ? It breathes ; it absorbs oxygen and
gives off carbonic acid, and almost in the same quantities.
The plant is placed on a tripod in the upper part of
the bell ; it will use the carbonic acid given off by the
apparatus, and will itself in its turn give off oxygen,

* In order to absorb oxygen in presence of carbonic acid Saussure’s well-
known mixture can be used consisting of iron filings, flowers of sulphur,
and water; as well as a solution of cuprous chloride (Cu Cl) and kitchen
salt, or a solution of chromous chloride (Cr Cl,). Phosphorus (dangerous

in summer) may also be used.
2 See chapter iii.

GROWTH 221

that will be absorbed by the liquid in the basin of the
apparatus B. To sum up: between the plant and the
apparatus B there will be a similar circulation of
matter as between a plant and an animal. The plant
under the bell will be provided with a periodical
automatic source of carbonic acid. The plant will thus
be supplied with carbonic acid until the little marble
ball is entirely dissolved, a matter of days and weeks.
Without coming into contact with the marble, the
plant is thus able to nibble it all up, and use its carbonic
acid for food.t4_ But in order to know whether every-
thing is right under the bell, z.e. that carbonic acid is
being given off and decomposed by the plant, the
apparatus B is supplied with the following adjustment.
Into the twice bent tube C (shown in greater detail in
fig. 64, C) a drop of mercury is introduced. Evidently
whenever, owing to the expansion of the air b (fig. B),
the level of the acid in the funnel rises from m to 7, the
level of mercury in the tube C will likewise rise from
m’ to n’ (fig. C). Two insulated conductors joined to a
common electric bell are introduced into the open end
of this tube. One of the conductors is always immersed
in the mercury, the other stops a little higher up,
and comes into contact with the mercury only when
the latter rises to 2’. The moment this happens the
electric circuit is closed, and the bell rings. It is pretty
easy to regulate the apparatus in such a way that the
giving off of carbonic acid will happen only when there
is very little of it left under the bell, and that the bell
will always ring whenever carbonic acid is given off.
Just as the giving off of carbonic acid ceases when the
acid returns to its former level ™, so the bell ceases to
ring when the drop of mercury returns to its former
position and opens the circuit. But if, owing to some

1 By weighing this little ball from time to time we can determine
approximately the amount of carbonic acid absorbed by the plant in so
many days or weeks.

222 THE LIFE OF THE PLANT

defect in the apparatus, carbonic acid should not
be given off in time, the bell will go on ringing without
stopping. I am sorry not to be able to show the
apparatus at work, for reasons I have already explained
more than once: its action depends on the giving off
of oxygen by the plant, and this process takes place
exclusively in daylight. But I can nevertheless give
you an idea of how it works. If we rapidly cool the
air under the bell (a) its volume will decrease as it
would have done in consequence of the absorption of
oxygen; and this decrease in volume, provided that
the apparatus is in good working order, must be followed
by the same result—the giving off of carbonic acid by
the marble ball and the ringing of the bell. In order
to quickly cool the air inside the bell, I have sprinkled
it with some ether. The air has cooled, has decreased
in volume; the column of acid has moistened the
marble. The marble has effervesced in giving off
carbonic acid, and the bell instantly rings. But the
effect of the momentary cooling ceases; the usual
order is re-established—and the bell stops ringing.

Thus every time the plant is threatened with a lack of
carbonic acid, the latter is given off by the apparatus B,
and this is accompanied by the ringing of the bell. If
the apparatus gets out of order, if carbonic acid ceases
to be given off—the bell rings without ceasing.

If I had asked you a few minutes ago whether it were
possible to oblige the plant every time it is hungry, every
time it is merely threatened by hunger, to inform us
of the fact by ringing a bell, you would probably have
considered it an untimely joke. And yet this is exactly
the significance of our apparatus. It is kept working
by the activity of the plant, by its power of decomposing
carbonic acid and giving off oxygen. We take advantage
of this faculty in order to oblige the plant to inform us
from time to time by a short ringing of the bell when
its feeding was going on successfully, and by beating

GROWTH 223

an alarm, calling for help, whenever it was threatened
by hunger. In a word we oblige the plant to let us
know by means of conventional sounds how it thrives.

We can now give a positive answer to the question
raised at the beginning of this lecture: we can not
only see but even hear how the plant vegetates.
The experiments by which we have studied the
various stages of the process give us at the same
time a clear idea of the contrivances to which we
must have recourse in investigating Nature. We are
not content with the passive part of the observer, but
enter into a struggle with her, during which the experi-
mental art offers us a whole range of tools and methods.
The plant is dumb, it does not answer our questions—
we oblige it to write; it cannot talk—we oblige it to
ring ; somehow or other we obtain from it an answer
to the question raised. It is vain to look upon this
experimental art, as many do, as almost a mechanical
activity, something inferior to abstract thought. The
mistake was made even by the great Goethe himself.
In distinguishing a certain duality, a certain discord
between the two ways of investigating truth, between
theory and experiment, he made Faust speak of Nature
in the following way :-—

‘Und was sie deinem Geist nicht offenbaren mag,
Das zwingst du ihr nicht ab mit Hebeln und mit Schrauben.’

Certainly it was neither lever nor press that extorted
the mystery from Nature—it was the far-sighted medita-
tion and the stubborn will of the investigator that have
done it. The tool for investigation is as much the
product of a creative mind as the theory confirmed by
it: it is the very same thought in palpable form. Is
it not strange that while one might have seen for
centuries in almost any old city of Europe? collections

1 In Nuremberg, for instance.

224 THE LIFE OF THE PLANT

of those disgraceful instruments of torture by which
man vainly tried to extort truth from his fellow
creatures, it was only in 1876 that for the first time
the idea occurred of collecting in one place the glorious
implements man has used during three centuries in his
struggle with Nature, in the course of which he has
triumphantly come to wring from her one great
truth after another. Is it not strange still to hear
the naiveté with which people wonder that educated
folk can choose some frog or blade of grass as a subject
of study; or their open lamentations that the study
of Nature, by engrossing the human mind with material
subjects, diverts it from higher problems, makes it
narrow and causes it to degenerate. In so saying they
frequently cast sad glances on the past, as if the
human mind used then to be concentrated exclusively on
topics more worthy of its attention. Is such a reproach
justified ? Is it true that natural sciences narrow the
mind, degenerate it? Is it true that they are less
worthy of man’s attention than other sciences, when, as
we have just persuaded ourselves, these natural sciences
occasionally give man access to regions where in the
good old times only the immortal gods were given to
tread ?

1 In 1876 there was an exhibition in London of instruments and
apparatus used in the experimental sciences, with an interesting historical

section devoted to apparatus that had served famous scientists in their
investigations.

CHAPTER VIII
THE FLOWER

THE nutrient substances spent in the building up of
the solid parts of the plant reach their final destination
in the phenomena of growth. Thus the life of a plant
resolves itself into nutrition and growth. A plant feeds
in order to grow, grows in order to feed, z.e. to enlarge
the surface of its food-absorbing organs. These two
conjoint processes may last a very long time; in some
plants they last even thousands of years; yet they
always reach a limit, though as a matter of fact we are
unable to explain the necessity for such a limit, or to
understand why one and the same vegetable organism
should not exist for an indefinite length of time. Let us
imagine a plant that produces surface runners, like
those of the strawberry, or underground stems, so-called
rhizomes, like those of the couch-grass (Triticum repens) :
these new parts will spread out and cover an ever wider
area; old parts will die away, and consequently the
connexion between them and the young parts will
break : they will separate, but nevertheless they will
continue to be parts of one and the same plant, which,
while destroyed at one end, will go on growing at the
other. Or let us take another example from among
trees: a well-known Indian fig-tree, the banyan tree,
produces adventitious roots from its outstretched
branches. These roots reach the ground, thicken, and
form pillar-like supports to the branches, furnishing
them at the same time with necessary food. In this
way a single tree may cover whole acres of land. The
main trunk may get destroyed here also; but I do not
think this fact would prevent branches which have
P

226 THE LIFE OF THE PLANT

taken root from continuing their existence for an
indefinite length of time. The capacity of the plant for
reproduction is not limited to such spreading; it is
also manifested in another way. Whole parts of
plants, such as stems with leaves, may acquire a special
form and then free themselves from the plant that has
produced them ; such are, for instance, thé little bulbs
formed in the angles between the leaves and stems of
lilies, and also the tubers that appear on the underground
stems of potatoes, in which we can recognise branches
only changed in form. We may consider all the plants
which spring from these organs as individualised
ramifications which have separated from one and the
same plant, in consequence of its own rapid spreading.
It might seem that these and similar methods of so-called
vegetative reproduction are quite sufficient to make the
life of a single plant secure for an unlimited length of
time ; but matters turn out otherwise. It happens that
vegetable life cannot be infinitely prolonged in one
direction ; it is bound from time to time to interrupt
its course, to ascend again to its source, so that, starting
once more from the very beginning, as a single cell, it
may retrace the same course in the same order of con-
tinuity. In a word, we notice in the life of plants, as
well as in the life of animals, a necessary succession
of generations, and in each generation an invariable
sequence of different stages of development, that we call
age. Moreover, it happens that for this periodical
renovation not one, but two beings must participate
in the formation of a new organism. This is the
phenomenon of sexual reproduction. At all levels on
the organic ladder, beginning with the weed and ending
with man, it presents one and the same phenomenon,
which is the blending into one of two beings, two lives,
ultimately two cells.

This inference, that in order to maintain vegetable
life, periodical sexual renovation is necessary, is proved

THE FLOWER 227

by the fact that, apart from the very lowest repre-
sentatives of the vegetable kingdom which stand, so
to speak, on the threshold of the organic world, we do
not know a single vegetable group that maintains its
existence exclusively by means of a vegetative process,
by asexual reproduction as it is generally called; not
one that does not also go through the process of sexual
reproduction.

Let us see under what form this phenomenon takes
place in the vegetable kingdom.

At first, the existence of two sexes was noticed only
in some plants, mainly in those that had flowers ; hence
the name of Phanerogams given to them by Linnzus
in the eighteenth century in contradistinction to the
Cryptogams. To-day the name Cryptogam has lost its
meaning, because phenomena of sexual reproduction
have been discovered in all classes of plants, with the
exception of the very simplest organisms, where it
probably does not exist.

The idea that this process must take place in the
flower, and that the fruit and seed, ze. a young
embryo-plant, are the result of this process, ori-
ginated long ago; but as a definite scientific theory
it has not more than two centuries of history behind
it.

The idea must have been suggested by such plants as
have two kinds of flowers distributed on different
individuals. Such are many trees, e.g. the willow, the
aspen, the juniper, and also hemp. All these plants
produce two kinds of flowers : those that bear the fruit
and the seed, and those that possess only stamens, which
do not transform themselves into fruit, but are necessary
in that they contribute to the formation of fruit in other
flowers. The first plant which attracted man’s attention
in this connexion was probably the date palm. At all
events we read that people in the markets of Babylon
and the Arabs of later days used to sell the male flowers

228 THE LIFE OF THE PLANT

of this palm that they might be hung by the purchasers
among the female flowers to pollinate them ; for this
plan was seen to be followed by more abundant crops
of fruit. We call those flowers female that contain a
pistil which after the petals fall transforms itself into
fruit ; by male flowers we understand those that contain
only stamens which produce the fertile dust or pollen
and die when the flower withers. It is not, however, in
all plants that the male and female, the staminate and
pistillate, flowers are distributed on separate individuals.
In many cases they grow on one and the same plant, as
in the birch, the oak, the pine, and in maize; while in
the considerable majority of plants stamens and pistils
are found together
in one and_ the
same flower, z.e. the
flowers are herma-
<4, phrodite. Such is
the flower shown
in fig. Io.
Let us consider
the part played by
Fic. 10. stamens in the for-
mation of the fruit.
A stamen, as we already saw in our first lecture, is
in its most perfect form a more or less well-developed
filament, to the end of which are attached two oblong
sacks, which split longitudinally and shed a kind of
dust, as a rule yellow in colour. Every such grain
of dust represents a cell, commonly spherical in form,
with a double wall; the external layer is thick and
generally very elaborate, while the inner layer is thinner
and simpler. The external layer has usually some pores
closed with lids which can spring open on occasion.
The pistil in its simplest and most regular form looks
like a bottle (figs. 10 and 65, on the right-hand side).
Its large ovary, the inside of which is hollow, contains

THE FLOWER 229

ovules ; one, several, or even a great number as in the
poppy. The elongated part, the style, is occasionally
traversed by a canal; but in most cases the style is solid,
though its tissue is porous and spongy, its cells not
coming into close contact with each other but having
spaces between them. This style ends at its apex in a
broadened part called the stigma, which may be flat
like a button, delicately branched like a feather, or in-
deed of almost any shape. The surface of the stigma is
generally covered with short hairs and secretes a sticky
fluid. The ovule enclosed within the cavity of the ovary,
if split longitudinally, presents the following structure :
the central part, the so-called nucellus, is surrounded
by a double wall, through one end of which a canal
passes right down to the nucellus. This canal may be
directed upwards, or downwards as in fig. 65. In the
part of the nucellus near the canal, a very big cell
attracts our atten-
tion. This cell
has been called
the embryo-sac
(fig. 65), because
it is here, as we
shall see later on,
that the embryo
of the plant de-
velops.

Such in general
terms is the struc-
ture of these two
flower organs —
thestamenandthe
pistil. Their most
essential parts are
the ovule, which
has to undergo fertilisation, and the pollen grain, by
which the process of fertilisation is effected.

Fic. 65.

230 THE LIFE OF THE PLANT

In order that fertilisation may take place, the pollen
grain must first of all reach the surface of the stigma,
to which it easily adheres by reason of the hairs and
sticky fluid. The means by which this end is achieved
in Nature will be studied later on; in artificial breeding,
in horticulture, it is found convenient to convey the
pollen to the stigma with a brush. What happens next ?
The distance between the surface of the stigma and the
ovule is still great. In what way does the pollen affect
the latter ? This question preoccupied botanists for
a long time, and gave rise to a series of more or less
fruitless suggestions. It was supposed that the pollen
grains sink down to the ovary; that they burst on
the stigma and let out their contents which reach the
ovule ; even that they act from a distance by means
of some kind of emanation. Not one of these hypotheses
proved true when exact microscopic investigations at
last solved the problem.

Whenever a pollen grain reaches the stigma or a
suitable liquid, say a solution of sugar (but not water,
in which it generally bursts), it begins to germinate,
the inner wall protrudes through one of the pores in the
external wall as a tube (fig. 65, in the centre), into which
the contents of the grain are conveyed. The tube goes
on growing at the apex and reaches a considerable
length. As it grows forward, it often dies behind. These
pollen-tubes push their way down through the porous
tissue of the style (fig. 65, right-hand side), sometimes for
a long distance, as in the cactus, for instance, where the
style is several inches in length. Once inside the ovary,
the pollen-tubes reach the entrance of the ovule, make
their way to the nucellus down the canal and gain
entrance to the embryo-sac (fig. 65 right-hand side ;
at the bottom of the left-hand side is the upper part of
the nucellus, showing the pollen-tube applied to the
embryo-sac). The pollen-tubes reach the canal of the
ovule almost haphazard; but the accident happens

THE FLOWER 231

fairly frequently, because the number of pollen grains
which penetrate into the ovary is generally considerable.
Observations have, however, been made under the
microscope which point to the fact that pollen-tubes
are definitely attracted towards the tissue of the stigma
or style, if it lies sufficiently near to them. This tissue
seems to attract them even after it has been killed by
boiling.

Meanwhile events are occurring in the nucellus. Inthe
upper part of the embryo-sac three little cells are formed
out of its protoplasm. These cells have no cellulose
wall, and are therefore only spherical lumps of thick
protoplasm each with a nucleus. One of them is called
the egg-cell or ovum, because it is the actual origin of
the future embryo of the plant. This is therefore the
primary cell we were looking for; from which, as we
mentioned, every plant arises, be it a spore-bearing plant
or aseed-plant.1. The egg-cell is enclosed in the upper-
most part of the embryo-sac so that the tip of the pollen-
tube, on reaching the nucellus down the canal of the
ovule, comes into close contact with the egg-cell (fig. 65,
at the bottom, on the left-hand side).

Recent investigations have proved that the act of
fertilisation really consists in the passage of a nucleus,
from the end of the pollen-tube, through the softened
and dissolved wall of the tube into the embryo-sac (the
wall of which also becomes softened or dissolved), and its
‘fusion there with the nucleus of the ovum. It is curious
to notice that half of the chromatin substance involved
in this process, 7.e. half the total number of rods, belong
to the male and half to the female nuclei respectively, so
that the first nucleus of the embryo, formed by their
fusion, contains chromatin from both paternal and
maternal organisms. This fact explains very obviously
why the properties of the parent plants are blended in
their posterity. We shall soon see that in the flowerless

1 See chapter i.

232 THE LIFE OF THE PLANT

plants we can still more easily be convinced of the
fusion of the substance of male and female cells. The
nature of this phenomenon, the chemistry of the process,
is almost unknown to us. A ferment has been dis-
covered among the constituents of the pollen grain,
and moreover during pollination increased chemical
activity is known to be generally displayed by flowers :
they greedily absorb oxygen and give off carbonic acid,
and this respiration is accompanied by a perceptible
rise in temperature of the whole flower, and especially
of the stamens.

However, the result of this fusion manifests itself
in the awakening of formative activity in the embryonic
cell. It surrounds itself with a cellulose wall, becomes
divided by a partition, thus becoming two cells instead
of one (fig. 65, at the top, on the left). The first parti-
tion is followed by another, then by a third, and so on,
till a multicellular body is formed, which grows and
develops into the seedling which we met with in our
first lecture, in talking of the seed. Sometimes a
single seed has several embryos. This does not often
happen. There are several embryos in the seeds of
an orange, for instance, but they have quite a peculiar
origin, and it would be useless to enter here upon a
description of the process. Throughout the develop-
ment of the embryo stores of nutrient substances are
formed in other parts of the ovule, in the embryo-sac,
and in the nucellus ; this will be the part of the seed
which we have called the endosperm.! The endo-
sperm being a store of food, it may either be absorbed
by the embryo while the seed is still on the maternal
plant, in which case the ripe seed will not contain any
endosperm, like the seeds of beans; or, on the other
hand, most of the endosperm may be preserved in the
ripe seed, as in cereals, buck-wheat, the poppy, etc.

The effects of fertilisation are not limited to the

1 See chapter iii.

THE FLOWER 233

development of the embryo, and the transformation of
the ovule into the seed; its influence also extends to
the carpel, which grows after fertilisation and develops
into the fruit.

Such in very general outline is the formal morpho-
logical aspect—the only one known to us—of this
phenomenon of fertilisation. Let us now observe how
the same process takes place at the other pole of the
vegetable kingdom, among the simplest plants, like the
weeds and the moulds.

Fic. 66.

Here is a microscopic weed, Spirogyra, which we have
already studied, with its characteristic spiral bands of
chlorophyll (fig. 66, right-hand side). At a certain
stage of development the filaments, of which the
organism consists, become parallel to each other, as is
shown in the plate. In some cells the contents gather
into lumps, round or oblong in shape. Meanwhile the
walls of two adjacent cells form swellings, which grow
towards each other and meet; the partition dividing
them is absorbed and vanishes, and the contents of
the two cells fuse, the contents of the right-hand cell

234 THE LIFE OF THE PLANT

flowing into the left, or vice versé. The spherical mass
thus produced surrounds itself with a wall, and becomes
a spore. When set free this spore is capable of germina-
tion, and will give rise to a new oteagim, a new fila-
ment of Spirogyra.

Here, then, we meet with an extremely simple process
of fertilisation : two cells fuse in order to form a new cell
which serves the purpose of reproducing the organism.
This phenomenon, in a still simpler form, is met with
in a microscopic fungus—a mould. This mould con-
sists of a thin very much branched tube (fig. 66, left-
hand side) in which no transverse partition is to be
seen; which means that the whole organism consists
of asingle cell. In some parts of this cell short branches
appear, stretching towards each other. Whenever
they meet, the ends of the branches are separated from
the rest of the plant by partitions, and swell. Later
on the wall disappears where the ends meet, and the
contents fuse, thus forming a single cell—the spore.

Thus in the simplest spore-bearing plants, as well
as in the flowering plants, the phenomenon of fertilisa-
tion consists in the fusion of the contents of two cells.
This phenomenon is even more obvious in spore-
bearing plants, because we can actually observe there
the fusion of two cells. In the simplest cases we have
described, no difference has been noticed in the aspect
of the male and female cells, although a closer study
of their structure does reveal a distinction between
them; but in other more complicated spore-bearing
plants the male cell differs entirely from the female cell
in form as well as in character. While the female is
non-motile, the male moves about like a microscopic
animal, and penetrates (by its own activity) into the
organ containing the female cell. There it fuses with
it, dissolving in it, so to speak, and fertilises it.1

1 In 1897 such motile cells were found in the pollen-tubes of some seed
plants.

THE FLOWER 235

We can follow with still greater certainty the effect
of the pollen upon the ovule by means of experiment.
In the first place we know that if the stigma is not
pollinated the flower will fade without producing any
seed or fruit; further, when pollen-tubes have been
made to act under the microscope upon ovules, removed
from the ovary, it has been found that the effects of
fertilisation were manifested only when a pollen-tube
came into contact with an ovule. Finally, the partici-
pation of the male element has been most conclusively
proved by experiments in artificial hybridisation. If
the pistil of a flower is pollinated from another flower,
differing from the first, say in the colouring of the petals,
it is possible in some cases to produce a plant with
variegated flowers, i.e. flowers which have both the colour
of the petals of the one in which the pistil was fertilised,
and the colour of the other the pollen of which was used.
Obviously the effect of the male cell has manifested
itself in the plant which resulted from the process of
fertilisation.

In order to accomplish the requisite pollination,
plants are provided with a number of various adapta-
tions. Let us dwell on some of them. Here is a plant
(Pilea) rather insignificant in appearance, but grown
in hot-houses on account of the following curious pro-
perty : whenever the flowering plant, covered with
unsightly little flowers, is sprinkled with water, small
clouds of dust rise here and there from its surface as
if from minute explosions. This phenomenon is due
to the stamens of this plant (coiled inside the flower
and very hygroscopic) suddenly uncoiling like springs,
and shedding the pollen out of their broken pollen-
sacs. The pollen scattered in this way easily reaches
the stigma. Let us study another case, this time
a plant living in quite a different medium—a water
plant. All lovers of indoor aquaria are familiar with
their most common inhabitant, Vallisneria. The

236 THE LIFE OF THE PLANT

stamens and pistils of Vallisneria are found on different
flowers, and these are distributed on different plants

aes

= a

Fic. 67.

(fig. 67). Both the male and female flowers develop
under water. But fertilisation is impossible under

THE FLOWER 237

water. Gardeners know, for instance, that rain during
the flowering period interferes with fertilisation. Under
these conditions the flowers remain sterile and produce
no fruit. In order that the fertilisation of Vallisneria
may take place in the air the plant is provided with
the following ingenious adaptation. The female
flowers (left-hand side) grow almost at the bottom of
the water, on very long but tightly coiled stalks. When
the flowering season approaches these stalks uncoil
and grow, thus bringing the female flowers up to the
surface of the water. By this time the male flowers
which develop similarly at the bottom of the water
(right-hand side) are torn from their stalks and
also rise to the surface of the water. Floating among
the female flowers they open their anthers and shed
their pollen, some of which falls on the stigmas of
the female flowers. When the flowering period is
over, the stalk of the female flower coils up again,
carrying the fertilised flower down to the bottom of
the water, where the further development of the fruit
takes place.

The significance in a plant’s life of the pollen and the
ovule—the essential parts of the flower—is now quite
clear. The adaptations described above which make for
their mutual interaction are also comprehensible. But
another question springs up: what is the significance of
the remaining parts of the flower? What is the pur-
pose of the calyx? What is the use of the carpel
which only hinders the access of pollen to the ovules ?
Why have the petals such bright colours and some-
times such fantastic shapes ? What is the purpose of
the perfume of flowers, and, finally, of the sweet honey-
like fluid secreted at the bottom of the corolla by the
well-known clover, dead-nettle, and many other flowers ?
Let us try to answer these questions. The significance
of the calyx and the carpel is the most intelligible of
them all. The former, like the external scales of leaf-

238 THE LIFE OF THE PLANT

buds, guards the inner more delicate organs of the
flower during their development; the latter plays the
same part towards the ovules enclosed within it. In
the experiments on the fertilisation of ovules detached
from the ovary, that we have just described, the
experimenter met considerable difficulties in struggling
with minute parasitic organisms, bacteria, which have
so terrible a reputation owing to the infectious diseases
they produce. When we wish to preserve organic
matter from decay, we must protect it from becoming
infected with bacteria. This we do by keeping it in
hermetically closed vessels, or at least guarding it from
the access of the germs of these organisms, floating
about in the air, by closing the necks of the vessels with
cotton wool. The cavity of the ovary is exactly the
kind of vessel in which the ovule and the seed may
safely develop without coming into contact with the
germs of parasitic bacteria floating in the air. But
an objection may be raised here: if on reaching
the surface of the stigma the pollen grains germinate
and their pollen-tubes reach the ovule, why should
not bacteria flying about in the air likewise develop
on the stigma and reach the ovule in the same way ?
The same observer to whom we are indebted for the
explanation just brought forward of the significance of
the ovary, has also explained the special significance
of the stigma. Pollen grains developing outside the
flower (under the microscope) suffer from bacteria in
the same way as ovules. In order to prevent their
attacks he slightly acidified the liquid in which the
pollen grains germinated. This slightly acid reaction,
which was harmless to the pollen grains, did actually
prevent the development of bacteria. On testing the
stigma itself, he found it also had an acid reaction.
Thus, while it lets the pollen-tubes pass through, the
stigma apparently prevents the access of bacteria to the
ovary.

THE FLOWER 239

But how do the bright petals, the perfume, and the
honey glands serve the flower? At first sight their
function seems to be purely aesthetic. Formerly, when
man considered himself the centre of the universe,
when even the sun was believed to revolve around him,
it was easy enough to admit that everything existed for
the benefit of his eye, his sense of smell, or his taste.
But gradually, with the development of science, this
point of view lost more and more its hold. On the other
hand, failing to observe in these parts of the flower any
immediate purpose, botanists were in the habit of calling
them non-essential. They looked upon them simply
as the plant’s bridal dress, to use a poetical and meta-
phorical expression. Both opinions proved to be wrong.
To begin with, all these organs turned out to exist not
at all for man, but for insects, and, above all, for the plant
itself ; secondly, they proved to be very essential,
something even absolutely necessary for plants ; without
them the so-called essential organs themselves would
not have answered their purpose ; and, finally, they were
proved to be useful precisely in so far as they are bright,
perfumed, and sweet, 7.e. in so far as they serve as bait
for insects. Let us explain the matter.

The ecclesiastical and civil laws of most nations, except
those at the lowest stage of civilisation, censure, forbid,
and even persecute marriages between near relations.
Doctors and physiologists have tried to prove by statis-
tics the justice of this law so widespread over the earth,
and indeed there is abundant proof that near relationship
between parents has a very bad effect on their children’s
health. To-day such proofs are scarcely needed any
longer, because a whole series of investigations prove
that it not only applies to man and the animal kingdom,
but also to the vegetable kingdom; that this law is
general for all the organic world. Nothing so convinces
us of this truth as the plant.

We know, as a fact, that the fertilisation of the pistil

240 THE LIFE OF THE PLANT

by pollen from the same flower is less successful, and gives
a less vigorous posterity than when the pollen comes
from another flower. Moreover, there are plants in
which self-pollination is absolutely fruitless, e.g. Cory-
dalis. There are also plants with two or even three
kinds of flowers, e.g. the primrose, the purple loosestrife,
and the different species of flax. The pistils and stamens
of these flowers are of different lengths, and moreover
long pistils occur in the same flower with short stamens
(fig. 68 A) and vice versé (B). For successful fertilisa-
tion, it is necessary to transfer pollen to the stigma from

stamens corresponding in height, which means that the
pollen must always be taken from another flower.?
There are plants where fertilisation with the pollen of
another variety turns out to be more fruitful than self-
fertilisation. Finally, quite trustworthy investigators
have described cases in which self-fertilisation acts like
poison ; the stigma of a self-pollinated pistil appears as
if singed, and the flower dies without forming fruit,
whereas pollen taken from other flowers effects fertilisa-
tion. Thus a number of facts prove empirically the
existence of a general law, according to which cross-
fertilisation is beneficial and self-fertilisation compara-
tively harmful to the organism.

1 In the middle of fig. 68 the pollen grains, which are of different sizes,
are shown.

THE FLOWER 241

The existence of this law once admitted, the signifi-
cance of a number of peculiarities in the structure of
the flower, otherwise inexplicable, becomes clear. A
great number of facts have been accumulated in
botanical literature in support of the theory that
the structure of flowers is adapted to cross-fertilisa-
tion, especially through the agency of insects. Let us
here dwell only on the most prominent outlines of this
theory.

The following considerations can be brought forward in
support of the theory that a showy appearance, perfume,
and honey glands exist for the purpose of attracting
insects, which in flying from one flower to the other
promote cross-fertilisation. In the first place, all these
characters exist only during the period of pollination,
after which they disappear. Secondly, there is a marked
difference between plants in which pollination is effected
by means of wind and those in which it is effected
through the agency of insects. Flowers of the former
kind are generally small and unsightly, are never highly
coloured, and possess neither perfume nor honey. Such
are the majority of trees, e.g. the pine, the birch, the
aspen, and so forth. On the other hand, these plants
produce an abundance of pollen. Clouds of yellow
dust falling on the soil or on the surface of water have
even given rise to superstitious beliefs in rains of sul-
phur. It is noteworthy that such flowers generally
bloom in early spring, when the undeveloped leaves
cannot prevent the scattering of the pollen.

It is clear, nevertheless, that such a waste of material
probably precious to the plant, a waste inevitable during
pollination by the wind, must be a very great disad-
vantage to the plant. Apparently it is more profitable
to a plant to produce less pollen and to secure more
accurately its transference to the stigmas of other plants.
This economical purpose is evidently served by insects.
Honey glands, the so-called nectaries, entice insects to

Q

242 THE LIFE OF THE PLANT

visit flowers; the bright colouring, the size and the
perfume of flowers serve as means for attracting them.
It has been proved that bees possess the faculty for
distinguishing between colours. By smearing with honey
differently coloured surfaces, we can train insects to associ-
ate the impression of a certain colour with the presence
of honey. Cross-fertilisation takes place as a result of a
division of sexes in the flowers themselves ; the staminate
and pistillate flowers may be distributed either on one
and the same plant or on different plants: while in
hermaphrodite flowers cross-fertilisation is achieved by
the ripening of stamens and pistils at different times,
whereby self-fertilisation becomes impossible. But most
convincing of all are the numerous facts which show all
the parts of the flower to be adapted in structure and
disposition to the form and habits of the visiting insect,
so that in flying from one flower to another the insect is
bound to touch the
stamen of one flower,
and then with the
same part of its body
the stigma of an-
other. Let us limit
ourselves to a few
illustrations.

There are many
flowers the parts of
which, instead of
being arranged in the
usual regular way
round the centre, are
distributed in such a
way that right and
left sides, top and
bottom can be dis-
tinguished. Labiate flowers may serve as an example
(fig. 69, 2) Such a form is obviously very convenient

THE FLOWER 243

for insects; the lower lip serves as a platform or
balcony, on which the insect alights when in the
act of passing its proboscis into the tube of the
corolla at the bottom of which the sugary nectar
lies. But in doing so it rubs its back against the an-
thers (fig. 69, I and 2 m), and when it alights upon
another flower it rubs its back covered with pollen
against the stigma (). Self-pollination is quite im-
possible in this case, because the stigma develops later
than the stamens of the same flower, and it can
receive pollen only from another flower. In another
labiate flower, the common sage, the following curious
adaptation is found (the sage flower is shown in fig.
69). The stamens are of an unusual form, having the
anthers fixed to the top of a long cross-piece, which is
balanced ona short stalk (in fig. 69, I, a flower is shown
split longitudinally, with one stamen (m) ; in fig. 69, 3
the two stamens are shown separately). The lower ends
of these cross-pieces bar the entrance to the tube of the
corolla, at the bottom of which, as has been already said,
there lies some sugary liquid. The moment the insect
pushes its proboscis into the tube, it sets in motion these
lower ends of the cross-pieces, which like levers come
into position 4 from position 3 (fig.69). At the same
time the anthers descend on the back of the insect (fig.
69, 2 m) and cover it with pollen.

Another still more striking method of pollination is
found among the orchids, where in most cases pollination
would be almost impossible without the agency of insects,
and the flowers would be doomed to continual sterility.
The flowers of this family are distinguished by their fan-
tastic shapes, for which they are so commonly grown in
hot-houses. It may be sufficient to mention among the
wild representatives of the group the butterfly orchis
(Platanthera). Let us imagine such a flower with all its
petals torn off, with the exception of the lowest which is
in the shape of a lip and is elongated at the base into

244 THE LIFE OF THE PLANT

a tube, called the spur (fig. 70 sp.).1 The pistil and
stamens of this flower are equally singular in shape :
the pistil consists of a long twisted ovary (ov.), on the top
of which, without any style whatever, the stigma rests,
in the shape of a sticky spot at the very entrance into
the tube of the spur (stg.). The stamen has no filament,
but consists of an anther only (anth.), situated close to the
stigma. But all

on™ this. does not ex-
‘4 haust the pecu-
liarities of this

plant. Its pollen
is not crumbling
dust, but is
gathered in each
of the two lobes
of the anther into
alump, on astalk
which ends at the
bottom inasticky
knob on the out-
side of the anther (fig. 70, 2). Evidently since the
pollen is not set free by itself it cannot reach without
external assistance even the stigma of the same
flower, although it lies so close to it. This very
assistance is given by the insect. In settling upon the
lip (1), it thrusts its proboscis into the tube of the spur,
at the bottom of which a sugary fluid is generally
secreted, distinctly seen in’ Platanthera. The insect
invariably comes up against the viscous knob sticking
out of the stamen, and in flying off the flower carries
away its mass of pollen. This arrangement of the
flower is so accurate, and works so perfectly, that even a
needle cannot be thrust into the spur in the direction of

Fic. 70.

1 Fig. 70 shows the flower of an orchid, all the petals of which are
removed except the lip, which is split into two, to show the entrance into
the spur and the position of the stigma.

THE FLOWER 245

the arrow (fig. 70, 1) without being removed with the
mass of pollen adhering to it, as is shown in fig. 70, 3.
At first this mass of pollen stands upright, but in a few
minutes it bends forward (fig. 70, 3). The same thing
happens to the pollen mass adhering to the insect.
When it flies off to another flower, where the same process
is repeated, it leans with the pollen mass exactly against
the viscous stigmatic surface (sfg.) and leaves there part
of the pollen. The fact that everything actually takes
place in the way described above can be proved from
insects caught during their visitation of these flowers.
On their proboscis and head masses of pollen have been
often found adhering in considerable numbers. Conse-
quently, the fertilisation of such flowers cannot take place
without the participation of insects and resolves itself into
cross-fertilisation, always an advantage to the plant. The
importance to the plant that its floral parts should have
just these and not other forms becomes quite apparent.

Let us consider another striking example where all
the parts of the flower appear to be adapted to the
same end, i.e. to cross-fertilisation by means of insects.
This plant is Aristolochia. Its pale yellow flower has
a tubular corolla swollen into a ball at the base, expanded
and cut obliquely at the apex. The long ovary with a
number of ovules (fig. 74) passes immediately without
any style into a lobed stigma. Stamens without any
filaments and closely attached to the pistil are situated
right down under the stigma. The narrow tube of the
corolla is lined with stiff hairs pointing towards the
inside of the flower, as is shown on the plate (71, upper
section). The whole arrangement is like a mouse-trap.
Owing to this position of the hairs the insect easily
crawls inside the flower, but cannot come out again;
the flower is a trap where the insect finds a stigma already
mature and ready for fertilisation, together with stamens
not yet fully developed. If the insect comes from
another flower it rubs pollen on the stigma. Some

246 THE LIFE OF THE PLANT

time later the stamens ripen and split; the insects
restlessly move about in their prison (71, upper section)
and get covered with
pollen ; but this pol-
len on reaching the
stigma does not pro-
voke _ self-fertilisa-
tion, because the
stigma has already
withered. The hairs
of the corolla soon
afterwards wither
and fall off (71, lower
section), the door of
the prison is open,
and the insect,
covered with pollen,
flies out to be caught
again by a similar
trap in another
flower. Later on, the
upper lobe of the
corolla withers, and
bending over closes
the entrance into the
flower (71, lower sec-
tion) ; and the whole
flower, until then erect, droops. In this way fertilised
flowers escape useless visits from insects. This adaptation
works as successfully as the mechanism for the fertilisa-
tion of the orchids, so that we can tell almost with
certainty from the external appearance of the flower
whether we shall find insects inside it or not. The
number of insects thus temporarily imprisoned in a
flower is sometimes considerable.

We may, then, assuredly see in the so-called unessential
parts of the flower, such as the bright colouring of the

THE FLOWER 247

corolla, its fantastic shapes and honey glands, very
perfect adaptations of the flower to the purposes already
indicated. Perfume probably has a like purpose.
Like colour it serves to attract insects. It has even been
noticed that flowers, which exhale in the night a very
strong perfume, are visited by night insects. Yet the
exhalation of volatile, aromatic substances can serve
another purpose in the plant. To begin with, plants
need a more or less high temperature in order to flower,
which is partly supplied to them by the above-mentioned
rise in temperature as the result of respiration ; but, on
the other hand, during clear, calm nights plants are
known to be exposed to considerable cooling owing to
strong irradiation. To prevent this cooling we have
only to cover the plant with a glass bell which arrests
the heat rays emitted by the plant, and thereby prevents
unnecessary cooling. But this very property of the
glass belongs also to all volatile substances such as are
exhaled by flowers; they strongly arrest radiant heat.
In order to ascertain how considerable is the atmosphere -
of the volatile, so-called ethereal oils, surrounding scented
plants, we have only to apply a lighted match to the
strongly scented flower of Dictamnus. We see in amaze-
ment one flower and then the whole bush of flowers
enveloped in a bluish flame. This means that the
vapours of ethereal oils excreted by glands with which
the flower organs of this plant are supplied have become
ignited. Therefore, during calm, clear, summer nights,
z.e. exactly when there is a danger of cooling owing to
irradiation, the flowers are surrounded with a trans-
parent cloud, an atmosphere of these emanations,
which, by arresting like the glass bell the heat radiating
from the flowers, preserve them from excessive cooling.!

Thus one more disappointment has been added to the

1 Plants are known to be preserved from cooling in the night by the

lighting of a fire in their neighbourhood, which will make a cloud round
them and prevent them from losing too much heat by irradiation.

248 THE LIFE OF THE PLANT

many that science has been causing the pride of man
from the moment it proved that it is not the sun that
revolves round him, but he round the sun—namely,
that this variegated carpet of flowers, glowing with all
the colours of the rainbow and emitting the most delicate
perfumes, has never existed for his sake—the king of
Nature—but for midges and insects, and, above all, for
the sake of the plant itself.

But with every new achievement of science, although
it has involved his parting with unjustified claims,
man has only been gaining in real importance. So is
it in this case : if he has to admit that flowers were not
created for his sake, he can surely console himself with
the thought that they are partly created by him. We
have only to compare the plants in our flower gardens,
kitchen gardens, and cultivated fields with wild plants to
agree with this statement. A glance at any cultivated
plant, at any garden flower, or any vegetable reveals in
them the guiding hand and thought of man. His
claims, sometimes even only a _ passing whim of
fashion, have changed natural things in accordance
with those claims and whims. Fancy demands, for
instance, that a small irregular, pale, three-coloured
flower of heart’s-ease should become big, of one colour,
almost black, and round; and here, before our very
eyes, as if by some magic power, we see it actually
becoming bigger, darker, and rounder. The question
arises, by what means has man attained this result ?
how has he obliged Nature to contribute to his ends,
to follow obediently his indications ?

The procedure is very simple : man has been working
on these lines for ever so long without realising it ; and it
is only recently, after having grasped the treasures of
knowledge accumulated during centuries of practice,
that science has presented in their true light the sim-
plicity and universality of the method employed.
This procedure is as follows. Seeds obtained from one

THE FLOWER 249

and the same plant never produce two absolutely
similar plants ; differences invariably appear. If these
plants be left to grow and reproduce themselves, their
differences will soon vanish owing to cross-fertilisation,
and a permanent, mean, typical form will be produced.
Quite a different result will be obtained if a form
distinct from others in some point or other be removed
and isolated; its peculiarities will in most cases pass to
its posterity. If in the new generation we again isolate
the specimens which have attracted our attention by a
striking peculiarity, we shall emphasise the peculiarity
in every generation, and eventually fix it. This is the
method of selection.

In horticulture this method of selection is often
applied in the simplest and at the same time most
effective way. It consists in the destruction in every
generation of all the plants that fail to answer the
purpose in view. By repeating this operation of selec-
tion in every subsequent generation, and so strengthen-
ing minute and scarcely noticeable characters of the
plant, man seems to cast it in a new mould, feature by
feature, and to realise an anticipated ideal. It is worth
noticing that, while thus breeding plants and animals,
man has applied this principle of selection to himself as
well, although unhappily only in the reverse order.
For ages he has generally chosen the best representatives
of his own kind from the physical point of view and
condemned them to certain death. This experiment
with humanity proves, though of course in the negative
direction, the success of the principle of selection. Such
was, for instance, one of the results of Napoleon’s work.
His endless wars have resulted in the decrease of the
average stature in France.

Thus the principle of selection affords man a powerful
means for improving organisms, for perfecting them, and
the simplest application of this principle consists in the
extermination of organisms that do not correspond to

250 THE LIFE OF THE PLANT

his aim. Let us remember this conclusion, which will
later on prove to be the key to the explanation of
phenomena taking place in Nature.

Let us sum up what we have learnt in this lecture.
There is a law underlying organic Nature according to
which the cell though able to produce such giants as
the Wellingtonia and the Baobab, the age of which is
reckoned by thousands of years, is yet unable to reproduce
itself endlessly in the same vegetative way. The main-
tenance of vegetable forms requires that they should
occasionally be renewed by the union of two separate
cells. The significance, meaning, and necessity of this
law of the existence of two sexes is quite obscure ; it is
only an empirical law, based upon the conjoint testimony
of all the facts known to us.1. It may be that we are
entitled to see in this law only one of the many manifesta-
tions of a more general law—the law of the utility of the
physiological division of labour, which expresses itself
in the fact that the functions fulfilled in the simplest
organisms by a single cell distribute themselves over
different cells as the organism increases in complexity.
The cell may be unable to reproduce itself successfully in
all its parts in a long series of generations, and perhaps
this labour is divided between two cells, each of which
works out only a certain part of the future organism, and
taken by itself may be even incapable of further develop-
ment. But wherein does the difference lie between these
two cells ? Which is the element of development each
of them contributes ? These problems are the problems

‘ It is worth mentioning that certain seaweeds present a curious instance
of the sexual process taking place between three cells instead of two, a
phenomenon which has no analogy in the rest of organic nature. One
of these three cells, being the element fertilised by the second cell, is at the
same time the fertilising element of the third. This fact, quite authentic
although unique, together with the fact of the absence of any sex what-
ever in the simplest organisms, prevents us from too broad generalisations,
from metaphysical theories concerning the existence of a certain organic
polarity, and so on.

THE FLOWER 251

of the future. There is but one thing that can be
inferred from the facts we know, which is, that along with
the growing complexity of organisms, certain external
differences arise between the sexual cells, and at the
same time we observe that the degree of relationship
between them increases. So far as the significance of
the sexes is concerned, we are as yet completely in
the dark, and it is better to abstain from any explana-
tion at all than to indulge in vague hypotheses with
no facts behind them.

CHAPTER IX
THE PLANT AND THE ANIMAL

In the previous chapters we have studied three functions
of a vegetable organism, nutrition, growth, and repro-
duction, which, from a certain point of view, may be re-
garded as a particular case of growth. On glancing at
Nature superficially, and regarding only such forms and
phenomena as we meet at every step, we might easily
come to the conclusion that all the vital activities of the
plant are summed up in these three functions. This
idea has found expression more or less from time imme-
morial in some such definition of vegetable life as that
plants live (i.e. feed) but are deprived of motion, with the
occasional addition of voluntary motion. This absence
of motion and outward activity is looked upon as the
essential point of difference between plants and animals ;
and vice versé, this is why a man, whose life is spent
lazily, in little more than eating and sleeping, is said
to be vegetating. But is such a general statement
about the plant justifiable ? A broader outlook upon
the vegetable kingdom and a closer study of the plant
will soon prove how hasty such an opinion is. We find
with amazement that, far from being absent, the pheno-
mena of motion are even widely spread in the vegetable
world.

Let us turn first of all to the microscope and
study with its help a fully-developed uninjured cell
under the most natural conditions possible. We
choose for this purpose hairs found on the surface of
stems and leaves or of young roots, and consisting either

of a single cell or of a single row of cells ; or we may make
252

THE PLANT AND THE ANIMAL 253

a delicate section with a sharp razor of a leaf or stem .-
of a water plant, such as Vallisneria,1 so thin as to be
transparent, being at the same time careful not to
injure the uncut cells. Water plants are convenient
simply because microscopic investigations are generally
made in water, which means that the cell remains in
its natural medium. [If all the natural conditions are
fulfilled, z.e. if the temperature is not too low and the
cells are not injured, one of the most curious phenomena
ever presented by the organic world reveals itself in a
short time to oureyes. The cell-sap, or rather that part
of the cell-contents which we have called protoplasm,?
and which as a layer of thick liquid lines the inner
surface of the cell-walls, or stretches in strands across
the cavity of the cell, which is filled with a thinner sap,
—this protoplasm moves in every cell, slowly at first,
then more and more quickly. This movement is seen
especially clearly in cases (such as Vallisneria) where
bright green chlorophyll grains are suspended in the
protoplasm. ‘These grains, carried by the rapid current
of the protoplasm can be seen flowing along one of the
longitudinal cell-walls, turning along the transverse
wall to the other longitudinal wall, and from it again
along the second transverse wall, to come back to the
point of departure and repeat over and over again that
circular voyage. This rapid rotating movement of
the protoplasm can be observed in one and the same
cell for hours and even days. In cells in which the
protoplasm forms a general network of strands, the
movement is not limited to the circular current along
the walls, but is also seen to flow in thin streams across
the cavity of the cell. Such a movement can be
observed in any hair, such as the familiar hairs of

1 A plant to be found in any aquarium. The curious phenomena taking
place in this plant during the period of pollination have been described
in the previous chapter (fig. 68).

2 See chapter ii.

254 THE LIFE OF THE PLANT

Tradescantia (see fig. 15) or the stinging hairs of the
nettle, and also in the cells of the flesh of fruit, as, for
instance, the large cells visible to the naked eye making
up the ripest parts of a water melon. We have but
to take some of these cells with a needle and place them
under a microscope to notice this curious streaming
movement of the protoplasm in every one of them.
Thus the protoplasm in these cells is in continuous
movement. This movement is spontaneous: it is not
provoked by any external physical agents, although
such agents as heat or electricity may affect it, accelerat-
ing, retarding, or even entirely arresting it. So many
and such varied illustrations of this movement are
known to us, that it seems most likely it is character-
istic of the protoplasm of all cells, at all events at a
certain period of their existence.

Sometimes this movement of the protoplasm mani-
fests itself in a still more curious way, and so strongly
as to be seen even with the naked eye. There is a
group of organisms so peculiar that for a long time
scientists wondered whether they were to be con-
sidered plants or animals. Even nowadays some
people reckon them as a separate third kingdom,
although it might be more reasonable to range them
among the simplest plants, along with the fungi. These
organisms are called slime fungi, because during the
greater part of their existence they present but a mass
of protoplasm without any structure whatever, without
any cell-walls, and therefore like slime, colourless, or of
a brownish or bright yellow colour. These organisms
appear on the surface of decaying wood, leaves, etc.
One such organism specially well known occurs on
the piles of bark accumulated in tanneries. It appears
in masses without any definite shape, looking like thick
cream, only yellow in colour, penetrating amongst the
pieces of bark in thin filaments, or collecting on their
surface in variously branched or compact masses. If

THE PLANT AND THE ANIMAL 255

we mark in one way or another the position of these
semi-liquid masses (called plasmodia) and remember
their outlines, we shall be greatly surprised in a short
time to notice that they have moved considerably from
the place they occupied and have also changed their
shape. By observing one of the fine branches of a
plasmodium, or better still by examining it under a
microscope, we come to
the conclusion that it
actually does move.
These branches form
protuberances, pseudo-
podia, into which the
protoplasm of the neigh-
bouring parts flow. The
protuberance so formed
issoon drawn backagain
and becomes absorbed
into the general mass ;
another one appears
drawing the protoplasm
in its turn. Thus
stretching and contracting again the plasmodium creeps
about (figs. 72 and 73), tending for the most part in some
definite direction ; it changes its position, creeps towards
the light at the top of the pile, out from the inside of it,
where it was concealed, crawls over every object in its
path, e.g.a sheet of paper or glass—in a word it wanders,
until it is arrested by the approach of the period of re-
production. Then it transforms itself into an indefinite
scone-like shape, the size of the palm of the hand, with a
very brittle wall, and inside a very fine dust is formed,
reminding us of the dust we raise when we tread on a
ripe puff-ball. This dust consists mainly of minute
cells, spores, serving to reproduce the organism. In
germinating, the spores of our slime fungus shed their
cell-wall and soon transform themselves into micro-

Fic. 72.

256 THE LIFE OF THE PLANT

scopic lumps of protoplasm, which are continually
changing their shape (fig. 73). Though on a smaller
scale they exhibit the same creeping movement as
that described in the case of plasmodia, a fact which is
easily understood, since plasmodia themselves, masses
of protoplasm visible to the naked eye, are formed by
the coalescence of a very great number of these micro-
scopic lumps derived from the spores (fig. 73).

Thus we see that protoplasm, the foundation of every

DVAew

Fic. 73.

cell, vegetable as well as animal, is endowed with a
peculiar movement, inadequately accounted for as yet,
and manifested indifferently whether surrounded by
a wall or entirely free as in the case of the plasmodia
of the slime fungi.?

These instances do not exhaust the phenomena of
movement, exhibited by the vegetable cell. So far we
have studied one kind of motion, the streaming move-
ment of shapeless masses ; let us now study the pro-

1 There is a satisfactory attempt to explain this movement from the
physical point of view. Unfortunately we cannot dwell on it, because
it would take us too far into physics; we can only say that by mixing

two liquids we can obtain, under the microscope, forms and movements
exactly similar to these,

THE PLANT AND THE ANIMAL 257

gressive movements of whole cells. The spore plants
give us numerous illustrations of such phenomena.
Let us choose a few of them at random, taking them
mainly from among plants that we see every day.
If we pick up a dead fly and throw it into a glass of
water, we notice in two or three days a soft white
down, forming a kind of halo round the body of the
fly (ig. 74). This isa mould, z.e. a microscopic fungus.
If we examine under the microscope its radiating
branches, we shall observe at their ends oblong sacks,
filled with colourless grains (fig. 74). If we leave some
of these sacks in water under the microscope and look
at them occasionally, we shall almost certainly catch
one of them at the
moment when its
end breaks and lets
out the grains it
enclosed. These
grains will cluster
at the opening.
We shall notice
that every one
of them has two
cilia attached to
one side. But in
a few moments
the whole mass of
them will quiver :
firstone grain,then
another, and then Fic. 74.
all of them will
stir, rush round as in a whirlpool and disperse, moving
their cilia so rapidly that they are now scarcely visible.
For a long time they continue to rush about, swimming
across the field of the microscope, knocking against
each other or anything else they meet, then bounding
back and rushing in another direction.

R

258 THE LIFE OF THE PLANT

It is impossible to distinguish this movement from
that of the infusoria, and it is so contradictory to the
current ideas as to the non-motility of the plant, that
the first observers of similar phenomena refused to
believe the evidence of their own eyes; they would
not believe that these bodies were of a vegetable nature.
They attributed this movement to animals developed
inside the plant. These motile cells after a time come
to rest, germinate, and give rise to a new organism—
they are therefore spores. In order to mark their
resemblance to animals, they are called zoospores, i.e.
animal-spores, or rather motile, wandering spores.
Let us study one more zoospore, but from another class
of plants, the water-weeds. There appears on sub-
merged objects in ponds, streams, and ditches, occa-
sionally also upon the surface of very damp soils, a
bright green weed consisting of a single much-branched
tubular cell. If we leave such a weed in summer in a
glass of water, we shall notice every morning a curious
phenomenon: a narrow, bright green line will appear
on the surface of the water, at the side of the glass which
faces the light. If we move the glass so as to turn the
green edge away from the light, we shall observe that
the green line will disappear, to reappear again at the
side towards the light. We can repeat this experi-
ment many times and always obtain the same result.
This green matter is evidently capable of movement,
and moves always towards the light. Let us investi-
gate the constituents of this green matter and its origin.
We place it in a drop of water under the microscope
and notice that it swarms with green cells swimming
to and fro (fig. 75, I., on the top). The cells have no
walls ; they consist of a lump of protoplasm spangled
with shimmering cilia all over the surface. Let us now
turn our attention to the weed itself and see what is
its relation to these motile green cells. We shall
notice pin-head swellings at the end of its green tubes,

THE PLANT AND THE ANIMAL 259

filled with a darker green and thicker mass (fig. 75, I.).
If we observe such a swelling for some time (the obser-
vation must be made early in the morning, because in
the day-time this phenomenon ceases), we shall notice
that the green mass gathers into a round or rather an
oval lump, creeps out of the sack, which is ruptured
at the top, and begins to move (fig. 75, I.). This isa
large zoospore formed out of the protoplasm of our
water-weed.

The movements exhibited by spore plants are not
limited to the zoospores. We saw in the previous
chapter that these plants are clearly differentiated in
sex, but for the sake of simplicity we chose cases
where both male and
female cells are non-
motile, and come into
contact only by fu-
sion. But in a greater
number of cases the
male cell is motile,
and therefore seeks out
the female which is &
enclosed in a special %&
organ. In very rare
cases both male and
female cells are motile,
like the zoospores just
described : their move-
ments bring them to-
gether, make them meet
and fuse into a single
mass, into one cell,
one spore. As a matter of fact in weeds, mosses,
ferns, horsetails, and clubmosses it is only the male
cell which is motile; it also assumes most fre-
quently the shape of a rod twisted into a spiral and
provided with cilia, These so-called antherozoids are

FIG. 75.

260 THE LIFE OF THE PLANT

endowed with a double movement : they move quickly
forward and also rotate round their axis. Thus fer-
tilisation, secured in seed plants by complicated adap-
tations by which the non-motile pollen of a flower is
transferred to a stigma, is here accomplished by the
motility of the male cells themselves, the antherozoids.1
The antherozoids of mosses are most easily seen. If in
the spring we gather a stem of a big moss, which forms
round, soft, green tufts in woods and marshes, and if we
press between the fingers the unsightly brown clusters of
modified leaves, seen at that season at the ends of many
of the stems, small whitish drops will exude. Every such
drop will contain millions of antherozoids. Fig. 75, II.
shows the fertilisation of a female cell of a seaweed,
found in the Baltic Sea and called Fucus. This cell is
non-motile by itself, but antherozoids swarm round it,
often surround it with a thick layer, and thus carry it
away with them.

Thus, the vegetable world observed under the micro-
scope turns out to be full of motion : in the cells of the
water-melon the protoplasm moves; in every weedy pool
there swarm myriads of zoospores; in the drops of
evening dew there move the antherozoids of mosses
and ferns, finding their way to female cells in order to
fertilise them. But do we not notice phenomena of
motion in a more obvious form, in those organs and
plants that we can observe with the naked eye and
which are naturally associated in our minds with the
word ‘ plant’ ? Such phenomena can indeed be easily
demonstrated, although they do not occur as often
as microscopic movements. They are especially striking
in plants growing in warm countries or in our hothouses ;
the reason of which is easily understood. All kinds of
motion in plants are accelerated with rise of tempera-

1 We have noticed in the previous chapter that antherozoids have been
discovered in the pollen-tubes of some plants. In the next chapter we
shall be able to appreciate the importance of this fact.

THE PLANT AND THE ANIMAL 261

ture: thus, for instance, the movements of protoplasm
can be accelerated or arrested at will by subjecting the
cells under investigation to heat or cold.

We must differentiate two kinds of motion when we
speak of movement in the organs of the highest plants.
Some of them are slow and gradual; like growth they
can be observed only by their results, and they generally
depend on the influence of variable external conditions.
Others are rapid and abrupt like the movements of
animals ; and, as in animals, they are either provoked by
external irritation, or take place without any stimulus
whatever, apparently quite spontaneously.

Phenomena known under the name of sleep movements
belong to the first class. We mean by these the changes
in position of leaves and parts of flowers at different
hours of the day and night which are manifested by
nearly all plants, but most obviously by some of them.
If you look at a field of pink clover, you will get a different
impression of it according to the time of day. In the
day-time its surface will be more uniform, because the
leaflets are almost horizontal, and catch the light falling
right down upon them with the whole of their surface
turned up to the sky. In the twilight, on the contrary,
the surface of the field will look dishevelled, and if we
examine more closely the separate leaflets, we shall
notice that all the three blades of the leaf are razsed ;
they are now turning their edges instead of their
surface up to the sky; the two side blades are folded
together, while the third is pressed against their common
edge. Other plants have the lamina of their leaves
bent down in the nocturnal position, and so appear as
if withered ; in this case of the clover, on the con-
trary, they are raised, and so it is clear that we have
to do with quite a peculiar mechanical phenomenon.

Sleep movements in flower organs are even more
distinctly seen. Thus, for instance, in the early morning
or in the twilight we do not see any of the yellow heads

262 THE LIFE OF THE PLANT

of the dandelion which spoil so often the uniform green
of our lawns. This is because these flowers open only
in the light: on a dull day they may likewise remain
closed. Other flowers, on the contrary, close in the
day-time. This is the case with the goat’s-beard
(John-go-to-bed-at-noon), the yellow flowers of which are
very much like those of the dandelion, only larger in size.
They open early in the morning and close by ten or
eleven. These phenomena attracted much attention
among botanists of the eighteenth century. It was even
suggested that floral hours might be observed; the
hours of the day being defined by the opening and closing
of different flowers.

These phenomena are easily proved to depend on the
action of light and heat. The crocus is especially con-
venient for the purpose. Its large flower opens in the
day-time and closes at night ; but the same phenomenon
can be caused by placing it alternately in the light
and in the shade, or removing it from a warm place to
a cold one, and vice versé. A difference of temperature
of ten or twenty degrees makes it close and open again
in a few minutes. We can explain these phenomena
by unequal growth or by the tension of tissues in
the upper and lower, or outer and inner layers of the
moving organ. We have already seen that light
retards growth; consequently, under its influence, the
growth of the outer layers will be checked while the
inner layers will outgrow them, and the organ as a whole
will tend to curve outwards. Asa result the flower will
open. Now it is the inner (or upper) side that becomes
exposed to greater illumination, while the outer (or
lower) side, being shaded, will outgrow it and the flower
will close. Similar reasoning can be applied to the
effect of changes in temperature.

Such is the nature of these phenomena. They can
ultimately be attributed to irregular growth, and, as a
matter of fact, they are generally observed in organs

THE PLANT AND THE ANIMAL 263

that have not stopped growing.1 The movements of
another class are different. These take place quickly,
almost instantaneously, as a result of irritation, or even
without any stimulus whatever, and apparently quite
spontaneously. Let us
study a few cases of
this kind of pheno-
menon, beginning
with the _ simplest,
observed in the well-
known barberry. In
the centre of the
yellow flowers of this
plant, which are very
like small roses, there
is a pistil surrounded by six stamens (p, fig. 76).?
These grow normally in the position shown at st.
on the left. But as soon as we touch the base of
the filament with a needle (as shown in the figure), the
stamen suddenly moves and assumes the position sé’.
on the right-hand side, i.e. applies itself to the stigma.
It remains for a time in this position, then gradually
returns to the normal position to again apply itself
to the stigma as soon as irritated. Movements as the
result of irritation, though of a somewhat different kind,
are also characteristic of the stamens of the corn-flower,
the thistle, the artichoke, and other plants.

These are all movements of very small, if not exactly
microscopic, organs, and hence do not produce so start-
ling an impression as does the movement of the irritated

1 Some cases of these phenomena, however, fall rather into another
class; they depend on the presence of a special tissue in which the
quantity of water changes, hence also the tension of the cells. It is
obvious that in such cases the phenomenon may also be observed in
fully developed plants. Such are, for instance, the sleep movements of
leaves.

2 Fig. 76 is a longitudinal section of the barberry flower : pet. marks the
position of the petals; st. and st’., the stamens; , the pistil with the stigma.

264 THE LIFE OF THE PLANT

leaves of the sensitive mimosa, which grows in our
greenhouses. It is curious to witness the amazement
of a person who has never before heard of this
plant, and sees for the first time the way it folds its
leaves on being slightly disturbed. It is only then
that we realise how deeply rooted is the conviction,
based as it is upon daily experience, that motion is
not a characteristic of plants. Normally, the leaf of
the mimosa has the appearance shown in fig. 77 on the

al oe>
J} , LESSS
SHE A SSE
“UR LE SNe Whe
AEE | i SIN ZEN
ARE I= SZ
i 2 Sa,
KX <<
i i (Gs
U)
Fic. 77

right-hand side. This is a so-called compound leaf.
Its main leaf-stalk bears four stalks spread out like
a fan, and each of these in its turn bears a con-
siderable number of leaflets distributed in pairs. We
have only to touch such a leaf or irritate it in some
way and it will move. The leaflets will raise them-
selves in pairs, and fold like the wings of a butterfly
at rest. The four spreading stalks will lay them-
selves together, and the main leaf-stalk will eventually
droop, and sink downwards. The whole plant will
look like the leaf on the left-hand side of fig. 77.
The higher the surrounding temperature the quicker
the movement. When the irritation ceases the leaf
will gradually reassume its former position. Fresh
irritation produces again a similar phenomenon.

THE PLANT AND THE ANIMAL 265

Apparently we have here an impetuous movement,
caused by some external stimulus, reminding us very
strongly of the movement of an animal when it tries to
avoid some irritating contact. Can we give any
explanation of this phenomenon ? Yesandno. Wecan
indicate the proximate mechanism of the movement,
but as yet cannot explain the nature of the stimulation,
caused by irritation and in its turn producing the move-
ment. ‘This movement takes place at the points where
the leaflets are attached to the stalks, where these are
attached to the main petiole, and, lastly, where the
latter is attached to the stem. At all these points,
the joints or articulations, special swellings or cushions
are found. These leaf cushions or pulvini are formed
of a tissue, the cells of which are overfilled with sap,
and as a result these parts are in a constant state of
tension. ~The moment the leaves are irritated the tension
is suddenly released ; it is even reversed. Thus, for
instance, the tension of the tissues of the lower half of
the pulvinus which forms the base of the leaf-stalk
supports it horizontally, and even in a rather uplifted
position (as on the right-hand side of the figure). But
whenever irritated this part of the cushion loses its
tension, becomes floppy, loses its turgidity ; it is then
unable to support the petiole, which sinks or rather is
bent down by the upper half of the pulvinus, which
has preserved its turgidity. Thus the two parts of the
cushion—the upper and the lower—are in constant
antagonism. Ina normal condition the tension of the
lower part predominates and the petiole is held up ;
at the moment of irritation, when the tension of the
lower part ceases, the preponderance is in favour of the
upper part which bends the leaf down. At places
where individual leaflets are attached to the stalks, the
reverse phenomenon happens: the upper part of the
pulvinus (appearing here in the form of a white knob
the size of a millet seed) is always more strained than

266 THE LIFE OF THE PLANT

the lower; the leaves are consequently widely spread
horizontally, or even slightly bent down, but at the
moment of irritation the tension of this upper part breaks
down, and the leaves, being left under the influence
of the tension of the lower part only, rise and draw
together in pairs. So the cause of this movement lies in
the sudden, almost instantaneous loss of turgidity in the
tissue of one of the two halves of the pulvinus: from
being turgid it suddenly becomes flaccid, the balance
between the two antagonistic halves of the organ is
upset, and the leaf or leaflet moves in the corresponding
direction. But how can we account for this sudden
collapse, this loss of tension ? The microscope reveals
the fact that the tissue which has this curious property
of losing its tension consists of cells with thinner walls
than the cells of the opposite antagonistic side of the
pulvinus; and, moreover, that the cells of this irritable
tissue alternate with spaces filled with air. At the
moment of irritation these spaces become filled with
a liquid, as is easily proved. We have only to fix
our eyes on the thickened pulvinus at the base of the
petiole to see a sort of shade pass across that place
~at the moment of irritation ; the spot suddenly becomes
darker. The same thing happens even more distinctly
if several pairs of leaves are suddenly but gently clutched
with both hands in such a way that they are irritated,
but at the same time not allowed to fold. We shall
notice, then, that the thickened pulvini, compared
above to the millet seeds ‘and lying at the base of every
leaflet, will change in colour; from dull white they
become transparent green. The moment we let the
leaves go, they will fold. What causes this sudden
change in colour? It is the same cause which produces
a dark spot on the white surface of the snow, a filter
paper, or ground glass when we sprinkle them with
water. The whiteness in all these cases depends upon
the reflection of light by countless minute surfaces in

THE PLANT AND THE ANIMAL 267

contact with the atmosphere ; but whenever water is
substituted for the air, there is no longer the same
reflection ; the bodies become more transparent and
hence less bright. A direct experiment, however, shows
this explanation to be the true one. We have only to
make a slight incision on the lower side of the pulvinus
of the leaf-stalk to see that a drop exudes from the in-
cision at the moment of motion. If a similar incision is
made on a leaf, which has already drooped owing to
irritation, the drop of water will not exude. This water,
exuded from the cells and occupying the intercellular
spaces in the tissue, is absorbed or evaporated in the
course of time; the cells become refilled ‘with water,
and the tissue regains its tension until a subsequent
irritation.

Ultimately, therefore, the cause of the phenomenon
which has attracted our attention resolves itself into
the fact that water is rapidly exuded from the thin
walled cells of the irritable tissue overfilled with it, and
consequently this tissue as quickly loses its turgidity.
But why is irritation followed by the exudation of water,
and what is the energy that forces the cells to be over-
filled with water ? Weare unable as yet to answer these
questions, but very probably we are dealing here with
electrical phenomena, as we shall see further on.

Let us pass on to another example. At the end of
the eighteenth century a plant was discovered in the
marshes of North America, the movements of which are
more striking still. I mean the so-called catchfly
(fig. 78). The upper part of the leaf has the form and
function of a trap. Whenever we touch the hairs upon
ts surface, or whenever an insect imprudently creeps on
to it, the two sides of the trap immediately fold together
and do not let their victim out again. The more agitated
the entrapped insect becomes, the more tightly do the
walls of its prison close. This struggle between the plant
and the animal ends always in the death of the animal.

268 THE LIFE OF THE PLANT

The mimosa and the catchfly are instances of plants
capable of manifesting movements in response to the
slightest external stimulus; but here is also a plant in
which movements take place without any external
stimulus whatever—Desmodium or Hedysarum—coming
from the West Indies and
belonging to the so-called
Papilionaceae and therefore
akin to our bean, clover, and
other plants. Imagine that,
of the three leaflets which
form the compound leaf of
the clover, the uppermost
were to become greatly
elongated and the two side
ones were but slightly devel-
oped so that they were
much smaller than the
third. Such is the leaf of

Fic. 78. Fic. 79.

Desmodium, shown in fig.79. If now we take our stand
on a bright hot day before this plant, which is often
grown in our hot-houses, we shall notice in less than a
minute a slight shudder passing here and there over its

THE PLANT AND THE ANIMAL 269

leaves. Let us concentrate our attention on some par-
ticular leaf which arrests us by such unexpected move-
ment, and we shall soon perceive one of the most striking
phenomena in the vegetable world. At first the two
leaflets are in a horizontal position. Suddenly one of
them changes this position by an abrupt movement,
by a bound, so to speak, and it is found at a considerable
angle with the horizon. Another and yet another such
abrupt movement, and it is raised vertically. Mean-
while the opposite leaflet is lowered by a similar series
of abrupt movements, by internal shocks. Then the
leaflets change their parts. The raised one falls and
the lowered one rises. This movement seems to be
regular, produced by inner pulsation, provided the plant
has sufficient light and heat. As the temperature falls,
the intervals between every two movements will become
longer, and eventually the movement will be no more
jerky, but slow and continual, and only to be noticed
when attention is drawn to the relative position of the
leaflets. Finally, if the temperature fall, say, to 68° F.,
the movements will cease altogether, the plant will
become chilled. But warmed again it will recommence
to wave its small leaflets.

The study of all these phenomena of motion leads of
necessity to the question as to the purpose they serve
for the plant. Apparently their significance varies
according to the case. The movements of spores and
antherozoids, of the stamens of barberry, etc., are
obvious : they are useful, if not necessary, for purposes
of fertilisation and the reproduction of plants. The
sleep movements of flowers, the folding of their outward
wrappings during the night, probably saves them from
cooling, which is so dangerous to them. The sleep
movements of leaves have probably the same effect : by
folding or turning their edges upwards in the condition
of sleep the leaves present a comparatively small surface
from which heat can radiate ; consequently they escape

270 THE LIFE OF THE PLANT

too great cooling,and are less accessible to morning frosts,
during which plants are frequently frozen (actually
because of irradiation) although the thermometer may
not have fallen below 32° F. The purpose of the
movements of the catchfly is obvious: the very name
of the plant expresses it ; for, as we shall see later on,
together with certain other plants, the catchfly in reality
feeds upon captured insects. The use of motion in the
irritable leaves of mimosa is less obvious. Nobody has
apparently even tried to explain its purpose. Only
more or less probable suggestions can be brought forward
in this connexion. Any one who has observed the
effects of heavy showers and hailstorms will certainly
have noticed how it chips the foliage of our trees.
Such delicate organs as the leaves of mimosa would
suffer still more from tropical storms, if the first drops
of rain did not cause them to gather their outstretched
leaflets together and fold them against the stem. These
leaves, therefore, behave like the rush in the fable:
they weather storms which shatter oak-trees. I repeat,
this is only a conjecture, the accuracy of which can be
verified only by observation on the spot where these:
curious plants grow. It is much more difficult to explain
the object of the continuous movements of the leaflets
of Desmodium, unless we admit that these movements
serve the plant to scare away pernicious insects, attracted
by its sweet juicy foliage.t If this be true, plants
would seem to use their capacity for movement for two
different ends: for getting rid of enemies on the one
hand and for catching and feeding upon them on the
other.

1 The same explanation may hold with regard to mimosa; at all
events I have observed cases when mimosas in our hot-houses
perished from certain white lice, which found their abode at the very
articulations of the leaf. This is possible only in cases where the
leaf has lost its irritability. The tissue of the articulations must
specially attract insects, on account of the abundant sugary substances
it contains.

THE PLANT AND THE ANIMAL 271

Let us leave these conjectures for the present } and
study another question: Are we entitled to consider
the movements of plants described above as similar to
those in animals, or can some essential difference be
established between these two categories of phenomena ?
So far as the movement of protoplasm is concerned, it
does not present any difference whatever in the two
kingdoms. The same may be said of the movements
of the zoospores and antherozoids: here also no
difference can be established between the movements of
vegetable and animal organisms, which is proved by the
fact that the earliest investigators refused to believe
their own eyes and mistook for animals the motile
organs of plants.

The case is rather different when we compare the
movements of the higher plants with those of animals.
At all events we do not find in the plant any special
tissue adapted for movement, any muscular fibre
capable of contraction. Yet we can hardly base upon
this difference in structure a fundamental difference be-
tween the phenomena. A comparison of the conditions
which determine and accompany the movements of the
higher plants and animals points rather to similarity
than difference between them. We know, for instance,
that motion in animals is closely bound up with respira-
tion: the contracting muscle absorbs more oxygen, gives
off more carbonic acid than a resting muscle, and
it is this oxidising process that serves as the main
source of the energy used in muscular activity. Now,
does the plant present phenomena similar to the
respiration of animals? We have already met with
cases which prove this in the affirmative. During the
germination of seeds, the development of buds, especially
during the flowering period, these vegetable organs
greedily absorb oxygen and give off carbonic acid,

1 All these explanations brought forward as conjectures (in 1876) have been
proved by subsequent investigators and adopted by almost all botanists.

272 THE LIFE OF THE PLANT

while their temperature noticeably rises. This pheno-
menon is manifested by all parts of the plant during
its whole lifetime, but with the difference that the
green parts also decompose carbonic acid and give off
oxygen much more energetically under the action of
light, so that this process conceals the respiration
taking place simultaneously with it... But is respira-
tion connected with motion? Experiments answer
in the affirmative, although the nature of this connexion
is not as yet clear to us. If we arrest the supply of
oxygen to the plant, all the phenomena of movement
will cease accordingly ; the protoplasm will stop flow-
ing, the stamens of the barberry, the leaves of the
mimosa, will lose their irritability, and only after being
placed for a considerable time in an atmosphere con-
taining oxygen will these phenomena recur. Conse-
quently the movements of plants as well as the move-
ments of animals are closely connected with respiration.

Let us proceed with our comparison. Muscles
become heated by contraction; their temperature rises
to a measurable though insignificant degree: the same
is to be observed in plants. By applying to the pulvinus
of the leaf-stalk of mimosa a very sensitive thermometer
called a thermopile, it has been found possible to show
that the temperature rises at the moment of motion. In
muscles at rest, as also in a state of tension, the presence
of an electric current is noticed. If a circuit connected
with a sensitive galvanometer is closed (with all due
precautions) by means of a muscle of a frog, the needle
of the galvanometer will swing and manifest the current
in the circuit. A similar though weaker current will
become apparent, if instead of a muscle, a leaf of the
catchfly is introduced into the circuit. The similarity
does not stop there. If the muscle is made to contract,
a slight decrease in the strength of the current is ob-
served at the moment of contraction, the needle of the

1 See chapter v,

THE PLANT AND THE ANIMAL 273

galvanometer goes back: this is the negative variation
of the current. The same phenomenon takes place in
the case of the catchfly. At the moment when the
two parts of the leaf fold together a negative variation
in the current becomes apparent. Again a very
small but always measurable interval of time passes
between the moment of irritation and the contraction
of a muscle: this is known as the latent period of
excitation. A similar though more considerable in-
terval is noticed in the case of the leaf between the
moments of irritation and movement.

Thus the similarity between the movements of the
catchfly and of animals does not lie solely in the out-
ward manifestations of these movements but also in
the internal processes which accompany them. As
we have seen, it is true that the movements of vegetable
organs resolve themselves in the most obvious cases
into the exudation of water from the cells of the irritable
tissue overfilled with it, while the movements of animals
result from the contraction of muscles, a change in
their actual form; but the contraction of muscles is
not an elementary phenomenon. Will it not likewise
reduce itself ultimately into the mutual rearrangement
of the elementary solid particles and liquids which
enter into the composition of the muscle ?!

Thus the main wall of partition breaks down which
used to differentiate the vegetable kingdom from the
animal. Movement is not exclusively confined to
animals; it also takes place among plants. But if
this distinction does not hold good, can we not find
something ejise in its place? Let us consecutively
review the characteristics of the life of organisms,

1 It is possible that the overfilling of cells with water, its sudden
exudation, the presence of a current and its variation—all these details
of the process of motion in vegetable organs will find an explanation in
the phenomena, of electro-diffusion. It would be out of place to enter
into greater details about them at present.

s

274 THE LIFE OF THE PLANT

which were supposed to be peculiar to the represen-
tatives of this or that kingdom.

Let us commence with the mode of nutrition. Plants
are generally said to feed upon simple inorganic matter :
carbonic acid, water, salts; while animals feed upon
complicated organic compounds. Generally speaking
this is true, but the rule has nevertheless many excep-
tions. For instance, the large class of fungi comprises
plants which feed exclusively upon complicated organic
compounds, hence these organisms can exist only on
soils rich in humus—decomposing organic matter—
or else as parasites, feeding upon other organisms.
But fungi are not alone in feeding upon ready formed
organic matter; there are parasites also among the
highest representatives of the vegetable kingdom.
Some of these are lacking in green colouring matter,
such as the Dodder (Cuscuta), for instance, which
clings to our field plants, like the hop, and _ lives
entirely at the expense of its victim. Others, such
as the mistletoe, which attacks oaks, fruit trees and
other trees, although capable of independently pro-
viding themselves with food, probably also use in great
measure substances formed by the plant on which it
lives.

Moreover, modern researches, especially those of
Darwin, have acquainted us with a whole series of
plants which, though provided with green organs, feed
upon animal food and, what is more curious, digest
this food in the same way as animals. These are
known under the name of imsectivorous plants. Here
are some examples. The above mentioned catchfly
is the most curious of them all. If a leaf of the catch-
fly once gets hold of an insect, it does not reopen until
it has sucked out of it everything it can, leaving behind
a bare insoluble skeleton. A similar experiment can
be made by taking instead of the insect a piece of raw
or cooked meat, or the hard-boiled white of an egg.

THE PLANT AND THE ANIMAL 275

The leaf will immediately fold up and only reopen when
all trace of the food supplied has gone. The catch-
fly, as we have already said, lives in the marshes of
North America, but some of our own marshes provide
us also with a plant, which, though related to the catch-
fly, yet attains the same end, i.e. feeds upon insects,
by means of a somewhat different adaptation. This
plant is the Sundew (Drosera). Its small leaves are
covered with a certain kind of hairs, the ends of which
secrete drops of a viscid liquid. This liquid used to
be taken for dew, hence the name of the plant. The
insect which imprudently settles upon the leaf sticks to it.
The leaf then quickly manifests an extremely curious
kind of movement. The hairs from all sides tend
towards one and the same point, where the imprisoned
animal lies ; the glands at the tips of the hairs secrete
their sap profusely, and it dissolves the solid particles
of the nutrient substances, transforming them into a
condition in which they become easily absorbed by the
cells of the hairs. When the food is completely absorbed
the hairs expand again and are ready to meet another
visitor in the same way. The large peculiar leaves
of Nepenthes, Sarracenia, and Cephalotus found at
lower latitudes are not less curious, as well as the
minute leaflets of the bladderwort (Utricularia) in
our streams and ponds. One part of the leaf of the
three former plants develops into a pitcher, which
in the case of Cephalotus is, moreover, covered
with a lid, while in the bladderwort the finely sub-
divided submerged leaves are provided with similar
little organs. The pitchers were long ago observed to
contain a liquid. Formerly this liquid was thought
to be water, and only recently it has been proved to
possess the property of dissolving solid organic food
substances. Closer investigation of these pitchers
has revealed in them very complicated adaptations
for catching insects. They contain parts which secrete

276 THE LIFE OF THE PLANT

sweet sap and therefore serve to attract insects, and
also very smooth surfaces over which the insects cannot
help sliding down into the trap, and lastly, hard hairs
with tips turned inwards to prevent the victim from
coming out of its trap. But what is this liquid secreted
by the plant, what is this process by which the solid
food is dissolved, and has it anything in common with
what we generally call the digestion of food in an animal
organism? As we have already seen,! careful investi-
gations have proved that there is a remarkable similarity
between these two processes.

As in the gastric juice of animals, in which the dis-
solution of albuminoids takes place under the influence of
a special ferment, pepsin, so also in the liquids secreted
by all insectivorous plants the presence of a ferment
has been discovered. Just as there pepsin acts only in
presence of a small quantity of free acid, so also here,
at the moment when the plant is irritated, an acid
reaction of the secretion can be easily demonstrated.?

Thus the process of nutrition cannot afford us any
sure criteria for differentiating plants from animals:
fungi, parasites, and especially insectivorous plants,
present a complete analogy with the nutrition of
animals. In fact, if, taking into consideration all that
has been said about these plants, we had described an
imaginary organism, which by means of antennae
caught insects and conveyed them into a cavity of
its body covered with glands; if, moreover, we had
said that these glands secreted a juice that dissolved

1 See chapter ili.

2 Recently doubts have arisen as to the utility for the plant of this
absorption of organic matter. While paying a visit to Darwin I had an
opportunity of seeing some unpublished experiments of the great
scientist, which prove the significance of the process above described from
the point of view of nutrition. Several sundew plants were grown by him
under a glass bell, so as to save them from insects. Half of them received
meat, half did not. At the time I saw them (in July) the plants which

received meat were far bigger and more healthy in appearance than the
others (Note to the first edition.)

THE PLANT AND THE ANIMAL 277

albuminoids and then absorbed them as food, every one
would certainly have believed that we meant an animal
of some kind; yet all these features would have been
those of very distinct and typical plants. Ifthe nutrition
of plants can thus so closely resemble the nutrition of
animals, perhaps the nutrition of animals, on the other
hand, will never present us with a parallel to the pheno-
menon, which is characteristic of plants, of nutrition
at the expense of inorganic substances. But neither
can this be maintained, the property of decomposing
carbonic acid being, as we have seen, peculiar to a
special organ, namely the chloroplast, and we can name
several animal organisms containing chlorophyll.+

Now let us proceed to another supposed distinction
based upon the process of respiration. When the
interchange of gases taking place in plants, and resulting
in the decomposition of carbonic acid and the accumula-
tion of carbon, was erroneously compared to respiration,
the following antithesis used to be brought forward : the
respiration of animals consists in the absorption of
oxygen and the giving off of carbonic acid ; the respira-
tion of plants—in the absorption of carbonic acid and
the giving off of oxygen. We now know, however, that
the decomposition of carbonic acid cannot be compared
to respiration, that this is a case of nutrition—a peculiar
kind of nutrition by means of air; we also know that
another process takes place simultaneously—treal respira-
tion, but that this latter process can be observed only
when we investigate either colourless organs, or green
organs in the absence of light when the opposite process
of decomposition does not take place. This process of
respiration will certainly appear sluggish if compared
with the respiration of a mammal, or a bird. Carbonic
acid is given off in great quantities by the latter, and the
resulting rise of temperature above the temperature of

1 This chlorophyll belongs, however, probably to algae which have
found their way into the animal organisms in question.

278 THE LIFE OF THE PLANT

the surrounding medium is very apparent ; whereas in
the majority of cases plants passively acquire the
temperature of the medium. But if we compare the
respiration of plants with that of the so-called cold-
blooded animals, e.g. with frogs, or even with the
respiration of mammals in a state of torpor (for in-
stance during their winter sleep), we shall see that in
both cases the respiration does not differ so sharply
from the respiration of plants, either in the quantity of
gaseous interchange, or in the difference in temperature
of the organism from the surrounding temperature.

A question arises here; Is respiration one of the
processes absolutely necessary to the organism of the
plant ? We have seen that motion of any kind ceases
in the absence of oxygen. Fora long time the growth of
cells was also believed to be impossible without oxygen,
but it now appears that another chemical process can
be substituted for respiration, a process similar to it in
its results. This process is called fermentation, and
consists in the decomposition of sugar—glucose—into
alcohol and carbonic acid. This process underlies the
manufacture of alcohols, 7.e. the manufacture of wines,
the distilling of spirits, the brewing of beer, etc. In all
these cases fermentation takes place owing to the
development in the fermenting liquids of a peculiar
microscopic organism, a fermenting fungus known as
yeast. The yeast cells grow and multiply without
oxygen. The process of fermentation differs essentially
from respiration in the fact that it is not accompanied
by the absorption of oxygen, but is similar to it in that
in both cases carbonic acid and heat are given off.!

1 During respiration carbonic acid is formed at the expense of the
oxygen of the air, while during fermentation it is formed at the expense
of oxygen contained in sugar itself. Something like the combustion
of tinder or gun-powder takes place here. We know that both these
substances can burn without air at the expense of the oxygen in the
saltpetre which enters into their composition.

THE PLANT AND THE ANIMAL 279

It is probably this very heat that furnishes the plant
with the energy necessary for its development. Fer-
mentation appears to be a sort of succedaneum of
respiration. But this process is not as useful to the
plant as respiration, because for the same expenditure of
sugar much less energy in the form of heat is generated.
Formerly the process of fermentation was considered
essentially peculiar to the yeast fungus, but now it
appears that any plant or vegetable organ placed in
an atmosphere devoid of oxygen begins to give off car-
bonic acid, without absorbing oxygen, and forms
alcohol, z.e. begins to decompose its stores of sugars
and to ferment. The yeast fungus, which ferments
the sugars in the liquid where it develops, evidently
cannot suffer from this process in the same way as
the higher plants, which destroy their own substance
unproductively during fermentation. This circum-
stance, together with the accumulation of alcohol in
their cells—given off by yeast into the surrounding
liquid—probably explains why it is that higher plants
cannot maintain their life by means of the process of
fermentation; in the absence of oxygen all their
movements as well as growth come to an end, and if
they are kept a long time in such an atmosphere they
finally die.

Therefore fermentation can only maintain the exist-
ence of lower organisms, and even these but for a short
time, because they also appear to need respiration from
time to time ; whereas higher organisms cannot endure
fermentation even for a very short time. Happily
natural conditions in Nature do not expose them to this
danger. They begin to ferment only when exposed
to an artificially confined atmosphere, e.g. when they are
enclosed under a glass bell, from which oxygen has been
removed—in a word, when they suffocate. We cannot
simply say to an organism: Cease to live. It is true
it either lives or dies; but while it lives it clings to

280 THE LIFE OF THE PLANT

life. Failing to find in the surrounding medium the
necessary conditions for life, it reacts on itself, and
during this internal process breaks down, becomes
exhausted, and dies. But the moment we remove it
from the suffocating atmosphere, and by taking off the
glass bell give the organism the chance of breathing
freely, fermentation stops at once; the pathological
process of fermentation is replaced by the physiological
process of respiration, the work of destruction is replaced
by that of construction ; healthy and normal life asserts
its rights, and motion and development follow as fellow
travellers. Hence respiration conditions the very exist-
ence of organisms, be they animals or plants.

Much has already been said about the impossibility of
establishing any difference between the two kingdoms,
on the ground of the presence or absence of movement.
One more question remains to be discussed. Is the plant
capable of voluntary movement ? Before answering this
question we must agree as to what we mean by voluntary
movement, or speaking generally by a voluntary pheno-
menon. If the term implies a phenomenon produced
without cause science will not admit such even in the
sphere of animal life ; if the term implies a phenomenon
produced by internal, hidden causes, then voluntary
may also in the meantime be understood to connote
the movements of protoplasm, antherozoids, and the
leaves of Desmodium, because all these movements take
place without any apparent stimulus, under the influence
of internal forces peculiar to the organism. But if
capable of movement why may not a plant also feel ?
If we allow the response to stimulus, 7.e. irritability,
stimulation, to be a sign of feeling we are bound to re-
cognise this faculty in the plant. In fact if a man is
pinched, tickled, or pricked without responding to these
stimuli, we decide that he has become insensible (be-
come unconscious); but as soon as he begins to respond
to them, by some movement, we say that he has regained

THE PLANT AND THE ANIMAL 281

his senses (regained consciousness). If this is the
indication to be followed, then mimosa, the catchfly, etc.,
are apparently endowed with sensitiveness, because they
do respond to different stimuli, be it a prick, or a light
touch, a burn, an electric shock, or some chemical action.
There are some especially striking cases in which plants
do not respond to every stimulus in the same way, but
seem to discriminate between them. For instance,
contact with nitrogenous organic substances causes
quicker movements in the hairs of the sundew, and a
more energetic secretion of its digestive juices, than
contact with particles of inorganic matter, which cannot
serve it as food. Were it an animal we should say that
its mouth waters, that it greedily throws itself upon a
dainty morsel.

Granted that the plant is endowed with sensitiveness,
can we not deprive it of this property, make it in-
sensible to all kinds of stimulus ? Experiment proves
that we are actually able to do so; moreover, that
we attain this end by using the same means as when
we wish to bring a man into an insensible condition.
We can anesthetise a plant in the same way as we
anesthetise a man before a serious surgical operation.
We oblige it to inhale the vapour of ether or chloroform.
For this purpose we have only to cover a pot of mimosa
with a glass bell, and place a sponge under it filled with
ether or chloroform. After remaining a certain length
of time under the glass bell the mimosa will lose its
capacity for movement. However much stimulated, it
will no longer fold its leaflets, but after being exposed
again to fresh air, free from noxious vapours, it will
regain its sensitiveness, its irritability. In order that
this experiment may succeed we must not expose the

1 We know, however, that the reverse conclusion is not correct. By
the action of some poisons we can deprive an animal of the capacity for
responding by a movement to stimulation, without at the same time
depriving it of the capacity for feeling.

282 THE LIFE OF THE PLANT

plant too long to the influence of anesthetics, or it may
never revive, but will irrevocably perish. The same
thing happens with a man’s organism. Unfortunately
sad cases of death happen fairly frequently as a result
of the imprudent use of chloroform.

A nervous system has often been taken as the attribute
of an animal; but, as a matter of fact, a nervous system
is not found in all animals. On the other hand, if the
existence is established of special tracks in plants (as is
suggested by some scientists), by means of which irrita-
tion is communicated more quickly than by others, we
shall have to acknowledge that these are physiologically
somewhat analogous to nerves. Thus irritation in
mimosa, for instance, is transmitted by a special system
of tubes, by means of hydrostatic pressure. Such an
apparatus may best be compared with a pneumatic bell.
This case obviously does not present any real analogy to
the nervous system.

Now a last question: Is the plant endowed with
consciousness ? Weshall answer this question by raising
another: are all animals endowed with it? If we do
not refuse it to the lower animals, why should we deny
it in the case of the plant ? And if we do refuse it to the
simplest animal, tell me, where, at what degree of
organic development, does this threshold of consciousness
lie? Where is the limit beyond which an object
becomes a subject ? How can we escape this dilemma ?
Shall we not rather admit that consciousness is wide-
spread in Nature, that it smoulders in lower beings and
glows in a bright spark only in the mind of man ? Or,
had we not better stop at the point where the thread of
positive knowledge breaks off, at the border line beyond
which stretches the limitless province of speculation ever
captivating us by its elusive vastnesses, ever escaping
the limits of experimental inquiry ? 1?

1 Several botanists (Korjinsky and Famintzin in Russia) have recently
advocated the theory of the psychical activity of plants. I would only

THE PLANT AND THE ANIMAL 283

So, then, neither in the life of the animal nor in that
of the plant have we found a single feature specially
peculiar to the one or to the other ; not a single indica-
tion by means of which any and every organism might
be classified in the one or the other kingdom. Is there
then no difference between plants and animals ?

The difference is, in truth, very apparent; it is
too deeply rooted in our minds to be given up so easily.
Common sense based upon everyday experience persists
in repeating, whatever we may say, that a tree will
always be a tree and a horse a horse, that a whole abyss
lies between them.

How can we reconcile this contradiction ? Some-
times the difference is plain, sometimes it is not. The
issue is simple and the contradiction comprehensible.
It is based upon a logical fallacy, on the strength of
which man attributes real existence to abstract ideas,
the creations of his own mind. Unfortunately this
fallacy is very widespread, and has not a little thwarted
the success of natural science. As a matter of fact
there are no plants or animals as such, but a single
undivided world. Plants and animals are only aver-
ages, typical conceptions that we form for ourselves,
attributing special significance to some properties,
and neglecting, almost ignoring, the rest. These con-
ceptions, moreover, were formed at a time when only
the outstanding representatives of these groups were
known. So long as the comparison dealt with a tree
and a horse, no misunderstanding was possible ; but
the matter appeared in quite a different light when

point out that this theory has not brought forward a single argument
based upon fact. Only metaphysical, not scientific, considerations can be
adduced in its defence to-day, just as a quarter of a century ago when I
first raised the question. I would also remark that to explain com-
paratively simple phenomena of vegetable life by comparing them with
much more complicated phenomena of psychical life in animals is to leave
the track by which every science, every kind of knowledge, has advanced
until now.

284 THE LIFE OF THE PLANT

all living beings had to be taken into consideration
as a whole. Then the unity of the organic world
had to be admitted, and the realisation dawned that
all our divisions are only the production of our own
mind. I agree that the former conception was
among the greatest acquired by the human mind;
it would have been quite impossible to master without
it the chaos of individual forms. We must not lose
sight, however, of the real value of the logical method
we are using ; we must not identify types and abstract
ideas with real existences.

Granting, however, that such a dualism does not exist
in the organic world, that instead of considering plants
and animals as two absolutely different categories of
beings we are to realise them only as two typical concep-
tions, nevertheless once our mind has formed these two
conceptions we must do our best to describe them,
pointing out the peculiarities to which we give prefer-
ence, and which of them we connect with the conception
of a plant.

I do not think we can offer to-day any shorter
and more appropriate definition than that expressed in
the old saying, that ‘a plant grows, but is deprived of
voluntary movement.’ Let us try to discover a more
definite meaning for this saying. The movement of an
animal, as indeed any kind of motion, is governed by
general mechanical laws. The peculiarity of the animal
consists in the fact that the centre of the forces acting
upon it lies within itself, hence its independence of
external conditions. The source of these forces is con-
cealed in the process of oxidation. This takes place all
over the body, manifests itself in respiration, and
becomes the source of the heat and motion, which, on
the whole, characterise the animal in contradistinction to
the plant. I say ‘on the whole,’ because we have only
just found ample proof that these processes are also
met with in plants; but there they recede into the

THE PLANT AND THE ANIMAL 285

background, because completely screened by other pre-
dominant processes. We have already seen? that
under the action of light the green parts of the plant
manifest a phenomenon exactly opposite to that of
oxidation, i.e. the decomposition of carbonic acid,
accompanied by the accumulation of carbon. This
process is almost twenty times as energetic as the
respiration of plants, so that, for instance, to one pound
of carbon burning down in a plant, twenty pounds of it
are formed: the plant uses for its requirements only
one-twentieth of all the carbon deposited in it, hence
the accumulation of matter, the enormous increase in
mass, that startles us in the phenomena of growth.
Whereas, in the case of animals at the stage of full
development, a certain balance becomes established
between gain and loss of matter ; in the case of plants,
growth, z.e. accumulation of matter, takes place almost
as long as they live.2 However, this accumulation of
matter depends entirely upon the sun ; hence the utter
dependence of plants upon external conditions, and the
passivity which so sharply differentiates them from the
independent activity of animals.

It follows that the difference between plants and
animals is not qualitative, but only quantitative. The
same processes take place in both kingdoms, but some
of them predominate in the one and some in the other.
If in the end we have oxidation, waste of matter and
manifestation of energy, we have before us the type
of an animal ; if, on the other hand, we have deoxidation,
accumulation of matter, absorption of energy, we have
the type of a plant. Plants and animals have divided
labour between them. Animals use up the matter and

1 See chapter v.

* This comparison is, however, not quite accurate. It is more accurate
to consider as the individual in a plant a separate shoot which has a
limited growth, rather than the whole plant, which, like a tree for instance,

presents a complicated organism. It grows like a coral for an indefinite
length of time.

286 THE LIFE OF THE PLANT

energy stored by plants; plants in their turn derive the
energy they require from the sun. Animals depend on
plants and plants depend on the sun.

We arrive in this way at the most general conception
of the life of the plant, at the realisation of its principal
function, the part it plays in the organic world. It plays
the part of mediator between the sun and the animal
world. The plant—or rather, its most typical organ,
the chloroplast—is the link which unites the activity
of the whole organic world, all that we call life, to
the centre of energy in our solar system. Such is the
cosmical function of the plant.

When the type of a green oak rises before our imagina-
tion, rustling its luxuriant foliage in summer, bare and
frozen in winter, enduring all the fluctuations of the
external temperature; when we think of that oak year
after year, century after century increasing in organic
mass, yet always fixed to the same place, and then look
at a Russian trotter flying like an arrow, giving off in
winter clouds of vapour, and learn that it uses both in
winter and in summer quantities of hay and grain ; when
we subsequently learn that these opposite external
phenomena are only the necessary results of the chemical
processes which predominate in the one or the other case;
then the antithesis between plants and animals stands out
clearly before us. But when we venture to cast a general
glance not only upon these typical representatives, but
upon all plants and animals and upon the whole of their
functions, we are compelled to realise the inadequacy
of such an antithesis. This contradiction vanishes, and
everything becomes comprehensible the moment we ad-
mit that the stream of organic life, working its course
in the beginning along a single bed, has subsequently
divided into two branches; so that now, standing at
their mouth, we seem to see two independent currents.
It is only when we try to follow both currents along
their entire course till we rise to their common source

THE PLANT AND THE ANIMAL 287

that we come to the conclusion that they are only two
branches of one and the same potent stream of life.

With this we sum up the survey of the vital functions
of the plant. We have studied the structure of its
different organs, have learnt their significance, and have
thus solved a twofold problem, which is always facing
the physiologist : given an organ, to find its function ;
given a function, to find an organ corresponding to it.
We have observed how perfectly every single organ
fulfils its function, how well it is adapted to its environ-
ment, how indispensable is the reciprocal activity of
different organs, and how harmoniously they work
together in the general life of the plant ; how remarkable
is the co-operation of certain organisms, though belong-
ing to different kingdoms of the organic world; how
harmonious is the reciprocal activity of these two
kingdoms taken together. The study of all these facts
seems to justify us in saying we have reached the end of
our course. But it is precisely here, at this apparent
limit, that the physiologist begins to realise, though
faintly, that his subject is not exhausted, that beyond all
these particular problems there arises the most general
and universal inquiry : Why is it that all these organs,
all these beings are so perfect, so wonderfully adapted
to their environment and functions ? The more striking
the fact, the more perfect the organism, the more
haunting is the question: Why is it so perfect? By
what means has it reached this perfection? Is it
worth while to go through such a long course in order
to hear at the end the laconic answer : I do not:know,
nor understand, nor shall ever understand. It is true
that a naturalist is liable to say this, is even more
liable with sincerity to give this answer than any
other investigator; but, at the same time, he most
willingly grasps the first opportunity for an explanation,

288 THE LIFE OF THE PLANT

most jealously defends those provinces of knowledge
into which faint rays of light have succeeded in
penetrating.

The degree to which science is able to satisfy, in the
present case, the natural curiosity of the human mind,
and offer a key to what has been considered as the
essential feature of the organic world—its perfection,
harmony, or teleological finality —will be studied in
our next chapter.

1 Zweckmassigkeit of the German metaphysicians.

CHAPTER X
ORIGIN OF ORGANIC FORMS

WE came to the conclusion in our last chapter that
every thinking man, who turns his attention to the
phenomena of organic Nature, and still more so the
naturalist, who studies them more thoroughly, becomes
convinced that the organic world as a whole, as also
in its several parts, is marked by one common char-
acteristic which we try to express by the words
perfection, harmony, etc. This conviction in its turn
is succeeded by the involuntary desire, the irresistible
demand for an explanation of this most salient feature of
living beings. Formerly, on reaching this stage in his
investigations of Nature, the naturalist considered his
course had come to an end. He accepted this fact
of the perfection and harmony as a primary, an elemen-
tary phenomenon, beyond the reach of further scientific
analysis; and according to the particular turn of his mind
he either fell into silence, or gave vent to enthusiastic
effusions on this gratifying theme. But besides this
preponderating opinion some courageous pioneers in
the domain of science ever and anon gave voice to the
demand that this general characteristic of organic
forms should be treated in the same way as particular
phenomena, i.e. that, not satisfied with the simple
statement of the fact, science should try to explain it in
a more rational way, should treat it as a particular case
depending upon other more general laws; that, not
resting satisfied with the empirical knowledge that such
is the case, science should strive towards the deductive
conclusion, that such must be the case. What can
T

290 THE LIFE OF THE PLANT

these general laws ultimately be if, starting with them,
we arrive as a necessary conclusion at the startling
perfection of the organic world ? We shall devote this
our last chapter to answering this question.

Hitherto, whenever we have undertaken to explain
particular phenomena of vegetable life, we have always
tried to explain them by more general and compre-
hensive physical and chemical laws. In the majority
of cases we have succeeded more or less fully, without
even once having had recourse to the mysterious vital
force of which such lavish application was made by
earlier physiologists. We have not proved the inade-
quacy of this said vital force with its indefinite attri-
butes and intangible sphere of action; we have not
even ventured to refute its very existence; we have
simply not found room for it in our lectures—and we
have never had cause to regret it.

But now the question arises: Can this method of
explanation be applied to all the facts of vegetable life ?
Are we able, for instance, to explain by means of
physical forces alone the origin of the remarkable and
perfectly adapted forms which we studied especially
in our last two chapters ? Can we, for instance, by any
possible combination of physical forces at work at the
present moment, explain the formation of the flower of
the sage plant, so wonderfully adapted in all its details
to the co-operation of insects in the process of cross-
fertilisation so beneficial to the plant? Or can we
by the same means explain why the leaves of the
catchfly or the sundew possess all the necessary
mechanical and chemical properties for making them
perfect implements for catching and devouring insects ?
Apparently not. Evidently all these forms, or rather
their expediency, cannot in the least be accounted for
as a necessary outcome of the interaction of the sub-
stances and forces under the influence of which the
organism investigated has been formed. But if we

ORIGIN OF ORGANIC FORMS 291

cannot account for these forms by starting from the
conditions of their individual existence, can we not
find the desired explanation by some other method ?

When a historian or a politician studies the life of
a nation, and at a certain period of its existence comes
across a certain phenomenon which is not the direct result
of the morals and customs current at that particular
time, nor of the contemporary conditions of life, or when
he finds a very perfect and fully organised form of govern-
ment or society, he has recourse to historical causes in
order to explain them. Failing to find a ready answer in
the present, he looks for it back into the past. Are we
not entitled to use the same method for explaining pheno-
menain Nature ? When an organ appears extremely well
adapted to its function, when we see an organism in full
harmony with its environment, and yet feel that the
contemporary influences at work upon the individual
organism are inadequate to explain its origin, are we
not then entitled to assume that this perfection did
not arise suddenly, but has been accomplished by a
slow process of historical development, and that in this
way the adaptation has been in the long run wrought
by the same physical forces as are at work at the present
time ? Are we not entitled to assume that physical
forces which may be unable to affect so deeply a single
organism are yet able to cause a distinct change in the
course of a long series of generations ?

In order to admit such an interpretation of Nature,
we must begin by proving two propositions : firstly,
that the organic world has its history; and secondly,
that this historical process inevitably and _ infallibly
leads to perfection. If we succeed in proving the
truth of these statements, we shall obviously have
found the general key to the explanation of the per-
fection of organic forms.

Has a plant its history ? We have in passing already

292 THE LIFE OF THE PLANT

frequently answered this question in the affirmative, but
we have had no occasion as yet to consider the whole
body of proof that exists in support of such a state-
ment. This proof is given first of all by geology. We
have already seen in our first lecture that the vegeta-
tion of our planet is not the same to-day as it was
in former geological epochs, and that the more remote
is the epoch investigated the simpler are its forms.
Waterweeds appear first, then mosses, later still
horse-tails, ferns and club-mosses—all spore-plants ;
eventually seed-plants appear, and of these the conifers
come first; whereas the latest, the most complicated
and perfected in their organisation, are the dicotyledons,
which to-day predominate on our planet. Thus, in
course of time, to types already existing new types
of plants have been added which have overcome
them in point of numbers; and, moreover, the simplest
have been followed by the more complex. As we
have already seen in our first lecture, this funda-
mental geological fact can be explained by two con-
tradictory hypotheses: either the new types were
formed anew, quite independently from those that
existed before them, or they have arisen from them by
means of transformation, and therefore are directly re-
lated to them. I call both of these theories hypotheses,
and we cannot repeat it too often, because the exponents
of the first theory have applied this term with re-
markable persistence and assurance to the second
theory alone, forgetting that the one they hold is as
much a hypothesis as the other; that it is an arbitrary
interpretation and not a simple statement of fact.

Let us try to estimate the relative merit of these two
hypotheses. Let us see which of them agrees the
better with reality, explains the greater number of
facts, meets fewer contradictions—in a word, satisfies
the better the conditions we have to require of every
scientific hypothesis.

ORIGIN OF ORGANIC FORMS 293

The idea that one plant may have arisen from another,
an oak from a birch, a rose from a lily, appears so strange
at first, that the mind cannot easily grasp it. But is
it easier to realise that a cotyledon, a petal, a stamen,
a pistil have arisen from a leaf, so dissimilar to them
all? And yet when we discussed the theory of meta-
morphosis in our first chapter, we were driven to the
conclusion that all these organs so different in form,
structure and function, are nevertheless merely the
outcome of the transformation of a single organ, the
leaf. We arrived at this conclusion on the strength
of the following considerations. Firstly, on the ground
of the existence of insensible transitions: e.g. we have
seen a series of organs in the water lily, neither petals,
nor stamens, but similar to both, so that it is quite
impossible to say where the one ends and the other
begins. The second consideration in favour of the
gradual transformation of organs is based upon the
monstrosities to be found in plants, 7.e. cases in which
one organ accidentally acquires the form of another,
e.g. when the pistil of a peony assumes the shape of a
red petal with ovules at its edges. Cases are most
convincing in which such transformation is caused artifi-
cially, as for instance in double flowers, where stamens
become transformed into additional petals; as also in
experiments, where the outer scales of leaf-buds are
transformed into actual leaves. Since these considera-
tions force us to admit the possibility of the transforma-
tion of one organ into another quite different from it,
we are bound to admit more easily still the possibility
of such transition between similar organs in different
plants. Once we admit that a stamen has arisen from
a leaf, we can admit with greater assurance that the
leaf of one plant can arise from the leaf of another ;
the flower of one plant from that of another. We are
forced to do so on the strength of the very considera-
tions just brought forward, z.e. on the strength of the

294 THE LIFE OF THE PLANT

existence of transitional forms and of so-called mon-
strosities, z.e. direct transformations. Let us dwell
upon some examples.

When we were discussing flowers we had an oppor-
tunity of studying the sage, a plant used by chemists, and
remarkable for its adaptations for cross-fertilisation by
means of insects. We will try to demonstrate the existence
of gradual stages in the formation of this most interesting
flower from an apparently quite different flower, regular
and radially symmetrical in form. The sage belongs to
the family of Labiate flowers, so called on account of
their corolla having in most cases two lips. The wild
thyme, mint and other plants also belong to this class.
Botanists agree that the Boragineae are in many respects
very much like the Labiateae. We may take the forget-
me-not as a representative of the Boragineae. Everybody
knows that the blue corolla of this flower forms at its
base a short tube, and spreads out at the top, dividing
into five equal rounded lobes. If we peep into the
inside of the tube we notice five similar yellow anthers,
with their filaments attached to the tube (fig. 80).!
Can we admit that the two-lipped flower of the sage
with its two peculiar stamens could have descended
from this regular star-like flower of forget-me-not with
its five stamens ? If we succeed in demonstrating in
other representatives of the same two families a series
of intermediate forms between the extreme cases we
have selected, we shall make this supposition most
probable. To begin with, we must point out the fact
that among the class of Boragineae, all the flowers are
not as regular as the forget-me-not. In the viper’s
bugloss, for instance, the corolla already shows signs
of bilateral symmetry, 7.e. the top and the bottom have

1 Fig. 80—1 shows the corolla of the forget-me-not; 2, of the viper’s
bugloss; 3, of the figwort; 4, of the wild thyme; 5, of the sage. AU
the corollae are split along the lower lip and stretched out. The corolla
of the sage is also cut along the upper lip.

ORIGIN OF ORGANIC FORMS 295
begun to be differentiated, although not so strongly as

to form two distinct lips (fig. 80, 2) : at the same time
the five stamens are differentiated in size; the upper
one especially (fig. 80, 2 m) is considerably smaller
than the others. On the other hand not all Labiate
flowers have a distinctly two-lipped corolla; e.g. mint,
where the flowers are almost regular. Consequently,

UE SP

eZ,

the transition from a regular to a two-lipped corolla
might have occurred gradually. Let us pass:to the
stamens. The Boragineae have five, the Labiatae
four, stamens, of which the two lower are larger and
the two upper are smaller (fig. 80, 4, 2, n). What has
become of the fifth ? Whenever an organism lacks
an organ, the existence of which can be deduced by
analogy with other organisms, we generally find that one
of two changes has taken place: the organ has either
transformed itself into another organ, undergone a

296 THE LIFE OF THE PLANT

metamorphosis, or else has entirely disappeared, be-
come atrophied, and other organs developed instead.
The fact of such compensation, or correlation, in the
development of parts was observed by Goethe, to
whom, as we know, science owes its theory of meta-
morphosis. If one stamen, namely the upper one,
disappears in Labiate flowers (the very same stamen
which in the viper’s bugloss is already much smaller
than others, fig. 80, 2 m), what arises in its place? We
notice that the disappearance of this stamen coincides
with the strong development of the upper lip, and
hence we may suppose that the stamen has trans-
formed itself into the petaloid organ, which joins
the two upper petals and forms the upper lip. This
need not puzzle us, because the transformation of the
stamen into a petal and the fusion of parts in a flower
is a very common phenomenon. This can also be
proved by the fact that in mint, where the bilabiate
corolla is very feebly developed, a fifth stamen is not
seldom preserved. An example taken from among
other plants will make this inference still more con-
clusive. Two other families, the so-called Solanaceae
(e.g. potato) and the Scrophulariaceae (e.g. red-rattle,
foxglove, etc.,) are correlated in exactly the same way
as the Boragineae and Labiatae. The Solanaceae have
a regular flower and five stamens; the Scrophulariaceae
have a two-lipped flower and four stamens. How-
ever here, and especially in the figwort, we become
quite convinced that the uppermost fifth stamen has
been transformed into a petal, and has fused with the
two upper petals (fig. 80, 3 m).

We can, therefore, explain how the regular flower of
the forget-me-not with five stamens could gradually
have become transformed into a two-lipped flower with
only four stamens, characteristic of the majority of the
Labiatae. The sage has only two stamens. Let us
follow the fate of the other two. By examining the

ORIGIN OF ORGANIC FORMS 297

tube of the corolla of this plant, splitting it longi-
tudinally, we find, a little above the two developed
stamens—exactly at the spots occupied by smaller
stamens in other Labiatae—two scarcely visible atro-
phied stamens (fig. 69, fig. 80, 5 ”, 2). According to
Goethe’s theory, mentioned earlier, the remaining two
stamens have attained their greater size at the expense
of the undeveloped ones, and have thus acquired the
peculiar structure already familiar to us (fig. 69). This
peculiarity in the form of the two stamens presents in
its turn different degrees of complication in different
species of sage, which proves that it did not arise
suddenly but by a series of gradual changes. The
description of these transitional forms would, however,
require too much time as well as too many diagrams !
to be dwelt upon here. We might have made clear
by exactly similar arguments how another still more
curious flower—the orchid—could have arisen from
a regular flower such as the lily. Morphology or
the ‘Comparative Anatomy’ of plants is full of such
examples ; we may say it entirely consists of them.
Thus, if the theory of metamorphosis explains how,
by means of a range of insensible transitions, different
organs of one and the same plant have been derived
from each other, the anatomical study of similar organs
in different plants brings us to the similar conclusion,
that one vegetable form could have been derived from

™ We came to the conclusion in discussing the flower, that the whole
function of this complicated staminal apparatus consists in the promotion
of cross-fertilisation by means of insects. This cross-fertilisation would
evidently be more perfectly achieved were the flowers differentiated in
sex, 7.¢. if some flowers contained pistils and others stamens. In this case
this complicated and gradually developed staminal apparatus would
become useless, and in fact in other species of sage, e.g. in the field sage,
in addition to the hermaphrodite flowers female flowers are also found; in
these we are able to observe the way this curious and now useless
apparatus gradually became atrophied; how it has repassed in reverse
order through almost all the phases, which it must have passed through,
during its evolution.

298 THE LIFE OF THE PLANT

another ; otherwise, what other meaning can we attri-
bute to these rudimentary or rather degenerate organs
which meet us at every step, and demonstrate a gradual
transition between dissimilar forms ?

The study of organisms in the embryonic stage
proves this theory still more conclusively. All the
data of embryology testify that similarities and homo-
logies, which escape attention in fully developed
organisms, become comprehensible when the history
of their development is studied. Thus, for instance,
there is no wider or more fundamental difference in
the vegetable kingdom than that between spore-plants
and seed-plants; an impassable abyss seems to lie
between these two sub-kingdoms, and yet Hofmeister
managed to bridge even this gulf. The study of the
history of the development of the higher spore-plants
and of the lowest seed-plants has revealed the exist-
ence of a connexion between these groups, and has
even shown the course which this transition must have
taken. We have already seen that the most typical
spore-plants, such as ferns, are fertilised by means of
antherozoids, whereas seed-plants are fertilised by
means of pollen-tubes. Hofmeister predicted, on the
ground of his far-reaching investigations, that inside
the pollen-tubes of certain flowering plants anthero-
zoids would necessarily be discovered, and twenty years
after his death this prediction was actually fulfilled.
The exact sciences, such as astronomy, physics, and
chemistry, pride themselves upon such predictions.
Hofmeister’s prediction is the most brilliant in the
domain of morphology.t_| The Gymnosperms, to which
our Conifers belong, form a link between the two sub-
kingdoms of the vegetable world. It is in some members
of this group that antherozoids have been discovered.

1 It is very strange that Prof. Borodine has found it possible not even
to mention the name of the great scientist in his detailed work upon this
subject (The Process of Fertilisation in the Vegetable Kingdom).

ORIGIN OF ORGANIC FORMS 299

But we have already seen that, quite apart from this
deduction concerning the history of development,
geology had already demonstrated that such was the
chronological sequence in the appearance of these
groups upon our planet. Let us recall one of the
results of the preceding chapter, bearing on the im-
possibility of establishing any physiological border line
between plants and animals—the fact that the origin
of all organisms, the cell, or rather simply a speck of
protoplasm, is alike in all living beings, and we shall
inevitably admit the unity of the organic world, the
relationship, the immediate connexion between all that
lives on the earth.

It may seem strange, even incomprehensible, that this
conclusion could have met with opposition in the face of
such concordant and diverse evidence from all the
departments of biology. We do still meet such opposi-
tion even to-day.

In order to explain the origin of the divergence of view
among scientists, we must dwell for a short time upon
certain technical and perhaps tedious details, without
which, however, the reason for this controversy will
remain incomprehensible. An investigator of the organic
world very soon arrives at the conclusion that organisms
present different degrees of mutual affinity, as it is
generally called. In order to express the degrees of
affinity in the systematic description of organisms, these
are generally associated in groups, and the groups
marked by terms which indicate the degree of relation-
ship that exists between them; such are the terms
family, genus, etc. The smallest group in which
the forms are most similar to each other, the group
which represents the collective unit, so to speak,

1A recent discovery of a distinguished English botanist, Professor
D. H. Scott, has proved, from the palaeontological point of view as well,
the connexion between ferns and the Gymnosperms in which the
antherozoids were found—a new triumph for Hofmeister.

300 THE LIFE OF THE PLANT

out of which other groups are formed, was called by
Linneus a species. Out of species the genus is formed,
out of genera families, and so on. £.g. a violet and a
heart’s-ease represent, according to Linnzus, two species
of the genus Viola; two poplars—the black and the
white—are two species of the genus Populus ; a donkey
and a horse fall into the same genus Equus, the wolf and
the dog into the genus Canis,andsoon. The determina-
tion of the groups called species marked a great advance
in science: it rendered possible the strictly systematic
classification of organisms. However, after having
established this collective unit of their system, this
group the species, the systematists, not so much Linnzus
himself as his followers, declared that a species is some-
thing actually stable, invariable in space or time; that
species have always been and will ever be what they are
at present ; that the transformation of one species into
another is out of the question altogether, and hence that
the theory of the common origin of all organisms is quite
inadmissible. So far we have been discussing data which
have been gathered by the theory of metamorphosis,
by comparative morphology, embryology, and palaeon-
tology, and which testify to the possibility of the transi-
tion of forms of one family into those of another (e.g. from
Boragineae into Labiatae), the possibility of transition
from a spore to a seed-plant, the impossibility of finding
any line of demarcation between the vegetable and the
animal world, etc. But what can be the significance of
all these facts if it is true that no transition is possible
in the case of beings most closely related, in the case of
species of the same genus? If the violet and the
heart’s-ease have always been so different from each
other, if they are unable to vary, if species are immutable,
then certainly all our considerations as to transition
from one family to another, from one order to another in
the vegetable kingdom, as also from one kindgom to
another, are futile. Hence it is clear that the problem

ORIGIN OF ORGANIC FORMS 301

of the common origin of organic beings (and therefore,
as we have already seen, of the wider problem still, of
the cause of their perfection) is bound up with the
problem of the variability, or, speaking more generally,
of the origin of species. That is how Darwin’s book,
which caused a revolution in science, came to bear such
a dry and technical title.

Is it really true, as was steadily affirmed by a majority
of naturalists, that species are variable? In the first
place we know that no two exactly similar forms exist in
nature: plants grown from seeds of one and the same fruit
vary one from another. Hence entire similarity is out
of the question, and, as a matter of fact, no one has ever
suggested it. Moreover, we know perfectly well that
even within the limits of a species there may be narrower
groups of beings still more closely resembling one another.
Thus the species, serving as a unit for groups of a higher
order, in its turn breaks up into units of a lower order.
Everybody knows how different are the races of dogs,
how diverse the kinds of wheat, how numerous the
variety of flowers appearing year by year in gardeners’
catalogues. Where is the famous immutability of
species ? The exponents of this theory have a ready
answer. They say: ‘Within the limits of species
variation may certainly occur, but the range of these
variations is limited ; the degree of difference between
the varieties can never be as great as that between
species’: in other words, new species cannot be formed
in the same way as new varieties. Therefore the
question as to the immutability of species resolves
itself into the question of limitation in variation, or
rather into the significance of variations. After such a
categorical affirmation of the difference between a
species and a variety, those who uphold the immutability
of species might be expected to give an exact defini-
tion of both terms, and to provide a criterion whereby
we may know when we are dealing with two varieties

302 THE LIFE OF THE PLANT

of a single species, and when with two independent
species. Not at all: they not only give nothing of
the kind, but neither can they do so, because species as
well as varieties defy definition. We shall soon see
why, starting with the a priori conviction that one species
cannot arise from another, those who maintain their
immutability generally follow a rule which says that
two forms, connected by transitional forms, however
unlike they may themselves be, cannot be acknow-
ledged as independent species. Guided by the same
rule, however, the opponents of the theory point to
species between which transitional forms have been
discovered. The others answer: ‘ This means that we
have been mistaken, that we have taken for a species
something which in the main is a variation.’ Obviously
arguing in circles like this keeps up appearances. Affirm-
ing that species are invariable, they may easily assign the
term variety to every variable form, in the absence of any
positive sign which would differentiate a species from a
variety. There was a time, however, when those who
maintained the immutability of species did cherish the
hope of establishing the difference between species and
varieties upon some positive physiological feature. A
conviction arose that all the representatives of a species,
all its variations, however different they may be amongst
themselves, can intercross, producing hybrids capable of
further reproduction ; and that, on the contrary, species
are unable to produce hybrids, or, if they happen to do
so, the hybrids are bound to be sterile. It has even
been presumed that Nature has of set purpose made
species invariable. After having produced a certain
number of specific forms Nature has taken the trouble
to preserve them for ever invariable, by preventing
them from the possibility of varying as the result of
their forming hybrids. When we discussed fertilisation,
we stated facts which definitely contradicted this theory.
We have seen that fertilisation by means of the pollen

ORIGIN OF ORGANIC FORMS 303

of the same or a similar plant is sometimes less produc-
tive than that by means of pollen of another different
plant, and even that fertilisation with the pollen of
another species is sometimes more productive than that
with the pollen of the same species. Realising that it is
impossible to advance any positive criterion for differ-
entiating species from variations, the exponents of this
theory love to plead ‘intuition’ as a guide to investigators
in the solution of this problem. The precarious nature,
however, of this famous adjunct intuition is well proved
by the following instances. It so happens that as long
as a genus contains but few species botanists agree as to
their number ; but as soon as a genus contains more
than, say, four species they begin to disagree. How
far this disagreement may carry them is shown by
the following example: according to some botanists
the genus Hieracium contains twenty species, according
to others three hundred. The same disagreement holds
with regard to the blackberry, willows, and many other
plants. Evidently some botanists take for a species
what for others is only a variety. In view of these
contradictions, systematists have coined the expression
good species to differentiate the evident and universally
acknowledged species from doubtful ones. These mani-
fold contradictions unmistakably bring us to the con-
clusion that it is impossible to draw a hard and fast line
between a species and a variety, that it is impossible
always and infallibly to apply to reality conceptions of
this kind. This inference necessarily raises the question
whether some logical fallacy has not slipped into the
argument, as was the case when we discussed the
difference between plants and animals. Perhaps neither
species nor variety exist in Nature as two qualitatively
different categories. Perhaps they are but typical
conceptions, creations of our own mind. Let us try
to make this clear by an example. We clearly realise
the difference between a child and a grown-up man,

304 THE LIFE OF THE PLANT

Moreover, we differentiate between babies and adults,
youths, mature and old men ; and these conceptions are
entirely in accordance with reality, otherwise they
would not have arisen in our mind. It does not, how-
ever, follow that they must be applied without exception
to all cases. Nobody would ever think of affirming
that questions must or can be decided, in any and every
case, such as: Have we before us an adult or a youth,
a grown-up person or an old man ? and soon; and yet
it is with such problems that systematists struggle
when they have to decide whether a doubtful species
is to be considered a species or a variety. Species and
variations are clearly differentiated in most cases;
but it does not follow that they should be two categories
essentially different ; on the contrary, the difference
between them is entirely one of quantity. They are
two quantities passing gradually into one another:
at one extreme we have slight individual variations,
succeeded by sub-varieties, then obvious varieties,
doubtful species, and, lastly, good species.1. In a word,
the only logical solution to this problem of species
and varieties, so full of contradictions, consists in the
acceptance of Darwin’s formulae: ‘A variety is an in-
cipient species’ ; ‘a species is a strongly-marked variety’ :
just as a child is an undeveloped man, and a grown-up
man is a developed child, nor can any line of demarcation
be drawn where the child ends and the man begins. Let
us carry our comparison further. Supposing that a being
of some kind with a very short period of existence (a may-
fly, for instance) were to raise the question whether a
grown-up man develops from a child, or whether they
are both quite independent beings. It would be
impossible for our imaginary being to see this trans-
formation because of the shortness of its own existence ;

1 In fact, while systematists were able formerly to end their classifica-

tion with species, to-day four subdivisions more are admitted within the
limits of the species.

ORIGIN OF ORGANIC FORMS 305

yet by observing a whole series of slightly varying
transitional forms between a suckling and an old man,
by observing that all these beings vary before his eyes
though to a very small degree, and vary in one direction
only, z.e. become older, our imaginary being would come
to the conclusion that the child he sees before him will
in time become an old man, and that just as truly the
old man has also been a child in his time. Let us also
suppose that another similar imaginary being should
criticise this conclusion by saying: ‘I maintain that a
grown-up man has never been a child, and will maintain
this statement until I see the transformation take place
before my very eyes, which, as a matter of fact, could
never happen.’ Tell me, pray, who is in the right ? Is
it the one who reduces the whole of his experience to
terms of a strictly logical inference, or is it the other who
obstinately indulges in a kind of pseudo-philosophical
scepticism, repudiating both the testimony of experience
and the requirements of logic? Yet this is exactly
the position of the two opposite camps with regard
to the question of species. Not only the life of a
single man, but even many generations are as nothing
when compared with the period of time necessary
for the formation of a new species; yet scientists
who repudiate the immutability of species, seeing the
variability of organic beings and taking into con-
sideration the impossibility of establishing a difference
between a species and a variety, inevitably come to the
conclusion that species have arisen from varieties ; that
varieties are only consecutive steps towards the for-
mation of new species.

However conclusive this method of proof may be,
there is no doubt that the actual observation of the
formation of new species would have been more
conclusive still. If species do vary, may they not
within the confines of history have varied so much
as to give rise to new species ? This cannot possibly

U

306 THE LIFE OF THE PLANT

be proved with regard to organisms in their natural
conditions, since we cannot keep them under observation
for a sufficiently long period of time. The solution of
this problem, though still presenting great difficulties,
is becoming a little simpler in relation to cultivated
plants and domesticated animals, concerning which his-
torical data do exist. Our greatest difficulty is to prove
that different races of animals or plants, sharply differen-
tiated one from the other, do actually descend from
one and the same species. Darwin managed to prove
it fully in some cases, particularly with regard to the
breeds of pigeons. He proved that contemporary breeds
vary so much that had they been discovered in their
natural environment they would not only have been
classified as different species but even in different genera,
and yet they did undoubtedly arise from one and the
same pigeon.

Those who maintain the hypothesis of the immuta-
bility of species have one more argument in reserve, and
also a very important one. They say: Granted that
species descend one from another, how can we account
for the absence of all the minute transitional forms which
must have existed ? Why have they disappeared ? Why
is it that species generally represent groups of beings
entirely distinct from each other? In passing let us
remember that the very fact of the absence of transi-
tional forms between species is in many cases doubtful,
because, as we have already seen, whenever such
transition is manifested between two species, the
exponents of the immutability of species deny them
specific rank. However, this argument holds good in
the great majority of cases: really good species, as a
matter of fact, are not connected with each other by
transitional forms, and this fact was the main obstacle
to all the early endeavours to prove the origin of
species by means of variation; they all collapsed in
face of this crushing argument. As we proceed to study

ORIGIN OF ORGANIC FORMS 307

the very essentials of the conception of the origin of
species by variation, we shall see in what relation this
argument stands to Darwin’s theory. At present we
need only note the fact that this theory has employed
in its own defence the very weapon of its adversaries ;
it entirely accounts for the absence of transitional forms.
Indeed their very existence would have served as a
strong argument against it. This is one of its essential
points, the reason for its ascendency over other
attempts of the same kind.

So far we have brought forward arguments and done
our best to refute those who have contradicted the fact
that the organic world has a history. We have tried to
prove by means of concordant evidence from all depart-
ments of biology that the affinity of organisms, admitted
by all naturalists without exception, can only be explained
by their close relationship. Hence organisms have a
genealogy, a history. Turning to the second half of
our problem, we must now prove that this historical
process necessarily leads to the perfection of organisms,
implying by perfection the adaptation of the organ to
its function, of the organism to its environment. Observ-
ing that the organic world presents beings in all stages
from the simplest to the more perfect, and realising that
this perfection corresponds with the chronological
order in the appearance of these beings upon the earth,
many naturalists have even seen in this fact a proof
that the organic world is endowed with a tendency to-
wards perfection, and that this property needs no further
explanation. Others again have tried to give a more
rational explanation of the fact, though with ill success
for the most part. Darwin was the first to point out
the immediate causes, very general laws of Nature,
which result in the progressive development, the
evolution, of the organic world. He used for this purpose

308 THE LIFE OF THE PLANT

a method which appears paradoxical at first sight, and
the logical significance of which is even yet misunder-
stood ; or, rather, will never be understood by many of
its opponents, whatever explanations may be offered for
it. In order to find out how it is that by means of
historical development the organic world has reached
the degree of perfection we observe in it, Darwin started
by inquiring how man reaches the same end, how he
improves his artificial breeds of plants and animals—and
he came to the conclusion that the main factor in the
accomplishment of this end is selection, consisting, as
we have already seen,! in the selection from every
generation of only those organisms which correspond
most closely to the ends in view. In its simplest and
most perfect form selection consists in the extermination
of all unfit individuals. For instance, when a gardener
wishes to produce or to preserve a certain variety in a
plant, he simply exterminates all the plants which do
not correspond to his ideal.

Darwin next raises another question: Does not
Nature also advance towards perfection by means of
a similar selection ? One has scarcely time to word this
question before the opponents of the theory raise their
voices in premature triumph and make exclamations
such as,‘ Can there be anything in common between a
process directed by the rational will of man and the
action of the blind forces of Nature ? You undertake
to explain the origin of organic forms by physical laws,
and yet you start by personifying Nature, by endowing
it with rational activity, with a capacity for selection.’
Unmoved by exclamations of that kind, which are mere
words, let us study facts in order to understand the great
man’s idea. First of all Darwin dwells on cases of what
he calls unconscious selection. In. years of famine
savages are obliged to kill some of their domestic
animals. Asa matter of course they preserve the best,

1 See chapter viii.

ORIGIN OF ORGANIC FORMS 309

hence quite unconsciously improve the race. They
do this, indeed, against their will, for had they the
choice they would quite willingly preserve even the less
satisfactory ones. By selecting individual animals they
improve the race in course of time, and yet in respect
of the result attained they work as a blind, unconscious
elementary force. :

Can we then admit unconscious selection in Nature ?
In order to make the question less startling-let us put
it a little differently, and look at it in another light.
We have seen that in its simplest form the process of
selection reduces itself to the extermination of un-
satisfactory forms. Therefore we can substitute this
question by another: Do the unsatisfactory forms
become exterminated in Nature ? Such a process of
extermination would be equivalent to improvement, a
perfecting process. Science answers this question with a
most decided affirmative: yes, it says, they do become
exterminated in Nature to a very large extent, and with
inexorable strictness. This phenomenon is based upon
a property common to all organic beings. This property
consists in the fact that in the reproduction of
organisms is always involved their multiplication.
This fact is so universal, so constant, that the two
expressions are often interchangeable, multiplication
being used as a synonym for reproduction. In fact we
cannot name a single organism which would normally
produce only one single being during its life-time. On
the contrary multiplication generally takes place in
ever-increasing geometrical progression. This fact has
very important consequences, the significance of which
was first indicated by Darwin. We shall only fully
realise the rapidity with which organic beings reproduce
themselves if we take the trouble to calculate the
whole posterity of a single organism in a given number
of years. Thus, for instance, if all the posterity of a
single dandelion were preserved during ten or twelve

310 THE LIFE OF THE PLANT

years it would cover all the terra firma of our globe.
Yet the dandelion is not particularly productive.
According to Darwin our commonest orchid, the Spotted
Orchis, produces no fewer than 180,000 seeds a year,
so that even the grandchildren of a single plant would
cover the earth with a close green carpet. Nor is this
the limit to productiveness. There are orchids the seeds
of which are counted by millions. Let us, too, recall
the spores—the invisible grains of dust—formed on the
under side of fern leaves ; each of these is able to produce
a new plant.

What is the natural result of this enormous multiplica-
tion of organisms without exception, this tendency of
every one of them to occupy the whole earth ? It is
obvious : the majority of these organisms perish. We
may even say that the proportion which survives is
insignificant as compared with that doomed to perish.
A hard struggle sets in for the representatives of every
new generation, issuing in the arrival of a very small
number of victors. What determines the survival of
these selected forms ? What circumstances decide the
result of the competition in their favour ? Obviously
their own superiority, the perfection of their organisa-
tion—implying by perfection, as has been already said,
the adaptation of the organ to its function, of the organ-
ism toitsenvironment. In the majority of cases we are
not even able to realise wherein that superior adaptation
lies, because the advantage in the struggle for existence
may depend on a variety of properties, sometimes even
the opposite of one another. In one case the survival of
the plant is due to the fact that it germinated before its
fellows, appeared before them at the banquet of life,
and had time to seize a place in the universe ; in another
case, on the contrary, the selected, 7.e. the surviving, plant
will be the one which has germinated later than its
fellows, and thus has happened to be preserved from
late frosts which kill its too hasty rivals. The struggle

ORIGIN OF ORGANIC FORMS 311

for existence and its necessary outcome, the survival of
the fittest, or, as Darwin metaphorically puts it, natural
selection, is the necessary logical outcome of the law
of the rapid multiplication of organic forms.) Such
deductive proofs are not the only ones that can be
brought forward in support of the fact of the struggle
for existence and selection; immediate observation
brings us to the same inference. We have only to take
a mixture of flower seeds, say of sweet peas of different
colour, and gather all the seeds each year and sow them
again on the same bed, to find in a few years’ time that
some colours will oust the others from the bed. This
means that even such an insignificant character as colour
(most likely some property correlated with it, which
escapes our immediate observation) can decide the
victory in the struggle for existence. The same result is
observed in experiments in manuring natural meadows.
We have seen that nitrogenous manures and mineral salts
containing phosphoric acid and potassium constitute
beyond doubt useful and indispensable food for every
plant. But if we manure a natural meadow containing
a certain percentage of grasses and a certain percentage
of leguminous plants, we notice that when we use
exclusively nitrogenous manures the cereals get the
upper hand of the leguminous plants. On the other
hand, by using manures without nitrogen the advantage
is all on the side of the leguminous plants. Both
manures are useful for both kinds of plants, although
to a different degree, and according to that difference
the success in the struggle for existence falls to the one
or to the other. Lastly, as has been rightly observed,

1 The metaphorical use of the word selection has led many critics
astray (as has been already mentioned) : they said that the very expression
‘selection ’ points to the fact that Darwin was obliged to ascribe conscious
activity to Nature. If formerly it was only slow people who could be
led into this error, to-day, after the explanation given by Darwin, it
is only people unscrupulous in their choice of methods of argument who
can have recourse to such quibbling.

312 THE LIFE OF THE PLANT

we have but to recall the pains taken by a farmer to
save his fields from being overrun with weeds, to realise
the struggle our cultivated plants have to maintain, and
how easily they would perish if left to themselves.
Hence the fact of the struggle for existence, as the out-
come of the law of geometrical progression in the
multiplication of organic beings, obvious as any mathe-
matical truth, is proved by direct experiment. This
struggle in the same logically inevitable way leads to
natural selection, z.e. to more and more perfect adapta-
tion, although it may be liable to escape our notice
in any single generation. If we take the testimony
of geology into account as to the almost immeasurable
space of time that has elapsed since organisms appeared
upon the earth, we shall readily agree that the process
of selection, acting with such inexorable severity and
during such a lengthy period of time, can fully account
for both the variety of organic forms and the perfection
of their adaptation.

Thus the evolution of organic forms and their infallible
trend towards perfection may be considered as a neces-
sary logical outcome of the three fundamental properties
of organisms. These are the capacity for variation,
the capacity for transmitting variations to posterity,
z.e. heredity, and the capacity for multiplication, which
is invariably connected with reproduction.

The capacity of organisms for variation is indisput-
able. We do not know of any two absolutely similar
beings. Yet the causes of variation, and the relation
which variation bears to selection, need some further
explanation. The primary cause which produces
changes in an organism must lie in the indirect or
immediate influence of their external conditions; and
then comes the influence of secondary causes, such as
correlation in the development of parts, the exercise of
organs, and so on. In most cases it is, however, very
difficult to discover the connexion between a variation

ORIGIN OF ORGANIC FORMS 313

and its cause. We then call the variation accidental,
though science cannot admit accidental phenomena in
the literal sense of the word. We call it accidental
in so far as its primary cause is concealed from us.
The difficulties we meet in trying to unravel the con-
nexion between a variation and its cause depend mainly
upon two circumstances : in the first place it is already
rather late to look for a cause when the variation has
already manifested itself: secondly, external influences
very seldom produce a lasting effect upon a fully
developed organism; this probably happens more
frequently in the case of organisms in the embryonic
stage of their development, for it is obvious that the
earlier the influence is exerted the more far-reaching its
consequences. The influence of any disturbing factor is
generally very strong in the earliest stages of develop-
ment. This can be proved, for instance, by the fact that
it is impossible to propagate some sports otherwise than
by asexual reproduction, because the influence of the
second parent in the process of sexual reproduction is
strong enough to affect the whole organism to such an
extent as to interfere with the transmission of the
desired character. One of the secondary causes of
variation must be looked for in the action of a law called
the correlation of growth, which consists for instance in the
excessive development of one part being accompanied by
the under-development of another. Having at a given
time only a certain quantity of nutrient substances to
dispose of, Nature, as Goethe says, ‘in order to be lavish
in one direction is forced to economise in another.’
Lastly, once formed, the organ is apparently able to
go on developing precisely in so far as it is being
used.?

Variations when occurring under the influence of
physical conditions may equally well be useful or harm-

1 It has not yet been explained what variations obtained as the result
of exercise are inherited, and which are not.

314 THE LIFE OF THE PLANT

ful to the organism ; it is only the struggle for existence
and selection which control variation in a definite way
by destroying harmful deviations and preserving useful
ones ; so that scarcely perceptible variations, when
accumulated during a long series of generations, in the
end are considerably accentuated. Let us try to explain
by means of an illustration what part of the phenomenon
may belong to variation as such, and what part to the
subsequent action of selection. A while back we tried
to explain the course by which a symmetrical flower is
derived from a regular one. We pointed out a whole
series of transitional forms, but of course this does not
explain the primary origin of bilateral symmetry, the
primary deviation from regularity. We may conjecture
with a certain degree of probability that this transfor-
mation has taken place under the influence of the force
of gravity acting upon the branches as they developed.
We have seen } that growing organs change the direction
of their growth under the influence of the force of
gravity, and this change of direction depends upon
irregularity of growth in the upper and lower parts of
an organ. The same effect is also manifested in
another way: horizontal branches become unlike on
their upper and lower sides; the leaves, uniformly
distributed on all sides of the erect main stem,
in horizontal branches distribute themselves in one
horizontal plane, and so on.

A considerable number of similar facts concerning
flowers apparently justify this opinion. Flowers of one
and the same plant have been observed to be regular or
slightly symmetrical, according to the place they occupy
on the flower axis. Thus, for instance, plants with
regular flowers, having lateral flowers either in an
almost horizontal or in a drooping position, often
acquire a slightly bilateral form; while the apical
flowers of the same cluster, or erect flowers, preserve an

1 See chapter vii.

ORIGIN OF ORGANIC FORMS 315

entirely regular form. This phenomenon is observed
in campanulas and other plants. On the other hand,
in plants with entirely symmetrical flowers, such as
those of the Labiateae, Orchidaceae, and other families,
the apical flower often acquires a regular form. This
is the case in the Sage. All the lateral flowers have a
characteristic two-lipped form, while the apical flower
is sometimes quite regular. Recent experiments have
adduced direct evidence in support of this suggestion.1
On removing the effect of the earth’s attraction by
means described in chapter vii., it was found possible to
transform bilaterally symmetrical flowers artificially into
regular, radially symmetrical flowers. Thus the first
appearance of longitudinal symmetry in a flower may be
ascribed to the action of the force of gravity, the rest will
follow as the effect of selection. Since it is beyond doubt
on the one hand that cross-fertilisation effected by insects
is useful to plants and gives rise to a more vigorous and
healthy generation, and on the other hand that for
insects, which visit the flowers for their honey, the lower
lip of the flower presents a convenient platform, it is
clear that in every generation amid plants struggling
for existence those which possess this bilabiate form
more sharply differentiated have more chance of survival.
Similarly, partly owing to the force of gravity and
partly on the strength of the law we have just mentioned
of correlation of growth, first one, then all the three
upper stamens become atrophied, while the two lower
ones increase in size, and under the influence of selection
develop their peculiar shape, which is that most useful
to the plant. We gather from this example that in order
to explain the origin of a given form, be it even a very
complicated one, we have but to settle the following
three points: that the original variation might have
arisen under the influence of physical forces (acting upon
the organism generally in its embryonic stage, and only

1 H. Spencer was the first who advanced it.

316 THE LIFE OF THE PLANT

seldom when it is fully developed), that there exists a
series of transitional forms, and—the most important
of the three—that this transformation is useful to the
organism itself. Then it becomes obvious that under
the influence of natural selection such a form not only
might, but necessarily must, have arisen.

Hence the explanation of the harmony, of the per-
fection of the organic world, suggested by Darwin
does not turn out to involve any a priori endow-
ment of the plant with a tendency towards perfec-
tion, with any inborn progressive activity. On the
contrary, according to that theory, variations as
such may equally well be useful or harmful. But
owing to selection every harmful variation, precisely
because of its harmfulness, is doomed to extermination
sooner or later, whereas every useful adaptation is
transmitted to future generations. The general pro-
gression, the drift towards perfection, is effected by
exterminating everything that is harmful, and by
accumulating slowly and gradually useful properties.
Thus the perfection of the organic world no longer
appears in itself as an incomprehensible end, but as an
eminently conceivable result of authentic natural causes
well known to everybody,

Curiously enough another great thinker came to a
similar conclusion before Darwin ; with the difference,
however, that according to the current ideas of his time he
could not admit the theory of the mutability of species.
Auguste Comte writes as follows in the third volume of
his Positive Philosophy : ‘ Without doubt every organ-
ism finds itself necessarily related to a certain combina-
tion of external conditions. It does not at all follow,
however, that the former of these two correlated forces
has been produced by, any more than it could have
itself produced, the other. We have simply to deal with
two forces in a state of equilibrium, totally independent
of each other as also essentially different. If we imagine

ORIGIN OF ORGANIC FORMS ai

all sorts of organisms to be consecutively exposed
to the action of all sorts of external conditions in the
course of a sufficiently long lapse of time, we shall
clearly see that the great majority of these organisms
ought necessarily to disappear; only those that
satisfy the fundamental law of equilibrium mentioned
above should survive. In all probability such a system
of elimination has established the biological harmony,
which we observe on our planet, and which goes on
changing before our eyes.’ The similarity of both
opinions consists in that biological harmony, for both
Comte and Darwin, is the result of the elimination of all
that is inharmonious, and inconsistent with the funda-
mental law of equilibrium between the organism and its
environment. Comte does not indicate the mode of this
elimination of unsatisfactory organisms, nor the reason
of its inevitable and fatal necessity ; and besides, for him,
adherent as he was of the immutability of species, this
harmony ought to have appeared as something stable,
something that had already attained its end; while for
Darwin, the advocate of the unlimited variation of
organic forms, this equilibrium is unstable, a harmony
ever progressing and never reaching its end. Now, if
this harmony is unstable it cannot be absolute ; and this
is altogether consistent with reality. We never meet
with absolute perfection in Nature. The eye is rightly
considered the most perfect of organs, and yet it is of
the eye that Helmholtz, the greatest authority in his
subject, and at the same time an enemy of all idle talk,
said that had he received from an optician an apparatus
with similar defects he would have sent it back to be
repaired !

We see, therefore, that Darwin’s theory explains the
reason of the perfection of organisms by starting from
fundamental properties of bodies well known to every
one, and without having recourse to arbitrary premises.
Upon this rests its superiority to all former attempts of

318 THE LIFE OF THE PLANT

the kind. Its other great advantage consists in
turning to its own account one of the strongest
objections which former supporters of the theory of
the variability of organic forms were unable to over-
come. This objection consists in the absence of transi-
tional forms between really good species. In fact, if
species are related to each other there must exist links
between them, some transitional forms. To this Dar-
win’s theory says : such forms must indeed have existed,
but they have disappeared in course of time, and their
disappearance is one of the necessary results of the
struggle for existence and of selection. Before entering
into this question let us observe that an entirely false
representation is very often made of the transitional
form which connects two other forms. Such a form is
generally considered to be the middle form in the literal
sense of the word, a form which contains in itself the
attributes of both the forms which it links together ;
whereas in reality it may be almost entirely lacking in
the characteristic attributes of either of them. Very
often objections of the following kind are raised: if
the birch and the oak are related to each other show us
an organism that would be half one and half the other.
In all probability such an organism has never existed
in Nature. Organisms that exist to-day are related
to each other not because they may have originated the
one from the other, but because they proceed from
common ancestors ; and very probably had we found the
real link between two contemporary forms, 7.e. the form
of the ancestor upon which their relationship is founded,
we should not have recognised it as such, because it
would have presented in very slight degree, if indeed at
all, the characteristic attributes of its two descendants.
Let us take an example from among cultivated plants.
The cabbage, for instance, is distinguished by remark-
able variety in the development of its organs ; in some
kinds of cabbage the head is composed of thickened

ORIGIN OF ORGANIC FORMS 319

leaves, in others the stem presents a turnip-like swelling,
in others the inflorescences turn into fleshy organs well
known to everybody. Ina fourth kind the stem grows
tall, and hardens, so that sticks may be made out of it;
in a fifth the leaves acquire a bright colour, and so on.
Obviously no one would ever suppose that the progenitor
of all these forms, and hence also the transitional form,
which serves as the link between them, possessed all
these characters. As a matter of fact the wild original
form of cabbage does not present a single one of these
extreme characters. It is therefore very possible that
in some cases the real connecting form should escape
our attention. But it is beyond doubt, nevertheless,
that in the majority of cases transitional forms between
species no longer exist, and, as has just been observed,
according to Darwin’s theory this very absence of them
is one of the results of natural selection. For the sake
of clearness we shall again refer to a comparison with
artificial selection. When different varieties of cabbage
arose, people who cultivated them evidently began to
care for the most rare and extreme representatives of
the several kinds. The specialist in the cauliflower did
not worry about stems or leaves ; what he cared for was
only the inflorescence, which he desired to be large and
fleshy. A specialist in the decorative kind of cabbage
cared only about the colouring and the form of the
leaves ; one and the same plant could not produce at
the same time both a head and a walking-stick, and so
on. It is clear that plants which did not show an
extreme development of some special character, but
combined many of them less sharply defined, were no
longer cultivated, were even destroyed, and hence were
bound to disappear. Consequently the appearance
of more clearly defined representatives was inevitably
followed by the extermination of the less clearly defined,
as a result of which the connexion between extreme forms
would break down and a series of variations disconnected

320 THE LIFE OF THE PLANT

with each other would come into existence. Something
of this kind was bound to take place in the natural order
of things, where any new form can prevail only if it is
more perfect than others ; in such a case it is evidently
bound to thrust out, to extinguish the others. Darwin
also points out that it is to the advantage of every being
to become as different as possible from its own kind, be-
cause the less alike are the needs of two forms the less will
be the struggle between them, the greater the possibility
of their adaptation to the same environment, without
any struggle whatever. Farmers have long known
that the same plant cannot be grown for any length of
time on the same field, that it is necessary to change it
from year to year to another place; partly upon this
fact the rotation of crops is based. But what is true
in time appears to be true also in space: farmers also
know that from a certain area of land more hay can be
gathered when mixed grass is sown than when the grass
seed is uniform. Therefore it can scarcely be doubted
that new forms are bound to expel, to extirpate their
less perfect predecessors, and that of forms which arise
simultaneously the more strongly differentiated have
more chance of surviving. Thus, in varying, every
organic form tends to break up into subordinate groups,
and during the process the links between these forms are
lost ; so that the result is the appearance of a series
of divergent groups without any trace of immediate
transition, and yet bearing evident tokens of either very
close, or more remote, resemblance to one another,
which used to be vaguely denoted by the term affinity,
and which nowadays we simply call relationship. Ina
word, the present order of the organic world, with
its specific and generic as well as other still larger
groups, is but a necessary result of their common origin
by means of natural selection.

With this we close the long chain of argument which
contemporary biology, as represented by the great

ORIGIN OF ORGANIC FORMS 321

Darwin, can offer in explanation of the perfection and
harmony of the organic world. Let us cast a glance
back over what we have said on the subject. If we are
able to analyse the majority of vital phenomena into
their simplest physico-chemical principles, and can
explain them by causes now at work, we are nevertheless
obliged to go back to historical causes in order to explain
all that concerns adaptation. In order to explain the
perfection of organisms by this method, we must begin by
proving that they actually have a history, and then that
this historical development tends towards perfection.
The united testimony of all the branches of biological
science, of classification, comparative morphology,
embryology, palaeontology, goes to prove the common
origin of organic forms. The only objection to this
theory is the belief in the immutability of specific
forms; but the criticism of the very conception of
species, and moreover certain facts with regard to
domesticated animals, which have been established with-
in the limits of human memory, remove this objection.
Having proved that all the facts speak in favour of,
and nothing against, the conclusion that the organic
world has a history, we studied the very nature of
this historical process. Starting from such obvious
properties of all organisms as variability, heredity,
and the rapid rate of multiplication; we came to the
conclusion that this historical process inevitably leads
an organism towards perfection, through what Darwin
rightly called ‘natural selection.’ His theory does not
therefore give any ready explanation of the existence of
this or that special form, of this or that particular case ;
it indicates the general method by which this explanation
may be arrived at in any given case. If we are able to
discover the original cause of variation, and farther to
indicate the continuous series of transitional forms (as
we tried to do in the case of the sage), the origin of the
most complicated form, provided it be useful to the
x

322 THE LIFE OF THE PLANT

actual organism, will no more be puzzling ; it will be a
question of time and selection. This explains why
naturalists hail Darwin’s theory as the crowning of
the stately structure of modern physiology. It really
offers the key to the solution of the problem of the
origin of organisms, gives a reason for their perfection,
and solves the question we raised at the beginning of
this chapter.

Having undertaken the study of the life of the plant,
we tried in our first chapter to analyse this complicated
phenomenon into its elements, by showing that a plant
consists of organs, that these organs consist of simpler
organs still—of cells—which in their turn present an
aggregate of certain chemical bodies. On the ground of
this analysis we then studied in the opposite, ascending,
synthetic order, first the properties of these substances,
then the life of the cell, the life of organs, the life of the
whole plant, and, lastly, in this concluding chapter the life
of the whole organic world. This apparently exhausts
our problem, brings us to the end of the course through
which I undertook to be your guide; a long and toilsome
way, wearying at times, but not, as I hope, utterly fruit-
less. If for some of you ladies and gentlemen here
present, the plant ceases to be a lifeless object marked by
a Latin label, or an object exclusively of aesthetic enjoy-
ment, and becomes also a source of fuller intellectual
enjoyment: if by the discoveries of the microscope it
assumes enormous dimensions, and becomes sufficiently
transparent for you to look into the depths of the
numberless cells where you will perceive protoplasm—
the origin of all life—in ceaseless motion like the tide of
the sea: if the same mental glance shows you the root
buried deep in the ground, imbibing its liquid food and
corroding the particles of the soil all along its course of
many miles : if the green leaf revives in your imagination

ORIGIN OF ORGANIC FORMS 323

the idea of that insignificant chlorophyll granule, wherein
takes place the wondrous process of the transformation
of the sun’s rays into chemical energy, source of all the
manifestations of life on our planet: if you see in a flower
surrounded by busy insects something more than a curious
form, and involuntarily recall the wonderful ties which
bind together the two kingdoms of Nature: if, finally, a
dense mass of forest thicket or the luxuriant vegetation
of a meadow where wild herbs crowd and intertwine,
spreading the broad surface of their toothed leaves in the
sun, stretching up their narrow blades, coiling round some
chosen victim, stretching themselves from its summit to
another, everywhere and always manifesting but one
and the same tendency to take possession of the largest
possible stretch of land, of the greatest amount of air
and light—if this familiar everyday picture involuntarily
recalls to your mind a whole range of new ideas, of
the laws which inevitably and inexorably govern the
organic world, leading it on to perfection and harmony—
in a word, if a glance at a plant raises an endless file of
questions in your mind, questions that demand answers,
or better still if the desire comes to you to put these
questions to Nature herself and to extort answers from
her—lI consider that our time has not been wasted, and
I feel I may comfort myself with the thought that by
affording you in the future some moments of conscious
delight in Nature I have the opportunity of repaying,
were it only in small measure, the debt of gratitude I
owe to your long and indulgent attention.

APPENDIX

THE PLANT AS A SOURCE OF ENERGY?

“Can we really admit that all the sunbeams falling
on this our earth are lost without further use, or should
we not rather assume that they are but transformed when
absorbed by the emerald green of the leaves?’

Scherbin.?

WE all know that if a man is deprived of food he grows lean.
This fact is obvious and is understood by everybody, because
from the logical point of view it is not difficult to connect these
two phenomena, leanness and absence of food. A man’s body,
like everything else under the sun, becomes wasted ; it uses itself
up. This waste is replaced by food. It is not difficult to
imagine the food substance transforming itself into the sub-
stance of the body, though it will be long ere science arrives at
the explanation of all the details of the process.

Less obvious, though not less known, is the other aspect
of the effect of food, another side of its influence upon our
organism. Want or insufficiency of food causes loss of energy.
A man or an animal when hungry grows weak. Food restores
strength. The more work is done by an organism, the more
food is required. Everybody knows that when a horse is
expected to do hard work it receives an extra portion of oats
to help it to do that work. This fact is universally known,
and yet reflection alone is not sufficient to explain it.

1 Public lecture delivered before the Technical Society in St. Petersburg
in 1875.

2 These lines of our talented Russian poet may justly excite our wonder
if we consider that at the time they were written all the botanists, hav-
ing lost sight of Senebier’s remarkable intuitions, did not even suspect
the existence of Robert Mayer’s great generalisations,

824

APPENDIX 325

Thus we see that food not only serves to build up the vital
mechanism of our body, it also sets this mechanism in motion.
Feeding a man or an animal not only sustains his body—which
is obvious; it also maintains his strength—which demands
explanation. We ask: what energy can be concealed in a sack
of oats, a loaf of bread, a piece of meat? The answer is far
from being a simple one, and does not occur to every mind.

In order to get a satisfactory answer to this question it will
be necessary to study the properties of vegetable substance,
and also the conditions of its production. We say ‘ vegetable
substance,’ because we know that animal food proceeds in-
directly from vegetable food. Meat is only grass or grain
transformed by an animal organism.

But before we tackle this problem we must agree as to the
exact meaning of certain terms we are going to use. First,
what do we mean by energy? We will try to explain it by
some examples. Such a method of explaining scientific truths
is certainly not very exact, but on the other hand it is one of
the easiest and quickest, and therefore the most convenient in
the present case.

Man gets a conception of energy from his own experience,
from his own muscular sensations. Energy is defined in
mechanics as the ‘capacity of producing work,’ and ‘ work con-
sists in motion against resistance’ (Rankine). Let us study
first of all some instances of the manifestation of energy,
commencing with that most familiar to us, the energy of our
muscles.

Let us imagine two leaden balls, kept in the position C and
O? by means of two steel springs to which they are attached.
By overcoming the resistance of the springs, and moving the
balls to C’ O?’, I do work, as it is generally called in mechanics.
I do a similar work when, by lifting a weight, I overcome its
tendency to fall upon the earth when, so to speak, I tear it away
from the earth. The raising of weights is the very simplest
illustration of work ; such is the work of a porter, for instance.
We know that in that kind of work the greater the weight,
and the greater the height through which it has to be raised,
the greater the amount of energy which has to be expended.
If we raise a unit of weight, ¢.e. a pound or a kilogramme
through a unit of height, z.e. a foot or a metre, we do a unit

326 THE LIFE OF THE PLANT

of work: a foot-pound, a kilogramme-metre will therefore be
units of mechanical work, with which we compare and by which
we measure all work.

Thus I did work when I separated the two balls. I used a
certain amount of energy, which can be measured in terms of
work done. Let us see what happened to our balls. Their
relative position has changed, and they have acquired the
power to move, without the application of any external force.
I just remove my hands, thereby ceasing to exert any in-
fluence, and the balls are set in motion: they return to their
former position and knock against each other. Evidently
they possess energy in the new position into which I brought
them, and they had none in that from which I removed them.

Fic. 81.

This energy is latent, but ready to manifest itself at any moment
in motion.

This simple example clearly demonstrates the two different
states, the two types of energy, so to speak: the active,
apparent energy, manifested in the motion (of my hand, of
the balls), and the latent energy, accumulated or stored, like
the energy of a bent bow. We see familiar examples all
around us: the energy manifested by a falling weight and the
energy accumulated in a weight which may fall; the energy
manifested by an unbending bow, and that which is accumulated
in a bent one. All these are cases of active and latent energy
respectively. I must add that the weight once fallen, or the
bow unbent, does not possess any energy whatever, either active,
or latent.

It is evident, however, that these two types of energy are
essentially different. In the first case it is manifest, in the

APPENDIX 327

second there exists only the possibility of its manifestation.
We have just seen that bodies possess energy either as a
consequence of motion, or by virtue of their position (eg. a
weight raised, or our separated balls). The first kind of
energy is called actual or kinetic energy, whereas energy depend-
ing on position has been called potential energy, 7.e. energy
existing as tension. Therefore, energy becomes manifested in
motion, and is concealed in a state of tension. Both kinds
are mutually transformable; our balls present an obvious
illustration of this fact. Actual energy exerted in separating
these balls has not disappeared, but has transformed itself into
potential energy, into the tension of the springs. In that state
it can be stored and kept, and then used again whenever
wanted, retransformed into actual energy, into motion, and
moreover at pleasure either directly or gradually. Every day
of our lives we make such a storage of energy in winding
our watches: the kinetic energy of the hand which does the
winding transforms itself into the potential energy of the
watch-spring, which in the course of twenty-four hours
gradually passes into the state of actual energy, shown by
the moving hands. Something analogous to this happens
when a man saves money for his old age: he transforms the
superfluity of his actual energy, mechanical or intellectual,
into potential energy, so that he may use it when his actual
energy comes to an end. On all sides in Nature we see similar
transformations of motion into tension and vice versé. Keeping
this transformation in view we soon arrive at the conclusion
that energy as a rule does not arise anew nor disappear, that it
is eternal; in other words we become convinced that all the
work done, or which can be done in the universe by the forces of
Nature at any given moment, does not increase, nor decrease,
but remains the same. This broadest physical generalisation,
called the law of the ‘conservation of energy,’ is ‘ the highest
law in physical science which our faculties permit us to perceive’
(Faraday).

There are, however, cases in which this law seems not to
hold; energy seems sometimes to use itself up, and motion
instead of changing into tension seems to vanish altogether.
We have exactly such a case in our balls. I separate them and
let go. The balls knock against each other, and seem to lose

328 THE LIFE OF THE PLANT

in this very knock all the energy acquired from the movement
of my hand. They do not move any longer, neither have they
any capacity for motion, 7.e. tension. It seems evident that
energy has disappeared. But thisis only apparent. The moment
the balls knocked against each other, the moment their motion
disappeared, there appeared another kind of energy—heat. In
knocking, the balls became heated. It would be rather difficult
to prove this in the present case, because the rise in temperature
is only slight, but the fact cannot be doubted by any one who
has ever struck fire. Illustrations of this transformation of
energy are to be met with at every step. When metal is bored
the borings become very hot; a piece-of wood can be set on
fire by rubbing it against another piece of wood; sparks fly
from under the brake of a train when it is suddenly stopped ;
a leaden bullet partly melts when it hits against a solid obstacle.
These phenomena of the transformation of mechanical energy
into heat long ago attracted attention; they led the famous
Boyle, more than two centuries ago, to express an idea, which
has been scientifically developed only within the present genera-
tion: ‘ When we drive a big nail into a wooden plank,’ writes
Boyle, ‘ we notice that it requires a great many blows before
it becomes heated ; but when we drive it down to its head, so
that it cannot move any longer, a few blows are sufficient to
make it hot. While every blow of the hammer drives the nail
deeper and deeper into the wood, a progressive movement of
its mass is provoked ; but the moment this movement is checked
the shock produced by the blow, unable any longer either to
drive the nail any further or split it, is necessarily bound to
spend itself on the inner oscillation of particles ; and heat as we
know consists of such motion.’ Modern physics actually teaches
that heat is a rapid invisible but palpable oscillation of par-
ticles in a body. Thus the visible motion of balls produced
by the movement of my hand has passed into the invisible
motion of the particles of the balls. This motion, 7.e. heat,
was communicated first to bodies in the nearest neighbourhood
of the balls; then, spreading more and more, it is dispersed in
space. It is dispersed, but has not ceased to exist. The energy,
used by me to separate the balls, has not vanished entirely. In
doing this work I ultimately raised the temperature of the universe
though to an infinitely slight degree. Innumerable investiga-

APPENDIX 329

tions have shown that during this transformation of mechanical
work into heat or, vice versd, of heat into mechanical work, a
constant, strictly quantitative ratio is observed. A certain
amount of mechanical work on transformation gives rise to a
definite amount of heat and vice versa. The quantity which
expresses this constant ratio is called the mechanical equivalent
of heat. It can be determined in various ways: here is the
simplest and most obvious, although not a very accurate
method of determination, which was devised by the French
scientist Hirn. It consists roughly in the following: a heavy
iron hammer is made to fall from a certain height upon an anvil
on which a piece of lead is laid. This piece of lead becomes
hot from the blow of the hammer. We take a kilogramme-
metre for the unit of work, as explained before, and for the
unit of heat the rise in temperature of one kilogramme of water
by one degree Centigrade. Knowing the weight of the hammer
as well as the height from which it falls, knowing also the weight
of lead and the amount of heat it has acquired, possessing
moreover some other data, which we will not mention here,
we can find out how many units of work have been expended
as well as into how many units of heat they have been trans-
formed. Exact determinations give the value of 426 for the
mechanical equivalent of heat. This number indicates the
constant ratio by which heat transforms itself into mechanical
work or vice versd. This means that a unit of heat on trans-
formation into work gives 426 units of mechanical work, 7.e.
can do work equal to raising 426 kilogrammes to the height of
one metre, or of one kilogramme to the height of 426 metres.
On the other hand, by expending 426 units of mechanical work
to heat water, we can raise the temperature of one kilogramme
of it by one degree.

We have mentioned many examples of the transformation of
mechanical energy into heat ; examples of the opposite are also
often met with. The steam engine may be taken as a striking
illustration ; heat developed by burning fuel becomes transformed
through the medium of steam into mechanical work. The heat
of the sun evaporates water from the surface of the earth,
causes it to rise to a certain height whence it falls down again
upon the earth, runs from the mountains into the valleys and
thence into the ocean, producing all the way mechanical work,

330 THE LIFE OF THE PLANT

e.g. setting in motion our mills. The same energy of the sun
heats the atmosphere at certain spots, so as to produce those
terrible manifestations of mechanical energy known as whirl-
winds, hurricanes, etc.

So, then, heat transforms itself into mechanical work and
vice versa, and during these processes a strict quantitative
ratio is maintained. The same is true with regard to other
forces of nature, such as light, electricity, chemical affinity.
They are all capable of mutual transformation either immedi-
ately or by acquiring the latent form of tension, which subse-
quently manifests itself in one way or another. It is only as
we continually keep in mind this possibility of the mutual
transformation of different forces that we come to realise how
true is the law of the conservation of energy.

Let us dwell for a little upon the correlation that exists between
heat and chemical affinity, for it will naturally bring us back to
the question raised at the beginning of this Lecture. Chemistry
teaches that the atoms of the elements are endowed with
mutual affinity in various degrees. The atoms of hetero-
geneous bodies tend towards each other in much the same way
as bodies tend towards the earth, or as these balls tend towards
each other by reason of their springs. Our model is meant to
illustrate in an obvious way this very fact of chemical affinity.
The ball marked by the letter C represents carbon, the ball
O*? oxygen. The atoms of carbon and oxygen tend to combine
and to form carbonic acid, in which two atoms of oxygen are
combined to one of carbon (CO,). In the same way the atoms
of hydrogen (H) tend to combine with the atoms of oxygen and
form water, H,O, in which two atoms of hydrogen are combined
with one of oxygen. On the other hand, the atoms of carbon
and hydrogen are endowed with a comparatively much weaker
affinity, and therefore, even though combined with each other,
tend at the first opportunity to recombine each in its turn with
oxygen, thus forming carbonic acid and water.

At the moment of combination the atoms knock against each
other in the same way as these balls do. But whenever bodies
knock against each other heat is generated. The same takes
place in the case of the blows of atoms. These blows, this collision
of the particles of carbon and hydrogen with those of oxygen,
is exactly what we mean by combustion. Just as heat and

APPENDIX 331

light are generated the moment steel is knocked against a flint,
so the blow of the particles of oxygen of the air against the
particles of carbon and hydrogen in coal-gas generates the
heat and light that we observe in the gas flame. The only
difference between the two processes consists in that in the first
case we see the motion, the blow, as well as the accompanying
phenomena, 7.¢. light and heat ; whereas in the latter case we see
only these phenomena, and get an idea of the collision of particles
only from the results. Before combustion we have a hydrocarbon
(t.e. a compound of carbon and hydrogen), the coal-gas, and
oxygen, and after combustion carbonic acid and water.

Therefore every atom of carbon and hydrogen stands to
oxygen in the same relation as the ball C’ stands to the ball
O?. Like the balls they are in a state of tension, and possess
a store of latent potential energy which we call chemical affinity
or chemical attraction. In the separated atoms of carbon and
hydrogen we have a fresh illustration of the potential energy
of position, which at the moment of the collision of atoms in
combustion passes into the energy of motion, 1.e. into heat
and light.

This state of tension in the atoms of carbon, this tendency
of theirs to unite with the atoms of oxygen, does not attract
our attention in daily life, because an impulse is necessary to
produce their combination. In order to burn a piece of coal
we must set fire to it, z.¢. the process of combustion must be
initiated from without. This tendency of carbon to combine
with oxygen is, however, manifested more obviously in phenomena
of spontaneous combustion. It has long been known, for in-
stance, that rotting hay in stacks is capable of taking fire spon-
taneously, but a case of it has only recently been investigated
in Germany. When some large hay-stacks were opened in con-
sequence of spontaneous combustion being indicated by the
smoke coming out of them, it appeared that the hay inside was
already charred, and that the soft, shiny, graphite-like carbon
caught fire directly it came into contact with the air. It appeared
later on that such spontaneously combustible carbon could be
prepared artificially if hay was charred in the absence of air ;
in a sealed glass-tube, for instance. Carbon prepared in this
way catches fire the moment it comes into contact with the air.
This and similar examples obviously prove that combustion,

332 THE LIFE OF THE PLANT

i.e. combination with oxygen, can take place spontaneously,
z.e. without any previous setting on fire.

Both carbon and hydrogen have the property of combining
separately with oxygen, and of developing heat and light during
the process; hence they each possess energy stored in the form
of chemical tension. But the same is true with regard to com-
pounds of carbon and hydrogen as well as of any substance
capable of combining with oxygen, 7.e. capable of combustion.
Substances of which plants and animals are built—all organic
bodies, are combustible, and therefore are stores of latent energy.

We use these stores when we burn wood or coal in our engines.
The potential chemical energy transforms itself into actual
energy during the process, into motion of particles, z.e. into heat,
which in its turn transforms itself into external mechanical work,
and so into the visible motion of bodies such as the motion of
our locomotives.

This collision between the atoms of carbon and hydrogen and
those of oxygen can, however, take place without any such
ostensible liberation of energy as occurs in combustion ; they
can combine without any visible manifestation of light, without
the production of high temperature. This happens when com-
bination does not take place suddenly, but gradually. In
both cases the quantity of heat liberated by the combustion of
a certain quantity of carbon will be the same, but its liberation
in the first case covers a longer period of time, and hence is less
obvious. Respiration is a good illustration of this slow com-
bustion. Everything that breathes, whether man or animal,
slowly burns away. This is easily proved by placing a burning
candle, or a living bird or mouse, under a glass bell. We shall
soon see that the results will be identically the same: the
candle will cease to burn, the animal will die; while the air,
in which before the experiment oxygen was present and no
carbonic acid, will now contain carbonic acid, and the oxygen
will have correspondingly decreased in quantity. Thus the
carbon of every living organism continually combines with the
oxygen of the air, burning down into carbonic acid.

In order to restore this continuous waste of his body, man is
obliged to take in fresh quantities of carbonaceous matter in the
form of food. Food in the organism plays the same part as fuel
in an engine, 7.e. it burns down, though of course not directly,

APPENDIX 333

being first transformed into the substance of our body. What is
lost to the organism as matter is acquired as energy. But we
can accept the following statement as an axiom, says the famous
physiologist, Claude Bernard: ‘ every manifestation of activity
in a living organism ts necessarily connected with the destruction
of a part of its matter.’ In the organism as well as in an engine
a certain proportion of matter burns down, and this is accom-
panied either by the manifestation of heat, or by the mechanical
work into which this heat is transformed, such as the work of
our muscles. According to Frankland, a pound of wheat
bread stores up something like 75,000 foot-pounds of potential
energy. There is no doubt that an organism, just like an engine,
cannot transform into useful work all the energy stored up
within it in the form of fuel, z.e. all the potential energy of
the oxidisable parts of its food. Physiological experiments
prove, however, that in this respect the living organism far
outstrips any steam engine.

We have proceeded so far towards the solution of the question
raised above that we already know the kind of energy con-
tained in our food: it is the latent energy of its carbon and
hydrogen which are always ready to combine with the oxygen
of the air. A fresh problem arises at this point in the course
of our investigation. Wood burns, animals burn, man burns,
everything burns, and yet nothing burns right away. Forests
are burnt down, and yet vegetation is not exterminated. Genera-
tions pass away, but mankind is always alive. If everything
were only to burn away, the surface of the earth would contain
neither plants nor animals any longer: there would soon be
only carbonic acid and water.

Evidently another process must also be going on in Nature,
a process contrary to combustion, during which the substances
entirely burnt down are ‘ unburnt,’ transformed into substances
once more capable of burning. The formation of carbonic acid
must be accompanied by a reverse process, the decomposition
of the carbonic acid produced by universal combustion.

The first man whose attention was drawn to the logical neces-
sity for such a process in Nature was the great chemist Priestley.
As a matter of course this idea could not present itself to his
mind in the same form, or with the same precision and clearness,

334 THE LIFE OF THE PLANT

as it appears to us now; hence we are the more amazed at the
brilliant deduction, at the ingenious conception to which the
world owes one of its greatest discoveries in biology. Priestley
proved by a series of experiments that continual combustion,
or continual respiration in a limited volume of air, makes that
air unfit for further combustion, for further respiration: in it
a lighted candle goes out, an animal dies. Therefore, argued
Priestley, all the atmosphere should become unfit for combus-
tion, or for life; yet the many centuries of the world’s existence
testify to the contrary. Apparently there exists a process in
Nature which restores this bad air into good air. Is this not
due to plants? On the 18th of August 1772 Priestley made
the following experiment. He introduced under a glass bell
over water, where a candle had previously gone out, or a
mouse had died, a plant (mint), and kept it there for a time.
The plant did not perish ; it even continued to develop, and
when after a few days a mouse, or a burning candle, was again
introduced under the glass bell, it appeared that the air had
actually been renewed, that it had acquired once more the proper-
ties of maintaining combustion andrespiration. Hardly everinany
province of knowledge has a single experiment been followed by
greater results. The same stroke demonstrated the most charac-
teristic sides of the life of plants and animals, and the mutual
relationship which exists between the two kingdoms of Nature.
Priestley’s contemporaries appreciated the importance of this dis-
covery. The Royal Society conferred on him the coveted Copley
medal; and the President of the Society, Sir John Pringle, ex-
pressed the importance of Priestley’s achievement in the following
eloquent, though somewhat rhetorical, words: ‘ From this dis-
covery,’ says he, ‘ we are assured that no vegetable grows in vain,
but that, from the oak of the forest to the grass in the field, every
individual plant is serviceable to mankind ; if not always distin-
guished by some private virtue, yet making a part of the whole,
which cleanses and purifies our atmosphere. In this the fragrant
rose and deadly nightshade co-operate: nor is the herbage,
nor the woods that flourish in the most remote and unpeopled
regions, unprofitable to us, nor we to them; considering how
constantly the winds convey to them our vitiated air for our
relief and for their nourishment.” Priestley’s inference was that
the plant restored air vitiated by respiration, and made it again

APPENDIX 335

able to maintain respiration. His discovery of oxygen which
soon followed, and the determination of the composition of
carbonic acid, provided the explanation of the nature of this
connexion between the two organic kingdoms. The animal
inhales oxygen and exhales carbonic acid; the plant inhales
carbonic acid and exhales oxygen, retaining (precipitating)
carbon. Plants and animals present a chemical antithesis.
A series of further investigations showed that this process, which
decomposes carbonic acid and restores good air, has yet another
more important significance, which is that it provides the plant
with food. Carbon remains inside the plant, forms its organic
matter, and serves to build up its body. It follows that the
carbonic acid of the atmosphere must be considered the main
food of a plant. Although this function was long attributed to
the black particles of the soil, z.e. to humus, the inadequacy
of this view was established by exact experiments.

Priestley, however, had to experience one of the greatest dis-
appointments that ever befell a scientist. He failed some time
after, when he tried to repeat the experiment which made him
so famous: he could not obtain his former results; the plants
persisted in refusing to decompose carbonic acid and set free
oxygen. Although these disappointments did not shake his
confidence in his earlier experiments, it became evident, never-
theless, that some condition important for the experiment had
been overlooked, owing to which fact the experiment could
not be repeated. This condition neglected by Priestley was
soon after discovered by Ingenhousz. In order to fully appre-
ciate this discovery we shall dwell a little longer on the nature
of the phenomenon itself.

Let us for the last time turn to our balls. We have been
comparing chemical combination or combustion with the con-
cussion of two balls against each other ; heat and light liberated
during the process serve as a measure of the affinity or tension,
1.e. of the mutual attraction of these bodies (represented in our
illustration by the tension of the coils). In order to separate
them again, to break up the connexion between them, in order
to bring the balls into the former free position, we must on the
contrary expend energy, and expend as much of it as is liberated
at the moment of collision. Thus it becomes evident that a
phenomenon contrary to that of combustion has to be accom-

336 THE LIFE OF THE PLANT

panied not by liberation, or development of energy, but on the
contrary by its absorption and expenditure. While combina-
tion, 7.e. combustion, takes place spontaneously, decomposi-
tion requires the participation of an extraneous force. In order
to burn down a piece of coal we must set fire to it, after which
it burns without any external assistance. We have noticed .
that in some cases coal can also burn spontaneously, when it
comes into contact with the oxygen of the air. On the other
hand, in order to decompose carbonic acid and water, to ‘un-
burn’ them, we must expose them to a very high temperature.
Formerly it was supposed that the decomposition of such stable
compounds was impossible without the co-operation of a third
body, endowed with a still greater affinity for oxygen, and so
able to sever that oxygen from hydrogen and carbon. But not
so very long ago the attention of chemists was drawn to pheno-
mena of decomposition, or dissociation, as they were called,
apart from the action of any third body. In order to be
dissociated, carbonic acid or water must be passed through red-
hot tubes. Under the influence of the motion thus communicated
to their particles, 7.e. of heat, the connexion between them
becomes loosened, so to speak; the compounds break down
into their component parts, which must be immediately re-
moved, lest on cooling they should recombine and prevent us
from obtaining a complete separation. The amount of heat
liberated at the moment of combination and absorbed at the
moment of dissociation is strictly definite. There are exactly
as many units of heat liberated during the oxidation of a pound
of carbon into carbonic acid, as there are units of heat absorbed
during the liberation of this pound of carbon from carbonic acid.

Thus we come to the conclusion that the dissociation of carbonic
acid, which takes place in the plant, must be accompanied by
the absorption of heat, of energy speaking generally, and also
that the amount of carbon precipitated in this way in the plant
may serve as a measure for this absorption. But whence will
the plant obtain this energy so essential to it? It cannot create
it, because energy cannot be created. Apparently it must
acquire it from without. The dissociation of carbonic acid
in a plant can take place only on condition that there is a con-
tinual supply of energy from an outside source. This was the
condition which escaped Priestley’s attention, and the discovery

APPENDIX oor

of which made Ingenhousz famous.! Ingenhousz showed that
the dissociation of carbonic acid inside the plant takes place
exclusively in the sunlight. In Priestley’s later experiments
the plants probably did not get enough sunlight, and therefore
did not dissociate carbonic acid.

Sunshine, the rays of the sun, are the very energy which
loosens and separates the particles of carbon from those of oxygen
during the decomposition of carbonic acid. Such an expression
as ‘sunshine is a source of energy’ may sound strange at first. We
know from daily experience how pleasant it is to warm oneself
in the sun, and yet a long chain of arguments and calculations is
necessary to persuade us that this is not only a source of energy,
but even a very considerable source of energy—moreover, that
this is almost the only source of energy used by man. In fact,
apart from the energy of the tide, used in some parts of Europe,
and which depends on the attraction of the moon (and also of
the sun), all the other motive forces, all other sources of energy
directly or indirectly depend upon the energy of sunlight.
The flow of water in rivers and the circulation of air in the
atmosphere, which set in motion our mills, are due to the sun.
The latent energy of fuel, as we have already seen and shall
presently see in greater detail, proceeds from the sun. Even
phenomena so remote as those of electricity, which we use for
practical purposes, can be connected with the activity of the
sun. It is the sun that shines in a Voltaic arc obtained
by means of a galvanic battery, which is easy to prove. The
electric current which makes the carbon white-hot is the result
of the oxidation in the batteries of a certain amount of metallic
zinc. This zinc, however, is not found in Nature as a metal;
it is generally found in combination with oxygen, 7.e. entirely
burnt down. In order to deoxidise it, to restore its capacity
for combustion, oxygen must be removed from it. This is done
by means of coal, which combines with the oxygen of the zinc
ore, and burns down to carbonic acid. But this coal, be it pit
coal or wood charcoal, has been derived in the plant from carbonic
acid, dissociated by sunlight. Thus it is that the rays of the sun

1 Such is the current opinion, but lately, after a careful study of the
question, I have come to the conclusion that Ingenhousz’ priority is
more than doubtful, and that the fact of the dependence of this process
on sunlight was discovered by Priestley.

Y

338 THE LIFE OF THE PLANT

are connected with those of the electric light. The visible, kinetic
energy of the sunlight, expended in thé decomposition of carbonic
acid in plants, takes the form of latent potential energy, con-
cealed in carbon after it has been liberated from the carbonic
acid. This potential energy of carbon then passes over to the
zinc during the process of deoxidation of the zinc ore; carbon
burns down and metallic zinc is obtained capable of combus-
tion. Zinc oxidises in the galvanic battery, burns down, and
its potential energy transforms itself into the actual energy of
the electric current, appearing as light in the white-hot carbons.
Such is the complicated chain of transformations of energy, which
connect phenomena that take place on our planet with the
activity of the sun. We can, however, form a more definite
idea as to the significance of the radiation of energy from the
sun, by making an approximate calculation of the amount of
energy afforded by the sun. We can determine the number of
units of heat cast by the sun upon a given area of our planet ;
and then, knowing the mechanical equivalent of heat, we can
express the energy of sunlight in units of mechanical work.
According to the calculations of Mouchot the sunlight that
falls during eight or ten hours on a bright day upon the
surface of a square metre in Paris can do work approxi-
mately equal to one horse-power. Ericsson calculated that if all
the heat of the sun that falls upon the roofs of Philadelphia
were to be used, it would amount to the force of 5000 steam
engines, each of 20 horse-power. Further, on calculating
the enormous amount of heat that falls upon the earth, he
exclaims: ‘Archimedes undertook to lift the world by a lever,
whereas I maintain that by concentrating the heat of the sun
we could obtain a force capable of arresting the motion of the
earth.” Both Mouchot and Ericsson, however, did not confine
themselves to calculations: they made experiments, which
clearly demonstrated the stores of energy presented by the
rays of the sun. Mouchot made several very simple kinds of
apparatus, in which it was possible to boil water, soup and
vegetables as well as to bake bread exrlusively by means of the
heat of thesun. In the end he also made some steam and hot-
air engines set in motion by the sun. Of all the applications
of sunlight suggested by Mouchot, his pumps for irrigating
fields are perhaps the most curious. They not only act by

APPENDIX 339

means of energy obtained free of cost, but also work most effec-
tually. They are regulated according to the very need for
water: the amount of water is regulated by the amount of the
radiant energy of the sun, hence also according to the severity of
the drought.1

We can prove easily enough from what has been said that
sunlight is a powerful source of energy, and that this very energy
decomposes carbonic acid in plants. The plant is unable to
provide itself with the energy necessary for the purpose ; it only
plays the part of a mechanism, if I may so say, of a connector
for transmitting the energy of the sun.

The plant therefore presents a regular contrast to the animal
from the physical as well as the chemical point of view. The
life of the plant consists in a continual transformation of the
energy of sunlight into latent chemical energy ; while the life
of the animal, on the contrary, manifests the transformation
of chemical latent energy into heat and motion. The spring is
wound up in the one to unwind in the other.

We should be wrong, however, if we imagined that this function
of the sunlight became intelligible the moment Ingenhousz dis-
covered its participation in the process of the dissociation of
carbonic acid. More than half a century had to pass before the
actual mechanical details of the process were worked out. This
achievement science owes to Mayer and Helmholtz. While
light in former days was talked of only as an incomprehensible
though beneficial influence, Mayer was the first to state that sun-
light is actually used up in the literal sense of the word, and is
absorbed by the plant ; that the energy of the ray transforms
itself into chemical tension ; that in burning fuel, and in the vital
processes of our organism, we use the stored up energy of the sun.
It would be better to listen to his own eloquent way of putting it:
‘Nature, says he, seems to have set itself to capture the light
that falls upon our planet, to transform the most mobile of all
forees into an immobile form, and to conserve it as such. With
this end in view itefas covered the crust of the earth with
organisms, which, during their life-time, absorb the sunlight and
form at the expense of this energy the stores of latent chemical

1 See my article on the ‘Struggle of Plants with Drought’ in my-
Russian book, Agriculture and the Physiology of Plants. Moscow, 1906.

340 THE LIFE OF THE PLANT

energy incessantly accumulated. These organisms are plants.
The vegetable world is a kind of storehouse, where the sun’s
rays become arrested and stored for further use. The physical
existence of mankind depends on this economical solicitude of
Nature, and a single glance at our luxuriant vegetation involun-
tarily provokes the sensation of prosperity.’

Thus it is that in the dissociation of carbonic acid and the
formation of organic plant substance we have essential con-
ditions for any technical process. We possess in the sun-
light a motive power; in the plant—a machine to which the
motive power is applied ; in the carbonic acid—a raw material ;
in the organic matter of the plant—the manufactured product.

Let us investigate more closely the inner mechanism of this
process.

Let us study first of all the source of energy, the sunbeam.
We know that the sunlight, like any other white light, is not
homogeneous ; we know that it consists of many heterogeneous
rays, differing, among other things, in their colour. Rays of
seven different colours are generally discriminated ; they are
the colours of the rainbow—red, orange, yellow, green, blue,
indigo, violet. This decomposing of a colourless ray into its
seven component colours is best performed by means of a glass
prism. Ifa small aperture is made in a shutter facing the sun,
the sunlight in passing through this aperture will produce on
the floor an image of the sun in the form of around patch. Now
if we place a prism with its edge downwards in front of the
aperture, the image will move on to the wall; but instead of a
round patch we shall obtain a band showing the seven colours
of the rainbow just mentioned: the band will be red at one edge
and violet at the other. This rainbow band is called a spectrum.
Whenever a ray of white light falls upon the surface of a body
of any kind, it becomes partly or entirely absorbed. If all
the rays become absorbed by the body, its surface appears
black ; if all the rays are reflected in equal measure its surface
appears white. If some of the rays are absorbed and others
reflected, the body acquires the colour of those rays which are
reflected from the body and strike our eye. The same holds
true with regard to transparent bodies. If the body absorbs
all the rays it is not transparent, it is opaque ; if it lets all the
rays pass through it, it is entirely transparent, and as colour-

APPENDIX 341

less as water or glass; but if the body arrests some rays and
lets others pass through it, it will acquire the colour of the rays
it lets through. If we analyse by means of a prism the light
reflected by a coloured body, or the light which has passed
through a coloured body, it is obvious that we shall no longer
obtain an entire spectrum of seven colours, but one from which
the absorbed rays will be absent.

Vegetation presents us with a similar phenomenon. In
bright sunlight forests and meadows appear green. It is clear
that if the leaf reflects the green colour it must absorb part of
the white light which it received.

Before drawing any inference, however, from this fact, let
us investigate more closely the cause of the green colour of the
leaves. Whatever green part of a plant we may choose for

AaBbe D jaa)

Fic. 82.

investigation under the microscope, we very soon arrive at the
conclusion that in itself it is colourless; it consists of bubbles
called cells, the walls of which are as transparent as glass, and
the liquid which fills them as colourless as water. But this
liquid contains bodies or grains emerald green in colour. They
are generally called chlorophyll granules or chloroplasts. It
is to these granules containing chlorophyll that the plant owes
its green colour, in much the same way as the blood owes its
colour to the red corpuscles which flow in the colourless lymph.
Now let us observe what happens to the sunlight when it falls
upon the surface of the green leaf; which rays are going to pass
through the leaf and which will be arrested by it? For this
purpose we must let a ray of light pass through the leaf, and
then analyse it by means of a prism. When we do so we notice
the difference which takes place in the spectrum. The rays
absent in the spectrum, those in place of which black spaces
are observed, have obviously been arrested by the leaf, have

342 THE LIFE OF THE PLANT

been absorbed by its substance. We can perform this experi-
ment more exactly still. Since the colour of the plant
depends on the chlorophyll, we can study the absorption of
light by the chlorophyll itself. Chlorophyll can be extracted
from leaves by means of spirit. We all know that any kind of
infusion of leaves acquires a splendid green colour, which is
the colour of the chlorophyll. Therefore instead of nearly
opaque leaves we can use for our experiment a nearly transparent
chlorophyll solution. We fill a glass with this solution and place
it in the path of the sunlight, and then proceed to analyse with
a prism the light we thus obtain. This is the kind of spectrum
we get. The extreme red rays (from A to B, fig. 82) will have
passed through unabsorbed ; whereas in place of the brightest
red, the orange and part of the yellow rays, the spectrum will
have a black band (fig. 82 from B to D) ; + the green rays (between
D and a little to the right of 8) will not be absorbed, and will
give a green band in the spectrum ; the blue and violet rays will
be likewise absorbed. Hence, instead of all the seven colours
the spectrum of chlorophyll will show only two coloured bands:
a dark red and a bright green, with a black space between them.
Hence we conclude that the green colour of a plant is not pure,
but a mixture of green and red. This can be easily proved by
a curious experiment. The commonest blue glass absorbs
green rays and lets through some red rays. It follows that if
we look at green vegetation through a piece of this glass it will
arrest green rays on their way to our eyes, and let through only
red rays. German opticians have taken advantage of this fact,
and offered the public a rather amusing instrument called the
erithyo-phytoscope, which is simply a kind of blue spectacles, but
the moment you put them on the whole world changes its aspect.
A fantastic landscape with coral woods and meadows unrolls
itself under a deep blue sky. It might be useful to draw the
attention of some artists to this fact who are in the habit of
colouring their landscapes with that malachite green colour,

1 Fig. 82 represents a photograph of the absorption spectrum of chloro-
phyll. The blackest part lies within the red part of the spectrum. The
process of photographing the spectra has presented great difficulties,
even down to our own day. In the summer of 1893 I succeeded in
obtaining satisfactory photographs, which were demonstrated at a con-

gress of naturalists and physicians at Moscow in January 1894. The
letters mark the so-called Fraunhofer lines of the spectrum of the sun.

APPENDIX 343

never to be observed in Nature! In their unsuccessful en-
deavours to represent Nature, these artists probably tend to
portray her in the clearest green possible, whereas the colour
of our vegetation is in fact a mixture of green and red.

We must, however, return to the main object in view. We
wanted to know which rays are absorbed by the plant, and we
found that chlorophyll absorbs certain red, orange, and yellow rays,
as a result of which its spectrum presents a black band in place
of them. This fact can be tested even for a single chlorophyll
granule under a microscope. This time instead of throwing the
spectrum upon a wall, we can obtain it under the microscope by
means of a lens, and in this spectrum of the size of a pin’s head
we investigate our chloroplast. We notice then that it appears
transparent green in the green part of the spectrum, transparent
red in the extreme red, and entirely opaque, as black as soot,
in the red rays (marked by BC in fig. 82) absorbed by the solu-
tion. This means that the living grains of chlorophyll also
absorb these rays.

Thus when they fall upon a plant or rather upon the
chloroplasts enclosed in its cells, certain of the sun’s rays
become absorbed, cease to be light any longer. But there is
no loss of energy: it has only changed, passing into a state
of tension. What kind of work is done by these rays in
a plant? Let us recall the conclusion we have just arrived
at, that sunlight decomposes carbonic acid in plants. May
not this work take place at the expense of just those rays
absorbed by the chlorophyll granules? This suggestion gains
in probability when we learn that the chloroplast is the
very organ, the very apparatus in which the decomposition
of carbonic acid takes place. Priestley noticed that the de-
composition of carbonic acid and the giving off of oxygen
take place exclusively in the green parts of the plant, 7.e.
in leaves or green stems. He was even able to prove that
this activity is due to the green substance. If a vessel of

1 It is difficult to give any definite advice on the subject in the
absence of necessary technical information. The spectrum of chrome
green is nearest of all mineral green colours to the spectrum of chlorophyll,
its green colour being a mixture of red and green rays. At all events
we cannot obtain the green colour of foliage by mixing together yellow
and blue (the blue of the spectrum).

344. THE LIFE OF THE PLANT

water or some kind of extract is allowed to stand in the light,
a green deposit soon appears on the walls of the vessel. Now-
adays we know that this deposit is composed of microscopic
plants, of algae; but in Priestley’s time this fact was not known,
and the deposit was even known as ‘ Priestley’s matter.’
Priestley was able to prove that this matter gave off oxygen.
This experiment already showed that the green substance
decomposed carbonic acid, even outside the leaf or stem, and
that it was precisely to this green substance that these functions
were due. Other doubts, however, rose later on. There are
plants which are not green in Nature, and yet they also decompose
carbonicacid. Such are the numerous plants with red, black, and
other coloured leaves, which are more and more gaining a footing
in our gardens and greenhouses ; such are also the brown and
red weeds growing at the bottom of the sea. In the former the
matter has been easily explained. The variegated colour in such
plants depends on bright solutions which exist in their cell-sap,
and which conceal the green chloroplasts. These are easily seen
under a microscope, and can be also revealed in the following way.
We have only to dip a red or almost black leaf of Coleus, or some
other plant with similarly coloured leaves, into dilute sulphurous
acid, and it immediately turns green. This depends on the fact
that in decolourising the red pigment sulphurous acid does not
affect the chlorophyll. It was rather more difficult to prove the
presence of chlorophyll in seaweeds. It was impossible to find
the green grains in them even under a microscope; they were
all brown or red. Chemistry, however, showed that the green
chlorophyll is concealed behind another substance. It is easy
enough to observe this fact-simply by walking by the seaside.
Weeds cast on the shore very often manifest as they decompose
all shades of colour from their natural colour to green. This
is because in dead plants the more brightly coloured substances
are washed away by water, while chlorophyll remains insoluble.
Thus, even here the decomposition of carbonic acid takes place
only in parts which contain chlorophyll granules. This rule
has no exception. As has been already said, we must see in a
chloroplast the apparatus, the mechanism to which the energy
of the sun is applied. It was very interesting to test by means
of an experiment the truth of this hypothesis, and to see whether
the decomposition of carbonic acid actually did take place at

APPENDIX 345

the expense of rays absorbed by the chlorophyll. In order to
do so we had only to perform Priestley’s experiment simul-
taneously in different parts of the spectrum. The experiment
was performed in the following way: a series of glass tubes
(fig. 83 II, 1, 2, 3, 4, 5)

were filled with a mix- 7?

ture of air together | 4,
with a certain per-
centage of carbonic ,¢00
-acid. Green leaves
from the very same
plant and of similar |
size were introduced
into each of them.
Then the vessels were | yap
exposed to the spec-
trum of the sun ob- 79
tained in a perfectly
dark room. After a

700

+500:

+200

few hours it was de- 710 |

: ees ned Bl, uv
termined, by analys-
ing the gas, in which ‘a

tubes the carbonic acid
had decomposed and
in which not—those in
which it had decom-
posed more, and those

in plies lass, : (i Cut ally
hich es cee UL GD Z

tirely proved the hy-
pothesis. It turned _~ a eat z5 é
out that the decom- 0

position of carbonic
acid took place exclu- Fig. 83.

sively in rays which

corresponded to the black band in the spectrum of chlorophyll
(fig. 83, x Red.—and fig. 82 between B and D), which means
that rays which do not get absorbed by the chlorophyll do
not decompose carbonic acid; whereas rays which become
absorbed decompose it the more, the more they themselves

346 THE LIFE OF THE PLANT

are absorbed. This is graphically illustrated in fig. II. From
the line ab perpendiculars are drawn, the length of which corre-
sponds to the amount of carbonic acid decomposed in the
corresponding parts of the spectrum (I) in tubes 1, 2, 3, 4, 5 (II).
The broken line 1, 2, 3, 4, 5 (III) clearly shows the part of
the spectrum in the sphere of which carbonic acid is decom-
posed most energetically.

Thus spectroscopic investigation serves to prove on the one
hand that certain rays of the sun, after having crossed
without any modification immeasurable depths of space,
on meeting on their way a chlorophyll granule cease to be
light any longer, disappear in performing work of some
sort. On the other hand, the experiment in the spectrum
just described points to the fact that it is these very rays
which cause the decomposition of carbonic acid into carbon
and oxygen, and are used up in this chemical work. We
may conclude that a complete correlation is to be observed
between the accumulation and expenditure of energy in the
plant.

We have thus discovered the source of energy and also the
apparatus to which this energy is applied, z.e. the chloroplast.
We have seen the work done during the process ; we have now
to discover the product obtained, to follow the further fate of
the carbon liberated from the carbonic acid and to realise what
is produced from that carbon in the plant. Here our curiosity
can be satisfied by the microscope.

We take a green organ of some kind, say a leaf, and dissect
it so as to investigate it under a microscope; or, what is better
still, we take a vegetable body, such as a green alga, commonly
called green-slime, which can be directly observed under a
microscope. Having made sure that the chloroplasts do not
contain any foreign body to begin with,1 we expose the
green organ to sunlight either in the open air or in an artificial
atmosphere containing carbonic acid, 7.e. we place it under con-
ditions favourable to the decomposition of carbonic acid.
After some time we again investigate the chloroplasts under
a microscope, and discover in them colourless grains which were
not there before. It is easily proved that these grains consist
of starch. Among the properties of starch is that of turning

1 Which is ensured by keeping the plant in the dark for some time.

APPENDIX 347

dark blue under the action of a solution of iodine. It is by this
method that we detect starch in the chloroplast. The formation
of starch, however, is not observed in the absence of light or
carbonic acid hence we are justified in concluding that the
formation of starch is the result of the decomposition of carbonic
acid. This is confirmed by the rapidity with which the one
phenomenon is followed by the other. The decomposition of
carbonic acid is manifested a few seconds after sunlight falls
upon the surface of the leaf, and five minutes later starch is
already found in the chloroplast. This correlation of the two
processes becomes still more apparent if we take into considera-
tion the chemical composition of starch. Starch can serve as
a representative and type of the group of vegetable substances
known as carbohydrates. The carbohydrates contain carbon,
hydrogen, and oxygen. Their name is derived from the fact
that in them hydrogen and oxygen are in the same ratio as in
water, so that they seem to consist of carbon and water. In
order to form a carbohydrate out of carbonic acid and water,
we have only to remove all the oxygen from carbonic acid, 1.e.
we must perform exactly what takes place in a plant during
the decomposition of carbonic acid. Therefore carbohydrates
have precisely the same composition as would be expected of
substances formed in the plant from carbonic acid and water.

In this way the microscope fully confirms the results obtained
by means of analysis. Whenever carbonic acid is broken down
in a chlorophyll granule carbohydrates form inside it. The
following example is a good proof of this correlation of the two
processes. A bright spectrum of the sun is thrown in a dark
room upon the leaf of a living plant, previously deprived of
starch. In an hour’s time the leaf is removed, decolourised
with spirit, and treated with a solution of iodine. It appears
that starch has formed in those parts of the spectrum alone
which are absorbed by the chlorophyll, and the greater the
absorption the more abundant the formation of starch; in
other words, the leaf obtains an impression of the spectrum of |
chlorophyll in the form of starch coloured almost black by
iodine (fig. 83 IV—compare with fig. 82 and 83, I).

The group of carbohydrates forms the largest component
of our vegetable food. Thus starch forms three-fourths of
the weight of a wheat grain and four-fifths of the dry matter

348 THE LIFE OF THE PLANT

of the potato. There are many substances besides starch
which belong to this group, e.g. sugar, and cellulose, the sub-
stance which forms the solid skeleton of plants, from thin blades
of grass to the trunks of trees. All these bodies have a similar
composition, and differ only in their greater or less density
and other physical properties. Sugar, for instance, dissolves
in water; starch does not, it only swells and forms a kind of
semi-liquid paste; cellulose scarcely swells at all. In a sense
we can say that starch is condensed sugar, and cellulose con-
densed starch. Other carbohydrates can be easily derived
from starch. As a matter of fact sugar is artificially obtained
from starch in the manufacture of potato molasses. Cellulose
has not yet been prepared artificially, but certainly is derived
from starch in the plant: thus, for example, the starch of the
germinating seeds of cereals changes into the cellulose of which
the rootlet is built up. a

Albuminoids form the second predominant group of vegetable
substances after the carbohydrates. They are. called albu-
minoids from their likeness to the albumen of an egg. Wheat
flour, taken as an example of vegetable food, contains something
like 17 per cent. of albuminoid matter, called gluten. There-
fore, if starch and albuminoids are subtracted from cereal
seeds, there will be only a small percentage left for all other
substances. In addition to carbon, hydrogen, and oxygen,
nitrogen enters into the composition of albuminoid sub-
stances.

Though starch, as we have seen, cannot be formed otherwise
than with the co-operation of light, the formation of albuminoids
in a plant does not require light, or any other external source of
energy. It depends instead upon the presence of ready carbo-
hydrates. If some plants are only provided with a carbohydrate
of some kind, say sugar, and some source of nitrogen, say ammonia,
they are sure to form albuminoids even in total darkness. Accord-
ing to exact experiment, without touching the problem unsolved
as yet by chemists, as to the relation which exists between
carbohydrates and albuminoids, we may say that plants are
able to form albuminoids from a carbohydrate and ammonia.
A physiologist can say to a chemist: give me sugar, ammonia,
and a cell, and I will give you in return as much of an albuminoid
as you wish. Its manufacture may certainly not always be

APPENDIX 349

. very profitable, but in the present case its very possibility, if
even only in theory, is very important.

Without entering into details as to the origin of other vege-
table substances, less important to man as compared with
albuminoids, we may nevertheless apply to them what has been
said about the albuminoids, and thus arrive at the conclusion
that the agency of sunlight is necessary only for the forma-
tion of starch or, speaking more generally, of carbohydrates
from carbonic acid and water; no other substances require
sunlight for their formation.

‘We can only now appreciate fully the significance of the pro-
cesses taking place in the chlorophyll granule under the action
of sunlight.

In the first place, from the chemical point of view, it is here
that inorganic matter, carbonic acid and water, is transformed
into organic matter—here lies the source and origin of all the
heterogeneous substances out of which the organic world is built
up. On the other hand, from the physical point of view, the
chlorophyll granule represents an apparatus for capturing the
sun’s rays, which then are laid up in store for future use.

Plants form organic matter out of the air, and stores of energy
out of sunlight. They represent in every respect the machine
invented by Mouchot and Ericsson—the machine set in motion
by the energy of the sun, free of cost. This explains the result
of the farmer’s labour: by expending but a comparatively small
amount of substance in the form of manure, he obtains great
masses of organic matter; by expending a certain quantity of
energy he acquires great stores of it in the form of fuel and food.
He burns down a forest, feeds sheep on the grass of his meadows,
sells the corn of his fields, and yet everything returns to him
again in the form of air, which, under the influence of sunlight,
again acquires the form of forests, fields, and corn. With the
assistance of plants he transforms air and light which have no
market value into marketable quantities. He trades in air
and sunlight.

These considerations dispose of the theories occasionally
heard concerning the fate awaiting humanity when chemists
shall have discovered the secret of synthesising complicated
organic bodies, and have found means of preparing artificial

350 THE LIFE OF THE PLANT

food. On the strength of what has already been achieved by
synthetic chemistry, we can scarcely doubt that in the future,
it may be at no very distant date, science will realise these
expectations. At all events there is no such fundamental
difference between what has been already achieved and what
yet remains to be done as to make such a hope improbable. And
then if food actually becomes artificial, will not agriculture be a
thing of the past ? Will not land lose its value? Will not the
economic order of things entirely change? Let us see how far
these conjectures are true. We saw that to form an organic
body energy is required. On burning down a pound of bread
eight hundred and ninety units of heat are produced. Therefore,
to form it either naturally or artificially, a similar amount of heat
must be used, or speaking more generally a similar amount of
energy. Whence is this energy to be obtained? The only source
of energy free of cost is the sun. Therefore, in order to produce
artificial food our remote posterity will have to imitate plants
by covering the surface of the earth with artificial absorbers
of sunlight. Such imitation cannot be said to be an easy matter,
because from this point of view the plant presents a very perfect
apparatus. A glance at the thickness of the grass in any meadow
is enough to convince us of the fact that every bit of soil is at
present utilised. Calculations furnish us with data still more
eloquent. Thus, for instance, the total surface of the leaves of
a clover plant exceeds twenty-six times the area of the land
occupied by the plant, so that an acre of clover is equal to
twenty-six acres of green surface absorbing the rays of the sun.
Other plants occupy even larger surfaces still. The sainfoin
has a leaf-surface thirty-eight and lucerne eighty-five times
larger than the areas they occupy. Mixed grasses would
probably give still higher numbers.

Here another curious theoretical problem occurs to us: Can
we indefinitely increase by means of plants the amount of
organic matter obtainable from a certain area of land? Can
we expect that by means of improvements we shall indefinitely
increase the productiveness of our soil, or has it a limit? This
is the problem of the future fate of humanity. The data we
already possess permit us to decide this question in the affirma-
tive. There is a limit, and we are even able to determine it
approximately. We have already said many times that the

APPENDIX 351

formation of organic matter in a plant is accompanied by the
absorption of as much heat as is liberated in burning it down.
Thus, for instance, if a plant liberates 1.000 units of heat when
it burns, we may conclude that at least a similar amount of the
heat of the sun has been used up in its growth; and however
we may manure our soil and cultivate our land, if the sun does
not provide it with these I.000 units of heat we shall not
get our plant.

Thus it is that, knowing on the one hand the amount of
combustible matter (to be ascertained by analysis) contained
in a crop obtained from a certain area of land, and knowing on
the other hand the amount of heat cast by the sun upon this
area, we have all the necessary data for calculating the profit
and loss of the energy of the sun in our field, and hence to
decide what proportion of it we use and what there is yet to
be used. Such calculations for the crops which yield the largest
amount of organic matter, the richest harvest, bring us to the
following conclusions. The largest annual increase of matter in
woods represents something like -1,th of all the amount of heat
received by the area of land which they occupy during the
period of growth. The increase in bulk of the roots is not
taken into this calculation. The Jerusalem artichoke, one of
the plants which undergoes the most intensive cultivation, uses
in this way z¢,th of all the energy it receives from thesun. The
organic matter in the richest crop of hay (rye grass), including the
root remains, stores up 743th of the energy of the sunlight. Lastly,
the best crops of oats and rye (grain, straw, and root remains)
represent jth of all the energy they receive from the sun. Thus
by means of a plant we are able to avail ourselves approximately
of a quantity varying from z;455th to z4pth of the total amount of
sunlight which falls upon the surface of our forests and fields during
the growing period! Are we therefore entitled to say that by
improving our methods of culture we shall be able to increase
the crops by one hundred if not by one thousand times before we
reach the limit of productiveness? Is the plant able to store
up all the energy it receives from the sun? Certainly not.
We know that no mechanism or organism makes exception to

1 It is clear that these numbers are not very strict. The number
quoted for the amount of heat that falls upon a given area, taken from
Pouillet’s data, is only approximately true.

352 THE LIFE OF THE PLANT

this rule, or transforms into useful work all the energy obtained ;
and this consideration alone is sufficient to prove that the
physiological limit of vegetable productiveness cannot coincide
with the physical. Contrary to the figures just mentioned,
which have been taken from the results of various cultures, an
objection of the following kind can be brought forward : although
field vegetation, as we have seen, presents a highly developed
surface for absorption, we cannot, however, say that it absorbs
all the sunlight that falls upon it. The following experiment
will give us from this point of view more trustworthy statistics.
By exposing to sunlight green leaves with a surface of accurately
measured area, determining by means of analysis the amount
of carbonic acid decomposed by this leaf in the best light, say
during the space of an hour, and determining also the amount
of heat that falls upon the selected surface of the leaf during
that hour, we shall obtain all the data necessary for calculating
the correlation between the absorption of energy and its utilisa-
tion in decomposing carbonic acid. Direct experiments of this
kind gave on the average ;ijth of all the energy received,
zsth at the best. Some recent calculations show that this
quantity can be increased to th. This last figure may probably
be considered as approaching the limit of physiological pro-
ductiveness, because the plants in these experiments were
placed under the most favourable conditions possible. Thus
we see how closely all our most intensive cultures approach what
we have called the physiological limit, z7.e. the largest amount
of organic matter which can be obtained by means of a plant
from a given area of land.

Even at this limit, however, only ;3,th and in the best instance
goth of the energy received is retained. This will cease to puzzle
us if we consider the fact that apart from this uniquely pro-
ductive work from man’s point of view, other work takes place
in the plant, entirely unproductive for man.1 In the first place
the plant evaporates during the whole of its life-time such great

1 It is still more important to take into account the fact that a leaf can-
not even absorb all the light of the sun: otherwise it would be black
instead of green. Modern investigations prove that the leaf absorbs on
the average 25 per cent. of all the radiant energy received from the sun
—this is the physical limit; 3-3 per cent. are utilised in physiological
experiments, and 1 per cent. in the fields.

APPENDIX 353

‘quantities of water that in amazement we are inclined to dis-
credit the figures. The evaporation of this quantity of water
apparently requires much more heat than is used in the decom-
position of carbonic acid. Therefore, together with the pro-
ductive work in the formation of organic matter, the plant
uses still more energy in work useless to man—in evaporation.
But this, although the most important, is not the only other
expenditure of energy in a plant. The plant absorbs that water
‘from the soil, and therefore has to raise it to a certain height.
This work may be expressed in foot-kilogrammes. It may be
neglected in the case of our field-plants, but is considerable
in our trees! We can imagine what a large amount of
work underlies the raising of the masses of water evaporated
in forests by giant trees like the eucalyptus of Australia,
the tops of which, according to one botanist, might have cast
their shadow even on the summit of the pyramid of Cheops.
However, evaporation and the raising of water are not
achieved solely by the energy derived immediately from the
heat of the sun, though a considerable amount is certainly
supplied in this way. To these causes of unprofitable waste of
the sun’s energy must also be added another. We cannot
avail ourselves of all the organic matter stored up by the plant
during its life-time, because it uses, burns down, part of that
matter itself. We can say that it uses in this way as much as
goth of all the matter, so that as regards the accumulation of
organic matter the plant makes twenty steps forward and one
‘backward.

All these causes of waste of the sun’s energy which we
have enumerated illustrate for us, so to speak, the expenses
of the production of organic matter by the plant. We See,
therefore, that although the plant is a very perfect apparatus
for utilising the energy of the sun, it nevertheless leaves much
to be desired, since at the best it transforms into work useful for
man only ;3;th or 3}yth of all the energy it obtains from the sun.
Man has to face the problem either of perfecting the plant in this
respect, or of inventing in its place an artificial apparatus, which
shall utilise a greater percentage of the energy acquired, and

1 The raising of the sap to a great height may be considered as unpro-
ductive, only with regard to the production of matter, but on the other
hand it is all-important as it furnishes us with timber,

Z

354. THE LIFE OF THE PLANT

moreover work the whole year round. How far he will
succeed is for the future to decide. One thing is certain, that
when by means of his artificial apparatus man shall gather from
all the free area of land about a hundred times more organic
matter than is contained in the richest crop to-day, he will
be able to say that he has reached the limit; that he can
go no further. Man will then make no further demand of the
soil or his art, for more fuel or more food—he will not be able
to get any more, because the sun will not be able. to give
any more. Then the law of Malthus will manifest itself in
all its ominous cogency: mankind will have to keep a strict
account of life and death; it will have to take account of the
death-rate before reproducing itself, as has been already anxiously
suggested by perspicacious economists. No extra mouth, in the
literal sense of the word, will then find room at the banquet
of Nature. Will mankind ever attain this limit? “By what new
processes of synthesis will Berthelots of the future benefit it?
What new sun-machines will be furnished by future Mouchots
and Ericssons? Who can tell? One thing is certain, that our
planet will acquire then a very dismal aspect. When man shall
have arrived at the utilisation of all the energy of the sun
instead of only part of it as we do at present, then, instead of
the emerald green of our meadows and woods, our planet will
be covered with the uniform mournful black surface of artificial
light-absorbers. Lord Kelvin foretold that our planet will find
its death from cold, that our world would be wrapped in its
icy embrace; but I do not think this prophecy has alarmed
more than a very few. It will come to pass long after our day,
and we all know the proverb: aprés mot le déluge. Yet we
cannot help shuddering at the idea of what life will be like
when the earth is transformed into a universal factory, with no
possible escape into the open even on a holiday, even for a single
hour !

Let us turn from this gloomy and fantastic picture of what
I am happy to say is a very remote future, and go back to
the question raised at the beginning of this lecture, which
we are now able to answer fully and categorically. We can do
so best under the following figure. Once upon a time a ray
of sunlight fell somewhere upon the earth. It did not fall,
however, upon sterile soil, but upon a green blade of wheat,

APPENDIX 355

or rather upon a chlorophyll granule. The ray was extin-
guished when it struck the granule; it ceased to be light any
longer, but it did not cease to exist. It was used up in the work
it did inside the granule: it broke the connexion between the
atoms of carbon and oxygen which were combined as carbonic
acid. The liberated carbon in some way or other combined
with water and formed starch. This starch was transformed
into sugar, and after many peregrinations inside the plant was
precipitated again inside the grain as starch, or as gluten. In
either case it entered into the composition of bread, which
serves us as food. It was transformed into our muscles, into
our nerves. And now in our organisms atoms of carbon strive
to recombine with the oxygen which is carried by the blood to
all the parts of our body. The ray of sunlight, concealed in
these atoms during this process in the form of chemical tension,
reacquires the form of actual energy. It is this ray of sunlight
which warms us, and by which we move. May be it sparkles
in our brains at the present moment.

This illustration is the most detailed answer which science
can give in reply to our question. We can express it shortly
in three words. Food plays in our organism the part of a
source of energy only because it is a preserve of sunshine.

The scientific importance of this result is obvious. It
will also be appreciated by people indifferent to scientific
truths. A poetical dreamer who looks sadly upon the prosaic
labour of a scientist will be pleased to learn from him that he—
the poet himself—is much the same ethereal being, built of air
and light, as the immaterial productions of his fancy. The
haughty noble who prides himself upon his ancestry, and looks
down somewhat contemptuously upon the modest lot of the
toilers on the field of science, will certainly treat with more
respect this same science on hearing that she entitles him as well
as the Emperor of China to call himself ‘ the Son of the Sun.’ ?

1 Helmholtz: Ueber die Wechselwirkung dev Naturkvafte, p. 127.

Printed by T. and A. ConsTaBLE, Printers to His Majesty
at the Edinburgh University Press

a

Better ta
Sait eee
sae ge ed

ager

are

poeuies

eas
er
BSS 2