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Presented to the
library of the

UNIVERSITY OF TORONTO

by

awaroco to

prize,

to : ..^XcljU. S.ruu-2^-.

FORM

189

Prof. F.P. Ide

UNIVERSITY EXTENSION MANUALS
EDITED BY PROFESSOR KNIGHT

CHAPTERS IN MODERN BOTANY

GENERAL PLAN OF THE SERIES.

This Series is primarily designed to aid the University Extension
Movement throughout Great Britain and America , and to supply
the need so widely felt by students , of Text-books for study and
reference , in comiection with the authorised Courses of Lectures .
Volu?nes dealing with separate sections of Literature , Science ,
Philosophy , History , and Art have been assigned to representative
literary men , to University Professors , or to extension Lecturers
connected with Oxford , Cambridge , London , and the Universities of
Scotland and Ireland.

The Manuals differ from those already in existence in that they
are not intended for Elementary use , but for students who have ?nade
some advance in the subjects dealt with . The statement of details is
meant to illustrate the working of general laws , and the development
of principles ; while the historical evolution of the subject dealt with
is kept in view , along with its philosophical significance.

The remarkable success which has attended University Extension
in Britain has been partly due to the combination of scientific treat-
ment with popularity, and to the union of simplicity with thorough-
ness. This movement , however , can only reach those resident in the
larger centres of population , while all over the country there are
thoughtful persons who desire the same kind of teaching. It is for
them also that this Series is designed. Its aim is to supply the
general reader with the same kind of teaching as is given in the
Lectures , and to reflect the spirit which has characterised the move-
ment, viz. the combination of principles with facts , and of methods
with results .

The Manuals are also intended to be contributions to the Literature
of the Subjects with which they respectively deal , quite apart from
University Extension ; and some of them will be found to meet a
general rather than a special want.

Digitized by the Internet Archive
in 2017 with funding from
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Chapters

in

Modern Botany

By PATRICK GEDDES

PROFESSOR OF BOTANY, UNIVERSITY COLLEGE, DUNDEE

LONDON

JOHN MURRAY, ALBEMARLE STREET
1893

All rights reserved

For him the woods were a home and gave him the key
Of knowledge , thirst for their treasures in herbs and flowers.
The secrets held by the creatures nearer than we
To earth he sought , and the link of their life with ours :
And where alike we are , unlike where, and the veined
Division , veined parallel, of a blood that flows
In them , in us, from the source by man unattained ,

Save mark he well what the mystical woods disclose."

Meredith, Melampus.

PREFACE

This little book makes no attempt to condense a survey
of its science ; even within the fields through which it
passes it seeks only to be suggestive, not exhaustive ; its
chapters have actually grown out of the syllabus and
notes of University Extension Lectures, with their neces-
sary limitations. In matter and form its appeal is to the
general reader ; yet, in method and spirit, to the student
also, — in some measure even to the teacher. In botany, as
in other studies, educational methods alter with the times.
In the Linnean period the “ best botanist was he who
knew the most plants,” however little of each ; while a
later and still dominant school has founded upon Cuvier
a type-system which makes him know much, — but of
few. Hence the student has come no longer to load his
vasculum and memory in a single vacation, with all things
from the cedar to the hyssop ; but, seeing that cedar
and hyssop have been selected as types by the highest
authority, scrutinises these, and these only, for his term.
Analysis is great, and the anatomist is its prophet ; yet
such Elementary Biology is but Necrology, its so-called
“ life-histories ” being but histories of form.

It is the misfortune of biology that Darwin was not a
teacher. It is no easy matter for us professors, trained

VI

Preface

from our youth in Linnean or Cuvierian schools, to
change our thought methods, and see how, instead of
merely appending the dogma of evolution to our old
curriculum of morphological training, to organise one truly
Darwinian in spirit from the beginning. But, as teacher
and student usually end as they begin, let them begin as
they would end ; neither with conning an inventory of
plant-mummies, nor with the tissue-unwrapping of samples
of these ; but with childlike watching, scene after scene,
of the actual drama of nature, in which life interacts with
life, and fate with all. Only then, indeed, can Linnaeus,
can Cuvier, have his due, and do his work for us. It is
surely in the measure of our intelligent interest in the
play that we notice more and more of the dramatis
persona , or have keener scrutiny of each actor in his
hour. Hence the plan of this little book, which seeks
to lead from small scenes to great. Were it worthy, it
should be dedicated to the memory of Darwin.

PATRICK GEDDES.

CONTENTS

CHAPTER I

PITCHER PLANTS

Darlingtonia — Sarracenia — Origin of Darlingtonia Pitchers — In-
sect- Catching — Other Relations to Insects — Minute Structure of
these Pitchers — Australian Pitcher Plant ( Cephalotus ) — The
Pitcher Plant proper (. Nepenthes ) — Secreting Glands and
Nectaries ..... Pages 1-20

CHAPTER II

PITCHER PLANTS — continued

Use of Pitchers, “ Bionomics ” — Bionomics — Bionomics of Nepenthes
— Morphology of the Pitcher — Bladderwort — Bionomics of
Bladderwort — Allied Forms .... 21-35

CHAPTER III

OTHER INSECTIVOROUS PLANTS — DIFFICULTIES AND
CRITICISMS

Fly- Traps (. Dioncea and Aldrovanda ) — Sundews and Birdlime Traps
— Butterworts — Sundews proper ( Drosera ) — Details , Functional
and Structural — Digestion — Movements — A bsorption— Utility

X

Contents

— Other Insectivorous Plants — Legends — Difficulties — Further
Difficulties and Criticisms — Direction of further Investigation ;
Possible Compromise .... Pages 36-59

CHAPTER IV

MOVEMENT AND NERVOUS ACTION IN PLANTS

Climbing Plants — Darwin's Observations , with Summary — Inter-
pretation of Movements — Movements of Seedlings — Methods of
Observation — Theory of Circumnutation . . 60-75

CHAPTER V

MOVEMENTS OF PLANTS — Continued

Movements in relation to Gravitation — Light -seeking and Light-
avoiding Movements — Rationale of Light-seeking and Light-
avoiding Movements — The Sleep of Plants — Mr. Francis
Darwin's recent Discussion of Plant Movements — Summary
and Conclusion ..... 76-94

CHAPTER VI

THE WEB OF LIFE

Struggle among Plants — Perched Plants or Epiphytes — Parasitic
Plants — Mistleto — Dodder — Root - Parasites — Toothwort —
Broom - rapes — Saprophytes — Parasitic Fungi — Bacteria —
Symbiosis . . . . . . 95-119

CHAPTER VII

RELATIONS BETWEEN PLANTS AND ANIMALS

Plants and Snails — Plants and Ants — Domatia—Myrmecodia —
Galls — Plants and Aphides — Cats and Clover . 120- 142

Contents

XI

CHAPTER VIII

4|

SPRING AND ITS STUDIES — GEOGRAPHICAL DISTRIBUTION
AND WORLD-LANDSCAPES — SEEDLING AND BUD

Spring Studies — Mode of Study in Botany — Phenology and Dis-
tribution— Aspects of Nature, Vegetation and Landscapes of the
World — Germination — Buds and Bud- Scales — Arrangement of
Leaves in the Bud .... Pages 143-160

CHAPTER IX

LEAVES

General Facts in regard to the Life of Leaves — Experiments , rough
and exact — Sum7nary of L^eaf Functions — The Structure of
the Leaf — Palisade Cells and Chlorophyll Grains — Shapes of
Leaves — Leaves adapted to special Functions — Substitutes for
Leaves — Vitality of the L,eaf— Fall of the Leaf . 161-189

CHAPTER X

SUGGESTIONS FOR FURTHER STUDY

Root and Stem — Flower , Fruit , and Seed — The Web of Life once
more — Syste77iatic Botany — Morphology of Organs and Tissues
— Evolutio7i ..... 1 90-20 1

LIST OF ILLUSTRATIONS

A Tropical Forest ....... Frontispiece

FIG. PAGE

1. Darlingtonia calif ornica ...... 3

2. Leaves of Sarracenia purpurea . . . . . 9

3. Pitcher of Nepenthes distillatoria . . . . . 16

4. Pitcher of Nepenthes bicalcar ata showing downward -

directed prickles 27

5. Elk’s-horn Fern (Flaty cerium grande) .... 99

6. Patch of Lichen grown synthetically by Bonnier (from

sowing of fungus spores on algae) under bacteriological
precautions against entrance of foreign spores . . 1 1 7

7. Assai Palm (. Euterpe oleracia ) 156

8. Apparatus of Bonnier and Mangin, for analysis of gases

given off by plants . . . . . . .166

CHAPTER I

PITCHER PLANTS

Darlingtonia — Sarracenia — Origin of Darlingtonia Pitchers — In-
sect- Catching — Other Relations to Insects — Minute Structure of
these Pitchers — Australian Pitcher Plant ( Cephalotus) — The
Pitcher Plant proper (Nepenthes') — Secreting Glands as
Nectaries.

In attempting to arrange a suitable introduction to the
study of botany, a teacher may incline to one or other of
two distinct methods. The first, and in many ways the
more satisfactory, is to take the commonest plants around
one, begin with the simplest knowledge of these, extend it
by what is easily to be observed or obtained, deepen this
by closer study, and next extend to less familiar forms the
growing experience and practical power of the student.
Most courses of biological instruction now actually run
upon this principle : they place before the student some
common type, some frog or crayfish, some common fern
and flower, from which he may work his way towards a
wider survey of the science. The other method, which also
has its favourable side, is to start with something rare or
strange — at any rate unfamiliar, — and so not only evade
the prejudice that botany deals mainly in hard names

B

2 Chapters in Modern Botany chap.

or the like, but obtain the immense advantage of ministering
to some measure of reawakened curiosity, some freshened
feeling of the varied marvellousness of nature.

The plan here adopted is practically a compromise of
the two. Beginning indeed with some of the strangest
forms and processes of the vegetable world, it is not pro-
posed to exhibit these merely as a vegetable menagerie of
rarities and wonders, but to use them as a convenient
means of reaching, as speedily as may be, not only (a)
some general comprehension of the processes and know-
ledge of the forms of vegetable life, but also, and from the
very first, (b) some intelligent grasp of the experimental
methods and reasoning employed in their investigation.
For these purposes a very convenient beginning may be
made with pitcher plants.

Moreover, they will be found to lead us more rapidly
than would many more familiar types to the point of view
of Darwin, and the reading of his actual works ; this being
of course most central and characteristic in modern botany.

Darlingtonia. — Beginning then far afield, in the land of
big trees and vegetable wonders, we find not the least of
these in a marsh plant discovered just fifty years ago by the
botanist of an exploring expedition in the region of the
Sierra Nevada. A fresh expedition nine years later
gathered flowering specimens, but it was not until 1855
that the systematist Torrey formally introduced the plant
to the world as Darlingtonia californica (the surname
given in compliment to a friend). At first a rarity of
botanic gardens, it is now not uncommon in greenhouses,
and is very easy of cultivation. The flowers are large and
strange, resembling those of Sarracenia , described below
(p. 5). The leaves, however, are yet stranger ; they rise
in stemless clumps above their mossy bed to a height of
12 or 18 inches, slender tubes extending upwards like

I

Pitcher Plants

3

organ pipes, but each recurving into a large and well-arched
hood or helmet, with a framework of strongly - marked
veins. This helmet is brightly splashed with red, and
glistens with innumerable translucent spaces in which green

tissue is absent, and only the epidermis of either side
remains. Its small downward -directed opening is con-
cealed, not only by its position, but by a gaily-tinted and
banner-like streamer which hangs in front. To this open-
ing, however, there ascends a gently-curved pathway which

4 Chapters in Modern Botany chap.

runs upwards from the very ground, as if expressly built
for ants and other wingless creepers, while at its top
it is no less useful as a landing stage for winged explorers.
The finger-tip can easily be inserted, and finds the edge to
be incurved all the way round, while the descent into the
pitcher is of course close by. Slitting open one of the old
pitchers a gruesome sight presents itself — two, five, nay,
it may be more likely twenty or fifty mouldering corpses,
chiefly, in our greenhouses at least, those of bluebottles and
wasps, but with now and then also a moth or bee.

Sarracenia. — To understand all these peculiarities of
form and life we may best pass to the allied genus
Sarracenia , of which there are a good many different
species of similar habit and habitat, but wider distribution,
the genus ranging from Florida to Canada. The form is
less perplexing, the hollow leaves are now simply trumpet-
shaped, and instead of being rolled through a semicircle
and curiously narrowed, open widely towards the sky,
while the forked pennon of Darlingtonia is obviously
represented by an almost circular or somewhat pointed
leafy expansion, sometimes sloping over the mouth, like a
half open lid or cover, large enough to throw off rain, but
often also standing erect and conspicuous, the whole effect
being often no less attractive to botanist or bluebottle than
that of the Darlingtonia itself (Fig. i, p. 3). Here
clearly we have the simple form ; and passing from the
empirical facts of geographical distribution towards that
interpretation the healthy childish or scientific mind cannot
but demand, we can hardly fail to suspect that Darling-
tonia is but the most outlying of the Sarracenias, one
which has wandered to the farther side of the Rocky
Mountains, and in that region become so much specialised
beyond the ordinary type as to warrant re-naming as a
new genus. Comparing the flowers, we find essential kin-

I

Pitcher Plants

5

ship yet sufficient generic difference ; curiously enough, it is
the flower of Sarracenia which is the more specialised.
Each is solitary upon its lofty stalk, the long dull -red
petals quite surpassed in conspicuousness by the curiously-
dilated style, recalling the peltate leaf of the common
Indian Cress ( Tropceolum ), which climbs over so many
cottage walls, or in dwarf forms brightens the garden
border. In Darlingtonia the style merely shows the faintest
beginnings of such an arrangement. From this one of the
perplexities of evolution becomes evident ; we often cannot
say that one plant is more evolved than another as a
whole ; but only it may be on this or that respect, e.g. the
form of the leaf in Darlingtonia, of style in Sarracenia.

Origin of Darlingtonia Pitchers. — But how should such
a change come about ? the thoughtful student will ask, so
beginning to raise all the enigmas of evolution. Was it
by change of climate and soil ? or by spontaneous internal
variations, trifling differences of the kind visible in every
patch of seedlings, of which some, useful in some way to
the plant, helped their lucky possessors, which therefore
survived while their fellows died, and transmitted these to
a new series of similar divergent seedlings, which again
had to struggle for life in the same way ? Thus we see
how in course of generations we might obtain important and
obvious differences from the simpler parent form ; and this
of course would have been Darwin’s view ; it is a special case
of his famous theory of “ the Origin of Species, by means of
Natural Selection, or the Preservation of Fortunate Varia-
tions in the Struggle for Life,” while the speculation as
to the possible but unknown influence of climate and soil
represents the position of the earlier evolutionists, Lamarck,
Erasmus Darwin, etc. Here, then, at the very outset of
our studies, the riddle of origins comes up and refuses to
admit itself fully solved. For though each of these

6 Chapters in Modern Botany chap.

answers alone successively satisfied many minds, Darwin
himself at least inclined towards some varying compromise
of them. And as the student learns the really scientific atti-
tude, that of not learning answers but of asking questions,
new puzzles arise. Thus, what influence are we to place
upon the geographical isolation through many generations
of the incipient Darlingtonia from its kindred Sarracenias ?
How far does it modify our natural selectionist position
with its characteristic insistance upon adaptation to external
uses when we note that the odd hood of the Darlingtonia
is developed merely by increasing the relative rate of growth
of the outer and upper surface of the Sarracenia pitcher,
especially as we approach its mid -rib, so producing not
only the inflation but the curvature, and even the stretch-
ing apart of the leaf- tissue so as to leave the pretty
window-like patches. But if this be clear we have already
got a step below the conventional Darwinian level of
external adaptation, below the idea of progress merely
through the cumulative patenting of mechanical improve-
ments in our fly-traps. Helpful though that explanation is,
so far as it goes, we have in fact reached a new and deeper
plane of thought on which it may become necessary to
work out an entirely new set of evolutionary interpretations.
The outer world of external and mechanical adaptations
once left behind, we are at once brought face to face with
the internal and vital processes, and have to grapple with
the problems of organic growth, both general and special ;
in other words, it is in terms of the laws of growth that we
have to reinterpret the phenomena of development. We are
familiar with these differences of growth of different regions
of the leaf, as in young leaves of ferns, but the experimental
study in detail lies still before us. Deliberately to arrange
new conditions for our Sarracenia so that it shall at least
begin to roll its leaf into the form of Darlingtonia is, how-

Pitcher Plants

7

ever, at present “ impracticable,” i.e. remains a problem
for the experimental physiologist. Yet even the natural
selectionist most satisfied with accounting for the change
on the principle of the mechanically improving fly-trap is
notwithstanding the very man empirically to help, if not
anticipate us, by finding us a seedling varying in the
required direction among a patch of young Sarracenia ;
for on this and its offspring the experimentalist might best
begin. All such experimental researches are as yet only
in their infancy, but it is becoming admitted on all hands
that as the past of the science lies mainly in systematic
collections, in anatomical and microscopic analyses, so its
future has to be sought in the physiological laboratory, the
greenhouse, and the garden.

Insect-Catching. — But how, the active minded observer
will ask, does our curious helmet-like Darlingtonia pitcher
keep its victims, — for though it is natural for an insect to
creep into what doubtless appears to it a very inviting new
kind of flower, why should it not creep out again ? A
first difficulty is afforded by the incurved margins ; but this
would not be enough to detain it ; and here comes in the
use of the relatively widely distended helmet-space with its
innumerable transparent glassy panes let into its whole
upper surface. The insect has eyes on the top and sides of
its head, and sees abundant light above to spread its wings
and beat itself upon the resisting roof and walls of the
pitcher as persistently and as vainly as he does within our own
window-casement. No doubt when he becomes tired and
falls down over the entrance he may at times escape, but
is more likely only to rest over it till he can again begin
his struggles on the wing, while the adjacent opening,
that of the fatal oubliette, is not only of much larger
diameter, but of gently -sloping sides instead of repellent
recurved margins. The walls of the leaf-tube substantially

8 Chapters in Modern Botany chap.

correspond to those of Sarracenia, which we may therefore
describe more minutely.

The gaily- coloured lid of the Sarracenia pitcher is
bedewed in spring and early summer with drops of nectar,
which lie on its inward surface, at least for the most
part ; not on both, as in the pennon of the Darlingtonia. A
closer examination of its surface shows that these drops are
at once helped to form, and if sufficiently large to trickle
downwards by a coating of fine but short and stiff hairs
which arise from the epidermic surface. Here, in fact, is
in every way an admirably-constructed “ attractive surface/5
and it is obvious as well as natural that the insects which
sip the honey should travel down into the interior of the
pitcher to seek for more. Beyond the lid surface with its
hairs and nectar-glands they come upon the smooth and
glassy “ conducting surface,55 a well - paved path leading
indeed towards destruction. In S. purpurea there are
indeed a few fresh nectaries to be reached by this descent,
a new secreting surface below the conducting one — in S.
flava and other species not even this, — but in all cases we
soon reach the “ detentive surface55 of the whole lower part
of the pitcher. This is covered with long, stout, bristly
hairs, averaging say J inch long, all sloping downwards
into the cavity of the pitcher, and so presenting no
obstacle towards descent, but much resistance towards
return, as the finger can easily verify, or as the dead
inmates of the tubular prison still more conclusively show.
That so comparatively powerful an insect as a wasp or
bluebottle can be thus detained may be at first sight
perplexing ; but we see that there is no scope to use the
wings for escape, while legs and wings alike become
entangled and held back by the stiffly-pointed hairs, which
the struggling insect can at most only thrust along, and
thus not break. Another captive soon comes on top ;

1

Pitcher Plants

9

ventilation becomes checked, and the foul air rising from
dead predecessors must still further check respiration ; little
wonder then that life must fail. Even in our greenhouses

Fig. 2. — Leaves of Sarracenia purpu?-ea. A, attractive surface of lid ; B,
conducting ; C, glandular, and D, detentive surface, magnified. (A and D
are taken from S.Jlava.')

the leaf thus becomes filled, not only i or 2, but often
5 or 6 inches deep with dead insects ; while observers
on the spot, notably Dr. Mellichamp, to whom our know-

io Chapters in Modern Botany chap.

ledge is mainly due, have shown that there is normally a
considerable amount of fluid secreted by the pitcher,
although this does not seem to appear in European culti-
vation, and that this fluid has distinctly anaesthetic and
fatal properties to insects immersed in it.

Other Relations to Insects. — It is an odd fact that
while with us the bluebottle falls an easy and natural prey
to this unwonted trap, being doubtless attracted like the
wasp by that odour of decomposing carrion to which the
bee and butterfly in turn owe their safety, a shrewder
American cousin ( Sarcophaga sarracenicE) lays a few eggs
over the pitcher edge, where the maggots hatch and fatten
on the abundant food. In April three or four of these
larvae are to be found, but in June or July only one sur-
vives, the victor who has devoured his brethren. But
nemesis is often at hand in the form of a grub -seeking
bird, who slits up the pitcher with his beak, and makes
short work of all its eatable contents. For this bird in
turn the naturalist has next to lie in wait, and so add a
new link to the chain.

The larvae of a moth (Xanthoptera semicrocea) also
inhabit the pitcher, but devour its tissue, not its animal
inmates ; in fact, they spin a web across its diameter, as if
to exclude further entrance of these, and then devour the
upper part of the tissue, especially, it would seem, the
nectar-glands, finally passing through their chrysalis stage
within the cavity of the pitcher, and not, as in the case of
the Sarcophaga larva, making their exit into the ground.

It is said that spiders also spin their webs over the
mouths of the pitchers and wait to reap the profit of their
attractiveness — again a point of almost human shrewdness.

An American entomologist, Professor Riley, has de-
scribed the ways in which these associated living insects
( commensals , we may perhaps call them, by a not extreme

i Pitcher Plants 1 1

stretch of technical language) are adapted to life in such
dangerous conditions. The moth has long spurs upon its
tibise (second leg joints), which cross many of the hairs as
it walks, and so prevent its legs from sinking among them ;
while its larvae, destitute of this snow-shoe arrangement,
spin their silken strands over the tips of the detentive
hairs, and so keep out of danger. The larvae of the blow-
fly, on the other hand, have peculiarly long claws and
large cushions on the last tarsal joints, and so grip down
through the hairs and hook themselves firmly into the very
tissue of the trumpet-leaf itself.

The question naturally arose — are not these treacherous
plants victimising the very insects which fertilise them ?
But this seems little or not at all to be the case ; for
5. variolaris , at least, our good observer Dr. Mellichamp
has shown that fertilisation is effected by the “ melancholy
chafer 55 ( Euphoria melancholica ), nor has he ever beheld
the moth Xanthoptera so act. So far, at any rate, it seems
we have quite distinct and separate inter- adaptations of
flower and leaf, and to distinct and separate insects.

Minute Structure of the Pitcher. — Before leaving this
subject one may have a useful first lesson in “ vegetable
histology,55 since the tissues here are not only peculiarly in-
teresting and intelligible, but very easily handled. Opening
the pitcher with one’s penknife it is easy to make out
with the naked eye, and clear with the pocket lens, the
essential character of these surfaces, attractive, conductive,
and detentive respectively ; but to see the exquisite beauty
and perfection of their details we must multiply lens above
lens, so developing our simple microscope, noting, of
course, that we are passing to no separate “ science of
microscopy,55 but that we are merely adding in front of our
own eye lens first one artificial lens, and then more as we
need them. How these lenses need to be held together^

12 Chapters in Modern Botany chap.

and how one combination of these is brought near the eye
(“ eye-piece ”), so as to multiply still further the image
already magnified by another held nearer the object (and
hence naturally termed object-glass), is of course the ele-
mentary common sense of that exquisite marvel of detailed
perfection, the compound microscope. The further develop-
ments, as that of shutting off side light above the object-
glass by the microscope tube, and below it by the stage-
diaphragm, of placing the object upon a transparent stage,
and this upon a perforated one, or of getting the instrument
when we wish to examine a transparent object out of the
inconvenient horizontal position at first necessary into the
more convenient vertical or sloping one by the simple
device of reflecting the window light through the tube to
the eye by means of a mirror fixed below the stage, are
again no less obvious. This elementary instrument once
constructed, a new set of considerations would naturally
arise, among which the necessity of focussing and the diffi-
culty of getting rid of the prismatic colours which would as
yet enhalo our magnified image may be cited as specially
important. These have to be met by mechanical and
optical devices respectively, which are familiar enough ; and
so we might work on, a whole volume being needed to do
justice to the history of the instrument, as, indeed, are
special journals to its unending developments. The bare
outline given above is but to emphasise the idea, com-
monplace in phrase but too little habitual in practice, that
the scientific study of anything, be it a natural or social
product, ought always to proceed from the known towards
the unknown, and rationally from its beginnings onwards
wherever possible.

Given, then, the compound microscope, we may first
attempt to examine the epidermis more in detail, beginning
with the attractive surface of the lid by shaving off thin

I

Pitcher Plants

13

slices from its surface, and mounting them in a drop of
water between slide and cover-glass. These, however, we
probably find to be comparatively opaque and confused,
because too thick, and including much of the deeper
leaf- tissue or parenchyma as well as epidermis. We
may, it is true, improve our preparation by the use of
methods familiar to every microscopist, e.g. by washing
the preparation in spirit to dissolve out the green colouring
matter or chlorophyll, by treatment with caustic potash
solution to destroy even the protoplasm, by dyeing or
staining the cell-walls conveniently with a solution of one of
the anilines to bring out their outlines more clearly, and by
mounting in glycerine instead of water, so as to give greater
transparency to the whole. Instead of all this trouble, which
after all will not make a good preparation out of a badly-
made section, we may learn much from even a thick slice
in the fresh state by observing merely its edges, which are
sure to be somewhere thin enough. A better method how-
ever, which, with a little practice, will be found to give
excellent preparations, not only of all the tougher-leaved
insectivorous plants, but of any tolerably strong epidermis,
is to lay the morsel of leaf face downwards upon a slide in
a large drop of water, and then holding it firmly at one
edge, to scrape away with a sharp scalpel or penknife the
other epidermis and the green leaf-tissue, the veins too, as
far as they will come, washing the debris from the prepara-
tion from time to time, and scraping more carefully and
lightly as the lower epidermis is exposed, and of course
threatens or begins to tear. When tolerably clean the
preparation may be turned over and examined, a funda-
mental principle in all microscopic study being first to
make out all one can with the low magnifying power before
proceeding to a higher one, while the various operations of
histological “ cuisine ” above indicated may be applied if

14 Chapters in Modern Botany chap.

desired, and the preparation mounted permanently for the
collection, either simply in glycerine by putting a ring of
asphalt or gold size around the cover-glass edge, or by
mounting in glycerine jelly. Sufficient technical skill and
experience to make very fair botanical preparations will be
found to come very rapidly with practice, especially if the
beginner can obtain a practical start from any more
experienced student or amateur ; the help of any work on
the microscope is often of much value, although there is
nowadays a bewildering wealth of technical devices, each
no doubt useful in its own way, like the numberless
refinements of the microscope itself, yet like these quite
unnecessary until skill has been reached and special
problems undertaken.

Australian Pitcher Plant (Cephalotus). — Another
pitcher plant, farther-fetched than Darlingtonia, and less fre-
quent in cultivation in our greenhouse collections — indeed
one of the rarest and most peculiar plants in the world
— is the curious little Australian pitcher plant Cephalotus
follicularis , which occurs only in a small area not far from
the capital of Western Australia. It is by far the smallest
and least impressive of all the pitcher plants, yet is of
some beauty, and also of morphological interest in pos-
sessing at once ordinary leaves and well-formed pitchers
between which no gradations normally exist. In the large
collection of pitchered and other insectivorous plants in
the Edinburgh Botanic Garden the late Professor Dickson
(than whom these interesting forms have never had a more
keen and thoughtful student) was able to collect and figure
an interesting series of the monstrous leaves which occa-
sionally arise, and thus to show that the pitcher is but a
specialised modification of the ordinary leaf, a first em-
bryonic dimple near the point deepening backwards and
downwards into a pouch, the lid thus arising on the side

I

Pitcher Plants

i5

of the pitcher orifice originally nearer the base of the leaf.
The histological details of the pitcher are of interest, and
of exceptional beauty of colour.

The Pitcher Plant proper (Nepenthes). — From this
solitary and tiny Australian rarity we may now pass to the
abundant and magnificent pitcher plants proper, the genus
Nepenthes , of which not less than forty species are described
in Dr. Macfarlane’s recent excellent revision of the group.
They are widely scattered over the Oriental tropics, with
their headquarters in the hotter regions of the Malay
Archipelago, but thence range northward into Cochin China,
southwards into North Australia, and westwards into Cey-
lon, Bengal, and even Madagascar. In all the species the
pitcher is borne at the end of a long tendril-like prolonga-
tion of the leaf, and is not only of very beautiful form but
great size, varying from an inch to a foot or more in depth.
Two varieties of pitcher occur in many species, the first,
associated with the lower leaves and developed during the
younger state of the plants, are not uncommonly found
actually resting on the ground. This form is short and
broad, provided with broad, external, wing-like prolonga-
tions, up which ants and other ground insects readily make
their way to the lip of the pitcher. The adult and more
abundant form of pitcher is longer and narrower, with the
external wing-like appendages less strongly developed, or
it may be even absent. The anterior (lower) surface of the
lid stands well open, serving after maturity no longer as a
protective cover, save that it may serve to throw off rain,
but apparently as an attractive surface or insect lure, being,
like that of the forms already examined, more or less
baited with nectar. The rim of the pitcher rewards the
closest scrutiny, its surface being beautifully fluted and
turned inwards and downwards, so as not only to strengthen
the pitcher and keep its mouth always stiffly open, but to

CHAP.

1 6 Chapters in Modern Botany

lead the insect gently to the dangerous verge, at which the
fluid contents of the pitcher come fully into view, and the
glassy conducting surface can be easily reached. Dickson
discovered also the apparently constant presence of a row

Fig. 3. — Pitcher of Nepenthes distillatoria. A, honey gland from attractive
surface of lid; B, digestive gland from interior of pitcher, in pocket -like
depression of epidermis (opening downwards) ; C, transverse section of the same.

of very large flask-shaped glands along the very edge of
this incurved rim, and presumably of further attractiveness.
The edges of the flutings are often produced downwards
into stout hook-like processes, which are sometimes strong
enough to retain a small bird.

I

Pitcher Plants

i7

Scraping our microscopic preparations as before, we
may rapidly note the nectar-glands of the attractive lid,
the flask - shaped marginal glands just referred to, the
smooth internal conductive surface, and below this the
secreting surface. The former often shows small down-
ward - directed crescentic ledges, while when we come to
the secreting surface we find these suddenly becoming
better developed and crowded, each ridge bearing below
it a well -developed gland, which projects slightly, like a
watch just beginning to slip out of an inverted watch-
pocket.

The fluid of the pitcher stands at a tolerably regular
level, and so far as the insect visitors are concerned re-
places the detentive surface of Sarracenia. That it is a
normal and genuine secretion, and not mere collected rain,
is evident from its development before the young pitcher
has opened ; while its analysis by Voelcker shows the
presence of oxalic and citric acids, of chloride of potassium,
and of carbonate of soda, magnesia, and lime. Lawson Tait
again denies the presence of acid in the fluid of a young
pitcher. Of much greater importance, however, is the
interpretation of its nature and uses first promulgated by
Sir Joseph Hooker in a memorable address to the British
Association in 1874, in which he gave full details of his
experiments on the digestive properties of the fluid, which
he tested not only upon insects, morsels of beef, egg, etc.,
but even upon substances so resisting as cartilage. Lawson
Tait in 1875, and subsequently Rees and Will of Erlangen
in 1876; Gorup - Besanez, the well-known physiological
chemist of Strasburg, in 1877; Vines, and others, con-
firmed these results, and extended them by the separation
of a digestive ferment, apparently identical with the pepsin
of the animal stomach. Rees and Will actually found that
fibrin was dissolved even more rapidly by the secretion of

1 8 Chapters in Modern Botany chap.

the excited pitchers than in a test experiment with pepsin
from the pig’s stomach ! This, it must be confessed, seems
proving too much, and we shall do well to remember that
most samples of prepared pepsin are far from possessing
the same digestive potency, still less that of the fresh
stomach, not to insist on other sources of fallacy. Still
the existence of some appreciable quantity of pepsin seems
obvious. Hooker and Tait have shown that fluid removed
from a living pitcher into a glass vessel does not digest
unless some acid, preferably lactic, be added. During the
presence of food, however, they regard the pitcher as con-
tinuously stimulated to secrete acid, and to keep up the
supply of pepsin.

Tait also separated a very deliquescent substance from
the secretion of this and other insectivorous plants,
which he termed azerin. To this he ascribed digestive
and antiseptic properties, and also drew attention to its
remarkable power of wetting surfaces, just as glycerine or
paraffln does. Placing living flies in tubes containing
distilled water, Nepenthes fluid, and solution of prepared
azerin, he observed that “ when those in the tube contain-
ing the water touch the surface they remain there as long
as the water is undisturbed without ever getting completely
wetted, and that they live for a very long time, — as long,
perhaps, as in a perfectly dry tube. Those in the other
tubes, on the contrary, will become completely wetted in a
very few minutes after they touch the surface of the fluid,
soon become immersed, and seldom live more than a
quarter of an hour or twenty minutes.”

“ This must be due to the peculiar wetting property of
azerin, enabling the water to enter their tracheae and
drowning them. This method of death can be seen in
the case of flies placed upon the leaves of Drosera rotundi-
folia , for they become wetted in a way which was most

I

Pitcher Plants

19

astonishing to me until I knew the peculiar properties of
azerin. In the process of digestion this penetration of the
fluid must also be useful.”

Secreting Grlands as Nectaries. — So far we have been
looking at the glands of the Nepenthes pitcher as peculiar
organs special to their position ; but soon after the re-
examination of the pitcher by Professor Dickson, which
resulted in the discovery of those curious marginal glands
(which we may view as the highest specialisation of the
gland structures of the lid and pitcher — perhaps, indeed, of
the secreting gland yet known in the vegetable kingdom),
Dr. Macfarlane made an interesting step towards the
determination of the less specialised organ to which the
whole of these peculiar structures may be referred, and of
which they may be considered developments and modifica-
tions. Closely examining the whole plant, he noticed that
“ not only is honey secreted by the inside of the lid and
the mouth of the pitcher, as we already knew, but the
outer surface of the pitcher, as well as that of the lid, also
possesses honey glands. Further, the whole so-called
4 leaf,5 or expanded lamina, including the thong-like pro-
longation of the midrib, to the end of which the pitcher
is attached, may be regarded as a complete insect-lure,
seeing it also is found to be studded with honey-secreting
glands, thus presenting to unwary insects a long but
pleasant passage to the cavity of the pitcher below. The
stem, too, was found to possess glands for honey secretion
— in some species to a greater extent than in others.55
The curator of the Edinburgh Botanic Garden drew Dr.
Macfarlane’s attention to the viscid nature of the fluid
secreted by Nepenthes when flowering, and it was found
that this also was a honey secretion, and glands were dis-
covered to be present on the upper epidermis of the
sepals. Dr. Macfarlane then made a minute examination

20 Chapters in Modern Botany chap, i

of the other three genera of pitchered insectivorous plants
at present in cultivation — viz. Sarracenia, Darlingtonia, and
Cephalotus — with the result that substantially the same
condition of things was found to subsist in them all. “ The
pitcher-plants may thus be regarded as ingenious mechan-
isms for first attracting insects, in order to receive their
aid in fertilisation ; and next, for the capture of these
insects, and their subsequent appropriation for purposes of
nutrition.” These are in fact the “ extra-floral nectaries ”
well known in many plants, and which the reader may
most conveniently learn to know by looking for them on a
shoot of cherry laurel.

CHAPTER II

pitcher PLANTS — continued

Use of Pitchers, “ Bionomics ” — Bionomics — Bionomics of Nepenthes
— Morphology of the Pitcher — Bladderwort — Bionomics of
Bladderwort — Allied Forms.

Use of Pitchers, “ Bionomics.” — The view of the
economy of the Nepenthes pitcher held more or less strongly
by some older naturalists, that this was a benevolent provision
of nature to comfort the weary traveller or refresh the thirsty
bird, had of course given way ; not so much before the
distributional fact that these plants inhabit wet places in
tropical forest thickets (where even if travellers were wont
to pass, they with the birds would not need to seek so far
for water), as from the general decay of this cheaply
optimistic teleology. Yet so habitual was this way of look-
ing at things that we have even had in more modern times
the pitchers of Sarracenia and Darlingtonia described as
caves of refuge supplied by a benevolent Providence to
conceal insects from their pursuers.

Some better explanation was needed, and the new one,
in terms of ' that grim and all - pervading struggle for
existence, which naturalists were learning from Darwin
and the times to substitute along the whole line for the
old-fashioned “ harmony of nature,” could not but at once
arrest attention and quickly win its way to acceptance and

22 Chapters in Modern Botany chap.

approval, the more so because of its new and dramatic
form, the plant, usually the passive prey of the animal,
turning the tables and making the animal its victim. A
great step was thus made towards realising that view of
nature, that physiology not of the machinery of the indi-
vidual merely, but of species in their relation to all the life
around them, which it is probably the very greatest of all
Darwin’s services to have put before us in so many of its
scenes. This is what many old writers meant by u Natural
History,” and what too many modern German authors un-
fortunately confuse with the well-established general name
including all the fields of organic science as “ Biologie.”
Semper therefore prefers to speak of this his favourite study
(see his Animal Life in International Science Series) as
the u physiology of organisms,” of course in distinction to
the physiology of organs. Mr. Wallace terms this the
u higher physiology,” while Professor Ray Lankester has
suggested the convenient term of bionomics.

The last term has many advantages, not the least being
that its very sound and form helps us to realise its meaning
as expressing the economics of each of the innumerable
species with which we share the planet. It is, we trust,
likely to come into general use, and to supersede the
vague or confused terms above mentioned.

Bionomics. — It is important clearly to distinguish in the
work and influence of Darwin the various elements ; since
putting aside altogether his evolutionary theories, his work
in thus reopening the study of natural history in its widest
aspects, of constituting Bionomics as Cuvier did Compar-
ative Anatomy or Palaeontology, or Linnaeus Taxonomy,
must always remain of the first magnitude. It is thus
worth a little time fully to realise this. The child at first
delightedly watches the bees and butterflies upon the
flowers ; grown a little older he hunts and kills ; and

II

Pitcher Plants

23

by and by, when the civilised “ mania of owning things ”
has arisen, he collects. At school and college he learns
to name and to analyse ; normally too, alas ! to forget
all discontent though grammar be substituted for literature,
and form in all things for life, and though every outdoor
aspect of nature be forgotten during a whole youth
wasted in imprisonment between the whitewashed school-
room and the ball exercise -yard of his school. Thus
prepared, circumstances may make him £c a naturalist ”
again, but now with a difference from his childish starting-
point. His first impulse will be to seek his accounts of
nature in books and to comment on his predecessor, as
naturalists did all through the middle ages, and as most of
us do too much still ; if he go beyond this it is in the
first place to make a collection ; especially as he has here
the lucid logic, the consummate academic discipline
handed on from Linnaeus to guide him, and so he becomes
a systematist — it may be a Bentham or an Agardh ; but if
so, concentrating himself on his herbarium, group by group,
leaving the insects to their keeper over the way in the
Zoological Museum. Or a later medical education (itself of
course deeply influenced from the schools) may dominate the
preliminary one, and thus we get an anatomist — it may be
an Owen or a Huxley — and so far indeed our contemporary
university and school presentation of natural science has now
actually come — witness the accepted text-books (excellent of
course from their point of view) of “ Elementary Biology,”
“ Practical Botany,” and their various sources and imita-
tions. But all this time we have been taking no sufficient
note of Darwin. He, happiest as well as greatest of
naturalists, has gone straight through school and college,
but obviously with but an irreducible minimum of their
result upon him. Still fresh from the gardens and wood-
lands of childish and boyish home, he passes to his

24 Chapters in Modern Botany chap.

“ Naturalist’s Voyage.” Year after year, he watches for
himself the drama of organic nature, sees it more fully and
more deeply than ever great naturalist before had the
good hap to do ; and so returns to use indeed the museum
and the scalpel as well as another, but always as a mere
means to an end — that of watching the organic drama,
scene by scene, and if it might be deciphering the inner
mechanism of the plot. For him, as for not a few pene-
trating predecessors, the plot is Evolution (whatever that
may mean), and his special interpretation of its mechanism
is his world-famous “ theory of natural selection,” at which
we have already glanced, and to which we shall come more
fully by and by. Now it is plain that this reading of the
drama of the universe neither began with Darwin nor can
end with him ; it is indeed at the very outset frankly to be
admitted as one of the purposes of this little book to help
the reader towards getting beyond the Darwinian theory of
the progress of nature ; yet all the more must it be insisted
on, not only that we appreciate clearly what that theory is, —
and this, of course, in no mere literary fashion, — but as an
actual seeing of nature, scene by scene, as it appeared to
Darwin’s eyes : this too not merely for the general theo-
retic interest, much less the special controversial one just
hinted at, but for its intrinsic interest as well — indeed first
and foremost. At the drama of evolution mankind are
but awakening spectators ; here is one who, even if we
put aside his general interpretation of its nature and
mechanism for the moment altogether, we should still have
to appeal to as not only the most patient but the most
penetrating of observers. We cannot have, it is true, too
full a list of the kinds or “ species ” of the innumerable and
strangely varied dramatis persona; we cannot look too closely
into their corpses as they fall, else we shall fail to understand
much ; if we dry or pickle these sad remains they will be of

II

Pitcher Plants

25

use for reference. Even more useful is it to excavate ancestral
tombs, and bring out their fossils ; indeed none has set
us a better example than Darwin himself in all these very
ways.

But not the smallest living scene — not even this bee upon
its flower — is to be understood from our museums and her-
baria : for this exhaustive division of labour, with its ento-
mological and botanical specialists, in winning extension of
exact and detailed special knowledge, had lost sight of de-
veloping that vague general knowledge with which childhood
begins. Watching the bees among the flowers is an old and
happy occupation, not only for children but their elders, and
many a writer, from the prosaic economist up to the master
of all poets, has long ago said his say ; their points of
view, however, were alike always too impressionist or too
anthropomorphic in standpoint to contribute anything to-
wards exact natural science, and so mummy-labelling and
shelving went on indoors undisturbed. Once only a hundred
years ago a childlike old German botanist went out into
the garden and watched summer after summer, till he saw
what the bees were doing, all unconsciously, to the flowers,
and learned how they and the flowers were fitted one to the
other in every detail of form like hand and glove ; and
when he was sure of his facts he could keep the secret no
longer ; he noted down everything that he had seen, and
this too with excellent drawings, calling his book, in naive
childlike delight and pride, The Secret of Nature Dis-
covered ! But the botanists indoors would not look at his
book, save at most to say — What nonsense ! what childish
fancies ! what waste of time ! So it was soon forgotten,
and lay for a century unnoticed, until a naturalist, not con-
ventionalised in the museums, nor over- educated for his
intellect, but persistently childlike in questioning and
watching, and watching and questioning again, should

2 6 Chapters in Modern Botany chap, ii

come once more. And so Darwin wrote On the Fertilisa-
tion of Orchids , and Hermann Muller, Hildebrand, Kerner,
Delpino, followed suit ; while MacLeod and a whole younger
generation are following these nowadays.

Bionomics of Nepenthes.- — It is time to return from the
history of bionomics in general to our special scene ; that
of Nepenthes, luring and entrapping its flying and creeping
insect prey. We may now group around this some of
the minor incidents which naturalists have gradually
described. Thus a recent traveller in Borneo descants
upon the superior intelligence of certain ants, who refuse
to be inveigled into the pitchers, and succeed in drinking
its fluid contents, rich with the sapid juices of their less
wary congeners, by piercing and sucking the tendril-like
stalk upon which the pitcher hangs. He or another
even credits them with knowing that water rises to its
own level, and so with taking care not to pierce the stalk
higher than the level of the fluid in the adjacent pitcher !
It is a good story, and constructed on excellent, one may
almost say standard, lines ; the sceptical reader may wish,
however, to know whether the ants, however, were not
simply licking up the sugary exudation of those glands
which, as above mentioned, were left to Dr. Macfarlane to
notice outside the pitcher, and which, especially in a species
so carefully treated by the ants, might fairly be expected to
be more abundant as one descended towards it. Be this
as it may, we owe to the same observer another interesting-
picture, that of the odd little lemur ( Tarsius spectruni)
prowling over the Nepenthes pitchers, fishing out with its
long-clawed fingers their insect contents, and confiscating
them to its own use. One species, however (N. bicalcarata ),
he tells us, gets the better of the Tarsius, repelling, and if
need be punishing, the robber by help of a pair of long
strong prickles which grow from the lower side of the base

Fig. 4. — Pitcher of Nepenthes bicalcarata showing downward-directed prickles.
(After Burbidge.)

28 Chapters in Modern Botany chap.

of the lid downwards into the pitcher opening, their points
ending just where the intruder would naturally insinuate its
neck. And so on, as with Sarracenia, we see that nature’s
scenes are like Shakespearian ones : around the main in-
cident (or what we take to be that) there may be grouped
all manner of by-play, here quaint or picturesque, or again
laden with deadly issues.

Morphology of the Pitcher. — Baillon, and indeed first
of all Linnaeus, pointed out that by exaggerating the con-
cavity of a (peltate) leaf like that of the Water-lily ( Nym -
phcea) we might obtain a pitcher like that of Sarracenia.
Baillon has described intermediate forms — incipient
pitchers — exhibited by a variety of Peperomia arifolia , a
plant allied to the Peppers, and without going so far afield
we may see exhibited nearly every year at meetings of
botanical or horticultural societies specimens of monstrous
pitcher leaves in cabbage, lime, and other plants, where
elongated stalk or enrolled leaf forms a well for the rain-
drops. A recent writer describes such a pitcher upon a
leaf of vetch ( Vida septum ), of which he ascribes the
origin to the puncture of an insect.

Heckel’s theory of the pitcher of Sarracenia is that it re-
presents a hollowed leaf-stalk, the lid corresponding to the
blade of the leaf. However this may be, the interpretation
already given of the Darlingtonia pitcher as a further develop-
ment of that of Sarracenia of course remains unaltered.

Many authors have been wont to regard the broad leafy
portion of the Nepenthes as but a marginal expansion of
the lower portion of the leaf-stalk, the tendril being its
upper portion, and the pitcher thus corresponding to the
whole leaf, pouched as we have seen is actually the case in
Cephalotus. They trace the margins of the leaf in its ex-
ternal wings, and point out the little threadlike tip of the
leaf just behind the lid.

II

Pitcher Plants

29

For Hooker the tendril is a simple prolongation of the
leaf such as we see in various leaves, e.g. the lily-climber
Gloriosa , while he describes the development of the pitcher
as a simple dimpling and deepening of the upper surface
near the extreme tip of the tendril, which survives as the
mere rudiment already mentioned. He explains this
strange development as finding its possible initial rudiment
in those water-secreting glands common at the tip of so
many leaves, an apparatus which the reader may see at
work in the drop which often hangs at the leaf tip of his
white “ Lily of the Nile55 ( Richardia africana) ; still better,
in many greenhouse arums ; or, best of all, on the dew-
gemmed leaves of Lady’s Mantle ( Alchemilla vulgaris ) in a
summer morning’s walk. Applying our histological experi-
ence, too, we may prepare excellent microscopic specimens
of these water glands from the leaf tips of Saxifrages or of
Indian Cress ( Tropceolum ) by carefully scraping away the
lower epidermis and parenchyma upon a glass slide in a
drop of water, and then turning over the remaining epi-
dermis to show its upper surface.

Partly from his study of the pitcher leaves of seedling
Nepenthes, which appear immediately after the cotyledons,
and in which the pitcher seemed to him to develop from
the first as a much more important portion of the leaf,
Dickson was led to give up this doctrine of Hooker’s. The
curiously “ interrupted ” leaves of some Crotons (C. inter -
ruptus ), in which the flat portion, the intervening midrib,
and even the pitcher in a rudimentary form, are all present,
the latter as simply the upper third of the leaf, seemed to
him to afford the key to the difficulty ; the detailed develop-
ment of the pitcher as a leaf pouching seeming to him
essentially similar to that of Cephalotus, above mentioned.
Dr. Macfarlane’s conclusions as to the nature of the pitchers
are very different. He believes that in Nepenthes, Heliam-

30 Chapters in Modern Botany chap.

phora, Sarracenia, and Darlingtonia alike, the pitcher is
developed from what is originally a compound leaf, con-
sisting of from two to five pairs of leaflets. But there is a
marked tendency to dorsal fusion of these leaflets from
apex to base. Such fused leaflets are seen in the broad
basal part of the Nepenthes leaf, and in the flaps and lids
of the various pitchers. The pitcher itself is a deep dorsal
involution of the midrib just above the termination of the
fused upper pair of leaflets, except, indeed, in Cephalotus,
where, as Dickson clearly showed, it is an involution of the
leaf blade.

Professor Bower by no means agrees with Macfarlane.
He interprets the lid of Nepenthes as composed of a single
pair of leaflets fused together ; on the other hand, the lid
of Sarracenia as merely the flattened terminal portion of
the modified leaf.

Goebel lays stress upon the scantiness of the evidence
upon which both these ingenious rival theories of the com-
plex origin of the pitcher have been erected, and believes
that the structure of all the pitchers is very much the same,
that all may be derived from a peltate leaf in which a deep
involution of the upper surface has occurred. As to the
side wings, in which some see the vestiges of leaflets, he
regards them as entirely secondary growths.

We have cited all these opinions — and we might have
given others — just because in their puzzling divergence
they illustrate the difficulty, yet fascination, of morpho-
logical studies. It is not important to the student to “get
up55 this doctrine or that; indeed the teacher may with
advantage postpone or even refrain altogether from express-
ing his own judgment ; what really is important is that
the student should know how such a question is asked and
answered — partly by a study of the actual form alike in its
obvious and in its microscopic structure ; partly by com-

II

Pitcher Plants

3i

paring the form in question with that of nearly related
plants ; partly by observation of the young plants and their
gradual development ; partly by attention to those mon-
strosities which often reveal the secret of strange structures.

It is only when the student has learned to place himself at
the standpoint of several distinct theories, and to state and
weigh these impartially, that he becomes able to give his
adherence to one or other view. Nor can he fully do
this without reading for himself the original papers, and
perhaps reinvestigating the subject for himself after all.

Bladderwort. — Our tour round the world in search of
pitcher-plants may find appropriate completion in the dis-
covery of one not less interesting almost at our own doors.
In marshy lochs and mountain tarns the Common Bladder-
wort makes itself conspicuous for a month or two in
summer, when from the floating stem the flower-stalk rises
bearing quaint bright golden blossoms, somewhat orchid-
like in appearance, though »really akin to the primroses,
which are commonly considered to represent the simpler
regular ancestral form, much as do lilies to orchids, or potato-
blossom to foxgloves and snapdragon. At other times the
plant is not so readily seen, for it floats in the water, and
its leaves are small. Like some other aquatic plants the
water bladderworts have no roots, and the straggling stem
bears numerous, much -divided slender leaves. Among
these are hundreds of little bladders. From the main
submerged stem of Utricularia vulgaris , and yet more
markedly in tropical species, peculiar thin shoots, which
Goebel calls “aerial shoots,” rise to the surface, and bear
leaves slightly different from those on the other parts of
the stem. It seems likely that this part of the plant is
of special use in effecting interchange of gases with the
air.

Each bladder — shown by its mode of development to be

3 2 Chapters in Modern Botany chap.

a modified leaflet — is a simple but effective trap. It is a
hollow chamber, about of an inch in length, entered
by a thin transparent door or valve which opens inwards
only and allows of no egress, for it shuts instantly, as if
with a spring, against an anterior thickened collar or pro-
jection around the mouth.

These traps are very fatal to small Crustaceans, popu-
larly known as water-fleas, which swarm in every fresh-
water basin. Pursued by their enemies, or attracted
perhaps by a slight mucilage which is exuded from glands
on the door of the trap, or prompted it may be by wayward
curiosity, the water-fleas clamber on six or seven long stiff
bristle-like processes which project from the mouth of the
bladder. So far they are safe enough, but if they explore
farther, and push before them the inward-yielding door of
the bladder, they are within a prison from which there is
no escape. For a day or two they may live, but the trap
becomes crowded with prisoners, and they die. No diges-
tion occurs, but the bodies of the animals are decomposed
by Bacteria, and the products of decomposition are ab-
sorbed. That these products are useful to the bladderwort
is confirmed by Biisgen’s observation that those plants
from which all water -fleas were artificially excluded did
not flourish so well as those in normal conditions. Darwin
believed that the four-fold hairs which occur abundantly
over the internal surface of the bladder were absorbent
structures, but this is by no means certain. Indeed these
peculiar hairs are connected by intermediate forms with
the slime-secreting hairs found outside the bladders, for
instance on the door. It is worth noting that such hairs
are to be found in many aquatic plants quite innocent of
insect catching ; as, for instance, on the leaves of Calli -
triche verna. Chodat, and after him others, have shown
that they arise from cells which under ordinary circum-

ii Pitcher Plants 33

stances would have formed the stomata of an aerial leaf.
(See Chap. IX.)

The seasonal life of the aquatic bladderwort is interest-
ing. Throughout the summer it floats on the surface of
the water, and the straggling stem grows at one end as it
dies away at the other. Perhaps too many decomposing
Crustaceans, however good for the growth of the plant,
may not be altogether good for the leaves which most
directly receive this liberal manuring. As the summer
ends, and as the water-fleas cease to swarm in the pond,
the life of the plant becomes concentrated in a thick-set
terminal tuft. This, as in some other aquatic plants, sinks
to the bottom of the pond and passes the winter there.
In spring, lightened perhaps of what stores of reserve
material it contained, the stem rises again, and forming a
fresh set of bladders begins to grow vigorously at the
surface. The older botanists, e.g. A. P. de Candolle,
believed that the bladders were floats for the plants ; at
first they were filled with mucus, and the plant rested at
the bottom ; afterwards they were filled with gas, and the
plant rose to the surface. This was a pretty notion, but not
true. The plant will float without any bladders, or when
all are full of water ; the bladders have really nothing to do
with the floating.

Bionomics of Bladderwort. — It may be that the water-
fleas enter the bladders in search of Infusorians and other
small creatures on which they feed ; it seems likely, too,
that the bladderwort does really profit by the capture of
the water- fleas. There is another strand in the web.
Amid the Utricularia one commonly finds certain water-
spiders, who make for themselves a diving-bell with air
which they carry from the surface, their bubble glistening
like silver as they descend. Have not these clever crea-
tures come to recognise that the bladders of Utricularia

D

34 Chapters in Modern Botany chap.

are so many larders, and do not they rifle them ? Thus,
as in former cases, there would be a play within a play.

Allied Forms. — Not all the species of Utricularia, how-
ever, are aquatic ; some, especially in the Tropics, are terres-
trial plants. In these, though the booty is of course different,
the bladders sometimes borne by the creeping underground
stems may capture small terrestrial animals, larval earth-
worms, centipedes, and the like, much in the same way
as the aquatic species do. The same is true of an allied
terrestrial genus (Polypompholix). The terrestrial bladder
worts usually live in damp places, but some are perched
on the mossy stems of trees. For each kind of habitat
there are special adaptations : the aquatic forms have
sometimes air-reservoirs which act as buoys for the upright
flower -stalk ; the epiphytes often have water -reservoirs
which enable them to survive the dry season ; the ter-
restrial species, though rootless like all the others, have
little processes or rhizoids which descend into the ground,
especially at the base of the flower -stalk, and serve to
steady the latter as well as to absorb water.

Allied to the Utricularia there is another rarer insecti-
vorous plant, Genlisea, which is represented by several
species from Brazil, Cuba, and Angola. It lives in marshy
places, and, like the bladderwort, is rootless. The stem
rises upright from the ground, and is thickly beset with
leaves, most of which are spathulate, while others are modi-
fied into strange twisted descending staircases. These are
long-necked and lined with downward-directed hairs, which
at once aid an animal in its entrance and prevent its retreat.

Pinguicula, Utricularia, and Genlisea all belong to the
same order (Lentibulariaceae), and may be grouped, as
Goebel points out, in a series. “ We regard,” he says,
“ forms like Pinguicula with a rosette of slimy leaves and
a central flower- stalk as near the starting-point of the

II

Pitcher Plants

35

series. From this Genlisea has diverged in one direction,
Utricularia in another. That they have lost their roots is
not to be wondered at, for they are in great part aquatic
plants. Genlisea diverges but little from the hypothetical
ancestral form ; it has indeed very remarkable bladders,
but it retains the rosette of leaves seen in Pinguicula.
This is still represented in the terrestrial bladderworts, but
here there are most marvellous modifications of leaves
which obliterate the distinctions between leaf and shoot.
In the aquatic bladderworts the terminal floral axis of the
seedling is suppressed, but there is still a rosette of leaves,
though often only in rudimentary form. The strength of
the development lies in the floating shoots, which are
homologous with leaves.”

Before we leave the pitcher-plants and bladderworts, we
shall simply notice another strange plant — the Scaly Tooth-
wort (Lathrcea squamaria ), which is sometimes found in our
woods. It is a parasite on the roots of trees and shrubs,
and being without chlorophyll looks wan and strange.
We shall return to it in a subsequent chapter, but it is of
interest here to notice that its underground toothlike leaves
are not only solidly thickened stores for the food-reserve
appropriated from their hosts, but contain small hollow
traps in which many kinds of small terrestrial animals are
ensnared. The underground buds of Bartsia alftina show
a somewhat similar structure, and also imprison minute
animals. It is interesting also to note that these Bartsias
with some of their nearest allies, like the pretty euphrasy
and the curious yellow rattles and louseworts,1 and one
or two others, are all parasites upon other plants as well,
their roots sucking those of grasses ; while through them,
as we shall see later, we pass to true parasites.

1 Euphrasia officinalis , Rhinanthus Crista - gallic Pedicularis
sylvatica , and P. palustris.

CHAPTER III

OTHER INSECTIVOROUS PLANTS DIFFICULTIES AND

CRITICISMS

Fly-Traps (. Dioncea and A Idrovanda) — Sundews and Birdlime
Traps — Bultei'worts — Sundews proper ( Drosera ) — Details ,
Functional and Structural — Digestion — Movements — Absorp-
tion— Utility — Other Insectivorous Pla7its — Legends — Diffi-
culties— Further Difficulties and Criticisms — Direction of
further Investigation ; Possible Compromise.

Fly-Traps (Dionsea). — Besides those insectivorous plants
which we have already studied under the general title of
“ pitchers,” there are others more active in insect-catching,
which we may call “ fly-traps.” Of these the most famous,
cynically nicknamed u Venus’s Fly-Trap” ( Dion<za musci-
pula ), grows in damp places in the east of North America,
occurring in very local distribution in North and South
Carolina, especially near the town of Wilmington. It
was the first of the insectivorous plants to attract atten-
tion, for in 1768 Ellis, a London merchant, but a shrewd
naturalist withal, who discerned the animal nature of coral,
sent a description of the plant to Linnaeus, who in his
enthusiasm called it “ miraculum naturceP But he sup-
posed that the insects were captured accidentally, and
subsequently allowed to escape.

The Venus Fly-Trap, like its allies the Sundews, grows
on the wet moorland. A circle of more or less prostrate

chap, hi Other Insectivorous Plants 37

leaves surrounds the base of a flower-stalk which bears
numerous flowers at a height of four to six inches from the
ground. Each leaf is a fly-trap. The broadly flattened or
winged {spathulate) stalk is constricted to the midrib at
its junction with the bilobed blade, the halves of which are
movable on one another along the middle, closing together
with a snap, as a very tightly-bound book will sometimes
do. Around each margin are twelve to twenty long teeth,
and, when the leaf closes, those of one side interlock with
those of the other, thus forming a very perfect miniature rat-
trap. The centre of each half-leaf bears numerous rosy
glands, and on each side there are three hairs, which an old
naturalist described as spikes to impale the captured insect,
but which are really quite weak, and bend flat on a basal
joint when the leaf closes.

The student will find it easy and useful to make
a paper model, cutting it out the proper shape, folding
it along the middle line, carefully fashioning the teeth
at each margin, so as to interlock neatly, and gumming
on the three hairs on each half- blade, or more simply
snipping them from the texture of the paper. The glands
may easily be supplied with a red pencil, and a toy, not
to be despised by young or old, is the result. Let us see
how the trap works. If we go — preferably on a warm
and bright day — to our plant, which is common in green-
houses in this country, and with a finger touch the leaf the
closure follows. More careful experiment with anything
nearer the size of the insect’s leg, say a pencil-point, shows
that we may wander all over both surfaces of the leaf or the
marginal tentacles with safety until we touch one or more of
the projecting hairs ; and repeated experiment shows them
to be alone sensitive — the two halves of the blade close.
In the plant’s native haunts some insect does what our
finger did, and if in the rapid closure of the leaf the prey be

38 Chapters in Modern Botany chap.

caught, a secretion from the rosy glands immediately follows,
and the leaf remains closed for a week or two according
to the size of the insect. During that time the leaf acts
as a temporary stomach, and the insect is digested and
absorbed, as far at least as can be expected, and then the
leaf reopens, but remains for a time in a torpid state.
Sometimes, however, if the insect caught happen to be a
very large one, the leaf never opens again, its meal proving
too much for it ; and even in a state of nature the most
vigorous leaves are rarely able to digest more than twice,
or at most thrice, during their life.

The attractive rosy patches on the leaf, the rapid
closure of the blade on its midrib, the interlocking of the
teeth around the margin, the specialised sensitiveness of
the six jointed hairs, the copious secretion of the digestive
glands, combine to make Dionsea a very efficient fly-
trap.

The secretion poured out from the stimulated glands
contains formic acid and a digestive ferment. It further
resembles gastric juice in having marked antiseptic quali-
ties ; thus when Lindsay fed leaves with such quantities of
meat as to kill them with indigestion, the meat inside
remained fresh while portions hanging outside putrefied. So
abundant is the secretion that when Darwin made a small
opening at the base of one lobe of a leaf which had closed
over a large crushed fly, the secretion continued to run
down the footstalk during the whole time — nine days —
during which the plant was kept under observation. That
absorption follows digestion is shown by the disappearance
of the digestible substances, and Fraustadt was able, by
feeding leaves with albumen stained with aniline red, to
colour the contents and nuclei of the gland-cells.

The three pairs of hairs are exquisitely sensitive to the
contact of solid bodies, but are indifferent to wind and

hi ' Other Insectivorous Plants 39

rain. The triangular area between the bases of the hairs
is slightly sensitive, and if the general surface of the leaf
be wounded closure may occur. Inorganic or non-nitro-
genous bodies placed on the leaves without touching the
hairs do not excite any movement, but nitrogenous sub-
stances, if in the least degree damp, cause after several
hours the lobes to close slowly. Leaves which have made
a mistake and have closed over useless bodies, reopen after
a few hours, or at most a day’s rest, and are again ready for
action. According to Macfarlane, mechanical stimulus of
the fly-trap requires two touches, unless the stimulus be
very powerful, and the touches must be separated by an
interval greater than one-third of a second. If less than
one-third of a second be allowed as interval, no contraction
follows, and a third touch is then necessary.

As to the movement of the fly-trap, Darwin detected a
measurable contraction or alteration of form, and showed
that the movement follows a stimulus passing through the
cellular tissue from the sensitive hairs. There are really
two kinds of movement — one rapid, which follows the irri-
tation of the sensitive hairs ; the other slow, excited chemi-
cally, as when the leaves gradually tighten their hold on a
fly and bring the glands on both sides into contact with it.
Burdon Sanderson shows that the electrical conditions
associated with the rest and activity of the leaf are closely
like those observed in our muscles. Not only is there
a normal electric current in the leaf, but at the moment
of closure there is what animal physiologists know as a
“ negative variation,” due to the conversion of electro-
motive. force into mechanical work. These facts lead us
to believe with Burdon Sanderson that “ the property by
virtue of which the excitable structures of the leaf respond
to stimulation is of the same nature as that possessed by
the similarly endowed structures of animals.”

40 Chapters in Modern Botany chap.

Aldrovanda. — Allied to Dionaea, and with a some-
what similar leaf, is a water fly-trap ( Aldrovanda vesicu-
losa), which lives in clear well-sunned ponds in south and
central Europe, and also occurs in Australia and India.
Like the common bladderwort, Aldrovanda has a thin root-
less floating stem, which bears whorls of modified leaves.
It dies away at one end as it grows at the other, and is
reduced in autumn to a concentrated tuft, which sinks for
the winter to the muddy bottom of the pond, thence to rise
again in summer after it has exhausted its stores of starch
and has become buoyant with gas. The little leaves have
a spathulate stalk and a folding two-lobed blade with teeth
round the edges. Although in figures of the plant the
leaves are commonly represented as open, like those of
Dionaea, it is said that their lobes really open only about
as much as the valves of a living mussel, and are thus the
better fitted for capturing small animals. The surface bears
numerous sensitive jointed hairs and colourless stalked
glands, and the leaf closes on water-fleas, larvae of insects,
and even diatoms, very much after the fashion of Dionaea.
Darwin believed that the glands secrete a digestive fluid,
and that small four-lobed hairs situated on the outer thinner
parts of the leaf absorb decaying animal matter. Goebel
is inclined to think that these four-lobed hairs secrete some
slimy substance, perhaps attractive to small animals.

Sundews and Birdlime Traps. — It is said that Portu-
guese peasants use as a substitute for fly-paper the viscid
leaves of Drosophyllum lusitanicum , a common plant in
dry, sandy, or rocky places in Portugal and Morocco, and
hence, it is worth noting, with a much better developed
root system than its marsh-loving allies. It grows eight
or ten inches high, and has long narrow strap-like leaves
(which are interesting as being rolled up in the bud like
those of ferns, but backwards instead of forwards), beset

hi Other Insectivorous Plants 41

with stalked glands which secrete a viscid dewdrop-
like juice. On these leaves insects, attracted it may be by
the glistening drops and by the reddish colour, but more
probably by the honey-like fragrance, alight, and knocking
off drops from the stalked glands become besmeared and
choked. They sink on the surface of the leaf and then
small unstalked, uncoloured glands exude a dissolvent
secretion. On a plant a year old, which had lived in a
glass house with open doors, Goebel on one occasion
counted no less than 233 distinctly visible flies, distributed
over nineteen leaves.

Belonging to the same order as Drosophyllum and
the more familiar Drosera, there are two other sticky
plants in which the “ insectivorous habit ” is not more than
incipient. These are Roridula dentata from the Cape,
and Byblis gigantea , the latter with simple glands, scarce
differing appreciably from those on many other kinds of
plants, though with a more copious and glutinous juice.
They are interesting in showing the beginnings of the
peculiarity which becomes so marked in the sundews, and
in the same connection we should notice that leaves and
stems of some geraniums, sedums, and primulas have glan-
dular surfaces on which insects become entangled.

Butterworts. — The Common Butterwort ( Pinguicula vul-
garis) is very common on marshy grounds, especially among
the hills. It has a wide geographical distribution, repre-
sents a genus with about forty species, and belongs to the
same order as Utricularia (. Lentibulariacece ). It has long
been known, though not in connection with insect-catching,
for Linnaeus noted that the Lapps used it for curdling
milk. Every one who has tramped over high wet moor-
lands or followed the banks of a mountain stream up into the
hills know the appearance of the plant, — the rosette of plump
glistening leaves prostrate on the ground, the beautiful

42 Chapters in Modern Botany chap.

violet flower raised on an upright stalk. The plump leaves,
to which the plant owes its quaint name Pinguicula ( = little
fat one), have a characteristic fungus-like smell perhaps
attractive, and are covered with stalked and unstalked
glands, which exude a copious viscid secretion. This
obviously catches insects, and further, Darwin tells us,
also digests the small flies and midges which carelessly
allow themselves to be limed. To the touch of other
things, raindrops or sand-grains for instance, the butterwort
is indifferent, but a little insect provokes abundant secre-
tion. But this is not all ; the leaf moves, slightly capable
of motion as it seems. When an insect is entangled, the
edges of the leaf curl slowly inwards for an hour or two, not
fast enough indeed to catch an insect, but that is not neces-
sary, yet sufficiently to enclose the booty, or shift it inwards,
or at least expose it to the action of a greater number of
glands. The result is that the insect’s body is soon dis-
solved away, only indigestible chitinous shreds being left.

It seems that the butterwort forms the two ferments of
most gastric juice — a rennet-like ferment, to which the
plant’s power of curdling milk is due ; and a digestive or
peptic ferment, which dissolves the usable parts of the
bodies of insects. It is possible that the antiseptic pro-
perties of these ferments justify the old custom of applying
the leaves of the butterwort to the sores of cattle, but we
should not like to commit ourselves to any such apology,
since the coolness and dampness of such a plaster and its
special power of keeping flies from the sores are evidently
so far sufficient.

Professor J. R. Green describes a rennet-forming fer-
ment comparable to that of the calf’s stomach — not only in
Pinguicula, but in the flowers of the yellow bedstraw
( Galium verum ), in the stem of the Clematis, in the petals
of the Artichoke, and in other plants. A peptonising fer-

hi Other Insectivorous Plants 43

ment, like that of the gastric juice, occurs not only in
insectivorous plants, but in situations so different as the
seeds of the Vetch or the milky juice of the papaw-tree
{Carica papaya). In warm countries, where of course fresh
meat cannot be hung for any length of time, it is common
to make a joint rapidly tender by applying to it the leaves
of this tree, a practice which probably finds its explanation
in this presence of ferment. There is also in some plants
an emulsifying and saponifying ferment, which acts on fats
and oils as the juice of the animal pancreas does ; while
the diastase, which, as in germinating malt, turns starch into
sugar, is closely comparable to the ferment of the salivary
juice in animals.

Although from our present physiological standpoint we
have delayed mention of the butterwort until coming to the
sundews, its structural relations are with the bladderworts
and Genlisea. This is shown not only by the characters of
the flower, but in minute details, for, according to Goebel,
the two sets of glands in Pinguicula and the slime-secreting
hairs of Utricularia and Genlisea are all fundamentally the
same.

Sundews proper (Drosera). — Beside the butterwort on
the marshy moor, we may perhaps find one of the Sundews
(Drosera). The genus is a large one, and the species are
widely distributed over the northern parts of both hemi-
spheres. In almost all countries and languages they bear
the same pretty and, so far as description goes, appropriate
name — Rosso Its, Sonnentau , Sundew.

Our commoner British species — Drosera rotundifolia—
grows loosely rooted in marshy and peaty ground, often
embedded among the bog-moss which forms a fitting back-
ground for the rich red colour of the leaves. These, to the
number of half a dozen or so, lie prostrate, and from their
midst arises a small upright stalk with inconspicuous

44 Chapters in Modern Botany chap.

whitish flowers. Each leaf consists of a long narrow
stalk, expanded into a more or less circular blade, the
edges and surface of which bear scores of club-like “hairs ”
or “ tentacles,” apparently tipped with dew. These hairs
are complex structures ; the head of each is glandular and
well supplied with water-pipes (spirally thickened wood-
fibres or “ tracheides ”) ; it is the viscid secretion of the
gland which makes the apparent dewdrop.

These hairs or tentacles, then, are sensitive, mobile,
digestive, and absorptive — most marvellous little structures,
indifferent to the drops of rain which often fall upon them,
but responsive to the stimulus of a midge. An insect unwary
or deluded alights on the leaf, and is forthwith entangled ;
as it struggles the secretion becomes more abundant.
The tentacles too bend down upon the entangled midge ;
first one, and in a few minutes another, and another, till
all the two hundred may close upon the prey like so many
slow merciless fingers. The leaf may become more con-
cave, and after complete closure looks like a closed fist.
As the result of the secretion the booty is digested and the
products of digestion absorbed.

So far the usual general description of the sundew ; but
now let us take up Darwin’s Insectivorous Plants , and read
up the details, for there is no more characteristic example
of his patient elaborate way of working than his account of
the sundew.

Further Details of Functional or Structural Interest.

— There are on an average about two hundred glandular
tentacles. The stalk of each has the essential structure
of a leaf: a small “ fibro- vascular bundle,” consisting
mainly of spiral tracheides, runs up the centre, and is
surrounded by a layer of elongated cells lined by a thin
layer of colourless circulating protoplasm, and filled with
a purplish fluid. The glandular head of the tentacle con-

hi Other Insectivorous Plants 45

tains a central mass of spirally thickened cells in immediate
contact with the upper ends of the conducting tracheides.
Around these, but separated from them by an intermediate
stratum of elongated cells, there is a layer of cells filled
with purple fluid, and outside these another somewhat
similar layer. These two external layers form the really
glandular part. Goebel insists that all the glands of
Droseraceae, whether borne on tentacles, as in the common
sundews, or quite unstalked, as in Dionsea, have essentially
the same structure ; the tentacled and the sessile forms
are connected by intermediate gradations.

The leaves of the sundew seem to have some fascina-
tion for insects, but whether this is due to their colour,
their glittering secretion, their odour, or to all three,
remains uncertain. The drops are so viscid that an insect
may be caught if it but touch one or two of the outer
tentacles ; as they begin to bend and also secrete more
copiously the insect is carried inwards and more effectively
smeared. Even a large insect — such as a dragon-fly —
may be caught by several leaves, though in these cases it
is likely that the insect was to begin with in a weak state.
The bending of the tentacle takes place near its base, and
may be excited in various ways. For although the plant
seems to have grown accustomed to gusts of wind or drops
of rain, and is to its own advantage indifferent to these,
repeated touches will cause the tentacles to bend. On the
other hand, contact with any solid particle, even though
insoluble and of far greater minuteness than could be
appreciated by our sense of touch, will induce movement,
and one would think that in natural conditions movements
so induced must sometimes occur, and to no purpose. A
morsel of human hair, weighing only y-g-yy^- of a grain,
and this largely supported too by the viscid secretion,
suffices to induce movement. Finally, the absorption of

46 Chapters in Modern Botany chap.

even a minute trace of certain fluids, especially nitro-
genous, acts as a stimulus.

During the bending of the tentacle the secretion of
the gland becomes more copious, and its chemical reac-
tion changes from neutral to acid. Meantime within the
stalk of the stimulated tentacle a strange change occurs,
marked externally by a somewhat mottled appearance.
When examined under the microscope the formerly homo-
geneous fluid contents of the cells of the stalk are seen
to have separated in purple bead-like masses, of constantly
varying number, shape, and size, and suspended in a
colourless fluid. This change makes the layer of colour-
less circulating protoplasm which lines the cells more
distinctly visible.

Darwin attached considerable importance to this process,
which he termed “ aggregation of the protoplasm.” It
begins in the glands, and gradually travels down the
tentacle from cell to cell. After the action of the tentacle
is over, a reverse process of redissolution of the protoplasm
proceeds from the base upwards. Darwin believed that
it was a vital process, only exhibited when the cells were
alive and normal, not necessarily connected with either
bending or increased secretion, and quite different from
“ plasmolysis ” or the shrinking of the protoplasm from the
cell - wall, which is observed when parts of plants are
examined in any dense fluid which induces osmosis.
Darwin observed a similar aggregation in the sensitive
hairs of Dionsea, and in the roots of various plants, and
believed that it was of wide occurrence and of profound
importance in the physiology of the vegetable cell.

In connection with the sensitiveness of the Drosera,
one of the most interesting of Darwin’s observations was
in regard to the salts of ammonia. All the salts of
ammonia cause the tentacles to bend — the carbonate

hi Other Insectivorous Plants 47

strongly, the nitrate even more so, and the phosphate most
of all. But the remarkable fact is the sensitiveness of the
tentacles to infinitesimal quantities. Thus the immersion
of a leaf in a solution of the last-mentioned salt, so weak
that each gland could absorb only about -2o"oFU'o"o 0 a
grain, is sufficient to produce complete inflection of the
tentacles. Though the particles of solid matter which
stimulate the olfactory nerves of animals, and so produce
the sensation of smell in animals, must be very much
smaller than this, as Mr. Darwin remarked, the fact
remains truly wonderful that the absorption of so minute
a quantity by a gland should induce some change in it,
which leads to the transmission of a motor impulse down
the entire length of the tentacle, causing the whole mass
to bend, often through an angle of more than 180°, and
this too in the absence of any specialised nervous system.

It is not much to our present purpose to discuss the
numerous experiments which Darwin made on the action
of salts and acids, drugs and poisons on the leaves of the
sundew ; but the effects of organic fluids are important.
Darwin treated sixty-one leaves of Drosera with non-
nitrogenous solutions — gum-arabic, sugar, starch, dilute
alcohol, olive-oil, tea. The tentacles were not in a single
case inflected. He then applied to sixty-four other leaves
various nitrogenous fluids (milk, wine, albumen, infusion
of meat, mucus, saliva, isinglass), and sixty-three had the
tentacles and often the blades well inflected. Finally,
taking twenty-three of the leaves which had served for the
first experiment, and treating them with bits of meat or
drops of nitrogenous fluids, all save a few, — apparently
injured by exosmose caused by the density of the former
solution of gum, sugar, etc., — were distinctly inflected.

Digestion. — That the sundew possesses power of digest
ing the insects which it catches is evidenced in great detail.

48 Chapters in Modern Botany chap.

The secretion of the glands, like the gastric juice of animals,
contains a digestive ferment and several acids, as Frank-
land, Rees and Will, Lawson Tait, and others have shown.
But this is corroborated by what happens to little pieces
of organic material placed upon the leaves. Darwin fed
numerous plants with roast meat and minute cubes of
boiled white of egg, and by way of check placed similar
cubes in wet moss. Those in the moss putrefied, while
those on the sundew were dissolved. Pollen -grains had
their protoplasmic contents dissolved, and seeds were
usually killed. It is interesting to notice further that the
secretion obtained from tentacles stimulated by fragments
of glass was not able to digest, showing that the ferment
is not secreted until the glands have absorbed a trace of
animal matter. Moreover, while the leaves were able to
digest beef, egg, cheese, and the like, they could not
digest horn, chitin, cellulose, and other such substances —
thus completing the analogy with the gastric digestion of
animals.

Movements. — As to the movements of the tentacles,
experiments showed that pricking the leaf or the leaf-stalk
did not induce any response, that the stalks of the glands
were not stimulated by food, that in fact the glands alone
were sensitive. When a tentacle receives an impulse either
from its own gland or from the central tentacles, it bends
towards the middle of the leaf, the short tentacles on which
do not bend at all ; in all other cases all the tentacles,
even those of the centre, bend towards the point whence
the stimulus comes. Thus all the tentacles of a leaf may
be made to converge into two symmetrical groups by
placing a fragment of phosphate of ammonia in the middle
of each half of the blade.

Vivisection showed that the motor impulse travels
through the cellular tissue, and not through the fibro-

hi Other Insectivorous Plants 49

vascular bundles. An impulse thus travels more rapidly
along than across the leaf, since from the shape and lie
of the cells, fewer cell-walls have to be crossed in a given
distance. When the central glands are stimulated they
send some influence outwards, which reaches the external
tentacles and glands. There we can see one of the results
of its arrival in the process of aggregation which descends
the tentacles.

To explain the actual mechanism of bending is a most
difficult problem. Various suppositions have been made.
Thus if we suppose the cells at the base of the tentacle
to be in a state of high water-tension and to possess great
elasticity, a rapid outflow of water might cause shrinkage
and bending. Or it may be that the living matter of cells
is contractile, like that of muscles. What then causes the
outflow, i.e. how does the stimulus set it up ? What
meaning, again, are we to give to the “ protoplasmic con-
tinuity,” which has been traced between the living matter
of cell and cell, and how far shall we grant a share in the
matter to the contractility of this living network ? And so
we are face to face once more with perplexities to muse
over ; which, as they take form in definite questions, will
lead the reader towards the larger treatises and to new
reflection beyond their scope in turn.

Absorption. — It is difficult to make precise statements
in regard to the plant’s power of absorbing the digested sub-
stances. Clark fed Drosera with flies soaked in chloride of
lithium, and after several days found that all parts of the
plants when burned showed the characteristic spectrum of
lithium. But this did not conclusively prove more than that
the lithium salt had been absorbed by the plant. Lawson
Tait, by cultivating plants with roots cut off and leaves
buried in pure sand watered with an ammoniacal solution,
showed that the sundew can not only absorb nutriment

E

So Chapters in Modern Botany chap.

from its leaves, but can actually thrive by their aid alone,
if supplied with a little nitrogenous material. Bennett
described, not only in Drosera, but also in Dionaea and
Nepenthes, what he termed “absorptive glands” lying
beneath the epidermis, and sometimes furnished with
papillae, which rise above the surface.

Utility. — Relatively complete as Darwin’s study of
Drosera and other insectivorous plants was, it did not
adequately meet the scepticism of those who doubted the
utility of the habit. Though Knight in 1 8 1 8 had thought
that plants of Dionaea which he fed with morsels of beef
throve better than others not so treated, many observers
have since failed to see any improvement on insectivorous
plants when regularly fed, or any disadvantage when
prevented from obtaining animal food altogether. And
others have gone so far as to assert that animal food was
hurtful, having injured or killed their plants by feeding.

But it is obviously hazardous to draw conclusions as to
the utility of the insectivorous habit from plants under
cultivation. For it may be that plants living in the green-
house have a richer supply of nitrogenous food from their
roots than they can in natural conditions secure. Sundew
and butterwort very often grow among bog-moss on the
moors, hardly rooted, and therefore less adapted than
ordinary plants to absorb nitrogenous salts from their soil,
even supposing that these were present in abundance ;
which, it is worth noting, recent analyses of boggy soils
show they are not. Here indeed may be the grounds of a
fresh argument, since this relative scantiness of nitrogenous
supplies in the natural surroundings of insectivorous plants
may render them in part dependent on their peculiar animal
diet.

Again, although it has often been noticed that a leaf of
sundew or fly-trap may suffer, or even die, from the effects

hi Other Insectivorous Plants 51

of too large a meal, this can hardly be regarded as a
serious objection against the alleged utility of the insecti-
vorous habit until we learn that the casualty is common in
nature. A similar objection might indeed be urged against
eating dinner.

The gap left in Darwin’s work was soon filled by his
son. Francis Darwin took six plates full of thriving plants
of sundew, and divided off each by a transverse bar.
Then, choosing the least flourishing side of each, he placed,
on 1 2th June 1877, roast meat in morsels of about -i- of a
grain on the leaves, and renewed the dose at intervals.
Soon the plants on the fed sides were clearly greener than
those on the starved sides, and their leaves contained more
chlorophyll and starch. In less than two months the
number of flower-stalks was half as numerous again on
the fed as on the unfed sides, while the number and
diameter of the leaves and the colour of the flower-stalks
all showed a great superiority. “ The flower-stalks were all
cut at the end of August, when their numbers were as 165
to 100, their total weight as 230 to 100, and the average
weight per stem as 140 to 100 for the fed and unfed sides
respectively. The total numbers of seed capsules were as
194 to 100, or nearly double, and the average number of
seeds in each capsule as 12 to 10 respectively. The
superiority of the fed plants over the unfed was even more
clearly shown by comparing their seeds, the average
weights per seed being as 157 to 100, their total cal-
culated number as 240 to 100, and their total weight as
380 to 100.”

“ The fed plants, though at the commencement of the
experiment in a slight minority, were at the end of the
season 20 per cent more numerous than the unfed. In
the following spring the young plants which arose on the
fed side exceeded those on the unfed side by 1 8 per cent

52 Chapters in Modern Botany chap.

in number and by 150 per cent in total weight, so that, in
spite of the relatively enormous quantity of flower-stalk
produced by the fed plants during the previous summer,
they had still been able to lay up a far greater store of
reserve material.”

Similar results were independently reached by Rees,
Kellermann, and Von Raumer, who used aphides as food
for the plants.

It is noteworthy that the beneficial effect of insect diet,
although distinct in the vegetative system, is much more
remarkable in relation to reproduction — a fact which ex-
plains the unfavourable opinion of other observers.

Other Insectivorous Plants. — Besides the apparently
indubitable insect-eaters which we have described, there are
some in regard to which fuller information is still desirable.
Thus not only Dischidia, an Asiatic genus of Asclepiads,
whose pitchers contain internal roots, Martynia, one of
the Pedalineae, but Caltha dioncefolia , a species of the same
genus as our marsh-marigold, and several Aroids have
been called insectivorous. Some South American liver-
worts (e.g. Anomoclada mucosa and P.hysiotium cochleari-
forme') and a fern (Elaphoglossum glutinosum) have been
described by Spruce as capturing numerous insects. The
basally- united leaves of the common teasel ( Dipsacus )
frequently enclose moats of water in which insects are
drowned, and Francis Darwin described protoplasmic fila-
ments apparently emitted by the cells of certain glands within
these cups, and which he supposed to absorb the products
of decomposition. A similar process has been described by
Ludwig as occurring in Silphium , a genus allied to the
teasel.

Zopf has recently described an interesting fungus
(. Arthrobotrya oligospora ), which catches small thread-
worms in great numbers in its nooses, riddles their bodies

in Other Insectivorous Plants 53

with a growth of fine threads (hyphae), and absorbs the
tissues.

Legends. — The existence of insectivorous plants was not
recognised in days when fancy, ever nimbler than knowledge,
was allowed to trespass unrebuked in the domain of science ;
yet for that very reason our subject, which affords so many
convenient types, shows also how nature -legends arise
as of old, by the growing exaggeration and distortion of
some real image, as it is reflected from mind to mind.
Thus in a well -named medium, the Review of Review s,
quoting from “Lucifer ” (but it appears not verifiably even
there), we recently find this traveller’s tale and modern
myth : “ Mr. Dunstan, naturalist, who has recently re-
turned from Central America, relates the finding of a
singular growth in one of the swamps which surround the
great lakes of Nicaragua. He was engaged in hunting for
specimens when he heard his dog cry out, as if in agony,
from a distance. Running to the spot he found him
enveloped in a perfect network of what seemed to be fine
rope -like tissue of roots and fibres. The plant or vine
seemed composed entirely of bare interlacing stems, re-
sembling more than anything else the branches of the
weeping willow denuded of its foliage, but of a dark, nearly
black hue, and covered with a thick viscid gum which
exuded from the pores.” Hardly were “the fleshy mus-
cular fibres” severed; “the dog’s body was blood-stained,
while the skin seemed to have been actually sucked or
puckered in spots ; ” the “ twigs curled like living sinuous
fingers about Mr. Dunstan’s hand ” ; “ its grasp can only
be torn away with loss of skin and even of flesh ; ” “ as
near as Mr. Dunstan could ascertain its power of suction
is contained in a number of infinitesimal mouths or little
suckers, which, ordinarily closed, open for the reception of
food.” “ If the substance be animal, the blood is drawn

54 Chapters in Modern Botany chap.

off, and the carcase or refuse then dropped.” “ A lump of
raw meat being thrown to it, in the short space of five
minutes the blood will be thoroughly drunk off, and the
mass thrown aside.” “ Its voracity is almost beyond be-
lief.” Just as in this story we find a magnified arid dis-
torted image of the actual sundew, so in the same way the
sober and scientific natural history treatise of Aristotle
gradually, through alternately unthinking and fanciful copy-
ing, became all but unrecognisably transformed into that
marvellous compendium of fabulous natural history, the
Physiologus , whence herald or gargoyle-carver drew his
fantastic images. In earlier beginnings we may perhaps
detect the same process at work upon more of Darwin’s
volumes than the one we have been discussing.

Difficulties. — When Darwin published his book on In-
sectivorous Plants , there were many who disbelieved on the
ground that “ only animals have the power of digestion.”
But Morren and others more definitely soon showed the
mistakenness of this opinion, for indeed all plants have
digestive ferments, and many have two or three. The
peculiarity of the insectivorous plant lies then in the out-
pouring of the digestive juice, not in the possession of it.
For even the absorption of organic material is not unique;
it is exhibited by the fungi which live among rottenness
and by some parasites like the dodder.

How could this exudation of digestive juice begin ?
May we begin from glands such as those of some Saxi-
frages, Primulas, Geraniums, and suppose with Darwin
that the exudation of digestive fluid began with an exos-
mose induced by the juices of decaying insects caught
among the hairs, and that the habit once set up would be
perfected by natural selection ? Or may we suppose that
the glands and their exuded secretion have always had
and still have some meaning, apart altogether from in-

hi Other Insectivorous Plants 55

sects ; that they express some more direct physiological
peculiarity of the plant, and that the insect-catching is
after all a minor function ? Let us return to the pitcher
plants.

Further Difficulties and Criticisms— How far then are
we to regard the pitchers with Hooker, Darwin, and so many
others, as primarily insectivorous in function, and to account
for them as marvels of the perfecting action of natural
selection in progressive adaptation to that strange use ?
Scepticism and even controversy were rife enough fifteen
years ago, but gradually these diminished and disappeared,
and most botanists (with whom we may apparently reckon
the very latest author, Professor Goebel, although it is to
be regretted that the physiological part of his treatise has
still to appear) undoubtedly accept this alike as an estab-
lished doctrine of vegetable physiology and an admirable
special case for the Darwinian theory, explaining every
detail of elaborate structure and attractive colour in terms
of that view. To this persuasion also the writer was wont
fully to belong ; witness the undoubting orthodoxy of his
article on “ Insectivorous Plants ” in the Encyclopcedia
Britannica (vol. xiii. 1879). Yet, as better acquaintance with
the large Edinburgh collection went on year by year, his
faith, it must be confessed, gradually — at first almost in-
sensibly— diminished. Experiments on digestion did not
come off so well as Dr. Hooker, still less as Rees and Will,
would have it ; but this one at first put away as temptations
to unbelief — indeed, was ashamed to speak of, for were they
not more likely due to some defect in experiment or experi-
mentalist, or if not, to some unlucky dyspepsia of these
particular pitchers ? The old criticisms of the doctrine, too,
were often so unphysiological and in evolution so reactionary,
it seemed incredible that they could be of any value !

Timid comparison of notes with Mr. Lindsay, the curator

56 Chapters in Modern Botany chap.

of the Botanic Garden, than whom these plants have never
had a more experienced cultivator, led to a mutual confes-
sion of diminished certitude. Mr. Lindsay had often been
annoyed by the deterioration of specimens of Nepenthes
lent to flower shows, until it occurred to him that this might
be due to the loss of the fluid in transit. He gave orders
that the next plants so treated should have their pitchers
refilled to the proper level on arriving at the show, and
again on their return ; and when this was done he was
gratified to find that the plants no longer suffered. Hence
then the fluid is of importance to the plant itself ; the
pitcher seems a reservoir of the water of transpiration. It
is hardly necessary here to recall the distillation of dew
from plants by night ; the gemmed leaf-tips of the lady’s
mantle should be familiar to every one who has ever taken
a morning stroll along a dewy lane, while every grower of
hothouse arums and the like is familiar with the dropping
which goes on from the leaf- tips. In 1885 Kny and
Zimmermann called attention to the very considerable
development in the veins of the Nepenthes leaf of cells
of that spirally thickened type which is associated with the
carriage of water, and speculated as to its importance for
the internal water supply of the plant ; while Maury in 1887
insisted that the secreting glands of the Cephalotus pitcher
were not the special and essential adaptations towards insect
capture and digestion, as which they are commonly de-
scribed, but mere “water-stomata, which play the part of
regulators of transpiration, which give off water when there
is an excess, and take it up again when there is a deficiency.”
Again, “ the presence of fluid up to a certain level is the
sole cause of the uniformity and polish of the epidermis ;
one should not see in it a surface specialised as detentive
for insects.” He denies the digestive agency of the fluid,
finds indeed drowned insects, but also infusorians, green

hi Other Insectivorous Plants 57

algae, and zoospores all alive, and insists that if the liquid
were really very digestive, these could not survive its
action. He sums up that the physiological use of pitchers
is a general one (associated with transpiration), and not an
exceptionally specialised digestive one, as so commonly
believed, and lays considerable stress on the comparatively
frequent recurrence of pitchers, monstrous as well as nor-
mal, among unassociated plant -forms as additional pre-
sumptive evidence of their relation to some constant rather
than exceptional function of plant-life.

Again, M, Treub, the distinguished director of the
famous tropical garden of Buitenzorg (Java), has sug-
gested the internal roots of the Dischidia pitcher to be con-
nected with the reabsorption of water rather than of insect-
broth. In a book written before Hooker and Darwin had
made Nepenthes and its physiological analogues so famous,
Grisebach’s Distribution of Plants (vol. ii.), we find much
speculation on Nepenthes, apparently overlooked by subse-
quent writers, from which a couple of sentences may be
profitably extracted : “ So considerable a loss of water
should accelerate the circulation of sap much more strongly
than would mere transpiration from the surfaces of the
leaves. . . . All that the geographical distribution of
Nepenthes (Madagascar to New Caledonia) suggests as to
their organisation amounts to this, that they inhabit insular
climates, of which the atmosphere, abundantly laden with
water-vapour, impedes evaporation.”

A still more serious criticism is furnished by the latest
experiments on the digestion of pitchers and sundews —
those of Professor Dubois of Lyons, who has not only the
advantage of being apparently the only trained animal
physiologist who has yet worked at the question, but also
of having at his command the experience and the resources
of that vast development of bacteriological science which

58 Chapters in Modern Botany chap.

has grown up, one may practically say entirely, since
Hooker and Darwin, Tait, and others carried out the
experiments upon which current ideas became established.
Without denying then the existence of traces of digestive
ferment, such as may be prepared from all or almost all
living protoplasm, be it of a seed, a fungus, or a morsel
of muscle, he affirms that when the fluid of a pitcher is
sterilised so as to exclude the action of bacteria, no diges-
tion takes place — in short, for him such digestion and dis-
solution of the bodies of insects or artificially supplied food
material is simply the work of bacteria, and so comes
into line not with digestion, but with putrefaction and
decay. He even extends this to fly-traps and sundews.

Need of Further Investigation ; Possible Compromise.
— It may seem at first sight as if we were but returning to
the position of the ancients, yet from either side of the dis-
pute it is easy to correct that impression. Even if our Dar-
winism be vain, a new explanation has come in sight — that
associated with transpiration — and has to be applied in turn.
For if pitchers be reservoirs, how do they operate ? How
shall we explain the glutinous and deliquescent “ azerin ” —
say, as impeding evaporation, or even itself at times helping
to draw water from the atmosphere, as the aerial roots do for
an orchid ? Or does it act, say on sundews, by aiding the
transpiratory current, so necessary to the ordinary processes
of vegetative life and growth by compelling an exosmose of
water to dilute it ? Here, then, a new and as yet practi-
cally untried field of experiment opens out before us. In
fact, despite Darwin’s volume, the whole subject of tran-
spiration in sundews and other insectivorous plants, notably
of course Nepenthes , has still to be experimentally investi-
gated by the vegetable physiologist, before the function of
pitchers or tentacles can be really understood. Yet our
Darwinian interpretations will not be so easily dismissed,

hi Other Insectivorous Plants 59

for the movements, the increased and changed secretion of
Drosera, still more of Dionsea, are obviously not by mere
transpiration to be explained away. And even if the stress
hitherto laid upon digestion be more or less given up, and
bacteria be admitted as essential factors in the process,
must not, even on the new hypothesis, that of regulation of
transpiration, the absorption of the soluble products of decay,
be all the greater and the more regular, and the importance
of insect-catching as a source of nitrogen to the plant be
reaffirmed in an altered but even developed form ?

Here, indeed, to the writer’s mind, we are nearing the
probable solution of the case in a compromise, which may
indeed give up insectivorisrn as the main function, yet re-
instate it as a secondary one. In all the preceding descrip-
tions of different scenes of the organic drama we have been
noting, around what was at first described as the main action,
more or less of secondary incident. And now if this ap-
parently main action turn out to be itself but secondary and
incidental to a deeper lying and more general action, we
may be indeed for a moment confused and perplexed, but
our whole interpretation settles itself anew into a richer and
more complex form, to which all the preceding interpreta-
tions have contributed. Nor is even this new view in turn
necessarily final, yet the student-spectator has not lost his
pains who can feel and say that while nature’s art is long,
his time short, experiment fleeting, judgment difficult, yet

In Nature’s infinite book of secrecy
I can a little read.

CHAPTER IV

MOVEMENT AND NERVOUS ACTION IN PLANTS

Climbing Plants — Darwin's Observations , with Summary — Inter-
pretation of Movements — Movements of Seedlings — Methods of
Observation — Theory of Circumnutation .

We have seen that the sundews and fly-traps have a
power of movement as energetic as that of many animals,
and a sensitiveness to external stimuli which in its acute-
ness is not surpassed by that of our own nerves. In our
study we closely followed the work of Darwin, for his
researches, though by no means infallible, are funda-
mental, and moreover profoundly suggestive of that living
conception of nature which was so characteristic of his
work. In order to gain more complete possession of his
point of view — which is indeed that of Modern Botany —
let us still, as it were, continue to walk in his garden and
look at plants through his eyes. With this purpose we shall
first take a rapid survey of Darwin’s observations on Climb-
ing Plants as they are set forth in one of his volumes.1

Climbing Plants. — Among many different orders of
plants, and in all parts of the world, there are climbers
which reach the air and the light on the shoulders of their
stronger fellows. They insinuate themselves among the
1 The Movements and Habits of Climbing Plants (London, 1875).

chap, iv Movement and Nervous A ction in Plants 6 1

straggling branches of their neighbours, or twine them-
selves around the upright steins, or moor themselves by
sensitive elastic tendrils to the twigs of their bearers ; thus
reaching out of the crowded life of the ground herbage, or
out of the darkness and closeness of the jungle, to room
and fresh air and sunlight above.

Darwin arranged these climbers in four grades. Rising
a little above the crowd of those which merely scramble
over surrounding bushes, there are the hook -climbers,
such as Jack-run-the-hedge (Galium Aparine ), and root-
climbers, such as the Ivy. More efficient are the twiners,
like the Hop (Humulus) and the Honeysuckle (. Lonicera ),
but the climbing habit is . most frequently and most
perfectly exhibited by plants with sensitive prehensile
organs, either leaves or tendrils.

Let us consider these different kinds of climbers more
precisely, recognising, however, that the classification is
merely one of general convenience, for there are gradations
between scramblers and hook-climbers, between creeping
plants and root-climbers, between those which climb by
their leaves and the tendril-bearers.

The hook-climbers are least effective, being little more
than scramblers well equipped with hooks which are caught
up in the surrounding vegetation. Thus many brambles
and roses are merely scramblers, while the New Zealand
Rubus squarrosus and a rose known as Rosa setigera may
be fairly called climbers. The habit is very well illustrated
by those oriental palms which are often called Rotangs
(e.g. Calajnus extensus\ which with barbed branches in-
sinuate themselves and grow up through their more
vigorous neighbours. A more familiar example is the
Jack-run-the-hedge, whose stem and leaves are beset with
backward-directed hooks most efficient in binding the plant
to the growth of the hedgerow.

62 Chapters in Modern Botany chap.

Creeping plants, such as Ground-ivy ( Nepeta Glechoma)
and Strawberry, spread along the ground or up the
woodside bank, sending out long shoots which are at
intervals rooted in the soil. These suggest the next set
of climbing plants — the root-climbers, such as the common
Ivy. These ascend slowly, fixing themselves by rootlets
which grow away from the light and become glued to the
stems of trees or to the surfaces of rocks. We all know
the little brown roots by means of which the ivy clings
so closely that if you pull a piece off by force the roots
often break at their origin from the stem and not from
their attachment. There are many other root -climbers,
such as Tecoma radicans common in the Southern States,
some species of Bignonia, and many Figs ( Ficits repens ,
etc.) The beautiful night -flowering Cactus ( Cereus nycti-
flora ), often called “ queen of the night,” also affords a not
uncommon transition to these true climbers ; scrambling
as it does over rocks, and freely giving off at almost any
portion of its surface adventitious roots which soon fix the
plant, but no doubt also have a genuinely absorbent func-
tion as well.

The twiners, such as the Hop and the Honeysuckle,
differ from those already mentioned, for as they grow their
stems have a marked power of movement, bending and
bowing to all sides, and thus encircling their support.
Most twine in a definite direction ; thus the hop twines in
a right-handed spiral ( i.e . with the sun, or with the hands
of a watch lying face upwards), while the majority resemble
the French Bean ( Phaseolus multiflorus') in winding to
the left. The Bitter-sweet ( Solanum Dulcamara) seems
to twine indifferently in either direction, and the stem of
the Chili-nettle ( Loasa ) may change its direction in the
course of its climbing.

The next two sets of climbing plants are closely united.

iv Movement and Nervous Action in Plants 63

The leaf-climbers have clasping petioles as in Clematis and
Tropaeolum, or hook themselves up by the tips of their
leaves as in Gloriosa ; most of them also revolve like the
twiners, and in this way bring their leaves into contact
with adjacent branches. When they are young the leaf-
stalk or the leaf-tip, or even the whole surface of the leaf
(in the climbing fumitory, Corydalis claviculata :), is sen-
sitive to contact, bends towards the side on which pressure
is exerted, and thus clasps the plant to its support.

The tendril-bearers, such as the pea and the vine and
the passion-flower, are the most evolved climbers, for they
have prehensile organs specially adapted for this function.
These prehensile organs or tendrils may be modified leaflets
as in the pea, or modified leaves as in Lathyrus Afthaca , or
flower-stalks as in the vine, or even branches in some rare
cases. The shoots of the tendril-bearers revolve as those
of the leaf-climbers do, and the tendrils themselves move
round and round. Thus the tendrils are brought into
contact with surrounding objects, to the touch of which
they are often finely sensitive. They curve to what they
touch and link themselves around it, after which they
usually grow stronger and thicker, and by coiling into a
spiral raise the plant nearer to its support.

Darwin’s Observations on Climbing and Twining
Plants. — Having now classified the climbing plants as
Darwin did, we shall inquire more carefully into what he
has told us of their life.

In regard to twining plants let us quote one of Darwin’s
observations : “ When the shoot of a hop rises from the
ground, the two or three first-formed joints or internodes
are straight and remain stationary ; but the next formed,
whilst very young, may be seen to bend to one side and
to travel slowly round towards all points of the compass,
moving, like the hands of a watch, with the sun. The

64 Chapters in Modern Botany chap.

movement very soon acquires its full ordinary velocity.
From seven observations made during August on shoots
proceeding from a plant which had been cut down, and on
another plant during April, the average rate during hot
weather and during the day is 2 hours 8 minutes for each
revolution ; and none of the revolutions varied much from
this rate. The revolving movement continues as long as
the plant continues to grow; but each separate internode,
as it becomes old, ceases to move.”

The characteristic movement is a turning to all sides in
succession. In so doing the stem usually becomes twisted
upon itself, and perhaps thus gains in strength, as a rope
becomes firmer as it is more twisted. The revolutions
continue, though not with equal rapidity, during the night
as well as during the day ; the orbit described by the tip
of the shoot becomes wider and wider as the shoots grow
longer, and the chance of meeting with some upright
support becomes greater and greater. The shoot is
arrested at length by contact with a support, but the free
part goes on revolving. “ As this continues, higher and
higher points are brought into contact with the support
and are arrested ; and so onwards to the extremity ; and
thus the shoot winds round its support.”

There is a general uniformity in the behaviour of
twining plants, but the direction and rate of revolution
vary in different kinds. Thus, as we have mentioned,
the majority revolve in a direction opposite to that of the
hands of a watch, but many follow the sun ; one shoot
may make its revolution in little more than an hour, while
another may take a whole day. But at present we are
chiefly concerned with the general fact that these twiners
do move round and round.

Leaf-climbers are in their behaviour in some respects
intermediate between twiners and tendril-bearers. Like

iv Movement and Nervous Action in Plants 65

twining plants they show a revolution of young shoots, but
with a marked tendency to change the direction of circuit ;
they approach the tendribbearers in having petioles or leaf-
tips sensitive to contact and able to clasp their support.
This sensitiveness is often exquisitely fine, indeed it seems
more delicate than the tactile sense of most animals.
Thus Darwin observed a petiole responding to the exces-
sively slight but continued pressure of a loop of soft thread
weighing only Tk- of a grain. The response is a bend-
ing towards the side which is touched, and sometimes
begins a few minutes after contact. After clasping has
been effected the leaf- stalks become stronger and more
woody, often acquiring a stem-like internal structure. It
is interesting to observe that while most species of Clematis
and Tropaeolum are effective leaf-climbers, there are some
species of more sluggish constitution, in which both the
mobility and the sensitiveness of the petiole are enfeebled,
or even lost altogether.

Both mobility and sensitiveness reach their climax in
the tendril -bearers. In the common pea the tendrils
revolve in ellipses, taking about an hour and a half to
complete their orbit. “ Whilst young and about an inch
in length, with the leaflets on the petiole only partially
expanded, they are highly sensitive ; a single light touch
with a twig on the inferior or concave surface near the tip
caused them to bend quickly, as did occasionally a loop
of thread weighing T of a grain. ” The long and thick
tendrils of the vine — modified flower-stalks in structure —
are much less active, but move from side to side, or in
narrow elliptical revolutions. In both the pea and the
vine, and in most tendril -bearers, the tendrils contract
spirally a day or two after they have clasped some object.
In this way they become apparently shorter and obviously
more elastic, not only drawing the shoot nearer its support,

F

66 Chapters in Modern Botany chap.

but forming cables which do not easily snap. “ I have,”
Darwin says, “more than once gone on purpose during
a gale to watch a Bryony growing in an exposed hedge,
with its tendrils attached to the surrounding bushes ;
and as the thick and thin branches were tossed to and
fro by the wind, the tendrils, had they not been excessively
elastic, would instantly have been torn off and the plant
thrown prostrate. But as it was, the Bryony safely rode
out the gale, like a ship with two anchors down, and with
a long range of cable ahead to serve as a spring as she
surges to the storm.”

As to the more precise nature of the movement, it is
enough in the meantime to notice that the whole tendril
— excepting the base and the tip — is continuously curved,
bending in succession to each point of the compass. On
a thick tendril a line of paint may be drawn ; this line, if
drawn on the surface which chanced to be convex at the
time, would first become lateral, then concave, then lateral,
and finally convex as at first.

But we should also give some illustration of the great
sensitiveness of tendrils. To those of the passion-flower
( Passiflora gracilis) Darwin gave the highest place. “ A
bit of platinum wire ~ of a grain in weight, gently
placed on the concave point, caused a tendril to become
hooked, as did a loop of soft, thin, cotton thread of a
grain. With the tendrils of several other plants, loops
weighing of a grain sufficed. The point of a tendril
of Passiflora gracilis began to move distinctly in 2 5
seconds after a touch, and in many cases after 30
seconds.”

Summary. — To sum up after Darwin : the first action of
a tendril is to place itself in a proper position ; if a twining
plant or a tendril gets by any accident into an inclined posi-
tion, it soon bends upwards, though secluded from the light,

iv Movement and Nervous Action in Plants 67

the guiding stimulus being the attraction of gravity ; climb-
ing plants bend towards the light by a movement closely
analogous to the incurvation which causes them to revolve ;
the spontaneous revolving movement is independent of
any outward stimulus, but is contingent on the youth of
the part and on vigorous health, which again depends on
a proper temperature and other favourable conditions of
life ; tendrils, and the petioles or tips of the leaves of leaf-
climbers, and apparently certain roots, all have the power
of movement when touched, and bend quickly towards the
touched side ; tendrils contract spirally soon ^after clasping
a support, but not after a mere temporary curvature (due
to pressure which is not permanent), and they ultimately
contract spirally if they have not come into contact with
any object.

Interpretation of Movements. — But the student natur-
ally asks what interpretation Darwin put upon these move-
ments of climbing plants. It will be easier to answer this
after we have considered what he thought of the many
other movements which plants exhibit. Meantime, how-
ever, a partial answer may be given.

In the first place, it is plain that the climbing habit is a
useful one, such as would tend to persist in nature. “The
advantage gained by climbing is to reach the light and free
air with as little expenditure of organic matter as pos-
sible. ”

In the second place, the habit of climbing is not an
occasional freak ; it is of widespread occurrence among
plants. Of the fifty-nine alliances into which Lindley
divided flowering plants, thirty-five, according to Darwin,
include twiners, leaf-climbers, or tendril -bearers. More-
over, the most different organs — stems, branches, flower-
stalks, petioles, midribs of the leaf and leaflets, and
apparently aerial roots — all possess this power. Not un-

68 Chapters in Modern Botany chap.

naturally, therefore, did Darwin regard the powers of
climbers as inherent in all the higher plants. In crowded
surroundings, where it is of advantage to rise, some plants
have retained and developed the powers of moving and
feeling which are latent in all.

In the third place, while the powers of climbers are
often markedly influenced by temperature, by light, by
gravity, and other factors in their environment, it is certain
that these do not explain the movement, except, of course,
in so far as the health of the plant depends ultimately upon
the sufficiency of its surroundings. In short, the move-
ments are manifestations of the internal life of the plant.

But by what means within the plant are they produced ?
Of this in his volume on Climbing Plants Darwin says
little, nor can we even now say anything very complete.
This is not greatly to be wondered at, for as we have little
minute knowledge as to the processes of contraction in
animals where we know that these are located in definite
structures — the muscles — it is not surprising that we know
still less as to the movements of plants which have no
specialised contractile elements that we can recognise.

Sachs, Dr. de Vries, and others have suggested that the
rotating movement — say of a twining shoot — is due to un-
equal growth now on one side and now on another, and
that this again depends on altered water-tension or tur-
gescence in the growing cells. This suggestion was
accepted by Darwin, although he did not believe that it
could apply to those cases where rapid movement follows a
slight touch. This is obviously a difficulty. Furthermore,
the suggestion that unequal growth on different sides of the
stem explains the revolving movements has to face the fact
that the rotating movement may continue without there
being any observable growth.

Without denying that the altered water-tension and un-

iv Movement and Nervous Action in Plants 69

equal growth explain some of the facts, it seems to others
more convenient to take refuge in the hypothesis that there
are in longitudinal rows along the moving shoot certain
cells which retain the power of contracting and expanding
— of passing rapidly from one state of water-tension to
another — and that these determine the movement of the
whole shoot.

For our present purpose what is especially important is
that we appreciate the plant world as living ; instead, there-
fore, of here prolonging our discussion of the theories held
in regard to the movements of climbers, let us return to the
general standpoint of Darwin’s volume, of which the last
paragraph may be quoted : —

“It has often been vaguely asserted that plants are dis-
tinguished from animals by not having the power of move-
ment. It should rather be said that plants acquire and dis-
play this power only when it is of some advantage to them ;
this being of comparatively rare occurrence, as they are
affixed to the ground, and food is brought to them by the
air and rain. We see how high in the scale of organisation
a plant may rise when we look at one of the more perfect
tendril-bearers. It first places the tendrils ready for action,
as a polypus places its tentacula. If the tendril be displaced,
it is acted on by the force of gravity and rights itself. It
is acted on by the light, and bends towards or from it, or
disregards it, whichever may be most advantageous.
During several days the tendrils or internodes, or both,
spontaneously revolve with a steady motion. The tendril
strikes some object, and quickly curls round and firmly
grasps it. In the course of some hours it contracts into a
spire, dragging up the stem, and forming an excellent
spring. All movements now cease. By growth the tissues
soon become wonderfully strong and durable. The tendril
has done its work, and has done it in an admirable manner.”

;o Chapters in Modern Botany chap.

Movements of Seedlings. — We have seen how Darwin
began to believe that all plants in some degree possessed
the powers of movement which are conspicuously developed
in the climbers. To test this opinion he and his son
Francis began to watch and experiment with many kinds of
plants, and the results of their study are told in the sequel
to the work on Climbing Plants, a volume entitled The
Power of Movement in Plants (London, 1880). As before,
let us follow Darwin, and first of all in watching the life of
a seedling.

The seed lies on the damp ground, covered perhaps
with leaves which have fallen from the trees. As water
finds its way into the seed, as the life within begins to
gather strength, the young root or radicle makes its appear-
ance. It begins at once to move round and round. But
its movements are influenced by gravity, and it bends
downwards, following a more or less spiral course towards
the ground. Darwin believed that “sensitiveness to
gravitation resides in the tip, which transmits some influ-
ence to the adjoining parts, causing them to bend.” When
the tip of the root reaches the soil it bores into it, aided by
the continued movement of the radicle, and this boring is
easier if some soil has fallen upon the seed and fixed it, or
if fine root-hairs from the top of the radicle have moored
the seed to the surface. Then as the radicle grows longer
its tip is forced into the soil.

There it can no longer bend round and round, but it may
try to do so. And sometimes the tip will reach a crevice,
or it may be an earthworm’s burrow, in which it can move
more freely. “ When a tip encounters a stone or other
obstacle in the ground, or even earth more compact on one
side than the other, the root will bend away as much as it
can from the obstacle or the more resisting earth, and will
thus follow with unerring skill a line of least resistance.”

iv Movement and Nervous Action in Plants 71

The tip of the radicle is a very sensitive structure : “it
was excited by an attached bead of shellac weighing less
than of a grain ; 55 it bends towards moisture and

away from light ; it always responds to the attraction of
earth even when grown in entirely abnormal conditions.

“ If the tip be lightly pressed or burnt or cut, it transmits
an influence to the upper adjoining part, causing it to bend
away from the affected side ; and, what is more surprising,
the tip can distinguish between a slightly harder and softer
object, by which it is simultaneously pressed on opposite
sides.” “ It is hardly an exaggeration,” Darwin concluded,
“ to say that the tip of the radicle thus endowed, and having
the power of directing the movements of the adjoining parts,
acts like the brain of one of the lower animals.”

But the movements of seedlings are not confined to the
radicle. From the seed there also emerges the young stem
or plumule, almost always bent in the form of an arch, the
tender tip remaining within the protecting seed-coats until
the first obstacles to growth have been overcome. If the
seed be buried the arch breaks through the ground, and is
helped in doing this by slight movements. The young
stem grows blindly but unerringly upwards towards light
and air, as the root downwards towards moisture and dark-
ness ; we cannot yet give any full or sufficient mechanical
explanation why, though it is here only too easy to cloak
our ignorance under learned nomenclature.1

As the arch grows upwards, freeing itself from the soil,
the seed-leaves or cotyledons, if not already in use as store -

1 It does not seem necessary to encumber this narrative or the
student’s mind at this stage with the numerous technical terms applied
to movements in relation to gravitation, light, moisture, etc., since
these are only too apt to correspond to no definite mental images of
any kind, but be used as mere illusory explanations in terms of “in-
herent properties.” See next chapter.

72 Chapters in Modern Botany chap.

houses of food, will be raised above the ground and ex-
panded in the air. Sooner or later the arch straightens
into an upright shoot. Both in the young shoot and in the
expanded cotyledons there is power of movement ; they are
exquisitely sensitive to light and to gravitation.

The power of movement does not cease when the shoot
becomes a stem with leaves and branches. “ If we look,
for instance, at a great Acacia tree, we may feel assured
that every one of the innumerable growing shoots is con-
stantly describing small ellipses ; as is each petiole, sub-
petiole, and leaflet. The latter, as well as ordinary leaves,
generally move up and down in nearly the same vertical
plane, so that they describe very narrow ellipses. The
flower-peduncles are likewise continually circumnutating.
If we could look beneath the ground, and our eyes had the
power of a microscope, we should see the tip of each root-
let endeavouring to sweep small ellipses or circles, as far as
the pressure of the surrounding earth permitted. All this
astonishing amount of movement has been going on year
after year since the time when, as a seedling, the tree first
emerged from the ground.”

Methods of Observation. — But how can these move-
ments be seen and measured ? In some of the more marked
cases, as of twiners and climbers, there is no difficulty what-
ever. We need only observe the position of the plant at
intervals of hours and days. Or if we note the position of
a growing shoot in the garden as it bends over to one side,
and fix a piece of string, by means of a couple of stakes,
so that it lies in a line with the shoot when looked at
from above, we can, if we return in half an hour or
so, plainly see that the shoot has moved through a large
angle.

But there are of course more delicate modes of observa-
tion. Darwin gives many figures representing, e.g. the

iv Movement and Nervous Action in Plants 73

path of the young stem of a seedling cabbage during
ten hours. How did Darwin get this record ?

“ Plants growing in pots were protected wholly from the
light, or had light admitted from above, or on one side, as
the case might require, and were covered above by a large
horizontal sheet of glass, and with another vertical sheet
on one side. A glass filament, not thicker than a horse-
hair, and from a quarter to three-quarters of an inch in
length, was affixed to the part to be observed by means of
shellac dissolved in alcohol. The solution was allowed to
evaporate, until it became so thick that it set hard in two
or three seconds, and “ it never injured the tissues, even the
tips of tender radicles, to which it was applied.” (?) To the
end of the glass filament an excessively minute bead of black
sealing-wax was cemented, below or behind which a bit of
card with a black dot was fixed to a stick driven into the
ground. The weight of the filament was so slight that
even small leaves were not perceptibly pressed down. The
bead and the dot on the card were viewed through the
horizontal or vertical glass-plate (according to the position of
the object), and when one exactly covered the other, a dot was
made on the glass-plate with a sharply-pointed stick dipped in
thick Indian ink. Other dots were made at short intervals
of time, and these were afterwards joined by straight lines.”

Of course the result was not a picture, only a record of
the plant’s path, showing nothing more than the general
character of the movement.

Another method, which, however, is only in a few cases
practicable, is to allow the moving parts — radicles, for
instance — to trace their own paths, to write, as it were,
their own diary, on smoked plates of glass. Figures of
these root-tracks are given by Darwin in abundance, and
the student, in this case as the preceding, may profitably
endeavour to make them for himself.

74 Chapters in Modern Botany chap.

Darwin’s Theory of Modified Circumnutation —

Darwin’s observations led him to conclude that every
growing part of every plant is continually moving, though
often on a small scale. And as the most prevalent form
of movement is like that of a climbing plant, which bends
successively to all points of the compass, so that the tip
revolves, Darwin applied to it the term circumnutation.
This term he explains as follows : 44 If we observe a cir-
cumnutating stem, which happens at the time to be bent,
we will say towards the north, it will be found gradually to
bend more and more easterly until it faces the east ; and
so onwards to the south, then to the west, and back again
to the north. If the movement had been quite regular the
apex would have described a circle, or rather, as the stem
is always growing upwards, a circular spiral. But it gener-
ally describes irregular elliptical or oval figures ; for the
apex, after pointing in any one direction, commonly moves
back to the opposite side, not, however, returning along
the same line.”

In this 44 universally present movement ” Darwin found
44 the basis or groundwork for the acquirement, according
to the requirements of the plant, of the most diversified
movements.” His particular thesis was that the great
sweeps made by the twiners and by tendrils, the move-
ments of leaves when they go to sleep at night, the move-
ments of various organs to the light or away from it, and
even the movement of stems towards the zenith and of
roots towards the centre of the earth, are all modified
forms of circumnutation, which is omnipresent while growth
lasts. In regard to the two last sets of movement he
believed, of course, in the influence of light and gravitation,
but he regarded these as simply operating upon spon-
taneous processes of circumnutation, hastening, diminish-
ing, or otherwise modifying them. While maintaining this

iv Movement and Nervous Action in Plants 75

theory of circumnutation Darwin acknowledged that it did
not explain all movements ; he did not propose to apply it
to the collapse of the Sensitive plant’s leaves when they are
touched, or to the movement of the stimulated Sundew and
Venus Fly-Trap, or to the movement of the Barberry’s
stamens when they are jostled by an insect’s legs.

In regard to the means by which the circumnutating
movements are brought about, Darwin expressed himself
in his Movements of Plants more definitely than he had
done in the previous volume. Following several German
botanists, he emphasised the importance of the turgescence
or state of water-tension of the cells of the plant, and he
also recognised the extensibility of the cell-walls. “ On
the whole,” he says, “we may at present conclude that
increased growth, first on one side and then on another, is
a secondary effect, and that the increased turgescence of
the cells, together with the extensibility of their walls, is
the primary cause of the movement of circumnutation.”

Having stated the general thesis of Darwin’s book,
namely, that most of the movements of plants are modified
forms of circumnutation, and his general conception of the
internal processes involved, we may next briefly discuss the
movements of plants in their relation to gravitation, light,
and other external influences. In so doing we shall not in-
sist upon Darwin’s generalisation, for perhaps no part of his
work has met with more adverse criticism, as notably on the
part of Sachs, than this. It is indeed the opinion of most
botanists that Darwin’s theory of circumnutation was over-
strained.

CHAPTER V

MOVEMENTS OF plants — continued

Movements in relation to Gravitation — Light-seeking and Light-
avoiding Move?nents — Rationale of Light-seeking and Light-
avoiding Movements — The Sleep of Plants — Mr. Francis
Darwin's recent Discussion of Plant Movements — Summary
and Conclusion .

Movements of Plants in relation to Gravitation.

— We are so familiar with the fact that stems grow
upwards and roots downwards that we perhaps do not
think of it as in the least remarkable that one part of a
plant should persistently grow against and the other part
in the direction of the acting force of gravity. Of course
the student may be tempted to ask what roots would do
up in the air or what stems would do down in the ground,
or how they could grow otherwise than up and down, but
these questions are not exactly to the point — which is this,
that even when young plants are taken from their natural
conditions, when they are turned upside down or grown on
a rapidly rotating wheel, still the opposite tendencies of
root and stem assert themselves.

Let us see how the behaviour of roots and stems to the
force of gravity is experimentally demonstrated.

If seeds, of peas and beans which have germinated in

CHAP. V

Movements of Plants

77

loose soil, and have little straight radicles, be carefully
removed, and suspended in the air under a damp bell-jar
with the radicles pointing upwards or horizontally, in the
course of a few hours the radicles will all have turned
downwards.

In the same way the growing shoots of plants may be
placed or artificially forced to grow in a horizontal position,
but if the shoot be strong enough and be not, for instance,
naturally a creeper, its character asserts itself as soon as it
is free from restraint, and the stem grows upwards.

As long ago as 1806 Knight tried the effect of growing
young plants on a rotating wheel. When an apparatus of
this sort is devised, with seedlings suitably fixed on a
rotating disc, the young roots always grow outwards and
the young stems inwards. When the plane of rotation is
vertical the direct influence of gravitation is counteracted,
its direction being continually altered ; but in relation to the
so-called “ centrifugal force the roots and the stems grow
in consistently antagonistic directions, the stems against,
the roots in the direction of the acting force.

There is a more modern apparatus called a klinostat, a
clockwork arrangement by which germinating seeds are
slowly rotated in a vertical plane, and as the relation of
the parts to the earth’s axis is constantly changing, the
direct action of gravity is counteracted ; the result being, as
we would expect, that the young roots and stems grow in
totally indefinite fashion.

No one will suppose that in normal conditions the roots
simply sink downwards passively, for they will force their
way into quicksilver, and besides the stem is influenced in
an exactly opposite way. It is certain that the parts of
plants are influenced by gravity, but not passively ; they
are living organs.

It seems likely that the roots and the stems, differing as

78 Chapters in Modern Botany chap.

they do in structure, have the water-tension of their cells,
and secondarily, their growth differently affected by the
force of gravity ; but other explanations are in the field.
Some botanists go so far as to suppose a distinct stem-
protoplasm and a distinct root -protoplasm reacting in
opposite ways to given stimuli. Hence we see that after
all the research and discussion which has taken place the
subject is still far from being cleared up. Here, as in the
allied or, at any rate, intermingled problem of explaining
the mechanism of light - seeking and light -avoiding, the
student must guard against the too common habit of find-
ing an explanation in what is but a technical nomenclature,
and not cease to ask himself the child’s puzzling questions
of why does this stem grow up and that root grow down
merely because he is told to call them in longer words
“ negatively ” or “ positively geotropic.” For though we
may all laugh with Moliere at old-world medicine, it is
no easy matter to leave its virtus dormitiva out of our
minds.

Darwin laid emphasis on the very tip of the radicle.
“ In the case of the radicles of several, probably of all
seedling plants, sensitiveness to gravitation is confined to
the tip, which transmits an influence to the adjoining upper
part, causing it to bend towards the centre of the earth.”
When the tips were amputated the power was lost until a
new tip was formed. Subsequent experiments do not,
however, confirm this opinion, if indeed they have not
disproved it.

Darwin also made a large number of experiments show-
ing how a radicle on whose tip some minute object was
fastened, or some slight injury inflicted, bent towards the
free or uninjured side. But when the stimulus is applied
not to the side of the tip, but to the growing region a little
above the tip, the bending takes place towards the stimulus.

V

Movements of Plants

79

In regard to such experiments, however, critics have justly
pointed out that we must be exceedingly careful, with plants
as well as with animals, in drawing conclusions as to normal
life from facts observed when injuries are inflicted, however
apparently slight these injuries may be ; and this experiment
must therefore be given up.

Besides being affected by gravity and by light, roots are
very sensitive to moisture. They always grow towards the
greater moisture, sometimes even against gravity. Thus if
seeds be allowed to germinate in a sieve filled with damp
sawdust, the roots first grow downwards as usual, but after
they have descended through the sieve into the dry air, the
direction of their growth changes, and they grow up again
into the moisture.

Light - seeking and Light - avoiding Movements. —

Every one must have noticed that plants which have grown
in the recess of a narrow window are often very markedly
curved towards the light. The same light-seeking can be
readily shown by experiment, for if seedlings are grown in
a box which is illumined by a single aperture they all turn
their young stems towards the path of the light. Some
sedentary animals, such as the Serpula worms, which make
for themselves twisted tubes of lime, have the same habit
of bending to the light.

Some of Darwin’s observations show the extreme sensitive-
ness of certain seedlings to light. “ The cotyledons of
Phalaris became curved towards a distant lamp, which
emitted so little light that a pencil held vertically close to
the plants did not cast any shadow which the eye could
perceive on a white card. These cotyledons, therefore,
were affected by a difference in the amount of light on
their two sides, which the eye could not distinguish.” Dar-
win also noticed that if seedlings kept in a dark place were
laterally illuminated by a small wax taper for only two or

80 Chapters in Modern Botany chap.

three minutes at intervals of about three-quarters of an
hour, they all became bowed to the point where the taper
had been held. This convinced him that the excitement
from the light was due not so much to its actual amount
as to the difference in amount between that previously
received. “ Light seems to act on the tissues of plants
almost in the same manner as it does on the nervous
system of animals/’

Usually the movement is towards the light, especially in
the case of stems, but this is not invariably the case. Thus
the shoots of the ivy bend away from, not towards the light,
and so do the tendrils of the vine, of the Virginian creeper
( Ampelopsis hederacea ), and some other plants. We can
easily understand that it is advantageous for tendrils to
seek shaded recesses, for in so doing they will usually
come nearer the support to which they cling, although of
course this advantage must not blind us to the necessity of
a physiological explanation.

As most roots seek the ground their reactions to light
are not readily tested, except in artificial conditions. But
by means of the klinostat — the rotating apparatus which
we have already mentioned — it is possible to grow seedlings
illumined in one direction only, and with the influence of
gravitation eliminated. Then it is seen that some roots
bend towards and others away from the light. Of roots
which in natural conditions always bend away from the
light, the climbing roots of the ivy are the most familiar
examples. Or the following simple experiment may be made,
following the directions of Professor Detmer’s laboratory
manual of practical vegetable physiology — a work which
will be of great use to the student (and which can also be
obtained in translation) : A glass filled with water is closed
with a lid of fine muslin ; on the top of this are placed
germinating seeds of white mustard ( Sinapis alba) ; the

V

Movements of Plants

whole is covered with a bell-jar and placed in the dark.
The young stem grows straight up, the young root grows
straight down into the water. Then light is allowed to fall
upon the plant, in one direction only, through a narrow slit.
In a few hours the young stem has curved towards, the
young root away from the light.

Darwin’s experiments led him to conclude that the
sensitiveness to light was localised in the tips, e.g. of the
cotyledons of Phalaris and Avena, of the young stems of
Brassica and Beta, and of the radicles of Sinapis. There
seemed to be a transmission of influence from the tip to the
other parts, causing them to bend. “ It is an interesting
experiment to place caps over the tips of the cotyledons of
Phalaris, and to allow a very little light to enter through
minute orifices on one side of the caps, for the lower part of
the cotyledons will then bend to this side, and not to the side
which has been brightly illuminated during the whole time.”

The commonest position of leaves and cotyledons during
the day is one more or less transverse to the direction of the
light, and this also Darwin believed to be due to a modified
circumnutation ; but few would now agree with him in this
interpretation. In some plants in which the leaflets are
provided with little swollen cushions or pulvini at their
base, they move upwards or downwards or twist laterally
when the sun shines very brightly upon them, as in the
well-known “compass-plant” (Silphium) ; by directing their
edges towards the light they avoid the injurious effects of
too intense illumination.

It is easy to understand that it is advantageous for most
plants to bend towards the light, for thus their leaves are in
a better position to use the power of the sunlight on which
the life of the plant so much depends. On the other hand,
it is also advantageous that the aerial rootlets of the ivy or
the tendrils of the vine should turn away from the light.

G

82 Chapters in Modern Botany chap.

But it is difficult for us to form any conception of the
reason why different plants or different parts of the same
plants should be affected by the light in opposite ways.
Thus the flower-stalks of the ivy-leaved toad-flax ( Linaria
Cymbalaria ), which so often drapes our old walls with
beauty, at first bend towards the light, but turn in the
opposite direction after the flowers are fertilised, and the
time approaches for planting the seed in a crevice.

As to the causes of the light-seeking or light-avoiding
movements, in their usual forms they are possible only as
long as the moving parts continue to grow. Now as light
usually exerts a retarding influence on growth, the side of
the plant which is shaded grows more quickly than the
side which is lighted, hence the plant bends towards the
light. But this will not explain light-avoiding, and so we
are led to recognise, as before, that the effect of the light
on the living matter of different parts of the plant or of
different plants is not always the same.

Eationale of Light-seeking and Light-avoiding Move-
ments.— But when we inquire more precisely into the in-
fluence of the light, a hundred difficulties beset us. How far
was Darwin right in supposing that light simply modifies an
existing spontaneous movement of circumnutation ? How
far are others warranted in assuming that there are in
different parts of the plant specially contractile cells which
are affected in various ways by stimuli ? in other words,
how far may the movements be independent of growth ?
Or if the movements be entirely dependent upon growth,
how far is this the result of a change in the water-tension
or turgescence of the cells exerting pressure on their in-
ternal surfaces, as Sachs and De Vries would say ; or is
Klebs wholly wrong in regarding the turgescence rather as
a symptom than as a cause, and in referring growth to un-
known properties of the protoplasm ?

v Movements of Plants 83

And even if we accept the general conclusion that the
altered growth is due to a modification of turgescence in
the growing cells, to what is the change of turgescence due ?
— to a change in the elasticity of the cell-wall, as Sachs,
Pfeffer, Wiesner, and others suggest ; or to a change in
the osmotic properties of the cell-sap, the stimulus pro-
moting, according to De Vries, the formation of substances
which are osmotically active ; or to a change in the perme-
ability of the protoplasm, resulting directly from the influence
of light, as Vines believes ? Finally, according to Wortmann,
growth depends on three factors — the osmotic force within
the cell, the extensibility of the cell-walls, and the relative
abundance of surrounding water. Or if we turn to a recent
Botanical Journal, we find Noll maintaining that curvature is
due to a greater extensibility of the cell-walls on the convex
side and to a diminished extensibility on the concave side —
opposite effects produced by the mysterious activity of the
parietal protoplasm ; and we find Wortmann denying this,
saying that the curvature is due to a movement of the
protoplasm which in the region of growth alters the thick-
ness and tension of the cell-wells, distributing its materials
so that the two sides are unequally thickened.

Thus the student may see that as in regard to structural
problems, such as the real nature of the pitchers of pitcher
plants, so in regard to physiological problems, such as this
of the movement of plants in relation to light and other
stimuli, there is much difference of opinion. The reason
for this is perfectly obvious — the problem is so com-
plex, so many factors have to be considered. The history
of the investigation is, as in all other cases, one of pro-
gressive analysis. That plants move towards the light
in virtue of “ heliotropism ” is obviously a truism ; that the
“ heliotropism ” may be due to unequal growth expresses
the first step in the analysis ; the unequal growth is asso-

84 Chapters in Modern Botany chap.

dated with altered water-tension, etc., in the cells expresses
another step, and so on. The problem is to discover all the
observable conditions of the movement before finally con-
fessing that what remains unexplained is due to some
still unknown property or power of the living matter or
protoplasm.

This above all the student should recognise that the plant
is in all its actions no mere mechanical system, completely
explicable according to the facts of mechanics, hydrostatics,
and the like, but a living organism. And just as no one will
pretend to give a “ mechanical explanation 55 of how a
Serpula worm makes its calcareous tube towards the light,
nor pretend to be content with calling the animal “ heli-
tropic,” so we must avoid both extremes in regard to
plants.

The Sleep of Plants. — Every one has noticed how the
three leaflets of the clover and the wood-sorrel change their
position as the light of day grows and wanes ; they are
expanded during the day, and fold downwards in the even-
ing. Many may have observed similar movements in Lupine
and Melilot, Mimosa and Acacia. Indeed these movements
are very common, and were noticed by early naturalists such
as Pliny. Since Linnaeus wrote his famous essay called
Somnus Plantarum they have been spoken of as the sleep
of plants.

The movements are sometimes very marked, changing
the whole appearance of the plants ; thus “ a bush of Acacia
famesiana appears at night as if covered with little dangling
bits of string instead of leaves.”

Although sleep - movements are very general, their
amount and their nature are greatly diversified. Thus
Darwin enumerates 37 genera in which the leaves or
leaflets rise, and 32 genera in which they sink at night.
In some species the leaves sleep, but not the cotyledons ;

v Movements of Plants 85

in a larger number the cotyledons sleep, but not the leaves ;
in many both sleep, but in widely different positions. Yet
if sleep occur the general result is always the same ; the
blade is placed in such a position at night that its upper
surface is exposed as little as possible to full radiation.

The movements are without doubt associated with the
daily alternation of light and darkness ; but it is the
difference in the illumination rather than the darkness
which excites the change, for Darwin showed that in
several species, if the leaves have not been brightly
illuminated during the day, they do not sleep at night.
Moreover, in the North, where the sun does not set,
Mimosa still goes regularly to sleep ; and by artificial
light and darkness the daily movements may be reversed.
“ The presence of light or its absence cannot be supposed
to be the direct cause of the movements, for these are
wonderfully diversified even with the leaflets of the same
leaf, although all have of course been similarly exposed.
The movements depend on innate causes, and are of an
adaptive nature. The alternations of light and darkness
merely give notice to the leaves that the period has arrived
for them to move in a certain manner. We may infer
from the fact of several plants (Tropaeolum, Lupine, etc.)
not sleeping unless they have been well illuminated during
the day, that it is not the actual decrease of the light in
the evening, but the contrast between the amount at this
hour and during the early part of the day, which excites
the leaves to modify their ordinary mode of circumnuta-
tion. As the leaves of most plants assume their proper
diurnal position in the morning, although light be excluded,
and as the leaves of some plants continue to move in the
normal manner in darkness during at least a whole day,
we may conclude that the periodicity of their movements
is to a certain extent inherited. The strength of such

86 Chapters in Modern Botany chap.

inheritance differs much in different species, and seems
never to be very rigid ; for plants have been introduced
from all parts of the world into our gardens and green-
houses ; and if their movements had been at all strictly
fixed in relation to the alternations of day and night, they
would have slept in this country at very different hours,
which is not the case ; moreover, it has been observed
that sleeping plants in their native homes change their
times of sleep with the changing seasons.55

The movements Darwin explained in two ways : “ firstly,
by alternately increased growth on the opposite sides of
the leaves, preceded by increased turgescence of the cells ;
and secondly, by means of a pulvinus or aggregate of
small cells, generally destitute of chlorophyll, which be-
comes alternately more turgescent on nearly opposite sides ;
and this turgescence is not followed by growth except during
the early age of the plant.55 The movement may be
almost the same whether a pulvinus be present or not,
but in the former case the course of the leaves is more
regularly elliptical, and the movements are continued for a
much longer period in the life of the plant.

The position occupied by the leaves at night indicates
that the benefit they derive is “ the protection of their
upper surfaces from radiation into the open sky ; and
in many cases the mutual protection of all the parts from
cold by their being brought into close approximation.55 In
evidence of this Darwin showed that leaves compelled to
remain extended horizontally at night, suffered much more
from radiation than those which were allowed to assume
their normal vertical position.

“ Any one who had never observed continuously a
sleeping plant, would naturally suppose that the leaves
moved only in the evening when going to sleep, and in
the morning when awaking ; but he would be quite mis-

V

Movements of Plants

87

taken, for we have found no exception to the rule that leaves
which sleep continue to move during the whole twenty-four
hours ; they move, however, more quickly when going
to sleep and when awaking than at other times.”

Exceptional Developments of Plant Movement, the
Telegraph Plant, Sensitive Plant, etc. — The most re-
markable case of continuous movement is that of the Indian
Telegraph plant ( Desmodimn or Hedysarum gyrans). Its
leaves have three leaflets — a large median one, and two
lateral ones which are rudimentary. The main part moves
a little during the day and has a remarkable sleep move-
ment, but the lateral leaflets which do not sleep move con-
stantly, describing with a series of little jerks minute circles.
Each revolution sometimes occupies little over a minute.

The term sleep is often applied, as Linnaeus applied it,
not only to leaves, but also to the petals of many flowers
which close at night. This seems to depend rather upon
a difference of temperature determining differences in
turgescence than on a difference of illumination, and it has
the effect of sheltering the internal parts of the flower
from cold winds, rain, and over-radiation. Here certainly
Darwin’s suggestion of “ modified circumnutation ” does
not apply.

We have not spoken of one of the most familiar of plant
movements — that of the sensitive plant (. Mimosa ftudica).
As is well known, a touch is enough to make the leaves of
this plant suddenly assume their sleep position. This is
usually referred to a change in a cushion of cells at the
base of the leaf. Even Darwin did not think of explaining
it as connected with circumnutation, although he justly re-
gards it as an extreme case of exaggeration of the sleep
movements. Its outward or bionomic utility we cannot
fathom, still less its inner mechanism. It is tempting to
correlate it with that remarkable “ intercellular continuity

88 Chapters in Modern Botany chap.

of the protoplasm” which has been discovered in the tissue
of this cushion — at least so far as propagation of impulse
is concerned, for contractility of the network cannot be
assumed. The most recent theory on the matter is that of
Professor Haberlandt, who by means of very careful
anatomical researches has been led to conclude that the
stimulus travels down a continuous set of tubular conduct-
ing cells included in the bast portion of the leaf-strands.
He believes that the transmission depends upon changes
of hydrostatic pressure induced in the tubular cells and on
resulting movements of the cell-sap ; in other words, that
it is not due to any particular nervous excitability of the
living matter. But anatomical researches alone cannot
justify such a conclusion ; nor is it easy to see how to prove
or disprove it by direct experiment.

The movements of the tentacles of the Sun -dew, of the
leaves of Dionsea, of the irritable stamens of the Rockrose
(Helianthemum), Barberry, etc., the closure of the stigma
lips of a Mimulus upon a pollen-grain, the bending of a
tendril when touched, are also to be distinguished from that
series of movements to the discussion of which this chapter
has been devoted ; and no doubt reserve a wide and varied
field for a future generation of subtle experimentalists. For
instead of the animal world alone possessing movement,
and the plant standing passive, the phenomena of plant
movement, while of course less obvious in amount, seem to
be not only the more varied in kind, but perhaps also in
cause.

Summary. — The different kinds of movement may be
arranged as follows : —

A. Move7nents of Growing Parts —

( i ) “ Spontaneous v movements — that is to say those
whose conditions are not known, e.g. the revolving

v Movements of Plants 89

movements of twiners and climbers, young shoots
and roots, etc.

(2) Movements in relation to external influences :

( a ) Downward movement of roots and up-

ward movement of stems.

( b ) Light -seeking and light -avoiding move-

ments, especially of stems and leaves.

(1 c ) The movement of roots towards the
greatest moisture.

B. Movements of Adult Parts —

( 1 ) “ Spontaneous movements, e.g. of Desmodium
or Hedysarum gyrans.

(2) Movements in relation to external influences :

(a) In relation to light — sleep movements of

leaves.

(b) In relation to chemical and physical

stimuli other than light, eg. the move-
ments of the Sensitive plant and of Fly-
Traps.

Mr. Francis Darwin’s Discussion of Plant Move-
ments.— At this point it is appropriate that we should
turn to what Mr. Francis Darwin, who collaborated with
his father in writing The Power of Movement in Plants ,
has recently said in his Presidential address to the bio-
logical section of the British Association, 1891, which con-
tains an important discussion of growth - curvatures, a
problem towards the solution of which his detailed
researches have largely contributed. A brief summary of
this address, although including points already touched,
may hence be of service to the student.

He begins by distinguishing the two main questions : —

1. “ How does the plant recognise the vertical line ; how
does it know where the centre of the earth is ? ”■ — a ques-
tion of irritability.

90 Chapters in Modern Botany chap.

2. “ In what way are curvatures which bring the plant
into the vertical line executed ? 55 — a question of the
mechanism of movement.

The history of the answers to these questions may
conveniently begin with Hofmeister’s researches (1859)
on the effects of bending or striking a turgescent shoot.
He showed that when a shoot is violently bent the
elasticity of the passive tissues (cortical and vascular con-
stituents) on the convex side is injured by over-stretching.
£< The system must assume a new position of equilibrium ;
the turgescent pith (the active or erectile tissue) stretches
the cortex ; but as the passive tissues are now no longer
equally resisting on the two sides, the shoot must assume
a curvature towards that side on which the passive tissues
are most resisting.55 Applying the same conception to a
cell, Francis Darwin says: “ As pith is to cortex, so is
cell-pressure to cell-membrane.55

When a shoot is laid horizontally there is, according to
Hofmeister, a tendency for the resisting passive tissue
along the lower side to become water-logged, and therefore
more extensible. Therefore the shoot bends upwards.
So Knight, in 1806, supposed that roots penetrated down-
wards, because of the sinking downwards of the juices.
But both these explanations are crude ; they are too
mechanical.

As far back as 1824, Dutrochet, who was, however, by
no means consistent, had recognised the fundamental
biological fact that growth -curvatures were provoked by
external influences acting as stimuli, but “the botanical
mind took more than fifty years to assimilate Dutrochet’s
view.55

In 1868 Frank attacked the problem with true physio-
logical insight, showing that earth -seeking is an active
curvature, and that it depends, like other growth-curvatures,

V

Movements of Plants

9i

on unequal distribution of longitudinal growth. Moreover,
his experiments on the horizontal runners of the straw-
berry, and those of Elfving on rhizomes “ paved the way
for the theory that there are a variety of different organisa-
tions (or, as we now say, irritabilities) in growing plants ;
and that, whether a plant grows vertically upwards, or down-
wards, or horizontally, depends on the individual and highly
sensitive constitution of the plant in question.” Frank’s
views were, as we have seen, accepted by the authors of
The Power of Movement in Plants , although Frank’s
particular interpretation of the irritabilities as due to
“ polarities ” was not.

Francis Darwin then points out how the development of
our present views on irritability was delayed by the in-
sufficient theory of light-seeking, as implied, for instance,
in De Candolle’s explanation, that curvature towards the
light was simply due to the more rapid growth of the
shaded side. Fuller acquaintance with the facts, e.g. of
plants which curve away from the light, showed that this
again too mechanical theory was insufficient, and led on to
the idea, repeatedly expressed in The Power of Movement ,
that light and gravitation act merely as landmarks by which
the plant can direct itself. Pfeffer, Sachs, Vines, and
other botanical physiologists are at one in regarding
growth-curvatures as phenomena of irritability, as responses
to gravitation, light, and other stimuli.

So far the general question of irritability. Mr. Darwin
then proceeds to discuss that of mechanism.

“The first step in advance of Hofmeister’s views was
the establishment of the fact that the curvatures under
consideration are due to unequal growth — that is to say,
to greater longitudinal growth on the convex than on the
concave side.” Frank made important contributions to the
subject, and Sachs thoroughly demonstrated that the con-

92 Chapters in Modern Botany chap.

vex side grows faster, while the concave side grows slower,
than if the organ had remained vertical and uncurved.

“Then it began to be established, through Sachs’s work
(1871), that turgescence is a necessary condition of growth.”
“ De Vries (1879) maintained that growth-curvatures in
multicellular organs are due to increased cell-pressure on
the convex side : the rise in hydrostatic pressure being
put down to increase of osmotic substances in the cell-sap
of the tissues in question.” But, as Francis Darwin points
out, there are many serious objections to this explanation.

Many suggestions followed. Sachs directed attention
to the changes in the extensibility of cell-walls. “ Wiesner
held that the curvature of multicellular organs is due both
to an increase of osmotic force on the convex side, and to
increased ductility of the membranes of the same part.”
“ Strasburger suggested that growth-curvatures are due to
increased ductility of the convex membranes.”

More detailed explanations in similar lines are those of
Noll and Wortmann, which differ in this: “The former
lays the greater stress on the increased extensibility of the
convex side, the latter on the diminution of that of the
concave side. Again, Wortmann explains the difference
n extensibility as due to differences in thickness of the
cell-walls. Noll gives no mechanical explanation, but
assumes that the outer layer of protoplasm has the power
of producing changes in the quality of the cell-wall in some
unknown way.”

Francis Darwin concludes “there is a focussing of
speculation from many sides in favour of ‘ active ’ surface-
growth, or, what is perhaps a better way of putting it, in
favour of the belief that the extension of cell-membranes
depends on physiological rather than physical properties,
that it is in some way under the immediate control of the
protoplasm.” Beyond that we may choose between rival

v Movements of Plants 93

suggestions of Wiesner, Strasburger, Wortmann, Noll,
Vines, and others.

In the last section of his Presidential address Mr.
Francis Darwin states his position in regard to circum-
nutation. He cites the most important criticism of The
Power of Moveme7it , that of Wiesner, who denied that
circumnutation was a widespread phenomenon ; that some
stems, leaves, etc., grow in a perfectly straight line ; that
such curvatures as those of geotropism and heliotropism
cannot be interpreted as modifications of circumnutation.

Yet, for reasons given, Mr. Francis Darwin confesses
that he “ cannot give up the belief in circumnutation as a
widely-spread phenomenon, even though it may not be so
general as was supposed.” He adopts Vochting’s concep-
tion of “rectipetality” — “a regulating power leading to
growth in a straight line,” and says, “ The essence of the
matter is this : we know from experiments that a power
exists of correcting excessive unilateral growth artificially
produced ; is it not probable that normal growth is simi-
larly kept in an approximately straight line by a series of
aberrations and corrections ? If this is so, circumnutation
and rectipetality would be different aspects of the same
thing.”

“ A bicycle cannot be ridden at all unless it can
4 wobble/ as every rider knows who has allowed his
wheel to run in a frozen rut. In the same way it is
possible that some degree of circumnutation is correlated
with growth, owing to the need of regular pauses in growth.
Rectipetality would thus be a power by which irregularities,
inherent in growth, are reduced to order and made sub-
servient to rectilinear growth. Circumnutation would be
the outward and visible sign of the process.”

Phases of Botany, Pre-Hellenic to Neo-Hellenic. —

Hence, whatever detailed value be retained by the special

94 Chapters in Modern Botany chap, v

theories of Darwin, the world will always owe him thanks ;
for his books have a deeper use and significance. To
the dawning intelligence of the race, the forest is vaguely
astir with a life which man does not clearly separate from
his own — a mystery of growth which has left its mark
deep in the history of all religions. A later and more
self-conscious mind moulds this omnipresent life into
anthropomorphic shapes ; so a Dryad hides in every tree,
while Pan roams through the glade. These anthro-
pomorphic shapes are next formalised away from the living
realities they symbolise ; they become mere shadowy gods,
then fairies and fables. The tree (or what remains of it)
is now something economically useful ; it has also a
popular and a systematic name ; but to Utilitarian or
Linnsean alike, the form and substance seems the main
thing, not the life. <c Great Pan is dead ” ; the botanist
is as prosaic and unseeing as the woodcutter, in fact
essentially is one, at best with finer tools, and like him
does his best work away from the wild wood altogether.
But as the ages of fetishism, of Hellenic anthropomorphism
passed away, so now the formal and utilitarian and analytic
spirit is passing also in its turn. Science is entering a
new and brighter Hellas ; the Dryad, living and breathing,
moving and sensitive is again within her tree ; nay better,
the plant is herself the living Dryad, her naked beauty
radiant in the sun. And what of this old naturalist who
led us back into the forest sounding with the Protean
mystery-play of evolving Life ? Now that his rugged face
has vanished, it grows more strange yet more familiar in
memory ; we have seen it of old — we know it now for the
returning avatar of Pan.

CHAPTER VI

THE WEB OF LIFE

Struggle among Plants — Perched Plants or Epiphytes — Parasitic
Plants — Mistleto — Dodder — Root - Parasites — Toothwort —
Broom - rapes — Saprophytes — Parasitic P'ungi — Bacteria —
Symbiosis .

Struggle among Plants. — In our study of climbing plants
we saw that plants as well as animals had difficulties to
contend with, and that there was, especially in crowded
places, a more or less intense struggle for existence.

Of this the tropical forest of our frontispiece is a supreme
illustration. To rise out of the perpetual twilight of their
depths is a condition of success, and thus we have first tall
straight-stemmed trees, then twiners and climbers which
strangle and overshadow these, while strange high-perched
epiphytes and parasites grow and scatter their seed over the
whole tangled roof of verdure. Describing “ the struggle
for life in the forest,” Mr. James Rodway writes of a scene
in British Guiana: “We can almost fancy the magnificent
forest tree protesting strongly, as octopus-like, the Clusia
begins to compress and strangle it. . . . The Clusia grows
stronger and stronger, until by and by, as the strangler
opens its magnificent waxy flowers to the sun, and glories
in its conquest, the poor unfortunate victim droops and

g6 Chapters in Modern Botany chap.

dies. Then the trunk becomes diseased, wood ants begin
their work, and finally nothing is left but the hollow
cylinder of the strangler.”

In the crowded vegetation by the river- side, in the
meadow, along the hedgerows, the same struggle for
standing room, for air, for light, must occur, and there are
many peculiarities of plants which find partial explanation
as adaptations of structure which help plants in crowded
places to keep their foothold.

When we remember that plants have not such diverse
needs as animals have : that they all require very nearly
the same kind of food ; that most of them get this in
precisely the same way — by their roots from the soil, by
their leaves from the air — we feel that there must be what
may be called struggle or competition between them when
they grow crowded together. We get a vivid impression
of struggle for space in the crowded rosettes of a patch of
house-leeks (Sempervivums), which have been allowed to
grow for some time as they list ; we see the younger plants
budding from their parents, rising upon their shoulders,
and often not only smothering them, but soon coming to
crowd upon each other and compete anew. Again, only
a fraction of the seedlings which appear above the surface
in a plot of ground reach maturity. There is neither room
nor food for them all, and the less fit are eliminated. And
this is recognised practically by every farmer or gardener,
for does he not thin his turnips or onions, knowing that
thus alone can he ensure the success of individuals ?

No doubt there is some danger of exaggerating this
struggle for existence among plants, and yet more of
attributing to it results which may have some entirely
different origin. Experiment is much needed to sub-
stantiate what is often assumed.1 But on general grounds
1 A careful statement of facts was given by Mr. Walter Gardiner in

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it seems much more true of plants than of animals that
between those of the same kind the struggle for existence
in a crowded area is very keen. For plants cannot migrate
nor combine in mutual aid ; in a crowd the slightly weaker
must be smothered.

In this matter Darwin is again ready for us with exact
observation and experiment. “From observations which I
have made it appears that the seedlings suffer most from
germinating in ground already thickly stocked with other
plants. ... The more vigorous plants gradually kill the
less vigorous, though fully-grown plants ; thus out of twenty
species growing on a little plot of mown turf (3 feet by 4)
nine species perished, from the other species being allowed
to grow up freely. . . . Seedlings also are destroyed in
vast numbers by various enemies ; for instance, on a piece
of ground 3 feet long and 2 wide, dug and cleared, and
where there could be no choking from other plants, I
marked all the seedlings of our native weeds as they came
up, and out of 357 no less than 295 were destroyed, chiefly
by slugs and insects.”

Perched Plants or Epiphytes. — In temperate climates
the only perched plants are those mosses and lichens which
sometimes clothe the stems and branches of trees, but in
tropical countries many Orchids, Aroids, and other flower-
ing plants have this habit. Entirely isolated from the
ground, and yet not parasitic on their bearers, how do they
live ? In part, of course, like other green plants, on the
air and the power of the sunlight ; and it is interesting to
notice that they flourish best on trees, such as Cassia and
Caesalpinia, whose crown of branches is in the dry season
bereft of leaves. For water and mineral matters the

a lecture entitled * ‘ How Plants maintain themselves in the Struggle
for Existence,” delivered at Newcastle, September 1889. See abstract
in Nature , September 1889.

H

98 Chapters in Modern Botany chap.

perched plants depend on the rain and damp atmosphere,
for absorbing which the skin of the roots is often specially
adapted, forming a kind of sponge. And although the
roots do not come into contact with the soil, it seems that
in some cases they may absorb salts from the decaying
bark of their bearers, or from such debris as may gather
about the branches.

From the great Cypress swamps of Florida, or the small
but better known surviving fragment of one of these which
surrounds the palace-citadel of Chapultepec, the favourite
morning ride of every visitor to the city of Mexico, the
traveller brings back a unique impression of mournful
picturesqueness. The sombre coniferous foliage is draped
with long silver-gray streamers, the “ Spaniards5 beards 55
( Tillandsia usneoides ), one of the most conspicuous and
widely distributed examples of the perching habit. This
has not even roots, but fastens itself to its bearer by means
of its long thread-like stems, the scales of which seem to
absorb the necessary supply of water. Aroids of the genus
Philodendron , often cultivated in our greenhouses, and
Orchids belonging to the genera Dendrobium, Oncidium,
Phagus, etc., are also good examples of epiphytes provided
with aerial roots, which absorb water -vapour from the
moist atmosphere of the tropical forest. Here indeed is
the solution of a mystery which has often puzzled the
botanist : how transpiration from the leaves could go on
without any apparent source of water-supply. A micro-
scopic section of the root, however, shows a tissue of
characteristically thickened cell-walls, which take up mois-
ture from the vapour-laden atmosphere during the coolness
of night.

The best general account of Epiphytes is that given by
Goebel in his valuable Pflanzenbiologische Schilderungen
(Part I., Marburg, 1889). With this Schimper’s Epiphy-

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tische Vegetation Amerikas (Jena, 1888) should be com-
pared. Goebel discusses, for instance, the various ways

in which the epiphyte effects attachment to its bearer,
usually by a special disc with root - suckers, some-
what similar to that in mistletos. The spongy rind of
Aroids and Orchids is regarded as primarily an organ of

ioo Chapters in Modern Botany chap.

assimilation, but as a secondary function it absorbs mois-
ture. Sometimes, as in Tillandsia usneoides , the leaves
absorb water through their surface, and then the roots
tend to disappear. Another fact of much interest, repre-
sented in most collections, but which Goebel describes in
detail, is the manner in which some epiphytes, especially
ferns, e.g. “ the bird’s-nest fern ” ( Asplenhwi nidus-avis)
and the “ stag’s-horn fern ” {Platy cerium), gather nests of
humus about their roots, thus literally making soil for them-
selves upon the branches.

Parasitic Plants. — The perched plants which grow on
the shoulders of their fellows naturally suggest parasites,
which to a greater or less extent live at the expense of
their hosts. As among animals, there are endoparasites,
like some bacteria and fungi, which live within their
hosts, and ectoparasites, which, though vitally fixed to
their hosts, live outside of them.

Let us begin with the ectoparasites — the outside
hangers-on. Some of these, e.g. the mistleto, are only
in part dependent on their hosts, the parts which do not
pierce the host retaining the ordinary powers of green
plants ; others, e.g. the adult dodder, are wholly dependent
upon their hosts for the sustenance of their life.

Mistleto. — Of all parasitic plants the mistleto is
probably most familiar, as certainly also one of the most
remarkable. The species ( Viscum albuni) which grows in
Britain and throughout Europe is a parasite on a great
variety of trees, such as apple, pear, sycamore, lime, but its
favourite host is the black poplar ( Populus nigra), and one
of its rarest, in Britain at least, the oak. The strange
habit of the plant, the beautiful harmony of colour between
stem, leaves, and berries, its verdant and fruit-laden con-
spicuousness at the turn of the year towards lengthening
days and summer, of which the joyous celebration by the

vi The Web of Life ioi

northern peoples underlies the festival of Christmas, have
made the mistleto a favourite with men ; and, whether it be
the sacred plant of the Druids or no, it has become the
centre of many beautiful myths and customs.

A word for the life-history of the plant. In autumn and
winter the white seeds are eaten by birds, especially by
thrushes, and passing undigested from the food-canal are
voided on the branches, to the sides of which they eventu-
ally adhere. A tree in the botanic garden at Bonn is
thus specially noticeable ; its trunk crowded with seedling
mistletos just below a mass of branches whose sheltering
attractiveness is well marked by the remains of yearly nests.
From the seed a little root grows out, bends towards the
branch, sticks to it, and expands into a clinging disc.
From this there grows a modified rootlet which pierces the
bark and reaches the wood. There is no further growth
that year. But next spring the growth of new wood encloses
the rootlet, which at the same time increases in length.
In the second year the rootlet gives off lateral branches
which grow longitudinally between the wood and the outer
rind, and give off other rootlets. In proportion to the
number of these the mistleto plant flourishes in stem and
leaves. From the spreading roots fresh stems may arise,
so that from one seed a score of mistleto bunches may
arise. The rootlets penetrate into the wood and absorb
what that contains — namely, as we shall afterwards see,
water and salts, — while the mistleto stem spreads forth its
leaves, and behaves in regard to the light and air as any
independent green plant. It is in fact a natural graft.

Resembling our mistleto in most respects are other
species of the same genus Viscum, the American mistleto
often called by a different generic name, the more shrubby
Lora,7ithus europceus common on oaks and other trees in
many parts of the south of Europe. There are about

102 Chapters in Modern Botany chap.

three hundred species of mistieto-like plants included in
the order Loranthaceae, while the genus Henslowia — plants
of similar habit found in Southern Asia and in the Indian
Archipelago — belongs to the order Santalaceae.

It is interesting to notice that some of these mistleto-like
plants are sometimes parasitic on one another. Thus our
mistleto may grow on Loranthus europceus , or one species
of Viscum or of Loranthus may grow on another ; indeed
the common mistleto has been noticed — as is natural and
perhaps common enough — thus sprouting upon itself.

Dodder. — The dodders ( Cuscuta ) are parasites on such
plants as clover, flax, nettles, and hops. It is on the two
last that the commonest European species (C. europcea)
is usually found. In many ways it differs from the mistleto ;
thus the seed almost always germinates on the ground, and
the adult plant is practically destitute of chlorophyll and
leaves.

Let us follow the life-history of the common dodder.
Like most of the other species, it is an annual, dying away
in autumn. Before that, however, the seeds have burst
explosively from the seed-boxes, and have been swept hither
and thither by the wind. All through the winter they lie
dormant on the ground, sheltered in many cases by decay-
ing leaves, which supply a suitable bed for germination.
This does not take place till comparatively late in the
following year, not before the nettles and hops have acquired
some strength of stem, — a delay which is obviously of
advantage to the dodder.

Out of the seed there comes a little club-shaped root
which seeks the soil, but the young stem remains surrounded
by the seed-husks and the store of nutriment which these
enclose. It grows — thin as a thread, and somewhat spirally
— at the expense of the seed-store. This is soon exhausted
and growth practically stops, but the thin stem still circum-

VI

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The Web of Life

nutates in sweeping circles, as if seeking about for some
plant on which to cling. If this be not found, the stem at
length lies prostrate on the ground, and after a strange
dormant existence for a month or so longer, dies. But if
some plant be near at hand, the dodder stem slings itself
around it after the manner of a twining plant. As soon as
its stem has embraced that of its bearer — let us say a nettle
or hop — it gives off attaching papillae which penetrate the
rind and bud off numerous little rootlets. These come into
close connection with the bast portion of the hop or nettle
stem, and thence absorb nutritive materials. Now, in
all plants the vessels of the bast form the channels
by which the complex organic substances manufactured in
the leaves pass towards the root. Such are the spoils
which the leafless dodder, unable to manufacture organic
stuffs for itself, absorbs from its host. After suitable
attachment has been effected the stem continues its twining
growth with increased vigour, the basal part dies away and
all connection with the soil is lost, and eventually the wan
parasite bursts into flower.

Root-Parasites. — In some well-shaded part of the wood
we may find a large patch of the cow-wheat (. Melampyrum ),
a delicate plant with a pale yellow flower, akin to toadflax
and snapdragon. If we dig it up very carefully along with
the plants growing near, we may be able to see that its
roots are at intervals tightly bound to those of its neighbours.
The connection is a vital one, and effected by peculiarly
modified discs which grow round the roots of other plants
and send suckers into them. This is a clear case of root-
parasitism, but it is difficult to tell how much it amounts to,
for the cow-wheat has also independent roots which absorb
water, salts, and probably decaying vegetable matter from
the soil, and it also is a green plant. If this habit were
peculiar to the cow-wheat we should not be inclined to attach

104 Chapters in Modern Botany chap.

much importance to it, but it is far otherwise. What is
true of Melampyrum is true of several hundred species of
plants, especially of the orders Santalacese and Rhinan-
thaceas. Thus the louseworts (Pedicularis) have very long
surface-roots which attach themselves to the fibrous roots of
the grasses and other plants of pastures and meadows.
The length of the roots is in part explained, as Kerner
points out, by the fact that most species of Pedicularis
persist from year to year. For when a root has fixed itself
to that of an adjacent annual, and that has died, it becomes
necessary for the parasitic root to shift its anchorage and
to find another farther on. This case of Pedicularis is
further interesting because the root-hairs which are borne
by the roots of most plants are here absent, except at those
points where parasitic attachment is effected ; not but that
the skin of the root may without root-hairs absorb water
and salts and the products of decaying vegetable-mould.

Of the Alpine Bartsia (. B . alpina ), Kerner states that
some of the roots are parasitic, while others are adapted for
absorbing humus ; and among the same group of root-
parasites we have also to include the odd -flowered and
fruited yellow-rattles ( Rhinanthus crista-galli ), and even
the pretty Eyebright or Euphrasy ( Euphrasia officinalis ) of
our moorland pastures and roadsides.

The root-parasites are often blamed for spoiling pastures,
and even the milk of the cattle that graze there. They
certainly often occur in great abundance, and hence must
to some extent impoverish the plants whose roots they
suck ; as yet, however, there is no definite evidence Of
appreciable damage done.

One wonders what constitutional peculiarity distinguishes
the members of these two orders (Rhinanthaceas and
Santalaceas), so many members of which have this strange
habit of root-parasitism. Some hints of an answer have

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The Web of Life 105

been discovered by Professor Bonnier, who finds that in
Euphrasia, Bartsia, and Rhinanthus (though not in Melam-
pyrum and some others), respiration predominates over
assimilation (see chap, ix.), the oxygen which is liberated
in assimilation being completely masked by the absorption
of oxygen in respiration — the very reverse of that gaseous
interchange characteristic of the daily life of ordinary green
plants.

The Toothwort. — One of the strangest British plants
is the toothwort (. Lathrcea squamaria ), which has been
already mentioned at chap. ii. p. 35. It lives an almost
completely underground life upon the roots of poplars,
hazels, and other trees, hidden by a growth of ivy, or by
a heap of mouldering leaves. No young botanist will ever
forget his first finding of the strange pale plant, nor can an
old one ever lose his feeling of wonder as he digs down
from the thick drooping spike, with its faded lilac-tinged
flowers, to the still stranger underground stem, close-set with
thick sharp-edged white teeth, more like a witch’s necklace of
human incisors than a leafy shoot. The plant, though never
abundant, has a wide distribution in Europe and Asia. When
we bare the thin roots which spring from the underground
stem, we see that they are clasped by small adhesive discs
to the roots of the tree at whose base the plant grows.
From these adhesive discs, which in another European
species ( Lathrcea clandestine i) are about the size of split
peas, suckers penetrate the tree-roots, from which without
doubt the toothwort steals not only its salts and water, but
the abundant starch reserves which help to swell its
crowded leaves. Nor does the odd story of life end here ;
for each tooth-leaf has a strange recess, no mere hollow
of decay, but a normal cavity, narrow of entrance, gland-
lined, difficult of interpretation save as an insect trap,
as which it is commonly regarded, carrying back our

106 Chapters in Modern Botany chap.

thoughts to the terrestrial species of Utricularia (p. 31).
Few botanists imagine that this strange and local plant may
be cultivated ; it grows well under cherry laurels (“ common
bay,” Cerasus laurocerasus ) in the shrubberies of the
Edinburgh Botanic Garden, and hence at least suggests the
possibility of its introduction elsewhere.

Broom-rapes. — In our search for the toothwort we may
perhaps find one of the broom-rapes ( Orobanche ), which
grow parasitically on the roots of thyme, scabious, and other
common plants. As in Lathrasa, there is no chlorophyll ;
but the union between parasite and host is much more inti-
mate, so much so indeed that it is difficult to separate them,
or to tell where the tissues of the parasite end and those of
the host begin. The seedling of the broom -rape and
related parasites is very remarkable ; it bears no trace of
cotyledons, but is a delicate thread-like structure, one end
of which is hidden by the remains of the seed, while the
other grows in search of roots to which to fix itself. If
these be not soon found, the seedling shrivels and dies, for
it seems quite unable to absorb food from the soil. But if a
suitable host be found, the seedling becomes ultimately
united with it, thickening into a knotted tuber-like structure,
from which the flower-bearing stem with its brown useless
leaves will afterwards rise above the ground.

Thus from the non -parasitic toad- flax and its allies,
which we shall come to know better as the natural order
Scrophulariacece , commonly gay flowered and free growing,
often indeed mischievous as weeds, we have a regular
gradation through cow- wheat and Euphrasy, lousewort and
Bartsia, to full parasites like toothwort and broom-rape. For
still further modified allies of the broom-rapes (Orobanche)
we must travel abroad, especially to tropical countries,
where there are many strange plants (Balanophoreae) of
similar habit but strangely crowded and reduced flowers.

VI

The Web of Life 107

Best known among these, especially to tradition, is the odd
Cynomorium , a plant of fungus - like appearance, long
known as Fungus melitensis . It is found in the islands of
Malta and Gozo, and is remarkable for its scarlet colour
and blood-like juice, which, according to the “ doctrine of
signatures,” were interpreted as providential indications of
its value as a cure for all diseases accompanied by bleed-
ing. Even stranger are the Rafflesias, found in the Indian
islands and in South America. They have neither a
developed stem nor leaves, but are reduced practically to
a flower closely fixed to the roots of some other plant
(usually a species of Cissus). The largest species, dis-
covered in 1818 in Sumatra by Dr. Arnold, and sent by
Sir Thomas Stamford Raffles to Robert Brown, who hence
gratefully named it Rafflesia Arnoldi, measures about a
yard in diameter, and is, one need hardly say, the largest
of all known flowers. It seems, however, a repulsive giant,
fleshy and fungus-like, coarse and gaudy, worst of all, fetid
as carrion, and hence swarming with appropriate insects,
who even lay their eggs in preparation for its foul decay,
and so, too, doubtless render the service of fetching and
carrying pollen from flower to flower.

Saprophytes. — A large number of plants, especially in
the woods, some with and some without chlorophyll,
depend in great part at least on the abundant organic
material afforded by the decaying leaves and other parts
of plants — in other words, on the humus or vegetable mould
of the soil. This is true, for instance, of the Bird’s-nest
Orchis ( Neottia nzdus -avis), of the rootless Corallorhiza ,
of the little twae-blade (. Listera cordata :), and of many other
orchids. But it is very difficult to draw any hard and fast
line between parasites, which live on living organisms, and
the so-called saprophytes, which live on decaying organic
matter. Thus the species of cow- wheat (Melampyrum)

108 Chapters in Modern Botany chap.

and yellow rattle (Rhinanthus), which are undoubtedly
root-parasites, appear also to depend upon the humus of
the soil ; the orchid Monotropa Hypopitys is said to be a
parasite in the pine woods, but a saprophyte on the rotting
leaves under other trees ; and some of the epiphytes gather
so many decaying leaves around their roots that it is likely
that some of them should also be called saprophytes. The
staghorn - fern (Platycerium), so commonly grown as a
suspended ornament of our greenhouses, may here again
be an instructive case (see p. 99).

Parasitic Fungi. — Of infinitely greater importance as
mischief-makers in the world than the parasitic flowering
plants are the innumerable parasitic Fungi. Those which
cause mildew, potato-disease, some diseases of cereals, vine-
plants, and timber-trees, and so on, are often disastrous to
the prosperity of a fertile region, nay, as modern experience
as well as history too often tells, to a province, a whole
nation. Nor do they affect plants alone, but also animals,
and even man himself ; witness respectively salmon-disease
and ringworm.

Fungi being, like dodder, toothwort, and broom-rape,
plants without any chlorophyll, they are unable to use the
carbonic acid gas of the air, as green plants do, and are
therefore dependent upon ready-made supplies of organic
matter. Hence we find them living either as parasites on
living plants and animals, or as saprophytes on decaying
organisms, or in some cases indifferently on either.

As we do not propose here to do more than indicate the
part played by fungi in the economy of nature, either in
destroying living organisms or in utilising the products of
decay, we refer the student to that convenient and in-
teresting introduction to the subject supplied by Professor
Marshall Ward in his little volume on Timber and some of
its Diseases (Nature Series, Lond. 1889), and for syste-

vi The Web of Life 109

matic information to the text-book of Cryptoga7nic Botany ,
by Bennett and Murray (Lond. 1889). As an easy intro-
duction to the subject, the writer’s article “Fungi” in
Chambers' s Encyclopcedia may in the first place be read.

Bacteria. — Of all parasitic plants, the smallest — the
microscopic Bacteria- — are the most important. It is true
that not all of them are parasitic, for many live in rotten-
ness, but they all agree in being minute colourless units
unable to live, as green plants do, on water, salts, carbonic
acid, and sunlight ; able to thrive only when they find
organic substances of some sort ready-made for them.

We have already seen that their presence may help to
explain the disappearance of flies within the traps of some
of the insectivorous plants, and we can detect the work of
bacteria in a hundred ways all around us. If we leave a
piece of meat or fish (whether cooked or raw matters not)
in an open vessel filled with clear water, this becomes
turbid, and a scum gathers on the surface ; if we examine
this scum under a high power of the microscope we see
incalculable numbers of bacteria. On the other hand, if
the piece of meat be placed in a glass flask which is filled
with water, then boiled for a short time, and carefully
closed, while still boiling, with a plug of cotton-wool, the
water will remain quite clear, and the flesh will not decom-
pose. The only difference between the two cases is that
in the latter the bacteria in the water have been killed by
boiling, and are kept out of the flask by the cotton-wool,
therefore we may say that the decomposition observed in
the first case was due to bacteria. So it is always. As
Huxley in aphoristic style puts it, “ Putrefaction is the result
not of death, but of life.”

It is more than two centuries since the keen - eyed
Leeuwenhoek — that early devotee of the microscope to
whom, despite his rude and feeble instruments, the most

iio Chapters in Modern Botany chap.

abundant laurels of discovery must be awarded — discovered
this living dust, a knowledge of which has not only changed
some of our biological conceptions and created the dominant
school of medical theorists, but reformed and rationalised
surgery and hygiene, and exercised a potent influence
upon the greatest industries. To what is the importance
of bacteria due ? They are very minute, quite invisible
individually to our unaided eyes — minute spheres, rods, or
spirals they are, the smallest unit masses of living matter.
The secret of their strength is in their power of multipli-
cation. By repeated division and redivision one soon
becomes a thousand ; indeed, given sufficient food, in a few
hours a single unit may have become the progenitor of
millions.

But their importance depends also on their universal
distribution. Universal, indeed, for they live in the air
and in the soil, in food and raiment, in man, in beast, in
plant ; not even the water which we drink is free from
them. From the mouths of men to the walls of their
houses and the flies on the windows, from the hair of the
head to the toes of the feet, from the highway dust to the
recesses of the forest, and from wayside pool to sea, bac-
teria abound. But while they spread here, there, and
everywhere, they become most obvious wherever dead
organic matter is abundant, as in the refuse of our towns,
or in dead plants and animals. Rancid butter and rotten
cheese, “ high ” game-flavoured meats and over-stale bread,
blue milk and soured wine, as well as cesspools and dust-
heaps, sewers and slums, are their common habitats.

Most important, however, is the fact expressed in the
“ germ-theory,” that bacteria are constantly and intimately
associated with some of the most fatal of human diseases,
such as consumption, diphtheria, smallpox or typhoid,
malaria or leprosy. Bacteria, in fact, kill most of us,

vi The Web of Life hi

multiplying within us so as to choke the system, at once
feeding greedily on the tissues or fluids of the body, and
poisoning us in every cell with the waste-products of their
loathsome life. It would take us too far from present
botanical considerations to discuss the ways in which we are
saved from the bacteria which so easily beset us. There are
happily many drugs and reagents of inward or outward use,
from quinine to carbolic acid, and all the other ever-multi-
plying antiseptics — drugs which they cannot stomach. Like
the Acacia tree with its bodyguard of ants driving off other
dangerous ants, so, some tell us, we may have a protective
standing army of bacteria which save us from others ; at all
events, the widely accepted theory of Metschnikofif makes
us view the animal body as garrisoned by the uncoloured
blood-corpuscles or leucocytes which seize the intruding
microbes just as amoebae would do, and digest them
beyond the reach of mischief. But when the germs

become too numerous or the warders enfeebled, the

bacterial parasite begins its growth, and the disease has

set in. Such, for instance, Dr. Gulland tells us, is the

use of the tonsils which lie on either side of the pharynx ;
in health they furnish a continual succession of these
corpuscle sentries, while in the inflammations to which
they are so subject, the bacteria have been too many, the
defence too weak. This subject is at present under active
controversy, not a few bacteriologists maintaining that the
germicidal virtues of the blood lie wholly or mainly in the
serum, not in the corpuscles.

But the most potent preventive, universal and costless,
is a natural one — the sunlight. Miquel, of the excellent
bacteriological observatory of the Parc Montsouris at Paris,
and several French bacteriologists have shown that the sun’s
light, even without the heat, for a few bright hours is able
to kill the germs which float in the air, and leave them

1 12 Chapters in Modern Botany chap.

harmless. What it requires hard boiling to accomplish,
the sunlight does with rapid ease. For although the
spores or young bacteria which are carried in the air are
peculiarly hardy, able to nurse their dormant virulence for
months or even years, ready when they find a suitable
resting-place to multiply with appalling swiftness, they
cannot withstand the power of the sunlight. What is the
light of life to other plants, is to the bacteria death. Per-
haps it makes them live for a short time so rapidly that
their scanty speck of living substance is worn out and
consumed beyond possibility of repair. Quite literally
then “the pestilence walketh in darkness.55 Hence then
we may interpret much of the history of disease, and see that
we have here no mere mysterious visitations, to be crouched
before with submissive dread ; but a definite and intelligible
part of the order of nature not only to be analysed and
grasped by science, but which we may partly modify by art
and partly by adapting our other conditions. And thus it is
that we find so many dead germs in the air ; thus we regard
all dark places as evil, whether they be the lidless coffins
of the slums or the dungeon-like sunk flats of west-end
houses ; thus we begin to appreciate the hygienic importance
of sunlight — the most universal, the most economical, the
most potent antagonist of our subtlest and deadliest foes.

But the part which bacteria play in the economy of
nature is often beneficent. F or although we may not derive
much satisfaction from the fact that bacteria by thinning out
the weaker organisms help to keep up the standard of
vitality, a little observation leaves the more cheering con-
viction that many of them are among the great cleansers of
the world. Out of dead plants some form vegetable mould,
while others reduce the carcases of animals to simpler and
purer substances. Consider the large circle on which
bacteria occupy an arc ; green plants feed on air, water, and

VI

The Web of Life 113

the salts of the soil ; many animals feed upon plants, even
the most purely carnivorous are of course indirectly depend-
ent upon them ; dead animals are consumed by bacteria,
which in so doing set free carbonic acid gas and ammonia,
which steal off into the air, and nitrates and the like which
soak down into the soil, all to be utilised by the plant-
world anew. Here then the bacteria furnish the indispens-
able means by which the circulation of the materials of
organic life is perpetually renewed.

In this connection we may appropriately discuss an
interesting story in regard to the relations between bacteria
and some other plants. Pull up a bean-plant, wash gently
and examine the roots. Here and there may be noted
small rounded swellings or tubercles, sometimes as large
as peas. The same may be seen on the roots of vetches
and other leguminous plants, and also on some cereals.
That they are not normal parts of the plants is clear, for
there are roots without a trace of them ; and if the seeds be
grown in soil that has been calcined at a high temperature
or in water that has been boiled, the tubercles are not
formed. This becomes yet more evident when the tubercles
are examined microscopically, for then they are seen to
exhibit, not the tissues of a rootlet, but crowds of minute
rod-like or somewhat spherical bodies.

What then is the meaning of the tubercles ? The re-
searches of numerous botanists, with a controversy which has
been in progress since 1858, when these tubercles were first
noted, have at length convinced most of us that the little
rod-like bodies contained in the tubercles are bacteria, and
that the tubercles are due to the infection of the plants by
these micro-organisms. As to their physiological effect on
the plant, the investigators are not yet thoroughly agreed,
but the conclusions of Hellriegel and Willfarth, corroborated
by some others, are of great interest. By growing legum-

1

ii4 Chapters in Modern Botany chap.

inous plants and cereals in calcined earth and with a
supply of water that has been boiled, they obtained plants
without tubercles on their roots ; by watering these with
water which has been in contact with the soil in which
tubercled roots have grown, they were able to infect the
plants, or they could also inoculate them directly. After
infection or inoculation the plants acquire a new vigour,
and they increase in nitrogenous substances to a greater
extent than can be accounted for by the nitrogenous salts
in the soil. To explain this it seems necessary to believe
that with the help of their partner bacteria the legumes and
cereals are able to utilise the free nitrogen of the air. If
so, they are able to do what, as we shall afterwards see
(chap, ix.), is regarded by most botanists as quite impos-
sible for other plants.

Symbiosis. — Whatever be the final verdict in regard
to the alleged partnership between bacteria and their
leguminous or cereal hosts, there are other cases of
mutually helpful partnership in regard to which there is no
doubt. To this De Bary in 1879 applied the name
Symbiosis (literally, a living together). It is interesting to
notice the parallels among animals ; zoophytes clustered on
the backs of crabs are comparable to epiphytes ; among
animals as among plants there are many external and
internal parasites ; the well-known partnerships between
hermit-crabs and sea -anemones are examples of the
beginnings of a truly co-operative symbiosis.

The most important case of symbiosis among plants,
indeed by far the most fully-developed case in nature, is
that of the lichens. These used of course to be regarded,
as they no doubt popularly are still, as definite individual
plants united by common characters into a well-marked
group or “natural order,” which botanists were wont to
reckon along with algae and fungi, but no less distinct

vi The Web of Life 115

than these. Their immense variety has given no small
amount of labour to the systematists, and a well-developed
specialism of “ lichenology ” with a literature and with
collections voluminous enough to surprise any one who first
enters upon it, has arisen in consequence. In 1866 the
penetrating cryptogamist De Bary threw out the suggestion
that the many resemblances shown by the lichens on one
hand to fungi and on the other to algae, might really be
due to an admixture of these two constituents. The idea
was taken up and worked out carefully by his pupil
Schwendener, to whose practical insight and industrial
training the science owes other important ideas.

From the fungus-like tissue of a lichen he isolated little
greenish cells like Protococcus and other unicellular algae,
and in some cases these green cells were able to live and
multiply after they had been isolated. This remarkable
fact led Schwendener to develop the hint given by De Bary,
and to establish his so-called “ dual hypothesis ” of the
nature of lichens.

“ As the result of my researches,” he says, “ all these
growths (lichens) are not simple plants, not individuals in
the ordinary sense of the word ; they are rather colonies
consisting of hundreds and thousands of individuals, among
which, however, one predominates, while the rest in perpet-
ual captivity prepare the nutriment for themselves and their
master. This master is a fungus, a parasite which is
accustomed to live upon others’ work ; its slaves are green
algae, which it has sought out, or indeed caught hold of
and compelled into its service. It surrounds them, as a
spider its prey, with a fibrous net of narrow meshes, which
is gradually converted into an impenetrable covering ; but
while the spider sucks its prey and leaves it dead, the
fungus incites the algae found in its net to more rapid
activity, indeed to more vigorous increase.”

n6 Chapters in Modern Botany chap.

The lichenologists were indignant at this proposed
revolutionary demolition of their science, and defended the
traditional position stoutly from 1869 to 1873 \ but in that
year the leading algologist, M. Bornet, fully confirmed and
extended the results of Schwendener, showing, for instance,
that a single “ lichen ” might really contain three or four
distinct and familiar species of algae, overrun and woven
into a false tissue by a single mould ; while the lichenologist’s
contention, that the green alga-like cells developed from
the fungus-like filaments, was shown to be based on incorrect
observation, the former being really only sucked by close-
fitting or ingrowing protrusions of the latter. Another
important proof was given by Stahl, who succeeded in
making lichens artificially, i.e. by taking a known alga, and
sowing a known fungus upon it ; a lichen, a known lichen,
was the result. How obstinately the controversy raged
is oddly commemorated in the still current edition of the
Encyclopedia Britannica ; thus the article Botany retains
the older view, while Fungi states the modern, Lichens
(of course by an eminent and conservative lichenologist)
returns to the traditional standpoint, while this is finally
corrected at Parasitism. That, however, no shadow of
doubt any longer exists on the matter is well shown by the
recent researches of Bonnier, who has not only repeated the
synthetic experiments of Stahl with all bacteriological
precautions, but varied them by substituting the protonema
or filamentous alga-like stage of the life-history of a moss
for the ordinary alga constituent.

Since then it is not only possible to separate the algas
constituent, and to see it live independently while the fungal
one as naturally starves, but also to combine the two elements
into a unified life, we believe that a lichen is not a single,
but a double organism, — an intimate union of an alga and
a fungus, living in mutual helpfulness or symbiosis. After

VI

The Web of Life

117

we have considered the physiology of the leaf, we shall be
better able to understand how really helpful the two kinds
of elements which compose a lichen are to one another (see

Fig. 6. — Patch of lichen grown synthetically by Bonnier (from sowing of fungus
spores on algae) under bacteriological precautions against entrance of foreign
spores. (After Bonnier.)

chap, v.) Yet we already see that we have here a tiny
repetition of the organic universe, of the Balance of Nature
between plant and animal. The lichenologist is no real
loser ; if his specimen has lost its individuality, it has gained

n8 Chapters in Modern Botany chap.

a higher and more complex one ; and it is a microcosm of
nature to boot.

It is also interesting to notice the existence of what
Zukal calls “ half-lichens,55 in which the mutual partnership
has not been thoroughly established. For certain fungi
usually occurring as lichens may, in certain conditions, live
bereft of their partner-algae, as saprophytes ; while others,
which are usually parasites or saprophytes, are sometimes
found combined with algae, and forming lichens. We have
thus what we may fairly consider lichens in the making.

An interesting parallel to the case of lichens, in which
the part of the fungus may be taken by animals of very
various kinds, is that of the “ yellow cells,” first known
as practically constant features of the organisation of the
Radiolarians, and long supposed to form an essential part
of these, but also occurring in the tissues of many much
higher organisms, especially Ccelenterates. Notably the
large sea-anemone Anthea cereus , which prospers and multi-
plies greatly in consequence. These yellow cells survive
after their host dies, or even after being isolated ; they
divide like unicellular algae ; they contain starch, and a
pigment like that of Diatoms ; they have a wall of cellu-
lose, and they evolve oxygen during sunshine. There-
fore they are regarded with justice as partner algae. As
they live they remove carbonic acid and nitrogenous waste
from their partners, and evolve oxygen which acceler-
ates the vital processes of the animal ; they form starch,
which when dissolved passes out by exosmosis into the
animal tissues ; when they die they are digested. The
partnership is therefore one of benefit to both parties.
There has been considerable dispute as to details, but the
general facts of this symbiosis between plants and animals
are admitted by all.

Strasburger has discovered an interesting association

VI

The Web of Life 119

between a small aquatic plant called Azolla and one of the
freshwater algae called Anabaena. On the under surface of
each floating leaf of Azolla there is a small opening leading
into a cavity in the substance of the leaf. In this cavity
a colony of the alga is always found. The alga may indeed
live independently in the water, but the Azolla is never
without its partner. We are not, however, certain as to the
precise meaning of the association ; and it may be doubted
whether we have really here as yet any appreciable measure of
true co-operation at all. Yet through such mere mechanical
associations or juxtapositions true parasitism and sym-
biosis must largely have arisen.

One of the strangest kinds of reputed symbiosis is the
occurrence of fungi around the roots of certain plants.
Frank and others have shown that the root-tips of beeches,
birches, hazels, and the like, are invested by a net of fungus-
threads. They suggest that the net acts as a sort of sponge
intermediate between the roots and the soil, and this idea
of the “ mycorhiza,” as the fungus network is called, receives
some corroboration from the fact that in heaths and some
orchids the fungus -threads actually penetrate into the sub-
stance of the root. The question is, however, under hot
discussion, the veteran arboriculturist, Hartig, stoutly main-
taining that we have here nothing beyond a mere parasitism
of the mould filaments upon the tree-roots. The controversy
is one which the student may profitably follow, summarising
(and where possible checking by actual observation) the
arguments on either side.

CHAPTER VII

RELATIONS BETWEEN PLANTS AND ANIMALS

Plants and Snails — Plants and Ants — Doinatia — Myrmecodia —
Galls — Plants and Aphides — Cats and Clover

Relations between Plants and Animals. — Our study
of insectivorous plants and of plant-movements have already
shown us that the conventional distinctions between plants
and animals are by no means natural. Of this fundamental
unity Linnaeus had some conception when he united plants
and animals under the common title Organisata , in contrast
to the mineral world of matter merely aggregated, not
organised, as Conserta , and to have demonstrated the
essential unity of organic life is one of the most important
characteristics of modern botany. The reader will do well
to consult Claude Bernard’s work, of which the title is a
summary, Sur les Phenomenes de la Vie communs anx
Animaux et aux Vegetaux. Nor can it too clearly be
realised that the distinctness of three kingdoms of nature,
animal, vegetable, and mineral, however stereotyped in
school-books, is, like much else we have learned there, a
survival of mediaeval non-science. It is in fact a doctrine
of the alchemists, of which the whole science of biology,
the whole doctrine of evolution is the confutation. How
and where to divide the Organisata into Animalia and

CHAP. VII

Relations Between Plants and A nirnals 1 2 1

Vegetabilia , with or without a common debatable land of
Protista (or it may be a still more fundamental, albeit in*
its own way strangely specialised group of Myxomycetes ),
is an important controversy enough, yet after all a mere
internal problem for biologists to settle among themselves.

Recognising then plants and animals as two main
groups of living creatures, which in their life have to solve
essentially the same problems, we may profitably continue
to study the many points of contact between them, inter-
relations such as those which we have already described
between insectivorous plants and their prey. Illustrations
of these interactions are growing increasingly numerous.

The seedlings, whose struggle for existence Darwin was
fond of watching, were sometimes thinned by one another,
and sometimes by the weather, but often by slugs, insects,
and other animals. This is of course the most obvious of
relations ; hosts of vegetarian animals feed upon plants.
Nor is this relation so one-sided as it looks, for in so doing
the animals are often of real service. The thrush which
eats the mistleto-berries spreads the undigested seeds from
tree to tree ; the bees which rob the flowers of their nectar
are at the same time the bearers of fertilising pollen from
plant to plant ; even the cattle which wholly eat up some
kinds of herbage give others more room in which to grow.

That animals select one kind of plant and leave others
untouched is an undoubted fact, and it is easy to believe
that this selective process may have many important
results. Let us take the case of snails and plants which
was studied in great detail a few years ago by Professor
Stahl,1 the experimental lichenologist already quoted.

Plants and Snails. — There are two kinds of snails —
omnivorous eaters and “ specialists,55 as Stahl calls them.
The latter are epicures, feeding daintily on toadstools and
1 Pflanzen und Schnecken , 8vo. Jena, 1888.

122 Chapters in Modern Botany chai\

the like ; the majority, unfortunately for our gardens, will
eat almost any verdure they can find. They have large
appetites, able to devour an eighth part of their weight of
cabbage in three hours ; and as every gardener mourns,
they are also very abundant ; 150 have been seen around a
plot of but a square yard, and an industrious naturalist in
one day collected 1200 of a single species from a piece of
ground three-quarters of a mile square.

But while snails, always excepting the “ specialists, ” are
not fastidious in their diet, they draw the line somewhere.
Certain plants they have found do not agree with them,
and these they eschew. They try them, suffer for it,
remember their experience, and leave the disagreeable
plants alone for the future.

Here then we have a problem such as a German
professor loves, and which no one can tackle with more
painstaking industry than he, — not even Darwin with his
sundews, — to tempt innumerable snails with all manner of
meats, to find out their favourite menu and their index
expurgatorius of viands, and to make a theory out of it.
Thus Professor Stahl enumerates at least fifteen different
ways in which plants have become abhorrent to snails, and
are thereby protected. Some plants are too sour, others
are poisonous ; some are full of ferments, others are rich in
purgative oils. There may be bristling hairs which prick
the sole of the snail’s “foot” as it creeps on the plant ; or
limy and flinty armature which makes eating too slow and
laborious even for a slug ; or slimy secretions which prevent
the animals from getting a good grip; or, best of all, the
tissues may contain thousands of little crystal needles which
stick in the lips and make them smart.

If you chew a little piece — let it be only a little piece —
of the cuckoo-pint ( Arum maculatum ), which grows in the
shady corner of the wood, your tongue and lips will be

vii Relations Between Plants and Animals 123

painfully excited for some time afterwards. This property
is much exaggerated in some of its allies ; thus the
“Dumb-Cane” of the West Indies is associated with ugly
tales of slave-torture, and even nowadays with occasional
cruel practical joking among the usually gentle and kindly
race of gardeners. Even in Arum some acrid poison is
present, but the irritating effect is mainly due to the
myriads of minute needle-like crystals (the raphides of
old books on the microscope) which the plant’s tissues
contain, and which pierce the soft skin of the experi-
menter’s mouth. After this experience one remembers the
cuckoo-pint ; can you believe that the snails, which both
Darwin and Romanes credit with good memories, will
forget ? For years a taste or a smell will remain in the
memory, and just as the suggestion of a mouthful of flinty
horsetails gives one a “ goose-skin ” shiver, so the snail on
a renewed impression of a disagreeable plant writhes its
horns in disgust and turns away.

Stahl’s research is doubtless valuable ; it is interesting
to know of the fifteen different kinds of protection which
save plants from being eaten by snails, though when he
says that he finds no wild flowering plant — not even a tree
— without some kind of protection against snails, the
suspicion cannot be repressed that he is proving a great
deal too much. And when he goes on to interpret these
protective qualities as being in direct relation to the appe-
tite of snails, to credit snails with being important factors
in the evolution of these qualities, we emphatically protest
against his conclusion.

The notion is of course a familiar one, but the truth
that there is in it may be falsely exaggerated. Something
unusual happened within a plant and it became sour ; the
snails tasted it and left it alone, but ate up its relatives
which remained sweet. These eaten up, the sour plant

124 Chapters in Modern Botany chap.

was left to produce other sour plants, on which the snails,
this time a trifle less fastidious, have to begin anew. They
naturally select the sweeter, and hence “ natural selection ”
preserves and propagates the sourer ; and so on indefinitely,
and vegetation thus tends to grow sourer to all eternity.
For this the snails are responsible ! Meanwhile, too,
natural selection, her ministers this time the browsing
mammals, is at work producing thorny plants, and so on.
It is the Darwinian theory in a nutshell ; and its accept-
ance or rejection is of fundamental importance to our
whole conception of evolution. Not only to Professor
Stahl, therefore, but to Mr. Wallace, to Professors Weis-
mann and Ray Lankester, perhaps even to the mildly
divergent Mr. Romanes, as assuredly to Sir John Lub-
bock or Mr. Grant Allen, such interpretations seem
not only valid but necessary, and not only necessary but
sufficient.

You have two dozen apples in your fruit-basket just begin-
ning to spoil. Each day you take the two best, and at the
end of a week there are ten rotten apples left. To a slight
extent, it may be said, you are responsible for the growing
rottenness, for you might have periodically selected the
two worst, but the rottenness was there ; it not only arose,
but increased without you. . Your selection, it is true, has
been such as to accelerate this disastrous evolution ; a
different selection would have retarded it ; what selection
can do then is at most to accelerate or slow the progress of
the selected along its definite grooves of natural change.
So the acids and ferments, the oils and encrustations, the
hurtful hairs and crystal needles, are constitutional peculi-
arities which arise in plants, and increase in them apart
altogether from any snails. Nature is indeed a marvellous
web, but we must not make it more tangled than it is.
The sourness, the poison, the ferments, the crystals, are,

vii Relations Betiveen Plants and Animals 125

so to speak, “ in the blood.’5 Their occurrence is wide-
spread, and in some cases their primary meaning in the
internal economy of the plant is well known.

The idea that tannin has been developed as a protection
against snails and other animal enemies is no doubt at
first sight attractive, while we are ignorant of its real
nature, and also while we ignore the fact that a plant rich
in tannin, like the oak, may be peculiarly a prey to animal
enemies, or meet it by inventing a fresh hypothesis of the
adaptation of some animal enemies to withstand the
defence. But when it, like the protective crystals, becomes
viewed as essentially a waste product, and as therefore
necessarily turned out in definite chemical proportion by
the life-processes of the plant, the .natural selectionist argu-
ment recedes as far into the background as it would have
to do in explaining the origin of the crystalline form or
taste of any chemical product, the colour of any precipitate,
the lustre or specific gravity of a mineral.

We laugh at those who said, “ So are fleas black that
they may be caught more readily upon a white ground,55
but are we becoming wiser now, if it be true, as Professor
Stahl thinks, and we fear only with too much justice, that
the majority of modern naturalists would corroborate the
opinion that the protective characters of plants stand in
direct causal connection with the appetite of animals ? To
give snails credit for evolving plants with crystals, sourness,
and poison, to make cattle and the like responsible for the
thorns on plants, is like giving snakes the credit of evolving
boots which protect our heels. In all these cases alike the
possibility of some defensive utility is undenied, nor even
of some improvement through selective agency ; what is
contended for is, however, a change in our evolutionary
perspective, laying increased importance upon the definite-
ness and cumulativeness of the internal variation and con-

126 Chapters in Modern Botany chap.

sequently a diminished stress upon the external selection
which plays upon this.

Hence we have all sympathy with a recent critic, Dr.
Jumelle, who, in reviewing a recent endeavour to explain
not only the presence but the position of alkaloids, etc,, in
plants by showing how defensive they are, says : “ The
final causes to which so many authors constantly appeal,
have indeed the advantage of supplying an easy explana-
tion of embarrassing facts, but they are hurtful to the
progress of science, since the mind duped by an illusory
satisfaction is dissuaded from further investigation.55

Plants and Ants. — Both gardeners and botanists have
long been aware that ants are among the most frequent
guests of flowering plants, and such names as Myrmecodia ,
Euphorbia formicarum , refer to this. Are these visitors
hurtful ? In most cases only to a slight extent, for although
they rob the flowers of honey, they compensate for this by
eating other small insects which would do the plants much
harm. They are useful to the plant but not to its flowers,
for as the worker-ants are not winged, they are not suited
for carrying the fertilising pollen from plant to plant as
bees and flies often do. They devour pollen and nectar
without fulfilling any useful function, and it is said that
when there are crowds of them about a flower the bees are
apt to have their noses somewhat rudely pulled when they
thrust them into the recesses of the flower, and this will
obviously tend to frighten away the bees. But without
attaching much importance to this allegation that the ants
assault the bees* we see that it is advantageous that the
ants should be excluded from the flowers. In many plants
this is effectively done. There may be, as in some of the
teasel tribe, chevaux-de-frise of stiff, downward -pointing
hairs, like those inside of a pitcher plant, against which the
ants cannot climb. There may be sticky parts of the stem

vii Relations Between Plants and Animals 127

which the insects cannot cross, as we see in some of the
catchflies, e.g. Lychnis viscaria and others. There may
be very slippery stems on which the climbers can find
no foothold, “ and the flowers are often pendulous, as in
snowdrop and cyclamen, creeping qreatures being thus kept
out of them, just as the pendulous nests of the weaver bird
are a protection against snakes and other enemies.” Sir
John Lubbock quotes from Kerner, whose charming Flowers
a7id their Unbidden Guests is most full of information on
this head, the case of Polygojium amphibium , a common
pond-weed, which, as its name suggests, lives amphibiously,
sometimes in water, sometimes on land. “ So long as it
grows in water it is protected by the water, and its stem is
smooth ; but, on the other hand, those specimens which
live on land throw out certain hairs, which terminate in
sticky glands, and thus prevent small insects from creeping
up to the flowers. In this case, therefore, the plant is not
sticky, except just when this condition is useful.” In many
cases, too, the flowers, like those of the snapdragon, are
virtually closed boxes which can be opened by the bees,
but not by the small ants, unless indeed these bite a hole
through the base of the petals.

But it may be asked, if the ants are excluded from the
flowers in any of these ways, why do they visit the plants
at all ? Partly, no doubt, by way of experiment, partly for
the sake of the booty of smaller insects which they find about
the leaves, but also because the secretion of nectar is not
always confined to the flowers, but may take place on other
parts of the plant — in what are called extra-floral nectaries,
of which something will afterwards be said.

But the web has another mesh. As long ago as 1688
John Ray noted the constant occurrence of ants in the
hollow stems of the South American Cecropia palmata , and
many other naturalists had called attention to the same

128 Chapters in Modern Botany chap.

and similar cases. But precise knowledge of the meaning
of this partnership was not attained till 1874, when Belt,
the naturalist of Nicaragua, and Delpino, an Italian
botanist, cleared up the whole matter. Let us quote Belt’s
account 1 of his discovery : “ The thorns of the bull’s-
horn Acacia are hollow, and are tenanted by ants that
make a small hole for their entrance and exit near one
end of the thorn, and also burrow through the partition
that separates the two horns, so that one entrance serves
for both. Here they rear their young, and in the wet
season every one of the thorns is tenanted, and hundreds
of ants are to be seen running about, especially over the
younger leaves. If one of them be touched, or a branch
shaken, the little ants ( Pseudomyrma bicolor) swarm out
from the hollow thorns, and attack the aggressor with jaws
and sting. They sting severely, raising a little white lump
that does not disappear in less than twenty-four hours.
They form a most efficient standing army for the plant,
which prevents not only the mammalia from browsing on
the leaves, but delivers it from the attacks of a much more
dangerous enemy — the leaf-cutting ant. For these services
the ants are not only securely housed by the plant, but are
provided with a bountiful supply of food, and to secure
their attendance at the right time and place, the food is so
arranged and distributed as to effect that object with
wonderful perfection.” There is a sweet gland at the
base of each pair of leaflets on the bipinnate leaf, and a
little yellow pear-like body at the end of each small division
of the compound leaf which is carried off when ripe. The
young thorns are 'soft and filled with sweet pulp, so that
the ant finds its house full of food. As it hollows this out,
the thorn increases in size and bulges out towards the
base.

1 The Naturalist in Nicaragua. Lond. 1874.

vii Relations Between Plants and Animals 129

“ These ants seem at first sight to lead the happiest of
existences. Protected by their stings, they fear no foe.
Habitations full of food are provided for them to commence
housekeeping with, and cups of nectar and luscious fruits
await them every day. But there is a reverse to the picture.
In the dry season on the plains the acacias cease to grow.
No young leaves are produced, and the old glands do not
secrete honey. Then want and hunger overtake the ants
that have revelled in luxury all the wet season ; many of
the thorns are depopulated, and only a few ants live through
the season of scarcity. As soon, however, as the first rains
set in, the trees throw out numerous vigorous shoots, and
the ants multiply again with astonishing rapidity.”

A more recent traveller, Professor A. F. W. Schimper,
who has reinvestigated the whole matter, has gathered
some new facts of much interest, and the student may be
referred to Schimper’s book 1 not only on its own account,
but for the bibliography of the subject which it contains.

He tells us first of the destructive ravages of the leaf-
cutting ants, the sight of whose march soon becomes familiar
to the traveller in tropical America. In a minute or two a
sixpence-like circle is cut from a leaf and the ant marches
off with its burden. Only the dry and the very young leaves
are spared, and armies of thousands of ants soon make sad
havoc of a tree’s foliage. It is not quite certain what the
ants do with the leaves which they carry home : Bates
believed that they were used as a lining for the subterranean
galleries ; Belt supposed that the ants fed upon fungi which
grew upon the decaying leaves ; M‘Cook observed, in the
case of Atta fervens and A. septentrionalis , that a papery
material was manufactured from the leaves and used in the

1 Die Wechselbeziehungen zwischen Pjianzen und Ameisen in
tropischen Amerika , 8vo. Jena, 1888. Cp. E. Huth, Myrmecophile
und Myrmecophobe Pjianzen. Berlin, 1887.

K

130 Chapters in Modern Botany chap.

internal furnishing of the ant’s nest ; but the subject requires
further investigation.

The destructive leaf-cutters have certain preferences ;
imported plants, such as oranges, roses, coffee-plants, and
mango, are greedily attacked ; Solanaceae and grasses are
left untouched, but in South Brazil Schimper found that the
guava, a Caladium , Cassia neglecta , and A Ichornea iricurana
were peculiarly liable to be ruined. It is certain that the
presence of ethereal oils and ferments is not always a
deterrent to the ants, whatever it may be to snails, else
orange, guava, mango, rose, etc., would not be such
favourites. There can be little doubt that the leaf-cutters
would soon exterminate these and other imported plants if
no precautions were used, and it is likely that they have in
this way exterminated many indigenous species. The leaves
of the common orange and the bitter orange are eagerly
used, but those of the mandarine and the lemon are avoided.
Thus in the natural course of things the two first would be
eliminated, the two last preserved. This may be taken as
a good example of the action of natural selection, but we
cannot of course ascribe to the influence of the ants the
qualities which save the lemon and the mandarine.

In the province of Canton in China it is the custom to
place nests of harmless, tree-inhabiting ants upon the orange-
trees in order to defend these from the attacks of the leaf-
cutters. This is but an intentional imitation of what has
taken place in the course of nature. For, as Belt has told
us, the ants which inhabit various species of Cecropia , Cassia ,
and other plants serve as a bodyguard against destructive
intruders. Let us follow Schimper’s account of this
“ symbiosis.”

Schimper’s problem, shortly stated, is whether the plants,
which are so constantly furnished with a bodyguard of ants,
exhibit structures which can be definitely regarded as adapta-

vii Relations Between Plants and Animals 13 1

tions due to the symbiosis. “ Of late,” he says, “ naturalists
have become more and more accustomed to interpret all
structural peculiarities which are seen to be useful for a
certain purpose, as if they had originated for that purpose.”
This Schimper rightly regards as unscientific. All such
interpretations must rest on observation and experi-
ment.

Many ants live in the nooks which plants often afford —
in the axils of leaves, among the tangled roots of epiphytes,
inside old galls, and so on ; others build nests which they
fix to the branches ; others bore labyrinthine passages in
the dead bark of trees. “The fanatics of biology” ( i.e .
bionomics) “ are inclined to find adaptations in all such cases
of constant or usual symbiosis. The chambers of Tillandsia
bulbosa , the feltwork of aerial roots in the case of many
epiphytes, the cavities of the stem and branches in Triplaris,
are all for the reception of the protective ants. But we
know that these structures have quite another meaning :
the cavities in the base of Tillandsia are dried-up cisterns,
the feltwork of aerial roots collects moisture and humus,
the hollow stems of Triplaris serve to combine maximum
elasticity with minimum material.”

Where it can be shown that certain plants of which the
leaf-cutters are fond have a bodyguard of ants, there is no
reason to doubt that these are of protective advantage.
This has been repeatedly proved by observation, especially
in the case of some species of Cecropia (Imbauba or trumpet
tree). These trees have smooth upright stems, raised on
short aerial roots, and bearing simple branches, the whole
appearance .suggesting a gigantic candelabra. The leaves
are few but very large. Now when one shakes a branch of
the Cecropia, one rouses a wild army of ants, which with
poisonous jaws resent the intrusion. Where do they all
come from ? Closer inspection shows little round apertures,

CHAP.

132 Chapters in Modern Botany

especially on the upper internodes, which lead into the
hollow stem.

In walking with the well-known naturalist Fritz Muller,
Schimper saw a small Imbauba-tree which had been stripped
of its leaves by the leaf-cutters. Fritz Muller ventured to
affirm that in this case the bodyguard must have been
absent, and in slicing up the stem he found not one. This
was not an isolated case ; the same has been repeatedly
observed, so that we may safely conclude that a Cecropia
tenanted by its bodyguard is relatively safe from the leaf-
cutters.

The hollow stem with its horizontal partitions is plainly
a comfortable home for the protective ants, but does it
exhibit any structural peculiarities which must be referred
to the insects ? This cannot be said of the cavities them-
selves, but what of the doors ? Each internode has at one
time a door, which is obliterated by subsequent growth.
The door is always in the same position ; it is made by the
ants at a spot where the wall of the stem thins away in an
oval depression, and this is originally due to the pressure of
a bud. There is no doubt that the beginning of the door
arises quite apart from the ants, but on the other hand
Schimper has detected a number of minute structural
peculiarities connected with the door which he cannot
explain except as adaptations to the visitors. On Cecropias
which grow on the hill of Corcovado near Rio de Janeiro,
and have very smooth wax-covered skins, ants are unrepre-
sented, and the minute structure around the slight depression
at which a door, were there one, would be formed, is quite
different.

The ants find shelter within the Cecropia stem, but the
tree also affords them abundant food. Near the base of
the leaf-stalk there is on the under surface a small area
covered with brown velvet-like hair, and on the surface of

vii Relations Between Plants and Animals 133

this are numerous pear-shaped or oval little bodies which
look like insects’ eggs. These (“ Muller’s bodies”) are
eaten by the ants, and they must be very nutritious, for their
contents are rich in albuminoids and fatty oil. In their
young stages they resemble the glands of other plants which
secrete mucus or resin, and Francis Darwin has suggested
that they are homologous with glands, but they are very
peculiar, and their peculiarities Schimper would connect
with the ants, for, strange to say, they are absent on
Cecropias, such as those of Corcovado, on which there are
no ants.

FromCecropia Schimper passes to other myrmecophilous
plants, such as Acacia sfihcer ocephala, whose hollow thorns
afford shelter to a bodyguard, and which also bears little
food -bodies comparable to those on Cecropia. But we
have given sufficient illustration of this matter.

There is, however, another much-discussed subject on
which Schimper has something interesting to tell us. We
know that many of our flowers have honey-bags or nectaries,
and that these are attractive lures to the bees and other
insects which, in their search for sweets, carry the fertilising
pollen from flower to flower. But often, especially in the
Tropics, nectaries occur outside the flowers, as we have
seen in Nepenthes.

Both Belt and Delpino regard these extra-floral nectaries
as adaptations for the attraction of protective ants. Kerner
supposes that they afford such generous supplies of nectaries
that the ants leave the floral nectaries undisturbed ; but this
is not the way with ants ! Bonnier regards them as stores
of reserve-material ; but the sugar is almost always stolen by
ants or washed away by the rain. Johow has even suggested
that they are receptacles for the waste-products formed in
the movements of the leaves ! but this too is for several
reasons most unlikely, since it is enough to notice that

134 Chapters in Modern Botany chap.

they are especially large and numerous on the bracts which
exhibit no movements.

Schimper concludes that the theory of Belt and Delpino
is the only one which need be seriously considered. But
before it can be believed that the extra -floral nectaries
have arisen as adaptations to the protective ants, it must be
shown, as Schimper rightly observes : —

(1) That the visits of the ants thus attracted afford so
much protection, that without them the plant would be at a
great disadvantage ; and

(2) That the nectaries have not some other use in the
economy of the plant, to which they are primarily referable.

By removing the extra-floral nectaries from various plants,
Schimper has convinced himself that they are unnecessary
and not demonstrably important for the wellbeing of the
plant. They are most abundant where there are most ants.
They certainly attract the ants, and these visitors sometimes,
but by no means always ward off leaf-cutters. The plants
which possess them may be worsted, but none the less they
had an advantage as far as it went. In short, Schimper
accepts the suggestion of Belt and Delpino. This con-
clusion has been further corroborated by W. Burck,1 who
finds that almost all plants with extra-floral nectaries or
food-bodies are truly myrmecophilous, and proves by direct
observation that the bodyguard of ants attracted by the
nectaries are of important service in driving off marauding
insects which spoil the foliage or perforate the corollas of
the flowers without effecting fertilisation.

Domatia. — The homes which ants find inside the
Acacia’s thorns, or within the hollow stems of Cecropia,
are not unique ; Dr. Lundstrom has described under the
title of domatia (meaning little homes) a large number of
shelters on plants which are tenanted by harmless little
1 Ann. Jardin Bot. Buitenzorg , x. (1891).

vii Relations Between Plants and Animals 135

insects and mites. Sometimes they are like little wig-
wams formed from converging hairs, sometimes minute
caves or pits. They are not formed by the tenants, at
least not now, for they are natural to the plants, but they
are none the less well adapted to the use they serve. A
simple instance may be seen on the leaves of the lime-tree,
on the under surface of which, at points where two veins
of the leaf cross, there are little nooks tenanted by mites.
These do not injure the plant, but rather help it, for they
clear away minute fungi, and it is also possible that their
nitrogenous waste-products are absorbed by the leaves.

Myrmecodia. — A pretty tale is that of Myrmecodia
tuberosa , a rubiaceous plant from the Malayan Archipelago.
The worthy Rumphius, still memorable as the pioneer
naturalist of these regions, “ describes it in his Herbarium
Amboynense (1750) under the formidable but appropriate
name of Nidus germinans formicarum rubrarum , and
terms it “ prodigium itaturceA He seems to have been
uncertain whether the whole was a vegetable, or whether
the tuber was an ant’s nest from which the plant sprung ;
he says it is to be regarded as a zoophyte among vege-
tables ! It presents the form of a large irregular tuber
growing on the branches of old trees ; from this spring a
few thick fleshy stems, having a small number of smooth,
leathery, oblong leaves crowded together at their summits.
The small white sessile flowers are situated at the base of
the petioles, and almost concealed by the large persistent
stipules. The tuber is tenanted by small and very fierce
* red ants, which rush out upon the intruder if their dwelling
is attacked. The way in which these ants take possession
of the Myrmecodia , and the intimate relation which exists
between the plant and the insect, are thus referred to in
Professor Caruel’s recent paper upon the genus.1 The

1 Nuovo Giornale Botanico Italiano, iv. pp. 170-176 (1872),

136 Chapters in Modern Botany chap.

account is quoted from a manuscript note by Dr. Beccari,
who collected the plant in Borneo : —

“ I have carefully followed the development of this tuber,
having been able to observe the young plants in all stages
of growth from the period of germination. The seed is
surrounded by a viscid pulp, resembling that of our mistle-
toe, which readily attaches itself to the branches of the
trees upon which it falls. Its dissemination is probably
caused by means of the birds which eat the fruit, the
undigested seed passing through them and adhering to the
branches. The seed soon germinates and unfolds its
cotyledons, especially if it has fallen in an opening of a
branch where lichens have collected, or if it be placed in
mould ; the stem develops itself to the length of from three
to six millimetres, widening towards the base, acquiring a
somewhat conical shape, with the two cotyledons at its
apex. In this condition it remains until a particular species
of ant burrows a small lateral cavity at the base of the
stem ; if this does not happen, the stem does not develop
itself, and the plant dies. The wound caused by the bite
of the ant determines a great development of cellular tissue,
in the same way as the sting of the cynips causes the galls
on the oak. The tuber now enlarges and the stem
develops ; the ants soon find sufficient space for forming a
colony, and excavate galleries in the interior of the tuber in
all directions, thus making for themselves a living habita-
tion— a circumstance which is necessary to the existence of
the plant. The plant could not live or even arrive at
maturity unless the ants contributed to the formation of the
organ which must be the source from which it derives its
support, while in all probability the ants could not exist or
propagate themselves unless they had discovered this mode

quoted by Britten, c £ Ant - supporting Plants,” Popular Science
Review .

vii Relations Between Plants and Animals 137

of constructing so ingenious a habitation. The fleshy sub-
stance of this formicarium is formed of cellular tissue ; the
channels and galleries with which it is perforated have their
entrance near the lower part of the tuber.’5

Unluckily for Caruel and Beccari, the development of Myr-
mecodia has been reinvestigated lately by Treub,1 in Java,
who sadly diminishes the wonder by showing that the plant
can thrive without its guests, and that the galleries are
formed and grow as congenital peculiarities without the aid
of ants. By rearing plants from the seed, in the absence
of all ants, he has been able to study the growth of the
tuberosities. They seem to him to be at first reservoirs in
which water is stored for the needs of the plant, the firm
outer surface (devoid of stomata or lenticels) preventing eva-
poration. The ants simply use these when dry as dwellings,
without in any marked way benefiting the plant. This is a
fresh lesson of caution, and of the risks of Darwinising
over-much. A good specimen may be observed at Kew.

Galls. — These interesting domatia above referred to
must of course be distinguished from the galls which
many insects form on plants. Every one knows the large
gall-nuts (rich in tannin and gallic acid, both useful in ink-
making, etc.) formed on oak-leaves by wasp-like insects of
the genus Cynips, and the strange “ Bedeguar ” tufts so
common as mossy excrescences on wild roses. These
abnormal growths are produced by a remarkable vegetative
increase of the tissues of the plants. This is due to the
irritation produced by the gall-flies, or rather gall-wasps,
which lay their eggs in the soft substance of the plant.
Here again, as with lichens or ants, or what not, we are
but at the threshold of a new subject with its literature and
its specialists. For along every main road of the organic

1 “ Nouvelles Recherches sur le Myrmecodia de Java,” Annales
du Jardin Botanique de Buitenzorg , vol. vii. (1888), p. 191.

138 Chapters in Modern Botany chap.

sciences there branch off innumerable pleasant byways,
each leading into a tiny world of its own, a minor infinity ;
at first sight no doubt a hazy labyrinth, yet on deepening
study an ordered microcosm of evolutionary law.

Plants and Aphides. — From ants we naturally pass to
the plant-lice or Aphides, for these little insects which form
sweet juice receive much attention from the ants, who
sometimes use them as cows.1 Many of them, such as
those which infest roses, fruit-trees, and hops, are exceed-
ingly injurious to the plants, for they suck the sap, choke
the pores of the leaves with their honey-dew, and do other
damage. The honey-dew, of which the ants are so fond,
has been for long the subject of much discussion. Pliny,
and many later naturalists who should have known better,
said that it fell from heaven ; many have described it as
an exudation from plants, while other — perhaps most —
naturalists speak of two kinds — an animal honey -dew
formed by the Aphides, and a vegetable honey-dew exuded
in some way or other from the plants.

Biisgen’s recent observations 2 lead him to affirm con-
clusively that all honey-dew, excepting sugary exudations
caused by parasitic fungi, is an excretion of the Aphides.
The only flow of sap from the cells of the plants is into the
mouths of the insects ; the sweet juices are slightly
changed in the food-canal, but the quantity of glucose in
this is out of proportion to the animal’s wants, and hence
what is unused as food passes out. As a single aphis may
form as many as forty-eight drops of honey-dew in twenty-
four hours, it is not surprising that a very rain of nectar
should sometimes fall from trees (especially limes) which

1 See conveniently J. Arthur Thomson, The Study of Animal
Life. Lond. 1892.

2 “ Das Honig-Tau,” Jenaische Zeitschrift fur N a turw is sens ch aft ,

vol. xxv. (1891), pp. 339-428. 2 Pis.

vii Relations Between Plants and Animals 139

are infested by thousands of these insects. Besides
sapping the strength of the plant, and choking the leaves
with honey-dew, the Aphides by their secretion make it
easier for injurious parasitic fungi to establish themselves
upon the leaves. On the other hand, they attract ants,
whose presence in moderate numbers at any rate may be
useful to the plants.1 But as Herr Biisgen calculates that
in one case the quantity of carbo-hydrate material absorbed
by the Aphides from a plant was about one sixth of that
required to furnish the whole foliage, we must agree with
him that this is too high a price to pay for problematical
benefits.

The Ants and Aphides must serve as types of the
injurious insects, between which and plants there are
numerous interesting relations. We have to consider how
far various structures of plants, such as hairy stems, viscid
stems, pendent flowers, and the like, serve to save plants
from their enemies, as may be true in the case of unwel-
come ants ; we have also to notice what changes the
injurious insects, such as corn-insects, Phylloxera, Weevils,
etc., may effect on the plants which they infest ; and we
must also observe how the hostile insects, which affect
forest trees and vegetation generally, may occasion changes
which have far-reaching influences on the fauna, flora,
scenery, and even climate of a country-side. Readily
available information will be found in Miss E. A. Ormerod’s
valuable work on Injurious Insects (second edition, Lond.
1891), while the more advanced student would do well to

1 With all- respect to this observer, one may still maintain
that during prolonged fine weather, especially in June, while leaf
tissues are fresh and young, and Aphides not yet abundant, such an
excess of assimilation sometimes takes place as to create an overflow
of nectar from leaves ; the very ferns sometimes showing this with-
out Aphides upon them, or at any rate in adequate numbers.

140 Chapters in Modern Botany chap.

consult the researches of Riley, Packard, and others in the
Bulletins of the United States Entomological Commission,
from which many detailed illustrations of the web of life
may be gleaned.

Cats and Clover. — We have only given a few illustra-
tions of the infinite number of interactions between plants
and animals ; other examples and of more importance, e.g.
in connection with the fertilisation of the flowers and the
scattering of seeds, will be referred to again. As with
insectivorous or moving plants, the writer’s aim is to
familiarise the reader with the essential point of view at
which Darwin has placed us, — his appreciation of the dra-
matic complexity of nature ; as also of the task of the least.
Nature is no longer a mere confused multitude of specimens
to be collected and analysed, but each organism is linked
with others as consecutively as in the “ House that Jack
Built ” ; nay, with indefinite cross relations as well : what
seemed a unit is a link ; what seemed a chain is but a
thread within the labyrinthine web of nature. That in
thus opening out for us this new and fascinating study of
bionomics, he has exaggerated the importance of the selective
factor in evolution, and that the drama is not merely of
incident and adventure but of character also, is a secondary
consideration, though an important one ; since but for
Darwin we might hardly yet have realised that there is an
organic drama at all.

He takes a ball of mud from off the leg of a bird, and
finds that out of it no less than eighty seeds germinate !
He shows how cattle absolutely determine the existence of
the Scotch fir on the Surrey heath, or how insects deter-
mine the absence of wild cattle and horses in Paraguay.
“ If certain insectivorous birds were to decrease in Para-
guay, the parasitic insects would probably increase ; and
this would lessen the number of flies which destroy the

vii Relations Between Plants and Animals 141

young horses and cattle — then the latter would become
feral, and this would certainly greatly alter the vegetation ;
this again would largely affect the insects ; and this the
insectivorous birds, and so onwards in ever-increasing circles
of complexity.” So, too, with the terrible tsetse fly, which,
as Livingstone pointed out, renders pasturage and animal
transport impossible within its region, so condemning civili-
sation to a lower type.

But perhaps his best illustration is the most familiar
one, which should never become trite to us. “ Plants and
animals, remote in the scale of nature, are bound together
by a web of complex relations. ... I have found from
experiments that humble-bees are almost indispensable to
the fertilisation of the heartsease ( Viola tricolor ), for other
bees do not visit this flower. I have also found that the
visits of bees are necessary for the fertilisation of some
kinds of clover — thus, 100 heads of red clover ( Trifolium
pratense) produced 27,000 seeds, but the same number of
protected heads produced not a single seed. Humble-bees
alone visit red clover, as other bees cannot reach the nectar.

. . . Hence we may infer as highly probable that, if the
whole genus of humble-bees became extinct or very rare in
England, the heartsease and red clover would become very
rare, or wholly disappear. The number of humble-bees in
any district depends in a great measure on the number of
field-mice, which destroy their combs and nests ; and
Colonel Newman, who has long attended to the habits of
humble-bees, believes that more than two -thirds of them
are thus destroyed all over England.”1 Now the number
of mice is largely dependent, as every one knows, on the
number of cats ; and Colonel Newman says, “ Near
villages and small towns I have found the nests of humble-
bees more numerous than elsewhere, which I attribute to
1 Origin of Species, chap. iii. Lond. 1859.

142 Chapters in Modern Botany chap, vii

the number of cats that destroy the mice.” Hence it is
quite credible that the presence of a feline animal in large
numbers in a district might determine, through the inter-
vention first of mice and then of bees, the frequency of
certain flowers in that district.

This may seem a mere matter for naturalists, yet in this
connection we touch on practical politics. The British farmer
is at this day in many districts sorely exercised by a plague
of field-mice and voles with which sheep-pastures are notably
swarming. How far is this due to the relentless war waged
by the gamekeeper against owls, ferrets, weasels, and all
the other mouse-loving creatures, furry or feathered ? The
Board of Agriculture in the person of Mr. Chaplin, decides
not, suggesting climatic causes as the explanation of the
pests, if not almost hinting at occult ones, beyond the pene-
tration of human intelligence.1 But on what grounds, i.e.
on what actual observation and investigation does this de-
liverance rest ? If none, might not the Board of Agriculture
as profitably encourage it to be made by the farmers and
field-naturalists of each district ? The result could not but
be instructive ; perhaps unexpected.2

1 It is of course not inconceivable that we may have in animals
as in trees, good “ seed - years ” ; i.e. some internal rhythm larger
than the annual one, of unusual fecundity.

2 As this goes to press, I am glad to learn that the step has
been taken of forming a committee to investigate the questions.

CHAPTER VIII

SPRING AND ITS STUDIES ; GEOGRAPHICAL DISTRIBUTION
AND WORLD -LANDSCAPES ; SEEDLING AND BUD

Spring Studies — Mode of Study in Botany — Phenology and Dis-
tribution— Aspects of Nature , Vegetation and Landscapes of
the World — Germination — Buds and Bud- Scales — Arrange-
ment of Leaves in the Bud.

The study of plants is naturally begun in spring. It is,
indeed, the supreme advantage of a temperate climate, — one
which richly compensates in beauty and even in interest
for a verdure less exuberant, a variety less Protean than
that of the tropical forest, — that the procession of the seasons
is ever before us. Life is, indeed, universally rhythmic, in
animal as in plant ; but the plant is more passive and
plastic to its conditions, more under the sway of environ-
mental change, and hence this seasonal change of plant
life becomes the more impressive spectacle of living nature.
See the tide of life set in with a flood in spring, filling every
corner of the earth with sprouting seeds and shooting stems,
and crowding, spreading, rippling leaves ; how as the russet
underwood warms to the fuller sun through trees still bare,
it glows with bright golden patches of lesser celandine ;
how its dead leaves silently sink under a restless foam-
tipped sea of green anemone ; how every mossy bank is

144 Chapters in Modern Botany chap.

set with primroses in crowded constellation ; and how the
deep summer sky shows first in sheets of hyacinth. Soon
comes high tide of leaves in June : the full-robed year is
crowned and garlanded with exuberant blossom, to which
July brings the strongest chords of colour. Yet already
the tide has turned, the flowers are withering or fading, but
a new profusion of fruits, more strangely varied even than
the flowers, is rising in their place. These, too, ripen and
pass, and the seeds, each a young life, find, ofttimes through
strange adventure, their resting-place and sleep. The
shivering leaves surrender their life to the branches which
have borne them, and fall away, often strangely transfigured
in dying ; only their tiny nurslings the buds remain,
warmly wrapped away within their protecting sheaths.
Life has ebbed out of sight ; Proserpina is in Hades, and
sky and mother earth must mourn till her release.

Mode of Study in Botany. — Here then is the common
theme not only of poets old and new, of painters, but of
naturalists. Each, indeed, has his own way of treating
this ; and each way involves its special difficulties, its risks
of error. For the poet and the painter the detailed
exactitude are of subordinate importance, hence both have
often fallen into mere echoes and conventions : the town
poet in parrot whine of Proserpine without one true glimpse
of her yearly Paradise or Hades ; the painter (as if confined
to the latter) with his “brown tree.” For the botanist the
danger has been a different one. As his herborisations
have needs become keener and of more minute research,
his herbarium labours more vast and detailed, his laboratory
and microscope more engrossing and their new problems
more intricate, his library, too, more incredibly voluminous,
even his greenhouses better filled, his real intimacy with
living nature as a whole has often diminished rather than
increased ; until he writes manuals and treatises excellent

VIII

I45

Spring and its Studies

often as grammars, and in any case copious as dictionaries,
of the science ; yet with these most general of all phenomena
under consideration, left unmentioned in the one, nay, the
very technical name of their study — Phenology — alone for-
gotten in the crowded columns of the other. Whereas here
is the essential “ course ” in the subject indicated — say
rather openly revealed — to student and teacher by Nature
herself with clearness as in no other science. For the
child then, Florals Feast is as yet the best, almost the
only true primer of botany ; for the older student the
Natural Flistory of Selborne remains a central and
inspiring classic. The more developed and systematised
Naturalist’s Diary is the best vade mecum , to which a
Flora (preferably a district or county one) comes in as a
subordinate, though indispensable adjunct, and desirably
also any simple manual of garden flowers. “Am I to read
no text-books then ? 5J the dismayed student constantly asks.
The writer for one answers, with deeper and better justified
conviction every year, Read ? certainly not ; Consult ? yes,
constantly, by help of the index, for every point and diffi-
culty as it arises, for all information as it is desired. Thus
you will gradually get all the facts and results and methods
which it contains, while thus, and probably thus only, can
you avoid that elaborately formalist analysis of the subject
from which every science has such difficulty to escape.
Botany is a drama of nature, and this summer is your
opportunity of seeing it. That text-book is not really a book
of the play ; of that we have as yet only a multifarious and
scattered polyglot literature, the jottings and notes of many
hands ; and -though the text-book is for practical purposes
your first guide to the literature, — it may be almost your only
substitute for it, — you must bear in mind that its careful
summaries of results of research are but new patches upon
an old garment, new wine in old bottles. That is to say,

L

146 Chapters in Modern Botany chap.

that however useful and truly botanical its matter, the
arrangement and presentment of your text-book is not that
of either botanist or nature at all, but is directly and
historically (however unconsciously) derived from that
traditional discipline of grammar and analysis which in
the name of literature have been crushing out the love
of literature, even the knowledge of literature, in every
university and school of Europe for centuries, and from
which we are now only beginning to escape. Its list of
dramatis personce will be constantly useful for reference ;
its dictionary may help from time to time as well ; its
grammar of the science may be discussed later. Let the
evolutionary way of looking at things become gradually
familiar, then habitual, at last instinctive ; then you may
profitably consider the best way of arrangement of your
ideas — in fact, make your scientific grammar for yourself.
But this will be in sharpest contrast to the old one, no
longer fixed or static, but historical, kinetic ; rational
therefore, not merely empirical, synthetic instead of analytic
in spirit and result. And when you endeavour to make the
unending stream of things intelligible by the artifice of
viewing it in section as it were, as momentarily frozen and
photographed at this or that point upon its course, you will
sympathetically interpret and profitably absorb the pre-
evolutionary method, which has, of course, been doing this
all the while. In plainer language, instead of being (as the
established programmes have it) an anatomist first, dis-
secting a dead “ type,55 a physiologist afterwards making
its parts work, and an evolutionist by getting up Darwin’s
theory as an external body of dogma last of all, the right
course is precisely the opposite : Darwin’s habit of observa-
tion and interpretation first, physiological details afterwards,
with such anatomy as it is wanted to explain them. There-
after, of course, such pure morphology as you will.

viii Spring and its Studies 147

Phenology and Distribution. — Returning from the
manner to the matter of our studies, let us look more fully
at this most general aspect of the science — its “ pheno-
logical ” and geographical side. Astronomers have long
ago connected the waxing and waning of the redness of
Mars with the seasonal changes of its year ; and some
suggest that this change of colour may be explained not
merely as that of soil more or less exposed by the varying
area of polar snow-caps, but as connected with the annual
changes of a red vegetation, or what may answer to
vegetation. However this may be, if we imagine an
astronomer observing our earth from Mars, or of course
more conveniently from the nearer standpoint of the moon,
he could not fail to be impressed by the waxing and waning
of her verdant belt. And if we imagine him granted what
he would wish — the means of viewing this strange pheno-
menon more clearly and in detail — he becomes a botanical
geographer.

The botanist thus begins his studies where the astronomer
ends, and travels over the whole earth with the geographer ;
he is no mere student of detail gathering specimens to cata-
logue or anatomise, but, like each of these, holds the globe
between his hands ; nay, takes it from them not only for a
keener survey, but for a clearer, richer, and fuller one,
landscape by landscape. He is, in fact, twin brother to
the landscape-painter ; and while at one moment absorbed
like a pre-Raphaelite draughtsman amid the fascinating
wealth of foreground detail, is at the next an impressionist
melting down this detail into the richer tone and broader
colour of a larger truth.

Hence then, although we have entered the botanical
school by way of its greenhouse and collection, and looked
into its laboratories of physiology and histology, we are only
now reaching the larger aspect of the science. How shall

148 Chapters in Modern Botany chap.

we proceed to deepen our acquaintance with this ? Al-
though, as in every study, difficulties are not lacking, the
student may here start from common experience and with
advantages common to few sciences— those of observation
in his holiday walk and study in his general reading.

“ Aspects of Nature,” Vegetation and Landscapes of
the World. — Let him search out in the nearest library a
famous yet too much forgotten book, Humboldt’s Cosmos , or
at any rate run through his Aspects of Nature with its
passages of imperishable description ; let him turn over
the stately folios of botany and travel in an old library,
and look over the plates of Martius’s Palms and the like.
Let him read Darwin’s Naturalises Voyage , Wallace’s Malay
Archipelago , his Tropical Nature , and Island Life ; glean-
ing too the descriptive passages of books of travel, even
by non-botanists, so gaining detail from one book and
colour from another until he has some general idea of the
vegetation of the world so far as his accessible library
resources go. The plates of, say, the Botany of the
Challenger Expedition will thus get colour from the glow-
ing prose pictures of Kingsley’s dying voyage : Stanley’s
dark forest, Ruskin’s alpine flower- meadow and lichened
rocks, Wordsworth’s daffodils — all will help to fill this
gallery of the imagination. But why should this gallery
be in the imagination only ? To make these thought repre-
sentations science, they must be permanent and precise,
ordered and complete. Here begins the interest of the
collections of a botanic garden, especially where geographi-
cally arranged, as partly at Kew, or notably at Berlin. Yet
to make all this more real and vivid, we need pictures. For

We’re made so that we love

First when we see them painted, things we have passed

Perhaps a hundred times nor cared to see ;

And so they are better, painted — better to us,

viii Spring and its Studies 149

Which is the same thing. Art was given for that ;

God uses us to help each other so,

Lending our minds out.

Here Miss North’s collection at Kew is of special interest ; 1
and although the botanist is apt to turn away disappointed
with the lack of arrangement of this wealth of sketches,
and the artist with their pictorial quality, both will admit
at least that here is a beginning full of suggestiveness : and
if so, why should not both go to work together, — student
working for artist, and artist for student, — until a permanent
gallery of landscapes and vegetation is the result ; an ever-
widening panorama, by help of which we may vividly
realise what the world is like ?

But we cannot all travel like Miss North, it may be
said. Still, make the best of it ; look at our familiar walks
like geographical botanists, even within a couple of hours’
walk of a great manufacturing town, and behold, each little
scene stretches itself out over the world-map as if by en-
chantment. Shall we stroll along the sand-dunes blue with
the waving of their binding-reed ? What we see is half
the eastern shore of our whole island, and well-nigh the
whole western coast of the Low Countries opposite, whence
we may wander with the sea-birds along the whole length
of Prussia, and over to Sweden, or up to the Gulf of
Bothnia. Or to the cliffs, with their bright tufts of sea-
pink and bladderwort, their close-set turfy slopes ? This is
the coast of Brittany and Norway ; and these two pictures
will well-nigh frame for us the whole North Sea. Or shall we
sketch these stretches of unreclaimed moorland, here golden
with whin or purple with heather, and there with dwarfish
willows rising above an undergrowth of grass and sedges,

1 See the Kew Catalogue of North Collection by Mr. W. B.
Hemsley, and Recollections of a Happy Life , being the Autobiography of
Marianne North. 2 vols. London, 1892.

150 Chapters in Modern Botany chap.

with patches of turf-moss, their chilly gray lightened only by
the silver pillar of a struggling birch ? If so, this bit of
moorland is a surviving fragment of the great heath of
northern Europe, through which every one who enters
Germany from Rotterdam or Hamburg must still run for so
many weary miles, and of which the vast area of cultivation
up to the Ural may be almost described as an imperfect
clearing. East of the Ural the bogs and peat-mosses
widen, the heaths leave us, the willows and birches remain
— it is the gray tundra of North Siberia. So our after-
noon’s stroll, with its couple of sketches, gives the essential
general impression of five thousand miles.

Nor are these surviving fragments of natural landscapes
the only ones ; there are good artificial landscapes as well.
Would we roam through the deciduous forest scenery of
the lower Alps ? The neglected corners of that old park
are not to be despised. Or the sombre pine-woods of
Sweden ? That little loch among the hills, its shores close
planted to the very margin, will fairly serve. Or would we
see rhododendrons of America or Asia, the quaint conifers of
California, the evergreens of Japan or of the Mediterranean ?
They are in every shrubbery. Do we seek the richer
vegetation of the warm temperate zone, the ferny wealth of
the antipodes, the stately leafage of a tropical forest ? That
greenhouse, that botanic garden palmhouse, shows us these
in some ways at their very best. So too we shall come
better to understand the gardener in his highest capacity
as offering us the resources of an art vaster and grander
than that of cities.

For this geographical botany we need now only more
precise scientific guidance. The article Distribution of
Chambers’s Encyclopcedia may serve as a starting point ; then
the appropriate chapter of any good manual of physical geo-
graphy, conveniently Mill’s Realm of Nature in this series ;

VIII

Spring and its Studies

JSi

best of all of course D rude’s Pflanzenverbreitimg and his
section of Berghaus’s great Physical Atlas, which is
happily soon to be available in improved form for English
readers.

The student thus equipped, and the artist practised in
rendering these scenes in colour,, they may now with safety
avail themselves of the help of books and photographs in
extending their collection of paintings ; friendly criticism
can often be obtained from travellers who actually have
visited these scenes, and a growing collection of no little
interest and value to more than the pure botanist be thus
gradually obtained. Why should not any knot of friends,
of whom some love painting and others botany, work to-
gether in this way ? There is plenty of blank wall space
in the nearest school to hang their labours with endless
pleasure and usefulness to children, teachers, and natural-
ists alike.1 Nor is such a collection unworthy the attention
of the conservators of museums and art galleries, and of
the geographical societies.

Germination. — One of the many wise sayings of Eras-
mus Darwin was, that the offspring of any organism was
not so much a new creature as “a branch or elongation of
the parent.” This is the common characteristic of all kinds
of reproduction, whether of plants or of animals, that part
of the parent is separated off to begin a new life. In most
cases the new life begins from the union of two units, pro-

1 The writer has here to express his thanks to the Dundee Art
Club, whose members have gone far with the preparation of such a
collection of paintings for the botanical department of the College ;
to Mr. Maclauchlan, curator of the City Museum Library and Art
Gallery, for much help ; as also to the studio of the Edinburgh Summer
Meeting. With his friend and former pupil, Miss Etta Johnston, whose
share in this matter has been a very active one, he will be happy to
give all information and assistance to others who may wish to begin
such a collection.

152 Chapters in Modern Botany chap.

duced by two parent organisms, but the separated parts or
units are in a very deep way continuations of the life of the
parents, formed of the same living material as that which
gave origin to the parents, able therefore when separated to
grow into similar organisms.

When we examine seeds, such as those of beans or peas,
we at once see that they are not simple structures like,
for instance, the eggs of birds. It is easy to convince our-
selves that each ripe seed already contains a young plant,
with a little stem and root, and in the cases above mentioned,
with two seed-leaves or cotyledons packed full of what we
know to be nutritious matter. Seeds are not comparable to
the eggs but to the embryos of animals. To find out what
corresponds to the egg we must examine under the micro-
scope thin slices of unripe seeds or ovules, in the midst of
which we may be able to see a small unit — known as the
egg-cell. This it is which, after being fertilised by a pollen-
grain, develops into an embryo plant within the seed. A
seed is in fact the most complex and long-discarded marvel
within the field of botany, of which the full unravelment and
interpretation lies beyond the compass of the present volume.
In accordance with the bionomic or Darwinian principle of
study here advocated, the student may be advised first to
study its more obvious and external history of dispersal and
of adaptation by help of some such interesting and con-
venient volume as Sir John Lubbock’s Flowers , Fruits , and
Leaves , or Kerner’s Pflanzenleben , Leipzig, 2 vols., already
mentioned : the latter being by far the richest store of in-
formation accessible to the general reader, and, thanks to
its wealth of illustration both coloured and in the text, of
great use and interest even where there is no knowledge
of German ; after this, the minute anatomy of the seed, with
the comprehension of its morphological secret, and the evolu-
tionary meaning of this will be all the better appreciated.

viii Spring and its Studies 153

In the great majority of cases the embryo plant is fully
formed by the time the seeds are scattered in autumn.
Let us see what happens to the seed after it falls into the
ground.

Every one has heard stories of “ mummy wheat,” which
after lying for many centuries in the tombs was still alive,
and germinated when sown ; or of the raspberry grown
from seed gathered among the ribs of a Roman soldier.
Although these stories are not justified by sufficiently care-
ful experiment, they express, in exaggeration, a fact char-
acteristic of most seeds, that in natural conditions they
remain for a time quiescent before they begin to germinate.
Throughout the winter their life is dormant, and if condi-
tions do not change — as notably through depth of burial —
the dormant period may be lengthened in a degree varying
with the species, and not yet sufficiently determined by
experiment, but certainly often extending to many years.

This period of quiescence depends, however, only in part
upon external conditions, such as the depth or the frost-
bound hardness of the soil ; in part, too, upon the husks
which protect and imprison the embryo plant, and must be
softened before it can burst forth ; but also on internal
processes of fermentation, as the result of which the supply
of food becomes changed into more available form. In
fact, the process of digestion, which we recognised as an
apparently extraordinary thing in insectivorous plants,
occurs normally in the life of seeds. The stores of organic
substances — which make seeds important parts of our food
— are rendered soluble and diffusible before and during
germination.- There is not only a diastatic ferment which
converts starchy material into sugar, but there are others,
such as that which Professor J. R. Green has so carefully
studied in the seeds of Lupin — a ferment which acts like
the pancreatic juice of animals, converting the proteids of

i54 Chapters in Modern Botany chap.

the seed into diffusible peptones, and then into leucin,
asparagin, etc., quite as in the animal process.

The spring showers water the earth, the moisture softens
the husks of the seeds, and soaking inwards recalls the
living matter to activity ; the ferments which we have
mentioned convert the stores of food into a form available
for transport and use ; the temperature of the earth is
slightly raised, so helping the reawakening. Soon, as we
know, the radicle pushes its way out of the seed and into
the soil, the tiny stem arches its way above the ground,
on this the tender leaves (sometimes first the cotyledons)
spread forth. By all means watch how the germination of
the grain of wheat differs from that of the pea, and the
mustard seedling from that of the cedar ; meantime we
are concerned only with the general fact.

By means of a simple experiment we can get a hint of
the intensity of life in germinating seeds. Take a glass
vessel and put into it a small quantity of caustic potash ;
into the mouth of the vessel place a glass funnel, and after
inserting a piece of filter paper, fill the funnel with moist
germinating seeds, such as those of wheat or peas ; cover
the whole with a bell-jar, through the corked neck of which
a thermometer descends into the midst of the seeds. As
the cork is not air-tight, air enters the jar freely enough,
and the caustic potash absorbs the carbonic acid gas formed
by the living and breathing seeds. As these go on ger-
minating, the thermometer registers a marked increase of
temperature. Breathing and living imply oxidation of
some of the complex substances of the seeds, and, as in
ordinary combustion, the oxidation is associated with the
liberation of heat. In making this experiment it is well to
make at the same time a duplicate of it with this difference
only, that boiled peas or the like are substituted for the
germinating ones ; in this case it is hardly necessary to

viii Spring and its Studies 155

say the thermometer indicates no change. But what is
true of the living seeds would for a short time be true also
of flower-heads or of opening buds.

Buds. — We have spoken of seedlings as characteristic of
spring, and to a certain extent this may be said of buds
also. The bud is to the branch on which it grows what the
seed is to the entire plant. And just as most seeds in tem-
perate countries are formed during summer, are scattered in
autumn, lie dormant throughout the winter, and begin to
grow in spring, so most buds are well formed by autumn,
remain quiescent within their protective scales during winter,
and burst forth in spring into twigs and leaves. It is this
unfolding of the bud which is characteristic of spring-
time.

But let us seek to gain a more precise conception of
what a bud is. Every one is familiar with the appearance
of a split cabbage : in the centre there is a portion of stem
tapering to its growing point ; around this, springing from
different levels, are many layers of crowded crumpled
leaves covering the growing point in overlapping layer after
layer. Now a cabbage is an exaggerated bud, or, to put
it in another way, every bud is a sort of incipient cabbage.

Going back to the seed, we find in the plumule and the
minute leaves which often lie around its tip, the primeval
bud, a rudiment of the stem and its appendages. We see
the same thing at a later stage in the growing point of the
stem ; there is a central continuation of the axis, and around
this are several tiers, or rather spirals — in technical lan-
guage, whorls — of young leaves.

Very frequently among Monocotyledons (plants like
grasses, lilies, and orchids) the bud which forms the apex
of the stem alone develops, and thus we have the un-
branched stems of palms (fig. 7). But in most Dicotyledons
(the majority of flowering plants) the numerous buds, which

chap, viii Spring and its Studies 157

are usually situated in the axils of leaves, develop and
shoot forth as leaf-bearing branches or as flowers.

In our common trees, and indeed in most of the per-
ennial plants of temperate climates, the buds are well
formed in autumn. For a time there is active growth, but
this is checked by the advent of winter, during which the
buds remain dormant. When the frost is unusually keen
the tender life of the bud is sometimes killed, but this is
usually obviated by the protective scales with which the
bud is enclosed. Just as the outer older leaves of the
cabbage envelop the inner leaves, so is it in the bud. But
there is more than this ; the outer parts are modified — by
an arrest of development, or even partial dying — into firm
scales, which are often waterproofed with an exudation of
resinous varnish, or warmly lined with epidermic down.
Moreover, the inner leaves of the bud are kept close
together, bent over the growing point of the stem, for it
seems that at first their under surfaces grow more rapidly
than the upper surfaces — the very reverse of what after-
wards takes place when the buds burst and the leaves
unfold.

Bud-Scales. — If we look at a number of buds we see
various arrangements which secure their better protection
against the cold of winter. In the simplest cases, e.g.
lilac and rhododendron, the bud - scales are modifica-
tions of entire leaves : in fact, in a notably exceptional
case among our deciduous trees, that of the Wayfaring
tree ( Viburnum Lantana ), the lowest leaves serve as bud-
scales — that is, the bud-scales, thanks to their woolly coating,
are able to survive and develop as ordinary foliage leaves.
In other cases, they correspond simply to the broadened
stalks of leaves, and if we examine branches of the horse-
chestnut or walnut in spring we shall find occasional
examples of every stage between the foliage-leaf with its

158 Chapters in Modern Botany chap.

five leaflets and the usual reduced bud-scale. Finally, bud-
scales may be derived, as in the beech, from those little
appendages of leaves which we call stipules. When spring
comes, and the bud passes from a latent to an active period,
the protective scales — often almost dead — are thrown off,
and often carpet and tint the ground like a foreshadowing
of autumn. The autumn indeed it is for these sacrificed
leaves which are to know no summer.

Arrangement of the Leaves in the Bud (Phyllotaxis).
— It is interesting to dissect a few buds to see the various
ways in which the young leaves are packed together, a
great number being compressed into the small space. This
neat packing has the further importance that it mainly
determines the position of the leaves on the future stem or
branch. We have all observed the regular arrangement of
scales in a fir-cone or of leaves in a crowded rosette like
that of a house-leek, stiff and orderly as in a prize double
camellia, but so it is really in the longest stem. Mark the
knots upon a thorn walking-stick ; see the house-leek send
up its long scanty-leaved flower-stalk ; imagine the fir-cone
pulled out ; or reverse the experiment, and let a stretched
elastic cord stand for a stem ; wind round this, in ascend-
ing spirals at regular distances, a string bearing knots to
represent leaf insertions, or still better, holding paper leaves
or real ones, also of course at regular distances. Then
allow the elastic cord to contract ; the leaves will bend to
assume positions such as occurs in nature within a bud.
Twist the cord, and the leaf-spiral becomes more crowded
and complex, the leaves therefore more perfectly packed
one upon another ; so we may reproduce the spirals of
different plants. Here then, especially if of any mathe-
matical tastes, we have an interesting field of inquiry —
What are the different spirals to be met with in plants ?
how far are these constant or related ? what effect will an

VIII

TS9

Spring and its Studies

abnormality such as the fusion of two scales in a fir-cone
have upon the spirals ? and so on. Much new light upon
morphology used to be hoped for from this study, notably
by Alexander Braun of Berlin, half a century ago ; and in
our own day the late Professor Dickson has been its most
elegant and subtle expositor,1 as Schwendener its most
productive student ; it has, however, for most botanists a
diminished interest, although we may still hope it holds
some floral secrets.

More important, however, from our present point of
view, is the use of this spiral. For most botanists this used
to be explained simply as keeping each leaf out of the light
of its predecessor (or out of the shadow of its successor, if
we prefer to put it so) ; Airy, however, in an interesting
paper suggested by Darwin, inclines to interpret it
primarily as an adaptation to more perfect bud-packing, as
above indicated. Its deeper explanation of course lies in
some spiral growth-rhythm which we do not as yet clearly
understand.

From seeds and their germination, from buds and their
unfolding, we naturally pass to the life of the adult plant.
How is that life sustained from day to day ? The seed had
a store of food within itself, and the growth of the young
plant is for a time comparable to that of the embryo chick
within the egg. The store of food within the grain of wheat
is analogous to the yolk of the egg. But after that is
exhausted, how is life sustained ? We have already seen
that a plant — in some ways so like a sleeping animal — often
wakes up in movement, and although the movements are
not often so conspicuous as in the climbers, the facts that
plants do always move a little, that they force their way
into the ground and raise themselves into the air, that, in

1 The student may with advantage read his chapter on Phyllotaxis
in Balfour’s Elements of Botany.

160 Chapters in Modern Botany chap, viii

short, they live and grow, should be enough to convince us
that they must feed in some way, no less than animals. In our
study of insectivorous plants we saw some strange examples
of feeding, and in chap, iv., in considering the plant not as
an isolated thing, but as part of the great system of nature,
we got glimpses of some of the ordinary relations of plants
to the air, to the soil, and to radiant energy. Let us seek
to get a more precise conception of these relations, let us
see how the important parts of a plant — leaves, roots, and
stem — are adapted to the sustenance of the life. Let us
look closely at these different parts, and see how their
forms are suited to, and modelled partly by, the uses which
they have to perform, partly by the surrounding conditions
in which they live : in technical language, let us consider
the adaptation of plant-structure to function and environ-
ment. For this we can have no better type than is given
by a fuller study of the leaf, to which we may therefore
turn.

CHAPTER IX

LEAVES

General Facts in regard to the Life of Leaves — Experiments , rough
and exact — Summary of Leaf Functions — The Structure of
the Leaf — Palisade Cells and Chlorophyll Grains — Shapes of
Leaves — Leaves adapted to special Functions — Substitutes for
Leaves — Vitality of the Leaf — Fall of the Leaf \

General Facts in regard to the Life of Leaves.

— The leaf is the most distinctive part of the plant. There
are many rootless plants, such as the bladderwort which we
have already described ; there are many plants without
stems, such as the seaweeds ; but few plants can be called
leafless. Where the leaves are suppressed the stem is
modified to take their place.

The leaf may be also viewed as the primitive part of the
plant. For among the seaweeds there is nothing but leaf,
there are no true stems nor roots. The liverworts, which
spread over the damp banks of streams, are still mostly leaf,
provided, however, with suctorial rootlets on their under
surfaces. With the mosses and the ferns stems begin.

It is therefore natural to suppose that the leaf is the
most essential part of the plant so far as the sustenance of
life is concerned. We shall see how true this is. Let us
think over the familiar facts in regard to the life of plants
which may give us hints as to the nature of the leaf.

M

1 62 Chapters in Modern Botany chap.

First then in spring the bursting of the buds and the
unfolding of the leaves to the lengthening day ; then how
at the approach of winter, when the conditions of light,
heat, and moisture are no longer favourable, the leaves fall
off, and there is no obvious growth after they are gone.
Note too the many differences between the fresh growing
leaves and those which have withered and fallen off ; for
one thing, the former are rich in starch and sugar, and other
substances which the dead leaves lack. In dying at any
rate the leaves surrender the best of their substance to the
permanent body of the plant. We shall see that they are
doing this all their life long.

When voracious insects eat up all the leaves of a plant
and leave it bare, death often follows, although stem and
root are quite uninjured. And when a plant has received
some shock, as of frost or heat or poisonous smoke, or is
scarce of water, one of the first symptoms of ill-health —
often a very rapid symptom — is the drooping of the leaves,
so essential are they to the sustained life of the plant.

Again, we are all familiar with the eatableness of many
leaves, such as those of cabbage, spinach, and lettuce ; they
are swollen out in great part with water, but also with
nutritive materials ; so we get an idea of the leaf as a part
of the plant in which substances are manufactured and
sometimes stored up.

Experiments in regard to Leaves. — More precise
ideas about the uses of leaves may be gained by a few
simple experiments. Let us follow one of these described
by Detmer.

Take some seeds, such as those of wheat, maize, and
beans ; weigh them, and let them germinate and grow in
water to which some plant ashes have been added. But in
order to get a more precise measurement of the amount of
material with which you begin, take samples of the seeds used,

IX

Leaves

1 63

grind them to powder, thoroughly dry them at a high tempera-
ture, and weigh them. In this way the proportion of water
in the seeds may be estimated, and by simple calculation it
is easy to make an approximate statement as to the amount
of solid, and for the most part organic, matter in the seeds
used for the experiment. By and by, in favourable con-
ditions of light and heat, the plants (let us say of maize)
grow up and bear leaves. Let them flourish for several
weeks, and then remove them from the vessel in which
they have been growing ; dry off superficial moisture, cut
them into small pieces, and thoroughly dry a fair sample of
these at a high temperature sufficient to drive off the
internal water. It is thus easy to estimate how much
new organic matter the plant has made for itself during these
weeks. We say organic matter (starch and the like),
because the proportion of inorganic matter, estimated by
weighing the ash left after the whole is burned, is very
small, and may for the present be left out of account.
Now the experiment shows that in the course of its growth
the plant has gained greatly in organic matter. Where
has that come from ? Partly, no doubt, from the water
and salts ; but that can be measured, and does not account
for the total increase of weight, nor do the water and salts
account at all for the carbon, which is the essential element
in all organic compounds. The answer to our question
every one knows ; the plant has absorbed carbonic acid
gas from the air, and has built the carbon thus obtained
along with the elements of water, and in part also with the
elements of salts, into complex organic substances. But by
what means was the carbonic acid gas absorbed from the
air ? If we look at our maize plant there can hardly be
but one answer — by means of the leaves. But if one of
the vessels containing germinated maize is placed in dark-
ness and allowed to grow there for some time, the resulting

164 Chapters in Modern Botany chap.

plants, which are lean and blanched, actually weigh less
(when dried) than the seeds did. Instead of a gain in
organic matter, there has been a loss, due to the living
and breathing of the plant. In other words, in the absence
of light the plant is not green, and is unable to utilise the
carbonic acid gas of the air. So we may say of the leaves
that they are those green parts by means of which the plant
is in sunlight able to utilise the carbonic acid gas of the air
and to build up organic substances.

Another experiment of profound importance is readily
made. Take a piece of some vigorous water-plant, such as
the Canadian pondweed Elodea ( Anacharis ) canadensis , so
common in streams and canals ; place it in a vessel of
water into which some carbonic acid gas (easily produced
in a separate vessel by decomposing carbonate of lime with
hydrochloric acid) may with advantage have been intro-
duced. Place the vessel with the pondweed in the sunlight,
and watch the little bubbles of gas which appear on the
leaves and ascend to the surface. Place a funnel over the
plant, and catch the bubbles of gas in a test-tube filled with
water and inverted over the tube of the funnel. Gradually
as the bubbles ascend into the test-tube they displace the
water, and if you have patience you will at last have the
test-tube full of the gas. If you carefully remove the test-
tube and push into it a smouldering match, it will glow
brightly and perhaps catch fire again, or in other less rough-
and-ready ways you can show that the gas in the tube is
very rich in oxygen. The leaves, therefore, are organs
which in sunlight give off oxygen, this being another aspect
of their power of utilising the carbonic acid gas. The
oxygen which they liberate results from the breaking up of
the molecules of carbonic acid gas within the plant.

That the Canadian pondweed absorbs, like other water-
plants, its requisite supply of carbonic acid gas from what

IX

Leaves

i6S

is dissolved in the water, while most plants absorb it from
the mixture of gases in the atmosphere, is an unimportant
detail. These experiments can be readily varied and
extended ; thus we might put roots in place of the leaves
in our last experiment, but no bubbles of oxygen would be
formed ; or we might place the pondweed in water from
which the carbonic acid had been expelled by boiling, and
allow no air to enter the vessel except through a tube of
caustic potash, which will absorb its carbonic acid, with the
result that the evolution of oxygen would entirely cease.

More exact Experiment. — A rough analysis of the
evolved gases can easily be made by collecting them in
a graduated tube, shaking up first with potash solution to
absorb all traces of carbonic acid, and then adding pyro-
gallic acid, which absorbs the oxygen and leaves only the
nitrogen (with a little water- vapour). This method be-
comes of the greatest ease, rapidity, and exactitude, as also
delicacy in dealing with the smallest quantities of gases
by help of the apparatus recently introduced by MM.
Bonnier and Mangin ( Revue Generate de Botanique 1890),
which is figured below. It will be found of great use to
the teacher of practical botany, as also to the experi-
mentalist, especially in our uncertain climate.

Experiment can show us many other things about leaves.
It is commonly known that a starch solution turns a deep
blue colour when a little iodine solution is added to it. We
may use this fact, as Sachs recommends, to demonstrate
the presence of starch in leaves. If some leaves, e.g. of
sunflower or of potato, are placed for a few minutes in
boiling water, and then for a short time in warm alcohol,
they become colourless. The green colouring matter or
chlorophyll is dissolved by the alcohol. If the discoloured
leaves are then placed for an hour or two in a dilute alco-
holic solution of iodine, and then removed to a saucer full

1 66 Chapters in Modern Botany chap.

of water, those which were rich in starch assume the
characteristic blue colour. Of this test for starch we can
make many other uses ; by growing plants in darkness, or
in an atmosphere without any carbonic acid gas, we can
show that starch is not formed in the leaves unless they are
green, unless they are illumined, unless they are growing
in a medium where carbonic acid gas is available.

The leaves are able to manufacture many other organic
substances besides starch, and their presence is also of
course demonstrable by experiment, but starch is com-

Fig. 8. — Apparatus of Bonnier and Mangin, for analysis of gases given off by
plants, c, Reservoir of Mercziry’, m ?;z, graduated tube; v, screw of
piston, which enables the gas to be drawn into the graduated tube and
measured ; and after it the potash and pyrogallic solutions successively each
being thoroughly brought into contact with the gas in the dilatations a.

monest and simplest, and may well serve as a type of the
products built up by the life of the leaf.

It is also very important to show by experiment, by
the analysis of plant ashes, and by growing seeds in
artificially-prepared solutions, that certain salts, especially
nitrates and sulphates, are essential to the health of the
plant and to the leafs manufacture of complex organic
substances. The successive omission of each particular
ingredient gives rise to some recognisable defect of growth,
e.g. when iron is absent we have a blanched plant, while

IX

Leaves

167

verdure returns on the addition of a drop of ferric
chloride. It is also possible to prepare culture solutions
in which all necessary constituents are present, and thus to
grow many plants without soil. In many leaves it is easy
to demonstrate the presence of oxalate of lime — we can
taste it in the wood-sorrel and the sour-dock — and this salt
seems to be a by-product of chemical reactions which go
on in the leaf in which the nitrates and sulphates absorbed
from the soil are utilised in the formation of those complex
organic substances which chemists call proteids.

If we put the cleanly -cut stalk of a primrose -leaf
(still better, that of the Chinese primrose (P. sinensis),
so common and free -flowering in our greenhouses) into
our mouth, and keep the blade of the leaf immersed in
water, we can blow air through the stalk into the blade and
thence into the water. It issues from the leaf through
minute apertures, most numerous on the lower surface.
They are readily seen under the microscope, especially on
a leaf like that of the Iris, from which it is easy to peel off
a film of transparent skin. They are of great importance
in the life of the leaf, and are called stomata. To reverse
the above experiment, the leaf- stalk should be inserted
through a perforated india-rubber cork into a beaker half
filled with water ; the base of the stalk should be well
immersed in the water. Then if through a glass tube also
perforating the india-rubber cork we suck out air from the
upper half of the beaker, the altered pressure induces a
passage of air through the stomata of the leaf into the
blade and down the stalk into the water, through which it
issues as a stream of bubbles.

The recognition of these openings or stomata makes the
leaf more intelligible to us. They lead into spaces within
the substance of the leaf, and with a microscope it is easy
to prove that they are open when the plant is in full vigour,

1 68 Chapters in Modern Botany chap.

but that they close when drooping for lack of sufficient
moisture begins, and also that they are often closed at
night and open during the day. But the proof of this takes
us beyond the region of simple experiment to that of micro-
scopic observation.

By observing the rapid drooping of cut flowers, and
their recovery when placed in water, or the need of con-
stantly filling up our hyacinth-glasses, or more precisely by
fixing a leafy shoot in a narrow bent tube of water, and
noticing how the water diminishes when the plant is kept
in a warm room, and by other more delicate experiments,
we can convince ourselves that the leaves of plants give off
water-vapour into the air. So much of the water which
is absorbed by the roots is indeed used in building up
organic substances and in growth, but by several weighings
of plants it is easy to prove that much of the water simply
disappears into the air. And when we consider that the
surface of a leaf is often thick-skinned, and sometimes var-
nished with wax, and notice, for instance, how an apple
without its skin dries up much sooner than one which is
intact, and remember how much more numerous the little
apertures or stomata are on the under than on the upper
surface, we are warranted in concluding that it is by the
stomata, and therefore especially by the under surface of
the leaf, that the escape of water-vapour or transpiration
takes place. Nor is it difficult to prove this by direct
experiment. Thus we recognise another of the uses of
leaves ; they are the organs of transpiration, and the little
apertures on their under surface, which close in drought
but open when moisture is abundant, are the regulators of
this.

But we have not yet completed our experiments. Let
us place a number of growing seedlings in a glass retort
through the cork of which two glass tubes are passed.

IX

Leaves

169

Connect the end of the exit tube with a glass cylinder
containing clear lime-water or baryta -water, and beyond
that through a perforated cork with a large vessel half
filled with water. It is evident that if we allow water
to flow out from the base of the large vessel, and manage
the tubes rightly, there will be a passage of air from
above the seedlings, through the baryta -water cylinder,
into the final large vessel. As to the tube which enters
the vessel with the seedlings, connect this also with a
cylinder containing clear baryta - water, and beyond that
with an open tube packed with caustic potash. Now
set the apparatus agoing. As water flows from the
base of the terminal vessel, air passes — since the whole
system is continuous — through the caustic potash tube,
through the first baryta-water cylinder, through the vessel
with the seedlings, through the second baryta - water
cylinder, into the terminal vessel. Now the air which
enters is robbed of its carbonic acid gas by the caustic
potash, and the continued clearness of the first baryta-
water proves this. But the air which passes into the
second baryta -water clouds this (with a white precipitate
of insoluble carbonate of baryta), which proves that the
seedlings have been giving off carbonic acid gas.

This experiment is not by any means the best that can
be made in order to prove that plants, like animals, give off
carbonic acid as they live, but it is one of the simplest.
For conclusiveness it will be necessary to use leaves instead
of seedlings, to estimate accurately the precise change of
gases, to test the difference between plants growing in dark-
ness and those in the light, and so on. But we have at
least indicated how the student may convince himself that
plants, like animals, breathe, — for the liberation of carbonic
acid gas is associated with an absorption of oxygen and
with a slow combustion of the material of the plant, as in

i7° Chapters in Modern Botany chap.

all forms of respiration, — and furthermore, that the leaves
are the organs by which the breathing is principally
effected.

It is difficult to exorcise a persistent fallacy which has
long plagued the student, and for which tradition and text-
books are much to be blamed — that one of the great
differences between plants and animals is that plants take
in carbonic acid gas and give out oxygen, while animals
take in oxygen and give out carbonic acid gas. This is a
fallacy, all the more troublesome because both its statements
are true. It is true that animals take in oxygen and give
out carbonic acid gas — this is the essence of respiration.
It is also true that green plants in sunlight take in
carbonic acid gas and give out oxygen — this is one of
their essential characteristics. But it is also true that
plants, like animals, take in oxygen and give out carbonic
acid gas, for they must breathe. Only it happens that
in the daytime the process of respiration in green plants
is externally hidden, is virtually counteracted (since much
more than compensated) by the reverse nutritive process
peculiar to plants. The fallacy is the contrast between a
respiratory process, common to all creatures, and readily
demonstrable in plants which are not green, or which are
in darkness, and a nutritive process which is peculiar to
the green parts of plants during the light of day.

Summary of Leaf Functions. — We are now in a
position to sum up the chief functions of leaves : —

(1) Leaves are the principal organs by means of which
the plants give off surplus water, and that chiefly on the
under surface and through the minute apertures or stomata
which regulate the transpiration.

(2) Leaves are the principal organs by means of which
the plants breathe, by which the gaseous interchange which
is implied in all life is effected. But during the day in

IX

Leaves

I7I

green plants the absorption of oxygen and the liberation
of carbonic acid gas is counteracted by the reverse nutri-
tive process.

(3) Leaves are the principal organs by means of which
the plants absorb carbonic acid gas from the air, and by
aid of the radiant energy, which passes through the screen
of green colouring matter, split it up, liberating the oxygen,
and using the carbon as the foundation for the upbuilding
of organic products, in which water from the roots, and in
some cases salts as well, are also ultilised.

The student may indeed feel that these results might
have been reached much more rapidly by a priori reason-
ing. As plants live they must breathe, and the leaves are
obviously the organs fitted for this. As plants build up
carbon-compounds, and yet get no carbon from the soil, it
is evident that they must get it, as carbonic acid gas, from
the air, and the leaves are obviously the organs fitted for
this ; and so on. But this a priori method is never con-
clusive, and though we have done little more than indicate
the lines of experiment, we have shown how the functions of
the leaf may be experimentally demonstrated without taking
very much for granted. To do so even imperfectly is better
than to follow the short cut of dogmatic assertion ; and with
Professor Detmer’s convenient and well-illustrated manual
of experimental physiology, now Englished, the student,
even without a teacher, may fairly set to work, and
profitably read also, as his experimental course demands,
the appropriate passages in the manuals of Sachs and
Vines.1

The Structure of the Leaf. — We are now in a

position to advance to an examination — at first a very
general one — of the internal structure of the leaf.

1 Sachs, Lectures on Vegetable Physiology , Oxford, 1887. Vines,
Vegetable Physiology , Cambridge, 1886.

ij2 Chapters in Modern Botany chap.

The surface is often stiff and firm, and its outermost
coating thick. We recognise the use of this as a protection
to the delicate living parts within. When there is a waxy
varnish or a covering of hairs the protection from cold and
from loss of water will be all the greater. In many cases
the outermost layer, especially on the upper surface, consists
of units or cells with thick corky upper walls (the so-called
cuticle ), and we know how impermeable cork is to the
passage of water. But in many cases the cells of this
colourless layer of epidermis are very rich in water ; they
form a layer of little water-bags all over the leaf. We can
see the use of these ; they lie between the outer world and
the more delicate living cells in the heart of the leaf, so that
when a rapid loss of water occurs from the surface, it is not
the internal parts which directly suffer, but the hardier and
less intensely living epidermis.

In many cases the leaf is so well protected by its hard
external sheath that there can be very little transpiration
except by the regular apertures — the stomata. These are
exceedingly numerous, usually several hundred on a square
inch. In most leaves they are virtually restricted to the
under surface, but in those of plants like the water-lilies,
which float on the water, they occur where alone they can
be of use— on the upper surface, and in leaves which grow
upright they are equally numerous on both sides.

It is necessary to examine more carefully into the nature
of these stomata. Each aperture is bounded by two special
cells of the epidermis, called guard-cells. Unlike most of
the cells of the epidermis, these are green, and therefore
able to form starch like the tissue in the heart of the leaf.
Furthermore, they are so disposed that when they are rich
in water — in other words, when they are swollen, “ tur-
gescent ” — the stoma or aperture between them is opened ;
but when the guard-cells contain for the time being less

IX

Leaves

i73

water, and are relatively shrivelled, their walls are applied
to one another and the stoma is closed. The condition
determining the action of these ventilators is explained
then, in the first place, by reference to the state of water-
tension within the guard-cells. The state of water-tension
in the plant depends primarily upon the relation between
the water within the plant and the external conditions of
drought or humidity. Thus on dry days, when the plant is
necessarily losing much water to the air, when the leaves in
consequence tend to droop, the water-tension of the guard-
cells is necessarily lessened, and the stomata automatically
close, or partially close, thus beneficially retarding the drying
up of the plant. This explanation was for a long time alone
current : its insufficiency gradually became clear as obser-
vation showed that stomata in sunshine tend to open, in
darkness to close ; that light in fact is of more importance
to their movements than is the humidity of the atmosphere.
But how does the light act ? It does not usually affect the
opening or closing of ventilators : what can be the rationale
of this strange phenomenon ? This Schwendener has
explained by help of their chlorophyll already mentioned ;
they form starch and grape-sugar (glucose) as mere epider-
mis cells cannot do. Hence when assimilating vigorously
their glucose draws water from the neighbouring cells, they
become turgescent, and so open ; while, when light fails
and their activity and stores diminish or disappear they
close again. It seems, moreover (and thirdly), that the
light, apart from its influence on the guard-cells through the
formation of starch and consequently sugar, has a direct
influence o.n the state of water-tension in the cells. We
have already noticed a similar influence in the case of those
plants which bend towards the light. The rationale of
this has next to be sought for, and although neither so ob-
vious or certain as in the preceding case, need not be

174 Chapters in Modern Botany chap.

despaired of. The varying pull of the epidermic cells upon
the stomata has also to be considered, and so on.

We regard the stomata then as automatic ventilators,
which have especially to do with regulating the transpira-
tion of water-vapour from the leaf. They lead into spaces
between the loosely packed cells which form the lower half
of the leaf.

But where within the leaf is the important work of
building up starch and proteids carried on ? The answer
is not difficult, for starch is a very readily recognisable
substance, occurring in characteristic granules, and we
know that it occurs most abundantly in the layer or layers
of green cells lying immediately below the protective upper
epidermis, which are so closely crowded together towards
the light as to give a section the aspect of a palisade. This
“ palisade parenchyma 5J is the chief laboratory of the plant.
Here it is that the radiant energy of the sun streaming
through green colouring matter is used by the living matter
of the plant in the splitting up of the absorbed carbonic
acid, and in building up complex materials whose chemical
energy is well known to us when we use them as food or as
fuel.

When we think of the leaves manufacturing starch all
the day long and all the summer through, the question
must arise, Why do not the leaves become too full ; how
is it that they are not overloaded with starch ? It is plain
that the starch must in some way pass away from the
leaves, and it is not difficult to prove that leaves which
were richly filled with starch when the sun set may have
very little next morning when the sun rises. When the
leaves wither and die in the autumn there is again a dis-
appearance of almost all of their starch. How does the
starch disappear ? in what form ? by what path ?

At this stage in our studies we can answer these questions

IX

Leaves

T7S

only in general terms. The starch which is made in the
light cannot pass out of the leaf except into the body of the
plant, nor can it pass away as such, for the solid starch
grains could never get through the walls of the cells.
Before it can pass from the leaf the starch has to be fer-
mented, it has to be changed into sugar. The same is true
in regard to the starchy parts of the food that we eat ; it is
a simple matter to masticate and swallow a potato or
banana, but before the starch which these foods contain
can be of the. slightest use to us it must pass through the
walls of the food-canal into our blood. Now the starch as
such cannot pass through the walls 'of cells. The grains are
too large. If must be rendered soluble and diffusible ; in a
word — digested. This process begins in the mouth, where
a ferment contained in the salivary juice changes the starch
into a sugar. A precisely similar digestion (by means of a
ferment called diastase) takes place in the leaves, and it is
in the form of sugar (glucose) that the starch passes into the
stem.

But by what path ? We have spoken of the epidermis
or skin of the leaf above and below, of the loose arrange-
ment of cells on the lower side, and of the always greenest
palisade tissue in which the essential processes of the cell’s
life go on. But we have not spoken of one of the most
important and characteristic parts — the system of £< veins,”
which in most common plants forms an intricate net-
work through the substance of the leaf.

These veins form the most durable part of the leaf, for
in the skeleton leaves which we see rotting away in the
ditches the veins persist while all else, unless perhaps the
firm outermost sheath, has disappeared. There is no
doubt that the “ veins ” form a sort of supporting skeleton
for the leaf. They are the firm beams around which the
more delicate substance of the leaf has been built up. But

176 Chapters in Modern Botany chap.

the veins are more than supporting beams ; they include a
system of downcast pipes by which the sugars and other
materials manufactured by the leaves pass away into the
stem, and another upcast system of pipes, whose necessity
is equally obvious, by which the water and soluble salts
absorbed by the roots and passed up the stem irrigate the
leaf and supply some of the essential raw materials for the
manufacture which goes on there. These two systems —
bast and wood respectively — may be more fully studied when
we trace them into the stem.

Palisade Cells and Chlorophyll Grains. — To the ordi-
nary observer it seems as if the chlorophyll grains of the
palisade cells had no definite position ; but Stahl and others
have made the very interesting discovery that a movement,
at any rate a change of position of the chlorophyll grains
within the living cells, may be observed when cells are
brought from diffused light into intense sunshine. The
leaves of the Star of Bethlehem ( Ornithogalum umbellatum
and O. nutans ), of grape hyacinth ( Muscari racemosum ), and
other common plants, notably also the lesser celandine or
figwort ( Ranunculus Ficaria ), give excellent examples of
this, as also do the prothallia of ferns (although here, of
course, a palisade parenchyma is not specialised). In
figwort, for instance, the chlorophyll grains will leave the
upper and under surfaces of the palisade cell for its sides after
quarter of an hour of sunshine, thus taking up a position in
which they are as little as possible exposed, not only sheltered
one behind the other, but with their edges to the light. When
the sun goes off, they get back to their old positions, but
more slowly, needing several hours. This is, of course,
observed by cutting sections of leaves taken at different
times.

Now we know that too much light may be injurious ;
and Pringsheim has made striking experiments towards

IX

Leaves

177

the interpretation of this, showing that in a focus of intense
light (from which, of course, all possibility of the destructive
action of heat has been removed by stopping this off by a
light transparent, yet heat-opaque alum screen) chlorophyll
is at once destroyed and no assimilation can therefore go
on. That different plants have different optimum light-
quantities for assimilation, just as they have different optimum
heat-quantities or temperatures for germination or growth, is
obvious enough ; but nature does not constantly supply
these ; how beautiful this self-regulating mechanism, by which
the chlorophyll grains can utilise just their right amount
of sunlight, making the most of it when it is scanty, and
escaping its dangers when excessive ! Nay more, may we
not thus explain the peculiar shape and position of the
palisade cells themselves ? Is it not to give the chlorophyll
granules room to assume the most favourable position that
they have elongated perpendicularly to the leaf surface ?
In the same way, since the cells of the deeper-lying, spongy
parenchyma are shaded by the layers above, and need all
the light they can get for their chlorophyll grains, must not
they expand horizontally ? And hence we should be able
to explain why it is. that in shade-loving plants palisade
parenchyma may even be absent, and why in shade-grown
leaves the spongy parenchyma is often observed to be more
abundantly developed.

The explanation is beautiful, and seems a satisfactory
one ; a few years ago botanists were wont to teach it with-
out reservations. The critic, however, soon came in the
person of one of our most thoughtful and physiologically-
minded of vegetable histologists, Professor Haberlandt of
Graz, to whom, with his master Schwendener, belongs
especially the credit of attaching physiological interpreta-
tions to what were formerly too often mere microscopic
curiosities. He first verifies and establishes Stahl’s

N

1 78 Chapters in Modern Botany chap.

observations so far , i.e. in regard to some plants, insisting,
for instance, on the obviousness of the phenomenon in
figwort, but limits them also ; these plants of Stahl’s are all
shade-lovers, be it noted, and no doubt specially sensitive ;
but in not very many plants can his observations be made.
That palisade parenchyma is formed on both sides of leaves
growing vertically is also a serious difficulty to Stahl’s
theory. Moreover, as to chlorophyll grains, he tells us
they are found on the sides of the palisade cells whatever
the intensity or direction of the sunlight ; and that when
they are found on the upper surface parallel to that of the
leaf no change can be observed. Nor do they always
present their profile to the intenser light. Hence then he
gives up Stahl’s explanation of the characteristic form of
the palisade cells.

A new rationale, therefore, is wanted ; and this Haber-
landt finds suggested by a fresh observation, that chlorophyll
grains are never to be found upon the lowest wall of the
palisades (the bottom of the canister-shaped cells in short).
Now as the current carrying off the products of assimilation
(out from the cell towards the bast portion of the bundle)
must pass through this wall, he argues that this is kept
free for the purpose, and that this must furnish space for
the chlorophyll grains, which are therefore elongated in
the direction of the current. In short, the form of the
assimilatory tissue (as also the position of the chloro-
phyll grains) is essentially adapted to facilitate the most
easy {i.e. direct and rapid) transport of the products
of assimilation, and not their position relative to the sur-
face. The frequently radial arrangement of the green
parenchyma in leaves or stems, like those of Cyperus of
Equisetum , is cited to confirm this, and so on.

These two brief and necessarily scanty abstracts of
Stahl and Haberlandt respectively by no means exhaust

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Leaves

179

the discussion. Here, as everywhere, the aim has been to
interest the reader in it, or at least to make it clear that
there is one, and that scientific studies involve primarily,
beside wide observation, the critical reason, not the
credulous memory. Hence before leaving this subject it
may be profitable to cite yet a third writer. Thus Schrenk
finds cases in which chlorophyll grains do occur upon
those very cross -partitions to which Haberlandt denies
them, and not unnaturally urges that he cannot see
how the presence of a few granules at these walls could
materially obstruct the passage of the sap-current.

For him then “ neither Stahl’s nor Haberlandt’s theories
are necessary to explain the structure of the palisade cells.”
He reminds us of Frank’s observations, that the chloro-
phyll grains are especially abundant in the neighbourhood
of air spaces. There are of course vertical chinks in the
palisade parenchyma, hence also the absence (or com-
parative scarcity) of granules at the bottom of cells, on
which Haberlandt has so much insisted, becomes intelli-
gible for quite another reason, that these generally abut
upon another cell, and not upon an air space !

Discussion of this kind may seem to some unprofitable,
as if there could be no certainty in physiology. Yet we
should be losing one of the best lessons in biology if we
failed to see the variety and complexity of its problems,
and to see too how each new theorist may be doing good
service by bringing some new factor or aspect of the case
into view. Thus Schrenk goes on towards a compromise :
“ How can we imagine a plan better adapted to fulfil these
two conditions (i.e. of access to air and to sunlight) than
that on which the palisade tissue is constructed ? Into a
given volume of this tissue the greatest possible number of
cells is packed in such a manner that each presents its
upper surface to the incident rays, permitting them to

180 Chapters in Modern Botany chap.

pervade its whole interior, while at the same time it fur-
nishes ample accommodation to the chlorophyll grains, on
its long walls, where they have the best opportunity to
come into contact with the carbonic acid in the air-
passages.”

This compromise the student may now himself elaborate,
but also criticise afresh, so working towards familiarity
with the subject.

Shapes of Leaves. — “ The leaves of the herbage at
our feet,” Mr. Ruskin says, “take all kinds of strange
shapes, as if to invite us to examine them. Star-shaped,
heart-shaped, spear-shaped, arrow-shaped, fretted, fringed,
deft, furrowed, serrated, sinuated, in whorls, in tufts, in
spires, in wreaths, endlessly expressive, deceptive, fantastic,
never the same from foot of stalk to blossom, they seem
perpetually to tempt our watchfulness and take delight in
outstripping our wonder.”

We do not propose to describe these different forms of
leaves, nor to introduce the elaborate nomenclature which is
used in order to describe them with precision ; from the
present standpoint it is sufficient to give a few illustrations
showing how peculiar forms are adapted to special condi-
tions, referring the student for details to Sir John Lubbock’s
Flowers , Fruits , and Leaves (Nature Series, Lond. 1888).

Large and free-growing plants which obtain unobstructed
light most frequently bear simple or slightly lobed leaves,
while the smaller vegetation generally produces leaves either
long, simple, and narrow, as in grasses, or highly compound,
with small leaflets, as in ferns and many plants of the
woods and hedgerows, so as to seize as many as possible of
the broken sunbeams which have not been intercepted by
the loftier plants, while casting as little shadow as possible
upon each other. As the same amount of leaf material has
a greater surface when cut up than when forming a con-

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Leaves

181

tinuous blade, the advantage of leaf-lobing is as obvious as
is the breaking up of the coast-line of a country by many
fiords.

The leaves of aquatic plants, if floating, are simple and
largely expanded, so as to maintain their position and
obtain the maximum of light, as we see in the case of the
water-lily and the pondweed. But if the leaves are sub-
merged they are usually dissected into thread-like segments,
as in the water-primrose, so as to allow the water to pass
unobstructed, and thus constantly renew the supplies of car-
bonic acid gas.

Not unfrequently the leaves on the lower and on the
upper parts of the stem are in different circumstances, and
their form is also varied, as we may see, e.g. in Campanula
roltmdifolia , the familiar blue-bell of Scottish song ( Anglice
hair- bell, from its thin stem, often misspelt hare-bell), in
which the round leaves are only the lowest of the rosette,
gradually passing to narrow “ linear- lanceolate ” form.
A similar divergence of foliage may be seen in the
water -buttercup {Ranunculus aquatilis ), which possesses
both floating leaves, which are simple, and submerged leaves,
which are highly dissected. So, too, plants which grow in
dry and sandy places, and obtain scanty supplies of water,
either owing to drought or to the looseness of the soil, very
frequently store water in their leaves, which thus become
succulent, and preserve it from evaporation by a thick epi-
dermis with unusually few stomata. The stonecrops and
house-leeks of our rock-work, or the sea-shore sandworts
(Arenaria, Honckenya) the sea milk-wort (Glaux mari-
tima), and- other common species of the sea-shore are
familiar native illustrations ; while the Agaves and Aloes,
the Mesembryanthemums and Cactuses of a “ succulent
house ” show the same habit upon the grand scale appro-
priate to the more extreme climates from which they come.

1 82 Chapters in Modern Botany chap.

Let us take the case of the white water -buttercup
( Ranunculus aquatilis ) as a subject for interpretation. The
facts are that the floating surface -leaves are more or less
simple and rounded, and that the submerged leaves are
divided into hair-like segments ; and it is also known that if
the plants grow in swiftly running water all the leaves are
submerged and filiform, while if they grow altogether on
the mud by the side of the stream the dissected type of
leaf is lost and the appearance of the plant is very different.
So much so, indeed, that Lamarck believed that Ranunculus
aquatilis might thus be transformed into an allied species
R. hederacea , but this conclusion has been denied by
Godron, who has paid great attention to the variations of
buttercups. It is certain, however, that the predominant
type of leaf varies according to the surroundings, and that
each form of leaf is advantageously adapted to its condi-
tions. How is this to be interpreted ?

Sir John Lubbock says, “ Of course it is important to
expose as large a surface as may be to the action of the
water. We know that the gills of fish consist of a number
of thin plates, which while in water float apart, but have not
sufficient consistence to support even their own weight,
much less any external force, and consequently collapse in
air. The same thing happens with thin, finely- cut leaves.
In still water they afford the greatest possible extent of
surface with the least expenditure of effort in the formation
of skeleton. This is, I believe, the explanation of the
prevalence of this form in subaqueous leaves.”

Mr. Grant Allen, on the other hand, interprets the finely-
divided form of the submerged leaves to the relative
scarcity of carbonic acid gas in the water.

Our view of the problem is as follows : First, we must
take note of the manner in which the “ veins ” of the leaf
are distributed in different species of Ranunculus, especially

IX

Leaves

183

in those such as R. hederacea , which seem most nearly
related to the white water buttercup. For as it is likely
that the aquatic forms are derived from land forms, the
established type of venation will to a certain extent limit
the manner in which the leaves are cut up. The established
structure of the original type must be taken into account in
interpreting the derived variation.

Secondly, we should have to grow buttercups of similar
origin (reared from seed or cuttings from one plant) in
different conditions. One set should be placed in ordinary
pond water, and another set in water with an unusually
large quantity of carbonic acid gas. One set should be
grown in still water, another set in the same water flowing
rapidly, and so on. Only by experiments 1 more precise
than those which have as yet been made, can the problem
be satisfactorily solved.

Meantime we can only express our opinion — (1) that the
division of the submerged leaves is determined by the
established type of venation seen in related species and
in the surface leaves ; (2) that deficient nutrition, due to
insufficient supplies of carbonic acid gas, may cause the
tissue between the veins to remain undeveloped or to die
away ; (3) that currents of water exerting pressure upon
the leaves may help to develop the filiform state ; and
(4) that the form of the submerged leaves may thus, in a
natural and necessary way, have become well adapted to
the conditions.

7. Leaves adapted for special Functions. — Before we
pass from the leaf we must notice some of the special
functions which exceptional leaves may fulfil. Some of
these we have discussed already, for the pitchers of pitcher
plants, the bladders of Utricularia, the fly-trap of Dionaea

1 Cf. H. de Varigny, Experimental Evolution (Nature Series),
Lond. 1892.

184 Chapters in Modern Botany chap.

are highly specialised leaves. Again, we saw that the
Clematis and many other plants are leaf-climbers, and that
tendrils are in many cases modified leaves or parts of
leaves.

In not a few plants the leaves die away, in whole or in
part, often becoming reduced into mere spines. The Dar-
winian explanation of course is that these spines have
been produced by natural selection in consequence of
preserving the plants from being eaten up by mammals.
The Lamarckian explanation, on the other hand, is that
spiny plants are the product of the conditions of drought in
which the majority at least occur ; while the writer has else-
where 1 offered an interpretation complementary to this of
thorny species as being of ebbing vitality as compared with
their thornless congeners. The reader will find a vigorous
criticism of this view from the Darwinian standpoint in
Mr. Wallace’s Darwinism , and (if he thereafter retains
sufficient interest in these Lamarckian and neo-Lamarckian
views) a briefly summarised reply in the British Association
Report for 1890. The controversy would lead us beyond
the limits of this little volume ; suffice it here again to say
that in thinking of this and many similar problems in
evolution, the student should be careful to distinguish
between the primary factors, which really originate the
peculiarity in question, and the secondary factors, which
determine whether the peculiarities shall or shall not
persist. Natural Selection of course is the general name
for all these secondary agencies.

Here again is a matter for direct experiment, which has
indeed begun.2 Thus Lothelier has shown with barberry
and hawthorn (1) that the promotion of transpiration in-
creases the development of thorns which were absent when

1 Variation, Ency. Brit., and Life-Lore , 1889.

2 Comptes Rendus , cxii., 1891.

IX

Leaves

185

the plants were grown in a very damp atmosphere, and
(2) that intense illumination increases spininess, while
restricted illumination tends to suppress it.

In this connection may be read with advantage the plea
for experiment in Dr. de Varigny’s recent Edinburgh lec-
tures ( Experimental Evolution , Nature Series, 1892).

Very important as protective structures are the bud-
scales, which are often hard and varnished, and effectively
shield the tender leaves of the bud from frost and damp.
We have already spoken of their importance, and have
noticed that they are modified leaves or leaf- stalks, or in
some cases stipules. Thus on the horse-chestnut you may
find many gradations between the brown, sticky scale and
the leaf- stalk with its five leaflets. Some have simply a
little tip of green, in others this green tip is notched, in
others it is definitely divided into five lobes.

Another special function discharged by many modified
leaves is that of storing. Sometimes they are swollen with
water, as in the Sedums and Aloes. Oftener they are
receptacles for starch and other products which the plants
manufacture. Thus the leaf-bases of the primrose which
persist after the leaves die away are full of starch ; and
the crowded, much modified leaves which form the greater
part of the bulb of an onion or a hyacinth are also store-
houses, from which in spring materials for new growth are
absorbed.

Some leaves are so brightly coloured that when they
occur near the flower they may be mistaken for petals.
We see this very well in a common greenhouse plant,
the “Christmas flower” of Mexico, Poinsettia {P. ftul-
cherrima) ; and the great white spathe which surrounds
he flower of the Ethiopian “lily of the Nile” ( Richardia
( Calla ) africana) is also a modified leaf.

Lastly, as every one more or less clearly knows, at least

1 86 Chapters in Modern Botany chap.

since Goethe’s day (see chap. x. p. 199), the flower itself
consists of four whorls of “ modified leaves ” — the sepals pro-
tective, the petals attractive, the stamens producing pollen,
the carpels bearing the ovules which when fertilised be-
come seeds.

Substitutes for Leaves. — The functions of the blade of
the leaf are shared to some extent by the petiole or stalk
and by the green skin of the stem and branches, and often
by the sepals and ovaries • in short, every part of the plant
exposed to light tends to utilise it by producing chlorophyll,
excepting only those parts of the flower where, in current
phrase, more conspicuous colouring matters are required
for the attraction of insects. Thus the green stems of
Cactuses and some Euphorbias, which are practically leaf-
less, do in a measure discharge the ordinary work of leaves.
The green thorns of the gorse or whin are by no means
too hardened to be of some use as leaves, and there is no
doubt as to the use of the leaf- like shoots of Butcher’s
Broom ( Ruscus ), or of the reduced thread-like filaments
(really abortive flower-stalks) of the Asparagus.

Vitality of the Leaf. — We saw that in the Indian Tele-
graph Plant (Desmodium gyrans) the lateral leaflets are in
constant motion, but this is only a specially marked example
of the movement which Darwin observed in all young and
vigorous leaves. When we touch the sensitive plant, the
petiole sinks downwards and the leaflets fold together, but
this after all differs but little from the daily sleep-movements
of clover, wood-sorrel, and hundreds of common plants, ex-
cept in its being induced by a sudden shock. Other move-
ments of leaves in relation to the light, as they bend towards
it, or arrange themselves transversely or sideways to its rays,
are of constant occurrence. We saw too that the modified
leaves of insectivorous plants seemed to exude a digestive
juice, but it is more important to recognise that there is,

IX

Leaves

187

as Prof. Vines has of late conclusively confirmed, a starch-
changing (diastatic) ferment in all green leaves. It is
indeed in the ordinary processes of everyday life that the
vitality of the leaf is most marvellous, especially in that
process by which the kinetic energy of the sun’s rays,
entering the plant as a series of ethereal waves, passing
through the screen of chlorophyll, aids the living matter
to build up out of crude materials those complex organic
products whose potential energy may be again transformed
in heat and light or motion when we use them as fuel or
food. Nor in our appreciation of the life of the leaf
should we forget that it often contains enough of the
plant’s essential material and vigour to enable it in
suitable conditions to grow into an entire plant, for
every one knows how a piece of Begonia leaf, for instance,
may be planted so as to produce a new plant with stem
and roots.

Fall of the Leaf. — Throughout the summer the leaf
lives this intense life, thriving in the sunshine, and produc-
ing a store of food-stuffs laid up in reserve in different parts
of the plant. But in autumn the vitality is checked, the
supplies of water which the leaves demand is no longer
afforded, transpiration and the movements of the sap
become very slight, and the leaves begin to die. But
before they die they surrender all that remains of their
life to the plant which bore them ; before the breath of
approaching winter all that is worth having of sugar and
more complex stuffs ebbs in gentle current from the leaves
to the stem. Then the leaf, useful in dying as well as in
living, begins to be cut off, for while the retreat of the
residues of the leaf’s life is being accomplished, there has
also been preparation for the leaf’s fall. Across the base
of the leaf-stalk, in a region which is normally firm and
tough, there grows inward a partition of soft juicy cells,

1 88 Chapters in Modern Botany chap.

actively multiplying and expanding into a springy cushion,
which either foists the leaf off, or makes the attach-
ment so delicate that a gust of wind serves to snap the
narrow bridge binding the living and the dead. That
the scar should thus have been prepared before the opera-
tion is one of the prettiest points of the economy of
woodland nature. The student may profitably collect
instances of the gradual evolution of this, starting from
the ordinary monocotyledons, in which no such adaptation
exists.

Virtually dead the leaves now are, empty houses from
which the tenanting molecules of living matter have van-
ished, leaving little more than the ashes, on the hearth.
But these ashes — how glorious ! for in yellow and orange,
in red and purple, the leaves shine forth, glowing in the low
beams of the autumn sun.

Sometimes it is the green chlorophyll which breaks up,
and leaves little heaps of yellow grains which gladden our
eyes in golden leaves. Sometimes, on the other hand, amid
the flux of molecules inwards from the leaf, there appears
a special decomposition product — a pigment of death — an-
thocyan, which along with the acids so often present stains
the leaf with red, or without the acids gives us bluish-
purple, or along with the yellow grains above mentioned
shines out in bright orange.

Then the leaves, flushed in death, fall gently from the
trees, or writhing and rustling in the wind, as if loath to be
separated, are finally wrenched off and scattered. It is the
curfew of the year, and the poets listen mournfully to “ the
ground-whirl of the perished leaves of hope, the wind of
Death’s imperishable wing.”

But as the species lives on while individuals die, so the
tree is hardly impoverished when the leaves fall from its
many branches. Over the broken parts the partition mem-

IX

Leaves

189

brane completes its healing salve and protective scar, while
the fallen leaves, weathered, faded, and torn, are mouldered
by fungi and bacteria, covered up by the earth-worms, and
gradually form the soft mould in which are born the seedlings
of another year.

CHAPTER X

SUGGESTIONS FOR FURTHER STUDY

Root and Stem — Flower , Fruit , and Seed — The Web of Life once
more — Systematic Botany — Morphology of Organs and Tissues
— Evolution.

Root and Stem. — The general idea of the function
and structure of the leaf which we have now obtained,
obviously prepares us to ask questions as to the stem and
root. How does the leaf get its water of transpiration, its
salts of assimilation ? And as these have obviously to be
absorbed before they can be carried up to the leaf, it may
be literally as well as metaphorically most profitable to
begin “ at the root of the matter.” Let us watch then the
growth of rootlets in germinating barley, in a hyacinth and
a potato, each in a hyacinth glass and the like. Let us
carefully wash away the soil from a well-grown pot-plant, to
get an idea of the enormous development of the root-system,
while the attachment of the fine root-hairs to the particles of
soil may be readily studied under the microscope from
a gently washed seedling of grass. Thus questions will
accumulate : How do these root-hairs absorb, and what ?
What elements are essential ? Or again, in a different order
of ideas, How do roots grow ? The former is a matter for
physical and chemical experiment, the latter for observa-

chap, x Suggestions for Further Study 191

tion, helped of course by the microscope ; and thus the
student can work on, his scientific progress being in pro-
portion to the degree in which he finds he can ask reason-
able ordinary child-like questions ; a degree which he must
not be discouraged to find probably at first inverse to that
at which he could “ get up ” a large amount of ready-prepared
information upon the subject. In the same way let him
ask questions of the stem, and devise, even if he cannot
always execute, experiments. How is the stem constructed ?
will be best understood if he does not begin with the book,
but by scraping and slicing at a few twigs of laburnum, ash,
lime, and oak, till he has seen all that naked eye and lens
can see, and so on. Then the anatomical detail of the text-
books will become of interest, and therefore of true service ;
it helps to clear up how the stem works as well as how it
grows ; and thus the simple outline but complex detailed
discussion of the “ circulation of the sap,” of the “ thicken-
ing of the stem,” and the like, becomes gradually familiar
and interesting. From simple outlines, like those of Stem
and Root in “ Chambers,” the student may in fact pass direct
to Detmer’s experimental handbook, and to the convenient
survey of the question of sap-circulation in Marshall Ward’s
Timber and Tunber Trees (Nature Series, 1889), and to that
of stem-structure in his little book on The Oak (Modern
Science Series, 1892) ; thence returning to the larger text-
books.

Flower, Fruit, and Seed. — Leaving now the problems
of vegetative life for those of reproduction, of self-main-
taining for species maintaining, a new field of study, the
most fascinating of the science, opens before us. Here
again, beginning with the actual floral procession of the year
in fields and garden, we gather and inquire ; our books,
always kept in subordination, mere accessory helps to the
study of the phenomena themselves. If the student be by

192 Chapters in Modern Botany chap.

this time willing to take the writer for examiner, he will be
asked, as always, not to answer questions — with second-
hand information — at the end of his day’s work ; but to ask
questions — about such actual flowers and fruits and seeds as
he can see — at the very beginning ; this done, it is the books
and the teacher whom he has to examine, the limits of their
knowledge which it is his business to work up to and to
define. He must look how flowers are arranged before
“reading up Inflorescence” ; and take them to pieces before
reading of sepals and petals, of stamens and ovules ; he
must puzzle about what pollen and ovules are for, he must
watch the bees and butterflies among the flowers, and find
out which flowers no insects go to, before reading of insect-
and wind-fertilisation ; then the books of Grant Allen and
Sir John Lubbock may be read one summer as pleasant
introductions to the larger volume of Hermann Muller (the
standard work of reference upon the subject) for the next.
Finally its bibliography, supplemented by that of MacLeod,1
is exhaustive. Kerner’s Flowers and their Unbidden Guests ,
and his Pflanzenleben , now being translated, will also be
of special interest.

This widening knowledge of flower - function will of
course involve an increasing minuteness of observation in
detail, and the Darwinian interpretation of the utility of even
the smallest of these — the shape and relative position of
parts, the colour, the markings, the perfume — will give them
interest. Thus arises a knowledge of flower form not only
far more interesting and more genuine, but more per-
manent and more intimate and thorough than that of the
conventional “ anatomy before physiology ” method, against
which this little volume is a continuous protest. For it is
only when we have first seized the essential parts of the
flower (stamens and carpels), and seen how they are adapted
1 Botanisches Jaarboek , vol. ii. 5

x Suggestions for Future Study 193

to cross or self- fertilisation, and thereafter the petals and
sepals, as at best accessories to this main function, direct
or indirect, mechanical or attractive (if not largely
subordinate to mere external and protective purposes), that
our morphological interpretations can become either safe
or clear.

Coming in the same way to the study of the fruit, let
the student at first severely leave alone the competing
classifications, the arid and excessive nomenclature of the
text-books, and ransack the fields and garden, the fruit-
shop too, if it be summer-time, the latter at least if it be
winter. To crack an almond and find out what corresponds
to its shell and kernel in cherry, plum, and peach, is no hard
lesson ; precisely to compare any of these fruits with a pea-
pod may at first puzzle the beginner, but the pleasure of the
discovery will outweigh its pains; or similarly to ascertain
exactly wherein they agree with and differ from apples and
pears, and to make out for oneself the interest in this
whole connection of the fruit of Cydonia japonica , so com-
monly grown on walls for its beautiful scarlet apple-blossom.
What is a raspberry ? how does it resemble and differ from
the cherries and plums when ripe, from a strawberry when
young ? What is a strawberry? Is not its flower practically
the same as that of an apple ? Think out, then, how this
wide difference (yet deep resemblance) of fruit must arise.
Again, what is it that we eat in the orange? Dissect it care-
fully, and look at it in water. Return for more light on the
question to the unlikely pea-pod ; if this does not readily reveal
its open secret, go to the bean ! The subject abounds with
fascinating puzzles of this sort, some easy, some difficult ; in
fact, as the rose family well show, the fruit often tends to
be more Protean in variety than the flower. These asked
and answered, or at least honestly puzzled over, any new
set of specimens, say a handful of beechmast, hazel-nuts,

o

194 Chapters in Modern Botany chap.

and acorns, becomes as welcome and interesting as it could
be to a child. Sir John Lubbock and Mr. Grant Allen
again offer a pleasant guidance to the Darwinian interpreta-
tion of the fruit, while the simplification of nomenclature and
the common sense of classification may be conveniently
entered by help of Chambers’s “Fruit.” Similarly with
the study of Seed we must start with observation, say with
our almond again, and work out a new chapter of our
scientific education.

The Web of Life again. — Our chapter of the web of life
dealt with the vegetative system almost alone, yet was com-
plex enough; here the introduction of the reproductive
system makes the drama a far more complex and fascinat-
ing one. Each plant, in fact, like man himself, has many
relations to the world around, and the botanist thus becomes
a biographer of each ; yet though materials abound, the
full life -history even of the commonest plants has still
to be written ; and the student, as Prof. Balfour has well
pointed out, may do good service to science by following
this from seed to seed again, and year by year.

Here then is an incentive to the study of the complete
flora ; since every plant is not merely a new specimen to be
preserved or analysed, a new beauty to be admired, but a
new life, with an individuality and a history of its own.
From landscape we come back to foreground, from the idea
of vegetation to that of flora.

Systematic Botany : its Methods and Results. —
To examine our individual specimen and recognise it with
certainty is a matter needing no small precision in detail ;
we have to draw with accuracy, and to describe with no less
precision, the criterion of a good description being that a
drawing can be prepared from it by a draughtsman who has
never seen the plant. One is constantly asked by some
friend returning from a walk or greenhouse visit the name

x Suggestions for Future Study 195

of some plant whose beauty has excited a momentary
enthusiasm, but cross-examination leaves in nine cases out
of ten its beauty wholly indistinct : not a word of intelligible
(i.e. intelligent) description can be given. Here is a real
disclosure of deteriorated observing power, of utter vague-
ness of memory and language, on which the conventional
educationist would do well to ponder, and which is by no
means limited to things botanical.

This matter of systematic botany is in fact one which
the most strictly linguistic of educationists would do well to
consider. Limits of space prevent any adequate present-
ment of its advantages ; but the proposition may be formally
advanced, with definite offer of detailed evidence to any
teacher who cares to inquire further into the matter, that
here is assuredly a subject from which the accurate use of
language may be rapidly and surely learned (probably more
rapidly and surely than from any other study) ; clearness and
order, precision of details yet subordination of these in re-
spective rank being carried out here — thanks to Linnaeus — -
as in no other science. Here, however, a mere suggestion
must suffice ; so let the reader try to write a description for
himself of primrose and snowdrop, violet and hawthorn,
daisy and dandelion, so that a correspondent, say in tropical
India, should be able to tell what they were like ; then com-
pare these with the descriptions in the appendix of Lindley’s
School Botany (a treatise well worth hunting for through
old book-shops), and see whether botany has not something
to teach him in the use of language, and what is the real
need and justification of scientific nomenclature.

Systematic Botany. — We have not only to describe
our plants, but to name and classify and group them ; and
here again begins the vastest chapter — say rather literature
— of the science. How we are to name, and why we must
name in the universal language ; what are the difficulties

196 Chapters in Modern Botany chap.

of this problem, and what its history ; what the advantages
of the binomial nomenclature, defined and organised by
Linnaeus ; and what we are to understand by the much
disputed terms of species and genus — here are some of the
initial questions to be asked. Again, how shall we arrange
our genera into larger groups, of order, class, sub-kingdom,
and the like ? How did botanists classify before Linnaeus ?
What was his essential work, and what was the character
of his Artificial System ? How was his work continued
and developed by his school, and how far was it altered and
improved upon in the Natural Sy stein of De Jussieu ?
What of the subsequent development of this ?

Nor can we answer these questions without asking more.
The most concrete way of doing this is, returning to the
landscape and foreground, the vegetation and flora, of our
former problem to endeavour to make from these an ordered
whole — a botanic garden ; or if this may not be, at least
a hortus siccus , a herbarium, which, because of its very dry-
ness, may readily become far the completer garden of the two.
Still the garden has even more advantages than the at first
sight obvious ones, and as space forbids even outlining these,
it must suffice here to press upon the student the desirability
of discovering these by experience. A botanic garden need
not be on the scale of that of Kew or Edinburgh ; a small
college may have its acre or two, a school its rood or two,
an infant school its pole or two, even the dismallest of town
schools its window-boxes now, as by and by its little con-
servatory. For help as to practical details, the student or
garden-loving teacher may safely apply to any botanist of
his acquaintance, or failing such acquaintance, to the writer,
who has annual experience of this kind.

Some account of the vegetable kingdom can be obtained
more easily than any other botanical information, although
even here again, as so often, better in foreign text-books

x Suggestions for Future Study 197

than in English ones. Warming’s Systematische Botanik
will serve the student as the best guide to Baillon’s Histoire
des Plantes and Engler and Prantl’s more full and recent
Pflanzenfamilien , while a more modest acquaintance can
be obtained from any of the ordinary manuals, large or
small. The article, “ Vegetable Kingdom,” of Chambers’s
E?icyclop<zdia and the Britannica may also be consulted ;
with their minor articles on the particular natural orders
and on special plants ; while the historical evolution of
this field of science is outlined in Chambers at Biology,
Botany, and Botanic Garden.

Morphology of Organs (Comparative Anatomy). — With
our widening survey of plants increases our knowledge of
bionomics, our grasp of individual and general physiology,
but also our anatomical knowledge and skill. We thus
acquire a wide acquaintance with the forms of leaf, of stem,
and root, as well as with their life ; why should we not
generalise this knowledge also, and consider each of these
organs apart from its function; so developing a highly
abstract science of “pure morphology” — a crystallography,
as it were, of the organic world ? A leaf in the physiological
sense we saw to be the organ of transpiration and assimila-
tion ; but a stem may do the same, even an unearthed
potato. Shall we define it by its usual form, its bilateral
symmetry, its two surfaces ? But to these characters there
are many exceptions ; and, moreover, stems may flatten
out into what are, in general aspect as well as function,
excellent leaves. Seeking a yet more general character,
we say, morphologically speaking, leaves are appendages of
an axis — the stem, i.e. arise upon it, and never from each
other. Now arises a new set of physiological difficulties,
well seen in the prickly pear. Here the great flattened
joints, so like huge succulent leaves in appearance and
function, are branches, i.e. secondary axes ; the true leaves
O 2

198 Chapters in Modern Botany chap.

are here the prickles. True, however, is here an unsatis-
factory and only half-true term ; in this contrast between
form and function we have at the earlier planes of evolution
what in our own plane we recognise so constantly as the
difficulty between law and equity, letter and spirit. De jure
it is the prickles which are the leaves, since historically
and formally they represent them ; no matter though de
facto the stems have long ago taken up their whole
functions. In the same way the Merovingians are kings
de jure , long after their mayors are kings de facto . Here,
in fact, we have a curious result, carrying with it a fresh
result upon the theories of education ; for the student may
easily think out for himself the way in which we may either
start from the one side or the other. That the “ type
system of teaching,” with its precedence of anatomy over
physiology, is no mere survival of the authority of Cuvier,
but is in a piece with the antique academic programmes
generally, he will now more clearly see ; as further, that
the “ life-history” (as distinguished from “ form-description,”
or even “ form-history ”) system of teaching here advocated,,
with its precedence of physiology before anatomy, while
in the first place an attempt at utilising the method of
Darwin in biological teaching, is congruent with those wider
changes in education now happily germinating everywhere,
and of which this University Extension movement, with its
published volumes, its enlarging summer meetings, may
claim to be one of the largest seedfields.

Returning, however, to pure morphology, although no
outline can here be given, some notice must be taken of
that generalised conception which has been prepared for in
the preceding paragraph, since it is the most characteristic,
or at least the best-known, result of the science.

We have seen that the young flowering -plant — the
seedling — consists of an upward-growing axis or stem and

x Suggestions for Future Study 199

a downward-growing axis or root, and that the young stem
bears on its sides one or two seed-leaves or cotyledons.
Here we have the simplest form of higher plant — an axis
with appendages.

And when we study the various appendages which are
borne by the upward-growing axis or stem, we see that they
are all fundamentally the same. The seed-leaves are not
unfrequently of the same general type as the foliage-leaves ;
the latter are connected by gradations with the scales
which are wrapped round the buds, and the bracts which lie
at the bases of the flowers. The transition from bracts to
calyx may be conveniently studied in the mallow, that
from sepals to petals in the cactus, that from petals to
stamens in the water-lily or in almost any garden rose
(which, indeed, appears to have suggested the whole theory)
and that from leaves to carpels in many monstrous flowers,
especially the double cherry. Almost every one has seen
some flower or other in which all the parts had reverted to
the state of green leaves.

For further corroboration of this idea that all the
appendages of the axis are fundamentally the same in
structure, or are, in a word, “ homologous,” we must
watch the development of these parts. In so doing we
find the theory adequately confirmed ; leaves, bracts, sepals,
petals, stamens, and carpels all develop as precisely
similar processes of cellular tissue from the sides of the
axis.

This conception of the plant as an axis bearing variously
modified but homologous appendages, floated before the
eyes of Wolff and Linnaeus, was more clearly grasped by
the poet Goethe, was systematised by De Candolle, and has
since been corroborated and amplified by the progress of
morphology. See Morphology ( Encyclopedia Britannica.')

Morphology of Tissues. — But axis and appendages

200 Chapters in Modern Botany chap.

alike consist of first embryonic and then developed tissues ,
the same which in the leaf we have already learned to
know as epidermic, fundamental, and fibro- vascular. The
study of these in all their forms and modifications, from the
physiological and the morphological point of view alternately,
the continued analysis of tissues into cells , and the fuller
study of this ultimate unit-mass, and of the protoplasm of
which it is composed, is the new subscience of histology so
important to physiology and morphology alike. Were new
chapters (say rather volumes) available, we should work
out generalisations of no less interest and value than any
preceding ones ; we should study the growth and develop-
ment of the plant by the multiplication and differentiation
of its primitive embryonic cells ; and we should trace these
back to the fertilised parent cell or plant-egg, and though the
simple morphology of protoplasm would yield us little, its
physiology would carry us far. See Cell of “ Chambers,5’
with Bower’s Practical Botany as convenient laboratory
guide to the larger manuals of Van Tieghem and De Bary,
etc.

It is not, however, with this last product of analysis that
our studies would close ; we need a returning physiological
synthesis. That is to say, we should trace our initial egg-
cell onwards into organism again ; this, however, no longer
viewed from without as a specimen or form for analysis,
but as a working thought-model , and this in actual growth,
change, and evolution. The reader’s knowledge of the leaf,
for instance, has to be built up into such a mental image.
The up-stream of transpiration has not only to be seen
by itself, exhaling its rising fountain into unseen spray, the
stomata adjusting their openings meanwhile, but this must
be combined with a picture of all that we know of the process
of assimilation and its resultant downward stream ; so with
the whole plant, so with its changes throughout the year, so

x Suggestions for Future Study 201

with the living world. In this process of scientific imagina-
tion let the student by all means examine himself ; he
will thus not only find how little he knows more surely than
on more customary methods, but feel keener interest in
filling up the blanks in his knowledge ; nay, even make
far more rapid and more permanent progress, since we
remember what we want to know far better than what we
get without asking. This habit learned, the student may,
with increasing advantage, direct his own further studies.

Evolution. — The student who has thus from the outset
been at the evolutionary standpoint will have little difficulty
in continuing his reading. Summaries which may be of
service in reading the Origin of Species and other essential
literature of the subject, together with some outline of the
state of contemporary discussion, will be found in the
writer’s articles Darwinian Theory and Evolution of
Chambers’s Encyclopcedia , and Variation and Selection
of the Encyclopcedia Biitannica. Mr. Wallace’s Dar-
winism and Dr. de Varigny’s Experimental Evolution ,
both already mentioned, notably of course also Mr. Romanes’
Darwin and After Darwin , may next be read ; and from
these it is easy to gather references to the remaining (often
more or less anti-Darwinian) literature. But in this, more
than in any previous field of study, the student will need to
keep his own faculties, observant, critical, and reasoning,
fully alive.

THE END

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