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A POPULAR TREATISE

PHYSIOLOGY OF PLANTS

A POPULAK TREATISE

PHYSIOLOGY OF PLAITS

FOR THE USE OF GAPiDENEES

OR FOR STUDENTS OF HORTICULTURE AND OF
AGRICULTURE

BY

Dr. PAUL SOEAUER

DIRECTOR OF THE EXPERIMENTAL STATION AT THE ROYAL POMOLOGICAL
INSTITUTE IN PROSKAU (SILESIA)

TRANSLATED BY

F. E. WEISS, B.Sc, F.L.S.

PROFESSOR OF BOTANY AT THE OWENS COLLEGE,
MANCHESTER

WITH THIRTY-THREE ILLUSTRATIONS

LONDON
LONGMANS, GREEN, AND CO.

AND NEW YORK: 15 EAST 16'h STREET
1895

4U rights reserved

Printed by Ballantyne, Hanson & Co.
At the Ballantyne Press

TRANSLATOR'S PREFACE

In undertaking a course of lectures on Vegetable Physiology
at the Eoyal Botanical Gardens in Manchester, I felt very
much the want of a book which I could place in the hands of
the gardeners and students of horticulture who attended the
course, and which, while giving them a thoroughly scientific
account of the functions of the various organs, would at the
same time deal with the practical applications of the principles
of Vegetable Physiology.

Such a book was accessible to me in the German in
Professor Sorauer's admirable " Populiire Pflanzenphysiologie,"
and I should have been thankful if, at the time, a translation
had been available for my hearers. Professor Sorauer is well
known as an authority on the diseases of plants, but he had
an additional qualification for writing his book, namely, that
he was for many years the director of an institution which
had in view the scientific training of gardeners and agri-
culturists.

We are not so fortunate in this country as to possess such
well-appointed and State-endowed experimental stations as
those which exist in Germany, and we must for the present
be content with the more humble ventures instituted by the
County Councils. In all such educational courses as they may
institute, it is to be hoped the scientific principles which under-
lie the practice of horticulture and of agriculture will not be
lost sight of, and those intrusted with the teaching of tliese

\«»

^^

vi PREFACE

principles will do well to follow the lines laid down in this
treatise by Professor Sorauer, and to discuss the often very
direct bearing of the physiological truths upon the questions
which exercise the minds of gardeners and agriculturists.
For these the book is directly written, though other students
of botany will find much useful information in this treatise,
emphasis being laid by the writer upon the fundamental
truths by a repeated treatment of them under various aspects.
In conclusion, I would tliank Dr. Maxwell T. Masters for
information on some of the technicalities of horticulture,
always given so freely and so kindly.

F. E. WEISS.

TABLE or CONTENTS

CHAPTER I

INTRODUCTION

:c.

[. What conception should a gardener form of a vegetable or-
ganism ?

2. What are the functions of the various organs of a plant ? .

CHAPTER II

THE STRUCTURE OF THE ROOT

3. What is the structure of the al)sorptive organ, the root 'I- .
(re.) The absorj)tive portion of the root.

(b.) The root-tip

(c.) The conducting portion of the root

{(I.) The process of conduction ....

CHAPTER Til
THE NUTRITION OF THE ROOT

4. What substances must be present in the soil for the continuous

and effective nutrition of plants 1 . . . . . .30

5. What is the effect of the various nutritive substances on the

plant, and in what form do they enter it ? . . . .31

6. What part is played by the subsidiary nutritive substances and

the occasional admixtures to the nutritive salts '? . . -35

7. In what form does the I'oot find the nutritive substances in the

soil? 39

(a.) Potassium 41

(6.) Phosphoric acid 42

{(-.) Nitrogen 42

viii CONTENTS

SEC. PAGE

8. Why and how should we replace the chief nutritive substance

in the soil 1 .......... $2

(a.) Fallow . . 52

(h.) Inorganic manures 53

(c) Organic manures 55

(d.) Stable manures 56

9. How can the soil best meet the requirements of the roots for

air? 61

10. How can we improve our fields so as to obtain the best

possible crops ? 65

1 1. How is the nutrition of pot-plants effected ? .... 70

12. How do aerial roots nourish a plant? 75

13. How do ordinary roots obtain their necessary supply of air ? . 77

CHAPTER IV
THE TREATMENT OF ROOTS

14. How should roots be treated in transplanting ? . . . 81

(a.) A root system which has been considerably pruned . 81
(6.) A root system from which only the delicate roots

have been removed 85

(c.) The treatment of roots in re-potting .... 85

(d.) The treatment of roots in transplanting in the open . 87

CHAPTER V
THE STEM

15. What is the structure of the stem ? 94

16. What is cambium ? 98

17. "Wliat is the function of the cambium in the ordinary course of

growth? .......... 103

18. How does the stem as a whole perform its functions?. . . 105

CHAPTER VI

THE LEAF

19. Which cells of the leaf are the most essential ? .

20. How are the assimilating cells of the leaf protected ? .

21. How are the assimilating cells arranged within the leaf ? .

22. How is the leaf developed ?

23. What substances does the leaf chiefly form ? . . .

24. How does the leaf actually perform its assimilatorv function ?

108
no
118
120
122
127

CONTENTS ix

CHAPTER VII
THE TREATMENT OF THE SHOOT

SEO. PAGE

25. Why must the shoots of our cultivated jilants l)e pruned ? . .134

26. What is the least injurious form of a cut? 135

27. How does summer pruning differ in its efl'ects from winter

pruning? . • 137

28. What is the effect of the different methods of pruning? . .140

29. How may pruning be used to regulate the natural development

of the tree? 142

30. When is pruning harmful ? . . . . . . .146

31. In what way can the natural process of healing be accelerated ? . 149

32. By what means can we increase the effect of pruning? . . 155

(a.) The bending of shoots 155

(b.) The twisting of shoots 157

(c.) Notching 158

(d.) Ringing 159

(e.) Peeling the stems 161

33. Why do we slit the bark of trees ? 164

CHAPTER VIII

THE USE OF SHOOTS FOR PROPAGATING

34. What is meant by layering, and of what use is it? . . .169

35. What rules should be followed in striking cuttings? . . . 172

36. What objects have we in view in budding and grafting, and

how are these operations best performed ? . . . .183

37. To what extent do scion and stock mutually influence one

another? 194

CHAPTER IX

THE TREATMENT OF LEAVES

38. What is the effect of injuries to the leafy tissues? . . 199

39. In what cases can the leaf be used for propagation ? . . .201

CHAPTER X
THE THEORY OF WATERING

40. Why must we pay special attention to the watering of plants ? 205

X CONTENTS

CHAPTER XI

THE FLOWER

SEC. PAGE

41. Of what jiaits does the tiower consist? . . . .214

42. How are single and how are double flowei's developed '? . .218

43. Can a gai'dener determine the development of flowers ? . .221

CHAPTER XII
FRUITS AND SEEDS

44. How are fruits and seeds formed ? 226

45. How can the formation of fruit be influenced by ditterent methods

of cultivation ? 233

46. What are the conditions governing the production of seeds ? . 238

47. How should the ripe seed be treated ? 244

INDEX 253

THE PHYSIOLOGY OF PLANTS

CHAPTER I

INTRODU CTION

§ 1. What conception should a gardener form of a
vegetable organism ?

It is the business of the gardener and of tlie horticulturist to
cultivate plants. This term implies the guiding of the natural
development of a plant towards some special end. For a
gardener ought to know how to make use of the natural pro-
cesses and of the development of the vegetable organism in such
a way as to realise as completely as possible the end which his
cultivation has in view. His endeavours may be of various kinds.
In many cases he only tries to cultivate a plant in its normal
and natural form in some special locality. This is usually the
case in landscape-gardening or in the cultivation of plants
from warmer regions in our own climate. In other cases, which
we might call "true cultivation," the gardener seeks to make
a certain plant more useful for his purposes by increasing its
productivity and improving the quality of its fruits. This is
clearly the case in the culture of those plants which are ^rown
as vegetables, in which case, those organs which are used as
food are increased in size and number, and are considerably
improved in delicacy. Here the gardener interferes with the
normal development of a certain species, so as to increase
certain functions of the plant, and thus causes an increased
development of certain organs. In other cases, the time of
flowering or fruiting of a certain species is changed ; as, for
instance, when we wish to obtain spring flowers in winter

; newatrr ukurt

N. C. Suae CtUege

2 THE PHYSIOLOGY OF PLANTS

{forcing), ov, on the contrary, when we wish to produce spring
flowers late in the summer.

We are able, also, by certain methods of cultivation, to
change the colour of flowers, or to cause them to double by a
transformation of stamens into petals, or by the formation
of new whorls of leaves. AVe can cause hard roots to become
thick and succulent, as in the beets, and hard fruits to become
large and juicy by an increase of their softer tissues (apples
and pears) ; and we may therefore consider ourselves able to
a certain degree to modify the characters of the vegetable
organisms, and to partially change their normal development.

A plant must not, therefore, be looked upon as an unchange-
able organism, restricted to a definite form, but as a plastic
organism, capable of i'urther modification in all its parts. Its
usual shape can be altered as if it were made of wax, and it
can be remodelled within certain limits. This remodelling of
the form of a plant is, however, only possible if the gardener
understands how to regulate the conditions of its life, so that
without damaging the whole, i.e., the life of the plant, the
functions of the various organs may be increased beyond the
normal amount or reduced below the usual limit in favour of
some other organ. To effect these changes, it is naturally
essential to possess an insight into the various processes which
make up the life of a plant, and to understand how they are
affected by various external conditions — in fact, to possess a
knowledge of vegetable physiology.

But though our knowledge of vegetable physiology is by no
means complete, still we have obtained some insight into many
of the processes of plant-life, and a gardener must learn how
to make practical use of this knowledge. For this reason we
start by asking :

§ 2. What are the functions of the various organs of a plant ?

A plant presents itself to us as a complex organism, built
up of a number of chambers or cells, each of which has its
special and, to a certain extent, independent function. The
functions of all cells of the body have two common objects in
view, namely, the preservation of tlie individual and the pro-

INTRODUCTION 3

ductioii of material vvliich may be employed in the formation
of new individuals. These latter are formed in the seeds, the
function of which is to perpetuate the species.

Of the various members of the vegetable organism, the
iiowers have the function of producing and developing new indi-
viduals or offspring from the organic material with which they
are provided. The leaves, on the other hand, are concerned in
the production of the necessary food material. This is effected
in the green cells of the leaves under the influence of light
and from the raw material, which they obtain partly from the
atmosphere (carbonic acid), and partly from the soil in the
form of mineral salts dissolved in water. It is the function
of the roots to fix the plant in the soil, and to absorb from it
the soluble mineral salts.

CHAPTER II

THE STRUCTURE OF THE ROOT

§ 3. What is the structure of the absorptive organ, the root ?

We are first of all interested in the general structure of the
root. This organ represents the direct downward continuation
of the stem, and presents, like the latter, a central cylinder
(Fig. I G), which is surrounded by a softer tissue, the cortex
(Fig, I JR). As the woody cylinder of the uninjured root
never reaches the periphery, but is always surrounded by
cortical tissue, it is necessarily the latter which is especially
concerned in the absorption of nutritive substances from the
soil. The central woody tissue, in which the naked eye can
often distinguish large wide vessels {G), has the function of
conducting the dissolved substances to ail parts where they
may be required.

(a.) The Absorptive Portion of the Boot.

If we take a healthy plant carefully from loose soil, so as
not to injure the delicate ramifications of the root, and if we
wash the latter under a moderately strong jet of water, it will
be possible to free the roots almost entirely from the adhering
particles of soil. Only one portion of each rootlet will retain
its covering of grains of sand or particles of earth, and these
will seem to be stuck on to the root. With the aid of a
microscope we observe that delicate contorted root-hairs (Fig.
I H) have in reality fixed themselves to these small particles
of the soil and hold them very tightly. That region of the
root which is provided with this covering of sand is the portion
which is able to take up water from the soil. The absorptive
region of the root is therefore restricted to a certain youth-
ful zone, which begins close behind the smooth, often trans-
parent root-tip and reaches back about an inch. The older^

THE STRUCTURE OF THE ROOT 5

brown portious of the root are useless for the purpose of
absorption.

If we examine such a root-hair from its tip to its point
of origin, we find it to be a slender tube filled with liquid,
and projecting like the finger of a glove from one of the
external cells of the root. These cells, which form the outer
layer (epidermis) (Fig. i U) of the root, and which are very
thin-walled in this region, have, by elongating to form root-
hairs, immensely increased their absorptive surface. The
root-hairs, therefore, enable the root to present in a very
small region an enormously large absorptive area.

As the chief substance which is to be absorbed by the root
is water, we find that the development of the absorptive sur-
face of the root is often directly proportional to the need for
water. In the case of aquatic plants, for instance, in which
the entire plant is constantly in contact with water, there is
no need for an increase of the absorptive surface, and the
production of root-hairs seems unnecessary. As a matter of
fact, the roots. of these plants are often destitute of root-hairs.
The roots of the water-hemlock and other plants have been
observed to be smooth as long as they live in water, but as
soon as the plants are grown in dry soil, their roots become
clothed with root-hairs. In typical land plants, on the other
hand, the necessity for an increase of the absorptive surface
will become greater the larger the leaves are, and the greater
therefore the transpiratory surface becomes. Plants, on the
other hand, possessing narrow, tough leaves, which reduce the
transpiration, will be able to absorb sufficient water even if no
root-hairs are developed. This indeed is the case, and the
roots of various Conifers which bear needle-shaped leaves have
been found to be devoid of root-hairs, the thin flattened layer
of epidermal cells being able to absorb sufficient water for the
needs of the tree. Sometimes on the same plant some roots
may be devoid of hairs, while others are sparingly, others
largely covered with root-hairs, and this will depend upon the
facility which the roots experience in obtaining their water
supply. The more difficult they find it to obtain water, the
more perfectly will the absorbing hairs be developed. In
perfectly dry soil, too, the development of root-hairs will

6 THE PHYSIOLOGY OF PLANTS

naturally be stopped, as the root will scarcely grow at all.
The development of root-hairs is most abundant in a damp
atmosphere.

We have already mentioned that the particles of soil cannot
be removed from the absorbing portion of the root, but adhere
to it pretty firmly. If seedlings are grown in a damp atmos-
phere on moderately damp sand, in a short time many root-hairs
will be found which have come in contact with grains of sand.
Where this is the case, they will flatten themselves against
the particle, will curve themselves round it, and may grow
on to repeat the process with another grain of sand. The
cessation of growth in length and the expansion of the point
of contact are not, however, the only phenomena which make
their appearance when a root-hair touches a particle of soil.
The outer wall of the root-hair will be seen to lose its sharp
outline and swell up into a slimy mass. It is this mucila-
ginous coating which fastens the grains of sand so firmly to
the root-hairs. Now let us imagine that instead of a grain of
sand, the root-hair has grown round a particle of soil which is
covered with a thin deposit of the salts necessary for the plant.
As a matter of fact, the mineral salts which are most essential
to plant life are absorbed by the soil, and are mechanically held
by each particle of soil, together with a thin pellicle of water
which surrounds each particle. Now when a root-hair comes
into close contact with such a particle, the mucilage will help
to dissolve the salts, and the latter pass through invisible pores
of the membrane or cell-wall into the root-hair (see Fig. i H),
saturate the liquid or sap it contains, and permeate the
membrane which separates it from the next cell within, belong-
ing to the region B. If this inner cell of the cortex is in want
of more nutritive matter, it will absorb some from the root-hair
and satisfy its own wants. But that will not conclude the
matter, for the neighbouring cells will in their turn absorb
from the cortical cell, only, however, to pass on these substances
to still more eager cells which lie above them. In this
way a passage of the nutritive substances takes place to-
wards the upper regions of the plant, towards the growing
points, which are always in want of food material, and as these
substances are constantly being used up in these regions, the

THE STRUCTURE OF THE ROOT 7

lower cells, from which they have been withdrawn in their
turn, absorb from still lower cells, and these ultimately from
the root-hairs. Thus a continuous current of nutritive salts
is established through the cell-walls, beginning in the root-
hairs and extending to the vessels which occupy the central
cylinder of the root, and through which they are forced with
great rapidity to the upper regions of the plant.

But even solid mineral particles can be made use of by the
root-hairs. For these cells give off carbonic acid and also non-
volatile acids, which act upon the particles, dissolve them,
and absorb the solution. This process can be very readily
and effectively demonstrated by growing seedlings under a
very thin covering of soil on a polished piece of marble. After
a short time those rootlets which have come in contact with
the marble plate will be found to have etched their course over
its surface. This they will have done by dissolving away the
marble from those places where they have been in contact
with it.

(b.) The Boot- Tip.

It is of great importance to the gardener who wishes to
regulate the growth of roots to suit his own purposes, to
understand the structure of that part of the root in which
growth takes place. The latter depends, first, upon the forma-
tion of new root-cells ; secondly, upon the elongation of these
cells.

We should, however, be making a mistake were we to
assume that the formation of the tissues which cause the
elongation of the root takes place in the same way as it does
in the case of the shoot. In the latter the tip, at first pro-
tected by bud-scales, is afterwards generally uncovered, and is
built up of very delicate, closely packed cells, rich in proto-
plasm and divided by new transverse walls. The extreme tip
of this actively growing region (meristem) is continuously
giving rise to new cells during the whole vegetative period, and
the increase in length of the axis is due to the growth of each
of the new cells to its mature size.

In the case of the root, too, we find, it is true, the same
method of arrangement, and increase of the meristematic

8

THE PHYSIOLOGY OF PLANTS

cells of the root-tip, and the increase in length is therefore
also due to apical growth. But this root-tip is not free; it is
covered over by a cap of parenchymatous cells called the root-

FiG. I. —Lateral root growing out from the Stem of a Potato.
G young central cylinder, bounded by rows of wood vessels ; R, cortex ; E, epidermis ; H,
hairs. The separation of these tissues can be traced into the youngest portions of the
root-tip in this meristematic region. PI {Plerom) is the tissue which will continue the
formation of the central cylinder ; P (Periblem) the seat of formation of the cortex, and
D (Dermatogen) the initial cells of the epidermis. The delicate meristematic apex of the
root itself is covered by the root-cap (IF), the outermost cells of which (a) are gradually
becoming disorganised while internally (within;?;) new root-cap cells are being formed.

cap (Fig. I WW). The layers of cells forming the latter
are so arranged that the oldest layers (a) lie nearest the out-
side, the youngest layers (h) next to the root-tip ; here the
root-cap is connected with the root-tip by a layer of cells
common to both.

THE STRUCTURE OF THE ROOT 9

If we imagine a finger covered by a thimble, which adheres
to the finger by its internal surface, we have a rough but
pretty accurate illustration of a root-apex with root-cap.

Beneath this root-cap lie the tissues of the root-tip, and in
different arrangement according to the different genera. In
the majority of cases a central group of meristeni cells can be
distinguished (Fig. i PI), which give rise to the new portions
of the central vascular cylinder. This central portion is
surrounded by a number of layers of cells (P) which give rise
■to the cortex. A single layer of cells (D) covers in the cortex
over its whole extent, and will form the epidermis and the
absorbing cells or root-hairs. At the very extreme point of
the root-tip these tissues all converge into a group of cells,
■which is the seat of formation of all these layers, and is also
the point of connection between the root-tip and the root-cap.
The root itself, therefore, and the root-cap have in reality the
same point of origin, namely, the extreme apex of the root ; and
in the generality of cases, in dicotyledonous plants, we may look
upon the root- cap as merely a production or proliferation of the
youngest epidermal layer of the root itself.

This protuberance of the root is renewed from its inner
layer, i.e., from its point of connection with the root itself,
while its outer and therefore oldest cells {a) gradually lose
their contents, allow their cell-wall to break down into muci-
lage, and in the end separate from each other and break down.
During this process carbonic acid is given off, and becoming
absorbed by the water, it causes mineral salts, such as phosphate
of lime, which are insoluble in pure water, to become dissolved.

This we may look upon as one of the functions of the root-
cap. The salts which the decay of the older root-cap cells
causes to be dissolved are absorbed by the inner younger cells
and conducted to the centres of growth. To that extent,
therefore, the root-cap may be considered an organ of absorp-
tion ; its chief function, however, is that of a protective cap.
We can readily understand what obstacles a continuously
elongating root-tip has to encounter in its efforts to force a
passage through the firm soil. It requires the application of
special forces and special means of protection. The force
which enables the root to bore its way into the soil is the

lo THE PHYSIOLOGY OF PLANTS

elongation, already alluded to, of the cells lying behind the
root-apex, which have lost their meristeniatic character and
have become transformed into the permanent tissues of the
root. By their growth in length they push the apex forward,
as the older portion of the root is firmly fixed in the soil by
the root-hairs, and cannot therefore be forced back. But how
could a soft root-tip, composed as it is of the most delicate
cells, be forced between the sometimes very sharp particles of
the soil without sustaining severe injury ? Nature has guarded
against any such damage by protecting the root with a cap,
the oldest cells of which are turned towards the outside, and
protect the growing cells at the interior from any harm.

In the case of other organs (leaves, for instance) we can see
how their peculiar development is an adaptation to the sur-
rounding conditions, and we might presuppose the same for
roots ; nor shall we go far wrong if we assume that the pro-
duction of the root-cap is a necessary adaptation of the root
to its conditions of growth, and that it has thus gradually been
evolved.

The centre of growth, the elonfratin" and the absorbing regions
of the root, are all confined to the root-tip, and consequently the
conclusion we arrive at from our preceding observations is that
the first eondition of a rational method of cultivation is to
preserve and increase the number of root-tips.

(c.) The Conducting Portion of the Root.

After having examined the seat of growth in the root, and
having determined the region of absorption, it is necessary to
look for the tissue through which the absorbed water with its
dissolved food substances is conducted. This tissue is the
woody tissue of older portions of the root, which, as can easily
be seen in any root of a tree, forms a hard central cylinder,
and is surrounded by a soft cortical layer. To form the central
cylinder, the elements which we have mentioned as G in Fig. i
become more and more numerous, and group themselves into
thick strands arranged in a ring. These strands are termed the
"vascular bundles." In both Mono- and Dicotyledons they
consist of various elements, some of which are of the nature of

THE STRUCTURE OF THE ROOT

narrow or wider tubes (vessels), while others take the form of
long, narrow, and very mucli thickened fibres (wood cells).

The vessels are
formed of super-
posed cells, which
have become joined
by the loss of their
transverse walls,
and thus form con-
tinuous canals. The
lateral walls be-
come thickened by
the addition of
woody substance
on their inner sur-
face. Tins addition
of substance to the
original cell - wall
(primary mem-
brane), which only
becomes visible as
the vessels become
old, constitutes the
secondary mem-
brane, and appears
sometimes as a
fairly continuous
thickening, leaving
certain thinner
places, the pores or
pits (pitted vessel,
rig. 2 g). In
some cases the
thickening is net-
like (reticidate),
ladderlike (scalari-
form), or presents
the appearance of a spiral band on the inside of the pri-
mary wall (Fig. 2 g'). According to the manner in which

12 THE PHYSIOLOGY OF PLANTS

the thickening takes place we distinguish reticulate, scalari-
form^ spiral, or annular vessels. The fibrous elements which
run between the vessels and give the bundle its rigidity are
shown in Fig. 2 h. They are wood cells which have become
wedged together by their pointed ends, and which, by a chemical
change of their wall {lignification) have attained a great hard-
ness, but at the same time have become very brittle. Their
great length, thickness, and pointed ends distinguish them
from those cells which build up the soft tissues of herbaceous
plants, and which are usually six-sided, thin-walled, and richer
in contents. These latter are termed paFenehymatous cells
(Fig. I B, and Fig. 2 r), while the former constitute the pros-
enehymatous tissues.

The hard bast cells (Fig. 2 h) are a special form of pros-
enchyma ; they are usually much longer than the wood cells,
and often much more thick walled, but they are flexible and
tough, and run on the outside of the woody cylinder, but
parallel to it. The bast used so largely in gardening opera-
tions consists almost entirely of such cells. These hard bast
cells generally form broad longitudinal bands or strips. Pro-
tected by these cells, and between them and the woody
cylinder, we find long thin-walled cells, rich in protoplasm
or tube-like cells, which have this peculiarity, that the trans-
verse walls separating one from the other are perforated by
pores, and they have therefore received the name of sieve-tubes.
These tubes are accompanied by thin-walled, bluntly ending
cells, the bast parenchyma (Fig. 2 hp). In like manner we
find between the wood vessels and wood fibres parenchy-
matous cells, which have, however, also become lignified ; they
represent the wood parenchyma (Fig. 2 hp).

The vascular bundles which we find in plants running
along both the stem and the root, and branching into the
leaves to form a network of veins, consist therefore of two
very distinct portions. The most largely represented is the
wood, consisting of brittle lignified elements (Fig. 2, from g' to
hp, and h). The wood consists of vessels, narrow wood fibres
(lihriform), and wood parenchyma. In the case of Conifers
the first mentioned are replaced by wide vessel-like cells,
which are, however, not continuous, but resemble the wood

TPIE STRUCTURE OF THE ROOT 13

fibres. On account of their similarity with the vessels, how-
ever, which are sometimes called tracheae, they have been
termed traeheids, and they are further characterised by a
curious form of pores termed bordered pits. The traeheids
perform the same function as the vessels of other plants.
They conduct the nutritive salts dissolved in water from the
soil first into the leaves, where, under the influence of the
light and together with the carbonic acid of the air, they are
transformed into organic food matter. The other portion
of the fibro-vascular bundle lies towards the outside, and
consists of flexible, thin-walled sieve-tubes and cambiform
cells rich in protoplasm. This soft portion of the bundle,
which is concerned in the conduction of the organic nitro-
genous food substance, and is termed the soft bast, is generally
protected on the outside by strands of thick-walled hard bast
cells (&).

These bundles, whether they consist of real vessels or
traeheids, conduct along their woody portion the raw materials
which the roots have absorbed, and carry them into the finest
ramifications of the stem. Through their soft bast tubes, on
the other hand, they conduct the elaborated substances which
are necessary for the increase in root and shoot from the
leaves, where they are formed, to the root-tips. As these con-
ducting cells are very delicate, and might easily be compressed
and damaged by any external pressure, we find them generally
protected on the outside by a band of hard bast cells.'^

Let us now examine also in cross-section the structure of
the root which we have studied from a longitudinal section.
We choose as an example the root of a monocotyledonous
plant, namely, that of the Shallot (Allium ascalonicum) (Fig. 3),
and a root of a dicotyledonous plant, that of the Auricula
{Primula Auricula) (Fig. 4). In both figures the soft cortical
tissues, which are of secondary importance in the present con-
sideration, as well as the epidermis and root-hairs, have been
left out, and are only indicated by a few layers of cells at the

^ These protective strands of hard bast have been considered as part of the
vascular bundle, and the hard and soft bast together have received the name
of phloem, while the wood vessels together with the wood fibres have been
called the xylem. Both parts together form i\\e fibro-vascular bundle.

14

THE PHYSIOLOGY OF PLANTS

outside. Tlie chief conducting apparatus for the food material
which has been absorbed by the root-hairs and passed on
through the cortical tissue is the central cylinder, which con-
tains wood vessels, indicated in Fig. i G^ by narrow rows of
cells. These vessels, which appear in Figs. 3 and 4 as darkly
outlined spaces, do not lie singly in the transverse sections,
but are collected in groups. In the root of the Shallot, as in
many other Monocotyledons, the groups of vessels are arranged

Fra. 3.— The Central Vascular Cylinder op the Root op the Shallot
(Allium ascalonwum).
U is the very wide central vessel which joins the five dark rays of smaller wood vessels ;
p is the layer of cells which gives rise to new lateral roots (pericambium) ; s, the endo-
dermis or bundle-sheath ; d, the thin passage cells (after Haberlandt).

in rays, or like the spokes of a wheel round a central very
large vessel (Fig. 3 g), so that we seem to have a single group
of wood vessels running out into five arms or plates (Figs. 3, 4).
In the root of the Auricula (Fig. 4) the seven separate groups
of wood vessels (g) are arranged in a ring round a soft par-
enchymatous central tissue, which is termed pith or medulla.
In the Dicotyledons this pith often replaces the large central
vessel of the Monocotyledons, and the root structure resembles
more closely the structure of the stem. We need only imagine

THE STRUCTURE OF THE ROOT

that the groups of wood vessels (Fig. 4 g) have become en-
larged by the addition of new vessels and wood cells, so as to
touch each other laterally, and then we should have a con-
tinuous cylinder of wood such as we find in any first year's
twig.

Between the dark groups of wood vessels we see in Fig. 4 s
lighter groups of smaller cells. These are the more delicate
passages of the soft bast (lejjtom), which, we have previously
mentioned, conduct throush their sieve-like transverse walls

o|i^ii2&

-^"^oQ'o^n^A^ '

Fig. 4.— Seven-Rated Vascular Cylinder of an Adventitious Root of

Primula Auricula.

g, wood rays ; s, groups of soft bast cells ; p, pericambiiim ; u, bundle-sheath (after De Bary).

the plastic food substances formed in the leaves to the root
system, for the furtherance of its growth. Tlie tissues for the
upward passage of the raw mineral salts which are absorbed
by the roots lie at the side of the tubes, which have to con-
duct the formed substances down from the leaves. Both tissues
are surrounded by a ring of lightly coloured cells, p, the
pericambium. It is in this layer that the first rudiments of
the new lateral roots arise, and in such places the surround-
ing ring of thick-walled cells which protects all the conducting
tissues is interrupted. This ring (Fig. 3 s, and Fig. 4 w) is

i6 THE PHYSIOLOGY OF PLANTS

termed the bundle-sheath {endodermis), and when the soft
cortical tissues of the roots are destroyed, it keeps the central
cylinder intact.

As a matter of fact, the cortex of the root of many grasses
and other Monocotyledons dies off at an early period, and then
the root continues to live and function protected by the endo-
dermis alone.

But a matter of great importance in the structure of the
root of Allium ascalonicuvi (Fig. 3), and one deserving special
mention, is the fact that the interruption of this protective
sheath by thin-walled cells (Fig. 3 d) always takes place
immediately opposite a group of wood vessels.

For if we remember that these thin-walled cells offer an
easy passage for watery fluids, while the thick-walled cells
are rendered impervious by their thick cell-walls, we see that
these cells (d) constitute the points of passage of the water
which has been taken up by the root-hairs, and, after travers-
ing the cortex, has to make its way into the wood vessels, and
through them up into the stem and its branches.

(d.) The Process of Conductioii.

Having recognised the thin-walled cells of the endodermis
as passages through the thick-walled protective sheath of the
root, we have gained a sufficient insight into the structure of
the root, and can now pass on to the consideration of the
physical forces which must necessarily come into play in the
passage of the nutritive solution, in the first place into the
root, and thence upwards into the leaves.

But before we begin to examine these processes, let us recall
to our mind the conditions of Fig. i. The young root there
figured may be looked upon as a compound organ consisting
of various parts. First of all we have the actual body of the
root, the centre of growth of which lies in the tip near D, P,
and Fl. There the cells are in a meristematic condition, like
those shown in Fig. 5.

These cells are still very thin-walled and entirely filled with
an albuminous substance resembling the white of egg, which
is called protoplasm. Embedded in this mass we find a

THE STRUCTURE OF THE ROOT

17

denser body, richer in albumen than the general mass of proto-
plasm, and called the nucleus (Fig. 5 a). Within the latter
we see one or more refringent bodies, the nucleoli (Fig. 5 &).
These delicate meristematic cells, which are constantly dividing,
and are thus producing the elements of new growth, become
larger and more transparent the farther they are removed from
the actual apex. At the point where the first root-hairs are
developed, the cells of the region B in Fig. i will already have
the appearance of the right-hand cell of Fig. 6. Here the
viscid mass of protoplasm no longer occupies the entire cell
space or lumen. It still forms a thick lining {'primordial iitride)
on the inside of the cell-wall, as can be seen in every parenchy-

FiG. 5.— Young (Meris-
tematic) Cells with
Nuclei.

a, nucleus ; b, nucleolus.

Fig. 6.— lOUNG PARENCHiMATOlS CELLS.

matous cell as long as it continues to live. But in the centre
of the viscid protoplasmic mass spaces (vacuoles) (Fig. 6 a)
have made their appearance, and these are filled with a watery
cell-sap. As the cell grows, the vacuoles become larger and
more numerous, and the mass of protoplasm separating them
consequently thinner and thinner, until they present the ap-
pearance of delicate threads of protoplasm suspending the
nucleus in the centre of the cell (Fig. 6 z). Finally, °these
threads too break across (Fig. 6 b), the cell then contains a
large central vacuole, and the nucleus with its nucleolus (Fig.
6 k) slides towards the cell-wall, which is now evenly lined
by the primordial utricle (Fig. 6 c).

Cells in this condition, as we find them, for instance, in the
older portions of the root cortex, are able to store up other

B

1 8 THE PHYSIOLOGY OF PLANTS

substances, which may now make their appearance. Thus we
often find in these cells a certain amount of starch, which
makes its way down from the upper portions of the plant, and
may be looked upon as a reserve substance, which the plant is
able to store up, if the leaves are producing more of it than
can at the time be used up. In cells belonging to the upper
portions of the plant, which afterwards become green, the green
corpuscles {chlorophyll corpuscles) make their appearance in
the primordial utricle, as may be seen from Fig. 6 d.

In consequence of the storage of starch grains within the
cells, the cortex of some roots — the carrot, for instance — in
the autumn has a snowy white appearance and is full of starch,
so that we may look upon the cortex of the root as a storage
tissue. This storage is sometimes only of a transitory nature,
often limited to a single layer of cells, which surrounds the
vascular cylinder. In such a case this layer of cells is termed
a stareh-sheath, or, if it contains sugar, a sugar-sheath.

The function of the cortical cells as a storage tissue is chiefly
seen in that portion of the root where root-hairs have already
disappeared and the epidermis has become thick-walled and
has lost its absorptive power. Hence we can divide every
root functionally into three zones of very unequal lengths.
The apex, protected by the root-cap, is the region of cell-division,
and is concerned in the formation of new tissues. Then we
gradually pass into the absopptive region, which is also the region
of cell-growth. Here all the cells increase in size, especially in
length, and this region is consequently the chief seat of the
elongation of the root. A number of the epidermal cells, too,
grow out into root-hairs, which undertake the absorptive
function. The absorbed salts and water pass on through the
young cortical cells into the vascular cylinder, and through the-
vessels up into the leaves of the plant. Behind the absorptive
region begins the third zone, which reaches back to the insertion
of the root, and which comprises the fully developed portion of
the root. This older region of the root has now lost its absorp-
tive powers, and the parenchymatous cells of the root serve
only as a storage tissue for reserve substances, which are of the
nature of organic substances, and which have been elaborated
in the leaves and have passed down to the root structures.

THE STRUCTURE OF THE ROOT 19

Of these three zones of the root, the median one, in which
the absorption of salts takes place, is undoubtedly the most
important for the general economy of the plant. It is repre-
sented in Fig. I as covered with root-hairs. We must not,
however, forget that the production of root-hairs is not
essential for absorption, but that the tabular cells of the
smooth epidermis are quite capable of absorbing the necessary
solutions. But in that case the quantity absorbed by each
cell will naturally be very much smaller than that absorbed
by a cell which has elongated to form a root-hair, and may
increase its absorptive surface a hundredfold.

Every cell of the absorbing epidermis of the root represents
a closed space, and yet, as soon as the plant is in want of it,
water with its dissolved salts enters through the cell-wall into
the closed cell space. This method of passage of gaseous or
liquid substances through an uninjured membrane is termed
diffusion, and it can be explained by the assumption that the
cell-wall, just like all other organic structures, is composed of
very small particles (micellce), between which there remain
very minute interstices, so minute as not to be visible with
our strongest optical instruments. The presence of these
interstices makes every membrane into a very delicate filter,
and allows the passage of all bodies which are small enough
to pass through them, and which are attracted by the cell-
contents.

The attractive centre of each cell is the substance we have
termed protoplasm. The richer a cell is in protoplasm, the
greater will be its need for nourishment, and the stronger its
power of attraction. As the inner cells of the cortex are
richer in protoplasm than the outer ones, they will be stronger
centres of attraction, and consequently a current will be set
up in the direction of the central cylinder. The vessels of
the latter are always completely or partially filled with water,
and form a continuous, though sometimes curiously contorted,
system of pipes, leading to the leaves, and there they give off
to the green cells as much water and salts as the latter require.
The leaves lose daily large quantities of water by transpiration,
and the vessels which have to supply the leaves must make
up the deficiency by absorbing water from below.

20 THE PHYSIOLOGY OF PLANTS

We see, therefore, within the vascular system of the whole
plant a very active process of suction taking place. This
reaches right back into the absorptive regions of the root.
There a very active interchange takes place. The parenchy-
matous cells which adjoin the vessels are able, owing to their
rich cell-contents, to absorb large quantities of water, or rather
of a solution of nutritive salts. Filled with water in this way^
they become turgid, and their turgidity causes a pressure to
be exerted on the cell-wall. When such a turgid cell borders
on a vessel from which water is being withdrawn at one end,
as is the case here, it readily forces water into the vessel.
But as hundreds of thousands of such cells surround the
vessels of the root, and press their superfluous water into
the vascular cylinder, a considerable pressure arises, which
forces up the columns of water through the vessels into the
stem structures. Such a pressure (root pressure) actually
exists in all roots, and is often of considerable strength. In
the case of the vine, for example, a root pressure has been
observed strong enough to force a column of mercury to the
height of 39 inches.

The root therefore acts continuously like a force-pump.
The effect of this pumping action is of course not always so
marked as in the case of the vine, in which considerable quan-
tities of water will for days escape from the cut end of a stem
(bleeding of plants). A strong vine yields during the first day
after it has been cut about a quart of sap. If the soil is very
damp and the transpiration of the plant is reduced, the root
pressure will be increased ; the reverse conditions diminish the
root-pressure. At times it is so great that it will force drops
of water out of the apex or teeth of the leaves. This occurs
frequently in the autumn in hothouses which have been cooled
down considerably during the night, and also in cool houses
in which the heating has not yet been started. At such times
drops of water will be found to make their appearance at the
tips and on the teeth of leaves of many Aroideae {Calla
oethiopica, for instance), of grasses, Fuchsias, Pelargoniums, &c.

The suction due to the transpiration which takes place from
the surface of the leaves and the forcing action of the root
pressure are compensating factors in the passage of water

THE STRUCTURE OF THE ROOT 21

through the vessels. If the leaves exert but little suction, the
root pressure forces the water more strongly up the stem. A
given amount of pressure will act more vigorously in this way
the smaller the friction is within the vessels. The resistance
due to such friction diminishes with the increasing diameter
of the vessels. It is for this reason that climbing plants
(Vitis, Tecoma, &c.), which have to carry large quantities of
water to considerable distances, are provided with very wide
vessels.

In the case of narrow vessels, however, another force comes
into play to a considerable extent, namely, capillary attraction.
The attraction of water by the wall of very narrow vessels
causes the liquid to creep up in the thin tubes, where it is
often retained in considerable columns, even though air should
enter at the bottom. In such cases we often find small columns
of water interrupted by columns of air, the whole forming an
interrupted column of water and air, termed Jamin's chain.

This chain-like arrangement, which is formed very frequently
when rapid transpiration takes place above ground without
sufficient supply of water from the root system, makes it
possible for small quantities of water to be distributed over
the whole length of the vascular system, so that the various
organs will all be supplied, should they draw still further on
the limited supply. In this way they can be preserved from
drying up altogether. This would not be possible if the water
were to sink to the bottom of the vessels. Besides this, the
small quantities of water in the tubes can move about more
easily when forming a Jamin chain, and can therefore more
readily supply the thirsting cells at various levels of the stem.
For when the outer temperature rises, and consequently the
temperature within the plant gradually increases, the air ex-
pands within the vessels and raises the intervening drops of
water.

Thus, while the sucking and forcing actions of the con-
ducting tissues are specially efficient in the rapid conduction
of large quantities of water, the capillarity of the vessels and
the formation of Jamin's chains are of especial benefit in
the case of a reduced water supply. But besides this, a con-
tinuous interchange of water takes place within the walls of

2 2 THE PHYSIOLOGY OF PLANTS

the cells and vessels, except in the case of extreme drought.
The water which is contained in the interstices of the mem-
brane of one cell or vessel can be partly attracted by the wall
of an adjoining vessel, and so the moisture will spread evenly
through the walls of these two adjoining elements. When
the cell contents can part with no more moisture, the cell-
walls can still effect an interchange among themselves. This
is true not only of the vessels, but also of all lignified cells.

We cannot conclude the consideration of this part of the
subject without referring to the occurrence of special valves
which we find in this arrangement of suction and forcing
pumps. The valves are represented in this water-cou ducting
tissue by thin portions of the cell-walls, which are usually
termed bordered pits.

In the cells of all the tissues which make up the vegetable
body, we can easily recognise, even if the walls are only
sliglitly thickened, that they are made up of two layers, which
are physically and chemically different one from the other.
We can recognise an outer thin but hard membrane, which
was the first to be formed, and is hence termed the primary
membrane. Within the latter will be seen a less resistant
but thicker layer, which seems to form a padding on the
inside, and which is called the secondary membrane. This
layer, however, is not of the same thickness throughout, but
presents smaller and larger areas, at which it seems more or
less interrupted. At such points the thickening of the cell-
wall, which always takes place when a cell passes from its
youthful condition into its mature state, may have been
defective, or may not have taken place at all. If these areas
have the appearance of small holes or pits in the secondary
membrane, the cells or vessels are termed "pitted;" if the
apertures are numerous and large, so that the secondary
membrane has the appearance of a net spread over the inside
of the primary membrane, we speak of reticulate elements.
In scalariform, annular, or spiral cells or vessels, the secondary
membrane presents a ladder-like, annular, or spiral appearance.

We shall get an idea of the method of thickening of
the cell-wall from the representation of a thick vessel cut
longitudinally in Fig. 7 {lower g). The figure represents a

THE STRUCTURE OF THE ROOT

23

longitudinal section through the horizontal runner of the
Liquorice.

On the left are the narrow wood fibres tightly wedged
together (b), and adjoining a few layers of wood parenchyma
(Jqy). Then follows a thin pitted vessel {[/), with more wood

Fig. 7.— Longitudinal Section thiiough a Runner of Glycyn-hiza glabra (Liquouice).
g, vessels with slit-like pits ; hp, wood parenchyma ; h, wood fibres {aft-er TsciiiRCH).

parenchyma to the right of it. Still more to the right is a
large closed vessel {g), which is seen from the outside. Here
the thinner portions of the wall appear as somewhat spirally
arranged simple slit-like pits. How this appearance is formed
on the wall of the vessel we may gather from a glance at the

24 THE PHYSIOLOGY OF PLANTS

widest vessel lying farthest to the right (lower g), which has
been cut longitudinally, so that its interior is visible. First
of all we are reminded by the figure of the origin of vessels
from superposed cells ; for we can still see the several barrel-
shaped segments, the transverse ends of which run round the
inside of the vessel. Each such ring consists of two light-
coloured rings separated by a dark line. This latter repre-
sents the original cell-wall (primary membrane) which separated
the two cells, and upon it the lighter-coloured thickenings
(secondary membrane) have gradually become deposited. This
same layer of thickening has, however, not been so evenly
deposited over the remainder of the vessel as it was on the
rim of the original transverse walls, but is perforated by small
funnel-shaped pits, the structure of which will perhaps be
best understood by an examination of the lateral walls, which
are the only portion preserved in the upper portion of the
vessel. These walls appear to consist of a thin continuous outer
wall beset with a number of beads. This bead-like arrange-
ment is exhibited by the section of the secondary membrane
which is perforated by the pits, the space between two of
these bead-like bodies representing the pit itself, the broader
end of which lies towards the primary membrane. We see,
therefore, that these pits of the vessel are not actual pores,
but only holes in the inner secondary membrane, covered in
by the thin outer or primary membrane of the vessel.

This process of thickening, which takes place with the
maturing of the cells, is an absolute necessity. For how
should such a colossal growth of millions of cells piled one upon
the other preserve its shape if the cells remained in their soft
and youthful condition ? The whole structure would collapse
by its own weight into a shapeless mass.

Hence it becomes necessary to strengthen the separate cells
and vessels, and this is done by the formation of the secondary
membrane on the inside of the cell-wall. But it is also
useful, if not necessary, that this secondary membrane should
not be continuous, but should leave pores or pits at intervals,
so that the water may pass easily tlirough the thinner portions
of the cell-wall, for its passage through the thicker portions of
the cell-wall could only be accomplished with great difficulty.

n»m\ untAxr

N. C. State CoU^

THE STRUCTURE OF THE ROOT 25

Thus in the construction of all cells two ends have to be
kept in view : firstly, the rigidity of the organ, and secondly, the
facility of passage of the nutritive fluids.

The structure of the wall, however, shows us other very
useful arrangements, and to these belong the valves which
have received the name of bordered pits, and which regulate
the amount of water which may pass from cell to cell. First
of all we have to grasp the fact that in the case of two adjoin-
ing cells or vessels the pit in one cell always corresponds in
position to a pit of the adjoining cell. This is not shown in
Fig. 7, because at the side of the large vessel there is no
second vessel, but only thin parenchymatous cells. The current
of water has, therefore, only to pass through the thin primary
membrane, which is common to the two cells and separates
the two pits one from the other. The communication of two
pitted vessels or cells resembles, therefore, a room which has
been divided into two compartments (cells) by a piece of
outstretched calico. But it has been found desirable to
strengthen the calico by covering it with a layer of mortar on
either side. So as to ensure communication between the two
compartments, however, glass rods and glass funnels have been
placed at right angles to the sheet, meeting each other, so that
the space they protect should not be covered by the lime.
When the thickening is completed, we may consider the rods
and funnels broken away or withdrawn, and the wall will then
be ready for its functions. Wherever a glass rod was pre-
viously fixed, a pit will be found, at the bottom of which the
sheet will be visible. Two such pits on either side of the sheet
will form a cylindrical canal ; but where two glass funnels
touched the sheet with their broad bases, the canals will
become narrower towards the room itself and the two canals
will therefore form a lens-shaped hollow, in the centre of
which will be stretched a piece of the calico sheet.

If we were to introduce a hand into such a narrow canal,
and to push the sheet towards the other room, we should be
able, if the sheet were sufficiently extensible, to push our fist
so far that it would come in contact with the narrow neck of
the funnel-shaped opening on the other side. In that case
the fist would close the opening.

26

THE PHYSIOLOGY OF PLANTS

But suppose that, in the process of thickening, a small mass
of lime had fallen upon this portion of the sheet, and had
become hardened in the form of a small disk in the middle of
the separating membrane ; then if this mass is pushed towards
the other side, it, instead of the fist, would close the aperture.
In this somewhat rough illustration, the two halves of the
room represent two adjoining cells (Fig. 8), the sheet is the
primary membrane (o), and the deposit of lime the secondary
thickening (s and s), the funnel-shaped hollow towards the
other cell represents the widening of the bordered pit (c), the
portion of the sheet within the hollow the closing membrane
, . of the pit, and the

small disk of lime on
the separating mem-
brane stands for the
thickening of the
closing membrane,
usually termed the

tOPUS (t).

Now, if the water
is at the same pres-
sure in two adjoin-
ing elements of the
tracheal system — for
instance, in two
tracheids — the clos-
ing membrane will
remain in its original
median position, and
if there should be only a small excess of pressure in either
cell, the water will easily pass through the thin portion
of the closing membrane, and the equilibrium will be readily
re-established. Such an easy functioning of the valve-like
arrangement we may assume to take place in such wood as
that of Conifers, which has no true vessels, but in which the
wood consists only of wood tracheids with a row of bordered
pits (Fig. 9). In this figure we see the pits in surface view,
and there the light central ring represents the free portion of
the canal {t in Fig. 8), while the darker outer ring represents

Fig. 8.— Two Tracheids seen in Transverse
Section.

primary membrane ; s and s', secondary membrane ;
J, intercellular space ; t, thickening on the pit mem-
brane ; c, cavity of the bordered pit.

THE STEUCTURE OF THE ROOT

27

-7-^.><Tf?Y^:^^1T^'^-^-^^^^

the portion (c) below the overhanging margin of the secondary
membrane. This form of cell distinguishes the Conifers from
dicotyledonous plants, which have fully developed vessels. In
the latter, it often happens that in the summer the leaves
withdraw so much water from the vessels that the latter are
almost devoid of water and
filled with air. If this con- f||17l>'7^MTr?"
dition of drought lasts for a 1^1
considerable time, the deli-
cate membranes will be in
danger of drying up, and thus
of losing their power of con-
ducting water. But in the
case of the funnel-like con-
striction of the bordered pits,
the water is retained in the
pit by capillarity, even when
the vessel itself is already
dry, and consequently the
danger of destroying the
functioning of the valve is
avoided. If afterwards one
cell becomes tilled with
water while the other is still
devoid of water, a consi-
derable pressure is exerted
by the full cell ; this causes
the closing membrane to
bulge out towards the empty no
cell. In doing this, the
membrane, becoming ex-
tended, enlarges the area of "'• ™^'^^'''^^^:^^y tlhVtniTefnitf '"'''''''

' o sections of the bordered pits.

passage. If a vessel becomes

filled very suddenly and with great force (as, for instance,
if the air in one of the vessels becomes very much rarified),
the pressure can become so great that the closing membranes
are burst. In such a case the small thickening in the centre
lessens the danger, for as soon as the membrane is consider-
ably pushed in, the thickening presses against the wall of the

, 9.— Portion of a Thin Longitudinal
Section through the Wood of the Pine,
SHOWING Bordered Pits in Surface
View.

28 THE PHYSIOLOGY OF PLANTS

funnel, and the pressure is spent on the thickened portion,
which will resist any pressure likely to occur under such
conditions.

This useful system of valves is only developed between two
water -conducting elements; if, however, a vessel adjoins a
cell containing protoplasm or reserve substance, the bordered
pit will only be developed on one side. If the parenchy-
matous cells are permanently rich in water, while the vessel
is poor in water, or even filled with air, then the high pressure
(turgidity) of the parenchymatous cells will cause the cell-wall
to bulge out through the pit into the lumen of the vessel, and
the latter will become filled by protrusions on all sides from
the adjoining cells. These become divided off from the
original cells which gave rise to them by transverse walls, and
enlarge so as to completely plug up the vessel. This filling
of the vessels with bladder-like cell protrusions (thylloses)
takes place especially when the wood system becomes damaged,
and it affords a protection of the injured portion against the
effects of injurious atmospheric conditions.

Let us now sum up shortly the results of this paragraph.
The vessels and vessel-like cells, all tracheal elements there-
fore, are at the commencement filled with water, but can be
partially and temporarily emptied by the transpiration from
the leafy portion of the plant. In that case they become
filled again by the pressure of water from the parenchymatous
cells surrounding them. If the vessels become emptied of
water, the air within them must take its place, and fill the
cell or vessel. In doing this it becomes greatly rarified ; and if
it be cut under water, it will suck up the latter with great
avidity {negative pressure). Besides this rising of the water
by suction, an upward forcing takes place by root pressure.
Lastly, the capillarity of the vessels, and the changes effected
in the Jamin's chains which they contain, are of considerable
importance. These two physical forces make a continuous flow
of water in the vessels possible, if the leaf structures only use
up a moderate amount of water compared with the amount
absorbed. They also enable a passage of water to take
place when the vascular system is considerably emptied
by an inordinate amount of transpiration. The regulation of

THE STRUCTURE OF THE ROOT 29

pressure in a lateral direction of the columns of water of
adjoining vessels is effected by the valve-like action of the
bordered pits, and the slow movement of water through the
insterstices of the cell membrane.

These functions are only performed by the cells and vessels
of the wood {xylcm) contained in the central cylinder of the
root. The bundles, however, possess also a soft bark (phlcem),
which is concerned in the passage of the organic substances.
We shall discuss the method of functioning of this portion
of the bundle in dealing with the structure of the stem, and
shall now pass on to the consideration of the substances which
enter the plant dissolved in the water taken up by the roots.

CHAPTER III

THE NUTRITION OF THE ROOT

§ 4. What substances must be present in the soil for the
continuous and effective nutrition of plants ?

In order to find out what substances are necessary for the
growth of cultivated plants, a number of chemical analyses
have for years been made of the most widely differing plants,
and it has been found that only a very small number of
chemical elements are present in all of them. From this
small number, too, a few must be excepted, which occur in
some individuals grown in special localities, and which are
therefore, as we might say, accidentally taken up by some
plants, while equally robust specimens of the same species
are devoid of them. The results of these analyses caused
physiologists to try to cultivate plants in media the composi-
tion of which was known, but which in themselves had no
influence on the plant. The choice lay between distilled water
or quartz sand which had been sterilised by heat. To these
media were added the substances which had always been
found in the ash of plants. These substances were added in
various forms, and one or other was occasionally omitted, so
as to find out which substances must always be present in
order that the plant may grow successfully. These water and
sand cultures resulted in showing that even some of the sub-
stances which were present in the ash of all cultivated plants
might be absent without lessening the growth of plants ; and
hence we may assume that they are only present in all our
cultivated plants because they are present in every soil, and
are therefore taken up by all roots.

The knowledge thus obtained enables us to divide the
substances found in the ash of plants into three categories :

THE NUTRITION OF THE ROOT

31

{a.) the absolutely necessary or essential nutritive substances,
which must always be present in sufficient quantity and in
soluble form : these are oxygen, hydrogen, nitrogen, carbon,
sulphur, phosphorus, potassium, calcium, magnesium, iron,
and probably also chlorine ; (b.) subsidiary substances, which
are present in every ash, but are not indispensable for the
growth of plants : sodium, silica ; (c.) admixtures which are
due to the nature of the soil : such as zinc, copper, nickel,
aluminium, cobalt, manganese, lithium, strontium, and barium.

§ 5. What is the eflFect of the various nutritive substances on
the plant, and in what form do they enter it ?

All tissues of the plant contain oxyg-en, hydrog-en, and
carbon. The first two substances are not only necessary to
the plant in the construction of its tissues, but they are
essential for purposes of conduction ; for they make up the
water which pervades the whole plant. Tlie percentage of
water present in the various parts of the plant is naturally
very variable. Succulent herbaceous tissues possess 60 to

7 5 per cent, of water, while the fungi often possess only 5 to

8 per cent, of organic substance, the rest of their weight being
made up of water. As most of the nutritive salts are con-
tained in the soil, we can easily form an opinion of the
importance of water, which is the medium by which the roots
have to absorb the salts. These solutions pass through the
vessels of the plant, and the adjoining cells can absorb water
from the vessels, though they have often to pass it on to
neighbouring cells, and thus a constant absorption of the
water contained in the soil is brought about. Nearly all the
water which passes through the plant, carrying with it the
nutritive salts, is taken up by the root. This water is ulti-
mately given off as water-vapour, or is chemically decomposed
and used up in the construction of new material. Only in
cases of extreme dryness of the soil are the leaves able to
make use of the heavy deposits of dew.

In the case of carbon, however, which must be looked upon
as the chief constituent of the vegetable tissues, forming as it
does one half of their total dry weight, the soil is of no use to

32 THE PHYSIOLOGY OF PLAIs^TS

the plant as a source of this element. The atmosphere, how-
ever, M'hich surrounds the plant affords it all the carbon
which it requires. The leaves absorb carbon from the air
in the form of carbonic acid, and decompose the latter in
their cells under the influence of light, giving off the surplus
oxygen.

Nitrogen is the substance which is chiefly concerned in the
building up of young tissues, but it is also present throughout
the whole life of the cell in the substance which we termed
protoplasm.

Alkaloids, which are present in many plants, and asparagiu,
which is concerned in the regeneration of the albuminous sub-
stances, are formed from the protoplasm, and contain nitrogen.
Most of our cultivated plants have to take their nitrogen from
the soil ; it is only the leguminous plants which are able to
take up the nitrogen through their leaves from the atmosphere,
of which nitrogen is the chief constituent. As far as our
present scientific knowledge goes, the leguminous plants can
subsist on the nitrogen which they take out of the air ; while
cereals, fruit-trees, and indeed all other Phanerogams, must
obtain this substance in some soluble form from the soil.
Nitrates seem to be the most suitable form of salt from which
plants obtain their nitrogen. Ammonia, which can probably
be absorbed in minute quantities even in a gaseous state, is
less suitable.

Sulphur and phosphorus are of some importance in the for-
mation of albuminous substances ; both are taken entirely from
the soil, where they are found in the form of calcium salts.
This is especially the case with sulphur, which is widely dis-
tributed as calcium sulphate (gypsum), while calcium phos-
phate is of less frequent occurrence, and being only very
slightly soluble, it is advantageously replaced by a potassium
salt. In some places phosphoric acid is found very largely in
the soil in combination with iron. Plants which are deficient
in phosphorus generally assume a red coloration.

Potassium, which is chiefly found in very young tissues, and
disappears in the older ones, is probably connected with the
formation of the earbo-hydrates, i.e., with those organic sub-
stances which consist of carbon, hydrogen, and oxygen, and in

THE NUTRITION OF THE ROOT

33

which the last two elements occur in the same proportions as
they do in water.

Starch, sugar, and cellulose are such carbo-hydrates which
are transmutable within the plant. If a plant does not find
any potassium in the soil, its growth ceases, and the leaves do
not develop the power of forming starch within the green
colouring bodies, the chlorophyll grains. A plant which has
perhaps shown no growth for montiis, owing to the absence of
potassium, will recover in a few days after that element has
been supplied to it ; even after a very few hours starch will
begin to make its appearance in the chlorophyll grains. The
very soluble salts of this important nutritive substance are
liable to become washed out of the soil if watering is too
continuous.

Calcium, the foundation of all combinations of lime, is
chiefly concerned in the strengthening of the cell-wall, within
which it is often deposited in visible crystals of carbonate of
lime. But besides this, it is important in fixing the oxalic
acid which is produced by plants, and which is poisonous to
them. In this process of fixing, crystals of calcium oxalate
are formed, which in many plants are stored in continuous
rows in the cells of the cortex immediately surrounding the
channels through which the plastic material passes, and show-
ing therefore their intimate connection with the metabolic
processes. For as the organic substances passing downward
from the leaves through the cortex undergo manifold changes,
chiefly of the nature of further oxidation, it is probable
that the last function of the lime is to fix the oxalic acid,
which is formed by the oxidation of the carbo - hydrates,
and thus to render it harmless. Calcium is taken up by the
plant in the form of sulphates, phosphates, and carbonates.
In the case of water-cultures, calcium nitrate may be advan-
tageously used.

Magnesium, which of all the nutritive substances is most
nearly allied to calcium, but cannot replace it in the vegetable
economy, seems to have quite a different effect on the plant.
It seems to work together with the nitrogen in the formation
of protoplasm, and to influence the formation of chlorophyll ;
for plants cultivated without magnesium only form yellowish-

c

34 THE PHYSIOLOGY OF PLANTS

green chlorophyll granules, and do not form new cells very
readily. In the case of decomposition of succulent portions
of plants, very often only those parts which are very rich in
nitrogen will be found to contain crystals of magnesium phos-
phate. Magnesium, just like calcium, enters the plant in the
form of sulphates, phosphates, and other salts, especially in
combination with chlorine.

Iron is necessary in the building up of chlorophyll. As it
is the function of chlorophyll to form new plastic material
under the influence of the sunlight, it is natural that the
absence of iron, which is shown by the paleness of the
leaves, should cause a cessation of assimilation. But as
iron is very widely distributed in the soil, such paleness
{etiolation) is of much less frequent occurrence than might
be supposed. In diseases of all kinds yellow foliage may
make its appearance, and we must not be surprised if in
many cases the addition of soluble iron salts remains without
remedial effect.

To the subsidiary substances we must also reckon chlorine,
which is not looked upon as a nutritive substance by many
physiologists ; still many observations tend to show that this
substance is essential to the healthy growth of the plant.
Water-cultures devoid of chlorine compounds do not thrive,
and in some cases direct indications of disease make their
appearance. From the present data, we can only assume that
the presence of chlorine retards the absorption and ultimate
deposition in the tissues of calcium, but that, on the other
hand, it accelerates the passage of phosphates, and thus
increases the supply of food material at the growing
points.

If much phosphoric acid is used up, the nitrogen and
the soluble carbo-hydrates are more rapidly utilised by the
protoplasm for the formation of albuminous substances.
Hence we find that a supply of chlorine salts (calcium
potassium and magnesium chlorides) causes long and suc-
culent shoots to be produced, which, however, are poor in
starch.

THE NUTRITION OF THE ROOT

35

§ 6. What part is played by the subsidiary nutritive sub-
stances and the occasional admixtures to the nutritive
salts ?

Sodium must be looked upon as a subsidiary food sub-
stance, though it is absent only in very few plants, and is
taken up by some groups of plants (shore plants) in enor-
mous quantities in the form of sodium chloride (common
salt), and is found in their tissues together with potassium.
But in spite of these two elements being closely allied, they
cannot in any way replace one another. Thus, if a plant
is without potassium, a supply of sodium will be of no use
to it. The roots take up the very soluble sodium salts in
great quantities if they require the acids with which the
sodium is combined, and store up the sodium in another
form as a waste product. But we gather that this sub-
stance is of little importance in plant economy from the fact
that even those plants which always present a large amount
of sodium in their ash, such as the saline plants of the
shores, can be cultivated to perfect development in a soil
free from sodium.

Silieium, which is present in all soils in the oxidised form
of siliea, behaves just like sodium. Silica is generally found
in the soil in the form of crystalline (insoluble) substance,
but it occurs also in an amorphous soluble form, and can
in this condition be easily absorbed by the roots. Being so
widely distributed, it is found very generally in plants,
and, like lime, and often together with it, it is used to
strengthen the cell- walls. Such cell -walls are often so
strongly impregnated with silica, that in some cases (Bqui-
setum) these plants may be used for polishing purposes. The
stems (culms) of our cereals and the leaves of sedges are
provided with epidermal cells so stronly protected with silica
that a hand passed over them will be severely cut. The
enormous quantity of silica contained in grasses and sedges
was formerly adduced as a proof of the necessity of silica for
the strengthening of the cell-walls. But now that it has been
possible to cultivate cereals in solutions devoid of silica just as
well as in the open field, this inference breaks down. The laying

36 THE PHYSIOLOGY OF PLANTS

of cereals, which was formerly attributed to a deficiency of
silica in the lower joints or nodes of the culm, is known
now to be due to the incomplete thickening of the cell-
walls, in consequence of deficient illumination of the base
of the stalk.

Of the substances which are occasionally present in plants^
and which are due to the habitat of the plants, iodine and
bromine must be mentioned as always present in marine plants.
The sea-water contains only very small quantities of these sub-
stances, while more than i per cent, of the dry weight of some
sea-weeds consists of these substances (poisonous for other
plants) combined with potassium. Some chemists have found
both these substances in bog and also in land plants, while
others have not been able to trace them in other specimens of
the same species. This shows how dependent the appearance
of these substances is upon the nature of the locality in which
the plants grow.

Boron has been observed in traces in the Grass-wrack
(Zostcra) and in some sea-weeds, while fluorine has been found
in the Club-moss {Lycojjodium davatum), in the Horse-tail
(Uquisehtm), and in grasses.

Of the lighter metals which are usually found associated
with potassium, lithium and rubidium (not CECsium) have been
found in some plants, the former, for example, in the tobacco-
plant and in thistles ; the latter in turnips and in the leaves
of tea and coffee. But in spite of their affinity to potas-
sium, they can in no way replace the same, but are, in fact,
poisonous if they occur in any appreciable quantity.

Allied to calcium is barium, which has been recognised in
the ash of several trees and shrubs, and strontium, which has
been discovered in several sea-weeds. The former of these two
substances is harmless to many plants in fairly large quantities ;
the latter acts as a poison even in very small quantities. In
spite of the wide distribution of clay in soil and in rocks, its
chief constituent, aluminium, is confined in its occurrence! to
very few plants (lichens and club-moss). This element occurs
both in rocks and also in the product of their breaking up, in
the soil, almost exclusively in the insoluble form of a double
silicate of potassium and aluminium.

THE NUTRITION OF THE ROOT 37

Of the metals which are widely distributed in the vegetable
kingdom, we must mention first of all copper, which has been
found in the wood of various trees (oak, beech, lime, &c.), and
in some herbaceous plants. Not so widely distributed, but
occurring in higher percentages in the ash of plants, we find
zinc, which changes the entire appearance of some plants
{e.g., Forma calaminaria of Viola lutea), but only acts detri-
mentally when occurring in larger quantities. The leaves of
plants which suffer from ziuc-poisoning are discoloured by
small rusty-looking spots, which after a while spread over the
entire surface of the leaf. This metal generally enters the
plant as a soluble sulphate. Lead is less harmful to plants,
but arsenie is poisonous in very small quantities, decreasing
very rapidly the absorptive power (osmotic activity) of the roots.

From the above account, which is by no means exhaustive,
it will be seen what a number of elements can be present in
plants. Some of these substances are stored in large quantities
without injuring the plants in any way, while others prove
harmful when present in very much smaller quantities. From
this fact we may gather that roots are not able to choose their
nourishment ; if, therefore, poisonous substances reach the roots
in a soluble form and can penetrate through their cell-walls,
the plant will gradually be poisoned. But even if the sub-
stances are harmless in themselves, they will always have some
effect on the general economy of the plant ; sometimes this
influence will become apparent externally. Thus we have
already seen that zinc plants possess a different appearance —
in some cases a differently coloured flower. The same may be
gaid of plants growing in salty regions (halophytes) ; they are,
it is true, able to flourish without absorbing any salt, but they
only develop their succulent character when they have absorbed
some sodium chloride. Similar experiments with regard to
silica have been made on cereals, and they tend to prove an
indirect influence of this substance on the general growth.
The grains were better developed and the other food substances
were more completely used up.

We must, therefore, not look upon the substances only
occasionally absorbed by the roots as entirely useless for the
general nutrition of our cultivated plants. Their influence,

38 THE PHYSIOLOGY OF PLANTS

sometimes accelerating, sometimes retarding, will depend upon
the nature of the plant, and vary at different periods ; for one
species will be unaffected by a large amount of a certain sub-
stance, the same amount of which would be injurious to
another species or stop its growth entirely. If, therefore, we
hear of plants the ash of which is shown by chemical analysis
to contain a strikingly large amount of such a substance, we
must not suppose that this species needs so much of that
substance for its actual growth, but all that we can logically
conclude is, that this species can absorb without harm to itself
the stated amount of that substance. Plants rich in salt or
saltpetre, in silica or zinc, are therefore not plants which need
these substances in such large quantities, but which have the
power of resisting the over-loading of their tissues with these
waste products. The same may be said of the plants rich in
lime.

If, therefore, a plot of land, the soil of which has a special
character owing to the preponderance of one of these substances,
is to be planted afresh, those plants should have the preference
which can endure the largest amount of the substance in
question. What is said here of the several elements of the
soil is equally applicable to the other properties of the soil
(the facility of absorbing heat, the power of retaining water,
&c.). The constant occurrence of certain kinds of plants in
localities characterised by the preponderance of certain sub-
stances is explained by the greater power some plants have to
adapt themselves to the given chemical and physical conditions
of the soil.

For practical purposes the occurrence of lime-, silica-, salt-
petre-, or salt-loving plants should be noted by horticulturists
and agriculturists, as it affords a clue to the condition of the
soil. Such soils, characterised by the preponderance of one
substance, have the disadvantage that a number of cultivated
plants do not thrive on them on account of the excessive
richness in this one substance. If, therefore, a crop should
fail in such soil, we know where, in the first place, to look for
the cause.

THE NUTEITION OF THE ROOT 39

§ 7. In what form does the root find the nutritive substances
in the soil?

Every soil, representing as it does the product of disintegra-
tion of various rocks, consists of a mass of fragments of various
kinds, endowed with certain physical powers of attraction. A
mechanical analysis of the soil enables us to separate any soil
under cultivation into its coarser constituents, the skeleton or
framework, and finer earth, wliich can be washed out of it.
According to the size of the fragments making up the frame-
work, we can distinguish the coarsest constituents as coarse
gravel ; then follow gravel of medium coarseness, and fine
gravel or coarse sand. The fine sand would belong to the
earthy portion, which shows the characteristic physical proper-
ties that are of importance for the growth of plants, such as
the power of absorbing and conducting heat, capillary attrac-
tion, and the power to attract and retain gases and liquids.
It is the varying proportions in which the various constituents
of the framework are mixed with the earthy portion of the soil
which gives to every soil its characteristic structure. The
structure of the soil is important to the plant in the first place,
because it regulates the access of air to the roots, which require
air just as mucli as the upper portions of the plant. Some
plants, however, require a large amount of air circulating
among their roots, while others are content with a much
smaller amount of air, because they need only a very small
quantity of the really active constituent of the air, namely, the
oxygen. A loose soil, in which the several particles lie loosely
packed side by side, will not only stimulate the activity of the
roots more than a binding soil, in which the particles are closely
fixed together, but it will also favour the processes of decay
and decomposition. The most favourable structure of a soil
for cultivation is one which is not too loose to retain the water
which is necessary for growth, and yet is not so close as to
have its interstices filled with water under normal climatic
conditions, which would prevent the access of the necessary
amount of air to the roots.

Of the nutritive substances, the one of chief importance for

40 THE PHYSIOLOGY OF PLANTS

the roots is the water, which circulates in the interstices of the
soil, and is there retained.

The capacity of a soil to retain water witliout allowing it
to escape in the form of drops (the reteutiveness of the soil)
increases with increasing fineness of the particles ; but the
attraction which the particles of the soil have one for another
can be overcome even in fine-grained soils with great water
capacity ; large quantities of water are forced into the soil ;
the particles then become separated, and the soil becomes soft
and pasty. In the case of clays, less often of loamy soils,
a complete dissolution of the soil may take place. This may
be very serious in the case of seed-beds, as the soil between
the young plants is washed away, and the roots are laid more
or less bare. Such a condition of the soil will also cause the
sowing of crops to be postponed, and the yield of the harvest
in our country often depends upon the time of sowing. For if
plants have to produce a good stock or a number of large leaves
before they can yield a good crop, a retardation of the time of
sowing may prevent the proper development of such growth,
and an early dry summer will stimulate the development of
flowers, which will be scanty and poor. In hot and dry
summers we see the peas which have been sown late attacked
by mildew and smut, the kolrabi becoming woody, and the
lettuces running to seed.

The heterogeneous particles of the soil possess in different
degrees the power of attracting by physical and chemical
means, and of retaining the substances which are washed into
the soil. This can be demonstrated by a very simple experi-
ment. If a drain-pipe is filled with clay, the highly-coloured
liquid manure from a dunghill will be entirely discoloured by
passing through it. The organic compounds are retained by
physical forces of attraction, while inorganic compounds are
chemically fixed. This power of the soil to bind the various
organic and inorganic salts is termed its power of absopption.

The root of the plant will not alone absorb the water con-
tained between the particles of the soil, but can also make use
of the salts which have been absorbed by it.

THE NUTRITION OF THE ROOT 41

(a.) Potassium. /

Of the nutritive substances which have to be added to the
soil, potassium is most readily absorbed by the soil ; the next
in order are ammonia, magnesium, sodium, and calcium. Of
the acids, tlie very essential phosphoric acid is very energe-
tically retained. This is not so much the case with carbonic
acid, and still less with nitric acid. Sulphuric acid and
hydrochloric acid are not fixed at all by the soil, which will,
however, absorb silicic acid.

The absorptive power of the soil depends very much on
the quantity of basic hydrated silicates which are soluble
in hydrochloric acid (zeolithic). They contain the necessary
lime, sodium, magnesium, and potassium, but are almost use-
less to the roots on account of their insolubility. They are,
however, decomposed by the carbonic acid contained in the
soil or by compounds of nitric or sulphuric acid with alkalies
or alkaline earths, and the resulting carbonates, sulphates,
and chlorides are readily soluble, and can therefore be easily
absorbed. During the decomposition of certain silicates
hydrated iron oxide and hydrated aluminium oxide are formed,
and these compounds as well as the hydrated silicates enable
the soil to separate the potassium from its most stable com-
bination with hydrochloric, nitric, and sulphuric acid and to
retain it. These iron and aluminium compounds absorb very
little potassium, however, without the presence of such silicates,
but the soil absorbs it very actively if aluminium phosphates is
present. Potassium is, however, scarcely absorbed at all by the
humic acid compounds, by carbonate of lime and of magnesium.

The potassium salts which are absorbed by the soil are
able to form combinations with other nutritive substances,
and therefore to fix important substances in the soil. Thus
kainit (a sulphate and chlorate of magnesium) can be used
to fix nitrogen. The actual potassium salt (potassium sul-
phate) which takes part in this process plays a quite sub-
sidiary part. As soon as ammonium carbonate begins to
form in the soil, the acid of the magnesium sulphate combines
with the ammonia ; the ammonium sulphate so formed is still
very soluble. But if the application of kainit was preceded

42 THE PHYSIOLOGY OF PLANTS

by the addition to the soil of a manvire containing phosphates,
the more insoluble ammoniacal magnesium phosphate is
formed. In stables the giving off of ammonia is often miti-
gated by strewing gypsum, the sulphuric acid of which com-
bines with the ammonia to form the ammonium sulphate.
The same effect may be more speedily and more completely
attained by the addition of kainit ; and a similar action to
that of the magnesium sulphate of the kainit would be pro-
duced by sulphate of iron (green vitriol).

In light soils, therefore, in which there is little humus, clay,
or other absorptive substances, manuring with kainit would
enrich the soil by causing the ammonia, which becomes
volatilised in the decomposing manure, to be fixed and re-
tained in the soil. The common salt which is also present
acts as a solvent and distributing agent for other salts.

(&.) Fhosphoric Acid.

This very important nutritive substance, which counter-
balances the excessive addition of nitrates, is immediately
absorbed in the soil, if in solution, by the heavy metals and
the earths, as these substances will at once combine to form
insoluble phosphates. The acid is then generally found com-
bined in the form of calcium and magnesium phosphates, and
in smaller quantities as iron and aluminium phosphates. The
richer, therefore, a soil is in the first place in carbonates of
lime and magnesium, the more we shall be able to saturate
it with phosphoric acid. And the same effect will be pro-
duced by those silicates (sesquisilicates) which yield on decom-
posing hydrated aluminium oxide and iron oxide. If lime
is present in the soil in the form of gypsum, it can take up
phosphoric acid, gradually giving off sulphuric acid as it does
so. The phosphoric acid is only available to the roots
directly in the form of its soluble combinations with alkalies
(potassium and sodium).

(c.) Nitrogen.

This substance, as important as potassium to plant life,
represents 79 per cent, of the air we breathe, the remaining

THE NUTRITION OF THE ROOT 43

2 I per cent, consisting mainly of oxygen. This uneombined
nitrogen is only available in any appreciable quantity for
leguminous plants, which possess, as we now know, the faculty
of growing in a soil entirely devoid of nitrogen ; for though
it may possibly take part in the nutrition of other cultivated
plants, at least to a very slight extent, it must be absorbed by
their roots in a combined form. On the other hand, the small
green Algffi, which are present in every soil, seem to have the
power of making use of and of elaborating the free nitrogen,
and may therefore be looked upon as nitrogen purveyors to
the soil. The nitrogen compounds which are most important
for the nutrition of plants are ammonia, nitrous oxide, and
nitric acid. This combined nitrogen, however, which alone
can be assimilated by the roots, is not derived from the atmos-
phere except to a very slight extent. The decomposition of
nitrogenous organic substances liberates ammonia into the air,
and the lightning Hashes of a thunderstorm cause some free
nitrogen to combine witli ozone to form nitrous acid (ozone
and hydrogen peroxide also being produced). Besides this,
ammonium nitrite is formed in every process of oxidation, and
even during the evaporation of water.

When the water-vapour of the air condenses into drops,
ammonia is enclosed in them, and more is absorbed by the
drops when falling as rain through the air. Dew, hail, and
to a lesser extent snow, all contain ammonia. Eain, too,
especially during a thunderstorm, contains some nitric acid.
These available nitrogen compounds are thus washed into
the soil by natural agencies, or may be (but only to a very
slight extent) absorbed directly out of the air. But the
absorptive power of a soil for nitrogen depends upon its
composition. The several constituents of the soil do not
only vary in their capacity of absorbing nitrogenous com-
pounds, but they also vary according to the form in which
they occur. Humus compounds, gypsum, carbonates of lime,
and magnesia, for instance, take up very little nitrogen ; but
if humic acid combined with calcium occurs in the soil together
with decomposing silicates, a very considerable absorption of
nitrogenous compounds takes place. From experiments (of
Bretschneider) it has been calculated that if water absorbs

44 THE PHYSIOLOGY OF PLANTS

about 0.64 lbs. of ammonia per acre, gypsum will absorb only
0.23 lbs., carbonate of lime 2.62 lbs., pure quartz 0.93 lbs., but
iron oxide 10 lbs. If, however, pure quartz is mixed with a
brown humus compound {idmin), it will absorb, by the addi-
tion of only I per cent, of ulmin, 1 4 lbs. ; by the addition of
5 per cent, 92 lbs. of ammonia. This demonstrates the indirect
influence of humus compounds.

Although in artificial cultures in distilled water or sterilised
sand, plants thrive exceedingly well when supplied with in-
organic food substances only, and a proof has therefore been
given that these plants can grow without any decomposing
organic substances, still the figures just quoted indicate that
the organic substances of humus are of considerable value in
the nutritive processes. In certain cases humus is absolutely
necessary. To the plants which live entirely on organic food
matter belong all the fungi, which take their nourishment
either from living organisms {parasites) or from decaying
organic matter {saprophytes). Besides these cryptogamic or
flowerless plants, many more highly organised plants afford
us examples of plants which must necessarily be supplied
with organic food matter. The mistletoe illustrates the para-
sitic habit on a living host plant, while decaying organic food
matter is essential for the nutrition of the humus-growing
plants which have either very little chlorophyll in their tissues
or are completely devoid of it. Examples of these are the
Yellow Bird's-nest {Monotropa Hypopitys), and the yellowish-
brown Orchids {Neottia nidus avis, EpipogiLiii aphyllum, and
Corallorhiza innata). In connection with the mistletoe we
must not omit to mention those green plants, rich in chloro-
phyll, which are able to lead an independent existence, growing
and flowering in the soil, but which can on occasion further
their nutrition by absorbing the sap of other plants. To
these belong the well-known inhabitants of moorland meadows,
such as the Eyebright {Euphrasia), the Yellow Eattle {Rhinan-
thus), and the Cow-wheat {Melampyrum). The roots of these
plants, if they come into contact with other roots, develop
small suckers, which abstract substances from the roots
of the host plant. The desire to supplement the normal
nutrition by a parasitic absorption of organic nitrogenous

THE NUTRITION OF THE ROOT

45

substances and carbo-hydrates is therefore apparent from
these instances.

After the discovery of these facts, the next question to be
solved was whether any of our cultivated plants which do
not absolutely require humus for their growth, might not be
benefited by it. This problem led to some very interesting
culture experiments, which consisted in watering one set of
plants with an extract of humus, while another lot of the
same plants received the ash of the same quantity of humus.
Now, if only the mineral constituents of the humus were con-
cerned in the nutrition of the plants, both series of cultures
should have yielded the same result. But this was not the
case. The plants which had received the humus extract
yielded a crop twice as large as those which received the
salts only.

This experiment, therefore, must be taken as a proof that
plants in the open reap a distinct advantage if they receive
their food substances in part at least in an organic form, and
we have a scientific confirmation of a practical and popular
saying, that " The dung put into the ground accounts for nine-
tenths of the farmer's pound."

Of course it is not ascertained as yet whether the greater
vigour of growth is due to the absorption and the more rapid
assimilation of the organic compounds, or whether it is due
to the fact that combination of the humus compounds with
inorganic substances causes them to absorb more of the very
important ammonium salts. At all events, we must hold that
the presence of humus in the soil greatly stimulates the growth
of the plants, and that the view held up to the present
with regard to the importance of the humus compounds must
be considerably extended. According to this older view of
Grandeau's, the function of the constantly changing humus
compounds was to keep a sufficient quantity of nutritive sub-
stances in solution in the soil and make them available for the
roots. Chemically these humus compounds are still very
insufficiently determined.

But in considering the efficacy of humus, we must not only
take into account the substances from which it is formed, but
we must know at what stage of decomposition it has arrived.

46 THE PHYSIOLOGY OF PLANTS

This fact can be illustrated by experiments in which plants are
treated in one case with humus in its raw or fresh condition,
and in the other with humus which had been subjected to the
action of steam (212° Far.) for several hours. The same quan-
tity of soil after the action of the heat yields a crop many
times in excess of the former.

The mechanical effect of the humus upon the structure of
the soil we shall deal with in speaking of stable manures, but
to the general consideration of the efficacy of humus we must
append an account of the curious modification of the roots of
many of the humus-loving plants. This special form of root
is termed myeorhiza, and it differs from the normal root
structure by the fact that the lateral rootlets, instead of being
long and slender, are thick and short, more branched and
interlocked, and form a nest-like structure. These rootlets,
too, are usually devoid of root-hairs, so essential for absorption ;
but their entire surface is covered by a dense felted mass of
fungal threads, so that the root itself does not seem to be
directly in contact with the soil at all, but can only take up
its nutriment by means of the fungal threads. As the root
grows in length at its apex, the pseudo-parenchymatous threads
of the fungus increase in number too, so that the slightly
developed root-cap is constantly covered in by a dense cap of
fungal hyphae. This fungus being devoid of chlorophyll cannot,
as has been proved, absorb or utilise any raw or inorganic salts,
and probably derives all its nourishment from the organic com-
pounds of the humus. If, therefore, we do not like to assume
that this fungal covering becomes mechanically saturated with
the inorganic nutriment of the soil and passes this on to the
cells of the root, we will be forced to the conclusion that a large
number of cultivated plants actually derive their nourishment
from the organic substances of the humus, and do this either
by preference or by necessity.

How widely distributed in nature this arrangement of my-
eorhiza is may be gathered from the fact that all our forest
trees, such as oaks, beeches, birches, alders, Conifers, and also
poplars, willows, and limes, are provided, where they grow in
humus, with myeorhiza. They will, however, develop normal
rootlets and root-hairs if they are grown in solution or in

THE NUTRITION OF THE ROr)T 47

purely mineral soil, or if the humus has been sterilised by
heating, and the fungus is therefore killed. According to
A. B. Frank, upon whose extensive investigations these re-
marks are based, mycorhiza is also present in heaths, in most
orchids, in many Liliaceas and Smilaceas, and in a large
number of Compositte, Labiatae, Primulacea), Umbelliferse,
Rosacea, Leguminoste, and Ranunculacea?.

The fungi which form the mycorhiza belong to those
kinds which usually occur in the humus of woods and fields.
They do not seem to act in any way injuriously on the root,
but, on the contrary, stimulate it to renewed growth, and no
doubt the root in its turn stimulates the growth of the fungus,
nourishiog it with its own sap. We have here, therefore, an
example of the living together or partnership of two organisms
{symhiosis), each of which benefits the other. The fungus, too,
is not always restricted to the outside of the epidermis, or to
making its way between the epidermal cells (cdotro-phic mycor-
hiza), but it may live exclusive!}^ within the living cells of the
root (cndotropMc mycorhiza). In this latter case the roots
need not necessarily present the short, thick, and much-
branched condition, but may be long and very slender, and
devoid of cortical tissues. The fungus fills the epidermal
cells, which then lie immediately outside the central vascular
cylinder. In those roots which possess cortical parenchyma,
the endotrophic mycorhiza occupies a ring-like zone of cells,
from which fungal threads run towards the outer surface of
the root.

While the symbiotic association of these green plants with
a fungus is only formed when they grow in humus, and is
more largely developed as the soil becomes richer in humus, it
occurs always in plants devoid of chlorophyll, and there the
mycorhiza is either ectotrophic or endotrophic. In these
plants, therefore, the mycorhiza appears as a necessary means
of absorption, while in the chlorophyll-containing plants it is
only occasional and additional means of nutrition. Thus we
find in the absorption and assimilation of decomposed organic
matter a parallel to the conditions obtaining in parasitism, i.e.,
in the absorption of living organic matter. We have obligatory
humus absorbers (Monotropa), to which plants humus is indis-

48 THE PHYSIOLOGY OF PLANTS

pensable, and obligatory parasites (mistletoe), contrasted with
occasional absorbers of humus (forest trees) and occasional
parasites (cow-wheat, yellow rattle). As a similar and equally
important special method of nutrition in the vegetable king-
dom, we must, in our present state of knowledge, consider the
root tubercles of leguminous plants.

If we take up carefully peas, lupins, beans, robinias or true
acacias, and other Leguminos^e, which have been grown in
sand, and rinse the sand from their roots, we find the main
root in some cases, in others the lateral rootlets, bearing
curious fleshy tubercles, which are sometimes of characteristic
shape for different plants. These generally have the appear-
ance of a rudimentary tubercular rootlet, and in the first
stages of their development appear to the naked eye very
much like a lateral root breaking through the cortical tissue.
But in reality their origin is a very different one. These
tubercles arise by active cell division of the inner layers of
the cortex, while the lateral roots arise from the pericambium.
The pericambial layer, however, only takes part later on in
the formation of the tubercles. At the periphery of the
tubercles there will be observed a layer of cambiform cells,
which produce towards the outside large thick-walled cortical
cells, and towards the inside a smaller-celled tissue, and also a
ring of fibro-vascular bundles, which join on to the bundles of
the root. As the tubercles have for some time a meriste-
matic apex, by which they continue to grow, their similarity
to a short succulent rootlet is still further increased. But, of
course, they are not provided with a root cap. In the older
tissues behind the meristematic apex there gradually accumu-
late large quantities of oval or rod-shaped, undivided or forked
bodies, which are capable of moving about in water for many
days. These small bodies can be shown to consist of albu-
minous substances, and therefore characterise the tubercles as
storage tissues for nitrogenous material (albumens), which
are gradually used up by the plant during the ripening of
the seeds.

The tubercles appear in greater numbers the poorer the soil
is in humus and soluble nitrogenous substances, especially
nitrates. If the soil is sterilised by heat and soluble nitrates

THE NUTRITION OF THE ROOT 49

are added to it, leguminous plants will be able to grow, but
will not produce tubercles.

The most important fact, however, is that leguminous plants
exhibit a very healthy development and yield a good crop in a
soil which contains much too little nitrogen for such a crop,
or even when the soil is entirely devoid of nitrogen. In plants
grown under these conditions the production of root tubercles
is very great. It follows from this that these leguminous
plants must have absorbed the nitrogen necessary for their
development from the air, and that this power of absorption
of free nitrogen has some connection with the presence of the
root tubercles. We know, therefore, of some plants which
are able to make use of the air as an inexhaustible source of
nitrogen, and can absorb from it sufficient free uncombined
nitrogen to provide for an entire and rich crop.

This power of making use of the atmospheric nitrogen is, as
far as experiments have so far shown, not possessed by any
other cultivated plants ; they do not seem to derive any
appreciable advantage from it, and for their normal develop-
ment the presence of a large amount of nitrogen in the soil is
essential. The richness of their crop is approximately pro-
portionate to the amount of nitrates in the soil. But there
is in reality no fundamental difference in the methods of
nutrition between leguminous and other plants ; for the former
are able to nourish themselves entirely from nitrates if they
are plentifully present in the soil, but they possess in the
root tubercles a special provision by which they can supply
themselves with nitrogenous food when other plants would
not be able to do so.

This view is the outcome of extensive and conscientious
experiments of Hellriegel, upon which experiments we base
the subsequent remarks.

With regard to the faculty of the Leguminosa' to live upon
the atmospheric nitrogen, it has been proved that the plants
showed a complete and normal development if they were
grown uncovered in the open in a soil with little or no
nitrogen compounds, but in which the other food substances
were present. Their development was furthered, too, in a soil
devoid of nitrogen by the addition to the soil of an extract in

D

50 THE PHYSIOLOGY OF PLANTS

distilled water to the extent of 1-2 per cent, of a soil in which
the same leguminous plants had formerly been grown. After
such an addition, the development of the plants was not only-
normal, but often indeed luxurious ; but their mode of develop-
ment was very different from plants of the same species and
origin which absorbed their nitrogen in the form of nitrates
from a sterilised soil. For while the latter exhibited a con-
tinuous and even growth from the moment of germination,
the plants grown in a soil devoid of nitrates passed through
a very marked and characteristic phase, exhibiting the signs
of starvation, and this was followed, after a longer or shorter
period, by a very active growth.

Such a beneficial extract cannot, however, be made from
every soil, and even an active extract becomes powerless if it
is boiled, or only heated up to 70° C. There is a difference, too,
in the way in which leguminous plants of the same species
react to extracts from different soils. Thus an extract of a
soil in which peas and clover had been regularly grown for a
considerable period, but on which lupins and Seradella had
never yet been grown, was only beneficial for the growth of
peas in the experimental soil, but was perfectly useless for the
growth of lupins or Seradella. Lastly, the very interesting
experiment was tried of growing a leguminous plant in such
a way that one half of the root grew in a solution devoid of
nitrogen, but supplied with such a soil extract, while the other
half of the root grew in a nutritive solution of the same
composition to which the soil extract had been added after
it had been sterilised by boiling. The result was that root
tubercles were only developed in the solution which had not
been sterilised.

All these results point to the conclusion that the develop-
ment of root tubercles — that is, the formation of centres of
assimilation of atmospheric nitrogen — depends on the develop-
ment of organisms within the roots of leguminous plants.
These organisms are probably different for the different kinds of
Leguminosse, and multiply in the soil in which these leguminous
plants have once been grown. This supposition is strengthened
by the fact that for years various observers have described
within the substance of the root tubercles small bodies which

THE KUTRITION OF THE ROOT 51

have been looked upon as parasitic organisms. In the last
few years, too, successful inoculations have been made ; these
consisted in introducing the point of a needle into one of the
root tubercles and then pricking with the same needle the
healthy root of a plant of the same species. Even micro-
scopically the spreading of the organism from the point of
inoculation has been observed ; at least in the case of the
lupin the tubercle-producing parasite is known to form swarm-
spores, which pass over into a zoogloea stage, and this organism
is now known to science as Rhizobium leguminosariim (Bacillus
radicicola, Beyerinck).

In consequence of Hellriegel's experiments, it is probable
that several kinds of Rhizobium occur among the Leguminosae.
The curious circumstance that the plants, when grown in a
soil devoid of nitrogen, pass through a starving condition, in
which they draw upon the reserves of the most important
assimilating organs before they form the root tubercles, is per-
haps also capable of explanation. The roots of all plants con-
tain nitrates. (The upper portions of plants only contain these
substances occasionally, chiefly in the case of annuals.) If we
could, therefore, prove that the organisms which produce the
tubercles did not develop in the presence of nitrates, the follow-
ing would probably be the state of the case. If the soil is
well manured, the parasites enter upon a passive stage (zoogloea
stage); the plants grow without drawing upon the atmospheric
nitrogen. If the soil is poor in nitrogen, the parasites become
partially active, and the formation of tubercles commences.
Should the soil be entirely devoid of nitrogen, the roots will
only contain nitrates as long as those contained within the
seed last out. The moment the root becomes poor in nitrogen
(that is, the time when the plant draws upon its own organs),
it affords the best culture medium for the parasites, and these
produce large numbers of tubercles which assimilate the atmos-
pheric nitrogen.

Horticulturists may, therefore, in the case of pot-culture
of leguminous plants, dispense with any sort of nitrogenous
manuring.

52 THE PHYSIOLOGY OF PLANTS

§ 8. Why and how should we replace the chief nutritive
substance in the soil?

(a) Falluu:

The harvests which are taken from the ground in cultivat-
ing the soil consist either of fruits, seeds, or inflorescences,
in which case the remainder of the plant is returned to the
soil, or else the whole plant may be withdrawn from the soil
for economic purposes. No soil of ours can endure for any
length of time the loss of mineral substances, which form part
of the crop, without being replenished, for no soil contains
sufficient substances to cover the loss. The soil will therefore
become impoverished more or less rapidly, according to the
nature and requirements of the various crops. A certain
number of substances, such as hydrogen, oxygen, and carbon,
are replaced by the natural wetting and aeration of the soil.
Of the mineral substances, lime and iron can be renewed for
the new crops by the natural processes of decomposition taking
place in the soil ; but potassium, phosphoric acid, and nitrogen,
which are taken up in large quantities from the soil into the
younger organs (as in vegetables), or into the ripened storage
tissues (fruits and seeds), must be replaced in some way or other.
In former times, when agricultural methods were less exact-
ing and exhausting, the method employed to restore the pro-
ductive activity of exhausted soil was to let the soil lie fallow,
the natural means which the soil possesses to recoup its losses
by rest. Under the same climatic conditions, fallow land is
moister and warmer than cultivated soil ; but a greater moisture
and higher temperature increase and accelerate the processes of
decomposition which are going on in the soil, which is indicated
by the fact that fallow land is richer in carbonic acid than land
in cultivation. Now the more rapid decomposition of organic
substances on the one hand, and on the other hand the
greater amount of carbonic acid thus formed, which causes a
more rapid solution of the mineral constituents of the soil,
increase the amount of soluble substances by which the next
crop will be benefited. As fallow land has also a smaller

THE A'TJTRITIUN OF THE ROOT 53

covering of plants, it does not lose so much of the natural
condensations by evaporation from the surface of the plants ;
it will therefore also contain, when it begins to be cultivated
again, a larger amount of water, i.e., the means of transport of
the soluble substances. Lastly, a certain amount of enriching
in nitrogen takes place through the wild leguminous plants.

This beneficial effect of lying fallow is, however, only
noticeable in rich soils ; a sandy soil may, indeed, become
impoverished by lying fallow. For the soluble organic sub-
stances resulting from decomposition are not readily absorbed
by the sand, and may be washed down by strong rains into the
deeper layers of the soil, where they will only be accessible to
a few deep-rooting plants. Under such conditions, therefore,
the soil must always have a dense covering of plants. The
latter, however, will of course use up both water and nutritive
substances, and store up nothing for a future crop. A heavy
soil, too, may be impoverished under abnormal climatic con-
ditions by lying fallow. If, for instance, a continuous and
excessive rainfall has closed up the already very small inter-
stices of a clay soil for a considerable period, the process of
decomposition of organic substances may become altered, and
substances are formed which act injuriously on the root struc-
tures. We shall return to this subject in dealing with the
watering of pot-plants.

Lying fallow, therefore, only has the desired result on a
heavy soil, and under normal or dry climatic conditions ; and
the present methods of cultivation have caused this means of
enriching the soil to be entirely given up, and now recourse
is had almost exclusively to the direct addition of nutritive
substances by manuping.

(h.) Inorganic Manures.

Originally stable manure was the only food material directly
added to the soil ; now-a-days, however, artificial manures
have come more to the front. These do not, it is true, con-
tain, like stable manure, all the food substances of plants, but
they furnish the soil with all the most essential and most
important ones \\\ a more concentrated form. In practice it

54 THE PHYSIOLOGY OF PLANTS

will be best to add, in the first place, all the three most impor-
tant food substances, and then to judge from the nature of the
crops which manures should in future be given in increased,
and which in lesser quantities.

According to the poorness of the soil, potassium and phos-
phates should be used to the extent of 25 to 40 lbs. per acre.
The nitrogen in the form of soluble nitrates should be added
in half the above amount.

If you notice a tendency in the plants to run too much to
leaf, and to remain green for too long a time, decrease the
amount of potassium and nitrogen, or increase the amount of
phosphoric acid.

The chief forms of mineral manures are ammonium sulphate
and Chili saltpetre (sodium nitrate). The former, to be in con-
dition for absorption by the plant, must be transformed in the
soil into a nitrate, and should therefore be in the soil some
considerable time before the sowing takes place. The Chili
saltpetre, which is readily soluble, but is scarcely at all absorbed
by the soil, and is therefore liable to be washed into the deeper
layers by heavy rain, must be placed on the ground very shortly
before the sowing or planting of the field. As it is immediately
effective, it may be recommended for occasions where plants
which have suffered from external disturbances (cold, hail, or
drought) are just beginning to recover, and require some
stimulant to accelerate their growth.

Potassium is commercially obtainable in form of raw potas-
sium sulphate, or as a purer and more concentrated salt. In
whatever form it is used, it must be mixed sufficiently deeply
in the soil, not placed on the top, and must be introduced into
the soil at a time when a long damp period is certain to
follow (autumn or winter). If the soil is not rich in lime, it
is advisable, when manui'ing with the raw potassium sulphate
(which contains a large amount of magnesium chloride) to add
a considerable amount of powdered quicklime.

Which of the commercial potassium salts should be used in
any special case will depend partly on the distance of transport.
If the distance between the market and the estate is great, the
more concentrated forms are of course more advisable.

Phosphoric acid, too, can be obtained in various forms.

THE NUTRITION OF THE ROOT 55

The so-called raw phosphates and bone-meal contain this
nutritive substance in a very insoluble form, and must there-
fore be mixed with the soil a considerable time before sowing.
They are only to be recommended for fields known to be suffi-
ciently damp, and for autumn sowing, where the chief produc-
tivity takes place in the spring. If bone-meal is to act more
rapidly, it should be left to ferment in heaps before use. The
quickest effect is obtained from superphosphates. If it is
desirable to add nitrogen as well as phosphoric acid to the
soil, a mixture of ammonium sulphate and superphosphates,
so-called ammoniacal superphosphate, may be used. The third
chief nutritive substance, which contains potassium as well as
nitrogen and phosphoric acid in readily soluble form, is Peru
guano. These last two substances, on account of their being
immediately available by the plants, are used as top-dressing
for weak crops, and by gardeners for wholesale growing of
herbaceous plants, especially foliage plants. From the first of
these two preparations we must, however, distinguish the raw
ammonia superphosphate, which must only be employed in
fields which are well aerated and have a considerable moisture,
for this preparation contains a considerable amount of rhodan-
ammonium, which acts injuriously on plants.

A preparation which has more recently been largely used
is the Thomas-slag, which is derived from ironworks, and
which must be ground as fine as possible. It contains about
40 to 60 per cent, of phosphate of lime in different com-
binations (tricalcium phosphate, &c.), besides silica, clay, iron
oxides, and traces of magnesium, chlorine, and sulphur. The
lime, besides occurring in combination with phospliorus, is
present in considerable quantities combined with silica, and
also in the form of quicklime. This manure is also most
effective in a damp soil, and when added at an early period.
In a dry soil it is only rendered soluble after a considerable
time.

(c.) Organic Manures.

In spite of the evident beneficial results of inorganic
manures, and in spite of the fact that they are indispensable

56 THE PHYSIOLOGY OF PLANTS

in our present system of agriculture, they cannot replace
stable manures entirely. Our agricultural efforts must aim
at getting as much animal dung as possible, and to use the
mineral manures as additions to the former. For besides
taking into account the chemical constitution of the soil, we
must also bear in mind its physical or mechanical structure,
and the latter is greatly improved by stable manure, but is
rarely benefited — sometimes indeed deteriorates — through
mineral manures. As an example of the deterioration of good
soil under such conditions, the continuous manuring with
saltpetre and common salt might be cited. After successive
manurings of a clay soil with saltpetre, the finest particles of
clay will be found washed together and deposited in a solid
and compact form.

As the nitrates are so easily washed out of the soil, the
latter often becomes so caked that plants cannot be grown in
it. Manuring with saltpetre may, therefore, at first produce
fine green crops, but soon a sudden deterioration will be
noticed. Similar results follow on the manuring with common
salt, which is carried on in the case of certain crops. Soils
which contain basic carbonates in large quantities (alkaline
soils) have a consistency which renders their working quite
impossible.

(d.) Stable Manures.

As it is the first endeavour of an agriculturist to produce
the most favourable physical conditions of a field and to
preserve them, because otherwise chemical changes take place
which are detrimental to the life of plants, the chief point
which has to be considered in manuring a field is to find a
manure which will serve the above purpose best, and this is
certainly stable manure.

The ideal condition of a soil is one in which it resembles a
sponge, and in which it will retain the greatest amount of
nutritive substances and water, without losing its capacity of
absorbing air.

Heavy soils can be made to approximate this condition by
addition of straw-containing manure, while a light soil requires

T?IE NUTRITl():s' OF THE ROOT

57

the shorter decayed manure. Of course such manuring is
not always seasonable, and in each case the gardener or farmer
has to consider whether he ought to change the mechanical
nature of his soil, or need only enrich his field with concen-
trated mineral manures. For though stable manure contains
all the necessary ingredients, it does not contain them in so
concentrated a form as artificial manures. To give an approxi-
mate idea of the nutritive value of stable manures, we append
the mean values published in Mayer's " Agricultural Chemis-
try," and based upon Heiden's calculations. The following
amounts are contained in the excrements of the

Horse.

In tlie Dung.

In the Urine.

In a Mixture
of both.

Per Cent.

Per Cent.

Per Cent.

Water .

. 76

87-92

76-79

Organic substance .

21

6.9

19

Nitrogen

. 0.4-0.5

1-5

0.6

Potassium

• 0.35

1.6

'

Phosphoric acid

. 0.32

.0

l.

Total ash

• 3-15

Oxen.

3-1

3.15

Water .

. 82-S5

92-95

86-89

Organic substance .

. 14.6

3-2

10-12

Nitrogen

. 0.17-0.38

0.3-0.9

0.34-0.44

Potassium

• 0.05

1-3

Phosphoric acid

. 0.15

.0

Total ash

• 1-9

Sheep.

31

2.1-2.4

Water .

• 57-75

85-90

67.0

Organic substance .

• 24-37

5-IO

27.5

Nitrogen

. 0.5-0.7

1-3-2.5

0.9

Potassium

. 0.1-0.4

2.1-3-3

'I

Phosphoric acid

. 0.3-0.6

Trace

''

Total ash

• 3-0-5-7
Pig.

3.2-6

5-4

Water .

• 77-84

98

82

Organic substance .

. ic^i5

I

14

Nitrogen

. '0.7

0.23

0.6

Potassium

. 0.27

0.7

1

Phosphoric acid

. 0.4

'.

?

Ash . . .

. 6.5-7.5

I.O

1-7

5«

THE PHYSIOLOGY OF PLANTS

Pigeon Dung.

Fowl Dung. Duck Dung. Goose D

Per Cent.

Per Cent. Per Cent. Per Ce

Water .

. 62

65

S3 82

Organic substance

• 31-32

21-26

40 14

Nitrogen .

. 1.2-2.4

0.7-1.9

0.8 0.6

Basic salts

. 2.0-2.2

1.2-4.0
(0.9 Potassium)

0.4 3. 1

Phosphates

. 3.0-4.2

(I

2.0-5.0
2 Phosphoric acid)

3-5 0-9

Ash .

. 6.0-7.0

9.0-14.0

7.0 4.0

Fresh Human

Closet

Excrements.

Manure.

Per Cent.

Per Cent.

Water

• 92.9

97.0

Organic substance .

• 57

1-5

Nitrogen

. 1.06

0-35

Potassium

0.22

0.20

Phosphoric

acid

. 0.23

0.28

Ash .

• 1-37

1-5

If we compare with these values the most perfect of artificial
manures of organic origin, guano, we find that it contains —

Peru Guano.

iSaker Guano.

Sombrero Gviano.

Per Cent.

Per Cent.

Per Cent.

Water .

. 14.S

3-3-I0.5

2.9-10. 1

Organic substance
ammonium salts

and )

. 1 ^^-^

6.6-9.5

4.7-6.6

Nitrogen .

■ 144

0.3-1.0

?

Phosphoric acid

• 13-5

37.0-40.3

3^5-39-6

Alkaline salts-

Potass .

7.4

0.1-0.6

Traces

Lime .

39- 1-43-5

22.7-51.7

Ash.

'. 32.8

80.8-89.7

85.0-90.Q

Of the pure mineral manures, which only contain a few of
the nutritive substances, the percentage can be gathered from
their advertisements.

In stable manures, however, the animal excrements do not
form the chief mass, but the latter is formed by the substance
which serves as medium of absorption or litter. The latter
generally consists of straw, so we append an analysis of straw
and of other substances which are used for the same pur-
pose, such as pea-stalks, dry leaves, and fresh leaves of pine
or fir : ^ —

1 It cannot be considered the object of this book to fully discuss all the
materials used as manures, although the practical importance of that subject
is fully recognised. But we are here only concerned in illustrating the nature

THE NUTRITION OF THE ROOT 59

Corn-
stalks.
Per Cent.

Pea-
Stalks.
Per Cent.

Autumn
Ijcaves.
Per Cent.

I''re.sh I.eaveN

of Fir or Pine.

Per Cent.

Water .
Organic substance
Woody fibres .

. 12-21

• 75-83

• 29-53

12-17
82-83
34-52

13-15

78-81
II-16

47-5
52

Nitrogen .

. 0.3-0.9

2.0

0.8-1.4

0.5-0.9

Potassium

. 0.5-1. 1

I.I

0.15-0.4

0.03-0. 1

Phosphoric acid

. 0.2-0.3

0.4

0.2 0.3

0.1-0.2

Ash .

. 3.0-8.0

3.0 4C

4-2-5-7

0.8

In the use of stable manure,
a mixture of animal excrements and litter, our chief object is
to use it to its best advantage. This is only possible if we
allow the constantly occurring processes of decomposition to
run their proper course. This they will do if a sufficient
amount of oxygen is present. Then certain processes of oxida-
tion are set up, which we term decomposition, or which, in
the case of nitrogenous substance, we sometimes describe as
decaying. If, however, the supply of oxygen is deficient or
ceases entirely, other processes will take place, and the products
of these are often injurious to plant life.

The above processes of decomposition, which take place b}^
the agency of bacteria, break up the organic substances into
carbonic acid, water, ammonia, and free nitrogen, which sub-
stances are partly given off as gases, leaving the mineral con-
stituents in an easily assimilated form. The ammonia is
transformed by bacteria into nitric acid ; the formation of
carbonic acid takes place even in the absence of oxygen, but
is more plentiful up to a certain degree in the presence of
oxygen.

Light and heat are, of course, not without their effect. But
while light acts in such a way as to retard the formation of
nitric acid, probably because the formative bacteria are averse
to light, the increase of temperature up to T^y" C. causes an
increased amount of nitrification (formation of nitric acid) to
take place. The maximum evolution of carbonic acid takes
place about 60° C.

Moisture also furthers the decomposition of organic snb-

of manuring by the substances most widely used, and this only in so far as it
is of importance in the understanding of the physiological processes of the
plant. The same is true of other portions of the subject which belong more
especially to the chemistry of agriculture.

6o THE PHYSIOLOGY OF PLANTS

stance, as long as the soil remains porous enough to allow the
access of oxygen ; but still in a thoroughly saturated soil the
formation of carbonic acid does not cease entirely.

Of course the absolute amount of carbonic acid and nitric
acid produced depends mainly upon the nature of the substance
which is being decomposed and upon the substances with
which it may be mixed. The rapidity of the decomposition of
various substances used as manures can be roughly gathered
from the following figures, which represent the volumes of
carbonic acid contained in i ooo volumes of air after i gramme
of the substance has been decomposing for twenty-four hours :
— Steamed bone-meal liberates 3 1 ; Peru guano, 24 ; Pigeon-
droppings, 26 ; fresh swine-dung, 1 4 ; partially decomposed
horse-dung, 1 2 ; fresh cow-dung, 1 3 , and after three months
about 9 volumes of carbonic acid ; while peat gives oft' only 2 ,
sawdust 5, ground horn 6, dead leaves 7—8, pine leaves 9, corn-
stalks 17-19, and pea and bean stalks 22 volumes of carbonic
acid. Though, generally speaking, the addition of quicklime
during the later stages of decomposition becomes advantageous
on account of the combination of the lime with humic acid,
which decomposes more rapidly than the humic acid alone,
quicklime generally retards the initial stages of decomposi-
tion (Wollny). In the same way, mineral acids even in small
quantities retard decomposition, while weak solutions of alkaline
carbonates stimulate decomposition. The latter will also be
stimulated by small additions of sodium nitrate, but is retarded
by large amounts of this salt. Common salt (sodium chloride)
always retards decomposition.

As a general guide for the rapidity of decomposition, we
may place the manures in the following order. The most
rapid of decomposition are bone-meal, flesh-guano, flesh-meal,
the droppings of birds ; straw and stable manure are slower ;
still slower are leather scrapings, horn, dried leaves and saw-
dust ; peat is slowest of all to decompose.

Other things being equal, the heat developed will be pro-
portional to the rapidity of decomposition. Horse-dung heats
the soil most effectively, cow-dung least. Decomposing corn-
stalks give off less heat than the stalks of the pea, which
contain more nitrogen.

THE NUTRITION OF THE ROOT 6i

s^ 9. How can the soil best meet the requirements of the
roots for air ?

It is not only because the soil contains an extraordinarily
large quantity of the nutritive solutions in its pores that we
have compared the ideal soil with a sponge, but also on account
of its permeability for air. The roots must breathe, and there-
fore the soil on which our crops grow, must be of such a nature
that down to a certain depth there must be a sufficient inter-
change between the air contained in the soil and the outer
atmosphere. This explains the advantages of the addition of
organic humus-forming substances to all soils, whether they be
light or heavy. The nature of the air contained in the soil is
not, however, immaterial. If the processes of decomposition
in the soil take a wrong course, gases detrimental to the root
may be formed ; and as we are able, whenever we work or
manure the soil, to change its structure and its porosity, we
have every reason to pay attention to the interchange of
gases.

The air contained in the soil has not the same composition
as the atmospheric air ; for within the soil oxygen is continu-
ally being used up by the processes of decomposition of organic
substance, and the roots themselves constantly absorb some
and give off other gases. Besides these two causes, various
gases are physically fixed (absorbed) by the small particles
of the soil. Of these gases, we are at present only interested
in two — oxygen and carbonic acid. Nitrogen, and its com-
bination with hydrogen (ammonia), we have previously dealt
with.

We may assume that, generally speaking, the air of the soil
is richer in carbonic acid and poorer in oxygen than the
atmosphere. The rapidity of exchange depends on the atmos-
pheric pressure, and upon the moisture of the soil. The
greater the water capacity of the soil, the greater are the
variations in its permeability. If much water is present, the
permeability for air is greatly reduced. A loosening of the
soil will increase the permeability to a greater extent, the
finer the particles of the soil are.

62 THE PHYSIOLOGY OF PLANTS

A passing decrease of the permeability is caused by the
frost, which causes the water contained in the interstices of
the soil to expand and thus decreases the pores. The water
itself, in such a case, looses its mobility. If at this time coarse
organic substance is introduced, so as to enlarge the interstices,
we at the same time introduce a source of carbonic acid, which
lasts as long as oxygen has access to keep up the decomposi-
tion. A purely mineral soil contains air of about the same
composition as that of the atmospheric air. But even if the
oxygen of the air is excluded, decomposition still takes place,
partly at the cost of the existing organic substance itself, or
with the aid of oxygen gained from the reduction of inorganic
substances. This process, however, is comparatively unpro-
ductive.

Although the oxygen contained in the soil is of chief
importance for the aeration of the roots, the carbonic acid
present in the air and water of the soil is chiefly concerned
in the chemical changes, and therefore indirectly in the phy-
sical constitution.

Distilled water, of course, can only take from the soil the
soluble substances, and give them off again when it evaporates.
The cai-bonic acid, however, contained in the water enables
it to dissolve substances which are insoluble in pure water
— as, for instance, the carbonates of lime and of magnesium,
ferrous carbonate, and manganous carbonate. The simple
carbonate of lime is transformed into bicarbonate ; the phos-
phates of lime and magnesium become soluble in presence of
carbonic acid, and the same is the case with phosphates and
silicates of iron, ferrous oxide, and the silicates of potassium,
sodium, lime, and magnesium.

The carbonic acid of the air, in conjunction with the oxygen,
acts in the same way as the carbonic acid of the water. It
enables the oxygen to transform ferrous and manganous oxides
into the higher oxides, and the resulting ferric and manganic
oxides, as they take up more space than the lower oxides, help
to break up the mineral substances of the soil. The yellow or
brownish iron pyrites (FeS.,) is changed by the oxygen, in pre-
sence of water, into ferrous sulphate and free sulphuric acid.
When these products become dissolved in the water, they change

THE NUTRITION OF THE ROOT 63

the carbonate of lime into gypsum, sodium chloride into the
sulphate, and the magnesium contained in the insoluble
iron and magnesium compound (dolomite) is transformed
into the very soluble magnesium sulphate (Epsom salts). In
like manner the insoluble tribasic phosphate of lime is con-
verted into the soluble phosphate, and clay is split up into
a sulphate and a silicate of aluminium and magnesium.

By this breaking up of substances the number of soluble
substances necessary for the active functioning of the roots is
greatly increased. To cause them to function most actively, it
is necessary to maintain as even a supply of oxygen as possible,
and this can only be done by keeping the physical structure
of the soil as constant as possible. This is actually brought
about by the breaking up of the soil. The hardest mineral
cannot withstand a continued action of the carbonic acid. We
find large blocks of granite sticking out of the ground, the
surface of which seems eaten away, and when touched they
often break to pieces. The eroded surface is caused by the
action of the carbonic acid on one of the constituents of the
granite, the feldspar, from which it extracts in the first place
the potassium and the sodium. The feldspar is thus transformed
into clay, from which the more resistant pieces of quartz and
mica project. The water containing carbonic acid now sepa-
rates from the silicates the monoxides (ferrous oxide, lime,
magnesium, potassium, and sodium oxides), and by uniting
with them it liberates some hydrated alumina, which does
not readily combine with carbonic acid, and also a small quan-
tity of silica. This potassium containing feldspar (orthoclas,
3SiO^(K.,0 -I- Al^Og)) thus furnishes a large amount of the
necessary potassium. The rain then washes away the above-
mentioned monoxides, and also the sesqui-oxides (ferric
oxide and alumina), until only a soft finely-divided mass
remains, which forms the almost indestructible framework
of the soil, consisting of fine granules of quartz sand and
silicates.

To give some idea of the permeability of the soil for
air, which is constantly changing as the constitution of the
soil is being changed by the breaking down of its original
constituents, we append a table drawn up by Ammon.

64

THE PHYSIOLOGY OF PLANTS

This table shows how many litres of air pass in one hour
through a depth of 50 cm. of soil under a pressure of 40
mm. of water —

Clay .

Caolin .

Powdered lime mixed with humus

Chalk .

Pure broken limeston<

Peat .

Quartz sand .

Broken -up clay

Quartz sand .

Quartz sand .

Broken-up clay

Quartz sand .

Broken-up clay

in powdered condition

I size of particles up to
j 0.25 mm.

particles of 0.25-0.50 J 30.
mm. \ 41.

size of particles

0.5-1.0 mm.

size of particles

1-2 mm.

Litres.
1.62
2.84
3-32

3.78

4.24

5-04
16.80

04
f 92.24
112375

r 287.57
I420.16

This table is interesting in showing the effect of breaking
up a soil which is not readily permeable. The broken-up
clay is shown to be more porous than the coarse-grained sand ;
but a small amount of powdered clay added to the quartz sand
will greatly reduce the porosity of the latter. In fact, upon
the thickness of a layer of clay which lies between two layers of
sand depends the porosity of the whole soil, as all the absorbed
air must pass through it. The moisture of the air and of the
soil, too, control the amount of air which passes through the
soil in a given time. Dry air moves more rapidly through a
moist soil than air containing a large amount of water vapour,
and so air of a given nature passes more rapidly through a dry
than through a wet soil ; most rapidly, however, through a
slightly moistened soil. This latter phenomenon is probably
due to the fact that small particles of sand are broken up by
being moistened.

How the various physical properties of a soil are changed
by the varying proportions of clay and sand in a field, and
how important the presence of stable manure is in this connec-
tion, will be seen from the following figures of Masure : —

THE NUTRITION OF THE ROOT

65

Garden
Soil.

Sand.

Powdered
Lime.

Clay.

stable
Manure.

Saturation for rain-water of \

the various components of J-

0.39

0.30

0.44

0.68

0.56

the soil, reduced to i vol. J

The saturated soil loses per^

diem water to the amount -

4.20

3-70

3-50

4.30

4.50

of (in millimetres) ... J

The hygroscopic capacity of \

the soil : 100 grammes 1
of soil condense water I

5.60

2.10

3.60

7.0

41.0

(amount in granmies) . . )

Capacity of soil to absorb ^

heat, expressed by dif-

ference of temperature of
dry soil above temperature

14.2

10.7

9.0

11.50

14.70

of surrounding air (degrees

in centigrade) .... J

Cooling at night, difference
of soil behnv that of air .

-1.60

-1.7

-1.8

-1.8

-o-S

Permeability for air, expressed i

by the amount of oxygen 1
absorbed by loogrammesof T

14-18

1.6

10.10

15-3

20.3

soil (amount in grammes) )

§ 10. How can we improve our fields so as to obtain the
best possible crops ?

In practice, fields may be grouped, according to their consti-
tution, into those with a sandy soil, those with a lime soil, and
those with a clay soil. Let us examine them in turn.

The characteristic of a sandy soil is that it can be saturated
with a relatively small amount of water. The smaller the
grains of sand, the less porous is the soil, and the greater is
its power of retaining in its lower layers a small supply of
water and of condensing the atmospheric moisture at its sur-
face. Powdery sand is harmful on account of its density (im-
perviousness). Its great power of absorbing heat in its upper
layers from the direct rays of the sun is of advantage only
in cold situations, usually, however, harmful. Its power of
absorbing oxygen is small. To improve fields of this kind, a
large amount of stable manure is necessary, as the rain-water
is then retained in lai-ge quantities, and at the same time the
requirements of the plants in this direction are reduced. For
experiments have proved that in a well-manured soil the roots
need less water for the production of a given amount of vege-

66 THE PHYSIOLOGY OF PLANTS

table substance than in a soil poor in food substances though
rich in watei\ The roots must take up a certain fixed amount
of nutritive matter for every gramme of dry substance which
the leaves have to form. If the solution which the root finds
in the soil is very dilute, then a comparatively large amount
of this weak solution must be taken into the plant, and much
water must therefore be passed out by transpiration through
the leaves ; and, as a matter of fact, a square inch of leaf
surface of a plant growing in dilute nutritive solution gives ofi"
a great deal more water within a given space of time than does
a plant grown in a richer solution. After sowing it is very
advantageous to roll a dry and warm sandy soil, as its power
of retaining water is greatly increased by this procedure.

The chalky and marly soils are the chief forms of lime-con-
taining soils. The chalky soil consists of sandy lime mixed
with a varying amount of powdery lime. From the table con-
tained in the preceding paragraph we see that chalk can absorb
more water than sand, and does not lose it so rapidly by
evaporation. It is not so readily heated by the sun as sand,
and is only slightly more hygroscopic. Chalk soils must,
therefore, be treated very much like sandy soils, but they are
able to derive more benefit from manuring, and do so more
rapidly than sandy soils.

Marls contain an intimate mixture of pov^^dery lime and
clay ; the properties of the latter predominate and give to the
soil its characteristic features. It must, therefore, be treated
like a clay soil. It is more fertile than a clay soil, and this is
due to the presence of the lime, to which we shall refer again
in speaking of the addition of lime to the soil.

Every field may be said to consist of a clay soil if it contains
sufiicient clay, for the properties of this substance to predominate.
This may take place when it forms only 30 per cent, of the
soil. It is characterised by its extraordinary power of retain-
ing water, so that it very rarely drys up. If the atmosphere
is moist, its great hygroscopic powers prevent its becoming too
dry. If a long and hot drought sets in, the ground cracks,
and then of course more water is lost. It is as rapidly heated
by the sun as is sand, and has the power of absorbing a large
amount of oxygen. We see, therefore, that the clay soil has

THE NUTRITION OF THE ROOT 67

properties which would ensure a rapid decomposition of manure,
a process which is, however, retarded by the chief fault of this
soil, its great wetness. To counteract this, the soil must be
well drained, besides which, deep ploughing and repeated
harrowing, to cause an even breaking up of the soil, are very
beneficial. A large addition of straw-containing manure is
very paying, as it keeps up the porosity and permeability of
the soil.

The chief means of improving the soil is, therefore, as we
have seen, the use of stable manure, the nutritive properties of
which we have already discussed. Besides these, it is proved
to be useful on account of its faculty of retaining water. It
can absorb its own, and sometimes even more than its own,
weight of water. Being very porous, it is true that it loses
this water very rapidly, but its great hygroscopic capacity
causes it to absorb the necessary amount of this indispensable
substance. Dry stable manure can absorb from the atmos-
phere and retain 40 per cent, of its weight of water, and this
absorption of water vapour takes place in greatest quantity
towards the morning, and thus from the absorption of dew the soil
obtains carbonic acid, ammonia, and nitrates, which are of con-
siderable benefit to the vegetation. Besides its own nutritive
value, stable manure has, as we see, the power of further enrich-
ing the soil from the atmosphere, and this fertilising power is
enhanced by the fact that stable-manure has a greater power
than any other substance of absorbing the heat of the sun and
the oxygen of the air. It accelerates, therefore, the pi'ocesses
of decomposition taking place in the soil, and thus brings
about and keeps up the conditions necessary for the active life
of the roots.

Manuring with stable manure must therefore remain the
best method of improving the soil for every sort of cultivation,
while artificial manures may form useful additions.

Where market-gardening is carried on on a very large scale,
so that occasionally there may be a lack of stable manure, it
will be found useful to plough in some crop before it has
matured its seeds. Leguminous plants, as, for instance, lupins,
are most useful for this purpose, as their power of forming
nitrogenous subtances from the nitrogen of the air will

68 THE PHYSIOLOGY OF PLANTS

further enrich a poor soil. Spurrey and buckwheat are often
used, or on good soil white mustard may be grown for thi&
purpose.

Very often a field may be greatly improved by the addition
of lime, marl, or gypsum.

Heavy and caking clay soils which are poor in humu&
often stick to the plough and other implements when wet, and
when dry form large impracticable clods. Such a soil may
be improved by ploughing it up in the autumn and allowing
it to lie in its rough condition during the winter ; but the more
friable condition which it then assumes is soon lost during the
summer after heavy rains. If the addition of stable manure,
humus, or peaty soil is impossible, then lime or marls should
be added.

The loosening of the soil which is thus brought about is
probably due to the chemical action of the lime. If it has to
be employed rapidly during a dry season, so that the natural
breaking up of the limestone cannot be awaited, and if no
powdered form is available, the following procedure can be
recommended : — The burnt limestone should be filled into
baskets and these plunged into water until no further air
bubbles appear (3 to 4 minutes). Then it should be piled
in heaps. The pieces break up of themselves, and the lime-
stone (slaked), which had given off its carbonic acid when
heated, now forms a white powder of calcium hydroxide
(CaHgOJ or so-called slacked lime, which is soluble in 730
parts of cold, or 1300 parts of boiling water (lime-water).

The lime attacks the silicates of the clay, decomposes them
and liberates soluble potassium salts. It also accelerates the
decomposition of organic substances. This is the reason why
a field treated in this way uses up its organic manure more
rapidly than a field to which no lime is added. A lime soil
may therefore be said to be of a devouring nature.

With regard to the manipulation of the lime, it should be
strewn very evenly over the ground, either by hand or with a
spade, and this operation should not be carried on during
windy weather. It will be found advantageous to lay the
lime on the stubbles in the autumn and then to plough it in.
If forced to wait until the spring, it is best to strew the lime

THE NUTRITION OF THE ROOT 69

over the fields a considerable time before sowing, as soon,
indeed, as the soil has become slightly dry. Small quantities
(5 to 10 cwt. per acre) added after every five years are more
beneficial than a single heavy addition of lime, as the decom-
position of humus becomes so active that very little is left for
subsequent harvests. Of course the above-mentioned quantity
must only be looked upon as an approximate value ; the actual
quantity will depend greatly on the nature of the soil. Great
care is needed in treating light sandy soil with lime, while on
a tenacious clay soil lai'ge quantities may be used. The most
conspicuous and rapid results of this treatment will be shown
by a soil rich in humus but poor in lime upon which the
Sheep Sorrel {Bumex Acetosclla) grows abundantly. In such
soil the lime will act directly as food material.

As in the process of marling the lime contained in the
marl is also the active principle, it is evident that a soil rich
in humus and in clay will stand it much better than a light
sandy soil. To the latter a larger amount of clay marl than
of calcareous marl or shell marl may be added. The latter
two forms of mai'l are more advantageous for clay soils. The
much-dreaded over-marling will only occur when too small
an amount of stable manure is added ; we must remember
that this substance is the basis of the permanent fertility of
the soil.

Besides the increased decomposition of organic substances
by the lime, which results in an enormous increase in the
production of carbonic acid, the lime fixes many free acids
which are injurious to vegetable life, transforms the ferrous
into ferric oxide, and causes a greater absorption of basic food
substances. The bases are contained in the soil in the form
of hydrated silicates and humus salts. The bases, if they are
to combine with the humus compounds, must be united to
carbonic acid. Now the lime promotes the formation of such
carbonates. Besides these indirect effects of marling, there
are some direct actions which must be mentioned. A direct
enriching of the soil is effected by the potassium, phosphoric
acid, and magnesia which every kind of marl contains.

The addition of gypsum, which was already practised by
the Greeks and Romans, is effective in making the potassium

70 THE PHYSIOLOGY OF PLANTS

of the soil available to plants as a sulphate, while lime is de-
posited by this change. In practice it is usual to strew the
gypsum on plants freshly covered with dew or moistened by
rain. This method is justified by the fact that a solution of
gypsum is rapidly formed on the wet plants, and dropping
down from these into the soil, it becomes almost immediately
active in proximity of the roots.

§ 11. How is the nutrition of pot- plants effected ?

The points which are of greatest importance in dealing with
the manuring of crops in the open are of secondary im-
portance in speaking of pot-cultures, unless in some excep-
tional case there should be, by some gross neglect, an actual
deficiency of some food substance. The various soils which
are used for potting purposes all contain at least sufficient
food matter for the growth of the plants ; but even the
richest can only last for a certain time in the very limited
space of a pot. In all cases a renewal of the soil has to take
place by re-potting, and it is the gardener's object to select
the proper time for this proceeding.

If, therefore, the root of a pot-plant finds in every soil the
necessary quantity of food substances, the usual choice of
heather soil, leaf or peat mould, loam or turfy soil, for different
plants, is not made because one kind of mould contains food
substances which are absent from another, or because some
substance is present in greater quantity in one soil than in
another. Such differences exist of course in the different
soils,'^ but are of little importance in the case of pot-plants,
because usually the earth of a pot is renewed before the roots

^ The following table of Loges gives a comparison of the amount of the various
nutritive substances contained in a leaf-mould from various sources. The dry
weight of these moulds contained — •

Willow

Aspen Poplar

Scotch Fir

Pine

(Salix caprea).

(Po'p. tremula).

{Pimis silv.).

(Picea excelsa).

Per Cent.

Per Cent.

Per Cent.

Per Cent.

Nitrogen . .

. 1.46

0.97

0-95

0.81

Fats. . . .

. 4.0S

9.49

8.72

12.01

Carbo-hydrates

• 57-92

45-14

44-36

46-23

Fibres . . .

. 22.97

33-19

39.16

32.10

Potassium . .

1.60

0.31

0-33

0.06

Phosphoric acid

. 0.32

0.22

O.IO

0.12

THE NUTRITION OF THE ROOT 71

have exhausted the nutritive substances, and because we can re-
newthese by liquid manures during the life of the plant,and often
increase their amount above the original supply of food matter.

In the case of pot-plants, the choice of the various soils
depends upon their physical properties, upon their power of
retaining water, of absorbing nutritive substances, and allowing
a sufficient amount of air to enter for the aeration of the root.

With regard to the latter, our cultivated plants have very
diffei'ent requirements. As far as the development of the
plants is dependent upon the soil, we may say that : It is not
the total amount of nutritive substances contained in the soil
which determines the vigour of growth of a pot-plant, but it is
due to the degree of concentration of the nutritive solutions in
the soil, and to the intensity of its aeration.

It is the knowledge of this fact, gained by experience, which
distinguishes successful cultures from the attempts of beginners,
who are always searching for rich mould. The " richness,"
i.e., the amount of nutritive substance, we can easily supply to
every flower-pot, but we cannot do the same with the physical
conditions which are necessary for the active growth, and
especially for the respiration of the root.

We have at present no scientific knowledge of the actual
amounts of air necessary for our various plants, and we must
therefore at present make use of the practical experience of
gardeners. Now this teaches us that a root can never have
too much air, but often has too little. Without taking into
account those plants which are provided with aerial roots, we
need only remember the exposed roots of trees on rocky slopes,
walls, and roads, and think for a moment of the habit of many
plants in our greenhouses of sending up delicate rootlets out of
the soil into the air at the edge of the pot. All plants aie
also able to grow in moss or sand if the disadvantages of these
media are remedied by abundant watering with nutritive solu-
tions. As soon as we are able to satisfy the wants of roots
with regard to water and nutritive substances, we may choose
beads of glass or crumbled quartz instead of ordinary soil.
Indeed, by timely changes of tbe nutritive solution, and by
constantly renewing the supply of oxygen, we can cultivate
plants for years in water itself.

72 THE PHYSIOLOGY OF PLANTS

This fact, which has been proved by numerous experiments,
shows us how little we are tied to any special soil in the case
of potting plants, if we only pay sufficient attention to the
aeration of the soil. It is advisable, unless we know by experi-
ence that a certain species can stand a heavy soil, to choose in
the first instance some light kind of mould. To these belong,
in the first instance, heather soil, then follow the soils rich in
humus, such as leaf-mould, and lastly loam and clay.

Besides the costliness of employing heather soil in all cases,
the latter has the disadvantage of not absorbing nutritive sub-
stances, and of drying up very rapidly. In extensive potting,
therefore, leaf-mould is more largely used, as there is less
danger of the above-mentioned disadvantages, though in its
pure condition it too is liable to rapid drying up. The mould
resulting from the decay of leaves and twigs is therefore rarely
used in a pure condition, but is mixed according to necessity
with some animal manure and with loam. In what way this
changes the physical and chemical conditions of the soil may
be gathered from our preceding chapters.

As soon as we have realised that a considerable and per-
manent permeability of the soil is one of the chief necessities
in pot-culture, we have settled the vexed question as to whether
riddled or unriddled soils are most advantageous. By riddling
the soil only the finer particles are used ; this decreases the
size of the interstices of the soil, increases the capacity of
retaining water, and tends to clog up the soil, and thus cuts
off from the roots the proper supply of air. These dangers are
not counterbalanced by any advantages whatever, for the argu-
ment that a closely packed soil offers more nutritive material
to the roots than a soil with larger particles, and therefore
larger interstices, has no weight when we remember that by
a single application of liquid manure more food material can
be added to the soil than could possibly be contained in the
additional particles of the mould. In all pot-cultures, there-
fore, unriddled mould should be used. But we have mentioned
before that another point is of the greatest importance for pot-
culture, and this one is very often sinned against. We must
take into proper consideration the eoneentpation of the nutri-
tive solution which is offered to the roots. The water con-

THE NUTRITION OF THE ROOT 73

tained in the interstices of the soil contains a greater or less
amount of nutritive substances which it has dissolved on its
way to the roots, and constitutes therefore the nutritive solu-
tion. According to the amounts present in the soil, the solu-
tion will contain more of one substance and less of another, and
Avill therefore be concentrated in various degrees. Now, every
root needs for the proper development of the upper portions of
the plant a definite saturation of the nutritive liquid of the
soil. The plant will not die if this advantageous degree of
concentration is not reached, but it will not flourish so well as
it might do with a greater concentration of the nutritive fluid,
other things being equal.

If the fault lies in the fact that the solutions are too dilute,
the plant knows how to remedy the defect. The roots absorb
great quantities of the solution, and the plant gives off the
superfluous water through its leaves by an increased transpira-
tion, retaining the dissolved substances. The general appear-
ance of the plant betrays to some extent the method of its
nutrition, as large pale leaves are formed. No sickening of
the plant, however, is noticeable. The case becomes more
serious if the concentration of the water contained in the pot
becomes too strong, owing to a too rapidly repeated application
of manures. The growth of the upper portions of the plant
is then visibly retarded, the internodes are shortened, and the
leaves become puckered owing to the shortening of the midrib
and lateral veins, or are bent in various directions, spotted, and
fall oS" at an early period. The roots themselves are short,
thick, and bent up, and the newly-formed root-hairs are irre-
gular and shortened, soon become brown and discoloured, and
die away. As I know well from the many plants which have
been sent me by gardeners, these appearances are usually put
down to other causes, as gardeners are hard to convince that
plants may be easily overfed. This overfeeding of plants is
g-reatly on the inerease at present, and we shall prevent many
losses by being more moderate in the manuring of pot-plants.

We must not forget that the kind of soil and manure which
is sufficient, and perhaps the best, for a certain species of plant
may be much too strong for another one, and cause it to sicken.
Ericaceae, Myrtace^, and many Leguminosa3 require a com-

74 THE PHYSIOLOGY OF PLANTS

paratively weak solution of nutritive matter, while highly con-
centrated solutions are beneficial to Crucifer^B (especially our
vegetables), ResedacecB, Cucurbitacea?, Chenopodiacege, &c.

If we seek for sound guidance from the aspect of the
plant as to the amount of concentration which it requires, we
may take it as a general rule that plants with leathery leaves,
with hard and narrow leaves, and with hard wood, require
more dilute solutions than those with large, soft, and ex-
panded leaves. The period at which the manure is added is
also of considerable importance. During the period of leaf-
formation all plants can do with the greatest amount of nutri-
tive matter.

If pot-plants have suffered from over-manuring, the ball of
roots should first of all be examined, to see if the latter are
decayed. If this is not the case, the surface of the soil should
be loosened, the pots should be washed to keep them as porous
as possible, and the watering should be reduced, so that a
large amount of air can reach the roots. If the plants must
have water, add it in such quantity that it escapes by the
hole at the bottom of the pot. Pure water only should be
used, and the pots should remain in their ordinary position.
If, however, the roots be injured by decay, then the plants must
be re-potted. After the removal of the decayed parts the plants
should be potted in a loose soil in small pots, be given mode-
rate bottom heat, a close atmosphere, increased shade, and
diminished water supply. If bottom heat is not available, the
plants should be entirely screened from the mid-day sun, and
during the heat of the day the upper portions of the plant
should be kept moist by sprinkling, but the watering of the
roots should be reduced to a minimum. The plant must rest
as much as possible until the development of new leaves indi-
cates the renewal of its activities. Then the patient may be
removed to lighter quarters, and the water supplied to the
roots may be gradually increased. The efficacy of this treat-
ment I have had ample opportunity of testing.

THE NUTRITION OF THE ROOT 75

§ 12. How do aerial roots nourish a plant ?

This question is of considerable practical importance, now
that the cultivation of orchids and aroids has become such a
hobby. We must try to ascertain whether the aerial roots
are organs by means of which the gardener can improve the
nutrition and the development of his plants. This question
must be answered in the affirmative ; for the aerial roots con-
stitute an absorptive system, and can therefore derive some
benefit from an enriched supply of food material, i.e., from
manuring. The absorptive portion of the root consists here
of a fine but tenacious silvery layer, which surrounds the root
like a sheath {vclamen), and will be well known to any
gardener who has seen the strong aerial roots of a Vanda or
Aerides. The genus of Stanhopea lends itself best perhaps
to the study of aerial roots.

If a transverse section be made through the aerial root of
a Stanhopea, the central vascular cylinder will be seen sur-
rounded as in all roots by the thickened layer, previously
described as the endodermis. This layer of cells is surrounded
on the outside by the soft cortex, and the latter is enclosed
by a layer of thickened cells, which represents the outermost
layer, i.e., the epidermis of ordinary roots, the cells of which
would form the real absorbing surface, growing out to form the
root-hairs. In aerial roots, however, this is not the case ; for
this layer does not limit the root on the outside, but is covered
in by a tissue (very extensive in Stanhopea, but very delicate
in some orchids), and it constitutes, therefore, an outer endo-
dermis. The real soft cortex, therefore, is bounded in aerial
roots on either side by a layer of endodermis. Outside the
outer endodermis we find the root-sheath or velamen.

The cells of this sheath are fitted together without any
intercellular spaces, and form a radially or longitudinally
elongated parenchyma, generally strengthened by spiral bands
running in the cell-walls. A point of special importance is
the fact that the cells have often perforations leading from
one cell to another. Such apertures exist not only between
the cells in the inner portions of the velamen, but are also
found in the walls of the outermost layer. In cells which are

76 THE PHYSIOLOGY OF PLANTS

thus perforated, there naturally exist no liquid cell contents
of the usual nature, but they are filled with air. It is the
presence of this air which gives to the roots their silvery
appearance. In consequence of the perforations of this root-
sheath (which must not be confounded with the root-cap,
although both are formed from the protoderm), it is able to
function like a sponge ; it sucks up in a moment as much
water as it can contain, and condenses water-vapour and
other gaseous matter which is contained in the atmosphere
like any porous body. This power of condensing- vapours and
gases suggests the chief function of the aerial root under
ordinary circumstances ; it will use this power to nourish the
root-system and the whole j)lant, if it can pass the condensed
matter through the thickened layer of cells (the outer endo-
dermis) which separates the root-sheath from the cortical
tissues. The enormous power an aerial root possesses of con-
densing water-vapour is shown by an experiment with an
aerial root of Epidendrum, 1 2 inches long. In an atmosphere
saturated with moisture it absorbed in twenty-four hours an
amount of water equal to one-ninth of its total weight.

The passage of water from the root-sheath to the succulent
cortex is made possible by the peculiar structure of the outer
endodermis. Speaking generally, this layer forms a protective
sheath, built up of long thick-walled cells, without pits or
canals ; nothing practically can pass through their thickened
walls. But a more careful examination reveals the presence
between the thick-walled elements of many small thin-walled
cells filled with protoplasm, and frequently arranged in rows.
These will attract the liquid which is condensed in the velamen,
and will easily pass it on into the cortex. The outer endodermis
of the aerial roots unites, therefore, just as we have seen in the
case of the endodermis of the ordinary roots, two very useful
functions. Its chief function is to protect the aerial roots
from drying up during periods of drought ; but at the same
time provision is made to conduct the substances which are
absorbed during periods favourable to the vegetative processes
into the more central tissues, whence they will be carried by
means of the vessels to the leaves.

In some plants the root-sheath or velamen is thrown off

THE NUTRITIOJS" OF THE ROOT 77

when the aerial root attains a certain age. The same may be
observed to take place when such roots enter the soil, which
indicates that the velamen represents the special organ by-
means of which the root can derive its nutriment from the
atmosphere.

This being the case, the practical treatment of plants pro-
vided with aerial roots is readily deduced. In all cases,
whether the aerial roots supplement the ordinary roots, or
whether they alone exist, they must be protected and increased,
as being the means of collecting and absorbing food material.
Occasional sprinkling of the aerial roots with dilute nutritive
solutions may be recommended as the most suitable means of
manuring.

§13. How do ordinary roots obtain their necessary supply of air?

We have already dwelt several times upon the fact that in
all horticultural practices the great respiratory need of the
plant must be satisfied. Not only must the surface of the
plant be in constant contact with the most essential constituent
of the air, namely, the oxygen, but all the internal tissues, too,
must be constantly supplied with oxygen, so that the necessary
oxidation which constitutes the respiration of plants may take
place.

It is true that every green cell of a plant under the influence
of light is forming new organic substances from the nutritive
sap and the carbonic acid of the air (assimilation), and in so
doing liberates oxygen ; but this amount of oxygen is by no
means suflBcient for the respiration. This can be gathered
from the fact that in living green cells (cells of algge, for
instance), under favourable conditions of light and nutrition, the
functions cease, and the movement of the protoplasm is sus-
pended, if they do not receive renewed supplies of oxygen.
A living cell will recover from this state of suspended anima-
tion, if not very long after it enters upon it it is again pro-
vided with oxygen. Should the oxygen be withheld for some
time, the cell will become asphyxiated. During such a period
of lack of oxygen the living cell can take oxygen from other
organic or inorganic substances which may be at hand ; but

78 THE PHYSIOLOGY OF PLANTS

this destruction of various substances may become very disas-
trous to the plant, as we shall have occasion to see in dealing
with the " over-watering " of pot-plants.

If we are to take any account of the favourable conditions
for aeration of plants, we must first of all study the contrivances
which exist in plants for the interchange of gases.

In the first place, we must mention the breathing-pores
(stomata) which occur in all green organs of any considerable
size. These pores, which occur especially on the under-surface,
sometimes on both sides of leaves, belong to the external layer
of cells (epidermis), and consist of two semilunar or kidney-
shaped cells, facing each other with their concave sides, and
attached firmly together at their pointed ends. They contain
chlorophyll grains, which are generally absent from other
epidermal cells.

As the pointed ends of these sickle-shaped cells meet, they
leave an elliptical space between them, and this represents the
chimney or passage by which the air has access to the interior
of the plant. Every such passage opens internally into a
large cavity in the tissues of the leaf which lies immediately
beneath the epidermis, and is called the respiratopy cavity.
From this cavity a large number of small irregular passages
run in all directions, formed by small spaces (intercellular
spaces) which exist between the several green cells of the
leaf. The external air can pass, therefore, into the respiratory
cavity, and from this latter through the intercellular spaces to
every single cell of the leaf. Gases can therefore pass directly
into every cell or out of every cell. In submerged portions
of plants the cells derive the necessary oxygen from the oxygen
contained in the water.

In very delicate green structures, and in those portions of
plants which are devoid of chlorophyll, the stomata are absent,
while in some leaves they may number i 8o,000 per sq. inch.
In tissues Which are devoid of stomata the interchange of
gases is effected by the entire surface of the organ or by other
structures.

In older stems and roots these structures are the so-called
lentieels.

These are small structures penetrating into the cortex, and

THE NUTRITION OF THE ROOT 79

consisting of cork cells arranged more or less in rows. Between
these cork cells the air has free access to the cortical tissues.
To the naked eye these lenticels appear as minute warts on
the surface of the stem, and during periods of long-con-
tinued rain they assume a white mealy appearance {e.g., on
alders in the autumn, on potatoes in a wet summer). This
appearance is due to the excessive production and swelling up
of certain portions of the corky tissue, so that the outermost
cells split apart and the loose cork cells are pushed out of the
pore in the form of a loose powder.

The rapid movement of air in the interior of stems is
ensured by the presence in some plants of large irregular
but continuous passages between the cells (in submerged
plants and plants inhabiting swamps), while in others the
older vessels, which form the centre of the woody cyclinder,
are filled with air.

How eager all plants are to obtain sufficient air we can
judge from the behaviour of some which send up delicate
rootlets out of the soil into the moist atmosphere of a green-
house or forcing frame. In nature, too, this occurs in many
plants rooted in swamps. Avicennia, for instance, in its natural
surroundings of a mangrove swamp, sends up innumerable
lateral roots, which stand out above the surface of the water,
and are provided with lenticels for the purpose of taking in
oxygen.

Gardeners will all know well enough the upward-growing
roots of palms and Pandanus. The tips of these roots, which
project above the soil, but almost always remain short, have a
mealy appearance. Sometimes several inches of the root are
evenly covered with this powdery substance ; in other cases
the mealy covering is interrupted by several rings of the
original epidermis. The root-cap is shrivelled up to a small
brown cap.

Those regions which have a powdery appearance are no
longer provided with an epidermis, but are covered in by a
loosely arranged tissue, the intercellular spaces of which com-
municate freely with the intercellular spaces of the cortex.
This layer of "spongy parenchyma" is formed in the youngest
portion of the root-apex by an increase in number and a

8o THE PHYSIOLOGY OF PLANTS

rounding off of the cortical cells, which cause the root-tip to
become dilated into a knob. Below this spongy tissue the
usual sclerenchymatous ring, which forms a protective sheath
to the roots, is missing. If such an upward-growing root is
cut off and is hermetically sealed into the shorter arm of a
U-shaped glass tube, by pouring mercury or by blowing into
the other arm, air can be forced through the root-tip. This
proves that the aeration of the whole root takes place through
the mealy-looking tip. This means of aeration by specially
modified roots, which are termed " pneumathodes," occurs in
many species of Phoenix, Livistona, Cocas, Chamsedorea, Pan-
danus, &c. In the last-mentioned palms the respiratory
organs are found on thick horizontal roots in the form of
small wart-like lateral roots, scarcely I inch in length ; in
Pandanus the " pneumathodes " are very small warts on the
large prop-like roots.

The explanation of this phenomenon, to which little atten-
tion has been paid, of roots being sent up out of pots, is to be
found in the sensitiveness of these organs to gases,^ which has
now been established. The gas which produces the abnormal
growth in this case is the oxygen of the air, which is driven
out of the interstices of the soil by watering. The more it is
watered, the closer the soil, and the damper the pot is kept,
the more the roots will grow towards the upper regions of the
pot, where there is more oxygen, and even grow right out of
the soil. If the soil is only kept moderately moist, so that
the roots can obtain the necessary supply of air within the
soil, plants which usually direct their roots upwards will pro-
duce their pneumathodes underground. It is their desire for
oxygen which causes the roots to grow towards the inner
surface of the pots, and this points out the necessity of
occasionally washing" the outside of the pots to keep their
pores open.

^ Aerotropism.

CHAPTER IV

THE TREATMENT OF ROOTS

§ 14. How should roots be treated in transplanting ?

In the treatment. of the roots of our cultivated plants, we
must first consider what part the root plays in the economy
of the plant, and secondly, whether it is of economic value for
us. In the case of annuals, it is the rapidly growing absorp-
tive and fixing organ ; in perennials, besides absorbing the
water contained in the soil, it serves also partially as a store-
house for reserve material, which the plant wishes to keep for
the next vegetative period. In fleshy or tuberous roots, the
storage function lasts for the greater portion of the life of the
plant.

Its chief function is as an organ of absorption for the
nutritive solutions of the soil. It is self-evident that, other
things being equal, the development of the upper portions
of the plant, especially of the assimilating leaves, will depend
upon the amount of nutritive substances which are absorbed.
Conversely, the greater development of the leaves will result
in a greater production of organic matter (assimilated sub-
stance), and therefore more of this plastic matter will reach
the root system, and supply it with the means for producing
new ramifications.

(a.) A Boot System ivhich has hecn considerably Primed.

The above-mentioned reciprocity must always be taken into
account. If sickly plants with few leaves, or no leaves at all,
are able to form new roots, and if, on the other hand, plants
with feeble or damaged roots are able to produce new leaves,
it is obvious that such a growth must take place at the

8x p

82 THE PHYSIOLOGY OF PLANTS

expense of reserve material which is stored up in the main axis
of the plant. This will only take place, however, when an in-
crease of temperature stimulates the plant to increased activity.
This stimulus, which is so often applied in pot-cultures, may
be directly injurious if the newly developed organs, which are
always somewhat weakly at the commencement, are not suffi-
ciently cared for. This is especially the case with recently
formed roots of a sickly plant. As a rule, the pot is kept
too moist for the slowly and sparingly developing root system,
and as a considerable amount of organic matter is undergoing
decomposition in the pot, a dearth of oxygen soon occurs in
the soil saturated with water, and the young root-tips decay.
In the case of plants which are producing new leaves from
reserve substances, it often happens that those plants which
are just recovering are placed side by side with and under
the same conditions as healthy plants, which require a large
amount of transpiration. The new foliage is only able to
respond to this great activity if it is aided by an energetic
absorption of the root system, as is, for instance, the case in
the normal development of leaves in the spring. This, how-
ever, is not generally the case with the recovering plants ;
consequently the young leaves shrivel up, and the plant will
be entirely destroyed.

In the treatment of plants which are restoring their root
system by the production of accessory roots, the first rule is
to so reduce the work of the leaves that it is in harmony with
the activity of the root.

This rule is not confined to pot-plants, but applies equally
to plants grown in the open. The former may be placed
during the period of root development in closed damp houses
or frames, so that the amount of their transpiration is reduced,
and therefore corresponds with the reduced absorption of the
root system.

In trees and bushes which are transplanted the root system
is always injured; the most apparent injury is the absence of
the root-tips and of the absorptive region immediately behind
them. In the case of such a reduction of the absorptive root-
tips, it is evident that the plant would possess too large an
amount of foliage if all the branches which had been formed

THE TREATMENT OF ROOTS 83

were left intact. How can the root system, which has been
damaged and cut in taking it out of the soil, absorb sufficient
water for the full development of all its leaves ? However
much we may water the root, it will be of little avail ; it may
even be injurious to the plant, as the saturation of the soil
with water may cause decay to set in at the cut ends.

In consequence of what has been said in the previous chap-
ters about the function of the delicate root-tips, we must
emphatically contradict the view which is still held and acted
upon by some, that in transplanting trees and bushes the
branches should be left unpruned.

In support of this view it is often mentioned that pot-plants,
in which the root system has been considerably damaged by
transplanting, need not be pruned in. This argument, how-
ever, though it is actually true, is fallacious, for pot-plants can
be, and are, placed after transplanting either in a shady place,
or in the moist atmosphere of a frame, which reduces the
amount of their transpiration in accordance with the reduced
Absorption.

Transplanted woody plants must therefore have their crowns
reduced. It is only a question as to how the pruning should
take place, so as to assist as much as possible the speedy
formation of new roots. If we assume the roots to be pro-
perly pruned, the production of adventitious roots depends upon
the excess of food substance formed in the leaves over their
consumption. This excess will find its way down the stem
into the root system. Again, other things being equal, the
amount of assimilated food matter available for the roots will
depend upon the rapidity of development of, and upon the
amount of work done by, the foliage. The more rapidly, there-
fore, we can produce a large number of actively functioning
leaves, the sooner will the stem be in a position to pass down
material for the formation of new roots.

But with regard to a rapid and strong development of leaf
surface, the several buds of a branch behave very differently,
and we may take it as a rule that the uppermost buds of every
branch are the first to develop and produce the largest amount
of leaf surface. Now, for the purpose under consideration, we
require a large amount of foliage at the earliest possible period,

84 THE PHYSIOLOGY OF PLANTS

and we ought therefore to leave the ends of all the branches
intact, i.e., we ought not to prune at all. But opposed to
this requirement is the necessity to reduce the whole system
of branches to a certain extent, as otherwise, in the course of
the summer, some branches would die, because the damaged
root system is not able to supply all the water which would
be needed by the entire system of branches. But often those
branches which die off are just those which are most essential
for a regular growth of the crown, and therefore are an irre-
parable loss to the tree. Both requirements must therefore be
taken into consideration, and this is done by leaving intact
some branches which are essential for the formation of a good
crown, and reducing the intervening branches to one-half or
one-third of their length, according to the amount of damage
of the root system. In this way we secure the development
of a number of leaves at the ordinary time, and consequently a
certain amount of new assimilated food substance. For these
few branches the amount of sap absorbed is quite sufficient,
and the development of adventitious roots begins near the cut
ends of the rootlets before the buds of the pruned branches,
which only form a small amount of leaf surface, have begun
to open. The absorptive organs will therefore have increased
in proportion to the new leaf surface.

The present method is to cut back all the strong branches,
and to confine the production of leaves to the lower, lesa
vigorous, and later developing buds. Consequently, the un-
folding of the leaves takes place at a later and hotter period,
when the leaves will remain smaller, but in spite of that will
require more water for their transpiration.

The first-mentioned method of "partial pruning" deserves
the preference for transplanted trees and bushes.

It is of course taken for granted that the larger cut sur-
faces will be covered with tar or wax, and that the stem&
themselves will, if necessary, be protected from evaporation by
a covering of moss or straw.

THE TREATMENT OF ROOTS 85

(h.) A Boot System from lohich only the Delicate Roots have
been Removed.

In discussing the treatment of roots in transplanting, we
have purposely distinguished those cases in which the older
roots have been removed from those in which the younger and
more delicate roots only have been injured, for the methods of
healing are different in the two cases. The younger a root,
the more easily will it heal, and the more copiously and more
rapidly will it produce adventitious roots. This can be very
readily seen in trees which are taken up again a few years
after transplantation. Around the wound produced by the
cutting of a large root, there will be seen a ring of delicate
adventitious roots, some of which will have become stronger
roots. The adventitious roots, produced from younger struc-
tures, are perhaps not equally numerous, but their dimensions
are so perfect that they are often not to be distinguished from
the undamaged roots of the same age.

A root system, therefore, which has been pruned back con-
siderably will take a longer time to produce a sufficient
absorptive system, the stem therefore will have only a small
supply of water, and will require a treatment tending to re-
duce the transpiration for a longer period than a plant in
which the delicate roots only have been cut.

It is thepefoFB most advantageous in pruning- roots to avoid
if possible the cutting- of the older roots.

(c.) The Treatment of Roots in Re-Potting.

The necessity for re-potting occurs either when the root
system is diseased or when the soil has become exhausted by
very active growth.

In the case of diseased roots, the above-mentioned precau-
tions with regard to the upper portions of the plant should be
remembered. One is forced on these occasions to cut back
the roots to the entirely healthy portions, and therefore to cut
back to the old wood ; and this must not be done sparingly.
For if the smallest decayed portions are left, there is great

86 THE PHYSIOLOGY OF PLANTS

danger, should the watering be injudicious or the choice of
mould faulty (too heavy), of continuing and increasing the
decay. From the infected centre liquid products of decomposi-
tion are sucked up into the healthy tissues, cause the decay
of the stem, and in the end bring about the death of the plant.

It is essential to use very loose moulds and small pots for
plants with sickly roots.

First of all, no attention need be paid to a possible dearth
of nutritive substances which might occur after the plant has
made new roots. Such a want can easily be met by a subse-
quent transplanting. At present, the chief aim is to produce
as soon as possible a new supply of roots, and this necessitates
the preparation of a moderately moist soil, which can be kept
permanently well aerated. This even moisture must not be
brought about by repeated watering, but by a preservation of
the water contained in the soil in the first instance, and which
must be prevented from evaporating. This is most frequently
done by immersing the pots in a bed of cinders or sand, cocoa-
nut fibre, sawdust, tan, or other porous substances, which do
not easily decompose. It will be found of great advantage if
such a bed can be heated. Amateurs growing plants in an
ordinary room will find it advantageous to sink the pot in a
larger one containing river-sand.

River-sand is in all cases preferable to sand derived from
pits, as the particles of the latter are always more liable to
adhere together, owing to the presence of clay, loam, and iron.
If river-sand is not obtainable, the pit-sand should always be
washed before use. High temperatures, which would be very
beneficial in frames or green-houses, should be avoided in the
dry atmosphere of a room. Sickly plants should receive more
broken potsherds below the soil than healthy plants, as they
require a better and more rapidly drained soil.

The re-potting of a plant with a healthy ball of roots is
necessary when the soil becomes exhausted. A growing plant
often requires a larger amount of soil, the old soil being
entirely used up by the network of rootlets. The latter are
so closely pressed against the inside of the pot, that some are
always damaged in removing the pot. Now, it has become
the general practice to greatly increase this injury by ripping

THE TRP]ATMENT OF ROOTS 87

up with a pointed stick this dense felted mass of rootlets and
tearing it away. Experience has proved the utility of this
practice if carried out moderately. If this felt-work of rootlets
is left intact, and the whole ball of roots is immersed, just as
it is, in a larger pot, the earth at the centre has first of all not
been renewed ; and secondly, a tremendously dense mass of
roots partially damaged (by the removal of the old pot) is
replaced in a new, moist soil, containing a large amount of
humus. This dense mass of roots requires a very large amount
of oxygen, which it could formerly obtain, being pressed against
the side of the pot ; but now the conditions have changed.
In the new soil, which is richer in decomposing organic matter,
and therefore requires for itself a larger amount of oxygen, a
number of the pores are blocked by water, which absorbs the
products of decomposition, especially the carbonic acid. It is
this water which surrounds the felted mass of roots. Conse-
quently the injured portions of the root system often begin to
decay.

Secondly, the root-tips which remain pressed together retain,
in the first instance, the direction of their growth, and only
slowly bend towards the sides of the pot, whereas it is most
advantageous for them to push as soon as possible strong root-
tips towards the better aerated region on the inside of the
pot. It is therefore better to remove a portion of the old net-
work of roots, and so to open up a passage for the rapidly
growing new roots to the side of the pot. Especially in the
case of herbaceous plants will it be noticed that those with a
loosened ball will most rapidly re-establish themselves in the
new pot.

With regard to the mixing of a mould for pot-plants, we
may adopt the general rule that it must be coarse, but not so
light as to cause any danger of drying up. This is especially
to be borne in mind in the case of plants grown in dwelling-
houses.

(d.) The Treatment of Boots in Transplanting in the Open.

In the case of short-lived herbaceous plants grown in the
open ground, the treatment of the roots is very simple. In

88 THE PHYSIOLOGY OF PLANTS

pricking out the seedlings, the young tap-root is always damaged,
and consequently the lateral roots soon make their appearance,
so that the number of the absorptive organs is increased. We
are more concerned with the woody plants which the gardener
cultivates for commercial purposes or for his own use. In
both cases it is necessary in their culture to pay attention to
the portability of the plant. This is only attained when the
whole root system occupies a very small space. Trees which
are left to themselves develop where they are first grown a
long tap-root, by which they are best fixed to the soil, and
absorb from the deeper layers of the soil the necessary food
material. The development of these deep-rooting plants is a
relatively slow one, but correspondingly lasting. Trees which
have been transplanted in their youth, and have therefore
suffered an injury to their tap-root, produce near the wound
several lateral roots, which grow on unhindered in a horizontal
direction without much branching. To take up such trees
requires considerable labour. Besides, with such diverging
roots they are very troublesome to plant, as exceedingly large
holes must be prepared for their reception. The general
remedy for this is to saw ofi" the long lateral root, so as to
reduce the root system to a more convenient size. In doing
so, however, the old wood is cut, and only few adventitious
roots are formed, and their development is slow and weakly.
The few greatly reduced roots give to the tree only a feeble
hold on the soil, and produce only few absorptive organs, and
cause therefore a considerable interruption of growth, which
prevents the tree from attaining any considerable development,
and causes premature death.

A tree intended fop transplantation must have a root system
consisting- of a great number of short branches provided with
many rootlets, so that the whole absorptive system is limited
to a small area.

The large number of rootlets represents the greatest possible
area of absorption, and at the same time a network which will
take firm root of the soil, and cause the rapid formation of a
ball of roots, which enables the transplanted tree to establish
itself.

Such a much-branched root system, forming a nest or ball

THE TREATMENT OF ROOTS 89

of roots, can be systematically obtained by pruning the roots
from the very commencement. This demonstrates the necessity
of repeated transplanting of young* trees which are intended
for sale or for future transplanting. The treatment of young
wild stocks begins, indeed, with the pricking out of the seed-
lings. A discussion of the technicalities of this process would
be out of place in this book, but we may mention the most
suitable time for this process. In countries with a mild winter,
like England, November may be recommended, whereas in
Germany March is preferable, as this prevents any damage to
the young plants by the severe frosts, and the cut surfaces are
soon healed by the recommencing activity of growth.

Some woody plants have the tendency to produce only few
and sparingly-branched lateral roots, which resemble the tap-
root in growing almost vertically down to a great depth ; in
this case it is advisable to repeat the above-mentioned process.
All the roots must be pruned, and the same must be done for
the lateral branches of the stem. Conifers and other evergreen
plants must also be transplanted at the end of their year's
growth. But as this ceases at an earlier period than is the
case in plants which lose their leaves, we may proceed to
transplantation at the end of the summer in localities in
which a late drought need not be feared. Thus the plant has
the opportunity of forming new roots before the winter com-
mences.

To preserve the moisture of the spring, and to keep the
soil very porous, beds with such young plants should be covered
with loose straw, moss, fibre, or short manure.

Seeing that the formation of long roots must be prevented,
so as not to have to cut into the old wood, it is essential
that young plants should be transplanted every year, and that
(according to the species) the larger roots of the previous year
be reduced to about two-thirds of their original length. At
the commencement of the last third of the length there is
sufficient tendency to form lateral roots so as to ensure a con-
siderable number being formed, and the cut end is thin enough
to be very soon healed over. Large cut surfaces are dangerous,
because they are less rapidly covered over, and consequently
are easily attacked by Fungi, which will cause decay.

90 THE PHYSIOLOGY OF PLANTS

It is fi'om a dread of such consequences that the practice
has grown up of not pruning the roots at all in transplant-
ing, but of coiling them up spirally. This practice is further
strengthened by the observation that roots which have been
bent tend to produce numerous lateral roots immediately above
the bend. This observation holds good for seedlings at least, as
can easily be proved by growing them in water. When one
of the long roots reaches the bottom of the vessel in which the
plant is grown, it becomes bent, and lateral roots soon make
their appearance.

Still the above-mentioned practice is to be condemned, as
we will surely do after inspecting the roots a few years after
transplantation. Generally the tap-root will be found again,
only spirally coiled, so that its apex, which bears the rootlets,
is closer to the surface. The older portions will not have pro-
duced any branches, and a number of those found near the apex
will never be so great as in the case of a properly pruned root.
We must also not forget that the nutrition of the plant cannot
be so favourable under these circumstance, as the water has a
longer course to take through the spirally coiled root. Now
the bending of any organ always retards the passage of the sap ;
the shorter and more direct, therefore, the passage is, the more
advantageous will it be to the plant.

Older plants, even when well cultivated and nourished, are
always at a disadvantage as compared with younger ones,
because both the sap and the plastic substances have to pass
along much greater distances (sometimes no longer very pass-
able) before they reach the final goal, the growing points of
roots and branches. This causes the frequent dying off of the
tips of horizontal branches of old trees.

As a matter of fact, therefore, the nutrition of plants with
bent roots is less favourable than that of plants with pruned
roots ; besides this, the transplanting of the former is always
more difficult.

We must not close the chapter on the treatment of roots
without dwelling upon the important fact that fpeshly trans-
planted trees and shrubs are more sensitive than the untouched
ones.

Generally speaking, the roots are more sensitive than the

THE TREATMENT OF ROOTS 91

stem and branches, owing to their more delicate tissues,
and to the larger percentage of water of the former. The
branches may, indeed, be made more resistant by certain in-
juries to the roots. Several cases are kno^vn in which the
branches of fruit-trees which were transplanted in the autumn
were less damaged by frost than those of trees which had
remained in their original positions. Such a phenomenon is,
in all probability, due to the fact that the branches contained
less water, as the transplanting, by damaging the many root-
tips, would interrupt the absorption of water, and consequently
stop the growth of the branches, and accelerate the ripening
of their wood. Under normal conditions the shoots of apple-
trees in a heavy soil will go on growing until stopped by the
frost.

The greater liability to damage by frost in the roots of trees
transplanted in the autumn is not due to any greater sensitive-
ness of the root, but to the greater porosity of the soil, which
enables the frost to strike to a greater depth. The earth
around a newly-planted tree is much more porous than the
natural soil, which contains more water and allows the air to
circulate less freely ; and this is just what protects the roots
from frost. Hence, in transplanting in autumn, it will be found
advisable to heap up the earth one or two feet above the soil
around the tree, and open up the heap in the spring.

It is the porosity of the soil, which only settles down after
many months, which causes trees transplanted in the spring
to suffer from dryness. It is therefore always essential to
wash the soil well into the root system. We should not be
stopped from doing this by the fact that the soil is already
very wet, or that heavy and continuous rain is falling at the
period of transplanting. It is of prime importance that a
layer of fine earth should cover in the rootlets as soon as pos-
sible, and this can only be done by washing it in.

The practice of placing the roots in water previous to plant-
ing cannot always be recommended. The tissues absorb an
enormous quantity of water, and have afterwards to return to
a soil with a very much smaller supply. Experience, too,
teaches us that plants with great turgidity suffer most when a
dearth of water arises.

92 THE PHYSIOLOGY OF PLANTS

In the case of freshly-transplanted trees, again, manure of
any great streng-th should be avoided. The very highly con-
centrated solutions of nutritive substances which are formed
in the soil under these conditions cannot be utilised by the
injured root system. We always supply the roots as directly
as possible with some compost, if we do not fill the entire hole
with it, and this is quite sufficient to see the tree through the
first year. If further material is available, a little decom-
posed manure may be spread over the surface of the soil. This
will gradually become washed free of its nutritive substances,
which will find their way into the soil below. We do not,
however, wish to say that no manures should be put into the
trenches. If the soil is very poor, the addition of manure
(especially of animal dung) will probably be very beneficial ;
but it should be so placed that the roots will only make use
of it in the second year.

In manuring trees which have not been transplanted, mineral
manures are often preferable to animal manures, because the
decomposition of the latter takes place very slowly in the soil,
which is usually very dry under the trees. It is also more
diflScult to apply stable manures, and the effect produced by
them is not always the desired one. Supposing the trees
show a tendency to produce strong branches, and you wish to
produce fruit-buds, animal manures are of much less use than
the addition of phosphoric acid alone in the form of ground
Thomas phosphate, &c. If the tree does not form proper
woody shoots, a manure rich in nitrates and potassium should
be applied.

For large trees it is best to introduce the manures into
trenches dug round the tree.

Such a trench should be made about the breadth of a spade,
and carried round the tree at about the same distance as the
longest branches. It may even be closer up to the stem, but
it should be made so deep as to reach the network of roots,
the younger ones of which may be cut through. In the place
of the excavated earth, compost, decayed manure, or, in absence
of the latter, ordinary earth mixed with fresh dung, horn shav-
ings, or other insoluble refuse, should be placed. The best
effect is produced by rich compost.

THE TREATMENT OF ROOTS 93

The advantage of this method would be a decrease in the
development of branches, owing to the injury to the roots, and
the formation of numerous adventitious roots in the neigh-
bourhood of the cut roots, and finally, an even and adequate
nutrition.

There need be no fear of adding fresh animal dung in this
case, as it always takes some time before the adventitious
roots are formed, and have increased so as to be in contact
with the dung. If liquid manure is to be used, it will suffice
to dig a number of small holes in place of the trench. The
roots will be produced most profusely in the neighbourhood of
the centres of food substance ; they have partly the tendency
to grow towards the source of food matter (trophotropism), and
partly to grow in the direction of greater moisture {hydro-
tropism), if the water is unequally distributed.

Tupf under the trees should be avoided, as the roots of the
grass absorb the rain-water, and in dry seasons may even
draw away water from the deeper regions which should be
reserved for the roots of the tree. In periods of great drought
do not depend upon the protection of the turf, but remove it,
water copiously, and replace the sods, roots uppermost. Even
if no water is at hand for watering, lift up the turf, dig up
the ground lightly, replace the turf with the green side down-
wards. The larger openings in the soil prevent the rising of
the water by capillai'ity from the deeper regions of the soil,
which retain what little water they have for the roots. The
inverted sods, on the other hand, prevent or weaken the
suction which the dry atmosphere exerts on the air contained
in the loose soil.

CHAPTER V

THE STEM

§ 15. What is the structure of the stem ?

The root passes above the ground into the stem, which pre-
sents itself to us as an axis bearing the leaves, or in certain
comparatively rare cases, undertaking the function of the leaves
itself.

The external development of the stem axis varies very much
in size, duration, and in the composition of its tissues. An
axis which only bears flowers is spoken of as a scajje, the leaf-
bearing axis is called a culm or haulm (in grasses), a stalk (in
herbaceous plants), or a ste7n (in trees). In some plants the
main portion of the leaf-bearing axis remains underground,
and only the leaves and the tip of the axis are pushed up
above the soil. This, however, is not the place to discuss the
general morphology and anatomy of the stem, as the physio-
logist and gardener are chiefly concerned with the function of
the various organs, and their place and importance in the
economy of the plant. From this point of view, it is sufficient
for us to remember that we have already termed the leaves
the chief seat of formation of organic substance, and to observe
that the network of veins which is marked out on the leaf
surface collects into considerable bundles which pass down the
leaf-stalk and are continued down the stem.

If we follow the course of these bundles from a young leaf
into the stem, we see that it is these leaf-trace bundles which
form a ring of bundles in the stem, and which by their further
development form the woody cylinder of the older stem. The
number of bundles which pass from a leaf into the stem, and
which we have already termed the fibro-vascular bundles, varies
in different plants (3, 5, 7, &c.), as can readily be seen from

THE 8TE:\r 95

the leaf-scars on the branches of our trees. The darker, knob-
like processes which are seen on the smooth surface of the
leaf -scar are the broken ends of so many vascular bundles,
which passed from the stem to nourish the leaf. Each of
these bundles can be traced down in the stem, passing several
older leaves, until it becomes more and more delicate, and
finally splits up between the bundles of the older leaves, its
elements passing into those of the lower Ij'ing bundles.

If we examine a branch which has been cut across in the
region where it is beginning to get woody, we will notice in
the middle of the cut surface a soft parenchymatous tissue,
which we call the pith or medulla. This is surrounded by a
firm, white, and radially divided ring termed the wood-eylindep,
the radially running lines being the medullary rays.^ The
wood-cylinder consists of thick-walled elongated cells, which
interlock with their pointed ends, and are termed the wood-
fibres (libriform fibres). Between these are found the actual
wood-vessels. The medullary rays, however, consist of paren-
chymatous cells, which are often elongated in the radial direc-
tion, so that the passage of water within the medullary ray
from the pith to the cortex is retarded by fewer walls than if
it were to pass vertically through the ray. These medullary
rays, therefore, represent the most convenient passages in a
radial or horizontal direction. The wood cylinder is surrounded
on the outside by a thin layer of very delicate and easily-
damaged cells, the cambium ring", which passes over more
gradually into the external layers. These layers terminate
externally in those portions of the stem which are still soft in
a layer of tabular cells, which fit very closely together, and
are usually devoid of chlorophyll. This layer can often be
separated in the form of a thin skin, and is termed the epi-
dermis. In the older brown portions of the branch, new cells
will be found to be formed below the epidermis, which consist
of brick-shaped cells, and form the cork layer which constitutes
the protective sheath around the soft cortical tissue. But the
tissues outside the cambium are still further differentiated.
This can best be seen by scraping with the finger-nail a green

1 See Fig. 2, p. 11. In this longitudinal section through the wood and the
inner cortex, the medullary rays are indicated by the letter m.

96

THE PHYSIOLOGY OF PLANTS

shoot of the vine. It will then be noticed that only a little
of the soft green tissue will be removed, and then there will be
laid bare a number of firm whitish bundles of fibres running
in longitudinal direction. These are the hard bast fibres or
stereom. They consist of long spindle-shaped and thick-
walled cells, usually without contents, but very flexible. They
are united into strands running on the cortex, and form the
material which we use for tying up plants. These bundles of
hard bast run parallel to the vessels of the wood, and under
the microscope it will be seen that in a young branch one such
zone of bast lying in the cortex corresponds to each portion of

wood lying between two
primary medullary rays.
Such a group of hard
bast cells has often a
semilunar appearance, the
convex side turned towards
the outside of the stem.
Protected on the outside
by this group of hard bast
cells, we find a number of
strands of delicate cells,
which either have the
Fig. lo.— PART OF A Transverse Section THROUGH same appearance as the

cambium cells {camhi-

A Vascular Bundle of Lagenaria vulgaris.

m widely reticulate, ?! closely reticulate sieve-plate.

Each is the horizontal transverse wall of a sieve- JOrm), Or WblCfi represent
tube(«/^.rDEBARY). ^^^^^^ ^j^-^j^ j^^^^ ^^^^

formed by the fusion of a number of superposed cells. These
tubes are characterised by the fact that the transverse walls
which separated the original cells have not entirely dis-
appeared (as in the wood vessels), but are perforated after
the manner of a sieve. These tubes have therefore been
termed sieve-tubes, and together with the eambiform cells they
constitute the soft bast, as distinguished from the above-men-
tioned hard bast. It is along this soft bast tissue that the
organic plastic material (assimilated material) which has
been formed in the leaves passes to the regions where it is
required.

Fig. I o represents a more delicate and a coarser sieve-plate.

THE STEjSI

97

v::i

forniing the transverse walls of two superposed sieve-tubes.
In Fig. 1 1 is shown a longitudinal section through such a sieve-
tube and its sieve-plate. The pores on the left-hand side are
still filled with the protoplasmic contents, while on the right
hand the action of the alcohol has caused them to shrink and be
withdrawn from the pores. They have, however, been hardened
by the alcohol in the form of
finger-like processes, which at one
time penetrated the plate.

It is not without ground that
we have emphasised the fact that
these groups of soft bast run on
the inside of the cortex parallel to
the strands of woody tissue. They
are, in fact, a part of these fibro-
vascular bundles which traverse
the stem.

With this knowledge we are
now able to picture to ourselves
the structure of a stem or branch
about one year old. If we con-
sider such a shoot to be placed
vertically upright before us, it will
be found to be an elongate struc-
ture consisting of many stories.
Every story is marked by the in-
sertion of a leaf. The chief form
of brick in this tower-like struc-
ture is the parenchymatous cell,
which can best be compared with
the cell of a honeycomb. Of such cells the leaves and the
shoot are mainly built up. Every parenchymatous cell is a
small chamber, which shows at a certain time of its life a great
activity in forming new organised material. Such activity
can only take place to any extent if new raw material is
constantly brought to it and the assimilated substance is con-
stantly removed from these manufacturing cells. For this pur-
pose special passages are formed, the fibro-vascular bundles,
which consist of two chief portions. One part consists of

(f

I'iG. II.— Longitudinal Section

THROUGH A LARGE SlEVE-TUBE OF

Lagenaria vulgaris.

Its protoplasmic contents have been
contracted by alcohol into the strand c.
The protoplasmic processes which passed
through the pores g have been hardened,
and have shrunk away from the sieve-
plate {after De Bary).

98 THE PHYSIOLOGY OF PLANTS

woody cells and vessels, and constitutes the wood or xylem ;
the other contains the sieve-tubes, and is termed the bast or
phloem. The first-named portion conducts the raw solution
from the root to all the manufacturing cells, the second carries
away the organic material which has been built up in these
cells to all the places where new cells are to be formed.

In the leafy expansions we see the conducting bundles
forming a delicate network, which penetrates the whole paren-
chymatous mass and collects in the midrib into the more
compact bundles.

Each collection of bundles passes from the leaf into the
stem, runs down it through four or five stories, and then unites
with the bundles coming from a leaf at that lower level. In
this manner each leaf-trace bundle runs for a time between those
of the lower leaves, and at whatever point of the stem we
make a transverse section, we shall always find the conducting
systems of several superposed leaves.

The bundles which come from the leaves appear as it were
let into the parenchymatous ground tissue, of which the first
year's axis is mainly built up. As the bundles of dicoty-
ledonous plants and of conifers are arranged in a ring (Fig.
1 2 A), they form later on a continuous ring which separates
a central tissue from the external regions. This central tissue
is termed the pith or medulla (3f), the outer zone is called the
cortex (E). If the fibro-vascular bundles after their entrance
into the stem do not all run at a certain distance from the
periphery, but first bend in towards the centre of the stem in
a gentle curve, and then bend back towards the periphery, a
transverse section, would show no regular arrangement of the
bundles. It would reveal the bundles as dark, firm, irregularly
placed groups of tissue scattered through the ground tissue,
and not separating a pith from a cortical region. This is the
case in Monocotyledons, of which the stems of palms, Dracaenas,
and of the maize are easily procurable examples.

§ 16. What is cambium ?

But it is not only in the course of the bundles that we
notice a difference between Monocotyledons and Dicotyledons

THE STEM

99

and Conifers. The composition of each bundle also differs. It
is true in their first stages both are similarly made up of thin-
walled cells containing much protoplasm, and are then called
ppocambial strands ; but during their later development they
become differentiated in different ways. The differentiation of
the pi'ocambium cells into wood vessels {xylem) (Fig. 12, A, x),
and sieve-tubes or phloem (Fig. i 2, A, p) begins at the peri-
phery of each bundle, and proceeds towards its centre. But
in the bundles of Conifers, and of most Dicotyledons, there
remains in the centre an undifferentiated zone of the original
thin-walled cells with their protoplasmic contents, and these
retain and use their power of division, thus increasing in
number. This persistent zone of cells, which are able to divide
{meristematic cells), is absent from the bundles of Monocoty-
ledons, and the several bundles cannot therefore increase in
thickness during, the succeeding years, as is the case with the
vascular bundles of most Dicotyledons.

If, at the beginning of a new period of growth, long-lived
plants require an increase of their conducting tissues, this can
only be done in the case of Monocotyledons by the formation
of new vascular bundles, while in the case of Dicotyledons the
number of conducting elements can be increased within the
already existing bundle.

For the purpose of answering the question placed at the
head of this paragraph, we must remember that the stems of
our woody plants have a ring of bundles inserted in their
ground tissue But as all these bundles are arranged in the
same way, so that the xylem or wood is turned towards the
centre (Fig. 12, B, x), while the phloem or bast is turned
towards the outside (Fig. i 2, B, p, and h, h, h), the xylems
form a cylinder which is only interrupted by the narrow
medullary rays. The latter pass through the phloem too, and
their cells have the faculty of fitting themselves to the tissue
through which they have to pass. As long as the medullary
ray runs through the wood, its cells are lignified, but in the
bast the cells partake of the softer nature of this tissue.
Hence the two portions of the ray may be termed the inter-
fascicular xylem (Fig. 12, C, ifh) and interfascicular phloem,
respectively (Fig. 1 2, C\ ifp). By this adaptation of the

THE PHYSIOLOGY OF PLANTS

Fig. 12.— Diagrammatic representation of the
Gradual Formation of the Woody CriiNDER of
A Dicotyledon.

A, transverse section through the apex of a shoot; the
several vascular bundles are arranged in a ring in
the ground tissue, dividing it into central pith (M) and
an external cortex (R) ; x, wood or xylem ; p, bast or
phloem.

B, transverse section of a somewhat older shoot. The
lighter portion between the bundles represents the
medullary rays. In each bundle there lies between the
wood and the bast a layer of cambium, fc {fascicular
cambium). This cambium is continued into the medul-
lary ray, as interfascicular cambium (ic). Thus the
cambium ring is formed.

C, transverse section through a mature shoot. The wood
of the bundle (fh) and the interfascicular wood (ifh)
form together the wood cylinder ; ic and fc form the
cambiimi ring. Outside this are the bast areas {p and
ifp), the soft bast of the bundles being protected by
the hard bast groups, b (after Sachs).

medullary to the sur-
rounding elements, the
various zones of the
stem become homo-
geneous, and we can
distinguish with the
naked eye, an even
woody cylinder and a
correspondingly homo-
geneous ring or belt of
bast.

But each bundle
possesses, as we have
seen above, a layer of
cells lying between
the bast (Fig. 12, e,2),
and h, h, h) and the
wood (Fig. 12, C, X,
ifh), which layer is
able to, and does
actually, increase the
number of its cells
(Fig. 12,0, fc). This
layerof cells represent-
ingsome of the original
procambium cells,
which have remained
active, and continue
to multiply, we term
cambium. The layers
of cambium in each
of the several bundles
are also all equidistant
from the centre of the
axis. They do not,
however, touch each
other laterally, as a
medullary ray runs
between the bundles.

THE 8TEM loi

But tliese tissues of the medullary ray, where they separate
two cambium layers, assume later on the same cambiform char-
acter, and thus form a bridge (Fig. 12, C, ic), which connects
two adjoining pieces of cambium, and in this way a continuous
cambium ring is ultimately formed (Fig. i 2, 0, fc and ic).

What appears in transverse section as a ring is, however,
in reality a c^^linder of cells which reaches through the whole
length of the axis. The stem or branch, therefore, will now
appear to be a column, the centre of which is occupied by a
narrow but solid cylinder of pith. Around this we find a
hollow cylinder of wood, which is covered in by a mantle of
cambial tissue, bounded on the outside by the bast, still more
externally by the cortex.

The view, therefore, which is still to be found in some of
the older books that the cambium is a fluid, the so-called
formative fluid, is erroneous. Cambium is not a formative
fluid, but a formative tissue. Such formative tissues, which
are composed of delicate closely-packed cells, full of protoplasm
and plastic substances, and which have retained their powers
of division, are termed meristematic tissues. Cambium is there-
foPB a meristem.

So as to understand more clearly the position and the nature
of the cambium, let us examine the transverse section of a
one-year old shoot of the Laburnum (Cytisus Ldburnnni) which
was cut in the month of May (Fig. 13).

The portion of the section figured contains only the essen-
tial tissues. It does not include the cortex, which would lie
on the farther side of /•, nor the pith, which would be found
on the opposite side, i.e., beyond m. We have therefore
before us a portion of the wood cylinder which reaches to c:
c represents the cambium ring, which is made up of thin-
walled cells, fitting closely together, without intercellular
spaces, and rich in protoplasm. These cells increase in number
and become transformed towards the inside, i.e., in the direc-
tion of nh, into new wood elements, on the outside into new
phloem elements, while the median portion (where the letter c
is placed) remains the actual seat of formation of new cells.

The tissue lying outside the layer of cambium consists
chiefly of green cortical cells, in which are imbedded strands

THE PHYSIOLOGY OF PLANTS

y^Ua

i. 13.— Transverse Section through the
Wood and Bast of a One-Year Old Shoot
OP Cytisus Laburmiin, out AT the end of
May in the Second Year (after Luerssen).

parenchymatous cortex ; b, bast fibres ; st,
schlereuchymatous cells ; bp, bast paren-
chyma ; c, cambium ; g, wood vessels ; h,
wood cells (libriform) ; t, tracheids ; hp, wood
parenchyma ; g, vessels ; m, medullary ray.

of thick-walled hard - bast
cells, h, and other groups of
hard cells, st (selepenehyma).
The cells have the shape of
parenchymatous cells, but
have excessively thick walls,
penetrated by canals or pits,
and are very like bast
fibres.

The woody cylinder can
be divided into the firm
wood of the previous year
(ah), and the not quite so
thick-walled wood of this
year (nh) which has been
formed up to the month of
May, The elements which
make up the old wood and
the new wood are the same.
We recognise the large open
vessels (g) lying between
the wood fibres (h), with
their small lumen. These
masses of wood are radially
divided up by the lignified
parenchyma, which makes
up the medullary rays (m).

Fig. 1 4 represents a lon-
gitudinal section through the
same piece of twig.

Here we recognise that
the cambium layer (c) is
built up of thin- walled cells
with blunt ends, and which
in their transition to bast
cells {hj}) still retain their
original shape and size, but
have become slightly broader
and thicker-walled. On the
other side, however, towards

THE STIOI

103

g, they have already become elongated and pointed, and their
pointed ends are penetrating between the upper and lower

cells, so that the „_^„,.„

whole tissue be- mmmMm^^^^^^^Wi^^^^WW^

comes considerably

firmer.

If we imagine a
large number of
such segments as
Fig. 1 3 represents
fitted together later-
ally, we have a pic-
ture of the trans-
verse section of our
branch with its
cambial layer caus-
ing the increase in
thickness.

§ 17. What is the
function of the cam-
bium in the ordinary
course of growth ?

The ring of
meristematic cell
situated between
the wood and the
bast is placed in a
very advantageous
position for further
growth. It can
obtain from the
adjoining wood cells
as much of the
unelaborated sap as
it needs, while in the
bast at a very little

distance from the cambium are the sieve-tubes carrying down
from the leaves the fully elaborated plastic materials. The

104 THE PHYSIOLOGY OF PLANTS

cortex, too, and the medullary ray cells can take part in the
nutrition of the cambium, as they are temporary storage tissues
for reserve material (chiefly in the form of starch) with which
they can supply the cambium in case of need.

This advantageous position of the cambium explains its
continuous activity, which enables it to form on the inside
new layers of wood cells {splint-wood), and on the outside new
layers of bast, which push the older ones farther towards the
periphery.

From this continuous activity of the cambium results the
growth in thickness (the secondary thicliening) of the stem.

The wood increases by additions to the outside, the bast by
additions to its inner layers. Both regions of growth lie
close together, and are only separated by the narrow band of
cambium cells which are in process of division.

The new layers which are formed by the activity of the
cambium, as they become older, grow and expand, and need
therefore more room ; consequently they press the cortex
which lies externally to them towards the outside. But this
is not done without resistance, for the cortex is surrounded
by a belt of cork, the elasticity of which tends to retain the
cortex in its original position. This tendency of the cork
takes the form of a pressure, which makes itself felt as far as
the cambium layer itself, and is one of the causes in determin-
ing the form of the new wood cells. That this is the case
may be gathered from the fact, that immediately this pressure
is relieved by cutting thi'ough the cork, the form of the wood
cells is changed.

The increasing thickness of the stem finally ruptures the
cork in various places ; then new layers of rectangular cork
cells will be formed within the green cortical tissues. The
layers of cells outside these new cork layers are cut off from
their sources of nutrition, they dry up and peel off in the form
of scales. This is the mode of origin of bark scales.

Though the cork of a stem is very useful to the plant as a
protection for the green cortex (for it only lets the smallest
traces of water or gases pass through it), it is often very trouble-
some to gardeners. Trees grown on wet soil possess a very
tough cork layer, which retards the formation of bark and the

THE STEM 105

peeling off of the bark scales. This causes the pressure within
the stem to be greater, the aeration of the cortex is reduced,
and the thickening of the stem is kept back. In such cases
the bark will suffer considerable decay, and a large develop-
ment of lichens will take place on the trunk of the tree. These
stems covered with moss and lichens are generally due to an
excessive external pressure of the cortex, and should warn us
to take some measure against it. These consist in scraping
the stem or making longitudinal incisions as well.

The i^ractice, which is gaining ground, of seraping- the stems
of our cultivated trees, is very much to be recommended as a
means of preventing the excessive pressure of the cortex, and
has the secondary advantage of preventing the old bark from
harbouring insects and their eggs,

.^ 18. How does the stem as a whole perform its functions ?

We have sketched out in the preceding paragraphs the
structure, arrangement, and functioning of the wood and bast
systems, and of the cambium ; it is now necessary to add a
few words about the central cylinder of the pith. This latter
consists of parenchymatous cells, which in a few families of
plants is traversed by strands of soft bast, or even by entire
vascular bundles — that is, by tissues meant for the conduction
of formed substances. Such medullary phloem bundles are
present, for instance, in the tuberous Begonias, which have to
fill their storage tubers with reserve material for the leafless
period of rest ; they are absent in the ordinary Begonias.
Medullary bundles exist, therefore, in plants which withdraw
their reserve substances into the roots. From the fact that in
many plants the pith may die off, split, dry up, and disappear,
without affecting the stem, we may gather that the chief func-
tion of the pith must occur during the early development of
the stem. As a matter of fact, experiments go to prove that
the function of the pith is to swell up the axis, and therefore
to accelerate the growth in length of the stem. If we cut a
piece of a young stem or branch of the Elder (Samhuctis), and
place it in water, we see after a short time that the pith
bulges out from the cut end, which indicates that by taking

io6 THE PHYSIOLOGY OF PLANTS

up water, it increases much more in length than the other
tissues, especially more than the woody cylinder. If we
remove the latter, the pith will increase still more in length,
which shows us that it is its attachment to the wood which
prevents the pith from increasing to its full extent. The
rapidly elongating pith, however, drags the young ring of
wood with it, as long as the latter is still soft and thin-walled.
Besides this chief function, it has a secondary one, which often
lasts until the stem is very old, and that is of storing away
the reserve substances of the tree. In the winter, either a few
rows of cells, or the whole of the pith, may be found to be
filled with starch.

The stem, therefore, which is enormously developed in long-
lived plants, appears in spite of its great dimensions to have
only a subsidiary function in the economy of the plant. It
is primarily a framework which bears the organs of chief
activity, the leaves, the vascular bundles of which it connects
with the single central vascular cylinder of that organ, which
is second in importance to the plant — namely, the root. Like
all organic machines, as distinguished from mechanical ones,
the stem has the power of developing and completing its own
structure to suit new demands. By means of its pith it can
increase in length, and thus make room for new generations
of leaves ; its ring of cambium enables it to form new con-
ducting tissues for the increased number of leaves. The
conducting tissues are of two kinds. The wood contains
vessels which lift the nutritive solution taken up by the root
to the centres of assimilating activity, and carry them by
delicate ramifications (veins) to the most extreme points of
the leaves, the green cells of which have the power, under the
influence of light, of building up from the carbonic acid of the
air and from the salts of the soil new organic 'material which
can be used for further growth.

Whatever material the leaf cannot itself employ at once for
its own growth passes into the second part of the vascular
bundle, the soft bast, and through the sieve-tubes of the latter
into the axis. There the formed food substances pass slowly
downwards through the bast, giving off laterally (both to the
pith and to the cortex) whatever may be needed by these

THE STEM 107

tissues. This stream of assimilated food matter passing down
from cell to cell, and from sieve-tube to sieve-tube, regulates
the growth of the cambium — that is, the yearly increase in
thickness. In poor years, when the leaves are not very active,
or when they are not well developed {e.g., after attacks of
insects), it may happen that the plastic material is used up on
its way down, and nothing will remain for the cambium of the
roots, which requires it as much as that of the stem. In such
cases, the annual ring of growth tapers downwards, that is,
the annual ring will only be found so far down as the formed
material has been able to penetrate. In normal years, how-
ever, there is a surplus of food material. The tissues destined
for its passage are not sufficient for the downward current of
assimilated substances ; it overflows into other tissues and there
deposits the superfluous matter. In this case, the parenchy-
matous cells of the cortex, of the medullary rays, and of the
pith become filled with starch, which remains in reserve for
future use.

If trees are left to themselves, their upright trunk becomes
more imposing by the death and decay of the lower branches.
Leaving out of account the damage of the branches by herbi-
vorous animals, the tree is able to rid itself of its supepfluous
branches. Either through the overshadowing of its neighbours,
or through that of its own upper branches, the more delicate
lower branches die off" and are torn away by the wind.

The formation of a trunk is therefore advantageous to a
tree, and called forth by the natural condition of growth and
of its surroundings. In cultivated plants, however, it is not
always advantageous nor necessary. This has brought about
the cultivation of dwarf-trees.

CHAPTER VI

THE LEAF

§ 19. Which cells of the leaf are the most essential ?

A MICROSCOPIC examination of the leaf shows us that it is
mainly composed of green parenchymatous cells, which are so
arranged as to form a flat expanded surface. Parenchyma is,
as we have already stated, a tissue which consists of rounded
or polygonal cells, the diameter in various directions being
approximately the same, though the cells may sometimes be
elongated to form a cylindrical structure. The walls are of
moderate thickness, and the cells are, for a time at least, or
permanently, filled with protoplasm, in which the vital processes
of formation and transformation of organic matter take place.
The eells of the parenchyma are therefore the most important
organs of the plant ; all the other forms of tissue are only of
secondary importance. This is true of the thin- walled loosely
packed cells forming the merenchyma {e.g., the pulpy tissue
of fruits), which is poor in protoplasm, and also of the
sclerenchyma (hard tissue of nut-shells), the cells of which
resemble parenchymatous cells, but are very thick-walled and
entirely devoid of contents. The prosenchyma, too, consisting
of long, pointed thick-walled cells, such as form the wood and
the hard bast, is, functionally speaking, only of secondary
importance. All these elements have secondary functions,
and are not found in the simpler forms of green plants, such
as Alg« and Mosses. Those which are thick-walled serve
chiefly as protection and support for the parenchymatous cells.
The chief function of the green parenchymatous cell is the
production of new organic material {assimilation), of which we
have already spoken in passing. This formative process only
takes place under the influence of light. The carbonic acid

THE LEAF 109

which enters the plant is broken up within the cell, and a
portion of the oxygen is given off. This taking away and
liberation of oxygen from the carbonic acid is termed a process
of reduction. At the same time a very complicated process
of transformation of organic substances is going on {metabolism),
during which oxygen is taken up and carbonic acid is given
off, which shows that a process of oxidation is also going on.
The giving off of carbonic acid, due to the oxidisation of the
organic substances (combustion of substance), is of the same
nature as the breathing in of oxygen by animals, and the exha-
lation of carbonic acid, and is therefore termed respiration
also in the case of vegetable tissues. This process takes place
in darkness as well as in the light.

All parenchyma is not meant for assimilatory processes.
We have already studied the parenchymatous cells of the pith,
which rarely contain chlorophyll, and have a special function
to perform. In other cases, the parenchymatous cells may
have the function of a supporting or strengthening tissue. In
such a case, the cell-walls become thickened in the corners
where three or more cells meet, and the chlorophyll granules
are very slightly developed. Such a tissue receives the special
name of collenehyma.

The amount of assimilation which takes place depends upon
the number of chlorophyll granules present in the cell. For
it is these bodies which make use of energy latent in the rays
of light, and perform the actual assimilation. This chlorophyll
occurs in the phanerogamic plants in the form of granules or
corpuscles, while in some of the algse it assumes the form of
plates or bands. The form of granule seems the most prac-
tical, for it offers the greatest amount of surface for the smallest
amount of substance. In this way, within the smallest space
the largest amount of light is absorbed, and " liglit is life.''
The endeavour to absorb the greatest possible amount of light,
which is apparent in most organisms, is made evident in plants
by the phenomenon of positive heliotroplsm, i.e., the faculty of
placing their organs in such a position with reference to the
source of light, that the incident rays will be most advan-
tageously absorbed. Each chlorophyll granule consists of a
protoplasmic framework, of a spongy nature, which is itself

no THE PHYSIOLOGY OF PLANTS

colourless, but is impregnated with the green colouring matter
termed chlorophyll, which is soluble in alcohol. Sometimes
there are only a few chlorophyll grains in each cell ; generally,
however, they are pretty numerous (sometimes more than 30),
and they lie imbedded in the semi-fluid layer of protoplasm
which lines the inside of each cell-wall, and is termed primordial
utricle. In some cells of the leaf, which have a large lumen, a
movement of this protoplasmic lining will be noticeable, carry-
ing with it the chlorophyll granules. By this means they are
able to wander in each cell towards the point which is best
supplied with air and light. During the day they place them-
selves near the upper surface of the cells, and in the angles
which adjoin the air-containing intercellular spaces (position of
epistrojjJie), while at night they move towards the inner walls
which adjoin other cells {npostrophe).

§ 20. How are assimilating cells of the leaf protected ?

As we see that without chlorophyll granules a cell is not
able to form any new organic matter, we must come to the
conclusion that the most important and essential formative
body in the vegetable kingdom is the chlorophyll grain.

We are therefore completely justified in saying that "with-
out chlorophyll there can be no organic life." For, as a matter
of fact, the existence of animal organisms is dependent upon
the presence of vegetable organisms. Carnivorous animals
feed exclusively upon herbivorous animals, and the food of
the latter, the products of the vegetable kingdom, depend
upon the activity of the chlorophyll grain. The objection
which might be raised, that the Fungi represent a large group
of plants which may produce a considerable amount of matter
without possessing any chlorophyll, does not hold good, for Fungi
can only live on organic substances. If the latter still form part
of a living organism, we are dealing with a parasitic Fungi ;
but if the substance is no longer connected with a living
organism, and is in process of decomposition, we term the
Fungi saprophytic. In either case, the substance is of an
organic nature, and consequently the result (direct or indirect)
of the activity of the chlorophyll.

THE LEAF m

After this exposition we shall not be surprised to find that
nature has arranged these chief formative organs in such a

0

Fig. 15.— Surface View and Section of the Leaf of the Bakley.
E, a piece covered by the epidermis ; F, portion from whicli the epidermis has been
stripped ; o, long, thin-walled epidermis cells covering the mesophyll, which lies be-
tween two veins of the leaf ; 00, thick-walled epidermis cells covering in the vascular
bundles (,'/) ; j.^*, stomata leading to the spongy parenchyma, with large intercellular
spaces (i) ; in, mesophyll cells ; h, short hair on margin of leaf ; p, prosenchymatous
strand of cells strengthening the leaf margin.

Vf&j as to derive the greatest possible benefit from the smallest
amount of substance. This is done by building the assimi-

112 THE PHYSIOLOGY OF PLANTS

lating organs in such a way as to display the greatest super-
ficial area.

Tliis principle is carried out, in the first place, in the shape
of the chlorophyll-containing bodies themselves, which are
divided up into numerous small granules. Besides this, the
assimilating cells are united so as to form thin expanded sur-
faces, the leaf-blades.

In these it will be noticed that the assimilating cells (Fig. 15),
the actual green tissue (mesophyll) (Fig. i 5, m), is protected on
either surface by a layer of cells, which can often be stripped
off as a colourless skin. This protective covering (epidermis)
consists of closely fitting tabular cells, with a thicker outer
wall, and generally devoid of contents (0 and 00). These cells
are covered in on their outer surface by a continuous layer of
a substance which exhibits at times the character of cork, at
times that of wax, and which renders the cells more or less
impervious to gases and liquids. This layer is termed the
eutiele ; the more fatty matter this cuticular layer contains,
the less liable are the leaves to wetting. The bluish or
whitish bloom of many stalks, leaves, and fruits is due to a
vegetable wax contained in or covering the cuticle.

The cuticularised epidermis does not, however, entirely
close up the leaf. This would render the activity of the green
mesophyll entirely impossible. For the chlorophyll-containing
cells have the function of decomposing the carbonic acid when
they are in the light ; there must therefore be means by which
the carbonic acid gas of the air reaches the interior of the
leaf. This means exists in the form of a system of pores,
provided with a mechanism for opening and closing.

Between the cells of the leaf-tissue or mesophyll are
irregular spaces connected one with the other, which are
termed intercellular spaces {%), and these are filled with air.
Every green cell of the leaf can therefore take in gases from
the intercellular spaces, and give off gases into them. The
air contained within the leaf undergoes changes by the activity
of the cells, and collects in large and definite spaces or
respiratory chambers, which lie immediately beneath the
epidermis, and above which the latter is interrupted. The
openings of the epidermis are termed breathing pores {stomata.

THE LEAF 113

Sp). They are formed by the splitting apart of two semilunar
epidermal cells, which are formed specially for this purpose
(Fig. 16, Sp, scJi). These semilunar, or rather kidney-shaped
cells, have their concave sides applied so that the ends of the
cells alone are in contact, and a space, the actual pore, lies
between them (*S^, c). If these kidney-shaped cells are flaccid,
and are drawn out lengthways by the surrounding epidermis
cells, the bend of each cell is lessened, and the intervening
slit or pore is closed. Seen from above, the stomata will then
have the appearance of a coffee-bean (Sp). If, however, the
kidney-shaped guard cells become turgid through the absorption

.se^

Fig. 16.— Mesophyll Cells and Stomata of the Barley-Leap.

a stoma ; sch, guard cells ; c, aperture of stomata leading into the respiratory cavity of
the leaf ; M, parenchymatous cells of a leaf, taken from near a vascular l)undle ; Z,
nucleus ; chl, chlorophyll graius.

of water, the line (c) in the above figure widens out to form
an oblong canal. Through this canal a relatively large amount
of air can enter the respiratory cavity or escape from it. If
we consider for a moment that the leaf of an apple-tree has
over 100,000 such pores to every square inch of leaf surface,
we can form an idea of the ease with which an interchange
of gases can be effected between the atmosphere and the
inside of the leaf.

The structure of a leaf will become still more clear to us if
we examine the transverse section of a leaf (Fig. 1 7). We
will take as an example the leaf of the india-rubber plant
{Ficus elastica), which is suffei'ing from a disease very prevalent
in plants grown in dwelling-rooms, and termed intinnesrentia.
This disease consists in the appearance of numerous small

II

114

THE PHYSIOLOGY OF PLANTS

glandular protuberances on the under surface of the leaf.
These are formed by the tubular elongation of cells (int),
which in their normal condition have the appearance shown
in m, and are therefore provided with considerable intercellular
spaces. The diseased tissues on the under side of the leaf
become more like the normal pallisade parenchyma (p) of the

Fig. 17.— Transverse Section through the Leaf of Fims dastica suffering
FROM Over-Watering.

upper surface of the leaf, which is covered in by a triple
epidermis (E). Of these three layers, the outer one consists
of very small cells, which are, however, covered in by a very
thick waxy cuticle. The innermost layer of the epidermis
is formed by thin-walled comparatively large cells (vj), which

THE LEAF 115

must be looked upon as a water-storing protective layer.
Some few cells of this layer contain curious structures of
cell-wall substance {cellulose), which are incrusted with car-
bonate of lime. The part played by these grape- like
crystals, which are termed cystoliths (c), in the economy of
the leaf is at present still unknown. Such protrusions of
the cell-walls with crystallised deposits have only been found
in a few natural orders {Vriicacem, Ficoidem, Accmthacecc, Cucur-
hitacece).

The firm covering of the upper surface of the leaf is not
favourable for the passage of gases, for the passage of which
the under surface is peculiarly adapted. Here we notice the
large intercellular spaces in the tissue, which on that account
has been termed the spongy parenchyma. The air contained
in the intei'cellular spaces (i) communicates with the air con-
tained in the respiratory chamber (a), and through the stomata
{st) with the outer air, which can in its turn freely pass into
the tissues of the leaf

The water is supplied to the leaf through the veins, a small
one of which is seen cut through transversely {g). The letters
r represent the vessels of the wood, the actual canals along
which the water flows. The path taken by the organised
food material, which has been formed in the leaf, and of
which there is a superfluous supply, is along the bundle-sheath
{sell) down into the stem, k indicates the point where the
cells begin to round themselves off abnormally, owing to the
excessive supply of water ; the intercellular spaces become
obliterated, and they press the still more irritated tissue {int)
towards the outside.

A structure similar to that of the leaf of Ficus would
be presented by the leaves of most of our cultivated
plants. Many of them, especially the herbaceous ones, have
stomata on the upper side of the leaf, as well as on the under
surface.

The functioning of the stomata will call forth our astonish-
ment and admiration still more, if we reflect upon the fact
that the guard cells, which are considerably thickened on their
inner side, become more bent, and cause therefore an opening
of the stomata, when they become turgid. But they only

ii6 THE PHYSIOLOGY OF PLANTS

become turgid when the whole leaf is turgid, that is, when the
roots supply the leaf with a large amount of water, and when
there might arise some danger of a superfluity of water in the
leaf tissues. In such cases, the plant protects itself against
danger by enlarging the passages through which the water
vapour can escape. On the other hand, the tissues in the
inside of the leaf are protected from too great a loss of
water through evaporation ; for as soon as the guard cells
lose water and become less turgid, they close the aperture
which existed between thera. Thus the plant regulates
the amount of its transpiration according to its needs and
requirements.

In many plants such safety-valves are still more largely
developed. The tips and teeth of some leaves have very large
stomata, arranged singly or in groups, and their guard cells
are either rigid or sometimes quite destroyed. They lie
immediately above a specially formed very thin-walled paren-
chymatous tissue (epithem), which reaches up to the epidermis.
The vascular bundles of the leaf terminate in this epithem
by a number of lignified cells or tracheids, often arranged after
the manner of a brush. Now if the tissues of a plant contain
an excessive amount of water, it is given off in these places
in the form of liquid drops of water. Here, therefore, the
stomata are water stomata or water glands.

This explains the phenomenon, which is well known to all
gardeners, that in cool nights the tips and teeth of the leaves
of some plants {Primula sinensis, Calla cethiopica, &c.) are
quite regularly studded with small drops of water, and new
drops will make their appearance as soon as the first are
removed.

Just like the leaves, the green herbaceous stems are also
covered by an epidermis perforated by stomata, so that the
cortical tissues can be aerated. When the stem grows older,
and the simple epidermis is replaced by a more resistant layer
of cork, we find that below the stomata those more complicated
respiratory organs will be formed which we have already dis-
cussed— namely, the lenticels.

The description of the epidermis would be incomplete if we
omitted to mention its excrescences or triehomes. We under-

THE LEAF 117

stand by these first of all the hairs, which iu their simplest
forms are conical or tubular protrusions of the epidermal
cells, as we have seen in the case of the root-hairs. Just as
in the first stages of development, some cells of the epidermis
remain small and meristematic, and give rise by dividing to the
guard cells of the stomata, while others remain tubular, and
constitute the ordinary epidermal cells, so some other cells
which will not become guard cells also remain meristematic,
and retain the power of growing out, dividing and branching,
and represent the organs known as hairs. Sometimes a certain
connection may be observed between the hairs and the stomata ;
in some petals, for instance, the formation of hairs increases
as the formation of stomata ceases.

In the case of the formation of hairs, the plant again pro-
ceeds on the lines of increasing its superficial area. As long
as the wall of these outgrowths is thin and not cuticularised,
they will be able to absorb water and gases, and also to give
them off; they will therefore be able to serve the purpose of
transpiration. Later on, when the trichomes have grown to
their full size, and have begun to form firm, cuticularised
hairs, stellate, glandular, or woolly hairs, their function is a
different one. They now represent a covering to the organ,
and in the interstices of this covering they hold fast, very fast
indeed, a certain quantity of air. It is only with great diffi-
culty that the air can be driven out of such a felting of hairs,
and it therefore is a very considerable protection against
variations of temperature and of moisture.

Cold will take a long time to affect a leaf covered with such
a felted mass of hairs, and rain will not be able to drive out
the air from this covering, and will therefore not wet the leaf.
But the greatest protection which such a layer of hairs affords
is that it will greatly reduce the amount of transpiration from
the leaf surface. The wind which passes over a bare thin-
walled leaf carries away a great deal of water vapour, while
it will scarcely set the air contained between the dense
hairs in motion. The amount of transpiration, however, does
not depend entirely upon the dryness and the movement of
the air, but also largely upon the amount of light. The more
intense the illumination, the greater is the evaporation. But

ii8 THE PHYSIOLOGY OF PLANTS

in this case, too, the covering of hairs acts as a protective
coating, for it prevents a large amount of light from reaching
the leaf.

This explains why so many desert plants, and plants growing
in sandy or rocky places and exposed to a powerful sun, are
densely covered with hairs.

For horticultural purposes, too, we may deduce the rule that
plants with leaves densely covered with hairs require generally
a large amount of light and little water. We shall refer to this
again in dealing with the watering of plants.

.^ 2L How are the assimilating cells arranged within the leaf?

We have mentioned in passing that the cell of chief im-
portance is the chlorophyll-containing cell ; its function is to
assimilate. These cells are so arranged in the assimilating
oi'gans that the chlorophyll grains can perform the greatest
amount of work. The principle of increasing the superficial
area is carried out to the fullest extent. The assimilating
surface is protected by a well-developed and ingeniously
devised epidermal covering.

We are now able to examine a little more closely the posi-
tion of the assimilating cells themselves, and we shall find
that here too we have a very advantageous combination of
various contrivances, forming a most complete arrangement
for ensuring the maximum of efficiency.

Most of the leaves are horizontally expanded, so that we
can distinguish two surfaces, generally of different appearance,
one directed upwards towards the sky, and the other down-
wards towards the earth. This latter surface is generally of
a paler colour, which is due to the fact that the green cells
of the mesophyll are more loosely arranged and are less rich
in chlorophyll, and that the large intercellular spaces are filled
with air.

The assimilating cells, however, which lie beneath the epi-
dermis of the upper side of the leaf are very closely arranged,
like a number of stakes driven in side by side, with their
narrow ends towards the upper surface, and consequently with
their long axis at right angles to the surface of the leaf. These

THE LEAF 119

cells form what is called the pallisade parenchyma. The loose,
often horizontally elonuated, tissue of the under side of the leaf
is termed the spongy parenchyma. The pallisade cells, on
account of the greater number of chlorophyll grains which
they contain, and because the incident rays of light pass
through the entire length of the cell, and illuminate each
chlorophyll grain, are the most efficient assimilating cells. At
the same time, as they have only their narrow head end directed
towards the light, they are able to protect themselves from an
excess of light.

The product of their activity is recognisable very soon after
the light has been acting upon the tissues; it makes its
appearance in most plants within the chlorophyll bodies in the
form of starch g-rains. In some natural orders {e.g., in the
Com2)ositce), in the place of starch a soluble substance, inulin, is
formed, which also consists of hydrogen, oxygen, and carbon,
in about the same proportions as starch. If the water be
withdrawn from cells containing this substance — as will be
the case if the tissues are treated with alcohol — the inulin
attaches itself to the cell-walls in the form of solid crystalline
masses, the so-called sphseroerystals. Certain reactions, too, go
to prove that new albuminous (nitrogen-containing) substances
are formed within the chlorophyll grain ; so that the two chief
groups of the organic matter are represented within the chlo-
rophyll granules.

If the pallisade cell cannot itself make use of the newly-
formed organic material, it passes out of the cell during the
night in the form of sugar and other soluble substances.
These are either absorbed by the adjoining cells of the spongy
parenchyma and conducted towards the base of the leaf, or
they pass directly into the parenchymatous sheath which
surrounds the vascular bundle. These bundle sheaths (Fig.
17, Sch) are distinguished, according to their contents, as
sugar sheaths or starch sheaths, and form the actual canals
along which the assimilated substances pass and rapidly
reach the stem.

THE PHYSIOLOGY OF PLANTS

§ 22. How is the leaf developed?

If we examine the extreme tip of the stem, we find that it
is composed of undifferentiated cells, fitted very closely together,
with their delicate cell-walls, and containing a large amount
of protoplasm.

These isodiametrical cells form the primary meristem. But
very close behind the apex the tissues of the stalk begin
to become differentiated, and first of all the cells of the outer-
most layer elongate and begin to form a tabular row of cells,
which becomes the epidermis of the stem. Near the centre
of the axis a number of rows of cells become differentiated by
rapidly dividing by longitudinal walls, and forming therefore
strands of tissue consisting of very narrow cylindrical elements.
These strands, which go on dividing, are the rudiments of the
vascular bundles, which occur scattered in Monocotyledons,
but are arranged in a ring in Dicotyledons, where they will
afterwards form the wood cylinder. The remainder of the
primary meristem, which does not enter into the formation of
epidermis or vascular bundle, remains as a parenchymatous
ground-tissue, the central portion being called the pith, the
outer portion the cortex.

In this first period of differentiation of tissues, it will be
noticed that at certain points groups of cells which belong to
the outer ground-tissue, close below the epidermis, begin to
divide. The newly-formed cells elongate at right angles to
the axis, and therefore cause small protuberances from the
young stem.

The young epidermal cells which cover in these projections
also begin to multiply, and thus continue to form a protective
layer to the inner tissues. These begin to become differen-
tiated by the appearance of various strands of tissue, which
are the rudiments of vascular bundles.

This complex of cells which bulges out from the axis is the
first stage of development of a leaf, which takes its origin
from certain divisions of the young cortex, and may therefore
rightly be looked upon as an expansion of cortical tissue.

The young leaf very soon ceases to grow at its apex, and

TILE LEAF 121

its zone of gvowtli becomes limited to the basal portion. The
apex therefore represents the oldest portion of a leaf; only in
the fronds of ferns growth continues to take place at the apex,
and sometimes for several years {e.g., in Gleichenia). If a leaf
is to become pinnate, lateral projections begin to make their
appearance on the young leaf, beginning at the base and pro-
gressing towards the apex.

The further development of the leaf differs according to the
species and the locality, the only principle of construction
observed in all cases being, as we have already seen, the effort
to produce a great expansion of the assimilating tissue.

If this assimilating tissue is in any danger of being dried
up or damaged by too intense an illumination, nature protects
it by various contrivances. Thus the Helichrysce of the Cape
are covered with a tremendous development of felted hairs ;
the epidermal cells of Agave americana have an enormously
thickened cuticle, still further protected by a layer of wax ;
while Ficus dastica has, as we have seen, a triple epidermis,
one layer of which has the function of storing water. In
some cases the leaf is reduced to the form of a needle (3£cla-
leuca, Metrosideros) ; in other cases the leaf assumes a vertical
position, so that the sun illuminates the leaf edgeways {Euca-
lyptus). In this case a pallisade parenchyma is formed on both
sides of the leaf. Nature can also reduce the number of the
storaata according to circumstances. In many horizontal and
tough leaves the shining upper surface of the leaf is devoid
of stomata ; in other cases they are placed in protected pits.
Floating leaves {Nymphcca) have stomata only on their upper
surface ; the leaves of plants which always have a large supply
of water have very large guard cells, &c.

Whatever may be the arrangement of the assimilatory
tissues within the leaf, it always makes use of the first rays of
light which it absorbs to develop its real working organs, the
chlorophyll granules. The rudiments of these are present in
the protoplasm of every newly-formed cell, and have entered it
from the mother cell in the form of a small protoplasmic struc-
ture,^ which under the influence of the light begins to form the
green chlorophyll, and causes the young leaf to become green.

^ Trophoplast.

122 THE PHYSIOLOGY OF PLANTS

The fully-formed chlorophyll bodies grow, and become biscuit
or dumb-bell shaped. The narrow portion of the grain is almost
colourless, and by becoming entirely constricted the granules
increase in number. In this way the leaf attains its green
colouration, which is so characteristic of the fully developed
organ. By this time the cells will have attained their full
size, and the intercellular spaces will have made their appear-
ance. Even these must have definite positions to be of use to
the plant. In the case of the pallisade cells they occur be-
tween the lateral walls, and thus the newly-formed substances
cannot pass laterally into other pallisade cells, but must pass
out at the bottom of the pallisade cell into the spongy paren-
chyma, or into the bundle sheath.

.§23. What substances does the leaf chiefly form ?

The first product of assimilation which can be detected in
the chlorophyll body is the starch grain, which is the chief
representative of the so-called carbo-hydrates. This group of
substances, to which sugar, cellulose (cell-wall substance), inulin,
dextrine, and gums belong, consists of the elements carbon,
hydrogen, and oxygen, the last two substances being combined
in the same proportions as they exist in water (H^O). The
starch occurs in the form of curiously stratified grains, which
can be coloured blue, in some cases red, with iodine. Starch
may be looked upon as the solid equivalent of the liquid carbo-
hydrates which pass from cell to cell in the form of sugar.
When the light shines on the leaf, its transpiration is greatly
increased, and its cells lose some of their water ; then we may
imagine the starchy substance to be forced out of the thickened
{concentrated) cell-sap, and the latter will become more liquid.
The starchy substance is in most cases not entirely pure, but is
mixed with amylodextrin, and this causes the stratification of
the grain. The more this substance is present in the starch
the redder will it stain with iodine. 'As soon as darkness
sets in, the ferments become more active, and transform the
starch into dextrin. This latter is converted into sugar,
which passes from cell to cell through the cell-wall, and
from the leaf into the stem, where it is reconverted into starch,

THE LEAF 123

and remains as reserve material for future use. In some
plants the starch is replaced by fatty oils.

Ferments are substances, probably nitrogenous and of an
albuminous nature, which have the power of causing the break-
ing up of other substances. The most common of the starch-
splitting ferments (all of which are chemically little understood)
is diastase, which causes the ti^ansformation of starch into
sugar in the seeds, tubers, and stems at the commencement
of the vegetative period. The sugar itself often undergoes a
change by means of a ferment formed by fungi {eg., by the
yeast plant), and termed inveptin. The crystallisable cane-sugar
{saccharose), which is obtained from the sugar-cane or the
beetroot, is not able to be fermented, but is converted by the
action of invertin (also by acids) into grape-sugar {dextrose,,
glycose) and into Isevulose, both of which can undergo fermen-
tation. A mixture of these two substances (inverted sugar) is
of very frequent occurrence in vegetable tissues. The solution
of meat, in the case of the so-called carnivorous ^^/a^i^.s, is
brought about by "pepsin," which in the presence of acids
(formic acid, citric acid) is a very active ferment, and can
digest albuminous substances. Whether this vegetable pepsin
is identical with the peptic ferment found in the stomachs of
animals, which becomes active in the presence of hydrochloric
acid, remains to be finally settled.

In the formation of gums, too, a ferment has been discovered.
Gums often occur as transformations of cell-wall substance.
The gum arabic {arabin) is of very frequent occurrence, and is
looked upon as a combination of a carbo-hydrate with lime. In
plum and cherry trees a gum, eerasin, is formed in all the
tissues, and is looked upon as a combination of the carbo-
hydrate metagummic acid and lime. Basorin, or gum traga-
canth, is considered to be a pure carbo-hydrate formed in the
genus Astragalus. Together with gums and acids we often
find substances consisting also of carbon, hydrogen, and oxygen,
which form gelatinous masses on boiling, and are termed
pectin substances. They occur largely in the fruits of Ccratonia
siliqua (St. John's bread), in tamarinds, and in the pips of
apples, quinces, &c.

Glucosides represent a very largely distributed group of

124 THE PHYSIOLOGY OF PLANTS

substances in the vegetable kingdom, which are split up by
acids and ferments into sugar and an indifferent substance.
An example of this group is amygdalin, a nitrogenous form
which occurs in the stone of various fruits (cherries, almonds,
&c.), which have the flavour of bitter almonds, due to oil of
almonds. This oil, as well as prussic acid and sugar, are
derived from the amygdalin by the action of the ferment called
emulsin. Another well-known substance is the oil of mustard ;
it is formed in the seeds of Cruciferag from sulphur-contain-
ing myric acid, a glucoside, which is split up by a ferment
{myrosin) into oil of mustard, sugar, and potassium sulphate.

Very largely distributed in the vegetable kingdom, espe-
cially in young organs, are the tannin compounds, which are
chemically not very well defined. Of tannin itself we know
that it belongs to the glucosides. Allied to them are certain
colouring matters which are either dissolved in the cell sap or
occur in isolated drops. Other colouring matters ^ are also
devoid of nitrogen, soluble in alkalies and alcohol, but insoluble
in water. They occur only in the cell-walls, and are allied to
the resins which are rich in carbon, poor in oxygen, and devoid
of nitrogen, and which occur especially in the bud scales and in
woody tissues. Pure hydrocarbons occur in the form of etherial
oils, which in most cases give the plants their characteristic
odours. As an example we might mention the oil of camphor,
which yields camphor by oxidisation.

The albuminous substances {proteids) are much less known
because they are less easily separated one from another,
and occur in numerous modification and transition stages.
Chemically we may distinguish, just as in the case of animal
albumen, three chief groups, the albumins, the caseins, and the
fibrins. Albumin is the name given to the albuminous sub-
stances which are soluble in the cell sap, but which coagulate
at a temperature of from 6o° to 70° C. Caseins are soluble in
water, do not coagulate on heating, but do so when treated
with acids. Fibrins are insoluble in water but partially soluble
in alcohol. The various members of these gi'oups often differ
considerably in theii' characters. Among the caseins, con-
glutin, which occurs in almonds, is much more soluble than

^ Phlobaphenes.

THE LEAF

125

leguinin, wliich occurs iu the seeds of legniiiinous plants. The
aleuron layer of cereals contains gluten-casein and gluten-
fibrin. The products of decomposition, too, of the albumens
are as different as are their degrees of solubility. Thus gluten
fibrin, when boiled with sulphuric acid, yields very little
asparagin (a substance of great importance for the transit
and formation of new albumens), while the other groups
yield a large amount. Other products of decomposition are

FIG. 18.— A Section through the Outer Layers of a Potato Tuber.

k, cork ; pi, cells of cortex containing protoplasm and small starch grains ; cr, tabloid
protein crystals ; s, starch grains (after Tschirch).

leucin, tyrosin, glutamin, ammonia, and volatile oils. These
latter cause the unpleasant odour of decomposing albumens.
The albumens may also occur in a crystalline form in the cell
sap ; they are then distinguished from the inorganic crystals as
protein crystals. Such crystals occur in beautiful cubical form
in the end cells of tbe gland hairs of the potato shoots.

The protein crystals are perhaps most easily seen in sections
of a potato tuber, where they occur in the cortical tissues,

126 THE PHYSIOLOGY OF PLANTS

especially of the white varieties of potato. Indeed, the number
of these protein crystals can give us some clue to the amount
of starch present in the potato, for it has been found that
the greater the quantity of these crystals the poorer the tuber
is in starch. Fig. i 8 rej^resents a piece of a section through
the cortex of a potato. The actual peel (k) consists of a
number of cork cells which give it its toughness. Below the
cork cells are the outermost cortical cells (pi), which are rich
in albumens and poor in starch ; here we shall find the most
protein crystals (cr). The further we proceed to the centre,
the lai'ger the amount of starch contained in each cell. These
starch grains (s) have an oval shape, and have a very charac-
teristic and peculiar stratification.

Very often the crystalline albumens {crystalloids) occur
within other solidly formed albumens. In the seeds of
phanerogams they occur in the form of aleurone grains, and
form the reserve albumens which are used up on germination.
In these aleurone grains, which in some plants (Pceo7iia,
Ricinus) are soluble in water, some albuminous compounds
are mixed with phosphates of lime and magnesia, and assume
a spherical form (globoids).

We see, therefore, that the albumens occur like the carbo-
hydrates in form of insoluble reserve material, and in soluble
transportable substance.

We must here make mention of the groups of alkaloids,
characterised also by the fact that they all contain nitrogen.
They have organic bases, containing nitrogen, carbon, hydrogen,
and generally also oxygen, and are probably first formed by
the protoplasm, and contained in the cell sap, but are often
found in the cell-walls too. Among the alkaloids devoid of
oxygen we have trimethylamin, which gives the peculiar odour
to the ergot of rye, and some of the members of the Goose-
foot family, &c.

The oxygen- containing alkaloids comprise some of our most
valuable drugs, such as strychnine, quinine, morphine, atropine,
which occur combined with acids in the various organs of
certain plants.

TIIK LEAF

127

§ 24. How does the leaf actually perform its assimilatory
function ?

The above-mentioned albumens in various proportions make
up a portion of the protoplasmic substance which entirely fills
the young (meristematic) cell (Fig. 19), But very soon there
arise within the protoplasmic mass which fills up the cells
very small quantities of liquid substance (cell sap), which
collect together to form small vesicles (vacuoles), and these
give to the protoplasm a foam-like structure. Now, as the
protoplasm becomes used up during the elongation of the
cells, the vacuoles increase in number, run together, and force

^«;

H'iG. 19.— Young (Mkki-
STEMATic) Cells with
Nuclei.
a, nucleus; h, nucleolus.

Fig. 20. — V'oung Paeenchymatous cells.

the nucleus {cytoblast) (Fig. 20, z) and the remainder of the
protoplasm outwards towai'ds the cell- wall. The nucleus
which is visible in almost all cells is intimately connected with
the life of the cell, and plays an important part in the division
of the cells. It has the appearance of a dense albuminous mass,
in which one or more small refringent bodies are present,
which must probably be looked upon as reserve material, and
which are termed nucleoli (Fig. 20, /.).

With a high magnifying power the nucleus has the ap-
pearance of spongy framevvork, or sometimes of a number of
loops of thread intimately interlaced, consisting of a denser
substance, the chpomatin substance or nuclear thread, the
intervening spaces being filled by the nuclear sap or fluid.

128

THE PHYSIOLOGY OF PLANTS

The latter is a turbid, thickish liquid, whicli afterwards becomes
clearer, and takes up certain stains less actively than the
framework, which consists of the nuclear plasma {nuclcin,
chromathi).

In Fig. 2 1 we see in (i) a nucleus in the young mother
cell of the stomate guard cell before its division ; in (2) we
recognise more distinctly the chromatin framework, which we
see in (3) is formed of pieces of a thread. These pieces of
the nuclear thread are arranged (in 4) in the median plane
of the cell in which the new wall is to be formed.

They now form what is called the equatorial plate. The

-Nuclear Divisios in a Guard cell of 7/

ila (after Haberlandt).

nuclear substance now divides itself into two, and one-half
proceeds to each pole of the cell (5), where it becomes arranged
to form one of the daughter nuclei (6-9).

The movement of the various portions of the original
nuclear thread towards the centre, and the return of the
halves of these segments after division, takes place along a
number of threads which are formed in the protoplasm, and
are termed the spindle threads. These threads converge at
the two poles, but separate one from the other in the middle
of the cell (4-7)- By the time these threads make their appear-
ance, the former covering of the nucleus, nuclear membrane,
has disappeared, and the threads traverse the entire cell ;

THE LEAF 129

the figure which they form is termed the nuclear spindle
(4,5,6).

At the centre, where the nuclear plate had existed, after
the division of the nuclear matter a number of very small
albuminous granules (microsomata) make their appearance
(Fig. 21, 8). They and the central portions of the spindle
threads fuse together to form a plate, which forms a thin
transverse plate of protoplasmic substance in the original cell.
This protoplasmic plate becomes transformed into a cellulose
plate, which now constitutes the cell- wall separating the two
daughter-cells (9). These cells increase in size, and either
divide again or pass over into permanent tissues.

By this means the daughter-cells of the stomate mother-
cells are transformed into guard cells, the median wall split-
ting to form the aperture. In the same way, most cells
increase in number, and by this means the growth of leaves,
stems, and root-tips takes place.

Every daughter-cell has therefore one-half of all the con-
stituents of the mother-cell ; this explains the fact that all the
characters of the mother-cell reappear in the daughter-cell,
and the inheritance of all characters from one generation to
another can be explained by this means.

For we may imagine that every characteristic of a cell is
represented by some infinitesimal piece of matter, the atoms
of which have a definite relative position and movement, which
give it its character. Now, we have only to suppose that in
the cell division every such particle is divided into two, just
as, on a large scale, the nucleus is, and then each daughter-cell
will contain all these characteristic atoms, with their definite
structure and definite movement — will, therefore, have all the
characters of the mother-cell. The nutrition consists in this,
that the raw materials which enter the cell are attracted, now
by one, now by another of the constituents of the cell, and
adopt its specific movements, and are thus transformed into
the same substance (assimilated). Thus every cell of a seed-
ling contains in the innumerable microscopically still undiffer-
entiated particles of its protoplasm as many different groups
of bodies as the resting seed, but they have become active
during the germination, are continually increasing in amount,

I

130 THE PHYSIOLOGY OF PLANTS

and will give expression to all the characters which we recog-
nise in the adult plant.

Though we have stated that the various groups of sub-
stances of the protoplasm cannot as yet be microscopically
differentiated, we must modify the statement to exclude a few
cases. For we can recognise in every young cell of a leaf,
and of other tissues too, a number of small indistinct par-
ticles of a very dense protoplasmic nature, which are termed
trophoplasts,^ which have in many cases very important functions
to perform. A number of these bodies, for instance, develop the
green colouring matter, and become chlorophyll eoppuseles, i.e.,
the chief formative bodies of the cell ; others become brightly
coloured, and form the ehromoplastids which are found in
flowers and fruits ; and, finally, a third group, which remains
colourless, leueoplastids, can form starch grains out of sugars.

All these trophoplasts grow within the cell and increase in
number by division. A number of them are carried into the
daughter-cells during division, and we can therefore rightly
say that the chlorophyll granule, for instance, in the uppermost
cell of a Ricinus plant several yards in height, is derived from
one of the chlorophyll bodies of the embryo.

When the trophoplast becomes transformed into a chloro-
phyll corpuscle, it becomes more porous and sponge-like, and
in its interstices the actual colouring fluid makes its appear-
ance. The latter consists of two substances — namely, the
green chlorophyll and the yellow xanthophyll. The nitrogen-
containing chlorophyll (recently shown to be devoid of iron)
can be extracted from the cells by alcohol, ether, chloroform,
or by etherial and fatty oils. It then has the appearance
of a green solution with a blood-red fluorescence. It is a
very unstable substance even within the vegetable cell, and
as soon as the latter dies the acid cell-sap gives it a brownish
colour. By this action acid chlorophyll or hypoehlorin (cMoro-
'^hyllan, 'pliyllo-cyanic acid) is formed, and this is the cause of
the spotting of diseased leaves. If in the cooking of green
vegetables we wish to avoid such a discolouration, we need
only add a little carbonate of soda, as the presence of alkalies
will prevent the formation of the hypoehlorin.

^ They are also called plastids, chromatophores, leucites.

THE LEAF 131

This substance, however, is always found in some quantity
in tJie fresh extract of green tissues, which fact suggests that
at all times a breaking down and a new formation of chloro-
phyll is taking place.

The less soluble xanthophyll forms in the above-mentioned
liquids a yellowish non-fluorescing solution ; it does not con-
tain any nitrogen, and is less readily decomposed than chloro-
phyll. It makes its appearance in the cells before the green
colouring matter, and it remains longer in the cells at the
approach of autumn. Hence the leaves become yellow coloured
In the autumn, and in spring the young shoots and leaves are
yellowish-green.

As soon as the chlorophyll corpuscle is fully formed, it
begins to assimilate, forming in most cases starch, more rarely
drops of oil (in many Monocotyledons). It is certain that other
substances are contained in the chlorophyll corpuscle (in
some cases protein crystals and tannin masses have been
observed), but a micro-chemical demonstration of the same has
not as yet been possible. Speaking generally, however, of
the production of organic matter in the cell, we find that the
first products of assimilation are not very rich in oxygen, and
that in the further changes they become more and more
oxidised, until finally oxalic acid is formed, and then it returns
again to carbonic acid, which is set free and given off.

So we have in the life of the leaf a marvellous cycle of
changes. The leaf forms, under the influence of light, its
chief formative substance (chlorophyll), and the latter starts
at once its assimilation of substances, poor in oxygen, by
combining the raw solution of the soil with the carbonic
acid of the air. This gas, absorbed from the atmosphere, is
split up in the vegetable cell, the carbon being rapidly used
up, the oxygen being liberated. In this way sugar, starch,
and cellulose are formed. But the untiring oxygen exerts
its powers on the newly-formed substance. It unites with
these new bodies and forms more highly oxidised com-
pounds, and acids, such as oxalic acid, become more numerous.
The leaf renders the otherwise injurious acid harmless by
combining it with lime, and thus forming insoluble crystals of
oxalate of lime. These have either the form of needle-shaped

132

THE PHYSIOLOGY OF PLANTS

prisms (raphidcs), or flat lozeBge-shaped tablets, or rhomboid
crystals, often united in clusters. This oxalate of lime is
absent from very few plants (Ferns and Horse-tails) ; the
crystals are generally in the cell-sap, but sometimes in the
cell-wall substance (Fig. 22).

In this figure (22) we see two star-shaped cells taken from
a septum, which separates the air chambers of the stalk of
Musa. In each of the cells, which are separated by large
intercellular spaces (i), we see several of these prismatic
crystals. Besides these we find that some cells of Musa con-
tain the oxalate of lime in
the form of raphides (b).
While the latter occur most
frequently in Monocotyle-
dons, Dicotyledons usually
have octohedral crystals, or
clustered crystals of this
substance ; as an example
of these, we have the cells
(c) taken from the leaf-stalk
of a Begonia (after Luerssen),
It is more rare to find
crystals of calcium carbonate
in the tissues of plants ;
this generally forms an
incrustation on the outer
surface (many Algee), or it
enters directly into the
substance of the cell-wall
and renders it resistant. In some natural orders {Urticaceoe,
Acanthacece, Cumtrhitacecc), the epidermis and other tissues
have club-shaped or rod-shaped processes of cellulose {cystoliths)
(Fig. 17, c), which have become covered with carbonate
of lime, which, however, does not make its appearance if the
plants are grown without lime or without light.

Only in very rare cases does the lime occur in plants as
gypsum (sulphate of lime) or as phosphate of lime. This latter
compound sometimes makes its appearance when some tissues
which are very rich in protoplasm (potato-peels) begin to

Fig. 22.— Diffekent Forms of Crystals of
Oxalate of Lime.

THE LEAF 133

decay ; generally speaking, indeed, the occurrence of com-
pounds of lime indicates increase of age. It is only when
the cells become old that the cell-walls become strengthened
by salts of lime, and become less permeable ; and it is then
also that crystals of oxalate of lime are deposited.

Thus we find yellow autumn leaves rich in lime but poor
in plastic substances. They have served their purpose. The
yellow colour is due to the dissolution of the chlorophyll, the
xanthophyll still remaining in the tissues. In the falling
leaves the protoplasmic framework of the chlorophyll corpuscle
is also absorbed, and the nitrogenous compounds make their
way back into the stem. After the fall of the leaves, decom-
position alone takes place within their tissues, and among the
products of decomposition we have, in the first place, carbonic
acid, i.e., the substance which it has been the function of the
leaf to absorb and to transform into organic matter.

CHAPTER VII

THE TREATMENT OF THE SHOOT

§ 25. Why must the shoots of our cultivated plants be pruned ?

In all our horticultural efforts we desire a well-proportioned
plant. Furthermore, we seek to increase the economic value
of the plants by increasing the quality or quantity of its crop,
or by decreasing the time necessary for its production.

Lastly, we have to take into consideration the portability of
the plants.

Leaving out of consideration the short-lived herbaceous
plants, we find that most plants which are left to themselves
suffer considerably from accidental injuries (by storms or in-
sects, &c.), which spoil their appearance. Then the fashion
of the day often prescribes forms which nature would never
or rarely wish to produce. We need only refer to the artificial
forms of French trees, to the globular, pyramidal, or columnar
forms of flowering shrubs in pots and in the open. To pro-
duce such forms, or to repair the damages to the crown of a
ti-ee, it is necessary to regulate its growth by taking away old
or adding new branches.

To increase the productiveness, too, it is necessary to adopt
special methods of treatment. Even when a tree is grown for
the sake of its wood, it does not, when left to itself, always
produce the form most desired. In growing trees for masts
or tall poles, for long boards or beams, we cannot entirely
rely upon growing them in closely-packed plantations. Any
gap in the plantation is taken advantage of by the trees to
produce strong branches, which must be removed by the
forester. If with the French method of arboriculture a fruit-
bearing crown is to be confined to a very limited space, the
natural production of branches must be reduced, and the
134

THE TREATMENT OF THE SHOOT 135

development of the normally quiescent buds close to the
main axis must be stimulated. Similarly, if time is to be
saved in the cultivation of plants, they are not allowed to
retaia the long shoots, but these are removed so as to cause
the earlier development of flower-producing buds.

A tree or shrub left to itself often produces a crown of a
disadvantageous form, by producing too many branches, so
that the crown is too thick at the centre and consequently
barren. Here artificial means must restore the air and light,
and therefore also productiveness, to the centre. Then, again,
faults take place in the growth by the production of watery
shoots, which reduce productiveness of the tree (or entirely stop
it for a season) if rational measures are not adopted to cure
this defect.

Lastly, we have already pointed out, in dealing with the root,
that plants which are to be transported must be repeatedly
transplanted. Every transplantation ensures a more or less
complete root-pruning. In doing this, we must remember the
principle that a plant deprived of some of its roots must have
its shoots correspondingly diminished. Many more examples
might be brought forward to point out that the horticulturist
is very rarely able to let the development of the shoots take its
own course. The knife must be almost always brought into
requisition.

§ 26. What is the least injurious form of a cut?

This question can be answered very briefly. The youngest
possible shoots should be pruned. This rule is based upon the
fact that the youngest shoots heal most rapidly and most
completely. The danger of a wound lies in the effects which
may follow upon it, and of these the setting in of decay is the
most to be feared. This decay can take place without the
interference of fungi, though generally these organisms play
a part in the decay which sets in. The germination of fungal
spores on the cut surface is brought about and favoured if the
cut surface retains the necessary moisture for some time.

We must therefore prevent the accumulation of moisture as
much as possible. This consideration settles at once the best

136 THE PHYSIOLOGY OF PLANTS

form of cut and the proper time for performing it. The form
of cut to be chosen must be such that the atmospheric moisture
and the water which exudes from the branch are prevented
from collecting on the cut. If we cut off vertical branches
horizontally, the wound will be a horizontal one. But every
cut branch forms, owing to the greater contraction of the pith,
a cup-shaped depression at its cut end, and this depression
retaining the rain-water, dew, or the sap due to the bleeding
of the branch for a considerable time, will enable the germi-
nation of fungal spores to take place, with the result that the
fungal hyphcB will destroy the tissues. A wound with an oblique
position will therefore be less harmful, and all transverse cuts
should be diagonal.

The precautions we have to take to avoid the collecting of
moisture will also indicate that we should not choose as a
season for pruning the time when plants " bleed," that is, force
out much water from their tissues. Although the wounds will
be found to be provided with means of preventing the giving off
of water, we often see old wounds giving off a considerable amount
of water. The means of protection are of two kinds. In the
parenchymatous tissue a layer of separation is formed extending
from the cork right across the cortex. The vessels become closed
up by plugs of a peculiarly resistant gum-like mass, or by the
bulging out into the vessels of the surrounding parenchymatous
cells. These vascular protrusions are forced through the pits
until they meet each other, and so close the vessel {thylloses).
All these protective formations, however, arise more rapidly in
the younger tissues and in those which contain a large supply
of plastic substance. If we cut off a branch in a green
herbaceous condition, we notice that the shoot dies down as
far as the next bud, and here a layer of cork will be formed
separating the dead from the living tissues. In stronger
shoots, in which the wood cylinder is better developed,
a swelling will often be noticed immediately below the dead
tissues. Such a swelling occurs in many kinds of fruits, and
is no disturbing sign ; it is formed by an enormous increase of
parenchymatous tissue in proximity to the hard bast fibres,
which often die away for a considerable distance in the living
tissue and become enveloped by a layer of cork.

THE TREATMENT OE THE SHOOT 137

A cut is least dangerous when inflicted upon a leafy shoot,
first of all, because, owing to the transpiration of the leaves,
the shoot will not contain sufficient water to cause an accumu-
lation of it on the cut surface ; and secondly, because the leaves
will at once supply sufficient food material for the healing of
the wound. If ripened wood has to be cut, the early autumn
or late winter should be selected for the purpose. Early in
the autumn the branches have still sufficient activity to form
the closing layers, and thus to protect the shoots from the wet
of the autumn and the cold of the winter. If the pruning
takes place late in the winter, no injury from frost need be
feared for the ripened wood, and as the tree will soon enter
upon new activity, the healing of the wound will take place
at an early period.

Great care must be taken in the pruning of trees bearing
stone-fruits (cherries, plums, &c.), and in the South also in the
case of orange-trees ; in fact, in all cases where the plants are
liable to the exudation of gum. If the cuts are inflicted at
a time when a considerable formation of new tissues is going
on, gum is sure to flow sooner or later from near the cut. A
very easy way of demonstrating this fact is to make every
month during the winter an incision into a cherry or plum-
tree, reaching down to the wood. It will be found in the
summer that, from all incisions which were made in spring,
gum will be exuding. If one is forced in the case of other
trees to prune during the bleeding season, cloudy and cool
days should be chosen, as the pressure of the sap is least
on such days. Prune as early as possible trees producing
strong shoots, which, as a rule, are rich in water, and therefore
would suffer most from an excessive loss of sap. The conse-
quences of such a loss would be a retarded period of flowering,
irregular ripening of fruits, and an insufficient ripening of the
wood.

§ 27. How does summer-pruning differ in its effects from
winter-pruning ?

If the term winter-pruning is given to any removal of
shoots during the resting period of a woody plant, we may say

138 THE PHYSIOLOGY OF PLANTS

genei-ally that winter-pi'uning is strengthening, while summer-
pruning is weakening.

If any portion of the shoot system is taken away after it
has passed through one summer, the structure and activity of
the root system — that is, its power of absorption and of forcing
up water — is such that it can nourish all the branches. At
the beginning of the next period of activity, by cutting away
some branches the water- consuming area is diminished. The
same amount of pressure has therefore a reduced field of
action, and consequently the effect on the remaining branches
must be increased.

By pruning in the summer we remove soft shoots with
only recently developed leaves. The latter have yet their chief
work to perform. For at the commencement they are de-
veloped at the cost of the reserve material which is stored up
in the branch ; then comes a period at which the young leaf
requires all the substance it assimilates from without for its
own growth, and only after its full development does it begin
to work for the benefit of the branch. If, therefore, a soft
shoot is taken away, the older portions of the branch are robbed
of the materials which were used in the unfolding of the leaves,
without receiving anything in return from the leaves they have
developed. This causes, therefore, a loss to the general economy
of the plant ; but, with the increased productiveness of our
cultivated plants, such a slight weakening may be overlooked,
if any other special advantage is gained.

The taking away of tips of young shoots, which is called
pinching", is admissible in trees which are unproductive on
account of the excessive development of shoots. If we take
away the tip of a long shoot, we deprive it of the part which
makes the greatest demand upon its nutritive powers ; conse-
quently the pressure is increased in the lateral buds, the lowest
of which is increased in size while the upper ones will soon
grow out to new shoots. The cells of the lower buds which
are thus enlarged will store up a larger quantity of reserve
material, and this must be present if flower-buds are to be
formed.

The greatest success will attend this process if the pinching
takes place just at the period when the buds have still suffi-

THE TREATMENT OF THE SHOOT 139

cient time to swell up and become stored with food material,
but when the supply of water begins to diminish, so that the
upper buds do not grow out into long laterals.

In spring, and during the production of the so-called second
shoots in June and July, the general upward current of water
presents a strong difficulty. Any " pinching back " at this
period calls forth almost immediately the expansion of the
uppermost buds of the remaining branch. The younger the axis
is, the more readily will the buds grow out into shoots if the
supply of water is increased ; for the cells have only shortly
passed out of the stage in which their power of reproduction
was greatest, and they easily reassume that condition. Thus
in many trees (Peach, Ash, Acacia), in years of exceptional
activity, the recently-formed buds of that year's shoots will
grow out {prolcptic shoots).

To prevent disappointment, we state emphatically, as the
practice is very common, that no fixed rule can be laid down
for the commencement of summer-pruning. Trees may even
be pinched to death. The favourable time for this operation
depends upon the climate, the soil, the variety, and even upon
the individual characteristics of the plant. The cultivator
must himself judge whether the shoots have reached such a
stage of maturity that an elongation of the uppermost buds
will not take place.

If the pruning in August has not resulted in a sufficient
swelling up of the buds, in districts which have a long autumn,
an October or autumn-pruning may be resorted to. In so
doing, the tree may be cut back still farther, and the water
supply be restricted to a smaller number of buds without fear
of causing any of them to elongate. But here, too, in each
individual case only can it be ascertained by experiment how
far a tree may be cut back so as to produce the desired effect,
namely, the production in the following year of a large number
of " short shoots." In such shoots the amount of food matter
assimilated by a leaf spreads itself over a much smaller stem
area, and the shoot is therefore richly provided with reserve
material, which is an essential condition for the formation of
flowering buds.

If the pruning has been too close (which is sometimes

I40 THE PHYSIOLOGY OF PLANTS

purposely done in varieties producing weak shoots), the pres-
sure of sap may be so considerable in the following spring
that all buds grow out into long woody shoots. In this case
the tree would be pruned fop the produetion of wood.

§ 28. What is the effect of the different methods of pruning ?

It is not the function of this book to discuss the methods
by which a tree is forced to assume a certain shape ; that is a
pure technicality of arboriculture. We must, however, touch
upon the consequences which the use of the pruning-knife
entail to the various parts of the tree. The method of
obtaining trees of definite shapes depends upon a succession
of prunings carried out with a very definite aim. We need
not, however, conceal our opinion that this endeavour to force
a tree to assume a shape which is contrary to the natural
habit of branching has no horticultural value, and only results
in the production of a curiosity. This method of culture
condemns itself, because the best-formed trees usually produce
the least fruit. The advantages which the so-called French
method of culture is supposed to have, namely, the faculty of
making most use of the smallest available space and the
production of especially good fruits, are reached with greater
certainty if the tree is allowed more liberty to develop its
natural shape. This is the case in the cultivation of dwarf
varieties. The purpose of these dwarf trees is also to produce
the most beautiful fruits in a small space. By cultivating
a dwarfed stem, the crown of the tree is brought low enough
for a more careful inspection and treatment, and in both cases
the production of large fruits is attained by the reduction of
the total number.

One of the chief methods of treatment of fruit-trees, espe-
cially of pears and apples, is to cut a branch off 4 or 5 inches
above the bud of which the development is desired. Suppos-
ing this bud is to replace the main shoot as leader, all the
buds above it are removed, and the eyeless upper portion of
the shoot is made use of for fastening the tree to the wall.
Probably in this case the food material contained in this

THE TREATMENT OF THE SHOOT 141

upper portion sinks down into the lower portion when the
period of vegetation commences, and can be made use of by
the elougating bud. After this has grown out, the useless
portion of the shoot above it is removed.

If a branch is to be completely removed, however, we must
decide whether the basal dilatation at its point of insertion is
to be removed as well. This basal dilatation enlarges as the
shoot grows older, for that portion which is inserted in the
main stem also increases by secondary thickeniug. The shoot
which has sprung from a lateral bud was clothed at the com-
mencement of its base by bud-scales, which soon fell away on
the elongation of the shoot. But these bud-scales subtended
very small and rudimentary buds, which generally remain dor-
mant (dormant buds) ; now as the base of the branch enlarges,
they become covered in by the cortex of the basal dilatation,
and, though still present, are invisible from the outside. If,
however, the branch is cut away above this enlargement, the
two strongest of the dormant buds generally grow out and form
two weakly shoots.

It is therefore only advisable to remove the shoot in this
way if a too luxuriant branch is to be replaced by a weaker
one. In other cases, it is best to remove this basal dilatation
of the shoot as well, and thus get rid of the dormant buds.

This, however, is scarcely possible in the case of older
branches, and we therefore often see in such trees as poplars,
limes, and willows, where a branch has been sawn off, numerous
small shoots making their appearance. If they are cut away,
their basal shoots grow out until the number of shoots may
become so excessive that they are killed by mutual pressure.

In the case of cherries and other stone-fruited trees, a dif-
ferent treatment is needed from that applied to pears or apples.
In the case of the first-mentioned trees, the branches which
have once borne flowers will not do so again, but remain bare.
The formation of flowering buds, therefore, progresses gradually
towards the ends of the branches, while in pears and apples
fruiting spurs may bear flowers year after year, the spurs
increasing continually in thickness.

In some cases, indeed, this thickening of the fruiting spurs
becomes excessive ; and the branch differs anatomically, too,

142 THE PHYSIOLOGY OF PLANTS

from the normal condition. The wood cylinder is only slightly
developed, bat the cortex and pith are excessively enlarged,
the cortex of the stem being often continuous with the massive
cortex of the fruit-stalk. This form of branch is very sensitive
to frosts. If it becomes necessary, in the case of such trees, to
produce some long shoots, it is advisable to prune away the
deformed spurs to their base, and to stimulate the dormant
buds to farther development.

In the case of cherries, to prevent long sterile branches, it is
necessary to constantly renew the production of long shoots.
This can be done by pruning back the shoot which has just
borne fruit to about four basal buds. Two of these will in
general grow out next year, and the stronger of the two will
be retained as the fruiting branch. The same process will be
repeated with this branch.

The effect of pruning, therefore, is to stimulate a number of
buds to grow out which would otherwise have remained dor-
mant. The uppermost buds of each shoot are the most ready
to develop and produce the strongest lateral shoots; towards
the base of the shoot the buds become more and more difficult
to stimulate.

§ 29. How may pruning be used to regulate the natural
development of the tree ?

Among the large number of varieties of our fruit-trees and
decorative plants, there are many in which the branches are
not produced in a way that satisfies our wishes. Thus there
exist varieties in which the branches are always bent, others
in which the woody shoots are short and few in number, but
which very readily produce flowering buds ; others, on the
contrary, are inclined to run to wood, but only produce few
fruit-buds.

Similar tendencies may be produced by unfavourable situa-
tions. Pruning should in such cases be used to overcome such
defects. All our efforts then will be in the direction either
of producing more leafy shoots or in promoting the formation
of fruiting buds. In the first case we prune fop wood, in the
latter case we prune for fruit.

THE TREATMENT 0¥ THE SHOOT 143

If we find that a tree has the tendency to produce every
year a number of shoots with an excessive number of fruiting
buds, so that the tree is in danger of exhausting itself and not
producing a proper crown, the tree must be pruned for wood.
We know that a rich and continuous supply of water increases
the vegetative activity of a shoot, the leaves become larger
and the internodes longer. The supply of water to the various
buds can, however, be relatively increased by decreasing the
number of buds. The number of eyes will therefore have to
be diminished by pruning the branches, and the stronger the
shoots are to be next year, the shorter the branches must be
cut. Pruning for wood is therefore merely a cutting away of
branches, leaving only a short piece of each shoot.

Pruning for fruit, on the other hand, becomes necessary
when a tree produces long unbranching shoots of a vigorous
character, but which show no flowering buds. This production
of woody shoots can only be stopped by cutting away the tips
of the branches, leaving the greater portion, or at least the
half, of the shoot. The uppermost buds of the remainder of
the shoot soon grow out, but are pinched off while still young,
and by this means the lower buds are encouraged to swell and
to form flower-bearing buds. If this method of procedure
does not result in a sufiicient production of fruiting buds,
bending, twisting, or ringing the shoots may be tried. We
shall refer to these special means a little farther on.

The same variety will have to be treated differently in
different localities and also in different climates. Colder
climates have the same effect as damp ones of causing a
production of wood, and necessitate therefore a similar treat-
ment. Warm, dry, and sunny localities promote the formation
of short shoots, rich in food material, and are therefore favour-
able to the production of flowering buds ; the trees in such
localities must therefore be well pruned in, so as to stimulate
the growth of strong branches, which are necessary for the full
development of the crown.

Pruning should also be used to correct the faults of over-
manuring. If, for instance, a tree has been stimulated by an
excessive amount of nitrates or potash in the manure to
produce too large a number of long shoots, such over-produc-

144 THE PHYSIOLOGY OF PLANTS

tion of wood can be stayed by close pruBing. By doing so
the weak buds at the base of the shoot are caused to swell and
to develop more strongly than would normally be the case.
By this means strong shoots will later on be formed in the
neighbourhood of the main branches.

If the general constitution of the tree is weak, it is essential
to create as large an amount as possible of assimilating leaf
surface. In such a case, therefore, it is advisable to prune the
shoots only slightly, giving at the same time suflScient water to
the roots.

Varieties which flower sparingly should not be pruned in
closely, but this may take place in the case of plants which
produce a large number of flowering buds.

The same close pruning is applicable to varieties in which
the growth of the apical buds predominates and which produce
very few lateral branches, so that the crown when left to itself
consists of long unbranching shoots. If the varieties branch
freely they should be only slightly pruned.

The character of any variety is, however, greatly changed by
the stock on which it may be grafted. The so-called dwarf-
stocks of our pears and apples (paradise-stock and quince) cause
the production of shorter shoots, but, on the other hand, a greater
and earlier production of flowers, whereas the wild (crab) apple
and pear stocks favour the formation of strong woody shoots.
Grafts on the latter must therefore only be slightly pruned,
whereas the others may be closely pruned.

The age of the tree, too, requires to be taken into consi-
deration. All young trees tend chiefly to develop vegetative
shoots, i.e., to run to wood, and should therefore be pruned
less closely than old specimens of the same variety and in
similar situations.

In the case of trees grown against a wall, especially if the
artificial French shapes are the object of attainment, the pruner
must pay attention to the position of the branch. In the
"cordon" type the strong branches are kept horizontal. But the
more horizontal a branch, the smaller the supply of water to
the buds ; the nearer the branches approach the vertical, the
more copious is the supply of water and the larger the leaves and
interned es. The shoots coming off from the horizontal branches,

THE TREATMENT OF THE 8H0()T 145

therefore, must be more closely pruned than those of the vertical
branches. An exception to this rule must be made in the
case of the watery suckers which are produced in the case of
" cordons " at the bend of the leading axis. These soft shoots
are caused by the unnatural horizontal position which is forced
upon the main stem. On account of the bending, a certain
number of buds at the bend are more plentifully supplied with
water. When the branches are left in their natural position,
the buds near the apex develop most strongly ; but when the
branches are bent to the horizontal, the passage of water is
retarded, and the supply, therefoi-e, to the buds immediately at
the bend is abnormally great. This causes in the first place a
greater turgidity and enlargement of the cells of these buds, and
also supplies them more liberally with nutritive salts. When
once such a bud has started growing, its leaves will constitute
a further centre of attraction for the raw sap, and being in such
a favourable position, will elongate very rapidly, becoming a
strong and vigorous shoot. This, however, entails a loss to the
mother-stem and literally robs it of its nutriment. Thus the
new vertical shoots may often cause the death of the original
horizontal axis.

If by any inattention such a " robber " has been allowed
to develop, its tip should be pinched, but it should not be
entirely removed at the outset. Only those which most disturb
the crown should be pruned to about a third of their length,
and their laterals should be nipped off as soon as they develop.
No general rules applicable to all cases can be given ; this only
can be laid down as a guiding principle, that the development
of such branches is due to the fact that the amount of water
taken in by the roots is not in proper proportion to the
existing working surface, which is for the time too small.
If, therefore, the vertical shoot is suddenly taken away and
the surface again reduced, this procedure will only result in
the production of another shoot of the same nature. They
should, therefore, if possible, be used for the normal development
of the crown, or removed slowly after having encouraged new
and more suitable shoots.

146 THE PHYSIOLOGY OF PLANTS

§ 30. When is pruning harmful ?

It is contrary to good arboriculture to prune a tree too
much, so that it is covered with cuts. In our nurseries and
gardens trees are often too closely pruned, a procedure which
often diminishes productiveness and causes many diseases, not
infrequently indeed kills our pyramid trees.

The excessive pruning is caused as often by too frequent
application of the knife as by removing too great an amount
of wood.

We must give the tree an opportunity of developing a few
strong woody shoots, which will only be shortened after the
wood has ripened and the leaves have fallen, otherwise the tree
will become weakened. To remain in g-ood health and eon-
tinuously productive, the tree must have a sufficiently large
supply of leaves, and these must remain active as long as possible,
so as to store up a large amount of food material in the stem.
But if every strong shoot is pinched as soon as it exceeds
the desired length, and the lateral buds are thus caused to
grow out, and if they in their turn have their tips removed,
the tree will not have a sufficient supply of well-developed
leaves, and a large portion of the food material formed by them
will be used up by the developing lateral branches. It must
not be forgotten that at the commencement every young- shoot
draws like a parasite upon the food matter of the older branch ;
this applies as much to the consumption of water as to the
stored-up food material. The need of a strong root or shoot
system for a sufficient supply of foliage is often seen in the
case of cordon apple-trees. An exceptionally vigorous leafy
shoot will suddenly make its appearance, and if removed, will
cause the production of a number of vertically growing
suckers.

Pruning " to excess," however, may also be caused by the
untimely removal of older branches, as was formerly often the
case in the raising of standard fruit-trees. By cutting away
the lateral branches at a very early period, the stem, it is true,
would grow very rapidly in height, but the annual rings were
only feebly formed, and the stem was often too slender to bear the

THE TREATMENT OF THE SHOOT 147

weight of the crown. The modern practice is to reduce the
leading shoot, and thus cause a stronger development of the
lateral branches ; these will be able during the course of the
summer to send a suflScient supply of nutritive matter into the
young stem, which will be able to use it for the development
of broad rings of wood.

The tree may furthermore be injured by removing too many
branches, so that the tree will not have sufficient centres for
the use of the sap raised by the roots. If the branches were
removed bit by bit, growth would not be interrupted. It is
the sudden removal of the branches which acts injuriously.
Too many of the readily elongating buds near the end of the
shoots have been cut away, and this may cause excrescences
of the cortex and cambium, which may become centres of future
disease. This is the case in a disease to which the gooseberry
is very liable. If, in the winter grafting of Ribes Grossularia
on Bihes auremii, the stock is too much cleared of its lateral
branches, the cortex below the insertion of the scion will often
form swellings, due to tube-like elongation of its cells, which
may ultimately rupture the bark and prevent the graft from
growing on. Similar swellings, which do not, however, rupture
the bark, are formed on the spurs of soft varieties of pear ;
their formation is preceded here too by a removal of all the
buds of the spur.

If the cambium layer suffers from an excessive supply of
water owing to the removal of the lateral branches, the newly-
formed annual ring may become interrupted by transverse
bands of short soft parenchymatous cells in place of the
long and hard prosenchyma, and these bands may extend a
good way from the surface exposed by the cut. These bands
of soft wood parenchyma loosen the annual ring, and may be a
great source of danger if any signs of decay show themselves
at the cut end. For this softer tissue is very liable to decay,
and easily attacked by fungal growths. Trees which are
apparently quite healthy will often show, when sawn across,
brown markings in the white wood, very often a complete
brown ring of decayed tissue. In splitting the stem longi-
tudinally, the raoi'e central axis will split away from the outer
hollow cylinder of wood just at this brown ring. Such stems

148 THE PHYSIOLOGY OF PLANTS

are of course useless for cutting into boards or planks. Lastly,
we come to a point which in practice is very rarely suffi-
ciently appreciated, namely, the sawing off of thick branches.
There are, of course, cases in which this procedure cannot be
avoided, as, for instance, in the removal of the crown for
purposes of grafting or after damage by wind or snow ; but
there are many occasions when this always somewhat dan-
gerous experiment might be, but is not avoided. This is
especially the case in putting up telegraph wires along wooded
roads and lanes and in thinning woods.

The damages that ensue are of two kinds. First of all,
extensive wounds are made, which take years before they are
completely covered up, and during that time they give access
to wood-destroying Fungi. Secondly, the equilibrium of the
crown is disturbed in such a way that a sudden production of
numerous soft shoots takes place inside the crown, which
becomes choked and loses many of the horizontal fruit-bear-
ing branches.

Some, who attach little importance to the removal of large
branches, argue that in woods such branches are often re-
moved to obtain straight trunks, and often decay away natu-
rally on account of the want of light, and in neither case does
the tree seem to suffer.

But in both cases the argument falls to the ground. For
in the practice of forestry the removal of branches for the
benefit of the main stem often induces the decay of the tree,
and in the case of branches breaking off for want of light, the
branch is dead before separation takes place, and the cells and
vessels are sealed up with plugs of gum, resin, and by thylloses,
and are rendered unfit for fungal growths.

Of course the trees vary very much with regard to their
powers of resisting any such injury. For while in Conifers
the wound is covered up with resin and the stump of the
branch becomes soaked with resinous oils, which defy fungal
attacks, this is not the case with most Dicotyledons. In our
fruit-trees, a wound of two or three inches in diameter, which
becomes covered in in from six to eight years, always causes
brown spots of injured tissues in the stem. With larger
wounds this is still more the case. Decay will then almost

THE TREATMENT OK THE SHOOT 149

invariably set in in the branch, and subsequently in the stem.
Even artificial covering of the wound is not always sufficient
to prevent this, as the wood which is laid bare will produce
large cracks when it contracts in seasoning, and so offer a
point of attack for Fungi.

^ 31. In what way can the natural process of healing be
accelerated ?

First of all, we must see how the natural process of healing-
is effected. The age of the injured organ is of the foremost
importance, as the process of healing takes place more rapidly
the younger the injured tissues are. If a root, for instance,
is deprived only of its meristematic apex, the entire wounded
surface will begin to form new cells, and the root-tip will
be entirely renovated. If, however, the axis is cut below the
apex, where the tissues have already become differentiated and
the woody cylinder has made its appearance, we notice not only
a retarding of the process of healing, but we observe also that
all the tissues are not able to take part in the closing of
the wound. The outermost, that is, the oldest regions of the
cortex, the woody cylinder, and the pith too, are not able to
enter again into cell division.

In a branch of one year's growth the formation of cells which
will gradually cover in the wound is restricted to the cambium
layer. It is not, it is true, only the cambium layer of the
anatomists, i.e., the single layer of meristematic cells, but also
the cells adjoining it on either side, which I'opresent the
youngest cells of the bast and of the wood ; still it is only a
comparatively nai-row ring of cells just outside the wood which
possesses the power of forming the healing tissues. In some
plants (the lime), under favourable circumstances the pith too
of the first year's twigs can take part with its outermost
layers in the formation of new cells, but usually this faculty is
not of much consequence. The tissue formed by the cambium
and spreading over the cut surface consists at first of delicate
meristematic cells (callus) ; later on, the cells become differenti-
ated in such a way that those which are placed externally form

15°

THE PHYSIOLOGY OF PLANTS

bast, while the more internal ones which cover the wood form
short lignified wood cells. Thus the meristematic callus forms
gradually the closing layers of wood which cover in the cut
surface, and which may be looked upon as a continuation of the
tissues which formed the branch. The chief feature which we
have to notice is that the cambium of the branch is continuous
with the cambium which forms the closing layers. Upon the
activity of this cambium depends the annual advance and
increase in thickness of the layers of wood which ultimately
cover the cut like a cap (Figs. 23, 24, 2$).

The function of the callus and closing layers will be readily
understood from the adjoining figures. Fig. 23 shows the
formation of callus on the surface of a woody tissue which has
been laid bare ; this will take place under favourable conditions

in all stems in which
the bark and bast
has been peeled off
during the period of
special activity of the
cambium. In this
case the youngest
layers of cells on the
surface of the wood
will enlarge to form
rows of delicate cells
(c) which are united
into a continuous layer. This tissue is formed not only
by the cells (m) which form the continuation of the medullary
rays, but also by those (/) which adjoin the wood fibres. In
comparison with the fully developed elements of the wood (h)
the cells of the newly-formed tissue are exceedingly thin walled
and full of protoplasm ; but they chiefly differ from the former
by constantly growing in length and cutting off" cells at
their apex. Later on this active growth in length ceases,
and the last-formed cells and those immediately below them
become divided by closely set parallel walls, which become
corky, and thus form a layer of corky cells (k) which protect
the delicate tissue. This concludes the growth of this tissue
as callus. Then witbin the mass (c) a new meristematic

Fig. 23.
The Formation of Callus on the Exposed Wood
Surface of Tilia europcea.
(Explanation in the letterpress.)

THK TREATMENT OF THE SHOOT

151

layer makes its appeai-ance ; this is a layei* of constantly
dividing cells which runs parallel to the old wood {It), and this
layer behaves like a true cambium layer, forming new elements
of wood towards h, and new bast cells towards m, I, k. In
this way the surface of the stem which had been laid bare
covers itself afresh with a new bast.

Fig. 24 represents another method of callus formation,
which we shall refer to again in speaking of grafting. Here
the callus is formed entirely from the cells of the pith. The
figure represents one side of the innermost angle of a wedge-
shaped cut which has been made into the wild stock for the
purpose of inserting

the scion. The cut ^f//

has reached down to
the pith (m), and has
cut the woody cylinder
(h) along the line scJi.
The opposite side of
the wedge-shaped in-
cision has been left
out of the figure. In
consequence of this
cut a growth of rather
rare occurrence can be
noticed — namely, the
growing out of the cells
mJc at the juncture of
wood and pith. They

have not only grown out, but have divided to form a con-
siderable amount of callus, reaching from c to c. This
callus mass grows into the crack which exists between the
stock and the scion, and aids the growing together of
the two, which is, however, chiefly caused by a similar but
more abundant growth of callus from the cambium layer.
Crystals of oxalate of lime (0) are seen at intervals in the mass
of callus.

In Fig. 2 5 we see the method of healing and the formation
of callus from the bast and cortex. The cortex and bast
were cut through in the direction of the arrow, the cut

Fig. 24.— Formation of Callus in the Pith of a
One-Year-Old Twig op TUia europcea.

152

THE PHYSIOLOGY OF PLANTS

reaching down to the old wood (ah). The reader must re-
member that he is looking at a transverse section of the
branch. At the time the injury took place, the cambium layer
{camh) of the branch was in contact with the old wood (all) ; on
the outside of the cambium was the old bast (r). After the
cut had been made, those cells of the bast and cortex which
were still capable of division, as well as the cambium cells,
grew out into the gap and formed the callus layers (c to c).

K A CLT IN THE OUTER LAYERS OF A BRANCH OF Tilia europcea.

Very soon the callus formed a protective cork layer {k and k),
and the latter was continued backwards to the cork layer (kk),
which had been produced by the cortex cells immediately be-
low the original cork, and which thus separated the injured,
and consequently dying, bast fibres from the living tissues.
In this way a continuous protective layer was formed round
the injured tissues, and the cambium layer (camb) started
forming new wood on the inside. This new wood (7ih) was

THE TREATMENT OF THE SHOOT 153

of a looser nature, and contained more and larger vessels
{(j) than the old wood («./<-), and the latter became still more
separated from the old bast {r) by the formation of new bast
{nr) on the outside of the cambium. At the point where the
cut was inflicted, the cambium (camh) bends down into the
original callus and forms there too new wood and bast, which
form the closing lips projecting from the healing tissues.

If we imagine the branch to be cut through transversely, we
find this healing tissue starting on all sides as a ringlike out-
growth from the cambial and cortical cells, and gradually
approaching the centre of the cut surface, until the margins
of this circular crest meet in the centre. Here the mai^gins
fuse and form a complete cap, covering the stump of the
branch.

This process of covering may be completely successful, but
it must be remembered that no fusion ever takes place between
the old wood and the cap which covers it. Mature and com-
pletely thickened wood cells can never give rise to new cells,
nor ever fuse with new tissues. A protective growth of cells
may cover in a cut end of old wood as completely and as close
as you please ; but it can never nourish the wood nor prevent
it from decaying. This is best seen in the case of stumps on
oak-trees which have thus become covered up. The healing
tissue may here have formed a complete cap, but if the latter
be knocked off, the wood of the former branch will be found to
be discoloured and decayed.

The quicker a wound is covered in, the smaller is the
danger of decay to the woody tissues.

The rapidity of growth of the healing tissues depends upon
the proportion of the cambium layer to the area which has to
be covered. Consequently the conditions become more un-
favourable the larger the cross-section of the branch.

Furthermore, of two equally large wounds, the one in which
the cambial activity is greater will be the first to heal com-
pletely. The rapidity of cell division, however, depends upon
the amount of nourishment which the cambium receives, and
this again is dependent upon the position and form of the
wound.

The position of the wound is of importance, inasmuch as

154 THE PHYSIOLOGY OF PLANTS

different regions of the stem or branch are nourished in
different degrees. If above the wound there are plenty of
leafy shoots which are actively producing plastic material, the
descending food material will enable a rapid covering in of the
wound to take place.

The shape of the wound inflicted is also of some importance.
If we examine, for instance, the place where a piece of wood
has been cut out of a stem, we notice that the healing
tissue is more profusely produced at the upper end of the
injury. This is due to the fact that the plastic substances
descend from the leafy portion of the tree, and the upper edge
of the wound arrests this downward current and causes there
an accumulation of plastic material. If the wound is pro-
portionally very broad, a large amount of this material will
accumulate, and the healing will be more rapid than if the
greatest length of the wound were parallel to the long axis of
the stem.

With regard to the rapidity of healing, it is interesting to
notice the difference between a transverse and longitudinal cut.
If a branch is cut across transversely, there remains of course
no leafy tissue above it, and the cambium ring must draw all
the material it needs for callus formation from the immediate
vicinity. But plastic material does not travel upwards to any
great distance, and hence it will be observed that comparatively
small transverse wounds will not heal, or only do so very slowly,
if they are far removed from a lateral bud. If you desire to
cut a branch transversely, it is desirable to do so immediately
above a bud, because in this region there is a larger store of
food material, and also when the bud grows out into a shoot,
it will send down more food material which will become avail-
able for the healing tissues.

From these considerations we may deduce the general rules,
that wounds are covered up more readily, the less numerous
they are on any axis, the smaller their area, the more they
depart from the form of a transverse cut, and the greater the
amount of active leaves which remain above them.

Any artificial aid in the process of healing must therefore
be based on these considerations. Hence we generally prune
a branch by a long oblique cut close above a bud. It is

THE TREATMENT OF THE SHOOT

■55

also advisable to cut a branch with a sharp knife, and in the
case of thick branches, to pass over the sawn surface, at least
at the margin, with a knife. If the surface is very roughs so
that the injured and dying cells terminate at different heights,
the atmospheric moisture remains in contact with it for a
long time and in considerable amount, and offers facilities for
the spores of parasitic Fungi to lodge themselves on the surface
and to germinate there. In large wounds, which take many
years to cover, it is necessary to protect the wounded surface
from atmospheric influences. Of all the substances used for
this purpose, those should be preferred which produce the
same effect that is produced by natural means in the case of
Conifers. But as the resinous substances used for this purpose
are still too expensive for universal application, tar is most
generally used. On account of the cracks, however, which
gradually appear in an old wound which has remained open,
it is advisable to repeat the tarring".

§ 32. By what means can we increase the effect of pruning?

We do not always attain by pruning the end we have in
view, and this is especially the case when we seek to accelerate
or augment the productiveness of our trees. In consequence
of this failure, recourse is often had to operations the nature
of which we will now examine.

(a.) The Bending of Shoots.

The operation which gives least irritation to the economy of
the plant is the bending of shoots.

In most cases where these artificial means are employed,
branches are bent from a more or less vertical towards a more
horizontal position. From our preceding considerations we
know the effect of this procedure. First of all, the nutritive
sap becomes unevenly distributed. In their natural position
the water supply is most favourable to the buds near the apex
of the shoot. If the shoot is bent, however, so that the apex

156 THE PHYSIOLOGY OF PLANTS

is at a lower level than the middle of the shoot, the buds at
the highest point of the arch formed by the shoot will be the
most favoured. If the whole shoot is laid horizontally, the
supply of water to the whole branch is diminished, as the bend
at the base of the shoot retards the current of water which is
directed towards the apex.

If you examine the under surface of a greatly bent shoot
at the bend, you will see that the outer tissues are compressed
into considerable wrinkles. This is explained by the fact that
the cortical tissues have had to be reduced within the bend to a
smaller area. These outer wrinkles are, however, only an external
manifestation of an internal damage which consists in the split-
ting away of cortex and bast from the internal wood. The space
thus formed between the wood and the bast is, however, not
apparent from the outside, and later on a microscopic investi-
gation does not reveal the existence of a split within the bast,
but shows a whitish hardened tissue. This is newly-formed,
short-celled, and parenchymatous wood, which is rich in starch.
Any considerable bending of a branch entails, therefore, at the
bend a loosening of the tissues in the lower side of the branch
and an increase of tension on the upper side, so that the cells
on this side are stretched and consequently become narrower.
By this lateral compression of the cells of the upper side the
conducting tissue, both for the plastic material descending from
the leaves and also for the rising sap, has its activity diminished.
The kinking of the tissues on the under side produces a similar
interruption of the flow of substances. Considerable bending
of a shoot always diminishes the flow of sap towards the apex,
and also the conduction of assimilated material towards the
base of the shoot. The latter accumulate, therefore, at the
upper portion of the bend ; the raw sap, on the other hand, is
stopped on the lower side.

Every bend acts, therefore, as a barrier to the ascending and
descending currents. Above the bend the plastic material
accumulates and causes a more ready development of the buds
at that point into flower-buds. Below the barrier the buds are
more liberally supplied with water, and develop, therefore, into
strong leafy shoots.

The effect of the bending depends chiefly on the mechanical

THE TREATMENT OF THE 8H001'

'57

change which the tissues undergo at the bend, and must, there-
fore, show itself also when branches which naturally grow in a
downward direction are bent upwards.

(b.) TIlc Ticistiny of Shoots.

The twisting of shoots acts in the same way, but much more
energetically. In this case the shoot is twisted half-way round
during its period of growth and while still in its normal position.
By so doing the woody portion in which the twisting takes
place is loosened, and probably split up in the interior. After-
wards the shoot is also bent permanently downwards at this
point, so that the shoot forms a loop with the tip of the
shoot pointiug downwards. At the bend the under surface of
the twig comes to lie above, and the wood is split up into a
number of spirally twisted loops.

The region at which the twisting takes place forms a con-
siderable swelling, in which, as is revealed by the microscope, a
formation of parenchymatous tissue has filled up the splits. The
cambium, too, has gradually re-formed into a continuous ring,
producing new wave-like masses of wood round the broken
tissues.

The breaking up of the woody tissues and the twisting
the more delicate bast necessitates of course the loss of the
upper portion of the branch, and all that is effected is a dimi-
nution of the upward passage of water and a supplying of the
buds immediately above the bend with the assimilated mate-
rial which continues to be formed in the upper leaves of the
shoot.

In layering quinces, the shoots are often twisted about their
long axis at the point at which the roots are to be formed.
The plastic material passing down from the leaves accumulates
at the twist, and can be used for the production of adventitious
roots.

In the Caucasus it is usual to twist the fruit-stalks of the
ripe grapes, so as to produce an especially good soi't of wine.
The explanation of this procedure lies in the fact that the
supply of water to the grape is diminished, while the evapora-
tion from the berries remains the same ; consequently, the sugar-

158 THE PHYSIOLOGY OF PLANTS

containing sap within the berries becomes more concen-
trated, i.e., more sweet. The process of respiration, during
which a portion of the organic acids is oxidised to carbonic
acid, is not interfered with by twisting the stalks.

The greatest retardation of currents is produced by breaking
the branches. If a shoot which has already formed its secon-
dary wood is broken over the blade of a knife, a splintered
wound is formed. The tip of the shoot is connected with the
base by a certain amount of bast and cortex and a very small
amount of wood. It dries up more rapidly the more woody
the region in which the break occurred. If the shoot is soft,
the wood is not splintered, and the drooping end of the shoot
very soon raises its tip and begins to bend upwards, but grows
very slowly indeed.

(c.) Notching.

The fruit-grower is often compelled, so as to obtain trees of
a regular shape, to force a certain bud to develop without being
able to alter the position or form of branch to which it belongs.
In such cases, he has to resort to notching, i.e., to remove a
narrow sickle-shaped mass of cortex and young wood on one
side of the branch immediately above the bud. By so doing,
the upward current of water on this side of the branch is
diminished above the bud, while the latter, being more abun-
dantly supplied, will grow out very rapidly. If the notch is
made below the bud, the vascular bundles which supply the
leaf are usually severed, as they leave the woody cylinder of
the stem a good way below the leaf and run in the cortex for
a considerable way. The notch below the leaf stops the plastic
materials from passing downwards, and they accumulate chiefly
in the form of starch in all the parenchymatous cells in the
neighbourhood of the bud. This storage of food material is
considerable, as the woody cylinder is interrupted opposite the
bud, and a parenchymatous bridge reaches from the pith right
through the wood to the bud. The tissues surrounding the
bud thus become laden with food material at the same time
as the water supply diminishes. Such conditions are very
conducive to the formation of flowering buds. If a shoot is

THE TEEATMENT OF THE SHOOT 159

developed, it remains short, often indeed buncliy, and is very
prone to produce flowering buds.

If the notch heals up, it still remains of some importance
for the next year, as the wood first formed in that place is
large celled and will be able to store up starch. Still it is
advisable to repeat the notching if the desired result is not
obtained in the first year.

What has been said here about the bud is equally applicable
to a shoot or branch. The size of the notch must, of course,
depend on the size of the branch. In a one-year's shoot, the
breadth of the notch may vary from one-twentieth to one-quarter
of an inch, whereas in a strong branch it may rise to one-half
or two-thirds of an inch. The best season for performing this
operation is the late summer or beginning of autumn, as the
buds will have time to gain some advantage from the altered
conditions of nutrition.

(d.) Ringing.

Ringing is the term given to the removal of a narrow ring
of cortex and bast during the period of greatest cambial
activity, when the outer layers come away most easily from
the wood. The wound dries up very soon, as the outermost
layers of the young wood which is exposed (splint-wood) are
very readily dried. This reduces the upward conduction of
water to a slight extent, and in the same way the upward
passage through the cortical cells is stopped. That such a
passage from cell to cell does actually take place can be seen
when strips of cortical tissue are separated from the woody
cylinder, but are left attached to the branches at their upper
ends. If in such cases the transpiration from the wound is
prevented by surrounding the branch with a glass cylinder, it
will be found that the strips die off below, but remain turgid
near the upper end, and often form new wood cells on the
inside. Water, therefore, with the nutritive salts it contains,
moves along the cortex independently of the wood, and thus
supplements the action of the latter. But besides the reduc-
tion in the passage of water due to the removal of tissues, the

i6o THE PHYSIOLOGY OF PLANTS

upward current is further weakened by evaporation from the
exposed woody tissues.

In the cells above the excised ring of cortex the chief
factor in the elongation of the cells, the turgidity of the latter
is reduced. The elongation of the cells is lessened, and this
manifests itself in the reduction of the apical growth and the
diminished length of the internodes. The plastic material
which comes from the upper end of the branch accumulates
above the wound and causes an increase in the activity of the
cambium and a greater storage of reserve material. We
notice an increase in tbe diameter of the upper portion of the
branch compared with the portion below the ring-shaped cut.

A shortening of the internodes, however, and an increase
in the amount of plastic and of reserve material, is the first
condition necessary for the formation of fruiting buds ; thus
by ringing the branches become productive at an earlier
period. Branches which have been ringed assume the autumn
tints at an earlier period and ripen their fruit earlier. Such
a hastening of the ripening is especially desirable in damp
autumns in the case of late varieties of vines, as the grapes
would in such cases often not ripen at all. The success, how-
ever, of ringing these plants, especially in the case of American
varieties and hybrids, will only be assured if a considerable
amount of splint-wood is also removed. For the very wide
vessels of the wood will conduct very considerable quantities
of water to the growing points, so that the growth in length
will scarcely seem to be reduced. In the case of vines this
procedure is used against the dropping of the berries.

Ringing, however, must never be looked upon as one of the
regular operations in arboriculture ; it will always be an un-
natural treatment, which should only be used in exceptional
cases, as it generally entails the early death of the branch.
Only in those cases where the entire branch can be sacrificed
should one resort to ringing. For this operation does not
only affect the portion of the branch above the wound, which
easily breaks off", but the base of the shoot or branch is also
endangered. This latter portion receives no plastic material
from above, the cambium ring is therefore starved, and the buds,
owing to their deficient nutrition, do not form flowers and fruit.

THE TREATMENT OF THE SHOOT i6i

It is more advantageous to make only a very narrow circular
cut on very vigorous shoots, so that the wound may become
closed during one vegetative period. After the healing has
been effected the upper end of the shoot can continue its
growth in length, and the lower portion will again receive the
necessary material for the normal increase in thickness of the
wood and for the production of buds. This can be brought
about with certainty if the branch is constricted with a ring
of wire. The growth of wood at this point is in the first place
altered by the pressing together of the wood fibres, which be-
come laterally twisted. Very few true vessels are formed
between them. Afterwards, when the wire becomes enveloped
by the new growth, the cortex and bast are ruptured and a
soft wood is formed consisting of short, wide parenchymatous
cells. Of course, as long as the wire has not been grown over
the branch is liable to break off; but in other respects no
care need be taken for its further development. A deep black
discolouration of the wood will be noticed in the neighbour-
hood of the wire, but no attention need be paid to this appear-
ance. It is caused by the combination of tannic acid with the
iron and penetrates far into the wood. It is the layers of
splint-wood formed after the concrescence has taken place
which form the chief conducting tissue for the upper portion
of the branch. The sufficiency of the layers of splint- wood to
supply the entire leaf system is apparent from the vigorous
growth of hollow willows.

(c.) Peeling the Stems.

The most dangerous operation which is performed upon the
living tree, and which is a matter of life or death to the in-
dividual, is the process of peeling off the bark. In such cases
a large portion of the bark of a thick branch or stem, extend-
ing in length from six inches to three feet, is removed. It is
therefore a process of ringing on a large scale, and we only
adopt it as a last resource when all other methods of making
a tree productive have failed.

The danger of this operation is the uncertainty of the heal-

L

1 62 THE PHYSIOLOGY OF PLANTS

ing of the large surface which is exposed by the wound, but
it is by no means impossible.

The process of healing, however, is different from that of
all the other operations which we have so far considered, in all
of which the exposed tissues became covered in by an over-
growth from the margins of the wound. But in the case of a
wound the upper margin of which may sometimes be a yard
away from the lower one, it is evident that the margins will
never be able to coalesce. Yet this large area, if it does
heal, will heal more rapidly than that exposed by ringing,
in which operation the knife removes the youngest layers of
wood.

In peeling the stem, however, it is essential to leave the
new wood intact, for it is from this layer that the healing
tissues are formed (see Fig. 23). How very important it is
not to damage these layers of splint-wood we can see from
the appearance of those regions where the knife has made
longitudinal incisions to enable the bark to be lifted. The
operation, which can therefore only be executed when the
cambial activity is very great, and when a very delicate layer
of cells exists between the bast and the wood, is commenced
by making one or (in the case of thick stems) several longi-
tudinal incisions reaching down to the wood. These incisions
reach from an upper circular cut down to the point where the
lower cut is made. Where the circular and the longitudinal
cuts meet, we begin to lift the bark and then peel it off from
the underlying wood. If it comes off easily, the exposed wood
is smooth and moist.

If it is smooth and free from adhering portions of fibrous
bast, there is a prospect of the operation succeeding. If the
surface is not clean, the healing is incomplete, or may not take
place at all.

A few days will be sufficient to ascertain whether the tree
will live or die. If the latter is going to happen, the wound
turns grey and shows black lines. These are due to the de-
velopment of young fungal colonies, which attack and destroy
the woody tissues. If, however, the healing process is success-
fully established, the wound assumes a yellowish-green colour,
and in a few weeks we can ascertain by pressing with the

THE TREATMENT OF THE SHOOT 163

finger-nail upon the external tissues that a layer of soft tissue
has already been formed over the exposed wood.

This layer has been formed from the youngest cells of the
wood, which must have remained undamaged while the thin
cambium cells were ruptured, have thickened their outer walls
immediately after the operation, and have thus protected
themselves against the drying effects of the atmosphere. Some-
times this protective layer is strengthened from the beginning
by the collapse of the outermost layer of cells, while the cells
immediately beneath begin to multiply and form the new
tissue. We see the exposed medullary ray cells beginning to
divide ; the cells which were previously destined to be wood
fibres divide again and form short cells, and even in the young
vessels thylloses are formed and divide up to form new cells ;
in fact, all the cells constituting the new wood develop into a
homogeneous callus-like layer of cells, which at first looks like
a layer of cortex cells. But as these cells multiply a differen-
tiation begins to take place. The innermost layer alone remains
meristematic and forms a connection with the normal cambium
of the undamaged stem above and below the wound. A new
cambium layer is therefore formed over the entire exposed area,
and is continuous with the normal cambium. It forms the
new elements somewhat shorter at the commencement in
exactly the same way as the normal cambium, and thus a
new layer of bast begins to cover in the wood of the exposed
area.

This account of the process of healing explains two rules
which have to be observed. As the material for the formation
of the new layers of cells has to come from the reserve
material stored up in the wood, the most favourable time for
the operation is that period of cambial activity at which a large
amount of material is again becoming stored up in the wood,
and that is the period immediately preceding" the second period of
growth. In the spring the cambium, it is true, is very active,
and the bark can be easily removed, but the reserve substances
are already transported away, and are being used up in the
unfolding of the buds, and there is therefore too little material
for the healing of an extensive wound.

The second rule, to which also sufficient attention has not

1 64 THE PHYSIOLOGY OF PLANTS

been paid by gardeners, is the time of the day at which the
operation should be performed. It has been usual to prefer
the springtime for this operation, because it seemed desirable
to avoid the great heat of the summer. It was supposed that
the latter would dry up the wound too rapidly, and therefore
dull days were selected for this operation, or the wounds
artificially shaded or protected. But these assumptions are
wrong and have led to frequent failure. We have seen that
the exposed area protects itself by the thickening of the outer
cell-walls or by the collapse of these cells. Now both these
processes are accelerated by light and heat. Long-continued
moisture, on the other hand, will facilitate the attacks of parasitic
Fungi. It is therefore best to peel trees in hot summer weather
immediately before the second growth in August, and to do so
in midday heat.

§ 33. Why do we slit the bark ?

By slitting the bark we mean making longitudinal incisions,
in which the knife penetrates into the region of new wood,
but does not remove auy of the tissues. These incisions relieve
temporarily the pressure of the bark upon the wood in the
regions which are cut, and prevent the trees from becoming
hide-bound. The wounds thus inflicted heal very rapidly by
a closing in of the two margins of the cut, and in accomplish-
ing this a certain amount of the substances stored in the wood
is used up.

This operation may therefore be undertaken either when the
cortical pressure in stem or branch becomes so great as to
prevent the formation of the necessary amount of new wood,
or when more plastic material or sap is stored up in the stem
than can be used up.

The pressure of the bark can also become harmful when
the natural shedding of the bark of old trees is retarded. The
development of the bark in the cases of those trees which
present in their old age a rough bark is somewhat as follows :
— First of all, the stem in its young and succulent condition
is surrounded by a simple epidermis, consisting of parenchy-

THE TREATMENT OF THE SHOOT 165

matous cells. With the development of the woody cylinder a
strengthening of this outer protective layer takes place, by the
formation of several layers of tabular cork cells. When the
twig has grown to a branch, and has therefoi-e formed several
annual rings of wood, these layers split the cork mantle open
so as to be able to expand themselves. Close to these natural
splits new cork layers are formed, which project like arches
into the outermost layers of the cortex, and by cutting off
such layers cause their ultimate death. These dead pieces
are pushed outwards by the development of new bast and
cortex, and project from the surface of the stem. The repeti-
tion of this mode of procedure causes the formation of the
so-called bark scales. These scales, consisting as they do of
dead tissues, absorb water during moist periods, and give it
off again in dry weather, the cells contracting at the same
time. By this contraction the dead scales become gradually
separated from the living tissue beneath them. In this way,
the stems protect themselves naturally against an excessive
pressure of the bark, and the expansion or growth in thick-
ness of the stem is able to take place. The scales drop off
most readily when the weather is alternately wet and dry.
If, in some cases, the tree retains its bark for too long a
period, it becomes necessary to scrape the stems. The scraping
of the stems should therefore be looked upon as one of the
regular operations in arboriculture.

In those cases in which orchards have not been carefully
treated (especially in damp situations), we find trees from
which the bark scales have not been removed naturally or
artificially for several years, and by decomposing have formed
suitable places for an overgrowth of lichens or mosses. In
such cases the pressure of the bark is greater than where the
scales have been removed, and the growth in thickness of the
trees is retarded. We may here begin by scraping the stems
and then proceed to slit the bark along the whole length of
the stem.

Sometimes we only desire to strengthen certain portions of
stems or branches, where the growth in thickness has been re-
tarded by the pressure of other branches or of stakes. What
considerable tension is exerted upon the bark by the thickening

1 66 THE PHYSIOLOGY OF PLANTS

of the woody cylinder can be gathered by observing the longi-
tudinal slits some hours after they have been made. For while
the wound inflicted was at the commencement only as broad as
the back of the blade, a few hours after the operation the slits
will be twice or three times as broad. The margins of the
wound have become separated by the contraction of the strips
of bark which were previously stretched. Such a small slit
heals very rapidly, as the callus masses formed at the margins
very soon meet over the cut. The cambium layers of the
callus masses join, and soon complete the cambium ring which
was destroyed by the incision. This cambium ring does not,
however, at first form long wood fibres in the incisions and
in their vicinity, but gives rise to a large amount of wood
parenchyma, the cells of which are short and square. For
this reason the growth in thickness of the stem is more
rapid.

It is only when the wound has become covered in by a new
cork layer, and when the latter has become continuous with
the general cork mantle, that the pressure again becomes
greater on the cambium cells, and the new wood elements
assume an elongate form and become like the normal wood
cells. Under certain conditions, the slitting of the bark,
together with copious watering, may cause a renewed forma-
tion of spring wood, even after the autumn wood has begun to
be formed. If the wound heals very rapidly and a long dry
autumn sets in, then the ring marking the year's growth may
show two layers of spring and two layers of autumn wood,
and we have an appearance of two annual rings having arisen
in one year.

The bark is sometimes slit in order to cause a thickening
of a slender stem of a wild stock if the scion which has been
inserted is very thick. In this case the numerous incisions
which are made all round the stem commence in the scion
close above the juncture of scion and stock, and extend some
way down the latter ; thus a considerable increase in thickness
of the stock is obtained. In the case of grafts which have
been made close to the ground, and in which the scion is
strong and healthy but cannot develop jaroperly, owing to the
weakly condition of the wild stock, slitting the bark may be

THE TREATMENT OF THE SHOOT 167

resorted to for the purpose of stimulating the scion itself to
the formation of roots. In this case the numerous incisions
are made in the bark of the scion alone at its lowest end, and
the incisions are covered up tightly with stimulating soil.
Roots should then soon make their appearance, and thus the
independent nutrition of the scion would be commenced.
This procedure would be applicable in the case of dwarf trees,
such as pears grafted on quince stock, and apples on Doucin
or Paradise stock.

The slitting of the bark should be carried out soon before
the unfolding of the leaves, as the pressure of the bark is
greatest at this period, and the releasing of this pressure
will have the greatest effect upon the growth of the cambial
zone.

If, however, the bark is slit in order to get rid of an ex-
cessive supply of water and nutritive material, then no special
time can be recommended for the operation. It may be
resorted to if at, or shortly after, the commencement of the
vegetative period, a number of organs which would naturally
have used up the nutritive material have been removed. A
very well known instance is the disease of the gooseberry,
which we have discussed in a preceding paragraph. The
swellings on the stem of the wild stock below the point of
insertion of the scion are formed because the water which the
roots of the stock have been forcing into the stem during the
winter is not sufficiently used up. For it is, unfortunately,
often usual to cut away the lateral shoots too soon ; con-
sequently the bark is over-loaded with water, and the cells
elongate in the region where they are youngest and where the
pressure of sap is greatest, i.e., at the uppermost portion of the
wild stock. Thus a spongy swelling is formed, which bursts
and then dries up, causing also the drying up of the graft.
This unhealthy condition may be avoided by getting rid of
the superfluous water and of the plastic material which was
intended for the lateral shoots ; both these results can be
attained by slitting the bark.

Similar diseased conditions make their appearance, especially
in cherries and peaches, if too many superfluous eyes are
removed, or too many young shoots have their tips removed.

1 68 THE PHYSIOLOGY OF PLANTS

Swellings do not in this case make their appearance in the
bark, but the cambium forms a very spongy wood tissue which
is liable to gummosis. When this widespread disease is due
to the above causes, it can be successfully treated by slitting
the bark of the stem.

CHAPTER VIII

THE USE OF SHOOTS FOR PROPAGATING

§ 34. What is meant by layering, and of what use is it ?

A NUMBEK of shrubs have the faculty of readily producing
adventitious buds. Such buds are not formed like the regular
eyes in the young tissues at the tip of the stem, but take
their origin at a later period, and usually in consequence of
some external stimulus. The pith of these buds is not con-
tinuous with that of the shoot on which they are borne, but
joins on to those layers which were the formative tissue or
cambium in the year in which these buds took their origin.

Dormant buds, too, may grow out many years after the main
shoot is formed, but they were formed in the same year as
the shoot itself, and their pith is continuous with that of the
shoot.

Such adventitious buds may arise on roots and grow out
into shoots, and if separated with a portion of the root of the
parent plant, become separate individuals (Raspberry, Lilac,
some kinds of Plums, Cherry, Apple, Spirea, Rose, &c.).

On the other hand, many plants can produce adventitious
roots from leaf and stem structures if they receive a certain
stimulus. Gardeners use this faculty for purposes of propa-
gation by stimulating branches or shoots to produce adven-
titious roots, and then separating them from the parent plant.

Sometimes by merely increasing the supply of sap a branch
may be stimulated to produce adventitious roots. In other cases
more potent irritation (such as is produced by a wound) must
be set up to produce the desired effect, and it takes a longer
time until the result is attained. In such plants, in which
the formation of roots is difficult to bring about, the gardener
endeavours not to damage the shoots which are to be experi-

T70 THE PHYSIOLOGY OF PLANTS

merited with, in case the operation should not be successful,
and he tries, therefore, to produce a stimulus at various places
on the shoot, by enveloping them with damp soil, but does
not separate the shoot from the parent plant.

This process is termed layering. In certain cases this is
done (in the case of tall and rigid plants, especially if they are
valuable) by fixing some convenient receptacle to one of the
branches at a certain height and filling it with earth and
moss, which must be kept damp. After the branch has
become rooted, it is separated below the roots from the parent
plant. But all methods of layering" in which the branch remains
in its natural position are less reliable than those in which the
branch is artificially bent.

This may be taken as a guide for all cases of layering, and
the above-mentioned method of procedure is only to be re-
sorted to when regular layering cannot be employed. We
must remember, too, that the vigour of root formation in the
same species or variety of plant depends upon the amount
of assimilated food material stored up in that portion of the
stem which is being stimulated by moisture. From what has
been said in the chapter on pruning, we know that every
bend, notch, kink, or ring cut on the shoot hinders the flow
of food matter, and causes the plastic material to be stored up
in the portion of the shoot above the wound. The bending of
the branch therefore is in itself a favourable preparation for
layering.

The actual process of layering differs for different plants,
even when the shoots are all near the ground. If such shoots
are not produced in sufficient numbers, they must be called
forth by cutting off the mother-plant close above the soil, so
that strong lateral shoots may be developed. These can be
used already in their first year, when they are still herbaceous ;
they must then be bent outwards close to their insertion, and
the stump of the mother-plant and all the bases of the shoots
must be covered up with soil. If the branches have made
but few roots in their first year, it is advantageous to ring
the exposed places where they have been bent, and then to
cover them up again. The tips of the branches which have
now become woody are cut off, and the lateral shoots are

THE USE OF SHOOTS FOR PROPAGATING 171

nipped off, so as to let as much material as possible wander
downwards, and not to let it be spent on the formation of new
shoots.

The best time to cut down the parent plant is at the end
of winter, as there will be no more frosts to damage the
wound, and the dormant buds will soon begin to grow out.

This method is recommended for apples, quinces, and
plums. In the case of the former, in many varieties there
is sufficient root formation in the first year, while in the case
of quinces, wood of two years' standing seems to be most
advantageous. The branches may with advantage be twisted
before covering them with soil at the point where it is
desired for the roots to make their appearance. In the case
of crab-apples, the quickest method of propagation is to cut
in pieces the smaller roots, of about the thickness of a quill,
which may have dropped off from the roots of a larger tree
which has been ti-ansplanted, and cover them with soil. They
very soon produce adventitious buds and form very fine young
plants.

It is, however, more usual to bury the layered branches in
the soil than to heap earth up round the plant. The parent
plants, which are kept short, are surrounded by a trench, into
which the strong shoots are bent down in such a way that
their ends project far from the filled-up trench, and they are
then cut back to two or three eyes. According to the slowness
with which the branches form roots at the place where they
are bent, they may be further helped on by ringing, twisting,
or cutting. This method is employed in Gooseberries, Red Cur-
rants, Vines, Hazels, Quinces, and Mulberries, Elders, Guelder
Rose, Forsythia, and Magnolia. Among Conifers it succeeds
with Juniperus, Taxus, Thnja, and Sequoia. In France it is
emploj^ed for Aucuha, Rhododendron, and other more delicate
shrubs.

The long branches of climbers (Glycine, Arisfolochia,
Lonicera, Clematis, Hedcra, Tecoma, &c.) may be arranged so
as to form a series of arches, the intervening portions of the
branches being covered up with soil. Thus between two
covered-up pieces there is a leaf-bearing and food-providing
portion.

172 THE PHYSIOLOGY OF PLANTS

The separation of the rooted portions should take place at
the usual time of transplanting.

In the previously mentioned method of propagation by-
portions of roots with adventitious buds, as e.g. in Bhus,
j^sculus macrostachya, and others, the production of buds on
the roots can be fostered by continual pruning in of the
mother-plant and by injuring the roots. But such wild stock
produced from root buds is not so good for grafting upon as
seedlings, as it has a great tendency to produce more adven-
titious buds, and therefore to weaken the main stem.

§ 35. What rules should be followed in striking cuttings.

A cutting is a portion of a plant detached from the parent
stock, and which becomes an independent plant by the forma-
tion of new roots. The new roots take their origin either
immediately at the cut end or at some little distance from the
latter. In different plants the power of producing adventitious
roots is very different. Speaking generally, we may say that
the older the various organs of a plant are, the less inclined they
are to form adventitious roots, and that of the various cultivated
plants those are least able to be propagated by cuttings which
have a hard and brittle wood. Of our fruit-trees, apples and
pears very readily form a strong protective callus over a cut,
but do not easily produce adventitious roots.

From the Paradise stock cuttings can readily be struck,
however. The same may be said of the Myrobalan plum,
Quince, Vine, and Ribes. Of other trees, soft- wooded Poplars,
Willows, Ailanihus, Flatanus, and Pauloivnia readily produce
roots, while Eobinia, Elms, Beeches, Oaks, Walnuts are not
to do so.

If a cutting is to form roots, and therefore to develop new
organs, it must contain a sufficient supply of plastic matter for
that purpose. This material has either been formed in a
previous vegetative period and is stored up in the shoot (woody
cuttings), or the cutting must be able to form the necessary
substances after it has been detached from the parent plant
(herbaceous cuttings). The latter must therefore always be

THE USE OF SHOOTS FOR PROPAGATING 173

provided with leaves, while in the case of the former it is not
necessary. Woody cuttings always form callus over their
cut end ; in herbaceous cuttings it need not be formed. The
formation of roots in a cutting is not dependent upon callus
formation.

We may here repeat again that callus is a thin-walled colour-
less tissue, consisting of raeristematic cells arranged in close
rows, of which the end ones are still in process of growth and
which have not as yet become differentiated into cork or
•wood.

The first sign of life in a cutting manifests itself by an
alteration of the tissues near the cut surface, the cut generally
running obliquely across the shoot, and being close below a
bud. If we cut off a shoot, we thereby expose all the tissues of
which it is formed, and we then bring the latter in contact with a
damp medium (water, sand, earth, sawdust, fibre, &c.). Some of
the tissues which have been exposed are not able to form the
healing layers of callus ; this is always the case with the old
wood, often with the pith and the outermost layers of the
cortex. The layers which are capable of further division, and
are therefore charged with the production of the protective
callus, are the cambium, the very young wood cells, and the
innermost layers of the cortex. The larger, therefore, the area
of exposed wood as compared with the other tissues, the more
difficult will be the healing process.

It is therefore essential for the success of propagation by
means of cuttings to bring about a sufficient and natural clos-
ing up of the eut end of the shoot.

This closing takes place by two processes. In the older
soft tissues (pith and old cortex) there will be formed above
the wounded cells transverse layers of cork cells which protect
the cutting against excessive moisture. The woody elements
adjoining the damaged wood cells and vessels may become
plugged up with a very resistant brown mass (gum) or with
thylloses, which have the same effect in closing the apertures
of these cells and vessels. The second process is the covering
in of the cut by the formation of callus.

Both processes take place (with very few exceptions) more
completely when the cut surface is richly supplied with air.

174

THE PHYSIOLOGY OF PLANTS

Care must therefore be taken that the medium in which the

cutting is placed is very thoroughly aerated.

When the closing of the wound begins, the cells of the

cambium, of the young layers of the wood and of the bast

begin to absorb more water and to bulge out over the cut

surface. When the delicate extended portion of the growing

cell has attained a certain length, a transverse wall is formed

behind the apex, and the latter grows on farther. As the

cells, which bulge out in this manner and divide, are very

closely set, long rows of cells will soon be formed which go

on growing at one end, and

being firmly packed together,

form a delicate white tissue

(Fig, 26). As long as this soft

tissue continues to grow at

one end, and thus increases its

bulk, it is termed callus.

But, after a while, the

plastic substances which are

being passed down from the

cutting to the callus find the

path through all the callus

cells to the extremity too long

to traverse. The diffusion of
Fig. 26.— Very Yodng Callus, the Cells

OF WHICH CONTINUE TO GROW AT THEIR tho lood matter docs not take

r\S/crS(.)™^"'''''^"pl^'^« sufficiently actively to
6. bast ; /i. wood cells ; m. cambium. the margin of the Call US, and
the growth ceases at that
region. Instead of that within the callus, an arched strip of
tissue makes its appearance, and its cells continue to increase
in number. This strip of meristem becomes confluent with
the cambium of the cutting, and represents the continuation
of the cambium within the layer of callus. Within the latter
it now commences to form bast cells on its outside, and on
its inside new wood elements, and this forms the actual
covering layer. The covering layers at the lower end of the
cutting do not difier in any essential manner from those
formed in the healing of a pruned branch, except that no green
colouring matter is formed, owing to the absence of sunlight.

THE USE OF SHOOTS FOE PROPAGATING

175

Fig. 27 shows the process of healing in the case of a rose-
cutting ; s' is the line along which the shoot was cut ; every-
thing below this line is newly-formed tissue, or what is usually
called callus. This callus has grown out in the form of a
thick ring from the original cambium, and has spread from the
margin over the cut end of the shoot. We can distinguish in
this figure, which represents a longitudinal section, a ridge of

Fig. 27.— Formation of Callus in a Rose-Cutting.
(For explanation see the letterpress.)

callus (ca) cut radially, and a transversely cut ridge (ca") which
runs towards the front and has already coalesced with the ridge
(ca'). In this way the pith, it is true, has become covered in ;
but this has taken place without any active co-operation of its
tissue, as it has simply been covered in by the fusion of the
ring-like growths of callus, which have grown in from the
circumference. In other cuttings, as, for instance, in the case

T76 THE PHYSIOLOGY OF PLANTS

of the Fuchsias, the pith takes an active part in the formation
of the callus, and may, in some cases, form the greater portion
of this tissue. In Fig. 27, furthermore, m represents the
pith which has been cut by the knife. The groove u' is filled
by the callus which is growing forwards from the back ; h re-
presents the old wood which was formed before the cutting was
taken ; nil, the new wood which has been formed since then.
It commences with short, wide, porous, and thick-walled cells,
filled with starch grains, among which there soon make their
appearance some short reticulate vessels. These elements
become more and more compressed towards the periphery, and
resemble the normal wood more closely the nearer they are to
the present ring of cambium, which means the later they have
been formed after the cutting was taken. The cambium ring
arches over the cut end, and is covered on the outside by the
new bast and cortex, which layers are not shown in detail in the
figure. At the outermost portion of the cortex will be observed
the corky cells a, now disappearing, which were the first
spherical or pear-shaped callus cells which covered in the cut.
These rows of cells multiplied, as has been previously mentioned,
by the constant divisions occurring in the terminal cells.

In that portion of the callus ring which is directed forwards
(c«^), and is therefore cut transversely, g represents the short
reticulate vessels which are the commencement of the new
wood. Surrounding the latter we find the cambium layer (c') ;
1) is the old group of phloem formed before the cutting was
taken. It has been pushed far away from the old wood by
the formation of new cortical layers, and has died away at
its free end. Those cells which adjoined the hard bast cells,
however, have been liberated by the cut from the pressure of
the bark, and have become radially elongated (/), while in
their normal condition their long axis was parallel to that
of the shoot. The outer layers of the old bark (r) have not
become changed : (0) is the oxalate of lime, which occurs partly
in the form of single, partly in the form of clustered crystals.
In some plants in which there is little tendency to produce
adventitious roots, the formation of callus may become so domi-
nant, that all the material which finds its way down the stem is
used for the covering layers of callus, and the formation of roots

THE USE OF SHOOTS FOR PROPAGATING 177

does not take place (Conifers and Ericas). In these cases it
is advisable to cut into the large knots of callus tissue. By
this means the cambium layer receives a check, and a greater
massing of the plastic substances takes place in the regions
above the cut, and thus a stimulus is set up which may often
result in the production of roots.

This method of procedure succeeds in cuttings (often leaf-
less) taken from old wood, and has some similarity to the
ringing of shoots. In the case of ringed shoots, the tissues
forming the upper lip grow out into an expanding mass of
cells, which cover in the exposed woody cylinder. If we
imagine this woody cylinder to be cut through near the upper
end of the ring after the production of this callus mass, and
picture the latter covering in the cut surface, then we would
have what actually takes place in a woody cutting.

In the case of shrubs with deciduous leaves, the best time
for taking cuttings of woody portions is the beginning of the
winter, or, in the case of shoots which will not be damaged by
the frost, the latter portion of the winter immediately before the
active gi-owth of the spring takes place. Strong slips cut back
to three or four eyes should be bound up in bundles and placed
in a cellar, or only temporarily covered with soil, and when the
spring approaches be planted in rows in a north aspect with
only about two eyes projecting from the soil. In this way
old wood which has been pruned away in the early spring may
be used for cuttings. This at least answers in the case of
Rosa, Weigelia, Cornus, Deutzia, Lonicera, Bihes, Spirea, <&c.

The callus formation in the case of herbaceous cuttings is
somewhat different. Generally more tissues take part in the
healing process. It is here especially the pith which forms
the bulk of the callus ; the older cortical tissues may, how-
ever, be very active. Even the vessels of the wood may take
part in this formation of callus (Begonia, Thimbergia), as the
cavities of the vessels may become blocked with thylloses, which
may grow out over the cut surface.

Propagation by means of such herbaceous cuttings is the
most advantageous means of propagating plants, and this is
true of woody plants too ; only the treatment of the cuttings
is quite a different one. We must always remember, that

1 78 THE PHYSIOLOGY OF PLANTS

when the tip of a shoot is used as a cutting, its callus and
adventitious roots are not formed from reserve food matter
which has been stored in its tissues at some previous period,
but that the materials necessary for these growths have to be
formed by the cutting after separation from the parent plant.
As soon as a portion of a plant is dependent for its food upon
its leaves, we know that it needs light. Herbaceous cuttings
therefore need light, and comparatively much light, while
woody cuttings can do with very little light, at the outset
at least.

The herbaceous cutting is taken before its axis is much
lignified. The cut surface exposes tissues which have as yet
no thickened cell-walls ; the cells are rich in protoplasm and
cell sap, are more prone to changes and decomposition, and
require therefore an increased stimulus to continue the vegeta-
tive processes in spite of the wound which has been inflicted.
This stimulus is provided by the increase of temperature.
Herbaceous cuttings require, therefore, more heat than cuttings
of the same species taken from older portions of the plant.
In some cases indeed the temperature requisite for herbaceous
cuttings is harmful for the woody cuttings, because it calls forth
certain changes (possibly of a fermentative nature) the products
of which cannot be used up at the time, and therefore cause
decay.

We must remember, on the other hand, that the wound itself
cannot be healed at once, and that the soft cuttings lose con-
siderable amounts of water from their leaf surface by transpira-
tion, and this at the time they are without roots, which could
supply the requisite amount of water. We must, therefore,
reduce in the first instance the transpiration without taking
away the leaves. This can be done either by shading the
cuttings or by keeping the air saturated with moisture. Every
decrease of the amount of light diminishes also the amount of
transpiration. In a damp or saturated atmosphere, too, the
transpiration of the leaves is reduced.

Herbaceous cuttings require, therefore, at the outset a moist
atmosphere. We purposely say at the outset, because it is a
frequent source of error to continue this for too long a time.
Absence of light and a large amount of atmospheric moisture

THE USE OF HIIOOTS FOR rROPACxATING 179

reduces the assimilation of the leaves also to a minimum, and
therefore decay often sets in on the cut surface and the cut-
tings are doomed. Herbaceous cuttings should, therefore, only
be shaded during the first few days, and should very soon
become accustomed to the normal illumination. After that
allow the air to circulate among the leaves, and do not be
afraid of the drooping of the leaves when the sun is shining
on the cuttings. Gradually the cuttings will get accustomed
to a more sunny and drier atmosphere, even if they have as yet
no roots.

The great mistake in the treatment of herbaceous cuttings
is to water them too much and to keep them too much closed
in, in order to prevent them drooping. Even the most porous
substance in which the cuttings are placed will act deleteriously
if it becomes water-logged. The oxygen of the air is then
prevented from reaching the delicate cut surface, fermentative
changes commence in the cells, and the decay of the cut surface
begins.

Herbaceous cuttings always require a well-aerated soil.

The art of the cultivator consists in executing these
precepts in the way most suited to the individuality of the
cutting.

No general rules can be given as to the amount of light,
moisture, air, or heat necessary for cuttings, as the requirements
of various plants differ so considerably. If, therefore, cuttings
of many different plants are put into one propagating frame,
some of the cuttings will very soon damp off. Let us just
mention one or two of the most striking peculiarities. The
scarlet Pelargoniums are so fond of light, and can dispense with
water so well, that the cuttings may be put directly in the bed
near the mother-plant. The same is the case with Pdargonium
grandifiorum, which grows without much difficulty if the young
tips are placed in a pot with sand in the sunny position
occupied by the mother-plant. At the commencement they
will droop, and the oldest leaves will actually dry up, but the
young tips very soon recover, and remain turgid until the
cuttings have rooted. If you tried a similar experiment with
a cutting of the Heliotrope or Lobelia, the cuttings might easily
be killed by a single hot day. They will, however, grow in a

i8o THE PHYSIOLOGY OF PLANTS

warm saturated atmosphere which would cause the decay of the
tips of the Pelargonium.

Cuttings of the double white Primula can do with a good
deal of moisture and of warmth, but they must have a large
amount of light. Those who have no bottom heat at their
disposal should keep the cuttings fairly dry, and can place
them in the open in a fairly shady position. The leaves, it is
true, will wither, and the rooting will take a long time, so
that a gardener could not use his method with any degree of
security.

No time can be fixed for striking herbaceous cuttings.
Shoots may be taken as soon as they have received a certain
rigidity and have some fully developed leaves, supposing
always that there is still sufficient time favourable to vegetative
growth to enable the formation of roots to take place.

If, however, the season is too far advanced to make success
certain, it is better to take fully developed leafless woody
shoots for propagating. Besides this kind of woody cutting
mentioned, which may be called a winter cutting, in some
shrubs not liable to damage by frost autumn cuttings may
prove successful. Less hardy genera, too, if they are well
protected from the frost, give very good results with autumn
cuttings (Roses).

We may give the following general instructions for the
treatment of all cuttings made from dormant wood.

We want the shoot which we have put into the soil to use
its starch and the other stored up food material for the produc-
tion of callus and adventitious roots, instead of passing it into
the buds, as would have been the case if the branch had been
left attached to the plant. The conditions in which the cut-
ting is placed are very favourable for such a course ; for on
the one hand, the wound caused by the cut, when in contact
with the damp soil, acts attractively to the food matter, which
has become soluble during the vegetative period. Secondly,
one main factor in the expansion of the buds is wanting, and
that is the root pressure. The buds of cuttings, therefore, un-
fold much later than those of the parent plant.

These conditions must, therefore, not be disturbed, but
should be maintained until the callus covering the cut is able

THE USE OF SHOOTS FOR PROPAGATING i8i

to absorb sufficient water to give to the buds the necessary
turgidity, and to push forth the stem apex and the young
leaves. To commence with, therefore, we should ensure a
complete rest of the upper portions of the plant. These
conditions obtain especially in the autumn, when the air is
greatly cooled and the soil is still warm. If cuttings are
taken in the spring, it is advisable to plant them in a northerly
aspect, so as to prevent them from becoming heated, and to
prevent any premature development of the buds. In the case
of cuttings set in cold frames, the glass or other covering
should be removed as eai-ly as possible in the spring.

The neglect of these precautions is the cause of most
injury to cuttings kept in cellars, greenhouses, &c. The
warmth of these places stimulates the buds to premature
expansion ; the new shoot will then form the chief centre
of attraction for the reserve food substance, and will grow
out as long as the food supply lasts. Thus the formation of
roots is prevented and the cutting dies from want of root
nutrition.

In the case of valuable plants, which do not readily produce
adventitious roots, preparatory steps may be taken by ringing
or constricting the branch which is to serve as cutting, and
not separating it from the parent plant until the callus ring is
formed. The tip of the branch is then removed so that the
cutting has only three or four eyes.

In certain cases the stimulating effect of the warm spring-
weather acts beneficially. Quite a number of shrubs ( Weigelia,
Deutzia gracilis, Cydonia japonica, EiLonymus japonicus,
AuGuba, Biijcus, Laurus, Andromeda, &c.) grow best from cut-
tings made from the completely ripened first year's shoots
separated in July or September from the parent plant.

The cuttings may be placed in the open ground in rather
shady positions in sandy soil, or may be placed in boxes which
remain in the open. During the first few weeks it is well to
keep them out of direct sunlight ; later on, towards the autumn,
they may receive plenty of light. In this method of propaga-
tion most of the cuttings will have formed roots by the time
the winter sets in ; those which were taken very late in the
year will only have the protective callus. Of the latter

1 82 THE PHYSIOLOGY OF PLANTS

cuttings those which are iu pots or boxes should receive a
little bottom heat in the spring ; the roots will then soon make
their appearance. This is especially advisable in the case of
Thiijopsis, Thuja, Cnprcssus, Junijjerus, Sequoia. The more
delicate of these should be slightly forced, so as to ripen their
wood sufficiently by the end of June ; still the cuttings will in
most cases only have formed callus by the autumn, and will only
produce adventitious roots in the spring- when stimulated by
bottom heat.

The peculiarity of the cutting to continue the special mode
of growth attained by the parent plant can and is made use
of to form new varieties.

Variations, such as variegated plants, lasciated specimens,
or double-flowering varieties, can thus be perpetuated by
cuttings. Of special interest are the curious juvenile stages
through which some Conifers pass, and which have been
fixed by cuttings and brought into the market as new species
and genera. Thus it is mentioned that to obtain Cliammcy-
paris squarrosa, cuttings must be made of the small shoots of
Biota orientalis, which appear immediately above the coty-
ledons, and which bear cruciate leaves. In the same way
cuttings of the first lateral branches of Callitris quadrivalvis
give rise to a quite new form. The fixed juvenile condition
of Cupressus sempervirens may possibly have produced C.
Brcgeoni ; the first shoots of Cupressus Lawsoni have not got
imbricate, but horizontally expanded leaves, Retinospora ericoides
was obtained from Chamaxyparis splia^roidea var. Andalyensis,
&c.

Root-cuttings have been dealt with in the paragraph on
layering. They are portions of the underground axis, which
are stimulated by being cut in pieces to produce adventitious
buds. The shoot which arises from such a bud becomes an
independent plant of strong growth as soon as it produces
adventitious roots from its own tissues. Ailanthus, Aralia,
Paidowiiia, Rosa, Pints Malus, 3Iespihis, and others can be
propagated in this way, if strong pieces of their roots about
two inches in length are cut off either before the first shoots
are formed in the spring, or before the second shoots are
produced (July). These pieces are then planted in rows in

THE USE OF SII00T8 FOR PROPAGATING 183

the soil. Among- Conifers, Araucaria, Podoccajms, and Gingko
are mentioned as suited to such methods of propagation, espe-
cially if some bottom heat can be used.

In the case of some plants ( Vitis vinifera and Pcconia
arborea) a special and advantageous method of propagation is
practised by means of eye-cuttings. The buds are cut out of
the old wood in the spring, in such a way that they resemble
eyes used for grafting, attached to a small piece of wood, and
the latter is placed with its cut surface downwards in pots in
a sandy soil. The pots are then sunk in a hot-bed.

In plants, too, in which the axis is succulent, the eyes can
be cut out for propagating purposes (potatoes, &c.), or the
whole tuber may be cut in pieces. In this case we might
speak of tuber-cuttings. Their further growth is, as a rule, a
certainty, especially if the precaution is taken of keeping the
cuttings for a few days in a dry place before they are planted
in the ground. By so doing the foi-mation of a layer of cork
cells immediately below the cut surface is brought about, and
this constitutes the best protection of the soft and succulent
tissues against decay.

§ 36. What object have we in view in budding and grafting,
and how are these operations best performed

In budding and grafting, either one or several buds of the
mother-plant are detached and inserted on an older stock of the
same or of a similar kind. By this insertion of a younger
portion of a plant on an older stock the former can reap all
the advantages of the more advanced age of the latter, becomes
indeed older itself.

Both processes necessitate the cutting of the tissues ; that
method will, therefore, be the best one in which the healing
of the wound takes place most rapidly and most completely.

Tiie rapidity of healing depends (other things being equal)
upon the relative extent of the cambium, the layer which will
produce the healing tissue, as compared with the entire cut
surface. The greater the amount of cambium exposed on the
cut surface, the better are the chances of the graft or bud.

1 84 THE PHYSIOLOGY OF PLANTS

From this point of view budding- is the most advantageous
of the two operations, as it consists in the insertion of a bud
directly upon the cambium ring of the wild stock. To reach
this cambial region the bark is slit open by a T-shaped cut, and
the bud is placed under the raised lappets of the bark. The bud
may be attached either to a shield-shaped mass of bark alone,
or there may be some wood adhering on the inner surface
of the latter. In that case the operation is spoken of as
"budding with wood."

In the case of budding, the healing process can practically
take place at all the points of contact. As the bark of the
old stock has been split in the cambial region, the youngest
splint-wood remains on the surface of the wood, the youngest
bast cells line the lappets of the bark. From both the regions
normally new layers of cells will arise, which tend to fill up
the interstices between the scion and the stock. Later on the
scion itself will take part in the healing process, sending out
similar callus like rows of cells from its inner surface, just as
was done from the bast of the wild stock.

If the bud has some wood attached to it, the healing process
can only take place by means of the narrow cambium zone
bounding the scion. In this respect budding with wood is
less favourable, as the scion offers less surface for fusion of
tissues to take place. But this is of little importance in the
case of strong wild stock, as it will form healing callus so
rapidly that the participation of the scion in this process may
be neglected. On the other hand, this method of budding is
much more easy in the case of wood from which the bark cannot
readily be peeled, and is therefore much more successful at
the hands of an unskilled operator. For it often happens in
separating the bark from the wood that the fibro-vascular
bundles of the bud remain attached to the wood in the form of
a small conical protuberance, and the bud is only represented
by a hollow cap, which does not grow out even when the bark
has united to the stock.

How rapidly the fusion of tissues takes place in the case
of budding can be seen by the changes which soon make them-
selves apparent. Already after twelve hours the cut margins
of the bark, and the outermost wo jd cells, show a thickening of

THE USE OF SHOOTS FOR PROPAGATING 185

their cell-walls, the contents of the outermost layers of cells
increasing in amount.

But it often happens that all the cut surfaces do not take part
in the healing processes. If the splint-wood does not show
any new cell division, its cell-walls swell up and turn brown ;
the outermost cells of the splint-wood then collapse and form a
thick irregular brown band. This discolouration is, however,
quite slight or does not take place at all if the cells remain
capable of division, and its formative power is so great that
already after two days (in the case of the Ash) several layers
of callus cells will be formed.

In the case of the lappets of bark which have been raised
from the wood the cells actually forming the cut margins
usually die away ; those beneath them may protrude a little,
but rarely do more than form protective layers of cork, while
the tissues closer to the base of the lappet show active cell
division and the formation of healing tissue. But though the
exposed layers of cells within the wound only form a moderate
amount of new tissue, this is as a rule sufficient to cause a
complete temporary closing of the wound. We say a temporary
closure, because, as a matter of fact, this first formed tissue only
lasts for a short time. For as soon as this healing tissue has
attained a certain development and is exposed to a certain
pressure, a zone of meristematic tissue arises within it, which
unites with the cambium layers which grow out from the angles
of the lappets of the bark. This meristem now begins to form
the woody parenchyma which covers in the first formed thin-
walled closing layers, and which gradually becomes filled with
starchy reserve material.

Finally, the cambium layer of the scion unites with the
continuation of the cambium of the stock, and thus a new
continuous cambium ring is forn^ed, of which the scion forms
an integral part. The transplanted bud has united with the
stock and behaves as if it were a bud of the latter (Fig. 28).

In the adjoining figure, representing a transverse section
through a stem of a Rose upon which a bud lias been inserted,
these conditions are all represented. JFis the wild stock ; RL
are the lappets of bark, which have been raised by the T-shaped
cut ; E is the bud which has been placed within these lappets.

THE USE OF SH00T8 FOR PROPAGATING 187

In the case of the stock, h]i. represents the old wood of the
previous year, sli that of the current year, which was formed
before the budding operation was performed. In the bark, h
represents the bast fibres and t dead cells of the cut margin.
In making the T-shaped cut, the separation has occurred in
the youngest layers of the wood, a not infrequent occurrence
in practice, and the youngest wood vessels {y) and the cambium
(c) have remained attached to the bark. But often the reverse
takes place, and the cambium and young bast cells remain
attached to the wood.

Already, after twelve hours, a change has taken place at the
margin of the cut, the cell-walls having become thickened and
assumed a yellow colour ; very soon new cell formation begins,
and in the case of strong stock with the formation of callus
{oh) in the splint-wood. On the lappets of the bark the
marginal cells full of protoplasm (Jc) first bulge out, and their
activity increases the nearer they are to the angle, where the
bast is still attached to the wood, until they form a consider-
able amount of callus (o/i).

These masses of callus thus formed by the bast and the
wood and (under favourable conditions) by the scion unite into
a temporary covering for the wound. Later on the activity of
the cambial layer of the cortical lappets {cc), which represents
the continuation of the cambium of the uninjured portion of
the stem, becomes more pronounced ; this leads to the formation
of the actual and permanent wood parenchyma {kg) which
represents the commencement of a growth of wood which con-
tinues under slight amount of pressure. This parenchymatous
growth of wood, as it increases in amount, gradually crushes up
the thin-walled tissue which was first formed for the closure
of the wound. Gradually the whole of the tissue {ok) lying
between i and 2 is replaced by cells of the nature of kg, which
are filled with starch.

The scion {E) is in this figure a bud with an attached por-
tion of cortical tissue, but without any wood cells ; the actual
bud lies in the direction of 0, somewhat higher than the
section represented in the figure, which only contains the
large fibro-vascular bundle {gh) belonging to it. On the right
of the chief bundle we see another smaller one, the correspond-

i88 THE PHYSIOLOGY OF PLANTS

ing one to which on the other side has been removed. The
two smaller bundles are of no importance for the success of the
operation, but the large median one is of great consequence.
It is the small protuberance which must be present on the
inside of the bud, as every operator knows, if the bud is to
develop. If its place is taken by a small depression, then the
vascular bundle {gl) belonging to the bud, and which has to
form the woody cylinder of the new shoot, has remained
attached to the parent plant. The bud then is represented
by a hollow cap, which will not develop any further, even if
the wound should heal up.

The union of tissues takes place most favoui-ably if a layer
of callus is formed over the entire inner surface of the scion,
as is shown in the figure. The cambium zone lying beneath
the bast fibres (h) has formed new cells most profusely, as is
shown by the projecting group of cells at z. The cambium
layer of the scion {E, c) unites later on with that of the stock
(cc), and thus a continuous cambium cylinder once more exists
round the stem, and the scion appears inserted in the tissues
concerned with the normal nutrition of the stem.

The method of union of the tissues which takes place after
a successful operation we need not describe minutely, for we
know how an obliquely cut branch becomes closed in by callus.
The cambium layer produces at first soft callus- like layers,
which afterwards become hard and cover in the exposed
woody tissues. The scion behaves like a cutting, taking the
material necessary for its callus formation from the reserve
food material which is stored up in its tissues. The stock,
which is cut off obliquely above the point at which the bud-
ding has been effected, draws the material necessary for closing
this cut from the surrounding cells, the cut having been made
near to a bud which will soon grow out. The shoot produced
by this bud will soon form new plastic matter through the
assimilating energy of its own leaves, and the cut will soon be
completely healed over.

But the cut surfaces of scion and stock are in close juxta-
position ; the callus tissues formed by them must therefore
very soon meet and press one upon the other. The effect of
this pressure is to cause the fusion of the callus cells of scion and

THE USE OF SHOOTS FOR PROPAGATINO 189

stock, the cambium layers uniting- to form a continuous cam-
bium cylinder. The closer the cut surfaces are one to the
other, the more rapidly do the cells which are capable of fusion
meet, and the more quickly and completely is the union
brought about. Hence gardeners have adopted the rule that
in all processes of grafting- and budding the cambium layers
must be brought into contact. Tliis, however, is only then
completely possible when the scion and stock are of the same
thickness. If this is not the case, we must at all events
endeavour to secure that on one side the two cambial layers
will meet, so that the scion may from the outset be nourished,
and that it may at least receive the water which it needs.

If the scion becomes attached at one side, at all events, so
that it does not suffer from lack of water, it will soon be able
to help itself. The growing buds will form such quantities of
food matter that it will very soon form large masses of callus,
and these will cause a union to take place on those sides which
do not fit. This is most frequently seen in the case of cherry-
trees which have not been properly ligatured. The scion
will in these cases often be found to have an oblique position
owing to the fact that on the non- fitting side large masses of
callus have been formed which did not at first unite.

In such cases the advantage of properly binding up the
graft is seen ; the callus masses must not be able to push each
other away. If the scion cannot give way, the callus layers
will, under the action of the pressure, unite. The separating
of scion and stock after the operation can be prevented by
the so-called tongue grafting or tonguing.

The fusion of stock and scion takes place with the greatest
difficulty in the case of crown grafting.

The dangerous element here is the wound which is inflicted
upon the wild stock, which is either split longitudinally or
has at least a wedge-shaped portion of wood removed from its
upper end. Now the two cut faces of the wound expose the
old wood, which cannot give rise to any healing tissue. The
closure of the deep split, therefore, can only be effected by
the ingrowing of callus from the peripheral cambium layers.
This, however, only very rarely takes place completely ;
generally there remain gaps at the centre of the stem, and the

I go THE PHYSIOLOGY OF PLANTS

tissues wliich immediately surround these become brown and
die away. If water finds entrance to these cavities, decay
very soon sets in with far-reaching effect. Different kinds of
trees behave very differently when crown-grafted, and in
certain genera this operation may be undertaken without the
slightest danger, because they have (as, for instance, in the
Lime) the faculty of forming callus cells by the renewed
activity of pith cells. In such cases the callus will be found
making its way outwards through the splits, and fusing with
the callus formed by the cambium.

Such an instance of complete union of stock and scion in
the case of a crown graft is shown in Fig. 29, which represents
a transverse section through a two-year-old stock of the Lime,
Tilia Europcca, upon which a scion of the Silver Lime has been
grafted.

The operator had split the stock (Z;) very completely, had
cut the scion {Pf) iu such a way that some bark remained
attached at both sides, and had forced it right into the region
of the pith of the wild stock. The brown margins (s) denote
the boundaries between the stock and the scion. Throughout
the section hli stands for the old wood formed in the previous
year, nh for the new wood formed during the year in which
the grafting was performed, g for the wood vessels, c the
cambium layer, and mz for the normal thick-walled, un-
changed cells of the pith : rp is the normal parenchyma of
the cortex and phloem, with h the concentrically arranged
layers of hard bast ; re is the commencement of the callus
formed by the cortex.

As soon as the scion was placed in the split, the tendency
to close up the wound made itself manifest. The damaged
cells of the cortex died off, and became separated from the still
living ones by a layer of cork cells. The young cortex, the
cambium, the young wood, and here also the outer cells of the
pith, commenced to divide rapidly and formed callus tissue, which
forced itself in between the stock and the scion. The arrows
in the tissue indicate the direction in which the callus masses
moved, and the dark lines denote the line of juncture of the
young healing tissues. The chief amount of this tissue is made
up of the callus re, formed by the cortical tissue ; but both

THE USE OF SHOOTS FOE PROPAGATING

91

the young wood {nh) and also the pith have taken part in
the healing of the wound. The latter has, for instance, formed

'P

;>>,

A^

nil
-Pf

mz

hh

tfifc

W

Fig. 29.— Section of a Graft of a Two-Year-Old Twig of the Lime {Tilia, Europma).

The scion (P/) has been pushed into the pith of the wild stock. The arrows indicate

the direction in which callus layers have grown to close up the wound.

(The explanation of the lettering will be found in the letterpress).

the tissue {111c"), which has joined up with that formed by the
cortex {re"). That part of the split (s/;) which was not filled

192 TILE PHYSIOLOGY OF PLANTS

by sciou has become closed by the activity of the pith {nik),
which has formed a number of rows of callus cells {mc), which
continued to grow at their apex, until they met those produced
from the other side of the pith.

Thus we see all the cavities between the stock and the scion
have become completely filled by callus, and as the cambium of
both c and c have fused, we see that crown grafting may be
performed, in the case of the Lime, as readily as any other
method of improving the stock.

The healing is also very successful if the scion is not cut
into a triangular wedge, but if it is only pared down on either
side so that both sides are still covered with cortical tissue.
The centre of the split is in this case filled by cortical cells
which can form callus tissue to fill up the crack. Crown
grafting cannot, however, be entirely neglected. This opera-
tion has to be resorted to in the case of old stems, the cortex
of which does not easily separate from the woody cylinder,
and it is advantageous when grafting on root-stocks (Pceonia
arborea, Clematis, Bignonia, &c.). After the discussion of the
three chief methods employed for the improvement of wild
stock, we may now discuss the advantages of certain modifica-
tions of these methods.

Thus small graft shoots may be inserted into a T-shaped
incision into the cortex of the stem. This is most advanta-
geous in the late summer. The shoot is then trimmed like
an ordinary graft. This method is very advantageous, and
generally practised in the spring, as soon as the cortex can
be easily separated in the stock. It is of advantage in the
case of trained trees in which some branch has been injured,
and also in the case of an old stock which is too thick to allow
of a bud being introduced into it. This method might be
called budding with a shoot.

If the crown of the old stock is to be preserved, lateral
grafting may be practised. In the case of evergreen shrubs
(Camelias, Rhododendrons, Azaleas, Conifers, &c.), a last year's
shoot still provided with its leaves is used as a scion, and a
piece of the stock of similar size is cut away from the side of
the stem, so that the wound which is thus caused is covered
as completely as possible by the scion.

THE USE OF SHOOTS FOR PR0PA(L\T1NG 193

Instead of removing the piece of cortical tissue of the stock,
however, it may be left connected with the stem below, so that
the wound is really only an oblique slit in the stem. In this
slit the wedge-shaped end of the scion is fitted and bound up.
It is still better to make the slit in such a way as to have only
one cut margin. This is done by pressing the knife down into
the stem below the cortex, in such a way that its point does
not project through the cortex on the other side. Into this
tangential slit the pointed end of the scion is then inserted.

In the case of thicker stocks and slender scions, it is
best to substitute crown grafting by some form of graft in
which the scion is inserted into the cambium zone of the stock
plant.

How should gTafts be bandaged? This question is often
put, and is answered in various ways. To form a true judg-
ment on this point, we must consider what is the object of
binding up the wound, and also what material it is we employ.
We have already stated that energetically growing grafts do
not in some cases unite, because the callus masses formed by
stock and scion respectively tend to keep them apart. The
ligature has the function of preventing this, and of pressing the
scion against the cambium of the stock. When the fusion
has taken place, and the plastic matter formed by the leaves of
the scion begins to take its normal downward course, then the
thickening of the whole axis begins. If the material employed
in binding up the branch is sufficiently elastic, then no stoppage
or retarding of the thickening will take place at the graft. But
if we make use of very closely spun wool or of bass, the ligature
will soon be seen to constrict the cortical tissues, and this should
be prevented. From this it follows that it is better to bind up
grafts with flat than with twisted bands, and that cut surfaces
which are eoncave to one another should not be laced up to
meet. Such concave surfaces, however, should not and do not
generally occur in the case of skilful operators. Novices will
do well to prevent the transpiration from such badly fitting-
grafts by the use of a little wax. If the stock and scion are
full of sap, if the operation has been skilfully carried out,
and the ligature is iirm but not constricting, the time involved
by coating the wound with wax may often be saved.

K

[94 THE PHYSIOLOGY OF PLANTS

.^ 37. To what extent do scion and stock mutually influence
one another?

As long as it was supposed that cambium was a sort of sap
which circulated between the bark and the wood, it was firmly
believed that the characters of scion and stock became gradually
intermingled. Now, however, we know that the cambium is a
tissue, the young cells of which have inherited from their first
formation the tendencies of their mother-cells, and therefore
continue to function in the same way, forming the same sort
of cell-wall and cell-contents as their predecessors did. The
characters of the scion as well as those of the stock will develop
themselves separately in their several tissues.

But it cannot be denied that in those places where the
callus cells of scion have united with those of the stock, the
cells adjoining each other and belonging to two different plants
will influence each other to a certain extent. Each cell pre-
pares its substances in its own peculiar way, and those of its
contents which diffuse out into the neighbouring cells, or, in
case of the sieve tubes, pass over in a mass, will cause a dif-
ference of nutrition, and such a difference of nutrition will be
connected with differences of character. It is, therefore, not
strange that we should be able to demonstrate the influence of
scion and stock one upon the other. But a fusion of char-
acters never takes place, because the main functions of each
cell are too well fixed by heredity to be able to be changed by
mere differences of nutrition.

But apart from these considerations, the grafting will entail
a certain mechanical effect, because in the new individual,
consisting of the two parts, a transverse layer of short wood
parenchyma has arisen.

This layer causes a certain amount of difficulty to all con-
duction. The wood vessels of the wild stock, as far at least
as they were formed before the grafting was effected, are not
directly continued into the scion, because this layer of callus
lies between them. And even when the fusion of tissues has
taken place, and the continuous zone of cambium has formed
uninterrupted vessels which pass through the juncture of the

THE USP: of SH00T8 FOR 1>R0PAGATING 195

two individuals, they retain for many years a sinuous course
which retards the passage of substances.

The changes which take place by grafting cause for a number
of years a retardation of the transport of food material.

But several examples may be mentioned of apparent changes
brought about by grafting, one of the most remarkable of
which is the transmission of a white-leaved condition (albicatio)
from the scion upon the stock, and also vice versa {Passiflora,
Jasminum, Ahutilon)}

It is less common to find that the occurrence of red cell
sap in the leaves of the scion has extended to the wild stock.
This phenomenon has, however, been noticed in the case of the
copper-beech and the red-leaved hazel. The statements as to
any effect produced on the flower of the wild stock are very
few. A white flowering Jessamine upon which a yellow
flowering variety was grafted is said to have occasionally
produced yellow flowers.

But it is very well known that certain stocks have a very
pronounced effect upon the habit of growth of the scion.
Apples grafted on Paradise stock or dwarf stock (Firus j^rcccox)
remain of short stature and often produce flowers in the year
following the grafting. Grafted on the Doucin the varieties

' This occuiTence may perhaps, however, be attributed to mechanical in-
liuence. The appearance of white patches or of entire white leaves depends
upon the fact that in several portions of the tissues the cell contents appear
poor ; no chlorophyll grains are developed, and the protoplasm appears as a
colourless flocculent mass. The cells have, therefore, received too little plastic
material during their formation, and have not been able to increase the
amount they had received. But it is ]iossible in some plants which have a
tendency to white foliage to induce the formation of certain white-leaved
shoots by a continued pinching back of the young shoots. This may be
explained by the assumption that these shoots are induced to develop before
they have sufficient plastic matter, and had to develop very rapidly. Now,
grafting acts like pinching, and stimulates the lateral buds to growth before
they have sufficient food material. The leaves expand, and in the strong
illumination soon become old, and cannot produce new food material for
the impoverished cells, which therefore remain without chlorophyll. This
character would not be transmitted, but would be brought about by the re-
moval of the apex of the shoot in the operation of grafting. This also would
serve to explain why the appearance of white-leaved shoots does not always
take place close to the graft, but often at some distance from the latter,
while green-leaved shoots are found close to the point of union.

196 THE PHYSIOLOGY OF PLANTS

become bigger and fruit later ; on the crab-apple the tree
retains a normal growth, but the crown does not produce
flowers for a considerable time. In the case of pears, the
quince and the damp-loving hawthorn will form dwarf stock.
Trees with such stock, however, are known to have a shorter
duration of life than when true wild stock is used.

Quite a number of further statements exist, which we shall
partly repeat here, not so much to put forward these cases as
of general moment, but rather to give an impulse to new
experiments in this direction. Unfortunately there is a great
want so far of systematically arranged experiments.

It is generally held that tree-like forms succeed better on
bushes than the reverse. Sour cherries grafted upon sweet
ones succeed less well than the latter upon the former.
Cabanis states that late varieties of walnuts and chestnuts
never succeed upon early varieties ; while in the case of pears
and apples. &c., this method of grafting late varieties upon
early ones is said to have very good effect. The same holds good
with peaches, and causes an earlier ripening of the fruits.
Almonds on plums and the reverse will i-eadily grow, but
seem to deteriorate after a few years. The almonds grow
more rapidly and commence earlier in the spring, and they also
form a very large callus growth. It may be safely assumed
that a very early scion, which requires constantly more water
than the stock, will succeed upon a less vigorous stock only so
long as the stock is able to supply the requisite water. If
the scion cannot suit itself to the stock, it will soon perish from
want of food matter. But the nature of the soil, the water
supply, and the variety are three factors which cause a great
amount of variation in the results. On the other hand, a stock
which begins to function eai'ly in spring and forms a large
amount of wood will supply a more or less exacting scion with
more sap than it will be able to make use of. The superfluous
material of the stock will tend to the rapid production of new
growths. If the wild stock possesses many buds, its energy
will expend itself in the development of long shoots ; but if, as
is generally the case, the lateral branches and eyes have been
removed, the snpei-fluous material will become lodged in the
cambium of the stem, and will cause the formation of large

THE USE OF SHOOTS FOR PROPAGATING 197

masses of wood parenchyma in the place of the normal wood
cells. This parenchyma, however, is more delicate and sensitive
to external conditions, and in the case of stone-fruits liable to
cause the exudation of gum. Indeed, Duhamel has already
pointed out that branches of the almond upon which shoots
of the plum had been grafted were liable to gummosis at the
juncture of stock with scion.

If pears are grafted on quince or apple on Paradise stock, the
death of even healthy shoots of the scion will take place if the
soil is at all dry or there is an insufficient supply of roots.
In one case it has been proved that the injury to the roots
caused by transplanting was sufficient to cause the death of the
graft. Grafts made at the same time on similar trees, which
were, however, not transplanted, turned out quite successful.
Peaches grafted upon plums do not form successful grafts.
The scion forms a reddish wood and soon dies off.

It is, however, remarkable that pears and apples which result
in successful grafts with rather distant forms do not unite
well if the stock and scion are nearly related. Apples unite
very well with pears, and the apple scion bears fruit at a very
early stage, but it dies very soon. Pears grafted on apples
also yield fruit for a time, but the scion very often shows an
excessive formation of wood. (This depends, of course, largely
upon the tendencies of different varieties to form wood.) This
often results in knotty excrescences near the graft, in the
premature dropping of fruit, or the production of flowers
without fruits, and in a crippling or complete dying away of
the crown.

Evergreen scions are quite able to unite with stocks which
lose their leaves annually. Slips of the Cherry-Laurel (Prumis
Zaurocerasits) are said to unite with the stock of the Bird-Cherry
{Prunus Padus). QuermLS Ilex has been successfully grafted on
Quercus sessilijlora, and Cedrus Lebani on Larix eitropcca. We
have, however, no observations of the grafting of species which
lose their leaves on evergreen stock.

Lastly, we must mention the fact that hybridisation has been
occasionally brought about by grafting. Many instances of graft
hybrids will be found in botanical literature, but many of the
instances are of doubtful authenticity. But the possibility of

198 THE PHYSIOLOGY OF PLANTS

modifying certain characters which are easily liable to changes
(colour of flowers, &c.) by grafting must be considered a
reasonable one, and we must admit that there are some forms
intermediate between the stock and scion which may have
resulted by a form of hybridisation.

CHAPTER IX

THE TREA TMENT OF LEA VES

§ 38. What is the effect of injuries to the leafy tissues?

From what has been previously stated with regard to the work
done by the leaves, it follows that every injury to the leaves
means a decrease in the amount of new material formed. The
removal of the smallest amount of leafy tissue entails the
loss of some of the cells which should build up new organic
matter.

As the veins of each leaf lead downwards through the leaf-
stalk into the shoot, and as the soft bast and bundle sheath
conduct downwards the organic matter formed in the leaves,
the diminution in the organic matter formed will first make
itself felt in that portion of the shoot which is close to the
insertion of the leaf. If the shoot still requires the food
material for itself, the injury to the leaves will cause the shoot
to be less completely developed, i.e., will cause it to remain
short.

This is the explanation of the success which accompanies
the removal of the tips of leaves of a shoot before it has done
growing. The removal of the upper portion of the leaf causes
the adjoining internode to remain short. Peaches regularly
produce premature branches, i.e., the buds which have just been
produced in the axils of this year's leaves grow out immediately.
These premature {'proleptic^ shoots have the peculiarity that
their lowest internode grows several inches in length instead of
remaining short, as it does in branches which are produced
after a long resting period. The lower leaves and eyes of these
premature shoots are, therefore, also several inches from the
parent shoot. That is, however, a fault in trained trees in
which one of the objects to be attained is to have the fruit as

2 00 THE PHYSIOLOGY OY PLANTS

close as possible to the leading shoots. Now, if the tips of the
lowest leaves are removed, the internodes belonging to them
remain short, and the eyes will be in proximity to the leading
branches.

Kecently this method of procedure has been adopted in the
case of herbaceous plants (Dahlias), and short, bushy plants are
said to be produced.

The fruit-grower is sometimes forced to a partial removal
of leaves to ensure a better development of his fruits. In
summers with little sunlight, the almost entire removal of the
leaves will be beneficial to fruits which have already attained
the requisite size but need the ripening effect of the sun.

A considerable amount of defoliation is often useful in the
case of trees growing in a rich damp soil, to accelerate the
ripening of the wood. In such soils after a dry summer a wet
autumn will often prevent the formation of winter buds, and
the shoots will go on growing until December. Such shoots,
however, are very delicate, as they have little wood formed, and
are liable to be nipped even by moderate frosts. By pinching
off the apex of the shoot and removing some of the leaves, the
ripening of the wood is considerably accelerated. The removal
of the apex of the shoot removes the tissues which attract the
greatest amount of water, and the removal of the leaves allows
the shoot to receive more light, which is one of the chief factors
in ripening the wood.

We must now also consider those cases in which the foliage
has been damaged against our wish by climatic or other dis-
turbing influences. In such cases the amount of damage
done depends largely on the cause of the injury. The most
detrimental are the damages done by cockchafers or caterpillars.
This generally takes place when the leaves are still young.
The tree has expended the greater part of its reserve sub-
stances on the production of foliage, and has as yet received
little or nothing from them. Once more it is leafless, and
it requires more reserve material for the unfolding of those
buds which have remained uninjured. It has now to draw
upon the material stored up more deeply in the main stem,
for during the first unfolding the shoots and the roots have
used up all the material stored up in their vicinity. After the

THE TREATMENT OF LEAVES 201

first production of foliage there remains still a considerable
amount of starch stored up, chiefly in the base of the stem.
The formation of a second crop of leaves will use up the greater
part of this reserve material. By so doing the stem will be
greatly exhausted. There will also be no material at hand, for
a time at least, for the growth of the cambium, and conse-
quently only a very weak annual ring of wood will be de-
veloped.

It is probably less harmful to the tree to lose its foliage by
a late frost. The leaves killed by the frost remain attached to
the tree, and it is therefore probable that a certain amount of
the mineral substance, at least, which has been expended on
their production will be reabsorbed by the branches.

Such a reabsorption of substance from the leaves by the
stem has been proved in the case of foliage killed by excessive
heat and drought. This takes place when young trees are
growing in a very thin layer of soil covering rocks or scries.
The dried-up leaves will remain adhering to the branches
throughout the winter.

The nitrogenous constituents and the phosphoric acid, it is
true, remain in the dead leaves, but the starch, and with it the
potassium, is in part reabsorbed by the tree.

i^ 39. In what cases can the leaf be used for propagation ?

As has been mentioned in dealing with the shoot, some
plants have a considerable tendency to produce adventitious
roots and adventitious buds. This faculty is in many cases
shared by the leaves too, and causes them to be used as cuttings.
At present we know of no general rule as to what kinds of
leaves can be used for cuttings, and we can only base our
remarks upon experience. It does not indeed follow that a
leaf which has been observed to produce roots will be suitable
for propagating ; for we know many hard-leaved plants the
leaves of which readily produce adventitious roots and remain
fresh for months, but which will not form adventitious buds.

Begonias are perhaps most often propagated by leaf-cuttings,
and in their case it is chiefly the so-called leafy Begonias, like

202 THE PHYSIOLOGY OF PLANTS

Begonia Rex and its varieties. The strongest ribs are cut through
and the leaves are laid with their under surface on damp sand.
If care be taken to prevent the leaf from drying up by keeping
the atmosphere saturated, and if the temperature is sufficiently
high, it will be noticed that the severed ribs are very soon
covered in with callus. The wounds become surrounded by
an excrescence of tissue which at first produces only roots.
Very soon, however, adventitious buds make their appearance
on this tissue. These buds, however, do not produce their
own roots, but content themselves with those formed on the
callus.

It is especially interesting that these young shoots arise from
one, or only very few, epidermal and subepidermal cells in the
neighbourhood of the severed vein ; thus we see that even
tissues with very little protoplasm are able to enter again into
cell-division if they are well supplied with reserve materials.
The new roots take their origin much deeper, generally in
immediate pi'oximity to the cambium layer of the vascular
bundle, which runs along the vein. But even before the roots
are developed the leaf seeks to develop absorptive organs, and
we find the epidermal cells near the wound growing out into
hair-like processes, which somewhat resemble root-hairs.

As far as our observations go, we may assume that the for-
mation of young plants from leaf-cuttings takes place in other
plants in much the same way as in the case of Begonia.
Some differences occur, as in some cases (Peperomia) no callus
is formed over the wound, but the latter is simply protected
by a layer of cork cells. In other cases the young buds are
not formed directly from the tissues of the leaf, but arise
from the callus tissue {Achimenes). Such differences are, how-
ever, unimportant, and may occur in the same species under
different conditions of nutrition.

Among the Monocotyledons it is especially the bulbous
plants, the leaves of which are prone to form buds, and can
therefore be used as cuttings. It is well known that many
liliaceous plants may be propagated by bulb scales ; but it is not
generally known that, if properly treated, the green leaves will
produce new bulbs. The young buds which arise on the cut
end of a bulb scale or of a green leaf originate from the

THE TREATMENT OF LEAVES 203

epidermal or (in older leaves) from the subepidermal cells,
their appearance being preceded by the development of a
certain amount of true callus, the outermost cells of which
are the actively dividing ones. When the small tubercle of
callus tissue has attained a certain size, then a ring-like pro-
tuberance makes its appearance, which grows up in the form
of a sheath round the apex and forms the first leaf or scale of
the young bulb.

The formation of these small bulbs (hidbils) is best seen in
the case of Lilmm auratum or tigrinum. Here they make
their appearance on the inner surface of the leaf near the
margin, while the roots arise from the bast region of the
fibro-vascular bundle. The latter are, however, only of very
short duration, as the new bulbs very soon form their own
roots.

In most cases, however, it is not necessary to stimulate
the leaf to the formation of buds by wounding it, as the
leaves will readily form buds of their own accord. This
voluntary formation of buds is not in any way different from
the bud formation of cuttings. Examples of it may be
found in the groups of Mosses and Ferns, as well as among
the Lilies in the group of Monocotyledons. It occurs
frequently, too, among Dicotyledons, where the buds are
generally formed in the axils of the veins, and are larger
and stronger the larger the vascular bundle is next which
they lie.

It is very well known that the leaves of Bryophyllum
calycinum produce such buds in the depressions between
the teeth of the leaf, where a meristematic tissue occurs,
which soon begins to form buds.

As such buds, when carefully removed, may readily be
grown into separate plants, the knowledge of plants pro-
ducing such bulbils is useful to gardeners. We will, there-
fore, give a list of plants in which they will be found to
occur: — Hyacinthus Fauzolsii, Fritillaria impericdis, Ather-
urus ternatus, Ornilhogalum thyrsoidrs, Drimia, Malaxis,
Cardamine, Nasturtium, Tellima, Siegcshcckia, Utricularia,
Calanclioe, Begonia quadricolor and phyllomaniaca, Nymplima,
micrantha (according to Caspary), and its hybrids. If the

204 THE PHYSIOLOGY OF PLANTS

gardener desires to use the leaf-buds of these forms for
propagating, it is advisable to keep the plants from flower-
ing, and to place them in a moist and shady situation. The
vegetative activity will thereby be increased and more bulbils
will be formed.

CHAPTER X

THE THEORY OF WATERING

^ 40. Why must we pay special attention to the watering
of plants?

After having examined tlie structure and function of the stem
and leaves, we must consider the conditions which conduce to
a normal development of these organs. We have already dealt
with the subject of transplanting, and assuming that the roots
will find a requisite amount of food material in the soil, we
must consider how the supply of water should be regulated to
ensure the continued activity of all the organs of the plant.

It is a fact that the older a gardener grows the more care
he takes with the watering of his plants ; for year by year his
experience teaches him more definitely that careless watering
is the cause of an uncommonly large number of diseases of
plants.

The difficulty in watering lies in the fact that plants require
various amounts of water according to their species, their age,
their situation, the season of the year, and their actual state of
health or development. The amount of water which is suffi-
cient at any given time may be very much too large a month
later, and may therefore cause considerable injury. Above all
things, we must remember that the transpipatlon from the leaf
surface is not a merely mechanical, but a physiolog-ieal ppoeess,
and as such is regulated by the activity of different internal
organs, just as much in the case of plants as in that of animals.

It is, of course, true that the conditions which increase the
transpiration of an inanimate object in general accelerate the
transpiration of plants, but this is by no means always the
case.

The fact that the transpiration of living plants varies gene-

2o6 THE PHYSIOLOGY OF PLANTS

rally on the same lines as the evaporation of water from an
inanimate body will be understood if we reflect that the
factors which increase the evaporation in the latter case
(especially the rise of temperature) increase the vital activity
of the vegetable organism. An increase of the vital processes,
however, entails an increase of transpiration. On the other
hand, some conditions may occur under which tlie amount of
water given off by plants decreases, while inanimate bodies
would show an increase of transpiration. Thus sickly plants
may show a diminution of transpiration with a rising tempera-
ture. Inversely, however, an increase of transpiration will
be manifest even with a fall of temperature if plants are trans-
ported from a strongly concentrated nutritive solution to a
more dilute one. The roots will then appear to put forward
their best energies to absorb from the weaker solution as much
of the inorganic food matter as they formerly obtained from the
more concentrated solution. But this is only possible when in
the same interval of time larger quantities of the water are
taken up, which means, of course, that a greater amount must
be given off too.

The fact that sickly plants are not able to give off as much
water as healthy ones must not be forgotten when plants with
injured roots are re-potted and placed in a hot-bed. It is then
often supposed that the increased temperature of soil and air
will stimulate the plant to increased transpiration. The pot
is therefore often kept continuously wet, and the decay of the
roots, against which one is fighting, will continue unabated.

The amount of water necessary for plants is determined —

(1.) By the kind of plant under eonsideration.

(2.) By the intensity of the vital processes at any given
moment.

To know the requirements of any special kind of plant, it
is necessary to know something of the conditions under which
the plant grows in its native country. As far as practical ex-
perience has taught us at present, we may assume that plants
from rocky habitats, without shade, and from countries with
long periods of drought, are able to do with a very small
supply of water, and can only put up with an occasional
drenching. In those cases in which the climatic condition of

THE THEORY OF WATERING 207

the native country are unknown to us, the external appear-
ance of the plant rnay give us some clue to its pro])er treat-
ment. Plants with narrow and tough leaves, especially when
the leaf-blade is vertically placed, do not, as a rule, like
much water ; plants with broad leathery leaves prefer a damp
atmosphere to great moisture of the roots. Succulent plants
with hard epidermal cells (leafless Euphorbias, succulent Com-
posites, Aloes, and Agaves), and thin-leaved plants with a
strong woolly covering of hairs, are further examples of plants
which require little water.

Then the structure of the roots may often reveal the re-
quirements of the plants. Succulent roots in the case of
thin-leaved plants, and less succulent ones which taper away
and grow rapidly to the bottom of the pot, usually like, and
often require, a large amount of water. On the other hand,
a multiplicity of fine branching rootlets which tend to grow
out above the surface of the soil can often do with a small
amount of water, but require a greater amount of aeration.

In watering- we must attend quite as much to this desire of
the roots for air as to that fop water.

As we have already had occasion to mention, the roots
require just as much as the stem structures a thorough supply
of air. If this is not forthcoming, alcohol is formed in the
cortical tissue of the root, and later on fusel oil and acids
(acetic acid, butyric acid, &c.) make their appearance, and
these acting poisonously, generally cause the death of the
organ. The products of decay are then transported from
these dead portions of the plant to the still living organs, and
the inevitable result is the death of the latter.

The most frequent cause of the formation of alcohol in
roots and the subsequent decay of the latter is a permanently
wet ball of roots. The unfounded fear of damage arising
from a lack of water causes gardeners to water the pots as
soon as the surface of the soil begins to dry ; and it is not
taken into consideration that the centre of the pot still con-
tains a large amount of water. The consequence is that the
lower part of the pot is continuously saturated with moisture.
Thus the air is prevented from passing freely to the roots, and
these are gradually suffocated. This suffocation of plants

2o8 THE PHYSIOLOGY OF PLANTS

takes place very slowly, and is preceded by a long death-like
rigor, which is only overcome by a renewed supply of oxygen.
If death were to ensue rapidly in these cases, we should see
even a greater number of pot plants die than are actually
killed at present by too much watering,

"We need only observe the tremendous variations in the
amount of water supply to which these plants are exposed,
when they are dependent upon the rain, to see how little harm
these variations do. We are, therefore, generally in danger
of injuring the plants by a too anxious endeavour to supply
every small amount of water lost by the plant. Tndeed, it is
very good for plants, just as for human beings, to feel the
passing pangs of hunger or thirst, and they take no harm if
they are not watered until the pot sounds hollow. If water
is then supplied, it should be done so copiously that it runs
out of the hole at the bottom of the pot. The practice of in-
experienced persons of repeatedly wetting the upper surface of
the soil (especially of plants in peaty soil) has the disadvantage
that the lower portion of the soil remains dry while the
surface and the base of the stem become covered with moss.
Tiie above rule is especially important in the case of tough-
leaved plants, which do not require much moisture. It is
different in the case of herbaceous plants which are rapidly
growing. In their case every diminution of the water supply
is injurious, and in the case of these rapidly-growing plants
there is very little danger that the soil will be water-logged for
any considerable time, and consequently stop the respiration of
the roots.

But even if we know the natural conditions of the plant and
the requirements of the species, we will not be able to water
the plant rationally if we do not know or do not consider the
peculiar requirements of every individual. Even the same
individual with the same amount of leafy tissue may need
a larger or a smaller amount of water according to the vital
activity which it manifests at any given moment.

This difference in the amount of water required at different
periods becomes more comprehensible if we remember that the
water is not only needed as a food substance, the elements of
which become separated in the vegetable tissues, and used up

THE THEORY OF WATERING 209

ill the formation of new organic substance, but that it serves
also to transport the mineral salts contained in the soil to the
plant. These are used up by the plants more rapidly at one
time than at another, and the plant will, therefore, sometimes
take up more of the solution, sometimes less.

From experiments made on the transpiration of plants, we
know that every plant draws most largely on the soil during
the putting forth of new shoots and leaves, and takes up most
mineral substances at this time. At this period, even plants
which grow in arid regions like and require considerable sup-
plies of water. As soon, however, as the shoot is developed and
the formation of flower-buds is expected, the watering may be
decreased.

The fopmation of flower-buds is best initiated by preserv-
ing- a period of rest, and the latter is favoured by a diminished
water supply.

This is the reason why cactus-growers let their plants almost
shrivel up after the shoots have been produced, and for the same
reason pine-apples receive less water when the formation of
fruit is desired. If water is freely given, and no resting
period is allowed, leaf development alone takes place, as the
tip of the shoot grows on continuously, and remains the centre
of attraction for the ascending sap.

This condition is not desired by the grower, but does not
injure the plant. In other cases, however, untimely watering
may produce actual disease. This may occur, for instance,
when the period of rest at the end of the vegetative season has
been disregarded.

The activity of a plant during one vegetative season may
be divided into two periods, and this is the case whether the
vegetative season is our summer, or, in the case of tropical
plants, falls in with our autumn and winter. With the awak-
ening activity, the first purely vegetative period commences,
during which the shoot is produced, and begins during the
first few months to accumulate food material. In the case of
annuals, the organic material which has been formed in the
leaves is immediately used up during the flowering period,
which follows upon the period of leaf expansion. The food
material is immediately and continuously withdrawn from the

o

2IO THE PHYSIOLOGY OF PLANTS

leaves, and the latter begin to fade away, beginning near the
base of the stem, and die. In perennial plants the material
which has been accumulating during the lirst period is used
during the second, either in the production of a later shoot
(summer shoot) or for the formation of flower-buds which will
open in the subsequent spring. In the case of our trees,
a great part of the nutritive substance is conducted down
from the leaves and stored up in the base of the stem and
the main roots, and gradually, when these organs are filled, in
more peripheral structures, the lateral branches and the rootlets.
This process of storing food material comes to an end when
the fall of the leaves ushers in the period of rest. As this
partly depends upon the gradual decrease of temperature, the
upper portions of the stem, which are exposed to the atmos-
phere, may have completely entered the period of rest, while
the roots, which lie in the warmer layers of soil, are still con-
tinuing their growth in thickness. This activity may some-
times last until January, and then only can we say that the
plant is entirely at rest.

Such a period of rest occurs in all plants which last for
more than one year. It is either the cold, which stops all
vital processes, or, as in tropical regions, the excessive drought
of the hot season. The rest due to this latter cause is so
important that tropical evergreen parasites belonging to the
group of Phanerogams will lose their leaves when they grow
on trees which shed their leaves during the dry season. A
very excellent example of this phenomenon is Loranthus longi-
Jlorus when growing upon Stcrculia, Spondias, Terminalia, or
Erythrina.

Such a period of rest cannot be interrupted with impunity,
but it can be shortened by artificial means. Nature itself
allows the resting period to be of different durations, accord-
ing to the development attained during the preceding period
of activity. Tiiis is best seen in the forcing of bulbs, the latter
being able to develop very early if the preceding month of May
was warm and dry, so that the bulbs could ripen well and early.
During- this latent period plants require very little nourishment,
and even those green-house plants which retain their green leaves
require an infinitesimal amount of water and of nutritive salts.

THE THKORY OF WATERING 211

But how often is tliis experience disregarded. In the cold-
houses in which Frica, Epacris, Bhododendron, Azalea^ Acacia,
Melaleuca, and other Australian and Cape plants are placed,
you may see the careful gardener watering every day those plants
which show the slightest sign of dryness on the surface.
The consequences very soon show themselves. The stems of
the Ericaceas begin to rot, Leguminosse and Myrtaceje lose
their leaves, and other plants lose their flower-buds. Decay
of the roots sets in in all cases.

The disregard of the resting period makes itself felt in
other ways too. Many growers believe that by increasing the
temperature and moisture they can awaken any resting organ,
and by trying to do so they often come to grief in the case of
tuberous plants. If the tubers of Caladia, Cyclamen, Begonia,
Gloxinia, &c., are placed in a hot frame for forcing, are perhaps
covered up with soil and kept permanently damp, it is not ger-
mination but decay that will set in. It is generally the mould
{Botrytis cana) which causes the destruction of the tubers.
In some cases they can be saved by cutting out the rotten
portion, covering the cut with powdered charcoal, and placing
the tubers on the surface of the soil, where they will have
more light and more air.

It must be remembered that, with few exceptions, the
transformation of starch into soluble sugar can only proceed
slowly in the storage tissues. It is also only possible to
gradually induce this change, and in the first instance it takes
place in bulbs and tubers without much water. This we may
readily conclude from the fact that such succulent organs
begin to sprout at the proper time, even when they are not
buried in the ground and receive no liquid water.

The calling- of a dormant organ into life or activity must take
place gradually, and during the first development the supply of
water must be but small.

Lastly, there are still some symptoms to be mentioned which
should warn gardeners to be careful in the watering of their
plants. They make their appearance when a damp atmos-
phere accompanies a wet soil. For though a damp atmosphere
is useful and necessary at the time of active assimilation, it
can be very detrimental when it is excessive during the period

2 12 THE PHYSIOLOGY OF PLANTS

of rest. A saturation of the air with water vapour, especially
if the temperature is low, is favourable to the activity of
Fungi of decay in their attack upon plants. The activity
of the latter is decreased by the moisture of the atmosphere,
as assimilation and transpiration do not then take place to
any great extent. For the fungal spores, however, this tem-
perature is sufficient, and their development is accelerated by
the great moisture.

If the temperature of green-houses is raised in the winter
when illumination and aeration is insufficient, then the plants,
stimulated by the moisture to further growth, may develop
along wrong lines. Tlie different processes which take place
in a leaf make different demands upon the different factors,
such as light and heat. Thus the elongation of cells can take
place in feebler light than can the assimilation performed by
the leaves. The formation of sugar can take place at tem-
peratures at which it is no longer used up in the process
of respiration ; sugar may therefore often be stored up at low
temperatures in leaves, tubers, and other organs.

It often happens also that in green- houses the temperature
is high enough to permit of an elongation of the cells, but
the light may not be sufficiently intense to cause any appre-
ciable assimilation. If under these conditions the pots are
copiously watered, while only a small amount of evaporation
takes place from the leaf surface, a considerable accumulation
of water will occur within the plant, and this will cause a
pathological (abnormal) elongation of the cells. Some groups
of cells within the tissues of the leaf will begin to elongate,
and even grow out into long tubes, but as they receive no
material from without for the elongation of their cells, they
use up their own cell-contents. Thus the chlorophyll cor-
puscles become disorganised, and nothing but small yellow
granules will be found. This causes such impoverished por-
tions of the leaf to appear more or less yellowish, even by
reflected light, or at all events when the leaf is held up to the
light.

Such appearances are of much more common occurrence
than was formerly thought, and it must now be regarded as a
sign of a superfluity of water.

THE THEORY OF WATERING 213

lu other cases the discoloratiou is less intense, but tlie
elongation of the cells is so great that the epidermis is lifted
up into small wart-like protrusions on both the upper and on
the under surface of the leaf.

Tlie formation of yellow patches on leaves as described
above occurs in the leaves of Pandanus and Draccena ; the
wart-like protuberances of the leaf surface may be observed in
Ficus elastica, Aralia, Hedera, Solanum, and other plants.

The appearance of yellow patches with indistinct margins
or of small glandular dilatations on leaves during the resting
period may be taken as signs that the plants are probably
suffering from a superfluity of water, and the watering should
be decreased.

CHAPTER XI

THE FLO WER ^

§ 41. Of what parts does the flower consist ?

In the case of a complete flower we must distinguish two
groups of organs. We have first of all the essential organs of
reproduction, the stamens and the pistil, and secondly the less
important protective organs (sepals and petals), which may be
only rudimentary, or indeed entirely absent. If there is only one
set of protective leaves, the covering formed by them is termed
the perianth. The simplest flowers may consist of a single
sexual organ, as, for instance, is the case in the female flowers
of the Yew (Taxus). Usually, however, a number of different
organs are collected in a flower, and are often arranged in a
number of whorls one below the other, and then each group of
such organs has received a special name. All the male repro-
ductive organs (stamens) are collectively termed the andrceeeum,
while the female reproductive organs (carpels) form the grynse-
eeum. If the male and female organs are surrounded by the
same perianth, the flower is termed hermaphrodite, while a uni-
sexual (dicli?ious) flower will contain either stamens or pistil
only. The separation of sexes may be of such a nature that
both occur at least on the same plant (monaxious), or the separa-
tion is so thoroughgoing that each individual has only sexual
organs of one kind (dio.'cioiis).

^ At the commencement of this chapter we again point out that tlie reader
must not expect a description of the various modifications of flowers occurring
in the different natural orders of flowering plants. This book deals with the
functions of the flowers as a whole, and we shall discuss them in the case of
a flower in which they are most typically represented. The systematic
description of the peculiarities of the different families, orders, and genera
will be found in every text-book of botany, and we only mention here those
anatomical and physiological peculiarities which are necessary for a scientific
application of horticultural methods.

THE FLOWER 215

In most cases we are justified in regarding the reproductive
organs as leaves modified for a very definite object, and the
stamens have therefore been termed staminal leaves, while the
pistil may be looked upon as built up of one or more carpellary
leaves or carpels. The most important part of the stamen
are the pollen sacs, which run parallel on either side of the
median connective. Looking upon it as a modified leaf struc-
ture, the stamen represents a leaf consisting of a long and
delicate stalk {filament), and thick cylindrical blade {cmther),
into which the stalk is continued as a connective, the top of
which often projects above. On both sides of this eonneetive lie
the pollen sacs, two of which go to form eacli half of the anther.
The stamen has therefore four pollen sacs. The tissues in the
middle of each pollen sac will undergo certain divisions, and
the cells which ultimately result from these divisions round
themselves off one from another. They become covered by
a thick outer coat, which is often curiously sculptured, and
each cell is termed a pollen grain. These are generally libe-
rated by the longitudinal splitting of the anther, which opens
the pollen sacs. In some cases the anthers open by pores
{EricacccK).

The stamens have, therefore, in many respects, the same
structure as the normal green leaf, and we must, therefore, not
be surprised if at times, when there is a considerable tendency
for the development of green leaves, the stamens themselves
should become transformed into green leaves.

The anthers in that case do not become cylindrical, but are
thin and expanded, and coloured green. The loss of the
cylindrical shape of the anther generally entails the loss of the
power of forming pollen, and the stamen becomes a true green
leaf. This petpogressive tpansfopmation is at the bottom of
most doubling of flowers.

The female organ or pistil can be very well seen as the
central structure of the flower of the Tulip or the Cherry. It
consists of a dilated basal portion, the ovapy, a long tube-
like prolongation, the style, with a knob-like or lip-like ter-
mination, the stigma. We may conceive it to be formed by a
number of sessile leaves {carpels), the blades of which are drawn
out into points, and the margins of which have become fused

2i6 THE PHYSIOLOGY OF PLANTS

together. Of the three portions of the pistil, the lowest one,
the ovary, is the most important, as it contains the ovules with
their egg-cells. The stigma, which in some plants is very
extensive (feathery in Grasses, disk-shaped in the Poppy), may
be looked upon as an organ for catching the pollen, while the
style conducts the pollen tubes, which grow out from the
pollen grains, down to the ovules. In many cases there is no
style formed, and the stigmatic surface is directly on the top
of the ovary.

The sexual organs are, in most flowers, surrounded by the
leaves of the floral envelopes, which are either arranged in
several whorls or in a spiral manner. In complete flowers
the floral envelopes are sharply differentiated by form and
colour into two whorls of leaves, the inner one of which,
the eopolla, is built up of delicate cells, filled with a colourless
or a brilliantly coloured cell sap. The epidermal cells of these
leaves are often drawn out into papillae, and give the surface
of the petals a velvety appearance. The outermost whorl of
leaves, the calyx, is generally green, and resembles greatly in
texture, and often also in shape, the assimilating leaves.

If we cut through a flower longitudinally, we see that the
several whorls of leaves appear to spring from one point.
In reality they are, however, separated by short but distinct
internodes of the axis.

Each flowep represents, therefore, a reduced shoot with several
whorls of leaves, each of which is modified for a special func-
tion, but which, under certain conditions of nutrition, exhibit
the tendency to assume the nature of a green assimilating
leaf.

Such a retrogression of the flowering axis to a leafy shoot
we term proliferation.

The flowers occurring in some of the natural orders, how-
ever, diverge often very considerably from the typical flower
described above. One of the most frequent modifications
results from the disappearance of the differentiation of calyx
and corolla. The floral envelopes are completely alike, and
either resemble a corolla (Tulips and other Liliaceous plants),
or are green and have the appearance of a calyx {Juncacece
and Cupulifercv). In such cases we speak of a perianth.

THE FLOWER 217

Sometimes the sepals and petals may differ one from the
other in their appearance, but the leaves comjjosing these two
envelopes are spirally arranged, and gradually p;iss over from
one condition into the other, as is the case in the Water-lily
{Nymphwa), the Queen of the Night (Cereus), and the All-
spice (Calycanfhus).

Sometimes the petals are much reduced or transformed into
special organs for attracting insects by the secretion of honey,
and are then termed neetapies. In either case the calyx
will then undertake the functions of the corolla, and will
become brightly coloured, as, for instance, in the Larkspur
(Delphinium) and the Monkshood (Aconihim),

In some cases we find the food material available for the
development of the flowers is differently utilised in different
flowers of the same species. The female flowers, which re-
quire a large amount of nitrogenous food material for the
formation and development of their ovules, only produce a
very small and inconspicuous corolla, while the male flowers
use the food material for the development of a comparatively
large and corolla-like perianth, as may be seen in the case of
the Hemp.

On the other hand, we find occasionally a tendency in some
plants to produce a more elaborate corolla than is normally
formed. In such cases, at the point where the petal or tlie
perianth-leaf narrows down into a stalk, there will be pro-
duced on the inner surface of the corolla outgrowths of the
same texture and colour as the corolla, which form what is
termed the paraeoroUa or eopona. The Pheasant's-eye Nar-
cissus possesses a structure of this nature of a delicate red
colour, while in the Daffodil or Lent-lily it is larger, bell-
shaped, and more like the corolla in colour.

The thousands of variations which may occur in the form
and colour of flowers do not change the general conception of
a flower which we have put forward, namely, that the flower
is a modified shoot, the different leaves of which may become
transformed from one series of forms into another. It is the
business of the horticulturist to change the nature of the
floral leaves to suit his special purposes.

THE PHYSIOLOGY OF PLANTS

§ 42. How are single and how are double flowers developed?

If we examine the most perfect type of flower that is, one
provided with a calyx and corolla, we find the modified shoot,
which we term a flower, begins its development as a slender
meristematic papilla, at the base of which a number of small
ring-like outgrowths appear. The papilla, which first makes
its appearance, is the axis of the flower, and the first out-
growths from its young cortex {pcriUem) are the calyx
leaves. The first leaf rudiments grow more rapidly on the
outside than on the inside, and thus form concave structures
which arch over the apex of the axis and protect it. Pro-
tected by these leaves, another series of outgrowths appear
farther up the axis, agreeing in arrangement and number with
the petals. They are indeed the rudiments of the petals, but
they behave differently from the sepals in their development,
remaining rudimentary and scale-like for a long time, even
when the new outgrowths arising above them have been trans-
formed into stamens and carpels. It is only when the flower
approaches the period of expansion that the petals grow very
rapidly, become coloured, and by their increase in size burst
open the calyx.

According to this representation, therefore, the flower must
be conceived of as a shoot at the base of which the sepals are
formed, and which produces nearer its apex the other floral
leaves, the petals and stamens, while quite at the apex the
carpellary leaves arise and form the pistil. If we conceive the
axis which bears these floral organs to be made of a plastic
substance such as clay or putty, and imagine the apex of
the axis introverted, so as to form a cup-like structure, we
should find the pistil in the centre of this hollow receptacle,
and the other leaves higher up on the margin. This is the
case in the so-called perigynous and epigynous flowers, of
which Fig. 30 represents one in longitudinal section. It is a
young apple-blossom.

In this flower the actual apex of the shoot, which in the
previously described (hypogynous) flower occupied the summit
of the flower, is found at the base of the cup, and the cells

THE FLOWER

219

constituting it are shown in Fig. 30. The margin of the cup
is made up of the sepals (r), from the base of which springs,
as shown on the left-hand side, a tongue-like projection
directed upwards, one of the petals. Inserted at the same
point is one of the many stamens with only a very short fila-
ment. Lower down are seen two lyre-like arms, which are
the styles of two of the pistils,
the lower portion of which is
cut into on the right-hand
side and exhibits the ovarian
cavity, in which the seeds
would later on be contained.
The outer portion of the section
up to the point / consists of
the hollowed axis or recep-
tacle, which during the ripen-
ing becomes succulent and
represents the edible portion
of the apple. We see, there-
fore, that the apple is really
the cortical tissue of a succu-
lent shoot.

It is, therefore, not startling
to find that in some varieties
of pears the sweet and succu-
lent tissue extends some way
down the fruit-stalk. The
process of forming succulent
cortical tissue has only ex-
tended a little farther down-
wards in this case.

We can now also under-
stand how in some cases pears are formed which have no
core : they are derived from flowers on which no pistil was
developed, but in which the axis followed the normal course
of development.

In those cases in which the petals begin to elongate most
rapidly just before the flower opens, while the growth of the
calyx has been more rapid up to that period, the corolla

b"iG. 30.— Young stage in the De-
velopment OP THE Flower of
THE Apple.

2 20 THE PHYSIOLOGY OF PLANTS

functions usually only as an attractive organ. If, however,
the calyx is only small, and the corolla undertakes the function
of protecting the sexual organs, as is the case in the flowers of
the Vine, then it is developed as rapidly as the calyx and pre-
cedes in its development all the other organs.

On the other hand, it is rare for the calyx to undertake the
functions of the corolla. Instances of this have been mentioned
in the case of Aconitum and Delphiniujn. In some rare cases
the doubling of flowers depends upon this. Thus double
Primroses {Primula veris), Canterbury Bells {CamiMnula), and
Mimulvs become double by the transformation of the sepals
into petaloid leaves.

But it most frequently is the case in doubling flowers that
the calyx remains unaltered and that the number of petals
increases very considerably. This may take place by the
transformation of stamens into petals, as it occurs, for instance,
in the Eanunculacese. The tendency to produce leaf-like organs
may, however, be so great that each stamenal rudiment divides
into several pieces, and each stamen will therefore be repre-
sented by several petaloid leaves {Caryophyllacece). Sometimes
the stamens remain unchanged, and tlien the rudiments of the
petals are formed of such a breadth that these leaves divide
up into several leaves, as happens often in the Fuchsia. When
the flowers of Clarkia become doubled, an actual proliferation
takes place at the base of the petaloid leaves formed from the
stamens. In some cases indeed such proliferation may occur
on the axis itself between the staminal whorl and the petals
{Campanula).

In the case of Composites, the process of doubling is a
different one. What we generally call a flower is in this case
a basket-like head of flowers, i.e., a number of small distinct
florets, which with insufficient examination we should consider
as petals, are inserted on a cushion-like receptacle. The
florets may also be of two distinct kinds, as, for instance, in
the Cineraria, where the outer ones are brilliantly coloured,
the tube of the corolla being drawn out into a long tongue-
like structure formed by the fusion of the several petals {ray-
fiorets). The centre of the receptacle is occupied by incon-
spicuous tubular florets {dish-florets.) The so-called doubling

THE FLOWER 221

of sucli Howers may be brought about by the transfornuition
of the disk-tlorets into Horets sitnilar to the ray-florets, which
often, however, is accompanied by a deficient development of
the sexual organs. Similar transformations take place in the
production of the double Guelder Rose ( Viburnum Opulus) and
in Hydrangea. In the latter case, it is, however, the calyx
which is the conspicuously developed portion of the flower.

In the case of the capitulum of the Composites, the little
basket which encloses the flower, and which is often falsely
termed the calyx, is formed of a number of overlapping scales,
and should rightly be called the involucre. Besides these, a
number of bracts may be found between the several florets
and subtending them. Both these kinds of bracts may be
considerably enlarged and conspicuously coloured, and the
capitulum then has the appearance of a double flower. This
is the case with the " Everlasting Flowers " {Xeranthemum,
Helichrijsuin, Acroclinium roseum, and Rhodanthe Manglesii).
Such capitula very rarely divide, but occasionally small lateral
capitula make their appearance between the bracts of the
involucre.

The tendency to " double " is always less in the case of
plants with a corolla in form of a single tube {gamopetalous)
than when the corolla consists of a number of separate petals
(polypetalous) ; but among the latter there are some natural
orders which have no tendency to " double " ( Umhelliferce).
Speaking in general terms, we may say that it is easier to
produce double flowers in plants with radially symmetrical
than with bilaterally symmetrical flowers.

Double varieties have, however, been produced in the case
of Viola, Pelargonium, Impatiens, Pisum, Azalea, Lohelia,
Gloxinia, &c.

% 43. Can a gardener determine the development of flowers ?

This question really resolves itself into two questions : — i .
Can a gardener cause the formation of new flowering buds ?
and, 2. Can he cause a change in the course of development
of existing floral rudiments ? In answering these questions^

2 22 THE PHYSIOLOGY OF PLANTS

we cannot at present base our answers on precise experiments,
but we must make use of general experience, which has been
accumulated during years, or indeed centuries, of horticultural
practice. Science has yet to make many investigations into
such practical subjects.

In all cases in which the horticultural value of a cultivated
plant lies in its flowers, or in the resulting product of those
flowers, the fruit, it is desirable to increase as much as possible
the number of flowering buds. On this point we know by
experience that plants will only develop flowering" buds when
the food material formed in the leaves is copiously stored up in
the stem and branches as reserve material, and not when this
material is immediately used up in the production of new
vegetative organs (leaves).

In horticultural practice it is common to observe in the
case of perennial plants that a continuous and excessive for-
mation of leaves is detrimental to the production of flowers
(pine-apple and vine growing). Of our apple-trees it is well
known that in warm insular climates they grow into magnifi-
cent foliage trees, but remain unproductive of fruit. Growers
of cactuses will have found out that if the plants are copi-
ously watered during the winter in a warm house or room, they
will very rapidly produce new shoots, but no flowers. Many
Australian plants {Aktrosidcros, Cantua depe^idens, Co^^rea) are
generally kept in small pots by gardeners because they are
said to blossom better.

In all these cases it proves to be advantageous to prevent
the development of shoots and to bring about a complete
period of rest. The best means of doing this is by drought,
and under certain circumstances by diminution of the tem-
perature. We have already touched upon another method of
attaining the retention and storing up of food material by
ringing and notching the branches.

That a diminution of the supply of water accompanies the
production of flowers in nature may be gathered from the fact
that most trees and shrubs produce their flowers on short re-
duced shoots or spurs. The comparison of the anatomical
structure of such a short shoot with that of a long leafy shoot
confirms our statement, too, that an increase in stored food

THE FLOWER 223

material is necessary for the production of Jlowers. The
former shoots have far more storage tissue than the hitter ;
the development of the parenchymatous pith and cortex as
compared with the woody cylinder being very much greater
in the former. This is most conspicuously the case in the
development of the so-called " pouches " in some varieties of
pears, in which the spur is very short and almost succulent,
and has only half as much woody tissue as a leafy shoot of
the same age.

In forcing shrubs and trees, therefore, it is quite a rational
procedure to place the plants in pots quite a year before
the forcing begins, and to nourish them very plentifully, but
to begin to reduce the water supply towards the end of the
summer. In consequence of the gradually occurring drought,
the autumn rest and fall of leaves occurs at an earlier period,
and the changes which take place in the cells during the
resting period are as far advanced at the commencement of the
winter as they would be under ordinary circumstances in the
next spring. The withholding of water in such a treatment
prevents the use of the assimilated food substance for the
growth of new shoots and causes it to be stored up near the
buds.

The success in the cultivation of bulbs also depends upon
a period of dryness occurring at the proper time. A dainp
spring will cause the production of strong foliage in a well-
manured soil, and an early, dry, and warm summer following
upon this will prevent the leaves from growing too long, and
will cause the production of a large number of flowering buds.
If the summer continues dry, so that the leaves fade soon and
the bulbs ripen well, then we shall have richly flowering
hyacinths, tulips, &c., which will lend themselves to forcing.
The very copious manuring which has more recently been
applied to the growing and flowering bulbs does nothing more
than cause a luxuriant development of the flowers which are
already formed, but will not cause the production of a greater
number. Weak flowering buds, which, if badly nourished,
would not open, will freely expand if the plants are well
manured.

The use of manures will be beneficial for the solution of the

224 THE PHYSIOLOGY OF PLANTS

problem which horticulturists endeavour to solve, namely,
the production of changes in the various organs of the flower.
The desired change is generally the increase of the petals in
size and number at the expense of the sexual organs of the
flower.

All the processes which cause a doubling of flowers follow
one line of development, and that is a reduction of specialised
organs to their primitive leaf-like condition and the develop-
ment of new whorls of leaves in the corolla.

As we have already stated in speaking of the retrogressive
changes which may take place in flowers, all the diSereut
organs of the flower may become transformed into green leaves.
If we desire, therefore, a production of foliar organs in the
flower, we must use the means which are favourable to the
growth of leaves. The latter depends firstly upon the supply
of food material, and secondly upon the amount of water
available for its requirements. If there is a plentiful supply
of ibod material, especially of a nitrogenous nature, the multi-
plication of the cells will be a very active one. If at the
same time the organs are continuously and richly supplied
with water, the young cells will also be able to grow to their
utmost capacity. While for the production of flowering buds,
it is essential to decrease the supply of water and of nitrogen-
ous salts, to increase the phosphates supplied to the plants,
and to increase the illumination, double flowers require for
their development copious watering, an increase in the nitro-
genous salts in the soil, and, according to our experience, a
decrease in illumination. These factors, either singly or com-
bined, may be observed to have taken place where a leafy
development of flowers has taken place naturally, i.e., without
the aid or intention of man. On the other hand, we often find
that faults in the treatment of plants may produce a leafy
development neither desired by the grower nor desirable.
The proliferation of the catkins of the Hop, which is due to
the elongation of their axis and to the abnormal development
of leaves upon the latter, is caused by excessive moisture or
too rich a manuring with dung, or even with Chili saltpetre.

A change in the vegetative periods, too, has been shown
to produce doubling. In the case of the common Cineraria

THE FLOWKK 225

{Pericallis cruenta), weakly specimens which have not com-
pletely developed their inflorescence in the spring, and are then
bedded out in a shady place, will show in the course of the
summer some curious appearances of doubling. The connec-
tives of the stamens enlarge above the anthers and become
coloured like the petals, and the ray-florets develop excres-
cences which give them the appearance as if two ligulate
flowers were fixed back to back. Sometimes, too, adventitious
heads of flowers become developed in the axils of some of the
bracts of the involucre. These malformations are brought
about because the development of the flower takes place not
during the light and cool spring weather, but in shady places
with a considerable amount of moisture and heat during the
summer.

The effect of rich manuring in transforming flowers is best
seen in the case of tuberous Begonias. Here it is not only
the male flowers which have been doubled by the transforma-
tions of the stamens into petals, but the female flowers too have
become transformed in a similar way. These flowers, too, lend
themselves very well for the study of such transformation.
The branches of the style increase in number and expand like
leaves. Indeed, the transformation may be more far-reaching
still and extend down to the ovary. In this case the ovary
will open, and the ovules will protrude into the flower like
small white grains. Indeed, when the style and the placentae
themselves are transformed into leaves, the ovary will have
seemed to have disappeared, and the ovules will cover the base
of the petals as a fine white powder. The ovules themselves
under the microscope will often show a partial transformation
into leaf-like organs.

Such examples demonstrate sufficiently well the lines along
which the gardener has to work to secure changes in the
development of the various floral organs.

CHAPTER XII

FRUITS AND SEEDS

§ 44. How are fruits and seeds formed ?

In discussing the structure of a complete flower, we mentioned
that the centre of each flower was occupied by the pistil,
which consists of an ovary, a style, and a stigma. Of these
three parts the stigma is the apparatus for catching and
retaining the pollen, the style the channel along which the
pollen tube grows when the pollen grain germinates, and
the ovary is the receptacle where the ovules are produced and
develop into the seeds. The pistil is either formed of a
single carpellary leaf, the margins of which have been folded
in and have become fused, or several leaves arranged in a ring
have become fused together, in which case usually the ovary
will consist of several chambers. As already mentioned, the
tube-like elongation between the ovary and the stigma, the
style, is relatively of little importance, and is not developed in
some plants, the stigma being situated directly on the ovary.
Such is the case in the pistil of the grass represented in
Fig. 3 1 . Here the stigma is feathery and very delicately
developed ; its numerous filamentous branches rendering it
very efficient in capturing the pollen grains, which are easily
carried about by the wind. The main branches (g) which bear
the feathery processes are directly continuous with the tissues
of the ovary (/), which presents only a single cavity, the
latter being completely filled by the single ovule (o). Tiiis
consists of a very delicate tissue, which is differentiated into a
central portion and an enveloping layer. This latter is formed
by two integuments (i), which do not completely close in the
central nueellus, but leave a very small passage open to the
latter. This opening is termed the micpopyle.

FRUITS AND SEEDS

227

If some pollen grains chance to adhere to the feathery
stigma, they develop a long tubular process by the protrusion

Fig. 31.- Young Pistil of the barley.

St, stamen ; /, ovary ; n, stigma with feathery branches ; I, the tissue tlirough whicli the
pollen tube gi-ows; 0, ovule ; /, the integuments ; m, raicropyle ; e, embryo sac.

of the delicate inner coating (intine) through the thin portions
of the hard outer coat (extine). This pollen tube makes its

22 8 THE PHYSIOLOGY OF PLANTS

way down through a special conducting tissue (/) into the
cavity of the ovary, and then through the micropyle (m) of the
ovule. The more detailed representation of these processes
is shown in the subsequent figure (Fig. 32). In the preceding
one we were chiefly concerned with the general position of the
ovule in the ovary, at the side of which a young stamen (st) is
represented, and the figure was also intended to illustrate the
delicate and feathery structure of the stigma.
. The actual process of fertilisation in the receptive ovule
is represented (after Kny) in the case of the ovule of the
Pansy (Viola tricolor). The ovule has been detached from the
ovary wall at the point 'p, the placental reg-ion, which pro-
duced the ovule. The vascular bundle (r) of the placenta
is continued for the purposes of nutrition into the base of
the ovule. The tissue in which it terminates in the ovule
has been termed the chalaza. The nueellus is enclosed by
the inner integument {J J) and the outer integument {A J),
which however leave the small canal or micropyle open. The
pollen tube has penetrated into this passage, has grown through
the apex of the nueellus (I^Jl^ towards a large and almost
cylindrical cell, the embryo sae.

The embryo sac has become developed from one of the cells
of the nueellus by considerable eulargement at the expense of
the neighbouring cells, which broke down into mucilage. It
contains at the commencement a large nucleus, which divides,
the daughter-nuclei progressing to the two extremities of the
embryo sac. There, by repeated division, a group of four nuclei
results. A later stage of development is shown in the case of
the embryo sac of Polygonum divaricatum (Fig. 3 3 , after Stras-
burger). Two of the cells (s) at the apex of the embryo sac
exhibit each a large vacuole and towards the apex a nucleus ;
the third one (e) has the nucleus at the base and the vacuole
at the apex.

It is only this latter cell, the egg-cell, which gives rise
after fertilisation to the young plant (emhri/o). The other two
cells, characterised by the striped appearance they represent,
have been termed the synergidse. The group of three cells
is called the egg apparatus.

The cells at the opposite end of the embryo sac are sur-

FRUITS AND SEED8.

229

£«■/.;)

Fig. j2.— Longitudinal section through an ovule ok Viola trkolor {after Kny).

PS, pollen tulie; AJ, outer integument; JJ, inner integument; K\V, nucellus; /?i;i//,embiyo ;

Ends2>, enilosperni ; p, placenta; r, flbru-vascular bundle ; .>., stumata.

2 30 THE PHYSIOLOGY OF PLANTS

rounded by a thin cell-wall, and differ in this respect from
the cells of the egg apparatus. But we have stated that there
were four cells at each end, and we have so far only spoken
of three ; there remains, therefore, one nucleus at either end.
These will, however, be noticed to move towards the middle of
the embryo sac, and there to fuse into a new nucleus the
secondary nucleus of the embryo sac.

Having reached this stage of development, the constantly
enlarging embryo sac is now receptive, and the pollen tube
makes its way through the nucellus down to the embryo sac.
In some cases the pollen tube has less resistance to overcome,
as the tissue of the nucellus lying above the embryo sac be-
comes disorganised (Zabiata:), while the wall of the embryo
sac, lying directly below the raicropyle, breaks down or is broken
down by the synergidpe.

Before reaching the entrance to the micropyle, the develop-
ing pollen tube may find certain arrangements which make
its progress more easy. Thus we find ridges, or papillae, or
glandular excrescences, or specially loose tissues in the style
which lead to the ovules, and are therefore able to direct the
course of the growing pollen tube. Every ovule of the ovary
requires a special pollen tube for its fertilisation, and as there
are enormous numbers of small ovules in some ovaries (Orchi-
dacecc, Ericacew, Gesneriacecc), we can well perceive how it is
that sometimes bundles of innumerable pollen tubes will be
met with in the style, visible to the naked eye as a thread of
delicate silky hairs.

During this period of active growth the pollen tube is thin-
walled, and its basal portion, which is next to the pollen grain,
is transparent, because all the contents are collected near the
apex. As soon as the pollen tube enters the micropyle, it
begins to swell up and becomes mucilaginous. This process
of swelling up and softening of tissues, as we have seen above, is
no uncommon occurrence in the case of the nucellus, or indeed
of the wall of the embryo sac. It enables the contents of the
pollen tube to pass more easily into the egg-cell lying at the
apex of the embryo sac.

The transference of the fertilising contents of the pollen
tube must take place by a diffusion through the softened

FRUITS AND SEEDS

231

membrane, as a perforation or disruption of the cell-wall has
not been observed. One can only see that the contents of
the pollen tube become squeezed towards the embryo sac by a
constricting of the micropyle, and that the tube becomes firmly
applied to one of the synergidtc. Then the nucleus and vacuole
of this cell disappear ; it and its sister-cell (Fig. 33, s, s) be-
come dim and granular, lose their shape, and are ultimately seen
attached to the egg-cell as colourless and irregular viscid masses.

This completes the act of fertilisation, the immediate effect
of which is that the egg-cell (e), having temporarily two
nuclei (which afterwards fuse), surrounds itself with a delicate
membrane.

This stage of the development is shown in Fig. 32. Emhr
represents the fertilised egg-cell, and we
notice that during or shortly after fertili-
sation several changes have taken place
in the embryo sac. Before fertilisation,
as is mentioned above, a fusion of one of
the nuclei of the micropylar end with
a nucleus from the antipodal end has
given rise to a single new nucleus near the
centre of this embryo sac. Now by repeated
divisions of this nucleus, a large number F's- 33-

of nuclei (Endsp) have been formed, which
are chiefly found in the layer of protoplasm lining
the inner wall of the embryo sac. Here each nucleus
becomes the centre of a new cell, and by further division
the embryo sac becomes entirely filled by a new cellular
tissue. This tissue, which fills the entire embryo sac when
the latter has completed its growth, is called the albumen
or endospepm. It is the tissue in which are stored up the
reserve substances which have to nourish the young embryo
during its further growth, and it forms the greater part of
some seeds. In the case of the wheat-grain and other grass-
seeds, we see the small embryo attached to one end of the
seed, and the rest of the tissue filling up the former eml)ryo
sac is the starchy endosperm, very much more considerable in
bulk than the embryo. If we imagine the centre of this
mealy endosperm to be hollow, and not filled up with cells, we

232 THE PHYSIOLOGY OF PLANTS

should have a picture of what actually occurs in the cocoa-nut.
The hollow centre of the cocoa-nut, in which the milk is con-
tained, is nothing else than the central cavity of the embryo
sac, which has grown to so enormous a size, and becomes only
partially filled with endosperm cells, forming the edible coat
of the nut.

But all seeds do not possess a large mass of albumen at the
side of or around the embryo ; many seeds are exalbumiuous.
In such cases the development of the embryo greatly exceeds
the growth of any of the other parts of the ovule; the cells
of the endosperm become absorbed again, and are made use of
by the developing embryo, which grows so large that it fills
up the entire cavity within the two integuments. We then
get a seed of the same kind as that of the bean. In this case
the former integuments of the ovule have formed the hard
seed-coat (testa), and the latter encloses the embryo, which was
formerly represented by a single cell at the apex of the embryo
sac. The dark oval patch on the glistening testa is the point
at which the stalk of the ovule (funiele) was attached, fastening
the young seed to the placenta. This mark is termed the
hilum of the seed.

The main portion of the seed consists of the two large seed-
leaves or cotyledons, which will be found attached one to the
other at the base, the attachment being continued into a small
papilla (the radicle) at one end, while between the cotyledons
is enclosed the embryonic shoot (plumule) with its rudimentary
leaves. That these parts are really the young organs of the
plant can best be seen on the germination of the seed. The
seedling then bursts through the testa, the young root apex
growing down into the soil. Then the two cotyledons expand as
succulent leaves, and between them grows up the young shoot.
As the latter increases in size the cotyledons become poorer
in contents, wrinkled, and ultimately shrivel up and fall off;
they have performed their duty in supplying the young root
and shoot with the food material with which they were filled.
The cotyledons, therefore, have the same function in the exal-
bumiuous seeds which the endosperm performs in the albu-
minous seeds, namely, to nourish the seedling during the early
stages of its growth.

FRUITS AND 8EED8 233

The stimulus which the process of I'ertilisatiou imparts to
the ovule, and which results in the development of the seed, is
also transmitted to the tissues of the ovary wall. This is best
seen in the case of the flowers of pears and apples, where we
can distinguish a very few days after the flowering, with a con-
siderable degree of certainty, which of the flowers have been
fertilised. In these the flowering stalk is much more turgid
and begins to thicken very soon.

With the progressive development of the seed changes also
take place in the carpels which bear them, so that the green
ovary becomes changed into the ripe fruit. The wall of the
ovary often becomes differentiated into three layers : the
outermost layer {epicarp), a median one (mcsocarp), and an
inner one {endocarp). If they all three become hard and dry,
the fruit is termed a dry fruit (hazel-nut, pod of pea, or
capsules of lily and poppy). If, however, one or more layers
become fleshy, then we speak of succulent fruits (gooseberry,
fruit of the melon, drupe of cherry and plum). The cherry-
stone is not the seed of the cherry, but the hard inner
portion of the ovarian wall, and encloses the actual seed.
It is also important to know that the fruit of the pear and
apple is what is termed a false fruit or pseudo-carp, in the
formation of which the axis of the flower or receptacle plays a
part. In the case of the strawberry, too, we really eat the
fleshy receptacle upon which the actual fruits are borne as
numerous small nutlets. In the case of the raspberry and
blackberry, however, we are dealing with a number of true
succulent fruits which are crowded together on a common
receptacle. In the case of the pine-apple, mulberry, and fig,
the entire inflorescence becomes sweet and succulent. The
spiny coat surrounding the nuts of the Spanish chestnut is not
the ovary wall, but an involucre or cupule made up of four
bracts, and of the same nature as the cup of the acorn.

§ 45. How can the formation of the fruit be influenced by-
different methods of cultivation?

At the end of tlie last chapter we dealt with the mode of
formation of fruits and of fruit-like structures {p)sciido-carps),

234 THE PHYSIOLOGY OF PLANTS

which ill general parlance are termed fruits. Now we shall
discuss the processes by means of which the character of the
fruits may be changed and improved.

In the case of the fruits used for dessert, it is essential to
increase their number, size, appearance, and flavour.

An increase in the number of fruits demands an increase in
the number of flower-buds, and as these are generally formed
in the preceding season, the horticulturist must extend his
operations to that period. Everyday experience teaches us
that the period of sexual reproduction does not, as a rule, set
in until the vegetative growth (production of leaves) begins to
diminish or to cease entirely. The function of the leafy tissue
is to form new organic food material from the raw material
contained in the soil as inorganic salts and in the air as carbonic
acid. The stronger the young embryo, the stronger will be the
growing seedling — that is, the active plant. The larger and
more numerous the organs and the greater the quantities of
raw material which they can assimilate in a given time, the
greater will be the number of new assimilating organs which
the plant will develop. The longer these organs function, the
greater will be the amount of substances assimilated.

The duration of the assimilating period, and the way in
which the products are used up, depend partly upon the
season in which the vegetative period falls. In the case of
plants left to themselves, the germination of the seeds and
the development of leaves in trees and shrubs takes place
in spring, when the soil is moister, but during which period
there is less light and heat than in summer. This combination
of circumstances proves to be very beneficial for the develop-
ment of leaves. On the other hand, the foliage is smaller, the
shoots shorter, and the number of lateral branches smaller, and
their development less extensive, if the vegetative period is
artificially postponed until the summer, or if the spring is
abnormally dry and warm. The plant will under such cir-
cumstances very soon come to an end of its vegetative period,
and begins at once to use the assimilated substances for the
period of sexual development.

If, however, the first period is lengthened by artificial shading,
copious watering, and rich nutrition, or if a moist spring is

FRUITS AND SEEDS 235

followed by a damp, cool, and sunless summer, then the period
of leaf production will never really come to an end. In such
a case the plant uses the greater portion of the assimilated
matter for the further development of its leaves, i.e., for the
increase of its leafy tissues, and there is generally no opportunity
for storing up the large amount of reserve material necessary for
the production of flowers. In the case of our trees, we may also
observe that the formation of fruits is not commenced in the
earlier years of their growth, when the trees are occupied with
the development of their crown. Oaks generally attain an age
of fifty years, pines forty years, firs twenty years, and our fruit-
trees, with artificial aid, twelve or fifteen years before the
development of new branches has so far abated as to allow
the formation of short flowering shoots to take place. Only
when this youthful period of life is over does the apical growth
of the branches cease at the proper period and sufficient nutri-
ment is stored up in the axis. The greater the number of
leaves on each shoot, the larger will be the amount of food
matter stored up. In the case of plants, therefore, in which
we desire to increase the number of flowering buds, our method
of cultivation must aim at producing as large a number of
leaves as possible, and at giving them a sufficient time for
assimilation. But we must also ensure a period of rest at
the right time.

These conclusions, drawn from daily experience, will enable
us now to criticise the treatment of our dwarf and trained
fruit-trees. We can now see why, in many cases where
the branches are well trained, few or no flower-buds make
their appearance. The careless pruning of the branches in the
summer, and the too frequent pinching off of the tips, stimulate
many of the buds which should not develop till next year
to grow out prematurely. The tree exhausts its forces, there-
fore, in the production of new shoots without retaining the
leaves long enough to produce the requisite amount of food
material. We have already touched upon this point in speak-
ing of the treatment of shoots.

To annuals and biennials the same rules apply as to shrubs
and trees, but we are not able here to correct the faults of
one year in a subsequent season. We must remember here

236 THE PHYSIOLOCxY OF PLANTS

from the beginning that a very large number of flowers can
only be attained by the continuous functioning of a very large
amount of foliage. Assuming therefore that the soil is rich
in nutritive salts, as should always be the case in successful
horticulture, we must aim at producing as early a germination
of the seeds as possible. Furthermore, the development of the
leaves must be helped on by quick-acting manures containing
potassium and nitrates, which should be applied as top-dressing.
But above all, there should be a copious supply of water to
aid in the elongation of the cells. These artificial aids should
gradually be decreased as the hot season approaches. The
more powerful effect of the sun and the decreasing amounts of
water will diminish and ultimately stop the production of new
leaves, and give the plant the opportunity of forming flowering
buds and help on the ripening of seeds and fruits.

It depends entirely upon the specific peculiarities of the
plant and upon the aim of the gardener as to whether any
artificial aid should be resorted to after the flowering season.
If the production of seeds is the chief aim of cultivation, it
seems best to leave the plants to themselves and to let the
food material which has been formed by the leaves in the pre-
ceding vegetative period, but which has been stored up in the
stem, pass over into the ripening seeds. This passage from
the older to the younger portions of the plant takes place very
slowly, the leaves becoming gradually poorer in food matter
and turning yellow. Manuring during this period is directly
injurious.

If, however, it is the aim of the horticulturist to produce
luscious and edible fruit, in which either the ovary wall
{pericarp) or the receptacle is to become succulent and sweet,
it will be very beneficial to accelerate the natural processes
by artificial means. For in this case it is not directly the
sexual organs which have to be nourished by the already formed
food material, but it is the growth of part of the vegetative
structure of the plant which has to be promoted, and there-
fore those stimuli may be resorted to which tend to produce
foliage leaves.

In the case of dessert fruits, the full development of the
succulent tissues, their tenderness, lusciousness, and sweetness

FRUITS AND SEEDS 237

are objects of attainment. It has so far not been proved that
a fruit will develop many new cells during its growth to
maturity. This may take place to a limited extent ; but from
the fact that large fruits do not contain many more cell-layers
than small fruits of the same variety, we may assume that the
differences in fruits are mainly due to a difference in the
development of the cells. The size of the cells, however,
and the amount of their contents are exceedingly variable.
The development of the fruits can therefore only be acce-
lerated by such means as increase the elongation of the cells,
and cause weak cells to develop healthily. In horticultural
practice, therefore, repeated watering and moderate application
of liquid manure is resorted to.

The effectiveness of this procedure seems to be greater the
more the organ which develops into the fruit resembles a
vegetative axis. Liquid manure (in small quantities), or
water alone, prove therefore especially effective in the swelling
of apples and pears, figs, pine-apples, and strawberries. The
apple may be looked upon as a cup-shaped apex of a stem,
in which the carpels are sunk. The succulent portion of the
pear or apple is, therefore, mainly an excessively succulent
cortical tissue, as can best be seen in those cases in which the
fruit is continued on one side down the fruit-stalk. In the
case of the strawberry, the succulent axis is not hollow, but
forms a conical protuberance, and in its development we may
sometimes see the bad effects of too copious a supply of water
or nutritive salts. Many flowers remain sterile, and the
smaller number produce enormous fruits, which often become
hollow at their centre. The elongation of the cells is so ex-
cessive that a considerable tension exists at the centre, and
the cells separate one from another, as is the case in other
axial structures under similar circumstances (turnips and
potatoes).

In the case of stone-fruits, a judicious supply of water will
also aid in the swelling of the fruits, but here considerably more
care is needed in selecting the proper time for such operations,
as otherwise the fruits are liable to be thrown off. Especially
in the case of peaches, apricots, and plums it seems desirable
to abstain from the application of liquid manure or from

238 THE PHYSIOLOGY OF PLANTS

watering until the fruits have attained about one-half of their
normal size, and until the sclerenchymatous cells which form
the stone are sufficiently developed. During the period in
which the stone is formed the fruit does not increase much in
size, which seems to indicate that the fruit does not during
that time require so much water. It is just at this period
that the fruits drop if the plants are copiously watered. One
should also guard against beginning watering too suddenly in
case the trees have been passing tlirough a considerable period
of drought, which may have prematurely coloured the fruit.
A sudden and copious supply of water may cause a total fall
of the fruits ; the water supply has to be begun gradually, so
as to get the tree used to the more active transpiration which
this necessitates.

We may here add, that the usual treatment of plants
which have been suffering from drought is a wrong one ; as
a rule, they are copiously watered, shaded, and, where it is pos-
sible, they are placed in a saturated atmosphere. One forgets
that the plant has become unaccustomed during the period
of drought to any great transpiration, and this must gradually
increase before the water can be properly absorbed. The
transpiration can, however, be increased by placing the plant
after the first watering in a light place and in a dry atmosphere.
We can easily detect in a plant placed on a balance that the
amount of transpiration gradually increases and the plant will
recover, whereas a plant suddenly swamped with water and
placed in a dark damp place will recover very slowly indeed,
or may die altogether.

i; 46. What are the conditions governing the production of seeds ?

The ovules become changed by the act of fertilisation, i.e., after
the contact of the pollen tube with the egg-cell, and develop
into seeds. Without fertilisation no seeds are ever developed ;
but a fruit, especially a pseudo-carp, formed by a succulent
axis, may develop without any fertilisation having taken place.
Thus pears may become fully formed by abnormal development,
and will be found to be without a core, but edible.

FRUITS AND SEEDS 239

Fertilisation is dependent upon pollination, i.e., upon the
transference of pollen grains to the stigma of a similar
flower. Seeing that the gardener has in many cases, in order
to obtain seeds, to proceed to artificial pollination by transfer-
ring, with the aid of a fine brush, the pollen to the stigmatic
surface, we may point out that there are many hindrances
to this method of procedure. In nature, too, we may often
observe that the pollen from opening stamens may fall upon
the stigma of the same flower (self-pollination) but remains
without effect. This is the case in so called dichogamous flowers,
in which the two kinds of sexual organs ripen at different times.
In some cases the anthers ripen and disperse their pollen before
the stigma of the flower is receptive. Such plants are termed
protandrous (Geraniaceae, Malvacete, Compositse, &c.), and fer-
tilisation can only be effected with the aid of a second, younger
flower, by either natural or artificial means. In the reverse
case, namely, in that of the protogynous flowers, the stigma
which ripens before the stamens requires to be fertilised with
pollen from an older flower, because its stamens are as yet not
fully developed (some Graminea3). But even in the case of
both sexual organs developing at the same time, self-pollination
may often be difficult, if not impossible, because the stamens
are not long enough to reach the stigma. This is the case
with dimopphie flowers. In this case a plant will develop
hermaphrodite flowers of two kinds (Primrose). Some speci-
mens liave a long style, so that the stigma will reach to the
top of the corolla tube {pin-cijed). The stamens, however,
will be short, and remain hidden within the tube of the
corolla. In other specimens the reverse will be the case
(Jieterostyly).

Nature seems here to point out the way in which fertilisation
is to be effected. The flowers with long stamens are intended
to pollinate the long-styled forms. That this course of pollina-
tion is attained with good results has been proved experi-
mentally by Darwin. He found that the production of
seed is small, or sometimes even nil, when one (short or long-
styled) form is pollinated with its own pollen, or with pollen
from a similar flower, but that a large production of seed
results from the pollination of a long-styled form with pollen

2 40 THE PHYSIOLOGY OF PLANTS

from a short-styled form.^ We might add to these instances
of heterostyly a description of other mechanisms of flowers
which render it necessary for a plant to have special carriers
or distributors of pollen (fertilising agents). We need only
refer to the compact pollen masses (pollinia) of the Orchids,
which could never reach the stigma of their own accord, and
to the monoecious and dioecious plants. In the case of the
latter, e.g., the Conifers, Willows, Poplars, Oaks, &c., the wind
will transport the pollen from flower to flower. Such flowers
are therefore termed anemophilous, in contradistinction to those
in which insects involuntarily, no doubt, perform the pol-
lination (entomophilous). This is the case in the Orchi-
dacese, AristolochiaB, and many Papilionacese and Labiatae. In
some few water-plants (Vallis7ieria, Ceratophyllum) the water
carries the pollen from one flower to the stigma of another
one (hydrophilous).

Especially in the case of entomophilous flowers do we find
the most curious mechanisms for the employment of insects
for the purpose of pollination ; but we must content ourselves
with a single instance.

In the year 1885, the Society for Acclimatising introduced
into its gardens in Canterbury, New Zealand, the first humble-
bees. In the next year these insects had considerably increased
in numbers, and it was then found that the common red clover,
which had been introduced into that country, but had formerly
only produced very few seeds, now yielded very good seed-
crops. The humble-bees, which in this country fertilise the
flowers, had till then been missing.

Bees and humble-bees play a very important part in the
fertilisation of flowers, carrying the pollen from flower to
flower by means of their hairy bodies. They sometimes, how-
ever, bore holes into flowers, and then obtain the nectar or
honey without pollinating the flower. This is the case in the

1 The pollination of heteromorphic forms with pollen from a similar form
of flower has been designated by Darwin an "illegitimate union." The
legitimate union consists of the pollination of flowers with pollen from a dis-
similar form. Such a beneficial result of cross-fertilisation has not been uni-
versally proved — indeed, in some cases self-fertilisation results in a plentiful
crop of seeds ; yet gardeners will do well in all cases to transfer pollen of one
flower on to the stigma of another flower.

FRUITS AND SEEDS 241

Columbine {Aquilegia), the spurs of which are perforated by
humble-bees. The same is doue by them in the flowers of
Weigelia, whereas the honey-bee enters the flower. In the
different species of Orchis, the spur will often be found ripped
open, and in Tropccolum majus there are often two or more
holes in the spur, made by these insects.

Water-plants, too, are fertilised by insects, e.g., Nymphea,
Nupliar, Utricularia, Aldrovandia, Hottonia, Stratiotcs, Trcqja.
This, however, is only the case when the brilliantly coloured
petals are expanded above the water. If the flowering stalks
remain short, so that the flowers remain below the water, then
eleistogramy is resorted to. In this case small rudimentary
hermaphrodite flowers are formed which do not open, and must
therefore of needs be self-fertilised. In such cases self-ferti-
lisation is generally very effective, and in some cases these
cleistogamous flowers are the only fertile ones, while the con-
spicuously coloured and open flowers remain sterile. Among
land plants, the Violet is the most well-known example of
cleistogamy. Of our wild flowers, the Dead Nettle {Lammm),
the Eest-harrow (Ononis), the Toad-flax (Linaria), the Wood-
sorrel (Oxalis), and others, are provided with cleistogamous
flowers, which are not only smaller, but have rudimentary
petals devoid of scent and of nectaries.

Attention must be paid to all these conditions for any
success in one of the chief lines of horticultural enterprise,
namely, in the development of new varieties by crossing. In
spite of many successes, we must admit that the latter are due
more to chance than to any carefully planned method. It is
true that in this field science has as yet only a few data to go
upon and no general principles to lay down, and it might
derive much benefit from the observations of horticulturists.
In horticultural practice it is generally not possible to take all
the precautions of removing the stamens from the flower which
is to be fertilised, to protect the flower after fertilisation, and to
keep a detailed account of all the cross-fertilisation performed,
and very often an assumption of certain processes having
taken place has to do duty for actual observation. Of course
this leads to mistakes. If we acted quite honestly, we ought
often to assert that the origin of certain cultivated forms is

Q

242 THE PHYSIOLOGY. OF PLANTS

doubtful. The greatest certaiuty exists, of course, in those
cases in which specialists are occupied with a single genus or
species. Then the repetition of former results may take the
place of experiments performed with all the precautions which
ought always to be taken when a cross-fertilisation can only
be attempted once.

The Orchids seem to be a group especially well adapted for
such experimental work. They are now-a-days the fashion-
able flowers, and their fertilisation is carried out by gardeners
with great care ; besides which, their flowers can only be ^
fertilised by very special insects, which do not exist in this
country. Fertilisation by wind or insects while the plants
were under cultivation in Europe has only been recorded for
Phajus grandifolius, Aerides afflne, Vanda Eoxhurghii, Lcvlia
cinnabarina, Cypripedium ScJilimii, javanicum, virens, B^dleni-
anum. The signs which indicate that a flower is really
fertilised do not always appear in the same way or after the
same lapse of time. Gardeners must therefore not be too
rash in drawing conclusions from the non-appearance of
certain signs. The most well known of these is the swelling
of the ovary, which takes place in some cases very rapidly,
as has been observed in Oncidium {Papilio Forhesii), Odonto-
glossum, Lcclia, Cattleya, Phal<xno23sis, &c. Other orchids close
their flowers the day after the fertilisation has been effected.
Cypripedium remains fresh for a considerable period, and its
ovary shows no sign of swelling.

On the other hand, it must be pointed out that changes
which generally take place after pollination are not always a
sure sign that fertilisation has been effected. Of these, the
most obvious one is the growth of the pollen tube down the
style of another species or genus. Strasburger, for instance,
mentions that the pollen tube from a pollen grain of Lathyrus
montanus grew down into the ovarian cavity of Convallaria
latifolia, while tubes from the pollen of Agapanthus grew deep
down into the style of Achwieiies grandifiora (but not vice
versa). The pollen tubes of Fritillaria persica are said to
grow into the ovarian cavity of orchids, and to cause the
development and swelling of its ovules.

We have no rules of genepal application for hybridisation.

FRUITS AND SEEDS 243

We can at present only say that hybridisation is more easily
effected the more nearly allied the parent plants are, whether
varieties of the same species or species of the same genus.
We might explain this by saying that the substances con-
tained in the pollen tube, and which have to act upon the
egg-cell, will be most similar in the case of kindred forms,
and will therefore more easily combine. But we have all
sorts of curious exceptions. Thus it has been observed that
the pollen of Orchis Morio produces no tubes on the stigma
of Orchis fusca, while the pollen tubes of Orchis fusca not
only penetrate to the ovules of Orchis Morio, but effect a
fertilisation of the egg- cell.

With regard to the fertility of hybrids, it was formerly
held that their vegetative vigour was accompanied by sexual
weakness. In some cases the stamens, in others the ovules,
would be incompletely developed. Their fertility was, how-
ever, said to increase if they were fertilised by one of the
parent forms and became again more like it. These state-
ments must not, however, be regarded in the light of a fixed
rule, for they hold good, too, in the case of the production of
new varieties — that is, when flowers of the same species are
fertilised with pollen of the same species.

Our entire method of cultivation tends to increase the steri-
lity of our cultivated plants.

This is caused by the endeavour to grow vigorous plants,
in which the vegetative growth is stimulated by abundant
water and food supply, until the functioning powers of the
various organs are taxed to their utmost limit. Our culti-
vated plants exhibit the most luxurious foliage, the most
rapid growth, the most succulent tissues, the largest flowers,
and the most juicy fruits. But this all entails a retrogression
of the sexual reproduction, as is shown by the proliferation
and the doubling of flowers, in both of which processes the
stamens and carpels are transformed into sterile leaves.

Such tendencies are inherited, and are manifest in the
offspring of all the most luxuriant of our cultivated plants,
without having been caused in any way by hybridisation.

244 T^HE PHYSIOLOGY OF PLANTS

§ 47. How should the ripe seed be treated ?

The product of fertilisation, tlie seed, contains, as chief
constituent, the young plant or embryo. The latter is gene-
rally so far developed, that it shows the embryonic root or
radicle and the young shoot or plumule. Nature has also
provided for the early nutrition of the plant before the root
begins to function. The seed is for this purpose provided
with a mass of food material, which only requires to become
dissolved in order that it may be at once made use of by
the embryo. This food material, which is stored up chiefly
as starch or oil, is either contained in the seed-leaves or
cotyledons, if the embryo alone fills the seed, or in a special
parenchymatous tissue, the endosperm or albumen, in which
case tlie embryo is generally small as compared with the size
of the seed. In this latter case special arrangements are
made, by which the absorption of the dissolved food material
by the embryo is as easily effected as if the food material
were stored up in the cotyledons.

Besides their economic importance as food materials for
man, the seeds must be considered also with regard to the
future production of new plants. The chief value of seeds
indeed is the reproduction of the species to which they belong.
This reproduction is sometimes effected immediately after the
ripening of the seeds, but generally they pass through a
resting period of some length. We must, therefore, first con-
sider the resting and then the germinating* seed.

The fully mature and garnered seed is by no means lifeless.
A number of insufficiently known changes take place within
it, which cause it to give off water and carbonic acid, and also
to change its colour. We must also assume that during the
resting period ferments are formed which cause the rapid
solution of the food material during germination. This may
be gathered from the fact that seeds of the same kind and of
the same crop will under similar conditions require a longer
time for germination, the further off they are from the normal
time of germination.

The chief condition for securing germination is the requisite
supply of water, besides an increase in temperature and in the

FRUITS AND SEEDS 245

supply of oxygen. If, therefore, we wish to keep seeds healthy
during their period of rest, we must avoid dampness. The
latter will be the less harmful the lower the temperature is
and the less oxygen there is present. If seeds are not properly
attended to, they become musty and ultimately mouldy. If
we have the choice between a warm place liable to consider-
able variations of temperature and a uniformly cool place,
we ought always to choose the latter, regardless of the fact
that the seeds might be exposed to a frost. Most seeds of
our cultivated plants will resist comparatively low tempera-
tures if they are kept completely dry The storehouses for
seeds should be dry, cool, and airy.

During the process of germination three definite periods
may be recognised. First there is the swelling of the seed.
This process may be looked upon as a more or less mechanical
one, and is accompanied by a rise of temperature (due perhaps
to the condensation of moisture). This absorption of water
brings about the second phase, the solution of the food material,
which is a chemical process caused by the action of certain
ferments, and causing the third change to take place, namely,
the increase and expansion of the embryo.

The rapidity of the germination depends mainly upon the
rapidity with which the water is taken up by the testa. In
some plants the epidermal cells of the testa, and sometimes
also the adjoining cells, have a palisade-like arrangement, and
take up water so rapidly that in a few hours they form a
mucilaginous coating to the seed. This is the ease with the
seeds of pears and quinces, and with linseed. In other cases,
the structure of the testa is so firm, and the cuticle covering
its epidermal cells renders it so impervious to water, that
perfectly sound seeds may lie for years in a damp soil without
germinating. This is often the case in leguminous plants
(Bohinia, Clover, &c.). In the case of the latter it has been
shown that the palisade-like epidermal cells of the testa, which
contain the colouring matter of the seed, are so impervious to
water, that, after lying in water for several years, the seeds
were still alive and had not germinated.

Sometimes the loss which occurs owing to the non-germina-
tion of a considerable percentage of the seeds may be avoided

246 THE PHYSIOLOGY OF PLANTS

by sowing the seeds very soon after they have ripened. Tiiis
may be done with advantage in the case of Taxus, Ilex,
Magnolia, and Auricula. In the case of strawberries the fruits
should be dried in the sun and then be crushed by rubbing
them between the hands. They should then be sowed directly,
after having been mixed with a little finely sifted earth. If
seeds are to be germinated which usually require a considerable
time until their testa is soaked, it is best to treat the seeds
by some mecbanical process before sowing them. Such seeds
may be mixed with fine sand in a bag, and then be rubbed
or pounded. Experiments with different leguminous plants
showed that by such means the absorptive capacity might
be increased as much as 30 per cent. In the case of seeds
with still harder seed-coats a rougher treatment may be em-
ployed. In some cases, where larger and more valuable seeds
are to be dealt with, the testa may be opened up by one or
more cuts.

In nature, the seeds are sown immediately after they have
ripened, and in the case of those which are surrounded by
a stony envelope, and which do not germinate in the same
year, they are very often surrounded with a succulent layer
(Cherries, &c.). In this case it is obvious that the permanently
moist covering to the seed will prevent them becoming
hardened, as they do when they are dried. We must, where
possible, imitate this mode of procedure in horticulture, and
sow the seeds with their succulent covering. But in many
cases this is not possible, as the seeds have to be cleaned and
dried for purposes of sale and transport. We might, however,
at least place the seeds in the soil in the autumn, so that they
should make use of the moisture of the winter. But this is
not always possible on account of the attacks of mice and other
animals upon the seeds, and other methods such as the follow-
ing are usually employed.

The seeds are embedded in sand or sandy soil. They are
placed in boxes, baskets, barrels, or pots, according to circum-
stances, and placed in the cellar or kept in the open, according to
local climatic conditions. It is, however, essential in all cases
that the receptacle should be well drained, and that a consider-
able layer of sand should be placed at the bottom, covering

FRUITS AND SEEDS 247

a layer of potsherds or cinders. Then the seeds are spread
out singly over the sand and covered with another layer of
sand. The number of layers of seeds which may be placed
one above the other, and the thickness of the intervening layer
of sand, depends upon the porosity, and therefore upon tlie
coarseness of the sand. The more porous the latter is and the
larger the seeds, the greater the number of layers which may
be arranged one above the other. The layer of sand should
never exceed three inches, and more than six layers of seeds
should not be piled up, as one of the chief requisites, namely,
thorough ventilation of the soil, can then not be effectively
secured. The moisture must not be too great, but constant.
The time at which to begin this stratification depends upon
the time the seed requires for germinating. The more rapidly
the latter takes place, the later must this process be under-
taken, as the radicles must not have grown out much when the
seeds are transferred to the open. The effect of this process is
heightened by immersing the seeds in water before they are
laid in layers, or by keeping the temperature up. The cellar
is usually the best place for this process. If a very large
quantity of hard seeds whicli require to be moistened for a
considerable time (Ash and Sycamore) are to be dealt with,
they may be placed in the open in pits or mounds in a similar
manner.

It is, however, still an open question whether fresh seeds
are always the best for sowing. In most cases this will be
the case ; indeed, in some which soon lose their vitality
(Willows, Poplars, Elms) it will be absolutely necessary ; but
we find exceptions to this rule. Many practical gardeners,
indeed, affirm that old seeds of melons and cucumbers do not
run to leaf so much as fresh ones. This might be readily
explained ; for we know that plants which have always been
copiously supplied with water transpire more from their leaf
surface and have a more vigorous development of leaf surface.
It is also known that an artificial drying of seed-potatoes or
onions causes the production of short-shooted plants, which
more readily form tubers and bulbs, and this might also be
the case with the seeds of the melon. If seeds are kept for
a long time in a dry atmosphere and with little oxygen, they

248 THE PHYSIOLOGY OF PLANTS

retain their vitality for a long time. Of a number of pine
seeds which were kept in a glass jar in a room for five years,
3 2 per cent, were germinated ; after seven years i 2 per cent,
had sufficient vitality to germinate. Of seeds stored in the
same way the following percentages of germination were
obtained : — Pted clover after twelve years, 10.5 per cent, ; peas
after ten years, 47.7 per cent.; linseed after six years, 49
per cent. ; after eleven years, 3 per cent.

It is often stated that as the production of weakly offspring
increases with the age of the seeds, it is advisable to strengthen
their vitality by immersing them in dilute chloric, hydrochloric,
or some other acids. But the efficacy of this process has not
yet been established by experiment. It is possible, however,
that if the testa does not readily absorb water, the treatment
with acid might loosen the cells and effect a more rapid imbi-
bition.

The second stage of germination, the solution of the food
material, requires, besides the water supply, above all things
an increase in the supply of oxygen. The seeds may even
dispense with some of the water — that is, their tissues need
not yet be saturated with water — for the commencement of
the vegetative changes to take place. If liquid water is not
available at the commencement, the seeds may condense and
absorb water from the atmosphere, and even absorb hydrogen,
nitrogen, and oxygen, like some porous substances. Grain
which has been swelled can take up more oxygen out of the
atmosphere than nitrogen, and the giving off of carbonic acid
takes place so actively, that more carbonic acid is given off
than can be accounted for by the amount of oxygen taken in.
This indicates that internal processes of oxidation (respira-
tion) are carried on in germinating seeds. This oxidation is
accompanied by an evolution of heat, and the latter again
increases the solution of the reserve substances.

These now make their way to the seedling. If they are
contained in the cotyledons or seed-leaves, they will be
carried by the young vascular bundles to the root and stem
apices. If they are stored up outside the embryo in the
form of endosperm, the embryo will be provided with
special absorptive cells. In our grains (wheat, maize, &c.)

FEUITS AND SEEDS 249

the embryo is provided with u shield-shaped mass of cells,
the seutellum, which is applied to the starch-containing
tissue. The outer cells of the seutellum have finger-like
processes, and constitute the absorptive cells, taking up the
food material and passing it on to the embryo.

During this period of germination we must take care that
there should never be a dearth of oxygen or a superfluity
of carbonic acid. No gas is more injurious to germination
than carbonic acid. If there is an admixture of only a
few hundredth parts of carbonic acid to one part of oxygen,
the germinating process will cease.

Most of the mistakes in germinating occur from the fact
that there is a dearth of oxygen and an overplus of carbonic
acid. The direct cause of this is either too thick a covering
of soil over the seeds, or a closing of the soil to the free
access of oxygen by too continuous watering. There can be
no fixed rule about the thickness of the covering layer ; it
depends partly on the size of the seed, and partly on the
porosity of the soil.

The main consideration which will enable any one to
determine the depth at which his seeds should be sown
must always be this, that the soil is the medium which, in
the first place, is to keep the seeds sufficiently moist for
germination ; secondly, and only in those cases in which
the seedling will continue its development on the spot where
it has germinated, the soil must be looked upon as essential
for the fixing of the plant. In horticultural practice, where
the young plants are ppieked out, the covering of the seeds
with soil is not essential, if we only keep the seed-pans
covered with a pane of glass, or sow the seeds in an open
bed in a forcing-pit. But it is different in the case of seeds
sown in the open, where the dry winds or hot spring weather
may easily cause a temporary or a fatal stoppage of growth.
These evils must be overcome by covering the seed with
sufficient soil to prevent their becoming dry, but without
cutting off the supply of air.

If one is forced to use sunny beds for seedlings, it is better
to retain the necessary moisture in the soil by covering it
with loose straw, pine branches, &c., than by continual

250 THE PHYSIOLOGY OF PLANTS

watering. Even if the seeds are not sown too deeply, they
may not germinate if tlie soil becomes caked by repeated
wetting and drying, &c. Such caking of the soil prevents
the access of air to the seed at the time when the greatest
amount of chemical change is taking place within them,
and the air actually contained in their tissues is not sufficient
for any considerable length of time.

How essential the air contained in the tissues of the seed
is for germination, may be gathered from experiments which
were made with the seeds of turnips. These were placed
under the receiver of an air-pump, and the air they contained
was pumped out and replaced by water. The seeds took up
71.13 per cent, of water, but only 30 per cent, of them
germinated, while control-experiments showed that 90 per
cent, of the normal seeds germinated. These latter also
developed much more rapidly. In adult plants, too, death
may be brought about by extracting the air from the inter-
cellular spaces where it is contained, and replacing it by
water. The plants then have a transparent appearance.

A disturbing effect is often caused in seeds which have
been swelled, by an interruption of the process of germina-
tion by drought. The injuries thus caused vary according to
the nature of the plant and the stage of germination at which
the disturbance occurs. Speaking generally, the seeds of
Monocotyledons seem to suffer less than those of Dicoty-
ledons. The naked grains, like those of the wheat or rye,
are especially resistant. Those covered with glumes (bracts),
like those of the barley and oat, are more sensitive, and the
Indian-corn is especially delicate. Still, all these grains
suffer less than the linseed, rape, clover, peas, and beans.
Experiments made with a view to studying this phenomenon
showed that the cereals are able to withstand even repeated
interruptions of their germination by constantly forming new
adventitious roots at the base of the old ones, which died
off. This faculty is not possessed by most Dicotyledons ; the
dried-up roots rot away, and their decay affects the adjoining
tissue and spreads upwards. Even when decay does not set
in, and the seedling gradually recovers, it takes place much
more slowly.

FRUITS AND SEEDS 251

Seeds which have once been soaked and have then dried up
again will absorb water much more rapidly the second time,
but the testa is no longer the same. The increase of the seed
during germination may amount to 100 per cent, of its
volume, and this means that the testa becomes enormously
stretched. If it dries up, the testa shrinks again, and cracks
in innumerable places. When the seed becomes moistened
again, air and water will have a freer access to the tissues,
and the reserve material will become more rapidly dissolved.
It will, however, also pass more readily through the cell-walls
to the outside, and thus be lost to the embryo.

We see, therefore, that the swelling of seeds previous to the
sowing can only be advantageous if we can protect the sown
seeds from drought. If this is not possible, then it would be
better to leave the seeds to tliemselves. This rule, that vegetative
processes should only be accelerated by copious water supply,
if it is possible to continue such a supply, applies also to the
third stage of germination, and to all stages in the development
of vegetative organs. A plant which has been well watered
from its youth will give off more water from its leaves than a
plant of the same species, and with the same amount of leaf
surface, but which has always received only a moderate supply
of water. The former will, therefore, fade, and ultimately be
injured, under conditions under which a plant used to a smaller
water supply and less transpiration can thrive very well.
Hence it is that we find that luxurious lawns will suffer
more than meagre ones, and luxurious pot-plants more than
their small-leaved relations.

It is an agricultural rather than a horticultural practice in
some cases to manure the seeds, because it is believed that a
stronger seedling would be developed. For this purpose the
seeds are either coated with nutritive material or allowed to
swell in a solution of concentrated salts. Such a proceeding,
where it is not actually harmful, is at least useless. Experi-
ments with turnips, wliich can surely stand a lot in the way
of manures, have shown that ammonium or potassium sulphate,
even at a concentration of only 0.5 per cent., is deleterious to
germination. Cereals and leguminous plants are also not in
the least benefited and, indeed, germinate best in distilled

252 THE PHYSIOLOGY OF PLANTS

water. Ptape-seed alone was able to support a 2 per cent,
solution. In the case of seeds which have to lie a long time
m the soil before they germinate, the injurious effect of en-
crusting the seeds is less obvious, because the rain has more
time to remove the salts.

INDEX

Absorption by root, 4

Adventitious buds, 169

Aerial roots, 75

Albumin, 124

Aleurone grains, 1 26

Alkaloids, 32, 126

Aluminium in plants, 36

Ammonium sulphate as manure, 54

Amylodextrin, 122

Amygdalin, 124

Andrfficium, 214

Apostrophe, no

Arabin, 123

Arsenic in plants, 37

Asparagin, 125

Asphyxia, 'j'j

Assimilation, T]^ 108

Autumn pruning, 139

Barium in plants, 36
Bark scales, 104, 165
Bast, 13
Bast iibres, 96
Bast parenchyma, 1 2
Bending of shoots, 1 5 5
Bleeding of plants, 20
Bone-meal as manure, 5 5
Boron in plants, 36
Bracts, 221

Bromine in plants, 36
Budding, 183
Bundle sheath, 16

Calcium, function of, in plants, 33

Callus, 149

Callus in cuttings, 173

Callus formed from pith, 190

Calyx, 216

Canibiform cells, 96

Cambium, 98

Cambium ring, 95

Cane-sugar, 123

Capillary attraction, 3 1

Carbohydrates, 32

Carbon, 31

Carpels, 214

Caryophyllacea), doubling of, 220

Cell lumen, 17

Cell wall, thickening of, 22

Cells of leaf, 118

Cells of root, 1 7

Chalaza, 228

Chalky soil, 66

Chili saltpetre as manure, 54

Chlorine, 34

(Chlorophyll corpuscles, 18, 130

Chromoplastids, 130

Cleistogamy, 241

Collenchyma, 109

Connective, 215

Copper in plants, 37

Corolla, 216

Corona, 217

Crown grafting, 189

Cuticle, 1 1 2

Cuttings, 172

Cystolith, 115

Cytoblast, 127

Dextrose, 123
Diastase, 123
Diclinous flowers, 214
Diffusion, 19
Dimorphic flowers, 239
Dioecious, 214
Dormant buds, 169

254

Embryo, 232, 244
Embryo sac, 228
Emulsiii, 124
Endodermis, 16
Endosperm, 231, 244
Epidermis, 5, 95, 1 12
Epistroplie, 1 10

Fallow, 52
Ferments, 122
Fibro-vascular bundle, 1 3
Filament of stamen, 2 1 5
Flower, the, 214
Flower, development of, 218
Fluorine in plants, 36
Foliage, damage to, 199
Foliage, removal of, 200
Fruit, pruning for, 142
Fruits, development of, 226
Fungi, 44
Funicle, 232

Germination, 245
Globoid, 126
Glucosides, 123
Glycose, 123
Grafting, 183
Grape-sugar, 123
Growth in thickness, 104
Guano, 55
Guard cells, 1 1 3
Gum arable, 1 23
Gum tragacanth, 1 23
Gynsecium, 214
Gypsimi, use of, 69

Hadrom, 29
Hairs, 117
Hard bast, 96
Healing processes, 149
Heliotropism, 109
Hermaphrodite flowers, 214
Heterostyly, 239
Hybridisation, 242
Hydrogen, 31
Hydrotropism, 93
Hypochlorin, 130

INDEX

Integuments, 226
Intercellular spaces, yS, 112
Invertin, 123
Involucre, 221
Iodine in plants, 36
Iron in soil, 34

Jamin's chain, 2 1

Kainit as manure, 41

L^vuLOSE, 123
Layering, 170
Lead in plants, ^y
Leaf, the, 103
Leaf, development of, 1 20
Leaf used for propagating, 201
' Leaves, treatment of, 1 99
Lenticels, 78, 116
Leptom, 15, 29
Leucoplastids, 130
Libriform fibres, 12
Lignification, 12
Lime as manure, 68
Lime, oxalate of, 132
Lithium in plants, 36

Magnesium in plants, 33
Manures, inorganic, 53
Manures, organic, 55
Marls, 66
Marling, 69
Medulla, 95
Medullary rays, 95
Membrane, primary, 11, 22
Meristem, 7, loi
Mesophyll, 112
Micellse, 19
Microjiyle, 226
Microsomata, 129
Monoecious plants, 2 1 4
Mycorhiza, 46
Myrosin, 124

Nitrogen, 32, 42
Notching of stems, 1 58
Nucellus, 226

INDEX

255

Nucleolus, 17
Nucleus, 17, 127
Nutrition of pot-plants, 70
Nutrition by aerial roots, 75
Nutrition of roots, 30
Nutritive substances, liow replaced,
52

Ovary, 215, 226
Ovules, 216, 226
Oxygen, 31

Oxygen, lack of, 'j^

Palisade parenchyma, 1 19
Paracorolla, 217
Parasites, 48
Parenchyma, 12, 109
Pectin substances, 123
Peeling of stems, 161
Pepsin, 123
Perianth, 214
Periblem, 218
Pericambium, 15
Permeability of soil, 61
Phla^m, 13

Phosphoric acid, 42, 54
Phosphorus in plants, 32
Pinching, 138
Pit, bordered, 25
Pith, 95
Placenta, 228
Ploughing in of crops, 67
Pneumathodes, 80
Pollen grains, 2 1 5
Pollen tube, 227
Pollination, 239
Pollinia, 240
Potassium in soil, 41
Potassium as manure, 54
Pot-i)lants, nutrition of, 70
Pressure of bark, 104
Pricking out, 249
Primary meristem, 1 20
Primordial utricle, 17, no
Procambial strands, 99
Propagation by cuttings, 172
Propagation by layerings, 169

Prosenchyma, 12

Protandrous flowers, 239
I Proteids, 124
I Protein crystals, 125
I Protogynous, 239
j Protoplasm, 16

Pruning for fruit, 142
I Pruning for wood, 142
I Pruning, how to increase effect of,
155

Re-potting, 85

Resins, 124

Respiration of leaf, 109

Respiration of roots, ']'], 207

Respiratory cavity, 1 1 2

Retentiveness of soil, 40

Ringing, 159

Root cap, 9

Root cuttings, 182

Root hairs, 5

Root pressure, 20

Root tip, 7

Root tubercles, 48

Roots, absorptive portion of, 4

Roots, conducting portion of, 10

Roots, how nourished, 37

Roots, respiration of, 61

Roots, structure of, 4

Roots, treatment of, 81, 85

Roots, upward growth of, 79

Rubidium in plants, 36

Salpetre, Chili, as manure, 54
Salt, common, as manure, 56
Sandy soil, 65
Saprophytes, 44

Scion, influence of, on stock, 194
Sclerenchyma, 102
Scraping of stems, 105, 165
Scutellum of grasses, 249
Secondary thickening, 104
Seeds, formation of, 226
Seeds, manuring of, 251
Seeds, production of, 238
Seeds, treatment of, 244
Self-pollination, 239

2s6

IXDEX

Shoots, bending of, 155

Shoots, treatment of, 134

Shoots, twisting of, 157

Sieve tubes, 12, 96

Silica in plants, 35

Slitting the bark, 164

Sodium in plants, 35

Soft bast, 13

Soil, improvement of, 65

Soif, permeability of, for air, 6 1

Soil, power of absorption of, 40

Soil, retentiveness of, 40

Soil, structure of, 39

Sphaero-crystals, 119

Splint wood, 104

Sjjongy parenchyma, 79, 119

Stable manure, 56, 67

Stamens, 2 1 5

Starch sheath, 18

Stem, function of, 105

Sterility, 243

Stigma, 215, 226

Stomata, 78, 113

Storage tissue, 18

Stratification of seeds, 247

Strontium in plants, 36

Style, 226

Sugar sheath, 18

Sulphur in plants, 32

Summer pruning, 137

Superphosphates, 55

Tannij:, 124

Testa, 232
Thylloses, 28, 136
Tongue grafting, 1 89
Tracheids, 13, 26
Transpix'ation, 1 1 6
Trichomes, 116
Trimethylamin, 126
Trophoplasts, 130
Trophotropism, 93
Turgidity, 20, 28
Twisting of shoots, 157

Vacuoles, 17, 127
Vascular bundles, 10
Vessels, 1 1

Water-gland, 116
Water-stoma, 1 1 6
"Watering, theory of, 205
Wild stock, influence of, 194
Wood cells, 1 1
Wood fibres, 95
Wood parenchyma, 1 2
Wounds, injuries due to, 135
Wounds, healing of, 149

Xanthophyll, 130
Xylem, 13

Zinc in plants, 37

THE END.

nommr ifuuxr

ILC State C^lkm

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