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CAMBRIDGE BIOLOGICAL SERIES
GENERAL EDITOR : — ARTHUR E. SHIPLEY, M. A., F.R.S.
FELLOW AND TUTOR OF CHRIST'S COLLEGE, CAMBRIDGE
THE ELEMENTS OF BOTANY
CAMBRIDGE UNIVERSITY PRESS
FETTER LANE, E.G.
C. F. CLAY, MANAGER
100, PRINCES STREET
ALSO
ILonton: H. K. LEWIS, 136, GOWER STREET, W.C.
A. ASHER AND CO.
F. A. BROCKHAUS
$tfo lorfc: G. P. PUTNAM'S SONS
Bombag ant) Calcutta: MACMILLAN AND CO., LTD.
All rights reserved
THE
ELEMENTS OF BOTANY
BY
FRANCIS DARWIN, Sc.D., M.B., F.R.S.
HON. FELLOW OF CHRIST'S COLLEGE, CAMBRIDGE
WITH ILLUSTRATIONS
Cambridge :
at the University Press
1910
First Edition 1805.
Second Edition 1896.
Reprinted 1899, 1910.
PREFACE.
nnHE Elements of Botany appeared in 1895, and with
a few alterations was stereotyped in the following
year. I take the present opportunity of correcting one
or two obscurities or mistakes. If the book could have
been rewritten it might have been advisable to introduce
the conception of the stele, which helps to make clear the
identity of the central vascular cylinders of the Dicoty-
ledonous stem and root — a point in .which the older
terminology is less effective. For this purpose it is allow-
able to define the stele as a group of tissues characterised
by the predominance of conducting elements and contained
within an endodermis. Used in this sense the word stele
also coordinates the vascular anatomy of the Dicotyledon
with that of the fern-rhizome, whereas the term vascular
bundle, used in these instances, may confuse the beginner.
It seems to me that broad resemblances between different
types of vascular arrangement are to the elementary
VI PREFACE.
student of greater value than fine distinctions, and that
a more elaborate view of the stele may be deferred until
he has more knowledge of plant anatomy.
As this book originally appeared, the description of
the germination of the bean contained a blunder which
is now set right. I am indebted to Mr Heber Smith1
for pointing out that the part played by the micropyle, in
the emergence of the radicle, is often wrongly given. I
regret that I have not room in the text for a fuller account
of the process such as is supplied by Mr Heber Smith's
letter.
The substance of the book was given in the form of
lectures on Elementary Biology to Cambridge students.
This — the Botanical course for medical students — is now
given by Mr F. F. Blackman, who has introduced certain
improvements, notably in the addition of Fucus as a type
of reproduction. But I think it will be found that what-
ever value my little book had as an introduction to the
study of plants, it retains in relation to Mr Blackman's
course of instruction.
Except where otherwise specified, the illustrations
have been drawn from nature by Miss D. F. M. Pertz,
and by Dr W. G. P. Ellis, formerly Demonstrator in
1 Nature, Feb. 4, 1909.
PREFACE. Vll
Botany, to both of whom I desire to express my sincere
thanks. Dr Ellis not only undertook the chief part of the
drawings, but has also aided me in other ways in the
kindest manner. I am particularly indebted to him for
valuable help in the selection of laboratory material, and
for the arrangement of the Appendix containing instruc-
tions for practical work.
To Mr Shipley, the Editor of the Cambridge Natural
Science Manuals, I am indebted for much kindly co-
operation.
FRANCIS DARWIN.
BOTANY SCHOOL, CAMBRIDGE.
January, 1910.
TABLE OF CONTENTS.
CHAPTER I.
Yeast— Structure of the Vegetable cell— Keproduction — Nutrition-
Fermentation — Spirogyra — Chloroplasts — Assimilation of carbon — Cells
from Tradescantia, Elodea, Sambucus, Ranunculus — Protoplasmic circu-
lation pp. 1 — 13.
CHAPTER II.
Reserve materials — Seeds of the Bean and the Gourd — Germination —
Seedlings — Tuber of the Potato and of the Jerusalem artichoke — Starch —
Bulb of the Tulip pp. 14—32.
CHAPTER III.
The root — Geotropism as a phenomenon of stimulation — Structure of
the root — Conception of tissues — Vascular Cylinder — Root-cap — Growing
point — Meristem — Histology of vascular tissues — Secondary roots — Root-
hairs pp. 33 — 48
CHAPTER IY.
Stem structures — Sunflower and Jerusalem artichoke — Morphology —
Histology— Transverse and longitudinal sections — Vascular tissue — Struc-
ture of pitted cell-walls — Cambium — Cortex . . . pp. 49 — 64.
X CONTENTS.
•
CHAPTER V.
The Oak — Structure of the plumule — Wood — Annual rings — Medul-
lary rays — Cambium — Histology of the wood — Pitted vessels — Bordered
pits — Tracheids— Wood-fibres pp. 65 — 79.
CHAPTER VI.
The Oak— Structure of the bark — Epidermis — Cuticle— Cork — Phel-
logen — Secondary Phloem — Physiology — The arboreal habit — Geotropism
— Knight's experiment — Stability of plant structures — Turgidity
pp. SO— 93.
CHAPTER VII.
The leaf — Foliage- and scale-leaves — Phyllotaxy — Forms of leaves —
Stipules — Dorsiventrality — Histology — Mesophyll — Stomata and their
functions — Transpiration — Leaf-fall .... pp. 94 — 1O7.
CHAPTER VIII.
Beproduction — Struggle for life — Sexual and asexual reproduction —
Pleurococcus, reproduction by cell-division — Mucor, sporangia and spores
— Conjugation, in Mucor and Spirogyra . . . pp. 1O8— 117.
CHAPTER IX.
Eeproduction continued — Alternation of generation — The Fern —
Sporophyte and oophyte — Structure of the sporophyte of Pteris — The
Ehizome — Histology — Vascular bundles . . . pp. 118 — 128.
CHAPTER X.
Eeproduction of the Fern continued — Sporangia and spores — Germi-
nation of the spores — Prothallus — Archegonium — Antheridium — Anthe-
rozoids attracted by malic acid — Embryology . . pp. 129 — 139.
CHAPTER XI.
Spermaphytes or Phanerogams — Natural orders, genera and species —
The flower of Ranunculus — Floral diagram — Androecium and Gynoecium
—The papilionaceous flower — Fertilisation by means of insects
pp. 140 — 154.
CONTENTS. XI
CHAPTER XII.
Distribution of pollen by wind — Plantago — Self- and Cross-fertilisa-
tion— Dichogamy, protogynous and protandrous — Silene — Composite
flower — Dog-Daisy (Chrysanthemum leucanthemum) . pp. 155 — 167.
CHAPTER XIII.
Morphology of the inferior ovary — Flowers of the Cherry, Peach and
Gooseberry — Ovule of Caltha — Embryo-sac and egg-cell — (Termination of
pollen — Fertilisation — Embryology of Shepherd's Purse (Capsella bursa-
pastoris pp. 168 — 179.
CHAPTER XIY.
The fruit— Distribution of seeds by wind — Winged seeds and fruits —
Bignonia, Dandelion, Ash, Sycamore — Burrs, Geum urbanum — Seeds
which germinate after passing through the intestines of animals — Cherry,
Gooseberry, Pear pp. ISO — 196.
APPENDIX.
PRACTICAL WORK.
No. I. THE CELL.
Yeast, Spirogyra, Elder (Sambucus), Elodea, Tradescantia
pp. 199— 2O1.
No. II. THE SEED AND SEEDLING. TUBERS, BULBS.
Seeds of Bean (Vicia faba), Gourd (Cucurbita), Tubers of Jerusalem
artichoke (Helianthus tuberosus), Potato (Solanum tuberosum), Bulb of
Tulip (Tulipa) ........ pp. 2O2— 2O4.
No. III. THE BOOT.
Boot of Bean (Vicia Jaba), Root hairs of Mustard (Sinapis)
pp. 2O5— 2O6.
No. IV. THE HERBACEOUS STEM.
Stem of Sunflower (Helianthus annuus) or Jerusalem artichoke (H.
tuberosus) pp. 2O6 — 2O8.
Xli CONTENTS.
No. V. THE ARBOREAL STEM.
Wood of the Oak (Quercus) ..... pp. 2O8— 21O.
No. VI. THE ARBOREAL STEM.
Bark of the Oak (Quercus). Formation of cork in the Beech
pp. 21O— 211.
No. VII. THE LEAF.
Leaf of the Hellebore (Helleborus). Injection of Leaves. Phyllotaxy
of the Groundsel (Senecio vulgaris) .... pp. 212 — 213.
No. VIII. REPRODUCTION.
Pleurococcus. Spirogyra. Mucor . . . pp. 213 — 215.
No. IX. THE FERN.
Khizome of the Bracken Fern (Pteris aquilina) . pp. 215 — 217.
No. X. REPRODUCTION OF THE FERN.
Sporangia of Pteris, Aspidium, Polypodium, Germinating spores.
Prothallus. The young sporophyte . . . pp. 217 — 219.
No. XL THE FLOWER.
Flower of the Buttercup (Ranunculus), of the Bean (Vicia faba).
Anther of the Marsh Marigold (Caltha palustris) . pp. 219— 22O.
No. XII. THE FLOWER (continued) — DICHOGAMY.
Flower of the Dog-Daisy (Chrysanthemum leucanthemum). Flowers of
Plantain (Plantago) and Silene .... pp. 22O— 222.
No. XIII. THE SEED.
Stigma of Evening Primrose (CEnothera) with germinating pollen
grains. Ovules of Marsh Marigold (Caltha palustris). Embryology of
Shepherd's Purse (Capsella bursa-pastoris). Flower of the Cherry (Prunus
cerasus). Young fruit of the Gooseberry (Ribes grossularia)
pp. 222—224.
No. XIV. THE FRUIT.
Fruits of the Cherry (Prunus cerasus), Pear (Pyrus communis), Ash
(Fraxinus excelsior), Gooseberry (Ribes grossularia), Dandelion (Tarax-
acum dens-leonis), Herb Bennet (Geum urbanum) . pp. 224 — 227.
pp. 229 — 235.
LIST OF ILLUSTRATIONS.
no. PAGE
1 Yeast under a high power 3
2 Cell of Spirogyra 7
3 Various cells under a high power 12
4 Seed and seedling of the Bean 17
5 Germination of the Gourd 21
6 Rhizome of a Sedge 23
7 Aerial tubers of the Potato 24
8 Seedling Potato bearing tubers 25
9 Ivy, showing adventitious roots 27
10 Starch grains highly magnified 28
11 Bulb of the Tulip 31
12 Germinating Bean, the root curving downwards gcotropically . 33
13 Transverse section of the root of the Bean .... 35
14 Eoot-cap diagrammatically represented ..... 38
15 Diagram of merismatic tissue 39
16 Diagram illustrating the structure of the root- tip ... 41
17 Transverse section of the bean-root (high power) ... 42
18 Transverse section showing the origin of the secondary roots
of the Bean 46
19 Mustard seedling, showing root-hairs 47
20 Seedling Wheat with soil adhering to the roots ... 48
21 Stem of Pimpernel, showing nodes and internodes . . 51
22 Diagram of the transverse section of the stem of Helianthus . 54
23 Transverse section of stem of Helianthus tuberosus under a
high power 57
24 Longitudinal section through the stem of Helianthus tuberosus 59
25 Model of a pitted cell-wall , 60
XIV LIST OF ILLUSTRATIONS.
FIG. PAGE
26 Cells of a date-stone in section 61
27 Transverse section through a ridge on the stem of Clematis 64
28 Transverse section through a branch of the Oak . . 66
29 Transverse section of an Oak, showing the annual rings
and the medullary rays 67
30 Transverse section through the wood of the Lime-tree
showing the annual rings 69
31 Diagram illustrating two types of longitudinal section . 70
32 Longitudinal radial section of the wood of the Oak . . 71
33 Longitudinal tangential section of the wood of the Oak . 72
34 Cambium of the Scotch Fir in transverse section . . 73
35 Section through a branch of the Cork-oak to show primary
and secondary medullary rays 75
36 Part of a pitted wood-vessel from the Oak .... 77
37 Macerated oak-wood 79
38 Formation of cork in the Beech 83
39 Transverse section of oak-bark 84
40 Eadial longitudinal section of oak-bark .... 85
41 Horse-chestnut buds 94
42 Markings on a branch of Horse-chestnut .... 95
43 Diagram illustrating phyllotaxy 97
44 Phyllotaxy of the Plantain 98
45 Stipulate leaves 99
46 Transverse section through the leaf of the Hellebore . . 101
47 Stomata in surface view 103
48 Longitudinal section through the leaf-stalk of the Poplar to
illustrate leaf-fall 106
49 Mycelium, germinating spores and sporangia of Mucor . 113
50 Conjugation of Mucor 115
51 Conjugation of Spirogyra 117
52 Khizome of Pteris 121
53 Transverse section of Pteris rhizome (low power) . . 123
54 Transverse section of a bundle from the rhizome of Pteris . 125
55 Longitudinal section of the rhizome of Pteris . . . 126
56 Macerated rhizome of Pteris 127
57 Transverse section through a sorus of Pteris . . . 130
58 Development of the sporangium in the Fern . . . 131
59 Germinating spores and young prothallus of the Fern . 133
60 Archegonia of the Fern 134
61 Antheridia and antherozoid of the Fern . 135
LIST OF ILLUSTRATIONS. XV
FIG. PAGE
62 Development of the sporophyte of the Fern . . . 138
63 Flower of Ranunculus 143
64 Floral diagram of the Peach 144
65 Flower of the Cowslip divided longitudinally . . . 146
66 Nectary of Ranunculus 146
67 Flower of the Sweet Pea 149
68 Floral diagram of a papilionaceous flower . . . .150
69 Standard and one of the wings of the Sweet Pea . . 151
70 Flower of the Sweet Pea partly dissected so as to show the
keel, the androacium and the gynoscium . . . .152
71 Flower of the Wheat 157
72 Protogynous flower of Plantago lanceolata .... 160
73 Flower of Silene 162
74 Florets of Senecio . 164
75 Floret of Centaurea 166
76 Flower of the Peach 169
77 Flower of the Cherry . .169
78 Flower of Madder (Rubia tinctorum) 170
79 Flower and fruit of the Gooseberry 171
80 Longitudinal section through the ovule of Caltha . . 173
81 Diagrammatic sketch of pollen grains germinating on the
stigma of (Enothera, the Evening Primrose . . . 175
82 Embryo of Capsella, the Shepherd's Purse .... 177
83 Winged seed of a Bignonia 182
84 Fruit of Dandelion 183
85 Fruit of the Ash 184
86 Floral diagram of Sycamore 185
87 Ovary and style of Sycamore 186
88 Ripe fruit of Sycamore 187
89 Fruit of Herb Bennet (Geum urbanum) .... 188
90 Flower of the Cherry 190
91 Section through the ovary of the Peach .... 190
92 Fruit of the Cherry, longitudinally divided . ... 191
93 Flower and fruit of the Gooseberry , 193
94 Fruit of the Pear . 194
"I conceive it is a fine study and worthy a gentleman to be a good
botanique, that so he may know the nature of all herbs and plants, being
our fellow creatures."
The Life of Edward, Lord Herbert of Cherbury,
written by himself.
CHAPTER I.
YEAST — REPRODUCTION — NUTRITION — FERMENTATION —
SPIROGYRA — NUTRITION — TRADESCANTIA-HAIR — CELL
OF ELODEA-LEAF— CELL OF THE PITH OF ELDER.
YEAST is familiar as the cause of the process known as
fermentation, by which alcohol is produced from sugar.
It is obtained from the brewers for use in the labora-
tory as a muddy brown fluid. The muddiness depends on
the same general cause that gives turbidity to dirty water,
namely the presence of innumerable very minute particles
suspended in the fluid. In the case of yeast each of these
particles is a simply organised plant belonging to the
great tribe of Fungi, and known as Saccharomyces
cerevisice.
The plant is of the simplest possible structure, since it
consists of a single cell : it has nevertheless the attributes
of a more highly organised plant, it leads an individual
existence and is able to feed, to grow, and to reproduce
itself.
The yeast cell, like those which build up the tissues of
more complex plant-bodies, consists of a mass of proto-
plasm surrounded by a cell-wall. It is possible to make
D. E. B. 1
YEAST. [CH. I
the cell-wall separate from the protoplasm by pressure
applied to the cover-glass of a mounted preparation. The
protoplasm is squeezed out of the broken cell (just as the
flesh may be squeezed out of a grape-skin) and the torn
empty walls and crushed fragments of protoplasm remain.
The cell-wall is a colourless membrane made of a
substance called cellulose. Cellulose is a compound of the
greatest importance in plant-physiology. It forms a large
proportion of the substance of plants, and is the basis of
many products of vegetable origin. Cotton wool, which is
made from the hairs on the seeds of the cotton-plant is
nearly pure cellulose, and the same is true of filter-paper
which is manufactured from vegetable cell-walls.
In a wooden match are many thousands of cells of which
the walls are cellulose1. If such a piece of wood is dipped
in strong sulphuric acid or is charred by fire a mass of
charcoal is the result. This fact proves that cellulose
contains carbon, and as a matter of fact carbon makes up
nearly half the weight of this substance.
Cellulose also contains hydrogen and oxygen in the
proportion in which they exist in water, its formula being
C6H1006. This is the same formula as that of starch, — an
important fact, as will appear later on. In spite of the
identity of formula the two substances have not identical
reactions. Starch is characterised by giving a blue or
purple colour with iodine. Cellulose is characterised by
not giving this test unless it has been previously treated
with acid. In the laboratory it is usual to shorten the
process by the use of an acid preparation of iodine. The
1 More accurately lignified cellulose.
CH. l] YEAST. 3
purple colour given by this — Schulze's fluid — is character-
istic of cellulose. This reaction must be studied on the
cell-walls of the higher plants, because the cellulose of the
yeast plant, in common with that of the fungi generally,
only gives a purple colour after a certain preliminary treat-
ment.
FIG. 1.
YEAST UNDEU A HIGH POWER,
a — g successive stages of budding.
The yeast-cell as it appears under a high power of the
microscope is shown in fig. 1 a. The cells are seen to
contain a granular protoplasm in which clear spaces occur :
these are cavities in the protoplasm, containing fluid and
known as vacuoles : the fluid in the vacuoles is known as
cell-sap.
Reproduction.
The fact that yeast increases in quantity by reproduc-
tion can be demonstrated by adding a minute drop of the
1—2
4 YEAST. [CH. I
yeast-containing fluid to a dilute solution of sugar in
spring water. The increase of the organism is visible
by the increased turbidity of the culture-fluid. With the
microscope it can be seen that the increase is due to a
process of budding, as shown in fig. 1. The cells begin to
bulge or swell in places and the buds so formed break off
and begin an independent life. They may however remain
attached for some time, and by a series of buds give rise
to the chains of cells shown in the figure1.
Nutrition.
When a small number of yeast plants increase so as to
alter the appearance of the fluid in which they float, the
fact that a quantity of new protoplasm and new cellulose
has come into being forces itself on the observer ; and the
question whence and how it has arisen must be met.
When an organism grows, the new organic material built
on to the old body comes from the food supplied. The
food diminishes, while the organism increases; one turns
into the other literally, and absolutely. Nearly half the dry
weight of cellulose is carbon, it is certain therefore that
the yeast has been supplied with carbon in some form in
which it can be used as food. In the laboratory carbon is
given to yeast in the form of sugar2 : and if two jars are
prepared one (i) with, the other (ii) without, sugar, it will
be found that yeast increases rapidly in (i) but not in (ii).
In a similar way it can be shown that the increase in the
1 Another form of reproduction occurs in yeast; it is not described
because it is not met with in ordinary cultures.
2 The formula of Cane Sugar is C^H^Ou , of Grape Sugar C6H1206.
CH. l] NUTRITION. 5
amount of yeast does not simply depend on the sugar,
but also on the presence of certain other substances
which must be supplied to the plant in solution.
The reason is obvious : the cells contain nitrogen, sulphur,
phosphorus, potassium, lime and magnesium, and these
must be supplied in the culture-fluid. The solution used
for the growth of yeast is known as Pasteur's solution
and has the following constitution1:
Potassium phosphate 20 parts
Calcium phosphate 2 „
Magnesium sulphate 2 „
Ammonium tartrate 100 „
Cane sugar 1500 „
Water 8376 „
10000 parts.
The two chief points to notice are the conditions in
which carbon and nitrogen are supplied to the plant. Car-
bon is supplied as sugar and the yeast-cell cannot assimilate
carbon unless it is presented to it in an organic compound.
Yeast therefore resembles animals in regard to its carbon
supply, since like an animal it depends on a substance
(such as sugar) which has been manufactured in the leaves
of another plant, the sugar-cane.
But in regard to nitrogen the yeast differs from
animals : no animal could live if its only nitrogenous food
were an ammonia compound, whereas the yeast is able to
make use of the ammonium-tartrate.
1 Practical Biology (Huxley and Martin), 1888.
6 SPIROGYRA. [CH. I
Fermentation.
In the process of fermentation, sugar is broken up into
C02 and alcohol : the bubbles of gas become entangled in
the sugary fluid and give rise to the scum on the surface
so characteristic of fermenting fluids. Small quantities of
glycerine and of succinic acid are also produced.
It is easy to show that fermentation depends on the
life of the yeast-cell, for the process can be stopped by
boiling (and therefore killing) the plant. Into the difficult
question of the nature of fermentation and its relation to
respiration and to the source of energy generally I do not
propose to enter.
Spirogyra.
Spirogyra is a fresh-water weed, a representative of the
Algae, the great tribe to which the sea-weeds also belong.
It occurs in slimy tufts of delicate bright green threads.
Each thread is a Spirogyra plant which, although more
elaborate in structure than yeast, is yet of a very
simple construction, consisting as it does of a row of
cells united end to end. Each cell is precisely like its
neighbours, there is no division of labour, each cell being
responsible for its own nutrition, each growing indepen-
dently of the others, and each being capable of taking the
same share in reproduction. When one of the constituent
cells of Spirogyra has grown to a certain length it becomes
partitioned into two cells by the growth of a new
transverse wall. This process is called cell-division, and it
will appear later that it is of paramount importance in
the development of plants generally. It is important to
CH. l] SPIROGYRA. 7
notice that cell-division in this sense does not necessarily
mean that the cell is actually split into two free halves :
in the case of Spirogyra, and in growing plants generally,
the original cell is simply divided into two compartments
which increase in size and may again divide. It follows
from this manner of growth that a Spirogyra as it grows
comes to consist of more and more cells.
\ •, \
p. 71. C. p.U.
FIG. 2.
A CELL OF SPIROGTKA.
c, the spirally wound cbloroplast.
p. u, the protoplasm lining the cell (primordial utricle),
n, the nucleus suspended by protoplasmic ropes.
p, a pyrenoid with numerous small starch grains.
Each compartment of the plant is a good example of
the perfect vegetable cell. It has a cellulose wall often
coated outside with a layer of slimy material ; the cavity
of the cell is lined with a coating of protoplasm, inside
which is a large vacuole taking up nearly all the room
inside the cell. The fluid in the cell cavity is called
cell-sap, and is a very dilute solution containing certain
salts, vegetable acids, sugar and tannin. In the proto-
plasm a certain part is differentiated from the rest into
what is called a chloroplast — that is to say a piece
of protoplasm coloured green with the substance chloro-
phyll. When a green leaf or a Spirogyra plant is put into
spirit or ether it becomes colourless because the chloro-
8 CHLOROPHYLL. [CH. I
phyll is soluble in these fluids. It is important to remem-
ber the difference between chlorophyll, a substance soluble
in alcohol, and a chlorophyll-body or chloroplast, which is a
special kind of protoplasm. In Spirogyra the chloroplasts
are of a remarkable spiral form, winding like corkscrews
round the cell, as shown in fig. 2. It is the spiral arrange-
ment which has given the name Spirogyra to the plant.
In the cell-cavity is another organ, the nucleus, a part
of the protoplasm, denser and staining more easily than
the rest of the protoplasm, and having certain functions
which need not be discussed. It is suspended in the cell
cavity by ropes of ordinary protoplasm. The nucleus
contains one or more small bodies, the nucleoli.
The treatment of Spirogyra with glycerine or strong
salt solution is recommended in the Practical Work
in order to illustrate an important fact, namely that the
cell is tensely filled with cell-sap, the protoplasmic lining
being blown out with cell-sap, as an air-cushion is blown out
with air. The glycerine or strong salt solution takes away
some of the water from the cell-sap and the protoplasmic
lining collapses. The importance of this observation will
appear later on, in a section devoted to the stability of
plant structures.
Nutrition.
Since the Spirogyra increases in substance in the water
in which it grows it is quite certain that this water must
contain the food materials which are transformed into new
protoplasm and new cell-walls. If the water be analysed
it will be found to contain in minute quantities lime,
CH. I] ASSIMILATION. 9
potassium, magnesium, iron, — in fact the necessary mineral
constituents of the food. Nitrogen will be supplied as a
nitrate, sulphur as a sulphate, phosphorus as phosphate.
The water will not however be found to contain sugar or
any substance from which fungi can obtain carbon. It is
therefore clear that Spirogyra has some special method of
assimilating carbon. It is in the way that it gets its
carbon that Spirogyra (and all other green plants) differ
in nutrition, not only from fungi, but also from animals.
To yeast as to animals C02 is an absolutely waste product,
cast out in the process of respiration as of no more use.
But to the green plant it serves as an indispensable food-
supply, and it is because the ditch-water contains C02 in
solution, that the Spirogyra is able to live in it. The
process by which the carbon is taken out of the C02 and
built into living substance is known as the assimilation of
carbon.
The fact that CO2 serves as food may be proved by
observing the results of depriving green plants of this gas.
If a Spirogyra or other chlorophyll-containing aquatic
plant is cultivated in water, which except for the absence
of CO2 is precisely like that in which it naturally lives,
the plant dies. This experiment alone is not conclu-
sive as to the cause of death, but the conclusion is
strengthened by the result of another experiment. If
sugar is added to the water the plants do not die : from
this it would be rational to suspect that the absence of
C02 in the first experiment was injurious because it
meant the absence of carbon-containing food-stuff. Death
is not the only test of an organism being starved : if an
10 SPIROGYRA. [CH. I
animal is deprived of food, the degree to which it suffers
from the deprivation can be roughly gauged by estimating
the amount of fat in its body. When the degree of
starvation is severe the amount of fat is small. In a
green plant starvation may in the same way be estimated
by the amount which it contains of another carbon-com-
pound, namely starch. By applying this test it is found
that in water containing no C02 the Spirogyra soon loses
its starch, which reappears when C02 is added to the
water.
The same tests are of value in determining the conditions
under which assimilation of carbon from C02 can be carried
on. Thus no green plant can live permanently in darkness.
Even dull light is injurious, as may be seen in the
dwarfed miserable appearance of shrubs, etc. growing in
deep shade as compared with specimens in brighter light.
Here again the starch test is of value. If a green plant
is placed in the dark it soon loses the starch it possessed,
even though the water in which it lives contains C02:
and the starch will not re-appear until the plant is once
more exposed to light.
On the other hand a green plant can feed on sugar in
darkness, so that light seems to be a condition especially
connected with the extraction of carbon from CO2. The
fact is that the chloroplasts which give the green
colour to plants are machines, the motive power of which
is the energy of light, and whose special quality is the
power of robbing C02 of its carbon.
It is easily proved that this power resides in the
chloroplasts. In the leaves of variegated plants are
CH. I] ASSIMILATION. 11
certain patches or stripes which are yellowish-white in-
stead of being green, because they contain no chlorophyll.
If such a plant is placed in the dark the leaves will
after a time become starchless; if it is then exposed to
light, starch will appear, but only in the green parts where
chlorophyll is present. Moreover it is possible with the
help of the microscope to see that it is in the chloroplast
that the starch appears and disappears. This is
especially evident in Spirogyra, where the starch in the
form of minute granules is gathered round certain centres
in the chloroplasts which are known as pyrenoids.
The fact that the green plant is a machine driven by
the energy of sunlight can be made evident to the eye by a
well-known observation. When a water-plant, such as the
common river- weed Elodea, is placed in a beaker of spring-
water and exposed to sunshine, streams of minute bubbles
are seen to issue from the cut stalks. If the beaker is
darkened the bubbles cease and the same thing happens if
the water is freed from C02. The bubbles contain the
oxygen that is set free in the process of assimilation: it may
roughly be said that the plant seizes the carbon from the
C02 and lets the oxygen go. It is obvious therefore that
if there is no C02 in the water the production of oxygen
must cease, and the fact that the bubbling stops in the dark
shows that light is the power which drives the machine.
The stream of bubbles pouring from a water weed in
sunlight is, like the smoke coming from the chimney of a
cotton-mill, a sign of internal activity. The chimney
ceasing to smoke may mean either that there is a want of
cotton, a want of coal, or that the machinery is broken.
12
TRADESCANTIA.
[CH. I
In the same way the plant may cease to bubble for want
of raw material (CO2) or for want of driving power (sun-
shine) or because the machinery is broken, i.e. the
chlorophyll-bodies killed.
Tradescantia, Elodea, Elder.
The present chapter is intended to give a somewhat
wider introduction to anatomy and physiology of the
plant-cell than can be obtained from a study of yeast
and Spirogyra. Parts of certain higher plants have there-
fore been included in the Practical Work.
!
c
D
FIG. 3.
CELLS UNDER HIGH POWER.
A, B, young cells, C an older cell from the developing maize-root.
D, cell from the hair of Tradescantia.
E, parenchymatous cell from the cortex of Kanunculus.
A hair from the stamens of the Spider- Wort (Trades-
cantia virginica) consists of a row of rounded cells united
end to end. Under the microscope can be seen the
purple cell-sap which occupies the greater part of the
cavity of the cell The protoplasm is more easily visible
CH. I] ELODEA. 13
than in Spirogyra, because here there are no chlorophyll
bodies to obscure the view. There is not only a layer of
protoplasm lining the cellulose wall of the cell, but a
complicated system traversing the cell-sap and connecting
the nucleus with the rest of the protoplasmic cell-body.
The most striking fact visible in the Tradescantia hair
is the circulation of protoplasm, which is perhaps the best
ocular proof that can be given of the " aliveness " of a
plant-cell. The circulation is rendered visible by the
granules in the protoplasm which flow steadily along the
living ropes of which it consists.
The leaves of the river- weed Elodea are useful on
account of the visibility of the circulating protoplasm in
their cells. In Elodea the chloroplasts differ from those
of Spirogyra in being small round bodies instead of spiral
ribbons; it is these bodies which make the circulation
visible as they glide round the cells carried along in the
flowing protoplasm.
The young pith of elder (Sambucus nigra) is included
as a good general example of the plant-cell, in which the
cell-wall, the protoplasm, nucleus and vacuole can all be
studied *. The cell from the cortex of Ranunculus (fig. 3)
illustrates the same points.
1 As the pith becomes old the protoplasm dies and the cell-contents
are replaced by air. The pith is then dry, white and very light.
CHAPTER II.
RESERVE MATERIALS — SEEDS OF THE BEAN AND THE
GOURD OR PUMPKIN — TUBERS OF THE JERUSALEM
ARTICHOKE AND OF THE POTATO— BULB OF THE
TULIP.
IN the last chapter it was explained how a plant,
Yeast or Spirogyra, increases in size by manufacturing new
cell- walls and new protoplasm from the food material
supplied to it in certain nutrient fluids.
The present chapter is meant to illustrate the im-
portant fact that a plant may grow in one part, that is to
say that new cells may come into existence and these
cells may increase in size, by the rearrangement of food
material stored up in another part of the plant. This
principle is illustrated in the germinating seeds of the
bean and gourd, in the sprouting tubers of the potato
and Jerusalem artichoke, and in the bulb of the tulip.
The study of the form of these specimens will also
serve as an introduction to some of the simpler parts of
the morphology of plants.
A seed, e.g. that of the bean, consists of a young plant or
CH. II] RESERVE MATERIALS. 15
embryo contained in certain envelopes or wrappings. In
a dry seed this young plant is alive but it is a dormant,
quiescent form of life ; in a germinating seed it is on the
other hand actively alive, vigorously performing the func-
tions of a living thing, and on the high road to become
a full-grown plant. How is it that it is possible to
unlock the dormant energies of the bean-seed ? Certain
changes must be made in the surroundings of the seed.
In the first place it must be supplied with water. In the
laboratory beans are usually soaked for 12 to 24 hours,
during which time they absorb great quantities of water,
and increase considerably in weight and size. They change
in aspect, become softer and less brittle, while they no
longer show the wrinkled seed-coat characteristic of a dry
Secondly, a certain degree of warmth is necessary. A
bean seed which has been soaked in water does not grow
if kept at a temperature of 0° C. Nor does it grow if the
temperature is above 50° C.
A third condition is also necessary, the seed must have
free oxygen, it must have access to the atmosphere, or at
least to air dissolved in water. If placed in an atmo-
sphere of some indifferent gas such as nitrogen or hydrogen
it will not grow.
Respiration is necessary for the life of the seed, and
therefore for growth, which is one of the manifestations of
life. The respiration of plants is of the same nature as
that of animals : it is easy to illustrate this by a simple
experiment. A well-stoppered jar is partly filled with
germinating seeds; after 24 hours the stopper is cau-
16 BEAN. [CH. II
tiously removed and a lighted taper lowered into it is
found to be extinguished by the accumulated C02.
The point to which I wish to call attention is that
given water, free oxygen and a sufficient degree of
warmth, the growth of the young plant in the seed will
begin although no carbon, nitrogen, phosphorus, sulphur,
potassium, etc. have been supplied from outside. Thus a
seed will germinate although it has been soaked in
distilled water. The fact is that the seed contains a store
of food — (the very store in fact which renders seeds valu-
able as food for animals), and when the young plant grows
it does so by the transference of part of this food to the
growing regions. The store is known as reserve material,
and the capability of accumulating reserves and of using
them by transference is one of great importance in the
lives of plants: it is for this reason that a chapter is
devoted to its study.
The seed of the bean is covered by a smooth pale
leathery membrane called the testa or seed-coat, which
presents two special points of interest. At one end of the
seed, as shown in fig. 4, is a narrow elongated scar called
the hilum: it was at this point that the stalk grew by
which the bean was originally attached to the inside of
the bean-pod ; and it was through this stalk that the food
was transferred from the mother plant into the developing
seed. Near one end of the hilum is a hole known as
the micropyle which, when the seed was an ovule,
played an important part in the process of fertilisa-
tion. At present we need only note that it is
near the micropyle that the growing root escapes
CH. II]
BEAN.
17
Fm. 4.
A. SEED OF THE BEAN, Vicia Faba, in a dry state.
B. THE SEED riviDED LONGITUDINALLY.
C. GERMINATING SEED (adapted from Sachs).
D. SEEDLING PLANT.
/?, hilum. TO, micropyle. t, testa.
c, cotyledon. p, plumule. r, radicle.
In fig. C, c is the stalk of the cotyledon.
D. E. B.
18 BEAN. [CH. II
from inside the seed-coats; as it does so the testa is
seen to give way in the form of a triangular flap, which is
shown in fig. 12. But before this stage of germination is
examined the structure of the bean-seed must be further
described. On splitting it open, the young plant inside is
seen. By far the larger part of the plant is made up of
two thick fleshy lobes, whose inner faces are flat and lie
against one another, and whose outer faces are slightly
rounded and impress their form on the seed. These are
the two cotyledons or first leaves of the young bean-plant.
Similar cotyledons are familiar to most people in split
peas, which consist of little hemispheres, each being a
cotyledon ; in the almond too, the oval cotyledons, flat on
one side rounded on the other, are familiar enough. The
cotyledons of the bean are attached, by stalks at their
bases, to a minute stem, one opposite the other. This
axis is what will develope into the stem of the bean
at one end, and into the root at the other. The end
which grows into a stem and which lies between the
cotyledons is the plumule, the other end which terminates
in the primary root is known as the radicle. We see in
the bean our first example of the general plan of archi-
tecture common to a great number of the flowering plants.
The plant consists of a short axis or stem-like part, from
which spring side growths, — in this case primary leaves or
cotyledons. We have here, too, an instance of the division
of the plant body into two parts destined to have dif-
ferent functions and corresponding!}' different structures —
namely a roo£-half, and a stem or shoot-halt It is an
example of a general characteristic of plants that very
CH. II] BEAN. 19
early in their development we can draw a transverse line
across the embryo which shall divide it into two distinct
morphological regions, a point which will be more clearly
realised when the embryology of plants is studied.
In the growth of the seedling bean, the first thing that
happens is the elongation of the radicle: it is not until
the radicle has grown considerably that any striking
development of the plumule takes place. This order of
growth has a clear biological importance; the young
plant must get a hold on the soil before it can raise a
structure such as a stem above the ground.
An interesting fact about the plumule is its hook-like
form. When a bean is planted beneath the surface of the
ground, the part of the plumule which emerges is the
curved outline of the hook : it pushes its way through, and
makes a path for the delicate tip of the plumule which
follows it. If the plumule were straight, the tip would
have to make its own way through the soil at the risk of
being injured.
The most striking fact about the cotyledons of the
bean is that although they are undoubted leaves, they
never assume the appearance or functions of ordinary
leaves; they do not become green, and they are never
expanded in the air and light, nor do they increase in size l.
Without growing themselves, they give up their accumu-
lated reserve material to the radicle and the plumule.
It is not necessary to consider the nature of all the
1 The only growth is that of the stalks of the cotyledons, by which
the plumule is freed from its position between the cotyledonary lobes
and enabled to grow freely upwards : see fig. 4, C, c.
2—2
20 GOURD. [CH. II
reserve matters contained in the cotyledons, it will be
sufficient to call attention to one class of food — the
carbohydrates. If a thin section of the cotyledon is cut,
the cells which make up its tissue are found to be
crowded with starch grains which give the characteristic
blue colour with iodine1. The large quantity of starch in
the cotyledons may be roughly gauged by a simpler test,
namely, by touching the cut surface with iodine solution,
when the whole mass becomes dark blue or almost black.
Further details about starch are given in the section
devoted to the potato.
Gourd or Pumpkin (Cucurbita).
The seed of the gourd, shown in Fig. 5, is flattened,
oval in outline and marked with a characteristic thickened
border. At the square end is the hilum, or scar where
the stalk grew, and also the micropyle. The position
of this is shown by the outline of the radicle seen
through the closely fitting seed-coats, and pointing to a
spot close to the hilum. In the gourd, as in the bean,
the cavity of the seed is found to be occupied by a young
plant — and a plant, moreover, consisting of two large flat
cotyledons attached opposite one another to a central axis
made up of the plumule and radicle. Another resemblance
is that here as in the bean the cotyledons contain reserve
materials on which the growing plumule and radicle feed2.
But in other respects the process of germination is
1 Either the alcoholic tincture or iodine dissolved in potassium
iodide solution.
2 In the gourd, oil takes the place of the starch in the bean.
CH. II]
GOURD.
21
strikingly different from that in the bean. The cotyledons
do not remain inside the seed-coats, they throw off that
FIG. 5.
GERMINATION OP THE GOURD (Cucurbita).
A, the seed.
B, the seed laid open, showing the embryo ; one veined cotyledon and the
radicle are visible.
C, the radicle has grown out from the micropyle and curved downwards
geotropically.
D, the peg or heel has caught on the seed-coat while the growth of the
arched hypocotyl has nearly freed the cotyledons.
E, the cotyledons are freed and the hypocotyl has become straight.
F, the first foliage leaf has appeared.
covering, emerge from the soil in which the seed was
buried, and begin in fact to lead the life of true leaves.
22 GOURD. [CH. II
That is to say, they become green, and in gaming
chlorophyll they at once endow the young plant with the
power of earning its own living, because they give it the
power of gaining carbon from the air to be built up into
the store of organic material already existing.
The manner in which the gourd germinates is in some
ways unique. The radicle as it emerges from the seed
grows downward and fixes itself in the soil1. On
its lower side a sharp projection or peg grows out as
shown in fig. 5. The peg serves to hold down the seed-
coat while the cotyledons (with the plumule between
them) are extracted. This extraction is effected by the
growth of that part of the primary axis of the plant
which is just below the cotyledons, and which is known
as the hypocotyl. A simple proof that the peg is really
of value may be got by removing that part of the seed-
coat on which the peg should act; when this has been
done the cotyledons remain in the seed; although they
are finally freed by their own growth bursting the testa.
When the arched hypocotyl has made its way through
the soil it straightens itself, and points vertically upwards ;
the cotyledons increase in size, develope chlorophyll and,
instead of remaining face to face, open out and take up a
roughly horizontal position, thus exposing their upper
surfaces as efficiently as possible to the light. The
plumule then begins to increase vigorously and the plant
soon grows out of the stage in which it can be called
a seedling. The most striking feature in the developing
plumule is that it bears leaves having no resemblance
1 See the account of Geotropism in Ch. III.
CH. II]
POTATO.
23
to the cotyledons ; they are not only of different shape and
consistence, but are differently arranged on the stem.
Potato (Solanum tuberosum).
The accumulation of reserve material is by no means
confined to seeds, and it is especially well seen in those
underground parts of plants which are known as bulbs
and tubers. The potato is a good example of the tuber,
and the fact that it contains a store of food intended for
the future use of the plant, but diverted for his own use
by man (and by the potato-disease fungus) is sufficiently
familiar.
FIG. 6.
HORIZONTAL UNDERGROUND STEM, or rhizome of a sedge, sending adven
titious roots downwards and leaves upwards.
From Le Maout and Decaisne.
24 POTATO. [CH. II
Although the tuber of the potato is formed under-
ground, it is essentially a stem and not a root. It is only
one instance of a common state of things, namely, that
the underground parts of plants are not necessarily roots.
Many plants have creeping underground stems, like the
sedge shown in fig. 6. A similar morphological arrange-
ment will meet us in the fern. In the case of the
potato the thing is not so evident ; perhaps the most
striking proof that can be offered to one who has no
knowledge of morphology is that under certain conditions
tubers are formed on the aerial stem of the plant as in the
specimen sketched in fig. 7.
T—
Fio. 7.
FORMATION OF TUBERS ON THE AERIAL STEM OP A POTATO-PLANT.
T, T, tubers : L, the stalk of the leaf in whose axil TT appear.
Moreover fig. 8 suggests that the elongated organs
which end in tubers are branches, since they spring from
the axis above the cotyledons and therefore a fortiori
above the line dividing root from shoot.
CH. Il]
POTATO.
25
At present it may be taken for granted that the tuber
of the potato is a stem. It is a stem in which growth in
T
Fm. 8.
SEEDLING POTATO-PLANT BEARING TUBERS.
L, leaves. <7, cotyledons. TT, tubers.
R, root.
thickness has been excessive, as may be seen by comparing
the tuber with the stalk which bore it. The biological
meaning of the tuber is illustrated by the use to which
gardeners put it in the culture of the plant; instead of
sowing the seed of the potato they cut up the tuber into
bits, and plant these ; they take advantage, in fact, of the
part which the tuber is destined to play in the natural
course of the plant's life, namely, to provide for the con-
tinuance of the species.
It is a bit of everyday knowledge that the gardener
cutting up a potato for "seed" takes care that each bit
26 POTATO. [CH. II
shall contain an "eye." The eyes of the potato are
little crumpled or withered looking nodules sunk in
depressions on the surface, which the unwary might pass
over as diseased spots, or as due to other casual injury.
The bud-like character is apparent on cutting a section
(at right angles to the surface of the potato) through the
eye; such a section shows a dwarf stem and very small
leaves. The scars which occur at the eyes are the remains
of rudimentary scale-like leaves, which are plainly visible
in the young tuber. The fact that the " eyes " grow in the
angles (axils) of these leaves is another point demonstrating
the stem-like character of the potato. This point is more
fully dealt with in Chapter IV.
There is one point which strikes the observer who
compares the growth of a potato bud with that of a seed :
namely, that in the bean there is a radicle ready to grow
into the root-system of the plant, whereas in the potato-eye
there is a young stem but no young root. Nevertheless
the potato plant, which grows out of the eye, has roots,
and the question whence they come has to be answered.
They will be found to grow out of the stem of the
developing plant. When this occurs the growth is called
adventitious. A familiar example of adventitious roots is
to be found in the ivy, where the roots, by which the plant
adheres to and clambers up a wall or tree, grow out of the
branches, as shown in fig. 9.
With regard to the nature of the food-stores in the
potato we shall only consider the carbohydrate part of
the reserve — namely, the starch. The potato is one of
the commercial sources of starch, supplying, for example,
CH. II]
STARCH.
27
that used by washerwomen. In the laboratory it supplies
a material for the microscopic study of starch. A section
Fm. 9.
YOUNG BKANCH OF IVY,
showing adventitious roots.
From Le Maout and Decaisne.
through the flesh of the potato shows the following things: —
on the outside is a layer of cells arranged in a regular
manner like bricks one over the other. These form a layer
of cork serving as a protective layer on the surface ; it will
not be necessary to consider it in any detail, but it should
be noted that it is beneath the corky layer that the starch
is found. The main body of the tuber is made up of paren-
chyma, a tissue of simple cells fitting together like bees'
28 STARCH. [CH. II
cells in a honeycomb, and rounded or angular in outline.
It is in the cavities of these six-sided cells that the
starch grains are stored, and in such quantity that the
cells are crammed with them. A few grains are shown in
fig. 10. A drawing does not indicate the peculiar bright,
FIG. 10.
STARCH GRAINS FROM THE TUBER OP THE POTATO, highly magnified,
showing the stratification.
shining appearance of the grains, but it shows their othei
chief feature, namely, their finely striped appearance. The
fine lines indicate what is known as the stratification of the
grain, that is to say, that it is made up of a set of shells
one within the other. The shells are not loose, one from
the next, but are rather like the sheets of paper that go
to make up cardboard. Stratification is not peculiar to
starch, it is an important feature in the structure of
cell walls — as will appear in a later chapter, where too we
shall have to consider the origin and meaning of the
stratification.
I have spoken of the transference of food material
from reserve-stores to places where growth is going on
and where therefore food is needed. The transference of
starch from one part of a plant to another depends on
CH. Il] TULIP. 29
the power which the plant has of converting starch into
sugar. This power depends on the possession by the plant
of a ferment called diastase.
The action of diastase will not be considered in detail
but it is worth noting that the essential features of the
process may be studied in any brewery. Barley is made
to germinate, its starch changes into sugar and is trans-
ferred to the sprout of the grain. Finally this sugar is
used by man with the help of another plant — yeast — to
make alcohol.
Jerusalem Artichoke (Helianthus tuber osus).
The Jerusalem Artichoke supplies an example of a
state of things similar to that described in the potato ;
the tuber is a swollen underground stem stored with
reserve-material1 and bearing buds corresponding to the
eyes of the potato. They are relatively larger and more
obvious than those of the potato, and the leaves in whose
axils they grow are easily seen.
Tulip (Tulipa gesneriana).
When the flowering stem of the tulip appears above
the ground in the spring, it does so by means of growth
carried on by the expenditure of reserve material stored
up in the underground bulb ; so that from a physiological
point of view, the interest of the tulip-bulb is the same as
that of the tubers described above. Morphologically how-
ever it differs from these ; the chief bulk of the bulb is not
a solid mass like the potato, but is made up of fleshy scales
1 The carbohydrate is not starch but inulin.
30 TULIP. [CH. II
fitting closely over one another. These are morphologically
leaves, and it is for this reason that it bears a different name
— bulb, because its reserve materials are stored not in a
thickened stem but in specially modified swollen leaves.
Nevertheless its resemblances to a tuber are more
important than its differences, for the swollen scaly leaves
are necessarily borne on a stem, so that the bulb only
differs from the tuber in the predominant development of
leaves, which are insignificant in the last named.
It is well to begin by examining a bulb growing
in the garden during the summer. In the centre is
seen the stalk which, during the spring, bore leaves
and flower; it can be seen to be continuous with the
main axis of the bulb which bears the scales. It should
be noted that these scales are no longer plump and
fleshy, but dry and withered ; this is because they have
yielded their stores to supply material for the development
of the flowering stem. Since the bulb is exhausted it is
not obvious in what way next year's flowering stalk is to
be provided for. In fig. 11 (A) it will be seen that on
one side of the flower stalk a new bulb has formed:
the leaves on the flowering stem have, during the summer,
built up more organic material than the plant needed,
and this has been transferred downwards, and has led
to the growth (out of a minute bud hidden among
the scales) of a new bulb. Next spring this bulb
(B, fig. 11) will throw up a leafy and flowering stem,
will be in its turn exhausted, and will among its scales
give birth to another new bulb. The death of the old
bulb going on side by side with the development of a new
CH. II]
TULIP.
31
FIG. 11.
A, A TULIP-PLANT IN FLOWER : at the base of the flower-stalk and at the
right-hand side is seen next year's bulb developing.
B, LONGITUDINAL SECTION OF A NEXT YEAR'S BULB. (Early in September.)
(7, TRANSVERSE SECTION OF THE SAME.
/. Z, leaves borne on the flowering stem.
P. I, petals. a, anthers. gt gynoecium.
32 TULIP. [CH. TL
one produces, during the summer, a one-sided appearance
in the bulb, the flower- stalk is no longer central because
the main body of the structure is made up of the new-
born bulb, which has grown laterally and deformed the
symmetry of the whole.
CHAPTER III.
ROOT — GEOTROPISM — TISSUES — VASCULAR CYLINDER —
M ERISTEM — ROOT-CAP.
IN a bean-seed which has not begun to grow, the radicle
lies in the plane of the cotyledons and points towards the
micropyle. If a bean is sown (i.e. placed in damp soil)
with the micropyle downwards and the plane of the
cotyledons vertical (fig. 4 c), the radicle will grow straight
on in the direction in which it naturally points: but if
the bean is allowed to germinate lying on its side with
the plane of the cotyledons horizontal, this will not
happen, the radicle will bend at right angles to itself until
it points vertically downwards, and will then continue to
grow in that line as shown in fig. 12. In fact, in whatever
FIQ. 12.
GERMINATING BEAN.
The radicle (R) has curved geotropically downwards : fl", the hilum.
D. E. B. 3
34 GEOTROPISM. [CH. Ill
position the seed may be placed, the radicle will bend
until it reaches the vertical, and will go on growing
downwards towards the centre of the earth. This mode
of growth is known as geotropism, and is but one out of a
number of special powers which the plant possesses of
directing its growth according to external circumstances.
It used to be believed that the radicle attained the
vertically downward position in virtue of plasticity, that
it bent over by its own weight as a piece of sealing-wax
bends if kept in a warm place. This is quite a mistaken
view : we now know that the curvature of the root is an
active process due to a rearrangement of longitudinal
growth. That is to say the curvature results from one
longitudinal half of the root growing more quickly than
the other half. We further know that this rearrangement
of growth is a response to a stimulus quite as certainly
as that the movements of animals are brought about by
stimulation. It is not of course suggested that a plant has
consciousness, nor do we claim consciousness for muscles
or nerves. But botanists do claim for plants an irrita-
bility or sensitiveness by means of which the plant's
movements are directed to suit its environment: they
believe that by this sensitiveness the growth of the
plant is directed in the same unconscious way that the
flight of a moth may be supposed to be directed towards
a lamp. I shall return to this point when the upward
growth of the stem into the air is discussed, but I think
it is worth noting that at the very outset of the life
of the plant, in its germinating state, it is endowed
with and guided by a very remarkable kind of sensitive-
CH. Ill]
ROOT.
35
ness or irritability. This quality of growth which enables
a root to grow straight down into the ground is of
obvious use to it, for it thus fixes itself most quickly and
most effectively in the soil in which it has to play its part
in the plant's economy. Before going on to the functions
of the root it will be well to consider its structure.
FIG. 13.
TRANSVERSE SECTION OF THE ROOT OF Vicia Faba (semi-diagrammatic).
p.l, piliferous layer bearing root hairs.
c, cortex, the cells of which are not shown.
end, endodermis.
p.c, pericycle "\
x, xylem I central cylinder, or stele ; see Preface on
phi, phloem | the use of this term.
p, pith J
Fig. 13 represents a transverse section of the primary
root of a bean not far behind the tip, as seen with a
low power of the microscope. In the centre of the section
is a circular mass of cells differing in texture and aspect
3—2
36 ROOT. [CH. Ill
from the rest, which is known as the central cylinder ; the
region that surrounds it is known as the cortex, and the
layer of cells which limits the cortex, and at the same
time limits the outer surface of the root, is the piliferous
layer. If the central cylinder is examined a little more
closely it will be seen that it presents certain obvious
patches imbedded in substance not unlike the cortex in
general appearance. These patches are elongated masses
or ropes of tissue running longitudinally in the root and
known as vascular strands. Both the cortex and the sub-
stance in which the bundles are imbedded are made up
of tissue which like the parenchyma of the potato tuber
is built of cells whose length is not strikingly different
from their width.
In distinguishing the vascular strands from the rest of
the root, histologists make use of the word tissue: they
speak of vascular tissue and parenchymatous tissue. It is
extremely desirable, but by no means easy, to seize and
define the meaning of this important term. When a mass
of objects is presented to us, our impulse is to classify
them ; and the finer elements in vegetable and animal
structure are classified into tissues. But not every classi-
fication that can be made is a classification into tissues.
The conception is to some extent arbitrary, and has to be
learned rather than evolved from general principles. It is
possible, however, to give certain characteristics common
to tissues.
One such characteristic is that the cells or elements
making up a tissue obey a common law of growth.
Thus the vascular strands in the root, although made
CH. Ill] TISSUES. 37
up of numerous cells, have a sort of individuality : each
cell grows, and behaves generally, as if it were coordinated
with all the other cells of the strand. The cells making up
the vascular strand behave like the soldiers of a regiment,
and give to the strand the same sort of unity that comes
from the combined and ordered behaviour of drilled men.
On the other hand many tissues are chiefly character-
ised by being made up of a mass of similar cells. Thus
the tissue in which the vascular strands are imbedded
is a mass of simple rounded or angular cells, to which
the term parenchyma is applied, as in the case of the
similar tissue in the potato tuber; here the criterion of
unity of growth is not so obvious.
Lastly some tissues are more especially tissues by birth-
right : that is to say they are classified together because
they are found to be developed in a similar way from an
embryonic cell or group of cells. Examples of tissues in
which this character is strong will be met with later on.
I am now concerned to point out the difficulties which
meet the beginner in trying to seize the idea of a tissue.
On the whole it is best to let the conception grow
gradually : if he works out the histology of plants in the
laboratory, and reads books in which the terminology
is not incorrect, he will gain the idea in the best and
easiest manner.
Root- cap.
If a bean-root is held up against the light, it will
be seen that it ends in a conical point, 0, fig. 14, and that
inside the root a curved outline M can be dimly seen. The
38
ROOT-CAP.
[CIL III
main body of the root ends at M, and the part that gives
the conical form seems to be made of less dense material.
FIG. 14.
A ROOT- CAP DIAGRAMMATICALLY REPRESENTED.
R, root. M, meristematic region. C, root-cap.
These appearances, somewhat obscurely seen with the
naked eye, correspond to actual and important facts.
The cap-like part G is a structure highly characteristic
of roots, a region of the root of great interest and im-
portance, known by the name of root-cap. The surface
of the root-cap is slimy because the most outward
of its constituent cells are constantly becoming dis-
organised, and in the natural life of the bean they are, as
they die, rubbed off against the resisting soil penetrated
by the root. In spite of this wear and tear the root-cap
is not entirely worn away : this should suggest that, like
the skin on the human hand, it is renewed underneath as
it is worn away outside. This is the case, and it is at the
CH. Ill]
MERISTEM.
39
region M about the centre of the limiting line, that new
cells are being manufactured to replace the ones that are
lost. Not only is the region M the manufactory of root-cap
cells, but it is also the manufactory of cells which go to form
part of the main body of the root. Thus if we were to
examine a longitudinal section of a root, and if we carried
our observation along the centre line RMC in fig. 14, we
should discover this remarkable state of things : — well-
grown differentiated cells at R, and again at C differentiated
root-cap cells, and between them at .M" a small quantity
of meristematic tissue, minute, delicate, simple undiffer-
\
B t
d
* I
a
: D
IV
in
c
FIG. 15,
DIAGRAM ILLUSTRATING MERISTEMATIC (OR MERISMATIC) TISSUE.
I. a meristematic cell ABGD. II. a cross-wall db has appeared.
III. AabB has grown and again equals ABGD in size, while aCDb has also
grown.
IV. AabB has been divided by a cross- wall cd.
V. AcdB has again grown, it equals ABCD in size and is ready again
to divide. Meanwhile cabd and aCDb have increased in size con-
siderably.
40 MERISTEM. [CH. Ill
entiated cells, which will in time give rise in their turn
to root on one side and root-cap on the other. This
tissue makes up what is known as the growing point of
the root and of the cap. Several things have to be noticed
about meristematic tissue : one is what may be called
the quality of perpetual youth. Let ABGD, fig. 15, be a
meristematic cell, and let it be divided into two com-
partments by the transverse cell-wall ab. The lower half
will give up its embryonic character and will begin to
make part of the permanent plant-body. But the other
half AabB retains the embryonic merismatic character,
and when it has again grown to its original size
it again divides by a line cd. The process may be
repeated indefinitely, so that we get a row of cells of
which the topmost retains the capacity of continued
division and all the rest are on the way to become
permanent tissue.
I began by speaking of M (fig. 14) as a manufactory of
cells, and this is a convenient expression, but it must
always be understood that in such a manufactory, cell
originates from cell, and that the process of manufacture
is cell division of such a sort that half the divided cell
remains capable of keeping the work going.
In thinking over the growing point of a root we are
liable to fall into a false conception; we think of cells
being manufactured one on the top of the other like
bricks which make up a wall, and we may imagine,
when the new layers of cells have been made and laid on
the older layers, that they have done their work, that they
have increased the size of the plant by their diameter, just
CH. Ill]
ROOT-TIP.
41
as bricks put on raise a wall by a single course only. But
this would be quite wrong ; each cell that has been created
by cell -division at the growing point undergoes great
increase in size before it becomes a permanent member
of the root.
The root of the bean is not so easily understood or
so instructive as that of the maize, and the drawing
(fig. 16) which illustrates root -structure is therefore taken
from that plant.
FIG. 16.
DIAGRAM ILLUSTRATING THE STRUCTURE OF THE ROOT-TIP IN
LONGITUDINAL SECTION.
. c, cortex. v.c, central cylinder. r.c, root-cap.
The points to notice are (1) the central cylinder cc
seen in longitudinal section, ending a dome-like mass of
meristem. Then (2) the cortex which thins away to a layer
of meristem only one cell thick, which keeps forming
new cortex cells. Then (3) the sharply marked root-cap ro
whose new cells are made by another layer of meristem.
42 ROOT. [CH. Ill
The central cylinder is surrounded by an envelope or
cylindrical sheath, one cell in thickness, which in section
FIG. 17.
PART OF FIG. 13, TRANSVERSE SECTION THROUGH THE ROOT OF Vicia Faba,
under a high power.
c, cortex. p, pith. x, xylem. ph, phloem.
e, endodermis ; in this specimen the " spindles " on the radial walls were
only apparent opposite the phloems.
p. c, pericycle, which in the bean is of two layers opposite the phloems.
shows as a ring of cells. This is known as the bundle-
sheath or endodermis. It may be recognized because the
radial walls, those which separate cell from cell, have a
peculiar appearance, due to their being delicately un-
dulated, and producing the effect of a dot or spindle.
In the root of the bean however this character endodermis
is not easily seen.
Within the endodermis comes another sheath of cells
called the pericycle : this is usually a one layered sheath,
but in the bean it is in places several cells in thickness.
CH. Ill] VESSELS. 43
The eridodermis is the innermost layer of the cortex :
the pericycle the most external layer of the central
cylinder.
Within the bundle -sheath 8 or 10 patches of tissue,
4 or 5 being of one kind, 4 or 5 of another, alternate
with each other as shown in fig. 13, where phi and x
alternate as the eye travels round the circumference of
the axial cylinder. The patches marked x are known as
xylem, the alternate ones phi are called phloem. Xylem
and phloem are the constituents which, in vascular plants,
i.e. plants with vessels, make up the vascular tissues. At
present we are only concerned with the xylem ; it is made
up of vessels, a vessel being a pipe or tube built up of
cells placed end to end, the constituent cells of a vessel
being excessively long in proportion to their diameter.
The striking feature about them is that they have no
cell-contents ; the protoplasm which they originally con-
tained, and which regulated their behaviour whilst they
were developing, dies and disappears. Moreover the
cross walls, which are the end walls of the constituent
cells, become disorganised, and disappear, either in part or
completely, so that the vessels finally come to be elongated
tubes without protoplasmic contents. The walls of the
vessels undergo moreover a peculiar change, they are no
longer ordinary cellulose ; they have been lignified, —
changed in such a way that they no longer react chemi-
cally like cellulose.
The root is thus seen to be characterised by the
presence of elongated tubes running along its whole
length, which might suggest the transference of fluid
44 ORIGIN OF [CH. Ill
through the raot; and this in fact is their function, for
it is through these pipe-like vessels that the water
collected by the roots in the soil is transmitted to the
parts of the plant above ground. The absorption of water
by the root requires among other things that the root
shall present a large surface to the soil. It is only by the
extraordinary multiplication of surface that the plant is
able to perform what seems an impossibility : thus, if a
plant is kept for some time without water it is found,
just before it finally withers, to be obtaining water from
soil apparently as dry as dust. It is not usually realised
to what a depth and width roots extend; in a field of
winter wheat the roots have been found reaching to seven
feet beneath the surface, and in a single oat-plant it
was calculated that the length of the root including its
branches was 150 feet1.
This manner in which roots branch has therefore some
importance.
The roots which grow out from the primary root are
called secondary, these in their turn give off tertiary roots.
The first thing that strikes the observer is that it is only
on the older part of the root that secondary roots are seen.
Near the base (fig. 4), i.e. on the oldest part of the root,
are seen the longest, i.e. the oldest secondary roots ; and in
the region below they are shorter, while they are not to be
seen in the apical region. The figure also shows what is
characteristic of the secondary roots, namely, that they
are arranged in longitudinal rows, the roots in each ro\v
being accurately one above the other. This arrangement,
1 Johnson's 'How Crops Grow,' Dyer and Church's edit. 1869, p. 233.
CH. Ill] SECONDARY ROOTS. 45
which gives a curiously formal, symmetrical look to a
branching root, depends on the fact that the secondary
roots spring from opposite the xylems, and since the
strands of xylem run straight down the primary roots, it
follows that the bases of the secondary roots run also in
vertical lines. The primary roots, as explained before, have
a quality of growth which enables them to grow straight
down; the secondary roots have a similar, but not an
identical, quality of growth, in consequence of which they
grow, roughly speaking, horizontally, or rather somewhat
obliquely. In this way they parcel out space between
them, the four secondary roots emerging at any given
level, run out north, south, east, and west. It is clear
that there will be unoccupied soil between the secondary
roots, especially when they have grown to some length ;
this space is taken up by the tertiary roots, and these are
not guided by any directive quality of growth in relation
to gravity but run out upwards, downwards, right and
ieft, thus making the most of the vacant places.
One other characteristic of the growth of secondary
(and tertiary) roots must be described. In the fig. 4, at
the base of each secondary root, in the row facing the
observer, can be seen a vertical slit or cleft, through which
the root passes. This is explained by the mode of origin
of these organs, namely, that they arise in the pericycle.
That is to say, one or more cells in the pericycle of the
primary root begins to divide and form a mass of new
cells which constitute a very young secondary root. In
this stage, shown in fig. 18, it is obviously invisible from
the outside, since it is covered in by cortex. As it grows
46
ROOT-HAIRS.
[CH. Ill
in length it pierces the tissues and burrowing through the
cortex breaks out at the surface, leaving in the cleft
surrounding its base, evidence of its internal manner of
origination.
FIG. 18.
TRANSVERSE SECTION OF THE PRIMARY ROOT OF THE BEAN,
showing a secondary root developing.
p. I, piliferous layer. c, cortex. &. s, endodermis.
x, xylem. phi, phloem. p, pith.
The surface of root in contact with the soil is still
further increased by the growth of what are known as
root-hairs. These can be especially well seen in seedlings
of the mustard, cabbage, or one of the cereals. A seedling
mustard which has germinated in damp air gives the
appearance shown in fig. 19. The base of the root, where
it joins the young stem, bears a dense frill of delicate
colourless hairs ; nearer to the tip of the root they are
younger and therefore shorter, and at the tip of the root
they are not found. A transverse section (fig. 13) would
CH. Ill] ROOT-HATRS. 47
show that each hair is an external cell elongated by
growth at right angles to the surface of the root. When
FIG. 19.
MUSTABD SEEDLING,
showing the cotyledons (C), and the root covered in its older
part by root-hairs (R).
it is understood that each of the innumerable rootlets of
a well-grown plant bears root-hairs, it will be realised
how enormously the surface of the root is multiplied.
The nature of the contact between the plant and the soil
is extremely close ; if a plant is removed from the soil
and examined under the microscope it can be seen how
the root-hairs press against, even to some extent wrap
round and adhere to, minute particles. The adhesion of
the root-hairs to the soil can be simply demonstrated by
pulling a seedling up from loose soil, when it presents the
appearance shown in fig. 20. The apical region of the
root comes up clean and bare, while the basal region is
shaggy with its coat of earthy particles. In older plants
a further fact may be demonstrated in the same way.
In these the basal, as well as the apical part of the root
is bare, because the root-hairs are short-lived organs, and
where they are dead the root does not retain its envelope
of soil.
I have spoken of the root absorbing water, but it must
48 ROOT-HAIRS. [CH III
be remembered that the water so absorbed is the vehicle
by which the plant receives two-thirds of its food material,
FIQ. 20.
A SEEDLING WHEAT WITH SOIL ADHERING TO THE ROOTS. (After Sachs.)
for it is in this way that nitrogen in the form of nitrates,
sulphur in the form of sulphates, phosphorus as phosphates,
calcium, potassium, magnesium and iron reach it. To this
part of the subject I shall return.
CHAPTER IV.
THE STEM OF THE SUNFLOWER. MORPHOLOGY AND
HISTOLOGY.
THE present chapter deals with the structure of stems,
and for the sake of convenience this part of the plant-
body is studied in the sunflower instead of in the bean or
pumpkin. The sunflower, Helianthus annum, is a near
relative of the Jerusalem artichoke, H. tuberosus, and what
is here said applies, speaking generally, to both.
The germination of the sunflower is of the same
general type as that of the pumpkin; the embryo has a
pair of large fleshy cotyledons loaded with reserve matter,
which like those of the pumpkin expand above ground
and function as leaves, that is to say, they become green
with chlorophyll and they assimilate. Between the coty-
ledons is the minute plumule ; it bears a number of
undeveloped leaves crowded together, and surrounding
a growing point. It is in fact a bud which will lengthen
out into a tall stem on which fully developed leaves will
take the place of the semi-developed ones now clothing it.
The growing point at the extremity of the plumule differs
in detail from that of the root ; it has for instance
D. E. B. 4
50 SUNFLOWER. [CH. IV
nothing corresponding to a root-cap, but it has the
essential characteristics of places where new cells are
manufactured by division; it in fact possesses the em-
bryonic character, or quality of continual youth.
In a growing plant in which the stem has begun
the process of elongation several important points may
be noted: —
In the first place the leaves are markedly different
in form and texture from the cotyledons. But the most
striking point to be noted is one which, by its extreme
familiarity, tends to be forgotten, — namely, that the stem
is divided into alternate regions, (1) which have, and (2)
which have not, lateral outgrowths. The places where the
leaves spring out are known as nodes (fig. 21), and the
alternating leafless regions as internodes. Thus, like the
body of a worm or an insect, the plant-body is segmented
into a number of definite regions from which the lateral
appendages spring. The distinction into nodes and in-
ternodes comes out clearly in the growth of a bud, where
a certain division of labour is apparent : the nodes, which
bear the leaves, do not increase in length ; while the in-
ternodes, free from leaves, increase greatly in length. The
unfolding of a bud is therefore the simultaneous growth
of the internodes, and of the leaves at the nodes.
There is an important difference between the manner
of development of the lateral outgrowths of stems and roots.
The secondary roots have their origin deep down in the
tissues of the primary root. In the stem the outgrowths
arise on the surface. The growing point of the stem will
be found to end in a blunt, rounded end, and below this to
CH. IV] MORPHOLOGY. 51
present a series of rounded excrescences, those nearest the
point being the youngest and smallest, and those further
FIG. 21.
STEM OF PIMPERNEL (Anagallis).
To illustrate the alternation of nodes and internodes.
(From Le Maout and Decaisne.)
away being older and approaching more and more the
form of young leaves.
If the stem is split longitudinally in such a way
4—2
52 SUNFLOWER. [CH. IV
that the section also bisects a leaf and leaf-stalk longi-
tudinally, it will be seen that in the acute angle between
the leaf-stalk and the stem, i.e. just above the insertion of
the leaf, is a bud, similar in character to the small bud
which lay between the cotyledons and gave origin to the
main stem. This bud which, if it developes, will grow out
into a branch bearing leaves in its turn, is called axillary,
from its position in the axil, or armpit-like angle above
the leaf (see fig. 21), and it is a fact of broad general
significance that normal branches always spring from the
axil of a leaf. This general law is useful in understanding
the structure of certain plants : of this the potato- and the
artichoke-tuber have already supplied instances, for the
eyes or buds grow in the axils of leaves whose traces are
visible in the markings on the surface of the tubers. It
makes it possible also to understand such a plant as the
Butcher's Broom (Ruscus aculeatus). Here the observer
sees the stem beset with flat green outgrowths, which
he naturally takes for leaves, until a closer examination
shows him that each outgrowth springs from the axil
of a scale-like rudimentary leaf. This observation points
to what is the truth, namely, that the outgrowths are
flattened branches functioning as leaves, and taking the
place physiologically of the useless but true leaves in
whose axils they grow.
Although it is not always possible to tell a leaf
from a branch by its appearance, there are nevertheless
some characters by which the two can be distinguished.
In the first place, leaves differ from branches in the
limited character of their growth. A leaf soon ceases to
CH. IV] MORPHOLOGY. 53
have any embryonic tissue, in other words, all the cells
which compose the leaf soon take on a permanent
character. Compare for instance the leaf of an oak
and the branch that developes from the bud in its axil ;
the leaf increases until it attains a length of 2 inches
or so, and then it grows no more, but the branch grows
year after year and continues to bear, in the buds which
cover it, innumerable growing points.
Another point is of importance; the flowers, which
are the reproductive parts of a plant, are borne on
branches, whereas the leaves do not bear flowers. This
may be illustrated again by the Butcher's Broom, whose
flowers grow on the flattened leaf-like branches above
described. The position in which the flower is borne
has also a wider morphological importance. It is one of
the characters that distinguish the whole of the root-
system from that of the stem. At first sight it seems
absurd to appeal to such a character to mark off root
from stem, since a typical stem and a typical root are
so different in appearance. But in the potato an under-
ground rootlike stem has been met with, and in the fern
another instance will occur. Roots, on the other hand,
are by no means always underground : the aerial roots of
the ivy have been described, and many such cases occur ;
we must therefore look for more fundamental distinctions.
It has been seen that a stem, the potato-tuber, may be
devoid of chlorophyll, but it might have been hoped,
when a part of a plant is found to be green, that then at
least we should know it not to be a root. But certain
tropical orchids have flat green aerial roots which actually
SUNFLOWER.
[CH. IV
do the work of leaves in assimilating carbon. The cri-
terion of the absence or presence of chlorophyll fails
therefore, and we are driven to see that the absence or
presence of flowers may be of value.
The obvious external characters of the sunflower stem
may be summarised before going on to its microscopic
characters. The points to be noted are that it is a
vertical structure divided into nodes and internodes, the
nodes bearing opposite1 leaves, or in the case of the
lowermost node opposite leafy cotyledons ; it terminates
in a growing point and bears buds (which are potential
branches) in the axils of its leaves.
FIG. 22.
DIAGRAMMATIC SECTION OF THE STEM OF HELIANTHUS.
ep, epidermis. /, pericycle-fibres. x, xylem.
c, cortex. ph, phloem. mr, medullary ray.
e, endodermis. c6, cambium. p, pith.
A transverse section of a young Helianthus stem
presents certain resemblances to the section of a root, while
1 In Helianthus tuberosus the leaves are in sets of three, and there is
some irregularity in their arrangement in the sunflower.
CH. IV] CENTRAL CYLINDER. 55
it differs from it strikingly in detail. The resemblances are,
however, of a more fundamental nature than the points of
difference. The chief part of the section is taken up by
a cylinder, the central cylinder, which corresponds to the
region bearing the same name in the root, with which it
is indeed continuous. In the diagram, fig. 22, the cy-
linder is marked out by the double line e. Outside the
central cylinder is a region known as the cortex : the cortex
is covered by a single layer of cells ep, forming a special
tissue known as the epidermis. This tissue is of great
importance both morphologically and from the point
of view of function, and will be considered in detail
in a later chapter. The most internal layer of the cortex
is the line e, fig. 22, already referred to, which bears a
name identical with the corresponding layer in the root,
viz. endodermis. In the stem it generally pursues a
wavy course, as in the figure, and may be easily recognized
by the presence of starch grains in its cells. In the
centre of the central cylinder is a large mass of pith, p :
in the growing condition the pith is juicy, soft and greenish
in colour ; but after a time, long before the whole plant
dies, the pith changes its character : its cells die, that is
to say, the protoplasm inside them dies ; they no longer
contain cell-sap but become filled with air. It is now
no longer a green and sappy, but a dry, white, spongy and
very light substance, like the pith of a woody elder
branch, or like the pith used in the laboratory in cutting
sections.
Surrounding the pith is a broken ring, made of a
series of dots; this ring feels hard and woody to the
56 SUNFLOWER. [CH. IV
razor, and if the stem is split longitudinally it will be
obvious that each dot corresponds to a rope or strand of
fibrous tissue running down the stem. Each of these
dots is a vascular bundle, and is seen in fig. 22 to be made
up of a mass of vascular tissue (ph, oc) and of a patch of
fibres (f). These fibres1 were, until recently, called
bast fibres ; they are now usually described as pericycle-
fibres because they originate in the region known as the
pericycle, which forms the external limit of the central
cylinder.
It should be noted that between the vascular bundles,
avenues of parenchyma run towards the cortex : these
radiating paths by which, except for the pericycle, pith
and cortex are joined, are known as medullary rays, and
will be seen later on to be of great importance.
Each vascular bundle consists of three kinds of tissue,
as seen in figs. 22, 23, 24.
I. Xylem, nearer the pith.
II. Phloem, nearer the cortex.
in. Cambium, between the two.
The arrangement differs strikingly from that of the
root where free strips of xylem alternated with free strips
of phloem ; here each vascular strand contains both xylem
and phloem. (See Preface on the word stele.)
1 There seems to me no substantial inaccuracy in using the term
bast for the pericycle fibres as well as for the hard elements of the phloem.
The word becomes purely descriptive and does not assert a common
origin of the tissues to which it is applied.
CH. IV]
XYLEM, PHLOEM.
57
The word xylem indicates that it is the woody part
of the bundle, and it will appear later that the wood of
trees, in the ordinary sense of the word, is in fact chiefly
PP-
FIG. 23.
TRANSVERSE SECTION THROUGH THE STEM OF Helianthus tuberosus,
THE JERUSALEM ARTICHOKE.
e, endodermis.
s. t, sieve tube.
cb, cambium.
m. r. p, medullary ray.
x. /, xylem fibre.
c, cortex.
/, pericycle-fibres.
c. c, companion cells.
i. cb, interfascicular cambium.
d. v, dotted or pitted vessel.
p. x, spiral vessel.
p. p, pith.
made up of the xylem of a great number of vascular
bundles.
58 SUNFLOWER. [CH. IV
The word phloem on the other hand points to a
likeness to the bark of trees, and here again it will appear
that the term is well used, since the phloem part of the
bundle in the sunflower is a tissue allied to the external
tissues of trees.
The most essential character common to the xylem
and phloem is that which gives to both the quality
of vascular tissue, namely, the fact that they consist
largely of tube-like elements or vessels built up of long
cells placed end to end. In the xylem as well as in
the phloem there are also non -vascular tissues made up of
cells not fitted together into tubes. So that xylem and
phloem are made up of: — xylem vessels and xylem
cells, phloem vessels and phloem cells.
The points of difference are perhaps more striking
than the points of resemblance. The xylem vessels (like
the vessels in the root which were briefly examined)
are hollow tubes empty of protoplasm, whereas the vessels
of the phloem contain protoplasm. The xylem vessels
have but few partitions, the cross-walls of the constituent
cells having mostly disappeared. The cross-walls in
the phloem-vessels have not disappeared, and moreover
present a peculiarity which is especially characteristic,
and which has given rise to the name sieve-tubes by
which these vessels are known. The cross- walls of the
sieve-tubes are perforated by numerous holes (like a
sieve), and through these holes one constituent cell
communicates with the next in the row. Not that the
cavities communicate, for the minute holes in the sieve-
plates (as the perforated cross-walls are called) are filled
CH. IV]
XYLEM, PHLOEM.
59
by delicate ropes of protoplasm, by which means there is
continuity of the living element from cell to cell through-
out the sieve-tube.
Another point of difference between the xylem vessels
and the sieve-tubes (phloem vessels) is the character of
their walls : the walls of the xylem vessels are no longer
simple cellulose, but have suffered a change known as
lignification. They are firmer and more resisting than the
soft sieve-tube walls, but the difference between the two
is not merely one of texture, they are chemically different.
The xylem vessels no longer give the reaction of cellulose
with Schulze's solution, which colours them yellow, while
the sieve-tube walls still give the purple colour charac-
teristic of cellulose.
corf.
vase bimd.
LONGITUDINAL SECTION THROUGH THE STEM OF Helianthus
tuberosus, THE JERUSALEM ARTICHOKE.
corf, cortex.
«, epidermis.
p, parenchyma of cortex.
/, pericycle fibres.
cb, cambium.
sp, spiral vessels.
vase, bund, vascular bundle.
c, collenchyma.
&. st endodermis.
.<?. ty sieve tube.
d, d, dotted or pitted vessels.
p. p, parenchyma of pith.
60
STRATIFICATION.
[CH. IV
When a longitudinal section of the xylem is examined
with a high power (fig. 24) it will appear that the vessels
are of various kinds. Near the inner margin they are
very narrow in diameter and are marked by a spiral line,
further towards the circumference they are wider and
the walls are covered with dots instead of being spirally
marked. These two kinds of vessels are known as spiral
and dotted vessels. To understand the meaning of this it
is necessary to consider the way in which cell walls are
thickened. When a transverse section of a cell wall is
examined under a high power of the microscope it can be
seen to be delicately striped by numerous parallel lines, so
that it seems to be made up of concentric layers or shells
as described above in starch-grains. This appearance is
known as stratification and has been the subject of much
FIG. 25.
MODEL REPRESENTING THE STRUCTURE OP A PITTED CELL-WALL:
b represents a square sheet of paper pierced by a circular hole ; in the
upper figure a number of sheets like 6 are shown in section pasted
one over the other, the lowest being pasted to an unperforated sheet a.
CH. IV] PITTED WALLS. 61
research and of no little disputing among botanists. It
is now generally believed that the lines of stratification
represent successive films of cellulose added by the
protoplasm to the cell wall. A single sheet of paper,
which is thickened by pasting on to it other sheets, may
serve as a model of the thickening cell wall, and will in
transverse section give the same stratified appearance as
an actual cell wall. Fig. 25 represents a model of a more
complex case. The original sheet of paper (a) on which
the successive sheets are pasted is unperforated, while
each of the sheets (6) to be fastened over it is pierced
by a circular hole. The resulting thick sheet of paper
will when seen in transverse section have the appearance
shown in the figure.
If the perforated sheets had been pasted on to both
sides of the whole sheet the model would have represented
a dotted or pitted cell wall, in which a similar appearance
is produced in roughly speaking the same way. Fig. 26
FIG. 26.
CELLS OF A DATE-STONE IN SECTION,
showing thick cell- walls with numerous simple pits.
m.l, the middle lamella thickened on both sides, except where the pits occur.
62 BAST FIBRES. [CH. IV
represents what is known as a pitted cell wall ; the de-
pressions are the pits, and the thin layer, which separates
pit from pit, is the pit-membrane.
The dots seen on the walls of the large xylem vessels
are pits of a slightly more complicated kind than that
shown in fig. 26. The spiral lines and rings which
distinguish the narrow vessels at the central side of
the bundle are the result of still more complicated
thickening.
In the xylem the non-vascular elements are the cells
forming the parenchyma of the xylem, and the xylem-
fibres (fig. 23 x.f\ elongated, tapering, thick-walled cells.
In the phloem attention must also be called to certain
non- vascular elements, — minute elongated cells, called
companion cells, whose chief interest from our present
point of view is that they give a characteristic aspect
to phloem in transverse section and help us to learn
to recognize this tissue. The phloem also contains some
simple cellular elements making up the phloem parenchymat
which is of no special importance.
Fig. 24 shows in longitudinal section the pericycle
fibres to which reference has already been made. They
are narrow elongated elements with very small cavities
and thick, lignified walls, lying directly internal to the
endodermis. They form a tough resisting tissue possess-
ing in fact, to some extent, the quality that gives a
commercial value to the fibres of the flax, hemp and
other plants.
There remain to be considered the cambium and the
cortex.
CH. IV] CAMBIUM. 63
Cambium.
The cambium is a meristematic tissue lying between
the xylem and phloem. It is a cell-manufactory, where
by cell division new elements are added to the neigh-
bouring tissues, viz. xylem and phloem. Thus the new
cells which arise in the cambium go to increase the xylem
and phloem something in the same way as the meristems
at the growing point of the root yield cells for the
increment of root and root-cap.
The cambium will be studied in more detail in the
next chapter. In the cambium of the sunflower there is
one point of great importance as being introductory to
the study of the oak. It will be seen in fig. 23 that in the
spaces between the bundles, that is to say, across the
primary medullary rays, a tissue is forming precisely like
the cambium which lies in the bundle. It finally extends
across the medullary ray and joins the cambium of one
bundle to that of the next. Thus instead of there being
mere strips of cambium running longitudinally down the
stem between the xylem and phloem, there comes to be a
cylindrical sheath of cambium made up by the coalescence
of the cambium of the vascular bundles with the interfas-
cicular cambium that arises between the vascular bundles.
The origin of the interfascicular cambium is physio-
logically of interest ; it is due to a kind of rejuvenescence,
for the cells which lie between the bundles are mature,
and in beginning to divide once more and becoming
cambium, they regain as it were the quality of youth.
The architectural importance of interfascicular cambium
will be considered in the chapter on the oak. It will here
CORTEX.
[CH. IV
suffice to know that the cells to which it gives origin in
Helianthus go to form fibrous and vascular elements which
partly fill up the spaces between the original bundles.
Cortex.
The points to be noticed in the cortex are not many.
Under the single layer of epidermis are several layers
of cells of which the walls are thickened in such a way as
to give a certain clumsy look to the outline, and which
have moreover a peculiar gloss or sheen.
These two characters, the glistening texture and the
peculiar thickening of the walls, are common to tissue of
this kind, which is known as collenchyma.
FIG. 27.
TBANSVERSE SECTION THROUGH A KIDGE ON THE STEM OF CLEMATIS.
Beneath the epidermis the section shows a mass of collenchyma remark-
able for the thick walls separating adjacent cells : the protoplasmic
contents have fallen out of many of the cells.
Lastly, the cortex contains running through it a
number of ducts or tubes known, from the nature of their
contents, as resin ducts. In transverse section a duct
appears as a space surrounded by a rosette of 5 or 6
cells. The physiology of resin ducts is obscure and need
not be discussed.
CHAPTER V.
THE OAK — GENERAL STRUCTURE OF A TREE-TRUNK —
HISTOLOGY OF XYLEM.
THE seed of the oak (Quercus sessilis and pedunculata)
contains an embryo with two large fleshy cotyledons ; these
do not serve as assimilating organs, but supply food to the
plumule, which springs up above ground and developes
into the stem of the young tree.
In its younger stages the plumule bears hardly any
obvious resemblance to the woody trunk of the older tree.
It is herbaceous rather than tree-like ; its structure is
that of an annual plant, such for instance as the sunflower.
It has a considerable mass of central pith, a ring of
scattered vascular bundles, and a cortex covered by
epidermis. Compare this structure with that of an oak
trunk : here the epidermis has disappeared, the pith is
visible only as a relatively small speck in the centre of
the section, while the concentrically marked wood, which
makes up the bulk of the trunk, does not much resemble
the scattered bundles of the seedling. The problem is to
understand how the structure of the tree has developed
from the herbaceous structure of the seedling.
D. E. B, 5
66
OAK.
[CH. V
The development depends upon the activity of the
cambium, which in the seedling oak has the same form as
FIG. 28.
TRANSVERSE SECTION OF A FIVE-YEAR-OLD OAK-BRANCH in which the isolated
bundles are replaced by concentric shells of wood.
p, pith. rn.r, primary medullary ray.
Xj to x5, shells of xylem formed during successive years.
The secondary medullary rays are not shown.
in the sunflower, namely, a cylindrical shell looking like a
ring in transverse section.
As in the sunflower so here the cambium manufactures
cells on its inner side which become xylem, and cells on
its periphery which become phloem. It therefore follows
that the cambium manufactures a cylindrical shell of
xylem on one side and a cylindrical shell of phloem on
the other.
As already mentioned the pith can be seen as a small
speck (fig. 29) in the centre of the section of an oak-tree,
CH. V] WOOD. 67
and this tissue helps us to make out the general structure
of a tree-trunk : for though it looks small in comparison
with the diameter of the stem, it is the same pith that
looked big in the section of the plumule. The pith
has not grown, and the mass of new tissue has therefore
nothing to do with it. It ought to be possible to
discover another fixed point by which to guide ourselves.
The cambium ring should still be recognizable, since
it remains perpetually young, and therefore unchanged.
Between the bark and the wood there is found a layer
of cells (noticeable in the spring-time for its sliminess)
which proves under the microscope to be the cambium,
the direct descendant of the cambium ring of the seedling
and like it composed of delicate meristematic tissue.
With the help of these two fixed points, the pith and
the cambium, the tissues of the oak branch may be classi-
FIG. 29.
TRANSVERSE SECTION OF AN OAK-TRUNK, 25 years old.
From Le Maout and Decaisne.
fied. What lies between the pith and the cambium, and
is known as wood, must be xylem ; all outside the cambium
must be phloem and cortex. Again, there were in the
5—2
68 OAK. [CH. V
seedling, rays of tissue passing between the bundles ;
these should still exist, and they can in fact be clearly
seen (together with other medullary rays of later origin)
running radially outwards, as shown in fig. 29. One
other point can be made out in the same way ; the wood
of the stem or branch increases in size every year by the
conversion of a number of cambium cells into woody
tissue, and since the cambium is in the form of a hollow
cylinder, giving a ring in section, it is clear that a ring
of wood must be added every year ; these are the con-
centric markings seen on the section of the stump, from
which the age of a felled tree can be calculated. These
circles, known as annual rings, are shown in the section
of an oak stem, in figs. 28 and 29.
With a simple lens or a low power it can be seen why
the annual rings are so clearly marked out. The con-
centric circles visible to the naked eye are shown in
fig. 30 to consist of lines of large vessels. If the eye
travels from the centre to the circumference of the section,
it will be seen that it is the central margin of each annual
increment that is marked by a line of large vessels. In
each annual layer the vessels become smaller and less
frequent at the peripheral margin, till at the beginning
of the next year's growth the row of large vessels again
suddenly appears. But even in places where the vessels
are absent the ring can be detected by the close texture
of the non-vascular elements formed in autumn. It
should be noted that the mere fact of the tree growing in
summer and resting in winter does not necessarily produce
visible alternations in structure. A tree grows in length
CH. V] ANNUAL KINGS. 69
as well as breadth every year, but a branch split lengthwise
does not show transverse horizontal marks in the wood
TRANSVERSE SECTION OF THE WOOD OF THE LIME-TREE,
to show the annual rings.
P, pith.
R, E, lines of large vessels in the spring wood of the annual rings.
After Van Tieghem.
where the year's growth begins, the tissues of one year are
continuous with those of the next. Or to take a simpler
example : if a builder were to build for a week and rest
for a week and so on, it would not be possible afterwards to
point to the places where pauses had occurred. But if he
always began with a course of big bricks and ended with
a course of small ones, the resting places would be
revealed.
Much discussion has been held as to the physiological
meaning of the annual rings : it is clear why the tree
grows in summer when it has leaves with which to
assimilate, and light and heat with which it can work its
70 OAK. [CH. V
assimilating organs, but it is not clear why the vessels
formed in the spring should be bigger than the later
formed ones. The most probable explanation is that of
Strasburger — namely, that the large vessels are needed for
the rise of sap in the trunk, which occurs in the spring.
FIG. 31.
DIAGRAM ILLUSTRATING TWO TYPES OF LONGITUDINAL SECTION.
The line R would divide the cylinder by a radial section, T by a
tangential section.
Certain points can be made out by means of longitu-
dinal sections examined with a simple lens or with a low
power of the microscope. Longitudinal sections are of
two kinds. If a branch is divided longitudinally by an
incision which passes through the centre, the surface
exposed is a radial longitudinal section ; this is shown in
fig. 31, where R represents the line along which the
branch (here seen in section) is divided.
But the incision may be longitudinal, that is to say,
parallel to the axis of the branch, and yet may not pass
through the centre, it is then called a tangential section,
as shown by the line T in fig. 31.
If an oak branch of three or four years old is split down
the middle, and if the radial section thus exposed is
CH. V]
MEDULLARY RAYS.
examined with a simple lens, two things will be apparent ;
the surface is longitudinally striped owing to the general
longitudinal character of the tissue, but there are also
very evident transverse markings. These must correspond
to radiating lines in the transverse section, and they are
in fact the medullary rays. In a section examined under
a higher power their structure comes out as shown in
fig. 32.
X.J. X.9. 2C.3.
FIG. 32.
LONGITUDINAL KADIAL SECTION OF THE WOOD OF THE OAK.
a?j to #4 represent the xylems of successive years, the dotted vessels
appear at the left of the brackets x2 to #4 ; at the left of x± are the
spiral vessels of the protoxylem.
The medullary rays run like walls transversely across the section.
Each medullary ray is like a wall of brick-shaped cells,
the bricks being supposed to stand on their edges ; their
72
OAK.
[CH. V
walls are thickened, lignified and pitted, and enclose
living protoplasm "and a good deal of starch. The bigger
medullary rays are several cells in width, while the smaller
ones are but one cell wide, but this of course does not
show in longitudinal section. In either case the ray
is a plate of cellular tissue with its edges pointing
upwards and downwards. The medullary rays are of
various depths (i.e. in the direction of the axis of the
branch), the primary rays being the deepest. The exact
form of the ray can best be seen in a tangential section.
In fig. 31 the line T represents such a section, and it is
clear that, since the medullary rays run like radii from
the circumference towards the centre, they must be cut
by T. Thus a tangential section of the branch gives
approximately transverse sections of the medullary rays.
The rays represented in fig. 33 are one cell in thickness
FIG. 33.
LONGITUDINAL TANGENTIAL SECTION OF THE WOOD OF THE OAK,
showing dotted vessels and tracheids among which are
the medullary rays.
CH. V] CAMBIUM. 73
and from 6 to 15 or more cells in depth ; it will also be
seen that the top and bottom edges of the rays end in
ridge-like cells, triangular in outline, which give to the
rays the form of double-edged blades.
Cambium.
To understand the part played by the cambium it is
necessary to examine it under a high power. The
beginner will not find it easy to prepare sections of the
requisite amount of fineness owing to the delicate nature
of the tissue. But although he will not be able to see as
much as is shown in the figure (fig. 34) taken from
Strasburger, yet he ought to be able to make out some
of the chief points. The most characteristic feature about
the cambium is the radial arrangement of its cells. The
arrangement is so regular that it enables us to sketch
FIG. 34.
TRANSVERSE SECTION OF THE STEM OF THE SCOTCH FIB (Pinus sylvestris).
phi, phloem; s.p, sieve-plate ; m.r, medullary ray;
c, cambium ; the letter c is opposite to the initial cell i ; the youngest or
latest formed cell wall forms the right hand wall of the cell i ; it
may be recognized by ending flush against the radial walls.
In the xylem, 1, 2, 3 represent stages in the development of the bordered
pits which characterize the tracheids of the pine.
After Strasburger.
74 OAK. [CH. V
the cambium in the most diagrammatic way without being
seriously inaccurate.
Fig. 34 gives, much magnified, a small portion of the
cambium of a pine-tree. Towards the middle of the
ladder-like radial row is a delicate transverse wall which
abuts sharply on the radial walls. This is the last
cell wall that has been formed, and gives evidence that
this is the actual region of cell manufactory or as it is
called the initial layer of the cambium. The cells on
either side of the initial layer are on their way to
becoming permanent tissue, and the change in form
which accompanies increasing age can be clearly made
out. In the pine-tree the xylem is made up of vessel-like
elements known as tracheids, and at 3 such tracheids are
seen cut across in transverse section ; then comes a
younger tracheid (2) with thinner walls, and lastly a
tracheid (l)with thin walls and without the "bordered" pits1
characteristic of the fully developed elements. Between
such elements as (3) and the cells of the initial layer
there is a gradation of cells, intermediate in age between
the adult and the initial stage, and also intermediate
in appearance. There is a similar gradation from the
initial layer towards the phloem, but it is not so clearly
visible.
In longitudinal (radial) sections the character men-
tioned above is shown in a similar way. Namely, that
in the radial direction cells of equal length are arranged
one behind the other like books in a shelf.
A function of the cambium, which is sometimes
1 See the account of bordered pits in the next section.
CH. V]
CAMBIUM.
75
overlooked by beginners, is the production of medullary
rays. Certain of the cambial cells, instead of developing
into xylein or phloem elements, turn into medullary
ray cells; in this way the rays which already exist
are continued outwards as the trunk thickens, and at
the same time new rays make their appearance in each
annual ring. This will be understood by a reference to
fig. 35 ; it will be seen that only the original primary
rays run from pith to bark, while the rest (secondary rays)
arise in one of the annual rings, whence they are con-
tinued radially outwards by the addition of the medullary
ray cells manufactured year by year by the cambium.
M
FIG. 35.
PAKT OP A TRANSVERSE SECTION THROUGH A FOUR-TEAR-OLD BRANCH
OF THE CORK OAK.
(1), a primary medullary ray running from the pith (M ) to the bark.
(2), (3) and (4), secondary rays formed in successive years.
PC, phloem and cortex. (The medullary rays should be continued
into the phloem.) S, cork.
From Le Maout and Decaisne.
There remains to be considered the structure of the
wood as seen under a high power of the microscope.
76 OAK. [CH. V
Transverse Section.
The pith cells contain living protoplasm and also
starch at certain times of the year, their walls are thick-
ened and pitted. The pith presents an irregular outline
because the primary vascular bundles project into it all
round in the form of blunt wedges. At the ends of these
wedges, or apparently embedded in the pith, are the first
formed vessels, of narrow diameter and lined with a spiral
thickening. Spiral vessels occur nowhere else in the
wood1. The most obvious elements in the transverse
section are xylem vessels, looking like holes of various
sizes punched in the section, the largest being in
tangential lines in the parts of the xylem formed in the
spring. The medullary rays are seen running in radial
lines across the section, some of them one cell in thickness,
others consisting of many cells, and containing in certain
seasons large quantities of starch. The way in which the
rays bend round the larger vessels should be noted ; this
distortion is due to the great increase in size of those
cambium cells which turn into vessels, pushing the rays
out of their true radial course.
Besides the vessels there are a large number of
thick-walled woody elements which make up the rest
of the xylem. These are not easy to classify as seen
in transverse section, the wood-parenchyma may how-
ever be often distinguished by the starch grains which
it contains.
1 It must be remembered that the spiral character is not perceptible
in transverse sections.
PITTED VESSELS.
77
CH. V]
Longitudinal (Radial) Section.
The large xylem vessels are again the most striking
feature. They appear in longitudinal section as empty
spaces with here and there a remnant of an oblique
transverse wall. The markings on the walls can be seen
well where fragments of membrane come into the section
as shown in fig. 36.
Fm. 36.
PAET OF A DOTTED XYLEM-VESSEL FROM THE OAK.
These markings, from which the vessels take the names
of " dotted " or " pitted," are like so many screw-heads, a
disc traversed by a transverse line or elongated mark which
represents the groove for the screw-driver. The structure
of these pits, which are known as bordered pits, can be
explained by an imaginary model. Imagine a pair of
watch-glasses each pieced by a narrow slit, and imagine
them united face to face with a delicate circular piece of
78 OAK. [CH. V
paper between them, and then fixed into a hole cut in a
thick piece of card. The outline of the screw-head is the
outline of the united watch-glasses where they are let
into the card : the groove in the screw-head is the
oblique cleft which leads into the space between the
glasses. The structure will be understood from the
bordered pits shown in section in the walls of the
tracheid (3) in figure 34. A bordered pit is in fact a
thin place in the wall which allows water to pass laterally
from surrounding tissues into the cavity of the vessel;
the function of the protective "border" (the watch-glasses)
need not be discussed.
The remaining elements of the xylem are wood-cells,
wood-fibres and tracheids. The cells of the wood paren-
chyma, as seen in longitudinal section, or when isolated
by maceration, are not unlike the medullary ray, seen in
tangential section. That is to say, they consist of what
was originally a single cambium cell divided into chambers
by horizontal walls. The wood parenchyma retains vitality
in its constituent cells, which like the medullary rays are
loaded with starch grains, especially in the winter.
The tracheids (tr, fig. 37) and wood-fibres (f, fig. 37)
resemble the vessels and differ from wood parenchyma
and medullary rays in having no living protoplasmic
contents. The tracheids are in fact closely allied in
character and in function to vessels ; if in a line of
tracheids the transverse walls were to disappear such a
line would be a small vessel. Lake the vessels too, they
serve for water transport. In accordance with this
relationship we find that the pits (which are organs of
CH. V]
MACERATED WOOD.
79
water transport) in the tracheids are like those of the
vessels, namely, bordered.
FIG. 37.
MACERATED OAK-WOOD.
/, fibres. tr, tracheids. sp, spiral vessel.
d.v, dotted vessel. p, medullary ray.
The wood-fibres, as shown in fig. 37, are thick walled
elongated elements with narrow cavities.
CHAPTER VI.
THE OAK (CONTINUED)— BARK— GROWTH OF TREES.
THE bark in the everyday meaning of the word is that
part of the stem external to the cambium. I propose
to use the term in this sense in spite of the fact that
in English botanical books it is applied only to the
tissues external to the cork-cambium.
The bark increases in thickness in the manner described
in the case of wood, namely, by cambium cells, as they
develope, assuming the form and nature of phloem. And
just as the shells of wood formed by the cambial cylinder
are known as secondary xylem, so here the products of
cambial activity towards the periphery of the stem are
known as secondary phloem. But the growth of the bark
is more complex than that of the wood for more than one
reason.
In the first place it is complicated by the existence of
the primary cortex. In the young oak stem as in the
sunflower the cortex is the region outside the vascular
CH. Vl] BARK. 81
cylinder, and since the cambium-ring is formed in the
vascular cylinder, the cortex is obviously outside the
cambium, and therefore all secondary tissue formed by
the cambium towards the outside must at first be covered
by the primary cortex.
Secondly, the growth of the wood has an influence on
the bark. If the bark were to cease to grow while the
cambium continued to make new layers of wood, it is
obvious that the bark would be too small for the branch
and would burst by pressure from inside. Although this
is an imaginary state of things, it is worthy of note,
because in spite of the fact that the bark does grow, it is
nevertheless stretched by the growing wood, and this
helps to produce a distortion and compression of the
elements which are characteristic of the bark.
Thirdly, the structure of the bark depends partly on
the growth of certain tissues which have no connection
with the cambium, but which originate in a meristematic
layer in the primary cortex. It will be convenient to
describe this tissue — the corky layer — before dealing with
the secondary phloem.
In a young oak twig the epidermis is seen as a
limiting membrane, a pavement of a single layer of cells.
The outer wall of each cell c, fig. 38 (p. 83), has (as is
usual in epidermic cells) a special character. It is not
only thicker than the other walls but strikingly different
in its chemical nature; it is no longer pure cellulose,
but is cuticularised.
The layer forming the cuticularised outer walls of
the epidermic cells is known, as cuticle. It resembles
D. E. B. 6
82 OAK. [CH. VI
lignified tissue in giving a yellow instead of a blue colour
with Schulze's reagent. It is however especially re-
markable for its resisting power. If a section is placed
in strong sulphuric acid, ordinary cellulose walls are
destroyed, but the cuticle is not destroyed. Plants in a
state of nature are not subject to baths of sulphuric acid,
but this test shows at any rate a resisting power which
gives the cuticle value as an external armour-plating to
the epidermis.
In older branches the epidermis disappears, and its
place is taken by several layers of cork-cells, whose walls
have a similar but not identical resisting quality: the
walls of cork-cells are not said to be cuticularised, but
to be suberised.
The young oak twig is green, because the cortical cells
contain chlorophyll, but it begins to turn brown in its
first year, the brown colour being due to the growth of a
layer of cork covering up the green cortex like a veil.
This film-like appearance would suggest that the cork
arises on the surface of the cortex. It does not however
arise in the most superficial cells, i.e. in the epidermis,
but in the cells immediately under the epidermis. In
this layer a remarkable change takes place precisely like
that rejuvenescence which gives origin to the inter-
fascicular cambium. The sub- epidermal cells begin to
divide by tangential walls, and thus a cambium-like ring is
formed immediately inside the epidermis (ph in fig. 38).
This meristematic layer has, like the vascular cambium,
a double activity : it adds to the cortex on its central side
and manufactures cork on its external epidermal side. It
CH. VI]
CORK.
83
is often described as cork-cambium, but more technically
as phellogen. In the same phraseology cork is sometimes
FIG. 38.
TRANSVERSE SECTION THROUGH ONE-YEAR-OLD BEECH-BRANCH,
showing the development of cork.
c, cuticle. e, epidermis. p, developing cork-layer.
ph, phellogen. col, collenchymatous cells of the cortex.
cor, cortex. (The phelloderm is not yet formed.)
called phellem, and the cortical tissue arising from the
phellogen is called phelloderm. The three layers together
form the periderm1. Thus the bark of the oak comes to
be made up of three chief parts ; the original cortex to
which on the inside is added secondary phloem arising
from the cambium ring, and on the outside the periderm
arising from the phellogen.
The epidermis is stretched and cracked by the cork
growing underneath it, and ultimately dies and falls away
in flakes.
1 Some authors use periderm to mean cork only.
6—2
TRANSVERSE SECTION OF OAK-BARK.
cfc, cork. phel, phellogen. col, collenchyma of the phelloderm.
cr, crystals. pc, pericycle fibres. c&, cambium.
p, periderm. (N.B. the bracket p should extend more to the right so as
to include the phelloderm, col.) c, cortex.
phli to phl±, the phloems of four years, the youngest being next the
cambium ; the outer part of each phloem consists of a layer of bast-
fibre/; thick- walled pitted sclerenchyma cells are to be seen near p:
medullary rays run outwards from the cambium.
FIG. 40.
RADIAL LONGITUDINAL SECTION OF OAK-BAKK.
Lettering as in fig. 39.
Cubical crystals in vertical rows border the groups of bast-fibre/; inphlA,
sieve-tubes are to be seen ; medullary rays run across like walls.
86 OAK. [CH. VI
The cork-cells being formed by successive division
of the phellogen cells acquire the same regular pattern-
like arrangement that has been described for the cambium.
It is shown in figs. 39, 40, where the cork is seen in trans-
verse and longitudinal section.
The suberisation of the walls of the cork-cells is not
the only change that occurs ; an equally striking feature
is the disappearance of the protoplasmic contents :
so that cork, like pith, comes to be a mass of air-con-
taining cells. The fact that the cell walls are extremely
impervious to water, added to the fact that the cells
contain air, gives the floating power of cork. The
impermeability to water also gives the quality which
allows the periderm of the Cork Oak to be made into
" corks " for bottles.
The phelloderm need not be described in detail: it
consists of collenchyma in whose cells chlorophyll-bodies
are found.
As the oak-tree becomes older there is a more complex
formation of cork, which leads to the rough scaly look
observable on the trunk. Into this formation I shall
not enter.
Secondary Phloem.
A transverse section of the bark of a 4 or 5 year
branch of the oak shows, under a simple lens or low power
of the microscope, a stratified appearance. The concentric
lines which produce this appearance are due to the same
general cause which accounts for the annual rings in
the xylem, namely, that the products of cambial activity
CH. VI] SECONDARY PHLOEM. 87
are not always the same. In the case of the xylem, the
cambium in the spring develops large vessels, while
in the autumn smaller elements are produced It is a
corresponding series of changes that gives rise to the
alternate layers of tissue in the secondary phloem. These
layers in the bark are distinguished by a physical
character, namely, hardness, and are described as hard and
soft phloem.
Fig. 39 represents a transverse section of oak bark
highly magnified. At the upper end of the drawing
(which represents the outer side of the section) is the cork
ck, and phellogen phel ; at the lower edge of the drawing
(inner side of the section) are seen the medullary rays
running in radial lines. The layers of hard phloem
run concentrically at right angles to the medullary
rays, separated from each other by concentric layers of
soft phloem. Outside the region of alternate hard and
soft phloem, and inside the periderm, is the original
cortex, the limits of which are not clearly distin-
guishable in transverse section. In the longitudinal
section (fig. 40) the cells of the soft phloem are seen to
differ in size and shape from those of the cortex. The
cells, which together with sieve-tubes are the essential
elements of the soft phloem, are rich in tannin, a fact
which is familiar from a practical point of view in the use
made of oak bark by tanners. The structure of the hard
phloem will be understood from fig. 40; it consists of
elongated pointed fibres with thick walls and very minute
cavities. The layers of phloem fibres are bordered, as may
be seen in longitudinal section, by rows of cells, each
88 MEANING OF [CH. VI
containing a crystal of calcium oxalate. The same
salt occurs scattered in the soft phloem, but here the
crystals are more complex and have a star-like radiate
form, as shown in fig. 40.
The hard phloem is what gives the tough resisting
character to the bark of trees, and what in the lime tree
yields the strong rope-like material known as bast.
In oak bark there is another hard resisting tissue
shown in figs. 39, 40. The tissue is made up of rounded
cells with small cavities and thick ligoified walls of great
hardness, belonging to the type known as sclerenchymatous.
The sclerenchyma of the oak is easily recognised by tlw
numerous deep narrow simple pits which traverse the
cell walls.
Physiology.
It is not at first obvious why plants should have
developed into such huge structures as many trees are.
Why should there be Sequoias in America and gum trees
in Australia towering two or three hundred feet into the
air ? This question is asked from an evolutionary point of
view, and simply means: — What advantages, connected
with the tree-like habit of growth, have, by means of
natural selection, guided the evolution of plants in this
particular direction ? The answer to such questions must
be highly speculative ; we can never answer them
dogmatically. All that can be done is to point out
certain undoubted advantages which a plant, in taking on
the arboreal habit, gains in the struggle for life. The
chief gain is no doubt that a plant, in overtopping
CH. Vl] ARBOREAL HABIT. 89
its fellows, gains access to the light, and in shading
lower trees tends to starve a possible rival, and thus
better its own chance of keeping possession of the light.
At the same time in keeping back the aerial growth of
its rivals it starves their roots and thus keeps its own
roots free from undue competition. Many facts go to
prove that this struggle for light is an important feature
in the environment of plants. From this point of view it
is possible to understand the advantage of the climbing
habit in a plant, for it is thus enabled to reach the light
by a small expense of actual stem-production : it succeeds
by adaptation, instead of by the patient construction of a
column-like trunk of massive strength. The same thing
is true of epiphytes, i.e. plants which perch and root on
others, such as the innumerable orchids, ferns, Bromelias
&c. of tropical forests, which do not necessarily exhibit great
extension of growth, but possess adaptations for securing
themselves and for obtaining food in their aerial position.
Granted that trees grow up into the air in a com-
petitive search for light, how are they guided, and how
enabled to carry on the search ? The fact that plants
grow straight up, even when forced to germinate in the
dark, proves that there exists a directive tendency, in
which light plays no part. And when it is found that
all over the world the trees grow vertically, it is im-
possible to help suspecting that the force of gravity,
which all over the world acts in the direction of the
earth's radius, is the guiding influence.
This is the fact ; just as the root of a bean grows
vertically down, so the plumule grows vertically up. Both
90 KNIGHT'S EXPERIMENT. [CH. vi
are forms of geotropism, the root being positively, the
stem negatively, geotropic. A seedling bean placed on its
side gives evident proof of different kinds of sensitiveness
or irritability in its root and shoot, for under the influence
of one and the same force, viz. gravity, the root grows
towards, the stem away from, the centre of the earth.
The force of gravity is a stimulus to which different
parts of the plant react in different manners. Gravity
is as it were a sign-post by which the plant is enabled
to direct its growth in the most profitable manner.
The most striking proof that gravity thus plays the
part of stimulus, is supplied by the famous experiment of
Andrew Knight published in 1806. With the help of his
gardener he fitted up a small water-wheel which, being
driven by the stream in his garden, rotated rapidly and
exposed beans germinating on the circumference to strong
centrifugal force. If a flexible or ductile object is fixed
to a rotating wheel, it will bend until the free end points
radially outwards: in the same way when a bucket of
water is whirled violently round by a rope tied to the
handle, the water remains in the bucket even when it is
upside down, instead of flowing out in obedience to
gravity, as it would if the bucket were still. These
well-known results make one see that centrifugal force
replaces gravitation, and that it affects the object whirled
round like an imitation gravity acting in the direction
of the radius. Therefore if a stationary bean tends to
grow in the line of gravity, a bean whirled round on a
water-wheel must grow in the line of the imitation
gravity, that is in the line of the radius of the wheel.
CH. Vl] STABILITY. 91
This is what Knight found: the stems of the young
plants grew towards, the roots grew away from, the centre
of the wheel.
Geotropism is not only valuable in enabling a plant to
take the shortest line in its upward growth, but it is also
important in another way, it plays the part of the
plummet to the builder. If a tree had no power of
vertical growth it might grow upwards in an oblique
direction, and would therefore fall by its own weight.
The question how stability is gained in plants, how
they come to be strong enough to stand upright, is of
considerable interest. In the first place it should be
noted that the herbaceous plant, such as a seedling
sunflower, has a stability of a different order from that
which enables the oak to rear itself into the air. It
is well known that a delicate seedling plant withers if
exposed to the sun on a hot, dry day : it loses its stability
and droops towards the ground, but, if its roots are
supplied with water, it will recover when the damp
air of evening checks the evaporation from the leaves.
A woody stem, such as that of an oak sapling, is not so
affected, it does not collapse when dried.
The reason of this difference may be discovered by
experiments. Any juicy, actively growing leaf-stalk or
flower-stem will serve as material. A stem of this sort
cut from the parent-plant and allowed to lie on the table
in the dry air of a room soon loses its stiffness and
becomes flaccid. Or we may place it in a 5 per cent,
solution of common salt, which robs it of its water by
osmosis, just as dry air robs it by evaporation. After it
92 TURGIDITY. [CH. VI
has become flaccid in salt-solution it can be rendered stiff
by replacing it in water.
Or it may be made to collapse and become flaccid by
immersion in water at 60° C. But in this case the
flaccidity is permanent, because the tissues are killed.
When the cells were alive they were tensely filled with
cell-sap, which escapes as soon as the protoplasmic lining
of the cells is killed by heat. The flaccidity of the dead
tissue depends on a loss of fluid from the cells, and this is
likewise the cause of the similar though temporary loss
of stiffness produced by dry air or immersion in salt-
solution.
To understand the problem more fully it is best to
take the case of a single isolated cell capable of standing
up and supporting its own weight. The cell is stiff just
as an air-cushion, tensely filled with air, is stiff. The
air-cushion is filled by blowing air into it with a pair
of bellows. The cell is filled by osmosis, which depends
(i) on the fact that the cell-sap is denser than water,
(ii) on the physical properties of the protoplasmic lining.
When the fluid surrounding the cell is denser than
the cell-sap, the osmotic flow is from the cell to the fluid ;
when the reverse is the case, the flow is in the opposite
direction, and the cell gains, instead of losing, fluid.
Again, when the physical properties of the protoplasm
are changed by death, the cell-sap escapes just as the
air escapes from a ruptured air-cushion.
When a cell is tensely filled with fluid by osmosis it is
called turgid, and turgidity is the cause of the stiffness of
not merely the isolated cell, but also of masses of cells in
CH. VlJ TURGIDITY. 93
which every cell is turgid. Thus the flower-stem and leaf-
stalk used in the experiments on withering are stable and
rigid, because of the turgidity of the cells which make up
the central mass of pith. A woody stem is rigid not
from turgidity, but because of the rigidity of its lignified
cell-walls.
CHAPTER VII.
THE LEAF — TRANSPIRATION — LEAF-FALL.
IT will appear later that the parts of the flower (the
petals, stamens, &c.) have the morphological rank of leaves.
If these are omitted from consideration, leaves may be
classified into (i) foliage-leaves, (ii) scale-leaves. The first
are the ordinary leaves familiar to everyone; the other
kind of leaf is smaller, dry and hard in texture, colour-
less or dingy in tint, devoid of chlorophyll, and pro-
tective, not assimilative, in function. Scale-leaves of this
sort have already been met with in the potato, the surface
of which is marked by the remains of the scales, in
FIG. 41.
HORSE-CHESTNUT BRANCH, bearing a terminal and two axillary buds.
CH. VII] THE LEAF. 95
whose axils the eyes grow. Scale-leaves
again are what make the outer covering
of the buds of trees, they are familiar in
the horse-chestnut from their sticky outer
surface (see fig. 41). In the spring they
are seen unfolding and finally falling off
to allow the growth of the young branch,
i.e. the bud, shut up within them. The
markings on the surface of a horse-chest-
nut bough are instructive in connection
with both kinds of leaf. The most obvious
marks are broad triangular or shield shaped
depressions (L, fig. 42), which are the scars
left by the fall of the foliage-leaves in
former years : they occur in alternate pairs,
i.e. one pair of scars points N. and S., the
next E. and W. and so on. The scars are
marked near their lower border with a line
of dots or raised papillae1 which are the
scars of the vascular bundles. When the
leaf was attached to the plant, the vascular
bundles ran from the leaf-stalk into the
branch, and when the leaf was cast in
autumn, the bundles were broken like the
rest of the tissues. At the upper edge
1 Not to be confused with the lenticels scattered
irregularly over the bark.
FIQ. 42.
BRANCH OF HORSE CHESTNUT.
L, L, scars of fallen leaves.
W, Wt wrinkled places where the scale-leaves of terminal buds once grew.
96 PHYLLOTAXT. [CH. VII
of some leaf-scars are seen withered undeveloped axillary
buds.
At irregular intervals on the branch are seen finely
wrinkled places, about J inch in length ; these are made
by the scars of scale-leaves ; under a lens the scars can be
seen to resemble those of the foliage-leaves, except that
they are relatively wider and shorter, and that the scars
of the bundles are less evident, or indeed not to be seen.
Each wrinkled place represents the spot where a terminal
bud once existed, we have therefore evidence of how much
the branch grew from year to year. Thus we get by
different means the same sort of evidence of yearly
growth as is yielded by the annual rings in wood.
The horse-chestnut not only serves as an illustration
of foliage- and scale-leaves, it also serves to demonstrate
one of the common modes in which foliage-leaves are
arranged on the branch. When the leaves grow opposite
to each other in a plane at right angles to that in which
the pairs of leaves above and below are developed, the
arrangement is known as decussate. This decussate
arrangement is common, but it is by no means the only
one, great variety exists in this matter, and a special name,
phyllotaxy, has been given to this department of morpho-
logy. In many plants the leaves are alternate; thus
one leaf will be on the north side, then at the next node
the leaf will be southerly, then north again one stage
higher. In this case the leaves are arranged in two
vertical rows : in the horse-chestnut there are four vertical
rows, while in other plants a larger number exists. In the
groundsel for instance the leaves are in five vertical rows.
CH. VIl]
PHYLLOTAXY.
97
This plant may be used to demonstrate the fact of some
general importance, that the leaves also form a continuous
spiral line round the stem. This double arrangement may
be illustrated by a diagram, fig. 43, in which the dots are
arranged both in vertical and in oblique rows, the former
being the more obvious.
FIG. 43.
DIAGRAM ILLUSTRATING PHYLLOTAXY.
Take a shoot of groundsel and mark the base of any
leaf-stalk with a spot of ink by which it may be recog-
nized : the next leaf above will be slightly to the left and
the third again to the left, so that a line passing through
the 1st, 2nd, 3rd, 4th, &c. leaves in order of height will
make a spiral travelling upwards and in the direction
of the hands of the clock. When the 6th leaf has been
reached it will be found to be vertically over the 1st,
which must necessarily be the case when the leaves are in
five vertical rows. One other point must be noted; in
passing from the 1st to the 6th, the spiral goes twice round
the stem. These two facts are expressed numerically by
the fraction f . In the same way the fraction f means
there are eight vertical rows arranged in a spiral which
D. E. B. 7
98
PHYLLOTAXY.
[CH. VII
goes thrice round the stem, and other arrangements are
similarly expressed. The phyllotaxy (f ) of the plantain is
shown in fig. 44.
FIG. 44.
PLANTAIN (Plantago),
viewed from above to show the f phyllotaxy : the leaf-spiral follows the
course 1, 2, 3 13.
From Le Maout and Decaisne.
^07*771.
The typical form of leaf is flat and thin ; that is to say,
although many plants have fleshy, cylindrical, or almost
spherical leaves, yet the majority have the form familiar
to everyone in the leaves of our ordinary trees. The
biological meaning of this form is plain enough, the leaf
being the assimilating organ which enables the plant to
build up organic material, it is necessary that a green
surface as large as possible shall be exposed to the light ,
this will be realised when it is remembered how small
is the percentage of CO2 existing in the atmosphere.
CH. VII]
STIPULES.
99
In order to expose a large surface with a small expen-
diture of material the leaf must obviously be thin, just as
gold-leaf, which is required to expose a large area and is of
valuable material, is thin. Moreover if a leaf were not
thin, some of the cells would be so much shaded by the
others that they would be unable to assimilate.
The flat broad part of the leaf is the blade or lamina
(fig. 45), the stalk (which is often absent) is technically
known as the petiole. At the base of the petiole in
many leaves are a pair of outgrowths known as the
stipules (fig. 45), these are clearly seen in the leaf of the
rose, in the cherry, and in the pansy.
\
FIG. 45.
STIPULATE, i.e. STIPULE-BEARING, LEAVES.
On the left a pansy-leaf, on the right that of the cherry. The numbers
refer to the venation ; (1) mid-rib ; (2) and (3) secondary and
tertiary veins.
From Le Maout and Decaisne.
7—2
100 SYMMETRY. [CH. VII
A striking feature in the typical leaf is that it is dor si-
ventral, it has a back and a front differing from each other ;
in the parts of plant$ hitherto considered this has not been
the case, the stem and the root are not dorsiventral but
are symmetrical round an axis. In leaves the dorsiventral
character is seen in a number of points, even in external
characters ; thus in many leaves the lower surface is paler
than the bright green upper surface, or it may be more
hairy or marked W projecting veins. The internal and
microscopic structure is even more plainly dorsiventral.
Connected with this character is a capacity of growth, a
sensitiveness to light by which leaves are enabled to
arrange themselves with one particular surface at right
angles to the light. To a plant growing freely in the open
air the light comes mainly from above, and thus it happens
that leaves are more or less horizontal, i.e. with the upper
surface at right angles to the vertical light. The surface
which thus receives most light, and which we usually call
the upper surface, is physiologically considered the- assimi-
lating surface.
The power of adaptation is clearly seen in a plant so
placed that it receives light from one side, the leaves are
then twisted and tilted so as to make the most of the
light ; this may be well seen in a geranium growing in a
window, or a tropaeolum sprawling out of a flower-box.
Leaves vary extremely in shape ; what may be called the
typical form is well seen in the beech or the laurel, where
the stalk is continued as the midrib into the lamina,
which spreads out symmetrically on either side. From the
midrib a number of "veins" run right and left towards
CH. VIl]
VEINS.
101
the edge of the leaf, branching and becoming smaller till
the smallest branchlets are only visible to the naked eye
by holding the leaf against the light.j
The veins are the ramifications of the vascular bundles
and therefore contain xylem- vessels. Vessels, it must be
remembered, are the water-carriers of the plant, and
when it is considered how easily a leaf withers, or in
other words how great is its need of water, the fine
ramification of the water pipes is opt surprising. The
leaf may be compared to a country cut up into innumer-
able minute fields by an elaborate system of irrigation.
Fio. 46.
TRANSVERSE SECTION THROUGH THE LEAF OF THE HELLEBORE,
showing, from above downwards, the upper epidermis, the palisade
cells, the spongy tissue (in which a vascular bundle is seen), the
lower epidermis, in which is shown a single stoma opening into a
large intercellular space.
It must be remembered that the veins not only serve
for irrigation but also supply a framework for the support of
102 MESOPHYLL. [CH. VII
the cellular tissue in which the chlorophyll bodies are con-
tained. The cellular tissue or parenchyma of the leaf is
known as mesophyll, and is one of the points in which the
two sides of the leaf differ from each other. On the upper
side of the leaf there is, beneath the epidermis, a charac-
teristic layer of cells known as palisade tissue, because in
a section they look like planks placed side by side to make
a paling. The figure (fig. 46) shows this aspect ; it also
shows that the palisade tissue is the part of the leaf
which contains the greatest amount of chlorophyll. The
lower half of the leaf is seen in the same figure to be
made up of cells of irregular form, so arranged as to leave
large spaces among them. Those who have lived in a
chalk country must have seen walls built of flints in which
the large irregular spaces are filled with mortar. If the
mortar is imagined to be air, and each flint a cell, an idea
is obtained of the structure of the lower layer of the leaf,
which from its loose texture is called spongy tissue. It is
the presence of the air in the spongy layer which gives
the lighter colour to the lower side of many leaves. This
may be proved by a simple experiment. A leaf of the
lesser celandine, or an arum leaf, is placed in water and a
strong inhalation is applied to the cut stalk held between
the lips. In this way air is sucked out of the leaf,
and water finds its way through the epidermis and takes
the place of the air which has been removed. The
moment at which it enters is clearly perceptible by the
change in colour, the lower surface turning dark green
as the water fills up the air spaces in the spongy tissue.
If the epidermis is stripped from the lower surface of a
CH. VIl] STOMATA. 103
leaf and examined under the microscope, it will be evident
by what means the water passed this membrane.
FIG. 47.
THBEE STOMATA WITH SURROUNDING EPIDERMIC CELLS (E) :
(?, G, guard cells of a stoma.
Scattered thickly among the ordinary epidermic cells
are structures known as stomata, shown in fig. 47. Each
stoma is made of a pair of kidney-shaped cells called
guard cells, fitting together with their concave sides
inwards and leaving an oval cleft by which the inter-
cellular spaces of the leaf communicate with the external
air; it should be noted that the guard cells differ from
ordinary epidermic cells in possessing chloroplasts. Each
stoma is at first a single cell which is divided into
two compartments by a cross wall. The cross wall finally
splits into two layers between which the opening of the
stoma lies. The remarkable form of the guard cells is
well seen in transverse section (fig. 46).
The stomata have the power of opening and shutting
104 TRANSPIRATION. [CH. VII
in response to changes in the environment of the plant.
The mechanism by which they do so need not be described,
it must suffice us to know that when the plant begins to
suffer from want of water, or when it is exposed to some
other conditions, e.g. darkness, the cleft between the guard
cells, and therefore the passage from the outer air to the
intercellular spaces of the leaf, is closed. The most
important function of the stomata is the aeration of the
inner parts of the leaf. And since it is through the stomata
that the C02 enters the leaf, these organs are of great
importance in the nutrition of the plant. But they
also influence the degree to which the leaf loses water by
evaporation — a function known as transpiration.
Transpiration.
If a delicate leaf is gathered on a hot, dry day, it
withers almost immediately, that is, its cells collapse for
want of water in the manner already described. This
shows two things, (1) that leaves are constantly losing
water by evaporation, (2) that, since the leaf does not
wither if left on the tree, the loss of water is, under
normal circumstances, continually made good.
The means by which a current of water is earned from
the root to the top of a high tree is still an unsolved problem.
There are however certain fundamental experiments which
must be considered.
If a branch, such as that of a laurel, be cut and placed
in a coloured fluid, e.g. eosin dissolved in water, and left
there for some hours, it will be seen that the coloured
fluid has travelled up it, showing that there is a sucking
CH. VII] TRANSPIRATION. 105
power of some sort in the branch and leaves. The bark
will not be coloured, and if the branch be peeled, the
contrast between the white inside of the peel and the
wood stained pink with eosin is striking. The fact that
the fluid travels in the xylem is still better seen in a
young succulent stem where, on splitting the plant, the
vessels filled with eosin show out as delicate pink streaks.
There are various arrangements by which we can
accurately measure, from minute to minute, the amount of
water which a cut branch is absorbing. With an instru-
ment of this sort it is easy to prove that the amount of
absorption depends on the amount of evaporation going on
from the leaves. Thus if the leaves are placed under a
bell jar, and therefore in damper air, the instrument
records the fact that the absorption is less, and the
readings quickly rise again when the bell is removed.
The absorption may be diminished by cutting off some of
the leaves and thus diminishing the total amount of evapo-
ration. In this sort of way it can be shown that the
water-supply of a leaf by the vessels is a self-regulating
mechanism ; that rapid evaporation increases the upward
current, so that the greater the loss the greater is the
supply.
In thinking about the transpiration from a leaf surface
it must be remembered that the evaporating surface is
much greater than the surface which is visible, because
the spaces in the spongy tissue communicate with the
outer air through the stomata, so that the surface of each
constituent cell of the spongy tissue evaporates, not so
much as though it made part of the external surface, but
106
LEAF-FALL.
[OH. VII
still considerably. It will be more easily realised how
porous a thing the epidermis must be, when it is remem-
bered that there are thousands of stomata on a square
inch.
Leaf -fall.
That the leaves of many trees fall from the tree
in autumn is a familiar fact, but the physiology of the
process is not so well known. It is easy to prove, by a
simple experiment, that leaf-fall is a process requiring a
certain mechanism for its accomplishment, that it is not a
al:
FIG. 48.
LONGITUDINAL SECTION THROUGH A BRANCH AND PART OF A
LEAF-STALK OF THE POPLAR
The absciss layer and a layer of cork are shown at a.l.
ck, cork; c, cortex; /, bast fibres;
xy, xylem; p, pith.
CH. VII] LEAF-FALL. 107
mere tumbling down of withered leaves. If, during the
summer, a branch is half-broken so that it hangs on to the
tree by its bark only, its water-supply is cut off and it
soon withers and dies. It might have been expected that
these withered leaves would fall more easily than normal
leaves, — but precisely the reverse is the case : they hang
on to the tree after all the healthy leaves are cast. This
suggests that leaf-fall is an active, not a passive process, a
phenomenon of life which can only occur in a living leaf-
stalk. This is the case : the leaf falls because a layer of
cells (the absciss layer) forms across the base of the stalk
specially adapted to allow the leaf to free itself. Beneath
this layer cork cells are developed, which serve to cover
and protect the wound left by the fall of the leaf.
CHAPTER VIII.
ASEXUAL REPRODUCTION — PLEUROCOCCUS — MUCOR —
CONJUGATION — MUCOR — SPIROGYRA.
THE present chapter deals with a part of physiology
hitherto hardly touched on, namely, reproduction. Every
organism in the world is subject to a variety of risks,
and is constantly in danger of being destroyed: in the
case of the animal kingdom the preying of one animal on
another, and the contest among animals of the same
species for food are familiar, but we are apt to forget that
the struggle for life is quite as severe among plants. The
external dangers are evident enough, slugs decimate seed-
lings, and other and larger animals find their food among
plants ; while countless parasites — funguses and insects —
live on them : plants struggle with each other to get the
best of the light : and they have the severities of climate,
cold and drought, to contend with. Given the fact that
plants are subject to a struggle for life, reproduction at
once becomes of interest, for it is those species which
produce the best adapted offspring, in sufficient number
to make up for the constant destruction, that will survive.
So that the ways in which a plant can produce vigorous
CH. VIII] REPRODUCTION. 109
and numerous offspring come to be the most important
part of its physiological equipment.
The value of reproduction comes out clearly in those
plants which regularly exhaust themselves by yielding seed
and die in the process of reproduction. This is the
case with annuals, which start as seedlings in the
spring, yield seed in the autumn and then die. It is
clear that seed-production is what kills them, because if
they are prevented from setting seed, they will survive.
Here the individual is sacrificed to the good of the
community : the species (or group of individuals) lives on
in the seeds, while the individual plants die. All the
machinery of the individual, its manufacture of organic
material with the help of chlorophyll, and its consequent
storage of starch and other reserve material, are directed
to the very thing which kills it, viz. the production of a
big seed crop. The life of the species is the really
important thing, the life of the individual is important
because it renders reproduction, i.e. the continuance of the
species, possible. This sounds paradoxical, but I believe
the machinery of living things to be more comprehensible
if it is thought of as being directed to the preservation of
the species rather than of the individual.
Some forms of reproduction are comparatively simple.
In the tulip-bulb a bud developes in the axil of a leaf,
exhausts the old bulb, and carries on the life of the plant.
Again in the bramble the branches in autumn grow down
to the ground and put out adventitious roots, and in the
spring the bud at the end of the branch shoots out anew.
Again many water plants habitually detach parts of their
110 PLEUROCOCCUS. [CH. VIII
stems which float away and make new plants. These
forms of reproduction are precisely the same as the
artificial continuance of species practised by gardeners, in
making cuttings. In none of these cases does the question
of sex enter into the problem. The river-weed, Elodea,
which has spread in this way throughout the rivers of
England, consists entirely of female plants ; and the same
thing might have happened if the original parent plant had
been a male. For this reason these modes of reproduc-
tion are classed together as asexual or vegetative.
It will be seen later that many plants produce separate
and special cells which serve for asexual reproduction.
Such cases help us to realise that asexual reproduction
is quite as mysterious and wonderful as sexual reproduc-
tions. The essence of the thing is that in a single cell
there should be locked up the potentiality of a future
plant, i.e. that a single cell should be able to grow into
a perfect plant; and that this should be possible is
equally wonderful, whether the new growth originates in
an asexual reproductive cell or an ovum.
The difference between the two is that the egg-cell does
not normally develope until it has been fertilised (i.e.
infected or stimulated) by being fused with the male
element, whereas the asexual reproductive element re-
quires no such treatment.
Pleurococcus.
When an organism is simple in structure, when for
instance it consists of a single cell, asexual reproduction
may be, as far as observation is concerned, a perfectly
CH. VIIl] MUCOK. Ill
simple process. One example has been considered in
the case of yeast, where a young cell buds from the
parent and becomes a new plant. Here, and in similar
cases, growth cannot be distinguished from reproduction.
A cell gets bigger by means of growth, — the process
becomes reproduction when the increment breaks loose
from the parent. This comparison does not by any
means make reproduction easier to understand, it merely
shows that in growth the mystery of reproduction is really
present.
The green dust found on the trunks of trees is made up
of countless millions of the plant Pleurococcus : like yeast
it is a unicellular plant, but instead of being a unicellular
fungus it belongs to the great class of chlorophyll-con-
taining plants. It obtains its food from the air, and from the
water trickling down the tree. Its manner of nutrition
is like that of other green plants and need not be further
considered. Under the microscope it is seen to consist of
minute green cells, more or less massed together in
clusters, and presenting a number of different stages of
cell division. A single cell divides by a cross-wall into
two compartments, then into four, and then into eight.
Various intermediate stages are to be found, and it can
also be seen how the compartments, into which the parent
is divided, disintegrate into their component cells, which
finally fall apart.
Mucor.
For a study of specialised asexual reproductive cells
Mucor, one of the many fungi known as moulds, is
112 REPRODUCTION. [CH. VIII
convenient. Mucor like other fungi is devoid of chloro-
phyll, it does not earn its own living, but depends, like
yeast, on the material previously built up by some other
organism. Ripe fruit, jam, bread and other similar things,
when left to themselves and kept warm and damp, become
covered with a crop of some kind of mould. Such a result
is practically universal, because the reproductive cells or
spores of the mould, being small and light, float in the
air and are universally distributed; they settle like dust
on everything, and thus chance on the organic materials
which can support them. The spore germinates, that is,
it begins to grow and to take on the form of a delicate tube
known as a hypha. The hypha grows, branching as it
elongates, and covers the substratum with a delicate
colourless web or fluff made up of countless interwoven
tubes, sending other branches like roots into the sub-
stance on which it lives. This irregular web of branching
tubes constitutes the whole of the absorptive part of the
plant and is collectively described by the term mycelium.
The most remarkable point in its structure is that it is not
made up of a regular series of cells; it has occasional
cross-walls, but it is not divided into the numerous small
compartments or cells seen in the plants previously
studied. There is little to be seen in the hypha except
oily protoplasm containing vacuoles: by the use of
staining reagents numerous small nuclei can be seen.
After a time other structures make their appearance : —
minute rods grow vertically up in the air, each crowned
with a little ball, and looking like small round-headed
pins. These are called spore-bearing hyphce, and the pin
CH. VIIl]
MUCOR.
113
heads (in which the spores are produced) are called
sporangia. The sporangium is originally a swelling at
FIG. 49.
MUCOR.
A. Diagrammatic sketch showing the mycelium and the sporangia
borne on vertical hyphae.
B. Various stages in the germination of a spore ; /, mycelium with
vacuoles.
C. A ripe sporangium containing spores, and covered with a coating of
calcium oxalate crystals ; ID, the collar.
D. A burst sporangium; col, the columella to which two spores adhere ;
iv, the collar.
the top of a hypha cut off from the rest of the mycelium
by a cell-wall. The contents of this terminal cell become
separated into a large number of small masses of proto-
plasm, and these when they have clothed themselves with
cell- walls are the spores. But before this a change takes
place in the cross- wall at the base of the sporangium :
D. E. B. 8
114 KEPRODUCTION. [CH. VIII
it grows and bulges into the cavity of that receptacle,
filling up a great part of its cavity and forming a
structure known as the columella (see fig. 49). The
protoplasm, remaining over after the development of the
spores, degenerates into a slimy gelatinous substance in
which the spores are embedded. The wall of the spo-
rangium becomes brittle and is covered externally with
a coating of minute crystals of calcium oxalate. After a
time the wall of the sporangium breaks by the swelling
of the jelly, and the spores are set free. In this stage
the sporangium presents a characteristic appearance :
the remains of the wall look like a broken fringe or
cup at the top of the stalk or spore-bearing hypha, and
is called the collar: and on the columella (now fully
exposed) are usually seen a few scattered spores ad-
hering.
The cycle has now been completed, the spores will
germinate, they will give rise to a fresh mycelium, bearing
sporangia which contain spores ; and thus the plant may
be indefinitely reproduced.
Mucor. Sexual reproduction.
Mucor also has a simple form of sexual reproduction.
In the higher plants, as in animals, the male element is a
structure strikingly different from the egg-cell, which it
fertilises. This is obvious in animals where the spermato-
zoid is the fertilising agency ; also in the fern where a
similar motile male element, the antherozoid, conveys
the fertilising element to the egg-cell. But in Mucor
there is no such differentiation, the act of fertilisation is
CH. VIII]
MUCOR.
115
the coalescence of two similar protoplasts1. When this is
the case the process is known as conjugation, but it must
be recognised that it is essentially a sexual process.
~ff.
FIG. 50.
CONJUGATION OF MUCOR.
a, b, c, d, e, f, g, represent successive stages ;
h, fully formed zygospore.
The first thing that can be seen is the approximation
of two branches of the mycelium which are richly pro-
vided with protoplasm, and which finally meet by their
swollen ends (see fig. 50). The next stage consists in the
formation of a cross wall in each branch, and finally the
collections of protoplasm thus isolated are allowed to meet
by the degeneration of the ends of the branches. There
is thus formed a central cell containing the united
contributions from the conjugating mycelial branches.
This cell is known as the zygospore ; it is characterised by
a rough, dark-coloured outer coat, and it remains attached
to the now empty mycelial branches, which are sometimes
1 Protoplast means the protoplasmic contents of a single cell.
8—2
116 CONJUGATION. [CH. VIII
called suspensors — an unnecessary term. The zygospore
is endowed with a certain persistence of vitality, so that
after the crop of Mucor has died and disappeared, the
zygospore is left alive, isolated in the nutritive sub-
stratum. After some weeks of rest it germinates, i.e.
begins to grow. The thick outer coat bursts and the
inner cell-wall grows out into a stout mycelial tube or
hypha. This hypha may either at once proceed to form a
sporangium, or it may branch once before it does so, but
in any case it does not form a complex web of mycelium
like that produced from a sporangial spore.
Spirogyra. Conjugation.
Spirogyra (as described in Chapter I.) is an Alga,
having the form of a delicate filament, each filament being
made up of a simple row of cells. The process of conju-
gation takes place between the cells of neighbouring
filaments. A number of cells in each of the filaments put
out processes, simple tubular outgrowths from the cells,
which meet, coalesce and finally become converted into
tubes uniting cell to cell as shown in fig. 51. The contents
of the conjugating cells contract and the rounded masses so
produced are the elements which fuse together in the act
of conjugation. The balling of the protoplasm begins in
one of the conjugating cells before it is perceptible in the
other. The protoplasts which are thus early in contracting
have a certain masculine character, inasmuch as they are
more active than the protoplasts with which they conju-
gate. They travel through the connecting tube and by
fusion with the stationary protoplasts they form zygospores.
CH. VIII]
SPIROGYRA.
117
The zygospore clothes itself with a thick resisting cell-
wall, and after a period of rest germinates and gives origin
to a new Spirogyra filament.
FIG. 51.
SPIROGYRA.
A. — G, represent various stages in the process of conjugation.
H, fully formed zygoapores.
Note the loop-like folds in the cross-walla ; they are connected with
the mode of growth of the cells.
CHAPTER IX.
ALTERNATION OF GENERATION — THE BRACKEN FERN
(PTERIS)— STRUCTURE OF THE SPOROPHYTE OF PTERIS.
Alternation of generation.
The study of the fern is introduced in this place
because of the remarkable manner of reproduction —
known as alternation of generation — which this plant
presents. Alternation of generation is especially inter-
esting because it gives a key to the relationship of the
higher plants, such as the sunflower and the oak, which are
known as Phanerogams, to the great class of which the
fern is one, known as Cryptogams. The knowledge of the
process enables us to form a guess at the line of descent
of the flowering plants, thus for instance it tells us that
they have probably been evolved from fern-like plants. To
the professed botanist this speculation is of the greatest
value. He studies all plants, and it is this kind of
knowledge which enables him to classify them in a
rational manner. The subject cannot have this sort of
interest to the elementary student. I have therefore
determined to treat alternation of generation only from a
general point of view, as a remarkable form of reproduction,
CH. IX] ALTERNATE GENERATION. 119
and not to insist in any detail on the connection between
it and the reproduction of Phanerogams.
The fern exists under two quite distinct forms, as
different in appearance as the caterpillar and butterfly;
these two forms may for the moment be called $ and 0.
The alternation of generation consists in this : both S and
0 have reproductive organs, but S only produces 0,
and 0 only produces S. So that the pedigree of a g
fern would be represented by 0
The process is further remarkable for the working ^
together of two types of reproduction, sexual and
asexual. The essence of sexual generation is that a cell,
the ovum or egg-cell, is fertilised by a male cell which
unites or melts up with it, and the fertilised egg-cell
thus becomes capable of developing into an embryo
or young plant. The essence of asexual reproduction
is that the parent produces a cell which developes into
a young plant without being stimulated by a process of
fertilisation. In the fern, the form 0 bears an egg-cell
which when fertilised developes into $; S produces,
without a sexual process, certain reproductive cells called
spores, and these produce the form 0. The
pedigree may thus be amplified by adding the
letter ra to express the union of the male
element.
The diagram shows that the act of fertilisa- i "*
tion is confined to the form 0, so that instead of S
there being (as in quadrupeds) a regular series .L
of sexually produced generations, there are alter-
nate sexual and asexual generations.
120 SPOROPHYTE [CH. IX
The form 0 is known as the Oophyte* or egg-bearing
plant ; it is a minute moss-like organism which no one
would suspect of being a fern, and is commonly to be
found growing in the flower-pots in ferneries.
The form 8, known as the Sporophyte or spore-bearing
plant, is what is familiarly known as the fern plant.
In the next chapter the details of the reproductive
acts by which oophyte produces the sporophyte and vice
versa will be considered. We now pass on to the general
structure of the sporophyte.
Sporophyte of Pteris, the Bracken Fern.
The part which we see above ground, with an up-
right stalk subdividing and bearing leaflets, is a leaf;
the stem from which it grows is underground and, as
in the case of the sedge (fig. 6), is called a rhizome.
The subterranean stem creeps horizontally below the
surface and sends up leaves year by year. The fern serves
as an example of a manner of life differing from those
hitherto studied, and one that is common among plants.
The sunflower is an annual, dying down after it has borne
fruit, and beginning next year's cycle in the seedling stage.
The oak is a woody perennial, — in which the parts above
ground are permanent. The fern is a herbaceous peren-
nial,— in which the parts above ground (the leaves) behave
like the stems of annuals and die down to the level of
the ground; but they differ from the leaves of annuals
in springing from a stock or perennial underground stem.
1 The term gametophyte is commonly used as an equivalent for
oopJiyte,
CH. IX] OF PTERIS. 121
Physiologically the manner of life of the fern is similar
to that of such flowering plants as pseonies, larkspurs,
columbines and other garden perennials, as well as bulb-
plants, tulips, daffodils, &c. It is a manner of life especial-
ly well adapted to withstand severity of climate — for the
perennial stock is hidden away underground safe from
frost and drought.
The rhizome of Pteris is shown in fig. 52, it is a
rough looking, irregularly branching stem which grows
G— ~
Fm. 52.
HORIZONTAL UNDERGROUND STEM OR KHIZOME OF Pteris.
G, the growing point ; Llt a developing leaf ; L2, the leaf of
the current year ; L3, a decayed leaf of the previous year.
The rhizome bears adventitious roots ; from L2 a young rhizome B
is growing.
more or less horizontally. It ends in a conical point,
the growing point, which resembles the growing point of
122 RHIZOME [CH. IX
a sunflower in being a place where cells are manufactured
by cell-division, but it differs from it in certain details
which need not be considered.
The leaves come off right and left from the rhizome
and bend up to emerge above ground. A rhizome, dug up
in autumn, will show leaves in various stages ; at the basal
end, i.e. away from the growing point, are the dead and
withered stalks of last year's leaves, and nearer the apex
come the present year's leaves, nearer still are very young
leaves which will remain dormant through the winter and
shoot up in the following spring. The most noticeable
point about the young leaves is that the stalk is strongly
developed while the lamina is small and folded down on
the top of the stalk. Two other facts must be noted:
namely, that buds are formed on the leaf-stalks ; and that
the rhizome bears adventitious roots — the original true
roots having long ago disappeared.
Pteris. Histology.
The histology of the rhizome is interesting because it
supplies a type of vascular bundle differing from anything
previously described.
The characteristics of the bundle in the sunflower and
oak are two : (i) it possesses in its cambium the power
of increase in thickness : (ii) xylems and single strands of
phloem run side by side; these features are technically
expressed by calling the bundle open and collateral.
In Pteris the bundle is closed, i.e. the cambium is absent,
and there are two layers of phloem running with the
xylem and almost surrounding it.
CH. IX] OF PTERIS. 123
In a transverse section of the rhizome we have ex-
ternally the epidermic layer, which presents no special
PIG. 53.
KHIZOME OF Pteris, transverse section slightly magnified.
p.sc, peripheral sclerenchyma. Z.I, lateral line.
p, parenchyma.
sc, sclerenchyma.
sc.st, scattered strands of sc. v.b, vascular bundle.
points of interest, and from a physiological standpoint it is
unimportant, for the protective function is practically
taken over by a layer of hard-walled sclerenchyma. This
layer makes a dark-coloured border round the section as
seen with a low power (fig. 53); it is wanting at two
opposite points where the subjacent tissue comes to the
surface. These places are visible as streaks running down
two opposite sides of the rhizome and are known as
lateral lines. Their function is believed to be connected
with aeration. In the cork of the oak stem and of the
potato tuber a,re certain spots known as lenticels, where the
124 RHIZOME OF PTERIS. [CH. IX
cork-cells are loose and traversed by intervening spaces,
through which the internal parts of the stem receive air.
So that the lateral lines of the rhizome, although of
different morphological value, would seem to have the
physiological character of lenticels.
The inside of the section presents three obviously
distinct tissues. It has patches and dots of a dark colour
and hard consistence which are irregular strands and
plates of sclerenchyma running longitudinally. Secondly,
there are yellowish spots of rounded or oval outline.
These, the vascular bundles, are not arranged regularly,
although they make, with the larger sclerenchyma bands,
a more or less defined mass in the section. The rest of
the rhizome is made up of soft pith-like parenchyma.
These various tissues must be examined in detail.
The cells of the sclerenchyma are many times as long as
broad, and fit close together without intercellular spaces.
The walls are lignified, and have simple oblique slit- like
pits.
The soft parenchyma is made of polygonal cells
roughly hexagonal in transverse section, with cellulose
walls not fitting closely together, but leaving inter-
cellular spaces. It is this tissue which comes to the
surface at the lateral lines.
The parenchyma cells are crowded with starch grains
and serve as the storehouse of the rhizome.
Vascular bundles1.
Each bundle is surrounded by a bundle-sheath or
endodermis consisting of a single layer of small cuticu-
1 See Preface on the word stele.
CH. IX] HISTOLOGY. 125
larised brown-coloured cells, which in longitudinal section
are seen to be only slightly elongated ; they do not
contain starch.
FIG. 54.
TRANSVERSE SECTION OF A BUNDLE IN THE RHIZOME OP Pteris.
e, endodermis, outside which is parenchymatous tissue p.
p.c, pericycle. p.phl, protophloem.
s.t, sieve-tube. sc.v, scalariform vessel.
Inside the bundle-sheath is an irregular layer of
colourless softer cells, which differ from those of the
bundle -sheath in containing starch : these form the peri-
cycle. The rest of the bundle is made up of the same
two main classes of tissue as those seen in the sunflower
and oak, namely, xylem and phloem : the centre of
each bundle is xylem and is surrounded, not completely
but on two opposite flanks, by the phloem. There is
no cambium.
What was said in an earlier chapter as to the general
characteristics of vascular tissue, holds good in the case
of the fern.
126
RHIZOME OF PTERIS.
[CH. IX
Both xylem and phloem contain vessels and paren-
chyma.
In both the vessels are built up of elongated cells
one overlapping the next.
The xylem vessels have lignified walls and no proto-
plasmic contents.
The phloem vessels, or sieve-tubes, have cellulose walls
and intercommunicating threads of protoplasm piercing
the sieve-plates.
Inside the pericycle is a layer of small cells, the
protophloem1, and inside this again is a layer of large
A
jt scu.
Fia. 55.
LONGITUDINAL SECTION OF THK RHIZOME OP Pteris.
p, parenchyma.
e, endodermis.
p.p, protophloem.
sc.v, scalariform vessels.
s, sclerenchyma.
p.s, pericycle.
s.t, sieve-tubes.
1 In the rhizome of Pteris the protophloem is sometimes continuous
all round the bundle, although the fully developed phloem is discon-
tinuous and does not completely surround the xylem.
CH. IX]
HISTOLOGY.
127
sieve-tubes. In the fern the sieve-tubes differ from the
typical sieve-tubes of the flowering plants in which a large
transverse sieve-plate separates two contiguous elements.
In the fern the sieve-plates are on the longitudinal walls,
and are therefore most easily seen in longitudinal section.
Fig. 55 shows irregular elongated areas of a granular, or
faintly dotted aspect, which are the sieve-plates pierced
by strands of protoplasm passing from one element to the
next.
Inside the phloem are the vessels of the xylem im-
scl
spy
:T-T/ seal.
sv.t.
FIG. 56.
MACERATED RHIZOME OF Pteris,
showing the isolated elements.
scl, sclerenchyma. seal, scalariform vessels.
sp.v, spiral vessels. sv.t, sieve-tubes.
128 RHIZOME OF FTERIS. [CH. IX
bedded or packed as it were in a small quantity of
parenchyma. There are a few minute spiral vessels (not
shown in the figs. 54, 55), but the great mass of xylem
vessels are of large diameter and are known as scalari-
form vessels.
The scalariform, i.e. ladder-like character of these
vessels comes out clearly in longitudinal section. The
horizontal markings which represent the rungs of the
ladder are thickenings of the walls.
CHAPTER X.
REPRODUCTION OF THE FERN — SPORANGIA — PROTHALLUS
— EMBRYOLOGY.
Sporangia and spores.
The plant of which the general structure has now
been described, and which is known as the sporophyte,
bears certain reproductive cells which are called spores.
The development, and structure, of the spores, must now
be given in detail, together with the history of their
germination ; this will be followed by an account of the
oophyte to which the germinating spore gives rise.
The spores (as in the case of Mucor) are found in
receptacles — sporangia — of which a group is shown in
fig. 57. The sporangia grow in groups and patches
known as sori, which in the majority of ferns are found
on the lower surface of the leaves. Each sorus is pro-
tected by a covering, — the indusium. In Pteris the
indusium is the edge of the leaf folded back so as to roof
over the linear sorus running down the marginal part of
the leaf. In Aspidium it is a specially developed mem-
brane covering the sorus like an umbrella, and therefore
P. E. B. 9
130
FERN.
[CH. X
differing morphologically from the indusium of Pteris.
Fig. 57 shows that the sporangia arise from a cushion
(the placenta), which is simply a swollen vein.
FIG. 57.
TRANSVERSE SECTION THROUGH A SORUS OF FTERIS :
P, the placenta, bearing hairs and sporangia, two of which contain
spores : R, the annulus or ring of the large empty sporangium.
The sporangium consists of a hollow head mounted on
a delicate stalk. Within the cavity of the head are
contained numerous minute brown cells, which are the
spores. The spore-wall is differentiated into two layers,
an inner cellulose membrane and an external cuticularised
layer, resembling in fact the outer wall of an epidermic
cell. To understand the development of the spores it is
necessary to have a general idea of the development of
the sporangium.
Each sporangium is the product of a series of cell
divisions occurring in a single epidermic cell. This mode
of development gives a certain morphological value to a
sporangium, which is technically expressed by saying that
CH. X] REPRODUCTION. 131
the sporangium of the fern is a trichome, i.e. a hair-like
structure ; the multicellular hairs which occur on the
surface of many plants being each similarly developed from
a single epidermic cell.
The chief stages in the development of the sporangium
are shown in fig. 58. The epidermic cell divides into a
stalk and a head, which are the parent-cells of the stalk
and head of the future sporangium. The stalk cell
a
FIG. 58.
DEVELOPMENT or THE FERN SPORANGIUM.
a. The out-growth from an epidermic cell is divided into two cells ; a
rounded apical cell giving rise to the head of the sporangium, and a
basal cell from which the stalk is developed.
6. Two out of the four oblique walls, by which the archespore is marked
out, are shown.
c. A third oblique wall has appeared, and the wall of the sporangium
is seen to be developing.
divides by numerous cross-walls and comes to consist of
several stages or storeys one over the other, each layer
consisting of four cells. The fate of the head-cell is more
complex : it will be enough to say that by four cell-walls
(of which two are shown in fig. 58, 6) a large triangular
cell, the archespore, is marked out in the middle of what
was once the headlike half of the original epidermic cell.
From part of the archespore what are known as the
9—2
132 FERN. [CH. X
mother-cells of the spores develope by cell division, and
finally each mother-cell divides into four spores.
The development of the sporangium and spore is
here given in an abbreviated and diagrammatic manner,
and the formation of what is known as the ring or
annulus (fig. 57) has been left out. It is a line of strong
cells running like a crest three-quarters of the way round
the head of the sporangium. These cells are sensitive to
changes in the dampness of the air; when they are dried
the ring tends to uncurl and exerts a tearing force on the
thinner parts of the sporangium wall, which gives way
under the strain in the form of a gash or cleft running
across it. The place at which the sporangium opens will be
understood from fig. 57 (R) in which the cleft is not quite
closed. Through this gash the spores are able to escape,
and here their small size and lightness comes in as a
valuable quality, since they are borne on the wind like
dust, so that some at least, out of the great quantity
produced, hit on situations suitable for their future
growth. In the laboratory spores are sown on peat, or
better still on tiles, where they grow well and handily.
With regard to the germination it will suffice to know
that the spore increases in size, and by the formation of
cross- walls becomes a cellular body instead of a single cell.
Fig. 59 b represents an early stage in the germination, or
in other words it represents a very young oophyte. The
upper part p, consisting of two cells, is green from the
presence of chlorophyll and carries on the work of as-
similation, while the lower, thinner part r.h. is colourless
and is a root-like organ or root-hair. Thus in the earliest
CH. X]
PROTHALLUS.
133
stages the young oophyte is differentiated into a green
aerial or assimilating part and terrestrial root-like part.
The green part now grows and by a series of cell divisions
FIG. 59.
DEVELOPMENT OP PROTHALLUS FROM THE SPORE.
a, germinating spore ; s, the cell wall of the spore ; the new growth has
already divided into r.h., a root-hair and pt which contains chloro-
plasts and by further division forms the prothallus.
6, c, dt older stages, c and d, less highly magnified,
a and &, Dicksonia antarctica, c and d, Aspidium filix mas.
(After Luerssen.)
forms a flat, heart-shaped body, which is known as a pro-
ihallus. The central part of the prothallus is thickened
into a cushion-like ridge several cells in thickness, the
rest of the expanse consists of a single layer of cells. It
grows with the cushioned side downwards attached to the
soil by numerous root-hairs and is now a full-grown
oophyte leading an independent life, and as above pointed
out, of a form extremely distinct from that of the
sporophyte which gave it birth.
134 ARCHEGONIUM. [CH. X
Sexual reproductive organs.
These are of two kinds : the archegonia, which contains
the egg-cell, and antheridia, in which the male elements
are developed. The archegonia (fig. 60) are found near
the notched end of the prothallus and on its under surface.
Each archegonium consists of a rounded cavity sunk in the
tissue of the prothallus and contains the egg-cell: the
cavity of the archegonium communicates with the outer
world by a canal, a curved chimney-like tube projecting
A. YOUNG ARCHEGONIUM OF THE FERN (Polypodium vulgare).
B. THE SAME MATURE AND OPEN.
C. THE EXPULSION or THE SLIME AT THE MOUTH OF THE ARCHEGONIUM
(in Pteris serrulata).
p, p, cells of the prothallus ; o, egg-cell ; v.c.c., ventral-canal-cell
m, neck-canal-cell. (After Strasburger.)
beyond the surface of the prothallus. This canal is seen
in section to be made up of four rows of cells, as though a
chimney were built of tiers of four bricks each. In the
immature archegonium the free end of the canal is shut,
and its cavity is filled up by a long cell or cells called
neck-canal-cells. Between this and the egg-cell at the
bottom is another cell called the ventral-canal-cell, which
fills up the rest of the cavity of the archegonium.
CH. X]
ANTHERIDIUM.
135
When the archegonium is fully ripe the canal-cells
break down into mucilage, which swells and bulges out at
the opening now formed at the free end of the canal by
the separation of the terminal tier of cells.
The anther idia (fig. 61) are small green papillae (which
afterwards become brown) found principally among the root-
hairs, and further from the notched end of the prothallus
B C
FIG. 61.
A. MATURE ANTHERIDIUM OF A FERN (Polypodium) AND CONTAINING
NUMEROUS MOTHER-CELLS OF ANTHEROzoiDS. Chloroplasts are visible
in the cells constituting the wall of the antheridium.
B. AN ANTHEROZOID.
C. THE ANTHERIDIUM BURST AND EMPTY.
(After Strasburger.)
than are the archegonia. Their architecture is very re-
markable. Each antheridium is built of three cells : one
forms the roof, and the remainder form the circular walls
which limit the cavity within. Imagine a. piece of india-
rubber tubing bent into a ring by the union of its ends :
if such a ring be placed on the table it will make a low
circular wall which may be doubled in height by the
superposition of another similar ring. This is precisely the
structure of the antheridium, its wall is made up of two
hollow, ring-like cells with a third flat cell on the top.
136 ANTHEROZOIDS. [CH. X
Within the cavity a number of spherical cells are seen,
and inside each of these is developed one of the male
elements known as antherozoids, motile organisms resem-
bling the spermatozoa of animals. The antheridium
bursts by the rupture of the lid-cell, and its contents,
the rounded cells, escape. The process of bursting only
takes place when the antheridium is wetted, as for
instance with rain or dew in a state of nature, or under
the coverglass in the laboratory. Water has moreover
a special effect on the rounded cells, which are rapidly
disorganised, and thus set free the antherozoids, which
swim about in the water. Each antherozoid (see fig. 61)
is a tapering rod bent into a corkscrew, and bearing at the
smaller end where the coils of the spiral are closer a
number of long cilia, by means of which it swims1.
Antherozoids are found in certain water plants, for
instance in the stone- worts Chara and Nitella, and here
it seems a natural and fitting adaptation that the male
element, which has to find its way to the egg-cell, should
be a swimming organism. But that in a land plant the
male element should be forced to swim to the egg-cell is
remarkable. It seems only possible to explain it as an
inheritance from an aquatic ancestor; just as the gill-
clefts of the mammalian embryo are such inheritances.
Biologically the fact is of interest for it seems to throw a
light on the mode of life of the prothallus. We can
understand the advantage which the prothallus gains
from its habit of growth, clinging as it does to the soil
1 Each antherozoid bears a protoplasmic vesicle of unknown function.
CH. X] EMBRYO. 137
and thus making a damp chamber in which a film of
water, coating its lower surface, may persist until the
antherozoids have swum to their destination. That the
antherozoid does reach the egg-cell is not a mere matter
of chance. The slime which fills the cavity of the canal
of the archegonium contains malic acid, and it has been
shown that the antherozoids are attracted by this acid.
If a capillary glass tube containing malic acid is introduced
into a drop of water in which antherozoids are swimming,
these organisms are found to direct their course towards
the tube and to force their way into the opening. In
the same way they force themselves into the slime in
the canal and ultimately make their way to the egg-cell,
which they fertilise.
The fertilised egg-cell divides and subdivides and
grows into a complex of cells, — an embryo or young plant.
This young plant is the sporophyte, the plant which will
bear sporangia, will produce spores and will thus com-
plete the cycle of development. It is not necessary to
give a complete account of the process of cell division
by which the embryo grows out of the egg-cell. It will
be enough to know that the first-formed cell-wall is more
or less parallel to the axis of the archegonium and divides
the egg-cell into an anterior and a posterior half. From
the anterior half the young stem and the first leaf are
developed, from the other half are formed the primary root
and a structure known as the foot. The foot has an
important function, for it is by means of it that the
embryo sporophyte is nourished in the early stages of
its existence. The prothallus has finished its share in
138
FERN.
[CH. X
the life-history of the plant and is of no further use except
as a supply of food material for the embryo; and this
supply is drawn by the foot acting like a sucker or root.
FIG. 62.
DEVELOPMENT OF THE SPOROPHYTE OP THE FERN FROM THE
EGG-CELL, diagrammatically represented.
In the upper figure the embryo is made of a number of cells, the four
thick lines represent the cell-walls by which the egg-cell was
partitioned in the early stages.
Of these four cells, s and Z develope into stem and leaf;
r and /, into root and foot, as may be seen in the lower figure.
a, a, unfertilised archegonia.
rh, root-hairs on the lower surface of the prothallus.
The embryo is contained in a swollen and distorted archegonium.
(After Mangin.)
We thus get this remarkable state of things ; that the de-
veloping sporophyte remains attached, by an absorbing
organ, to the oophyte which gave it birth ; the sporophyte
lives in fact like a parasite on its parent. This arrange-
ment is only temporary, after a time the prothallus dies
and the sporophyte grows into a massive leafy plant
capable of self-support.
CH. X] EMBRYO. 139
It is interesting to note that the embryological develop-
ment of the sporophyte begins by a cell-wall cutting the
egg-cell into two halves, one of which has the general
character of stem, the other of a root. It is one of the
many instances of the early differentiation of plant-
embryos into what are known as a shoot-half and a
root-half.
CHAPTER XL
THE FLOWER OF THE BUTTERCUP (Ranunculus)
AND OF THE BEAN ( VlClO, foba).
THE bean and the buttercup, whose flowers form the
subject of the present chapter, belong to the important
division of plants known as Phanerogams. They are
separated from the class of plants known as Cryptogams
(in which are placed Spirogyra, Mucor and the ferns) by
certain well-marked characters connected with repro-
duction. Phanerogams are sometimes known as Sperma-
phytes or Seed plants, and this is a happily chosen name,
for the production of seeds is the most characteristic
feature of the class. The name Flowering Plants, which
is a familiar equivalent for Phanerogam, is not so appro-
priate; while the term Phanerogam, implying that the
process of reproduction is obvious or plainly visible, is
particularly inappropriate. In reality the Cryptogams,
whose title suggests obscurity in the matter of repro-
duction, have reproductive processes far more simple and
more easily detected than those of the Phanerogams.
The bean and the buttercup both belong to a division
of the Seed-plants characterised by the possession of two
CH. Xl] CLASSIFICATION. 141
cotyledons, and for that reason known as Dicotyledons;
the tulip, on the other hand, which formed the subject
of an earlier chapter, belongs to the Monocotyledons) or
plants with a single cotyledon.
The Seed-plants are classified into a number of
divisions known as Natural Orders, and the arrangement
of flowering plants into these groups is an important part
of the systematic botanist's work. The student of Ele-
mentary Biology is not expected to know this part of the
subject, but he ought to have a rough idea of the general
plan on which plants and animals are grouped. In collect-
ing material for the study of the flower, the student
will come across two kinds of buttercup, not identical in
appearance, but both obviously buttercups. This relation-
ship is technically expressed by saying that both plants
belong to the genus Ranunculus, but that they are of
different species : — for instance Ranunculus acris and Ra-
nunculus bulbosus. Besides the genus Ranunculus there
are other plants whose flowers are plainly built on the
same general plan, for instance the Marsh Marigold (Caltha
palustris) and the Globe Flower (Trollius Europceus).
Other flowers such as the Columbine (Aquilegia) and
the Larkspur (Delphinium) do not obviously resemble
buttercups, but are found by analysis to be of the same
structural type. All these genera, Ranunculus, Caltha,
Trollius, Aquilegia, Delphinium and many others are
massed together into the Natural Order Ranunculacece,
so named after one of its constituent genera, — Ranunculus.
In the same way the bean (Vicia faba), the pea (Pisum),
the lupin (Lupinus), the clover (Trifolium) and scores of
142 THE FLOWER. [CH. XI
other genera constitute a Natural Order, the Legumi-
nosce.
In classifying, botanists are guided chiefly by the
structure of the flowers, — by the form, number and
position of the petals, and of the other floral organs,
so that the morphology of the flower comes to be the
key to the science of systematic or classificatory botany.
And it should be noted that in classifying plants we are
not simply satisfying the instinct which leads us to sort
our possessions into like and unlike. The classification of
living things has an interest which does not attach to the
arrangement of artificial objects such as postage-stamps.
Living things are not merely placed in groups as an
expression of resemblance, they are classed in natural
groups, that is to say they are ranged according to blood-
relationship. Thus in the case of the Ranunculaceae it is
believed that all the genera are descended from a single
ancient plant, and a wide field for speculation is open to
us, as to how and why the primaeval Ranunculus has left
such varied descendants as the Larkspur, the Marsh Mari-
gold, &c.
The flower.
The flower is essentially a shoot or axis bearing leaves
on which the reproductive elements are produced. The
proof that the petals and other floral organs are morpho-
logically of the rank of leaves cannot here be given in any
detail. A few of the arguments in favour of this belief
may however be sketched. The development of the parts
of the flower as superficial outgrowths from the growing
CH. Xl] BUTTERCUP. 143
point is a leaf-like character; so is their arrangement in
spirals or in circles (whorls) on the axis. Deformed or
monstrous flowers supply interesting evidence : almost any
part of the flower may abnormally take on an obviously
leaf-like form. And in some cases the axis which bears
petals below is prolonged beyond the flower and bears
ordinary green leaves.
A FIG. 63. B
A. KANUNCULUS FLOWER FROM WHICH THE SEPALS, PETALS, AND ALL BUT
TWO STAMENS HAVE BEEN REMOVED.
B. KANUNCULUS FLOWER DIVIDED LONGITUDINALLY.
(From Le Maout and Decaisne.)
The floral leaves are divided into two main groups.
(1) Those which are essentially reproductive; and (2)
those which are not essential to reproduction. Fig. 63 A
shows a buttercup stripped of the non-essential parts and
retaining part of the reproductive leaves grouped round
the central axis. Fig. 63 B shows a longitudinally divided
flower in which the non-essential parts of the flower are
also shown. They consist of ten flattened leaf-like organs
arranged in two groups of five each. In the horse-
chestnut we had an instance of leaves springing from
the stem opposite one another at the same level. In
144
THE FLOWER.
[CH. XI
the buttercup five floral leaves spring from the axis at
practically the same level ; and five others form a second
group just above the first. In fig. 63 B two of the lower
group1 are visible, they are seen to be smaller than the
leaves of the inner group (of which three are shown) and
are also distinguishable by their hairy outer surface. This
outer group is known as the calyx, and each of its con-
stituent leaves is a sepal. The inner group of leaves is
known as the corolla, and is made up of petals.
It is important to note that the petals are arranged
alternately with the sepals : in other words the petals are
not vertically above the sepals, but each petal springs
from the floral axis on a line which if prolonged down-
wards would pass between the points of origin of two
sepals. This arrangement will be understood from fig. 64,
FIG. 64.
FLORAL DIAGRAM OF THE PEACH.
(From Le Maout and Decaisne.)
1 In many flowers the floral leaves are spirally arranged on the axis,
just as the foliage leaves are so disposed on the stem. This may be the
case even when the floral leaves are obviously divided into groups, the
members of which appear to spring from the axis all at one level. Such
groups are conveniently called whorls, although this term strictly implies
that the members are not spirally disposed,
CH. XI] COROLLA AND CALYX. 145
which gives a floral diagram, or simplified bird's-eye view
of a flower1; the two outer whorls are represented by
brackets, the sepals being shaded, while the petals are
black. In spite of the overlapping of the parts it is clear
that the centre of any petal is half-way between the
centres of two sepals. The importance of the alternation
of petals and sepals will appear when the structure of the
bean-flower is examined.
In the buttercup the petals are bright yellow, while
the sepals are less bright in tint ; in the bean the petals
are black and white, while the calyx is nearly colourless.
This is a general but by no means an absolute rule,
namely that the calyx is green or dingy in colour,
while the petals are conspicuous. It is also commonly
the case that the sepals are of a simpler, less elaborate
pattern than the petals: this is not obvious in the
buttercup, where both are of a simple form; but in
the bean the contrast is plain, the petals being of
a specialised type, while the sepals are simple. A com-
parison of these flowers brings out another important
point, namely that each sepal may be free, i.e. not
united to the neighbouring sepals, as in the buttercup ;
or the sepals may be united into a cup or tube, as
in the pea (fig. 67). Similar differences in regard to
cohesion occur in the petals : thus in the cowslip they are
united into a tube (fig. 65), while in the buttercup they are
free. Other opportunities will occur of considering this
point, which is here merely noted as one of the most
1 The floral diagram, though not that of a Eauunculus, serves equally
well to illustrate alternation.
D. E. B. 10
146 NECTARY. [CH. XI
striking characters in which the architecture of flowers
is modified.
FIG. 65.
FLOWER OP THE COWSLIP DIVIDED LONGITUDINALLY.
From Le Maout and Decaisne.
Before passing to the reproductive parts of the flower
a point in the structure of the buttercup petal must be
noted, — a minute notched scale (fig. 66) at the base of the
FIG. 66.
PETAL OF EANUNCULUS,
showing the scale-like nectary at the base.
From Le Maout and Decaisne.
inner surface. This is known as a nectary and secretes the
sugary juice called nectar, the importance of which in the
natural history of the flower will be considered later.
CH. XI] ANDRCECIUM. 147
Androscium and Qyncecium.
Above or within the petals the floral axis bears the
parts of the flower which are essential to reproduction.
The lower group of floral leaves is known collectively as
the androBcium because it is connected with the male or
fertilising part of the process of reproduction. Above the
andrcecium conies the gyncecium, where the egg-cell is
found.
The andrcecium is made up of stamens, of which two
only remain in fig. 63 A, the others having been re-
moved; in fig. 63 B it may be seen that the stamens in
the buttercup are numerous, the precise number being
unimportant. Each stamen consists of a stalk — the fila-
ment, and a swollen elongated head — the anther. Within
the anther are developed minute bodies — pollen-grains, by
means of which the male element is conveyed to the egg-
cell contained in the ovule. The pollen-grains are carried
by the wind or by insects or other means to a part of the
gyncecium where they germinate and by a long hypha-
like tube transfer the male element to the egg-cell (for
further details see Chapter XII). The pollen occurs in
large quantities, and is familiar to most people as a floury,
dusty material, frequently orange or yellow in colour,
coating the ripe anthers. The pollen is developed in four
cavities hollowed out in the anther : these pollen sacs are
afterwards converted, by degeneration of two dividing
walls, into two cavities. The fully developed anther opens
or dehisces by two longitudinal fissures through which the
pollen is set free. Above the stamens are the floral leaves
known as carpels, constituting the gyncecium ; and these,
10—2
148 GYNCECIUM. [CH. XI
like the members of the androecium, are arranged spirally
on the axis of the flower. Each carpel may be considered
a leaf folded so as to include a cavity. The hollow of the
carpel is known as the ovary and contains an ovule. The
ovule is simply a young seed; for our present purpose
the only point of importance about the ovule is that in
it is developed the egg-cell which afterwards gives origin
to the embryo. In fig. 63 B one of the carpels is laid open
so as to show the ovule within. At the hook-like upper
end of the carpel is an organ called the stigma, whose
function is to receive the pollen-grains and transmit, in a
way to be described, the fertilising element to the egg-cell.
The ovary and stigma1 are the essential parts of the carpel,
but usually there is a distinct stalk, the style, on which the
stigma is borne ; it is absent in the buttercup, but in the
cowslip (fig. 65) the style runs up the centre of the flower
as a delicate column, bearing a rounded stigma at its free
extremity.
Bean-flower.
The structure of the bean-flower will be understood
from the sketches of the very similar flower of the Sweet
Pea (fig. 67). The flower stands with its axis more or less
horizontal instead of approximately vertical as is the axis
of the buttercup flower. It differs from the last named
in the matter of symmetry ; it is not uniformly symmetrical
round its axis ; this is clear when it is noted that the big
petal marked S in fig. 67 has no counterpart on the opposite
1 The term pistil is used to express the ovary, style, and stigrna
collectively.
CH. Xl] BEAN-FLOWER. 149
side of the flower1. If however the flower is split into two
by a median section in the plane of the paper, it will be
FIG. 67.
FLOWER OF THE SWEET PEA.
S, standard or vexillum. W, wings or alse.
.K", keel or carina. (7, calyx.
(The flower had been preserved in alcohol, hence the keel was visible
through the semi-transparent wings.)
divided into similar halves; it is in fact symmetrical
about a median plane which, in the natural position of the
flower, is a vertical plane.
The sepals, as mentioned above, are united into a
tubular calyx the edge of which bears five teeth, and
these indicate the number of coherent parts forming
the calyx. When the calyx has been dissected off, the
parts of the corolla are thoroughly exposed. The upper-
most petal is the standard or vexillum, whose narrow
horizontal base covers over the bases of the other petals,
and whose broad apical part stands obliquely upwards.
The name standard has been given to this petal because
1 The flower of the bean, sweet pea, clover and other allied plants is
said to be papilionaceous because its irregularity gives it a fancied
resemblance to a butterfly.
150 BEAN-FLOWER. [CH. XI
it is raised like a flag, making the flower conspicuous.
Next come two petals, the wings or alee, standing sym-
metrically right and left of the median plane1.
When the wings have been removed a hooded boat-
like structure is seen which is called the keel or carina.
The keel consists of two coherent petals, as is obvious
FIG. 68.
FLORAL DIAGRAM OF A PAPILIONACEOUS FLOWER.
From Le Maout and Decaisne.
when the floral diagram (fig. 68) of a pea-flower is
examined. Within the five sepals (which are shaded) are
shown — in black — the parts of the corolla ; these are four
in number, the lower one, representing the keel, being
partly divided to indicate the union of two petals. The
point of union of the two halves of the keel comes opposite
the centre of the lower sepal ; in other words this sepal
alternates with the petals which make up the keel. In
fig. 69, which gives a back view of the standard, it is seen
that this is a single petal, since it falls between two
sepals : the alternation of the standard may also be seen
in the floral diagram.
1 In the bean each wing is marked with a black spot. The wings
require some slight force to detach them since they are superficially
attached to the part of the corolla within, i.e. to the carina.
CH. Xl] ANDRCEC1UM. 151
Within the keel are contained the androecium and the
gynoecium. The stamens show a remarkable arrangement
FIQ. 69.
FLOWER OF THE SWEET PEA.
S, the standard viewed from behind.
C, the calyx. W, one of the wings.
which is described by the technical term diadelphous. The
horizontal bases of the filaments of nine of the stamens
are united into a broad plate which, being longitudinally
folded, makes a trough, while the tenth filament is free
and roofs in the trough above. The nine stamens are not
coherent throughout their entire lengths, their free apical
parts bend upwards and terminate in anthers.
The gyncecium is contained in the .trough of the
united filaments; it consists of a single carpel, of which
the horizontal part (in the trough) is the ovary, while the
vertical part is the style which bears the stigma. The
ovary of the pea differs from that of the buttercup in
containing several ovules, as may be seen in fig. 70, where
however only part are shown. The cavity of the ovary is
made by the folding of the carpellary leaf, and the ovules
152
GYNCECIUM.
[CH. XI
are borne along the leaf's united edges. The ovary
finally developes into the pod and the ovules into peas,
FIG. 70.
FLOWER OF THE SWEET PEA.
In the central figure is seen the keel (K), through the walls of which cnn
be seen the swelling ovary and some of the stamens. The style
0 projects from the apex of the keel. C, the calyx.
The lower figure gives the 9 united filaments, the 10th or free stamen,
and the projecting style.
The upper figure gives the horizontal ovary containing ovules, and the
vertical style G.
when it is a familiar fact that the peas are attached along
one edge partly on the right and partly on the left-hand
valve of the pod,
CH. XI] INSECT VISITORS. 153
Fertilisation by means of insects.
In order that the egg-cell may be fertilised it is
necessary that pollen shall reach the stigma. The
question therefore how the pollen reaches this position
has to be met, and the flowers of the papilionaceous type
are well adapted to illustrate one of the chief means of
pollen-distribution, namely by means of the visits of
insects. When it is understood that it is advantageous
to the species that its flowers should be so visited, we can
understand the meaning of many parts of the flower which
without the knowledge of this fact would be meaningless.
Thus the bright colours and sweet scents of flowers
undoubtedly serve to attract insects, while the sugary
juice or nectar supplies a more substantial attraction. In
the buttercup the scale-like nectary has been described ;
in the pea-flower the receptacle is more elaborate, being
in fact the trough made by the united filaments. The
freedom of the tenth stamen gives the visiting insect
access to the nectar, and that this is the meaning of the
arrangement is clear from the fact that where (as in the
Broom) there is no nectar (the flower being visited by
bees for the sake of the pollen), the tenth stamen is united
to its nine fellows1.
The flowers of the bean and pea are especially adapted
to be fertilised by bees, and the manner in which these
insects visit them presents some points of interest. In
settling on the flower the bee uses the alae as" a stage to
alight on, and these petals being intimately in connection
* The stamens are then said to be monadelphous,
154 INSECT VISITORS. [CH. XI
with the keel, the weight of the insect is brought to bear
on the keel and forces it downwards so that the anthers
and the style emerge and touch the under side of the bee's
body. The union of the wings and keel is effected in
the pea-flower by an interlocking of protuberances and
depressions which can hardly be understood without
examining the flower. In the bean the adhesion of the
wings to the keel has a similar use. The bee not only
carries away pollen from the flower visited, but also
brings to it pollen which had adhered to its hairy coat
during previous visits. In this way the insect will smear
the stigma with pollen and at the same time carry off a
fresh supply for future fertilisations. When the bee, after
having sucked the nectar, flies away, the keel, relieved
from its weight, springs up into its former position and
once more covers up the androecium and gynoecium in its
hood. In a wet climate like that of England this
arrangement must be of service to the plant in protecting
the anthers from wet, — for it is a matter of experience
that pollen is injured by rain: the nectar too is tho-
roughly sheltered and cannot be diluted or washed away
by a shower.
In books1 especially devoted to this subject many
other details are given as to the adaptation of papi-
lionaceous flowers to the visits of insects. What is here
given must suffice for our present purpose.
i See The Fertilisation of Flowers, by H. Miiller, 1883.
CHAPTER XII.
DISTRIBUTION OF POLLEN BY THE WIND AND BY INSECTS
— SELF AND CROSS FERTILISATION — DICHOGAMY —
PLANTAGO — SILENE — DOG-DAISY OR CHRYSANTHEMUM
LEUCANTHEMUM.
OF the flowers which form the subject of the present
chapter, two, namely Silene and the dog-daisy (Chrysan-
themum), are visited by insects, and the distribution of
the pollen is carried on by their agency. In the remaining
flower, the plantain (Plantago lanceolata), the pollen is
carried by the wind. A number of other plants are in the
same case, for instance fir trees, the yew, hazel, oak and the
great class of grasses : such plants have certain characters
in common, which may be demonstrated on the plantain.
The " heads " of the plantain are made up of a number of
minute flowers massed together, each flower consisting of
four simple sepals, and a tubular corolla of four mem-
branous brown petals.
Here we have one of the chief characteristics of wind-
fertilised plants, namely that the flowers are small, simple,
and inconspicuous, presenting a striking contrast to the
brightly coloured petals of insect-fertilised flowers. The
156 POLLEN DISTRIBUTED [CH. XII
plantain has no scent, and does not secrete nectar; in
fact it has none of the qualities which were referred to
above as serving to attract insects1.
Another point is the production of great quantities of
pollen ; this is not so striking in the plantain as in some
other members of the wind-fertilised class, for instance in
the yew or pine, in which the clouds of dusty pollen, which
may be shaken out of a branch, are familiar to every
one. In this way pollen comes to be widely distributed,
and has been found in the dust collected at considerable
heights in the air.
The biological meaning of this profusion of pollen is
clear enough: the plant has to trust to chance for the
conveyance of pollen from stamen to stigma, instead of
to the visits of insects by which small loads of pollen are
transferred directly from flower to flower. Thus to make
sure of all the countless stigmas on an oak tree being
dusted with pollen, enormous and apparently wasteful
quantities of the material must be manufactured. The
pollen-grains of wind-fertilised plants are smooth, dry and
incoherent, and seem especially adapted for floating like
dust in the air.
On the other hand the pollen of insect-fertilised plants
is coherent like a damp powder. This quality is generally
due to the coats of the pollen-grains being sculptured into
minute prickles so that the grains cohere in groups and
masses; the dog-daisy supplies an instance of rough-
coated pollen-grains.
1 Some species of Plantago are visited by pollen-collecting insects,
and are both scented and conspicuous in colour.
CH. XII] BY WIND. 157
In the plantain the filaments of the stamens are
enormously long in proportion to the size of the flower ;
this is frequently the case in wind-fertilised plants, for
instance in the wheat-flower shown in fig. 71.
FIG. 71.
WHEAT-FLOWER,
showing the large anthers hanging far out on long flexible filaments.
Above are the two large branching styles. sq, the scaly floral
leaves. From Le Maout and Decaisne.
In consequence of this character the anthers are well
exposed and easily shaken by the wind, and the distribution
of the pollen correspondingly favoured. In the " catkin "
of the hazel the same end is brought about by other
means. The catkin is an inflorescence, — a stalk bearing
numerous minute flowers, the stamens are short, but the
whole inflorescence is pendant and flexible, and easily
shaken by the wind. In the nettle the filaments are at
first bent inwards towards the centre of the flower, but
later on they uncurl with a sudden movement, scattering
158 CROSS-FERTILISATION. [CH. XII
their pollen in a minute explosion. The same thing is
seen in the " artillery plant " (Pilea) which is sometimes
grown in green-houses, and receives its name from the
puff of smoke-like pollen given out from its exploding
stamens.
Wind-fertilised plants also show a certain resemblance
to one another in the character of the gyncecium. Since
the stigma receives the pollen fortuitously, the chance of
fertilisation is increased when the stigma is large. The
stigmatic surface in the plantain is great in proportion to
the size of the flower, and the same thing is particularly
striking in the wheat-flower figured above (fig. 71). This
is a general character of the class of flowers we are
considering, though the extension of surface is brought
out in different ways, for instance in the walnut the
stigma is a broad plate-like structure, instead of being
papillated or divided, as in the plantain and the grass.
Self- and cross-fertilisation.
When a flower is fertilised by pollen from its own
anthers or from the anthers of a flower on the same plant,
the process is called self-fertilisation. When the pollen
comes from a distinct individual, it is known as cross-
fertilisation. Some species of plants, for instance the
nettle, are divided into two classes of individuals: (1)
consisting of plants whose flowers have stamens but no
carpels ; (2) of plants whose flowers have carpels but no
stamens. Reproduction must in this case (if it occurs at
all) be the result of cross-fertilisation. But in the flowers
whose structure we have been considering it is obvious
CH. XII] DICHOGAMY. 159
that either cross- or self-fertilisation may occur. There
are however a variety of characters found in flowers which
are apparently adapted to favour cross-fertilisation, that is
to render it more probable that the plant shall be cross-
than self-fertilised.
Experiment has shown that the offspring of cross-
fertilisation is more vigorous than that of self -fertilised
flowers, so that any adaptation which favours cross-
fertilisation is an advantage to the species. These
experiments make it possible to understand why so
many flowers present arrangements by which cross-
fertilisation is favoured. Such modifications will be
preserved in the struggle for existence because they
increase the general effectiveness of the species.
Dichogamy.
In the nettle, as above mentioned, cross-fertilisation is
a necessity, because the pollen-grain and the egg-cell are
the product of different individuals. The gyncecium is
separated in space from the androecium. In the pheno-
menon known as dichogamy the separation is one of time,
not of space.
The plantain is a good example of this state of things.
The head or inflorescence of the plantain bears a series of
flowers of graduated ages, those at the base are the oldest,
while the free end of the spike bears the youngest flowers.
The younger flowers (fig. 72 F) show a stigma projecting
beyond the corolla, but no stamens are to be seen. On
dissection they will be found in an immature condition,
tucked away within the flower. In this stage the flower
160 PLANTAIN. [CH. XII
does not, as far as fertilisation is concerned, differ from a
flower devoid of stamens. If it is fertilised the pollen
PBOTOOYNOUS FLOWER OF PLANTAGO LANCE OLATA.
Fig. T, in the younger stage with the style S projecting. Fig. 0, in
the older stage with full-grown stamens (A) and withered style (S).
From Mliller's Fertilisation of Flowers.
must come from another flower; it may of course be
pollinated by a flower on the same inflorescence with
itself, but at any rate its chance of cross-fertilisation is
increased, since the pollen may come from another plant.
As the flower gets older the stigma withers, it no longer
has the fresh velvety look of a receptive stigma (i.e. one
capable of pollination), and it does in fact cease to
function. But the flower as a whole has not ceased to
function, for as the stigma withers the stamens develope
and the older stage, shown in fig. 720, comes on, in which
it produces pollen, not for its own fertilisation, but for
CH. XII] SILENE. 161
that of another flower. The particular form of dicho-
gamy in the plantain is known as protogyny1 because the
gynoscium matures before the androecium. When the
reverse is the case, as in Silene and in the dog-daisy,
the term protandry is used, and the flowers are called
protandrous.
Silene.
In fig. 73 the partly dissected flower of a species of
Silene is shown. The calyx has been removed with the
exception of two torn strips at the base.
In an undissected flower it is seen to be a deep tubular
cup made of five united sepals. The rest of the flower is
raised on a stalk (visible in the figure below the ovary G)
as though an internode were interpolated between the
calyx and the rest of the floral leaves. The petals are
five in number and are free from each other, — that is to
say they do not cohere into a tube. Each petal has a tall
thin vertical stalk, the claw, and a broad horizontal lamina
or limb ; it is the limbs of the petals which make up the
conspicuous disc-like face of the flower.
Within the petals are the ten stamens, of which seven
only remain in fig. 73; they will be found to be sticky
with nectar, or indeed dripping with the sweet fluid
excreted by glandular nectaries inside their bases. The
gynoecium (G, fig. 73) is the first instance which we
have met with of the coherence of more than one carpel-
lary leaf to form a single ovary. Here there are three
1 The flower is said to be protogynous.
D. E. B. 11
162 PROTANDRY. [CH. XII
such leaves so fitted together that the resulting ovary is
divided into three compartments ; this may be seen in a
A
- />> «==
A
FIG. 73.
YOUNG FLOWER OF A SlLENE, PARTIALLY DISSECTED.
C, one of two remaining fragments of the calyx.
G, the internode (anthophore) which is surmounted by the ovary, and
bears the petals and stamens.
P, P, the two remaining petals.
A, A, anthers ; three of the stamens have been removed.
S, the immature styles.
transverse section of the ovary, which will also show the
ovules springing from the point of union of the three
component carpels. The existence of three carpels is indi-
cated, not only by the three compartments of the ovary,
but also by the three styles which surmount it.
In the young flowers (fig. 73) the styles are seen to be
CH. XII] DOG-DAISY. 163
only half grown while the anthers (A) are mature and
project from the mouth of the corolla; in the older flowers
the anthers having played their part wither and fall from
the filaments, while the styles, having become mature and
capable of pollination, have grown so that they project
at the mouth of the corolla and occupy the position of the
stamens in the younger flower.
Dog-daisy (Chrysanthemum leucanthemum).
What is ordinarily called the flower of the daisy is in
reality an inflorescence, — a number of minute flowers
massed together on a button-shaped stalk1. The white
rays springing from the edge are not petals, as they are so
often called, but each is a minute flower or floret, and the
same thing is true of the minute round-headed pegs which
make up the yellow mosaic-work in the centre of the
flower-head. We have in fact a state of things essentially
the same as that in the plantain, the shape of the axis on
which the florets grow being the only point of difference
between the two forms of inflorescence. In the dog-daisy
the flower-head is surrounded by a number of green scales
(bracts) which help the deceptive likeness of the head to a
flower, by resembling a calyx.
In the daisy the yellow florets which make up the
centre of the head are known as disc-florets ; each has a
minute tubular corolla edged with five small teeth indi-
cating the five coherent petals. A floret of this kind from
a Senecio (a species allied to the common groundsel) is
shown in fig. 74, A. In fig. 74, C is shown a floret in
1 The expanded axis on which the florets grow is called the receptacle.
11—2
164
DOG-DATSY.
[CH. XII
which the tube of the corolla is open down one side so
that it ends in a flat expansion, from which it takes the
name of a ligulate or strap-like floret. The white florets
of the Chrysanthemum, which are known as ray-florets,
are of the type shown in fig. 74, (7, but the ligulate part is
proportionately much longer than in Senecio. Not only
do the disc and ray florets of the dog- daisy differ from
A.
B.
C.
B
FIG. 74.
TUBULAR FLORET or A SENECIO.
THE SAME DIVIDED LONGITUDINALLY.
LIGULATE FLOBET OF THE SAME.
From Le Maout and Decaisne.
each other in form and in colour, but in their reproduc-
tive organs. The ray-florets have an ovary, a style and
a stigma, but no anthers, while the disc-florets possess both
andrcecium and gyncecium. In some allied plants the
CH. XII] DOG-DAISY. 165
differentiation is carried a step further, — the outer florets
lose the gynoecium and become sterile or sexless : this is
the case with the blue corn-flower (Centaurea). In the
dog-daisy the florets have no calyx, but in most plants of
the natural order Composite1 the calyx is present although
greatly metamorphosed. In fig. 74, B fine radiating hairs
are seen springing from the base of the corolla: these
make up what is known as the pappm, which is in reality
the metamorphosed calyx. A simpler pappus is seen in
fig. 75.
In figs. 74, 75 it is plain that a structure of some
kind projects below the point of origin of the calyx and
corolla ; this is the ovary, — which in fig. 75 is laid open so
as to show the solitary ovule contained within its cavity.
It is a striking morphological character of the florets that
the ovary is below the point whence spring the calyx and
corolla, instead of being, as in the buttercup, above that
point. The ovary of the dog-daisy is said to be inferior,
that of the buttercup superior : in the following chapter it
will be shown that intermediate cases connect these types
of floral structure.
The stamens2 are five in number and instead of spring-
ing from the axis of the flower they arise from the internal
surface of the corolla, as may be seen in fig. 74, B and in
fig. 75. A similar state of things may be seen in the
cowslip flower given in fig. 65, p. 146. The characteristic
1 The natural order Composite comprises the sunflower, dandelion,
groundsel, dog-daisy and many other common flowers.
2 For the structure and arrangement of the stamens it is well to
dissect a floret of one of the garden Centaureas.
166 DOG-DAISY. [CH. XII
feature of the androecium is the coherence of the anthers
into a hollow cylinder1 while the filaments are free. It is
FIG. 75.
FLOWER OF CENTAUREA DIVIDED LONGITUDINALLY.
From Le Maout and Decaisne.
a state of things the reverse of what is seen in the bean-
flower, where the filaments of nine of the stamens are
coherent while the anthers are free.
The pollen of the daisy is shed, and collects inside the
tube made by the united anthers. The anthers ripen and
1 When this is the case the anthers are called syngenesious.
CH. XIl] PROTANDRY. 167
discharge themselves (as in Silene) before the stigma is
ready for pollination, so that while the pollen is being
discharged the style is hidden within the anther tube.
The two branches of the bifid extremity of the style have
not yet opened out into the F like form shown in fig. 74,
which would indeed be impossible within the anther tube.
The branches of the style are closely appressed to each
other; they point vertically upwards and bear at their
upper ends a tuft of short hairs (faintly visible in fig. 74 A).
The lower part of the style begins to grow in length,
so that the pollen is gradually pushed or swept out at the
mouth of the anther tube by means of the brushes at the free
ends. All this time fertilisation is impossible, in spite of
the fact that the end of the style is covered with pollen,
because the stigmas are still unripe, and incapable of
pollination. As the pollen is pushed out it is carried
away by insect visitors and part of it appears adhering to
the style, when by continued growth it emerges from the
anther tube. The stigmas become receptive and the
branches of the style open out as shown in fig. 74. All
the various stages of this process may be studied in the
flower-head of the dog-daisy. In the centre of the disc
are the youngest florets still unopened, further towards the
circumference are florets, the anther tubes of which are
crowned with emerging pollen, and further still from the
centre are seen the extruded styles with widely opened
branches.
CHAPTER XIIL
MORPHOLOGY OF THE INFERIOR OVARY — THE CHERRY —
THE GOOSEBERRY — THE OVULE — THE EGG CELL —
THE POLLEN GRAIN AND FERTILISATION — EMBRYO-
LOGY.
THE present chapter deals principally with the
structure of the ovule, and the development of the
embryo from the egg-cell. It also serves as an introduction
to Ch. XIV. in which the fruit is considered in detail ; for
this reason the morphology of the ovary is illustrated by
two examples, — namely the cherry (or peach) and the
gooseberry.
Cherry or Peach.
If the flower of either of these species is divided
longitudinally as shown in figs. 76, 77, it will be seen that
the stamens, petals and sepals arise close together from
the edge of a cup, in the bottom of which the ovary is
seated. We might imagine a flower of this type to be
evolved from a flower like that of the buttercup by the
fusing together of the basal parts of the calyx, corolla and
CH. XIIl]
CHERRY FLOWER.
169
of the filaments of the stamens. The cup which con-
tains the ovary would be formed of these adherent parts
FIG. 76.
FLOWER OF THE PEACH,
divided longitudinally.
From Le Maout and Decaisne.
and the edge of the cup would simply be the place
where the various whorls of the flower were no longer
adherent.
FIG. 77.
CHERRY FLOWER,
longitudinally divided.
R, the hollow receptacle. C, fche calyx.
170 INFERIOR OVARY. [CH. XIII
This, however, would not be the correct way of
describing the architecture of the flower.
In reality the cup is the axis or receptacle of the
flower which assumes this remarkable form. The hol-
lowing out of the receptacle brings the points of origin of
the calyx, petals and stamens above the ovary, reminding
the observer of the state of things in the florets of the
dog-daisy. If the edges of the cup in figs. 76, 77 were
brought together, the ovary would be contained in a
cloced cavity instead of an open cup, and the calyx, corolla
and stamens would spring from the roof of the cavity.
We should then have a flower like that of the Madder
shown in fig. 78, which only differs from our imaginary
Fm. 78.
FLOWER OF MADDER (Eubia tinctorum),
divided longitudinally to illustrate an inferior ovary.
From Le Maout and Decaisne.
case in this : — that the space between the ovary and the
enclosing walls of the closed cup has disappeared, or in
other words the walls of the hollowed-out receptacle have
coalesced with the walls of the ovary. What is here
described as an imaginary case is believed to have really
CH. XIIl]
GOOSEBERRY.
171
taken place in the evolution of the inferior ovary, of which
the gooseberry supplies an example. Fig. 79 shows the
FIG. 79.
GOOSEBERRY.
On the left, the flower longitudinally divided.
o, the cavity of the ovary ; p, petals ; s, sepals ; /, filaments ; st, the
bifid stigma.
In the centre, a transverse section of the ovary.
On the right, a transverse section of the ripening fruit,
a, transparent cells of the testa (see Ch. XIV.).
inferior ovary surmounted by the rest of the flower, of
which the most characteristic feature is given by the five
minute petals alternating, on one hand, with the five
calyx-lobes, and on the other with the five stamens. The
ovary is made up of two carpels as is indicated by the bifid
stigma, and by the two opposite placentas or regions
which bear the ovules : this is especially well seen in the
transverse section of the ovary1.
1 The fruit is described in the following chapter.
172 OVULE OF [CH. XIII
Ovule.
The structure of the ovule may be studied in the
marsh-marigold (Caltha palustris), a plant which has
already been utilised for the study of the anther. Caltha
belongs to the Ranunculaceae, and like the buttercup it
has a group of free carpels in the centre of the flower.
Each carpel resembles a miniature pea-pod, and contains
several ovules borne on the united edges of the carpellary
leaf. The ovule or immature seed is attached to the
carpel by a stalk known as the funicle, by which food is
supplied to it from the mother plant. The scar left at
the point of attachment of the funicle to the seed has
already been described, under the name of the hilum
in Chapter II. The ovule consists of a mass of simple
cellular tissue, the nucellus, n in fig. 80, within which is
contained the egg-cell. The nucellus is covered by an
integument, i, which on the left side of the nucellus (away
from the funicle) is seen to be made up of two layers.
The integuments do not completely shut in the nucellus,
a narrow gap is left at m leading from the cavity of the
ovary down to the nucellus. This passage is the micropyle
which persists in the adult seed as a hole in the seed-coats
(see the drawing of the seed of Vicia faba, fig. 4, p. 17).
In the ovule its function is to admit the pollen-tube by
which fertilisation is effected. If a line is drawn along
the funicle as far as the base of the ovule, and then
through the longer axis of the ovule to the micropyle, the
result will be a curved line f| like the letter U reversed.
When this is the case, so that the micropyle is close to
the point of origin of the funicle, the ovule is described as
CH. XIIl]
CALTHA.
173
anatropous. The biological meaning of the inversion of
the ovule is not clear, but, like many other characters of
FIG. 80.
LONGITUDINAL SECTION THKOUGH OVULE OF CALTFJA.
c, wall of ovary. ov, cavity of ovary. /, funicle.
t, integuments of ovule. TO, micropyle. n, nucellus.
s.e, secondary nucleus of embryo-sac. o, egg-cell.
s, one of the synergidaB. a.c, antipodal cells.
unknown physiological importance, it is a distinction of
value to the systematic botanist. Thus certain groups of
plants are characterised by possessing an anatropous
ovule, others by the presence of an orthotropous ovule, i.e.
one in which the funicle and the axis of the ovule are in a
straight line.
174 EMBRYO-SAC. [CH. XIII
The embryo-sac and egg-cell.
In a young ovule — younger than that sketched in fig.
80 — a single cell can be detected as differing in size and
appearance from its neighbours : this cell is called the
embryo-sac. The embryo is developed in its cavity, which
ultimately developes into a large hollow in the substance
of the nucellus. In fig. 80 the embryo-sac is shown as a
white space in the middle of the dark nucellus. The
embryo in fig. 82 lies in the embryo-sac, the limits of which,
however, are not shown.
The nucleus of the embryo-sac undergoes a certain
process of division which leads to the state of things
shown in fig. 80, where a secondary nucleus has arisen,
together with certain other structures of even greater
importance. The primary nucleus divides into two
halves, and these halves again divide so that there come
to be four nuclei at one end of the embryo-sac, and
four at its other extremity. Two of the nuclei, viz. one
from each group of four, travel to the middle of the
embryo-sac and there unite to form the secondary nucleus
of the embryo-sac. One of the three nuclei remaining at
the micropylar end of the embryo- sac becomes the nucleus
of the egg-cell, while the other two form what are known as
the synergidce. The three nuclei at the opposite end of
the embryo-sac form a group known as the antipodal cells.
The last-named cells are of no further importance, the
interest now centres in the egg-cell, and in a much less
degree in the synergidae.
To make the further history of the egg-cell clear, it is
necessary to return to the pollen-grain. The germination
CH. XIIl] GERMINATING POLLEN. 175
of pollen may be watched by cultivating the grains in
sugar solution, or the pollen may be made to germinate in
a natural manner on the stigma, which must then be
examined in longitudinal sections. A section of this sort
is diagrammatically represented in fig. 81. The pollen-
Fm. 81.
DIAGKAMMATIC SKETCH or POLLEN GRAINS, germinating on the stigma
of (Enothera, the Evening Primrose. The tissue in the interior of
the section is not shown.
grains of the Evening Primrose are triangular in outline,
and the angles are the places whence, in the process of
germination, the hypha-like pollen-tubes grow forth. It
passes between the superficial cells of the stigma and
burrows like a fungus in the tissues of the style. It feeds,
as it grows, on the tissues through which it passes, so that
it not merely resembles a fungus hypha in appearance,
but also behaves like one, being in fact, for the time being,
176 POLLEN-TUBE. [CH. XIII
a parasitic growth. In this way the pollen-tube travels
down the style, emerges into the cavity of the ovary, and
finally grows down the micropyle. By this time the tissue
of the nucellus has been so much encroached on by the
growth of the embryo-sac that the pollen-tube at the inner
end of the micropyle is ck>se to the egg-cell. The act of
fertilisation, — the transference of something from the
pollen-grain to the egg-cell is not yet completed, but it at
last seems to be a possibility.
The pollen-grain although it looks like a single cell is
in reality a compound structure. By appropriate treat-
ment two nuclei are revealed within the pollen-grain,
indicating the presence of two cells, which however in the
majority of the Phanerogams are not separated from each
other by cell-walls. Of the two nucleated protoplasts
contained within the wall of the pollen-grain, one is
called the generative, the other the vegetative cell. The
functions of these cells are indicated by their names, the
generative cell is essentially the reproductive part of the
grain, while it is the vegetative cell which germinates and
produces the pollen-tube. The generative nucleus divides
into two nuclei which travel down the pollen-tube and
finally escape, through the wall of the tube, into the
embryo-sac. One nucleus usually fuses with the secondary
nucleus of the embryo-sac (a process with which we are
not further concerned) while the other nucleus unites
with the egg-cell and fertilises it.
Embryo.
The development of the embryo from the egg-cell may
be studied in the Shepherd's Purse (Capsella bursa-
CH. Xlll]
EMBRYOLOGY.
177
pastoris) in the manner described in the Practical Work,
No. xiii.
FIG. 82
A. OPTICAL SECTION THROUGH THE OVULE OF THE SHEPHERD'S
PURSE (CAPSELLA).
F, funicle ; M, micropyle ; E, embryo.
B. STAGES IN THE DEVELOPMENT OF THE EMBRYO.
1, suspensor, bearing below the undivided embryo-cell.
2, embryo (i.e. excluding the stalk or suspensor) consists of eight cells.
3, the primary epidermis has appeared: h is the hypophysis, i.e. the last
cell of the suspensor.
4, the primary vascular cylinder (shaded) has appeared: the hypophysis
has divided, part goes to make part of the embryo.
5, 6, older stages : 6, with well-formed cotyledons (C).
The first stage (which is not shown in fig. 82) is the
division of the egg-cell into two parts ; one, which may be
called the upper cell, being next to the micropyle end,
while the lower cell points to the cavity of the embryo-sac.
The latter, which is called the embryo-cell, gives rise by
cell-division to nearly the whole of the embryo ; the upper
half gives rise to a simple row of cells called the suspensor,
because, by it, the main body of the embryo is hung as by
a stalk. The minute swollen head at E in fig. 82, A, is the
very young embryo and the stalk, by which it hangs from
the micropylar end of the embryo-sac, is the suspensor.
D. E. B. 12
178 EMBRYOLOGY. [CH. XIII
The next stage of interest is shown in fig. 82, B 2,
where the embryo-cell at the lower end of the suspensor
has divided into eight cells, of which, however, only four
are visible. Of these eight cells, the four lower ones, i.e.
the four which make up the free rounded end of the embryo,
give rise to the cotyledons and plumule, while the four
next the suspensor give rise to the radicle. Thus when
the embryo consists of no more than eight cells, it is
possible to distinguish in it distinct morphological regions.
In fig. 82, B 3, it will be seen that the lowest cell of the
suspensor h projects slightly into the spherical body of
the embryo. This projecting cell is called the hypophysis,
and its encroachment among the cells of the embryo
indicates its further history: for the hypophysis takes a
share in the architecture of the embryo, by dividing and
supplying a group of cells at the upper end of the embryo.
Thus the embryo-cell gives rise to plumule, cotyledons
and part of the radicle, while the hypophysis gives rise to
the tip of the root and the root-cap. In fig. 82, B, it may
be seen how the eight cells1, of which the embryo consists
in B 2, have produced curved superficial cells in B3:
these are the primary epidermic, or as they are called,
the dermatogen cells. The eight dermatogen cells give
rise by continued division to the superficial cells over
the whole of the plant, except in the region of the root
built up by the hypophysis. This is a good instance of a
"tissue by birth right2," the epidermis comes to be one
of the fundamental divisions of plant-tissues because it
originates thus early in the history of the embryo.
1 Only four being visible. 2 See p. 37.
CH. XIII] ENDOSPERM. 179
In fig. 82, B 4, a central core of tissue is beginning to
be marked out in the centre of the embryo, as indicated
by shading ; this core, which is seen increasing in fig. B 5,
is the beginning of the vascular cylinder, which in the
embryonic condition is known as the plerome. These
two points — the early appearance of the primary epi-
dermis, and the early appearance of the vascular cylinder —
are the most important features in the histology of the
embryo.
The growth of the embryo is provided for by a supply
of reserve material, stored up in a specially developed
mass of cellular tissue which forms inside the embryo-sac.
This, which is known as endosperm, is the product of the
secondary nucleus (s.e. fig. 80) of the embryo-sac. In
some cases, e.g. in grasses, the endosperm is formed in such
quantities that it is not all used in the growth of the em-
bryo ; thus in the mature seed the embryo is accompanied
by a mass of endosperm, which is not utilised until, on the
germination of the seed, the embryo begins to grow. In
the seeds which have been studied in earlier chapters,
namely those of the bean, the gourd and the sunflower,
the mature seed contains no endosperm. There the endo-
sperm has a temporary importance, supplying food to the
embryo as it grows: by the time the cotyledons have
reached their full size, the endosperm has disappeared,
and the whole cavity of the seed is occupied by the
embryo. What occurs on germination, when the embryo
wakes from its resting stage, has already been described
in Chapter II.
12—2
CHAPTER XIV.
THE FRUIT — DISTRIBUTION OF SEEDS BY WIND — BY
ANIMALS — WINGED SEEDS AND FRUITS — BURRS —
EDIBLE FRUITS.
IN the last chapter the development of the seed has
been traced, and in one of the earlier chapters the ger-
mination of seeds has been described. But there is a gap
in the natural history of the plant between the ripe seed
contained in the ovary of the mother plant, and the seed
germinating in the earth. It is the object of the present
chapter to fill up this gap, by giving an account of the
methods by which seeds are sown in nature; while the
examples on which these methods are studied will also
illustrate the morphology of the fruit.
When it is considered that a plant is a stationary
object, it is obvious that the seeds must be in some way
or other supplied with the means of locomotion, otherwise
it would be impossible that the seedlings should hit on
suitable habitats. The means by which pollen travels
have been described, and the distribution of seeds is an
equally important section of the natural history of plants.
CH. XIV] WIND-DISTRIBUTION OF SEEDS. 181
The fact that seeds are widely scattered is proved by
the plants which grow on the walls of ruined buildings,
or in the mould accumulating in the tops of pollard trees,
where the seeds had certainly not been sown by man.
So numerous are the plants growing in such places that
Floras, i.e. lists of the vegetation, have been compiled for
Cologne Cathedral (in its unfinished condition), the Colos-
seum at Rome, for certain church towers in France, and
for the pollard willows near Cambridge.
The chief means by which seeds are scattered, are the
following :
I. They may be blown by the wind.
II. They may be carried in the form of burrs
adhering to the hair of animals.
III. They may be swallowed by animals, and may
germinate after passing through their bodies.
I. Wind- Distribution.
The spores of Mucor and those of the fern supply
instances of reproductive units whose distribution is
facilitated by minuteness. The seeds of flowering plants
are not generally so small as to approximate to the dust-
like character of spores, but the seeds of some Orchids are
exceedingly minute and are doubtless far more readily
wafted by currents in the air than is possible in the case
of more massive seeds. The more common adaptation
to aerial carriage is a specialisation in the matter of form.
Many seeds have a thin membranous border which
increases their area without perceptibly increasing their
weight, so that when freed from the mother plant they
182 WIND-DISTRIBUTION OF SEEDS. [CH. XIV
fall slowly through the air and may readily be carried
to some distance from the parent. A seed of this sort is
given in fig. 83.
FIG. 83.
SEED OF BIGNONIA ALBO-LUTEA,
showing the expanded membranous edge or wing. Life size.
The fall of these seeds is beautiful to see ; they swoop
and shift with a zig-zag flight, like a rook or peewit
" tumbling " in the air, or like a slate falling through water.
In some cases seeds which are not flattened, or winged
with membranous borders, are distributed by an arrange-
ment called the " censer mechanism." This may be seen
in the Larkspur (Delphinium) : the minute shining seeds
are found, when ripe, lying loose at the bottom of the
pod-like seed-capsules. They cannot fall out because the
capsule is closed except for a cleft near the top, but can
be jerked out by anything that shakes the plant, —
probably the wind or a passing animal would serve the
purpose in a state of nature. The poppy scatters its
seeds by the same mechanism, the seed-capsule being
pierced by a ring of small holes just below the radiating
stigmas which crown the capsule. In these and similar
cases the fact that the seeds are not easily thrown out of
CH. XIV]
DANDELION.
183
the seed-vessel prolongs the process of distribution : the
seeds are not all scattered at once, and are therefore
probably cast in a number of different directions.
In all these cases the ovary which serves as the
" censer," from which the seeds are swung forth, remains
on the plant, but in many plants the ovary adheres to the
seed and is cast off with it, from the parent plant. When
this is the case the fruit (i.e. the ovary together with its
contained seeds) is, in common language, described as a
seed; thus a grain of wheat or barley is generally
considered to be a seed, whereas it is in reality a fruit
containing a single seed. In the same way, what is
commonly called the seed of a sunflower is in reality the
inferior ovary in which the seed is hidden. The same is
true of the dandelion " seed " (fruit)1 shown in fig. 84.
Fm. 84.
FBUIT OF DANDELTON.
From Le Maout and Decaisne.
1 In botanical language the word fruit does not imply that the object
described is eatable.
184
ASH-KEYS.
[CH. XIV
The corolla has fallen off and the pappus or hair-like
calyx has developed into a delicate crown separated from
the ovary by a stalk. The crown of hairs serves as a
parachute which buoys up the fruit and enables it to float
on the wind to great distances.
The fruits of the Ash (Fraxinus) and of the Sycamore
(Acer pseudoplatanus) are also wind-distributed, although
not so effectively as the " clocks " of the dandelion.
The fruits or " keys " of the ash are familiar to every-
one, and are seen in the summer and autumn growing in
FIG. 85.
FRUIT OF THE ASH.
THE SAME OPENED, showing a single ovule developing into the seed
while the remaining three ovules do not develop further.
TRANSVERSE SECTION OP THE LOWER PART OF A, showing the cavity of
the ovary, and the vascular bundles of the two carpels.
A SMALL PART OF (7, more highly magnified, showing the thick-walled
cells next the inner surface.
CH. XI V] SYCAMORE. 185
large green bunches on the tree. Each key is shaped
something like the head of a lance, and consists of a thicker
basal part, the cavity of the ovary, and a thinner apical
part which serves as a wing, that is to say it serves, like
the winged border of the seed sketched in fig. 83, to
increase the area of the fruit, and make it fall slowly
through the air. It is certainly not a perfect flying
mechanism, but it is interesting as a rough approximation
to more complete adaptations. The ovary of the ash is
built of two carpels so united as to form a pair of cavities
in each of which are two ovules. Of these only one
comes to maturity, and when the ovary is opened in the
manner recommended in the Practical Work, a single
seed is found together with three undeveloped ovules.
Such a struggle for life among ovules is not uncommon
and another instance occurs in the Sycamore.
Sycamore, Acer pseudoplatanus.
Those who have lived in the country must be familiar
with the winged "seeds" of the Sycamore spinning and
FIG. 86.
FLORAL DIAGRAM OF THE SYCAMORE (Acer pseudoplatanus).
From Le Maout and Decaisne.
186 SYCAMORE. [CH. XIV
pirouetting through the air as they fall to the ground;
and those who can recognise a seedling sycamore by its
strap-like cotyledons can easily obtain evidence of the
distance to which the "seeds" are carried. The de-
velopment of these winged structures presents several
points of interest.
Fig. 86 gives the floral diagram of the sycamore, in
which it may be seen that the ovary has two cavities, in
each of which are a pair of ovules. In fig. 87 is seen
the bifid style, giving evidence that the ovary, — as in
the ash, — is constructed of two carpels. The same
FIG. 87.
OVARY AND STYLE OF THE SYCAMORE.
From Le Maout and Decaisne.
figure shows that each half of the ovary is growing out
laterally into what ultimately becomes the wing or flying
apparatus.
Finally, as shown in fig. 88, the ovary splits longi-
tudinally into a pair of wing-bearing capsules, in each
of which one of the ovules has aborted leaving a survivor
to develope into a seed. These are the bodies which fall
CH. XIV] BURRS. 187
with a characteristic rotation, and which are not seeds,
nor ovaries as in the ash, but half-fruits.
Fm. 88.
RlPE FRUIT OF THE SYCAMORE,
splitting into two winged compartments.
From Le Maout and Decaisne.
II. Burrs.
A burr is a fruit (or in some rare instances a seed)
armed with hooks, by which it adheres to the hair of
animals. Among English plants the most familiar in-
stances are the common "cleavers," i.e. the hook-bearing
fruit of a Galium, the large burrs of Arctium lappa, the
Burdock, and the hooked fruits of Herb Bennet (Geum
urbanum) which is included in the Practical Work, No.
xiv. A country walk is enough to convince anyone that
two of these, — cleavers and the fruit of Herb Bennet, are
effective burrs.
Burrs are so common in wool that they require special
188
GEUM URBANUM.
[CH. XIV
processes for their removal and form a serious in-
convenience to woollen manufacturers, who give them
distinctive names. The spread of certain plants from one
part of Europe to another has been traced to the
commercial carriage of wool. Early in the last century
a species of Xanthium was introduced into Wallachia by
the Russian army, being carried in the manes of the
Cossack horses, which are described as being deformed by
the accumulated burrs.
Herb Bennet (Geum urbanum) is a plant growing to a
H
FIG. 89.
FRUIT OP Geum urbanum (HERB BENNET).
Of the two upper figures the right-hand figure shows the doubly bent style.
In the left-hand figure the part between H and S has broken off : the
fruit should have been drawn reversed, with the hook H to the
right. The lower figure shows the bent style more highly magnified.
0, the ovary. H, the hook. S, the stigma.
CH. XI V] EDIBLE FRUITS. 189
foot or two in height and bearing an inconspicuous yellow
flower. In the fruiting stage the receptacle is crowned
with a number of carpels each bearing a hook (H, fig.
89) : the carpels are but loosely attached to the receptacle
so that a trouser brushing against the hooks easily carries
off the fruit.
The style is straight in the young flower, but with
age a bayonet-like bend appears which becomes exag-
gerated into the curious shape shown in the lower
drawing in fig. 89, as well as in the upper figure on the
right. Finally the terminal limb of the crook (which
ends in the stigma, S) breaks away and leaves a sharp,
hard hook.
III. Fruits which are eaten by animals.
If a seed is to be distributed by passing through the
intestines of an animal, two adaptations to such a mode of
distribution will be met with. (1) The seed must be
protected by a covering, supplied either by the seed-coats
or part of the fruit, of such a nature that the seed may
escape being crushed by the teeth of the animal, and may
also avoid the action of the digestive secretions in the
alimentary canal. (2) There must be something eatable
surrounding the seed, which makes it worth while for an
animal to swallow it. These characters will be studied
in the cherry, the gooseberry and the pear, and it
will be found that the attraction offered to animals
and the protection of the seed are insured by different
means and by different parts of the flower in these
three plants.
190
CHERRY.
[CH. XIV
Cherry (Prunus cerasus).
The flower of the cherry has been already described,
and is shown in fig. 90.
FIG. 90.
CHERRY FLOWER,
longitudinally divided.
R, the hollow receptacle. 0, the calyx.
The ovary seated at the bottom of the cup-like
receptacle is what develops into the fruit; it consists
of a single carpellary leaf, on the united edges of which
are borne a pair of ovules, as shown in the section of the
ovary of a closely related plant, the peach (fig. 91). Only
FIG. 91.
TRANSVERSE SECTION THROUGH THE OVARY or THE PEACH.
From Le Maout and Decaisne.
CH. XIV] CHERRY. 191
one of the ovules survives as a rule, but not infrequently
both the ovules develop into seeds, when the stone
contains a double kernel. As the ovary swells into the
fruit, the style drops off, and the rest of the flower
withers and falls away, leaving nothing but the green
unripe cherry at the extremity of the flower-stalk. In
this condition the noticeable external characters are the
scar at the free end of the fruit where the style grew, and
a longitudinal groove along one side, representing the
suture, or united edges of the carpellary leaf.
Fig. 92 represents a ripe cherry divided longitudinally
in the line of the suture just described. In the centre
FIG. 92.
THE RIPE FRUIT OF THE CHERRY,
longitudinally divided.
C, the vascular bundles running from the stalk to the seed.
EN, the stone. ME, the flesh.
The skin of the cherry, the flesh and the stone are developed from the
ovary-wall.
From Le Maout and Decaisne.
is seen one of the large cotyledons of the embryo, and
at its upper end the minute radicle projects: surrounding
the embryo is a membranous covering (the seed coat), and
from the left side at the upper end of the seed is seen the
delicate funicle by which the seed is attached to the wall
192 CHERRY. [CH. XIV
of the ovary. The funicle communicates by a vascular
strand C with the stalk of the fruit, and it is through this
channel that the developing seed is supplied with food
from the tree. To recapitulate : the kernel of the cherry
is the seed, and contains, within a soft seed coat, the
embryo, whose large cotyledons fill up the whole of the
cavity : the seed is attached to the inside of the stone,
which is not part of the seed but is the hardened inner
layer of the wall of the ovary. The rest of the ovary- wall
is developed into the flesh and " skin " of the cherry.
Thus the soft and sugar-containing tissue capable of
yielding food, and therefore of being attractive to animals,
is supplied by part of the ovary- wall, while the protective
layer of hard tissue is supplied by another part of the
same. In describing fruits it is found convenient to use
the word pericarp for the part which surrounds the seeds ;
the terms endocarp, mesocarp and epicarp are also used
when the pericarp is differentiated into layers of different
characters. Thus in the cherry the endocarp is stony,
the mesocarp fleshy and the epicarp membranous.
There are some interesting resemblances between the
distribution of seeds by animals and the fertilisation of
flowers by insects. In both cases the plant makes use
of the movements of animals to supply its own want
of locomotion. In both cases the animal is induced to
serve the plant, by a bribe of food, nectar or pollen in the
case of the flower, edible tissues in the case of the fruit.
In both, bright colours are developed, which only appear
when the flower is mature or the fruit ripe, as the case
may be.
CH, XI V] GOOSEBERRY. 193
Gooseberry (Ribes grossularia).
The flower of the gooseberry (fig. 93) has already been
described, the structure of the fruit may be made out by
sections of the swelling ovary in the green or unripe
state. The noticeable points are (1) the thickening of
the wall of the ovary by the growth of tissue which
ultimately forms the pulp of the ripe fruit, (2) the curious
structure of the external seed-coat, — a layer of elongated
FIG. 93.
GOOSEBERRY.
On the left, the flower longitudinally divided.
o, the cavity of the ovary ; p, petals ; s, sepals ; /, filaments ; st, the
bifid stigma.
In the centre, a transverse section of the ovary.
On the right, a transverse section of the ripening fruit,
a, transparent cells of the testa.
palisade-like cells, which swell up when the berry is ripe,
and form part of the pulp. Thus in the gooseberry the
D. E. B. 13
194
PEAR.
[CH. XIV
edible, attractive part of the fruit is formed by the ovary
wall and by part of the seed-coat ; the protective function
is performed by the inner, hard part of the seed-coat.
Small seeds, like those of the gooseberry, probably escape
the teeth of animals as a result of their minuteness; in
the same way, the " pips " of apples and pears escape, not
by being hard enough to resist the crushing action of the
teeth, but by the smoothness and slipperiness of the seed-
coat.
Pear (Pyrus communis).
Fig. 94 shows a pear flower in which the petals have
fallen and the fruit is just beginning to develope. The
FIG. 94.
On the left a young fruit of the pear. On the right a mature fruit : both
longitudinally divided. O, the ovary.
pear belongs to the same natural order (Rosaceae) as
the cherry and peach, and the architecture of the flower
may be described as an exaggeration of the floral
CH. XTV] PEAR. 195
structure of those plants. The wall of the cup-like
receptacle shown in fig. 90 must be imagined to be
greatly thickened and so much contracted that the
opening is almost closed above. If within this, five
carpels are placed, a model of the pear flower will have
been made. The wall of the receptacle may be recognized
by the stamens springing from its rim ; two of the five
carpels are seen at 0, each terminating in a style which
emerges at the contracted opening of the cup : an ovule
is visible in each carpel. It is especially noticeable that
the thick fleshy wall of the receptacle is adherent to the
carpels, so that a transverse section of the mature fruit
shows the seeds lying in five cavities in the flesh, — there
being no space between the wall of the receptacle and the
ovary : nevertheless the ovary wall is distinguishable in
the membranous substance known as the core. In the
mature fruit divided longitudinally as. shown in fig. 94,
the remains of the calyx are seen at the upper end,
but the passage through which the styles emerged is
practically obliterated.
In the pear and apple the edible part of the fruit is
supplied by the swollen fleshy receptacle, the walls of the
ovary being membranous, instead of juicy, as in the
gooseberry; or half fleshy, half stony, as in the cherry.
The protective function depends on the leathery coating
of the seeds.
There is a good deal of evidence to show that plants
are actually distributed by the seeds which have passed
through the bodies of animals. The most familiar instance
is supplied by the mistleto (Viscum) whose seeds are
13—2
196 DISTRIBUTION. [CH. XIV
conveyed from tree to tree by birds. The wild rose,
the elder, and the hawthorn are often found in England
growing on ruins or other places inaccessible except to
birds ; in southern Europe, too, the fig is said to spring up
in crannies of steep rocks, or the faces of precipices, where
doubtless the seeds have been left by birds.
APPENDIX.
PRACTICAL WORK.
No. I.
THE CELL.
I. Yeast (Saccharomyces cerevisice).
Put a small drop of actively growing yeast on a clean
slide and cover with a clean coverslip. Examine with a
high power and show on your sketch of a single cell, the
cell-wall, the protoplasm and numerous vacuoles. Make
sketches of budding cells, and of colonies of cells.
Run in iodine-solution and notice that the wall and
protoplasm become stained brown.
II. Spirogyra.
i. Mount in water a few filaments of Spirogyra, and
note under a low power that each filament consists of a
single row of similar cells. Make a sketch of a single cell
under high power, showing
a. cell-wall, sometimes covered with a layer of muci-
lage;
6. the spiral chlorophyll-body, showing "pyrenoids"
at intervals ;
200 APPENDIX.
c. the nucleus suspended by strands of protoplasm in
the centre of the cell ;
d. the nucleolus.
[Staining with iodine or with eosin may be necessary
for c and d.]
ii. Draw a drop of 5 °/0 salt solution under the cover-
slip with blotting-paper. Note the shrinking of the
primordial utricle, i.e. the protoplasm lining the cell-wall,
as water passes from the large central vacuole by diffusion
into the salt solution. Having made a sketch of the
contracted cell, draw water through as before and observe
the cell reassume its turgid condition as the cell-sap
returns to its former volume.
iii. Draw iodine-solution under the coverslip and note
that the light zone round the pyrenoids becomes nearly
black, which is due to the staining of the starch grains.
III. Elder (Sambucus nigra).
Cut a transverse section of a young stem of Elder,
keeping the razor wet. Stain for a few minutes in a small
quantity of iodine solution and mount in a drop of
Schulze's solution placed on a clean slide. Examine with
a low power, and note the pith in the centre and a similar
tissue (cortex) close to the periphery. Sketch a single
cell of the pith or cortex under high power, showing
a. the cell- wall, stained blue (especially well seen in
the cortical cells) ;
b. the primordial utricle, i.e. the protoplasm lining the
cell-wall : it is often somewhat shrunken away ;
PRACTICAL WORK. NO. I. 201
c. strands running from b towards d\
d. the nucleus, yellowish brown and very obvious ;
e. the nucleolus; one or more nucleoli may be present.
IV. Elodea.
Mount in water a leaf of the common water-weed
Elodea. Examine a single cell with a high power and
note the circulation of the protoplasm. It may be
necessary to wait half-an-hour or so before circulation
begins.
V. Tradescantia.
Remove with needles a few hairs from the central
parts of the flower of Spider- wort (Tradescantia virginica
or other Tradescantia, e.g. T. fluminensis). Mount them
in water, taking care to prevent the coverslip crushing
them. Examine carefully with a high power, and note
the passage of minute particles along the strands from
the primordial utricle to the protoplasm surrounding
the nucleus : show the direction by small arrows on your
sketch.
Should this circulation of protoplasm not take place at
once, slightly warming the slide as by holding it in your
hand may start it.
202 APPENDIX.
No. II.
THE SEED AND SEEDLING. TUBERS: BULBS.
I. Seed.
i. Examine a seed of Broad Bean ( Vicia faba) that
has been soaked in water. Identify the dark coloured
hilum, or point of attachment of the stalk (funicle) of
the seed: near its end the position of the radicle and
micropyle are easily made out; the latter by squeezing
the seed and observing that water is pressed out. Remove
the testa from the bean except near the radicle and hilum,
and then remove this small remaining piece as a whole,
and note, on the inside, the cavity in which the radicle
lies; also the micropyle near which the testa gives way
during germination. Split the bean and show on your
sketch one cotyledon, the radicle, and the plumule.
ii. Sketch a seed of Cucurbita. The outline of the
embryo is indicated on the testa, and will prevent you
mistaking for the micropyle a small hole marking the
position of the bundles of the stalk of the seed : in soaked
seeds this hole is often filled up with pulp. Remove the
seed-coat and cut off the broader end of the contents;
the remaining part is easily split (from the cut end) into
two parts. Show on your sketch
a. the radicle ;
6. one cotyledon, showing its veins ;
c. the plumule, a very small white spot at the base of
the cotyledon opposite the radicle.
PRACTICAL WORK. NO. II. 203
II. Seedling.
iii. Make a sketch of a germinating bean, showing
the radicle emerging near the micropyle. Remove the
seed-coat or testa and split open the bean longitudinally :
sketch your preparation, showing on your sketch
a. the radicle ;
6. the plumule ; .
c. the cotyledons or seed-leaves.
iv. Sketch a bean seedling, showing the ruptured
seed-coat, the root and its branches, the stem and the
leaves.
v. Show on your sketch of a germinating Cucurbita
seed
a. seed-coat or testa ;
b. radicle;
c. peg or heel which holds down one part of the seed-
coat to permit the cotyledons to leave it ;
d. cotyledons and the hypocotyledonary axis (hypo-
cotyl);
e. plumule.
vi. Sketch an older seedling, showing its stem, coty-
ledons, leaves differing in shape from the cotyledons, and
the growing apex.
III. Tuber.
i. Examine a tuber of the Jerusalem Artichoke
(Helianthus tuber osus), which is a swollen stem bearing
several buds. Make a sketch, showing that these buds
occur singly in the axils of scale leaves.
204 APPENDIX.
ii. Examine the "eyes" of a potato, noticing that
here two or three buds may occur in the axil of the scale
leaf. Sketch a single " eye."
Cut the potato and examine under a high power a drop
of the juice, which is turbid from the presence of numerous
starch grains. Sketch a single starch grain showing its
stratification. Let a small drop of iodine run under the
coverslip and notice that the starch grains turn blue or
blue-black.
A series of potatoes should be examined to show that
one, two, or more shoots may arise from each eye.
Make a sketch of a seedling potato to show that the
tubers are swellings of branches which arise above the
cotyledons.
IV. Bulb.
Cut a Tulip bulb in half longitudinally. Show on
your sketch of one half
a. the short stem ;
b. the fleshy scales acting as storehouses of nutritive
matter ;
c. foliage leaves (of next year's plant) ;
d. the flower ;
e. your preparation may also show a small bud near
the flower stalk, which during the year would have de-
veloped into the following year's bulb.
[Bulbs of various ages should be examined: for instance,
some while the tulips are still flowering, and others in
the autumn.]
PRACTICAL WORK. NO. III. 205
No. III.
THE ROOT.
i. Cut accurately transverse sections of a fresh Bean
root, or of one that has been well hardened in alcohol, keep-
ing your razor well moistened with spirit. Remove your
sections to a watch-glass of water, taking care to keep
them submerged. Mount a thin section in dilute glycerine
and sketch it under the low power, showing
a. piliferous layer ;
b. cortex ;
c. the central cylinder.
Make a sketch of the central cylinder under high
power, showing
a. the endodermis ;
6. the pericycle ;
c. the xylem strands ;
d. the phloem strands.
ii. Cut similar sections of an older part in which
lateral roots are just shown on the surface and mount as
before. Make a sketch of a suitable section, showing under
a low power the lateral roots, with their root caps, piercing
the cortex.
iii. Sketch a Mustard seedling to show its root-
hairs. They are well seen in a seedling grown in damp
air, but if by becoming wetted the hairs are matted
together, put the whole seedling into a glass of water and
the hairs then become obvious.
206 APPENDIX.
Cut off the end of the root including some of the
youngest hairs and mount it in a drop of water. Show
on your sketch that the hairs are outgrowths of single
superficial cells.
iv. Sketch a longitudinal section of the apex of a
Maize root, to show the very obvious root cap.
No. IV.
THE HERBACEOUS STEM.
i. Cut transverse sections of a piece of the young
stem of Sunflower (Helianthus annuus) or of Jerusalem
Artichoke (Helianthus tuberosus), preserved in alcohol, keep-
ing the razor well moistened with spirit. Soak the sections
in water for a minute or two and place a thin section on
a slide, add one drop of Schulze's solution, and cover in a
minute or two, when stained. Sketch your section under
low power, showing
a. epidermis ;
b. cortex, whose deepest layer is the endodermis
surrounding
c. the central cylinder, consisting of
(1) vascular bundles, separated by
(2) medullary rays, radiating out from the
(3) pith.
Sketch, under high power, a single vascular bundle,
showing, towards the periphery,
a. pericycle fibres \ each with very thick walls and a
small lumen,
1 The bast-fibres of the pericycle do not form part of the bundle;
see p. 56.
PRACTICAL WORK. NO. IV. 207
b. phloem, consisting of
(1) sieve tubes or phloem vessels, mostly empty,
except where sieve plates occur;
(2) much smaller companion cells filled with proto-
plasm ;
(3) a variable amount of phloem parenchyma.
c. cambium, consisting of small brick-shaped cells.
d. xylem or wood towards the centre of the stem,
and consisting of larger vessels next the cambium and
occasionally packed in with wood fibres, and radiating rows
of smaller vessels of the protoxylem, packed in with wood
parenchyma.
Your sketch should also show the beginning of the
interfascicular cambium, where a few cells in the medul-
lary rays next the cambium have begun to divide tan-
gen tially.
[A permanent preparation may be made by mounting
in glycerine a section that has been washed in water, and
enclosing the coverslip with a ring of gold size, stiff
balsam, or brunswick black.]
ii. Cut a small piece of stem in half longitudinally.
Hold a piece (about J inch long) in your fingers, and cut
longitudinal sections that shall pass through a vascular
bundle. Mount as before and show on your sketch
a. pith consisting of rectangular thin-walled cells
(parenchyma) ;
6. spiral vessels;
c. dotted vessels ;
d. cambium ; elongated cells containing protoplasm
and nucleus ;
e. phloem vessels or sieve tubes ;
208 APPENDIX.
f. companion cells ;
g. phloem parenchyma ;
h. pericycle fibres ; long narrow elements with thick-
ened walls ;
i. endodermis ; a single layer of cells containing
starch grains;
k. cortex similar to the pith.
I. epidermis.
[If you fail to get successful sections by this method
it is advisable to cut out a small piece of your tissue
containing a vascular bundle, and imbed it in pith. To
do so, slit a piece of Elder pith longitudinally with a
sharp knife and place your tissue in the slit so that the
radius of the stem passing through the bundle is level
with the pith edge. Pare off the pith, leaving only a
small area round the imbedded tissue, and cut sections
of the tissue and of the imbedding pith together : the
pith is easily separated on washing the sections from
the razor into a watch-glass of water. Do not use your
section razor for slitting or paring pith, which should be
done with the older razor used for rough work.]
No. V.
THE ARBOREAL STEM.
i. Cut transverse sections of an Oak twig of the
current year. Mount in glycerine and examine with a
low power. Show on your sketch
a. epidermis ;
6. cortex ;
PRACTICAL WOKK. NO. V. 209
c. vascular bundles forming an irregular ring ;
d. pith ;
e. medullary rays.
Sketch a single bundle, shewing its xylem, phloem
and cambium, and compare your sketch with that of a
bundle in the Sunflower, noting that in the Oak there is
a large amount of thick-walled wood fibre.
ii. Examine a transverse section of the stem of an
Oak seedling, and shew on your sketch that the bundles
do not yet form a ring, but are isolated somewhat as in
Sunflower.
iii. Examine an older stem with a simple lens. Cut
the surface clean with the razor reserved for rough work.
Make a sketch of the surface as seen with the simple lens,
shewing the annual rings of wood and the medullary rays.
The larger vessels in the spring wood are easily made out.
Peel the stem and from a piece of the peeled wood
cut transverse sections which must be mounted in dilute
glycerine. Shew on your sketch the annual rings due to
the approximation of the denser autumn wood with the
succeeding spring wood.
iv. Cut tangential longitudinal sections of a small
piece of the same stem, and mount in dilute glycerine.
Make a sketch of your section, shewing the medullary rays
as lenticular groups of cells, well seen in the harder parts
(fibres) of the wood between those lighter tracts which
are the large dotted vessels.
v. With a knife split the remnant of your stem
longitudinally into quarters. Then cut radial longitudinal
D. E. B. 14s
210 APPENDIX.
sections of one quarter. [You should attempt to cut a very
small piece only.] Shew on your sketch the medullary
rays as strands of from two to ten or more rows of cells
running from the centre outwards across the fibres and
dotted vessels. Your section may possibly shew also the
spiral vessels next the pith.
vi. Carefully examine specimens of old stems of
various sorts and identify the medullary rays (silver
grain), and the annual rings as seen in bulk.
No. VI.
PHLOEM AND CORK.
i. Cut accurately transverse sections of a small piece
of the bark of the Oak stem and of the wood attached to
it. Shew on your sketch drawn under low power, but
using the high power where necessary,
a. soft phloem, consisting of sieve tubes and com-
panion cells ;
b. hard phloem, isolated small patches of white, very
thick-walled elements ;
c. numerous cluster-crystals of calcium oxalate, more
numerous in the soft phloem ;
d. small groups of thick-walled, pitted, sclerenchy-
matous cells ;
e. primary cortex ;
/ cork on the outside; immediately beneath it the
cells are flattened and brick-shaped : these constitute the
PRACTICAL WORK. NO. VI. 211
cork cambium or phellogen, for the structure of which see
below ;
g. phelloderm ; oval, fairly thick-walled cells next
below the phellogen, and next to the primary cortex.
ii. Cut longitudinal radial sections of the same
material. Identify the above tissues; the phloem fibres
now appear as white thick-walled elements occurring in
strands runniog between groups of soft phloem. Cubi-
cal crystals arranged in longitudinal rows are very
numerous bordering the groups of fibres ; notice too the
cluster-crystals mentioned above. Scattered groups of
thick- walled pitted cells — sclerenchyma — also occur.
iii. Examine a twig of the Hedge Maple (Acer cam-
pestre). In the lower, and older, part notice the furrows
in the cork due to the cracking caused by growth.
Proceeding towards the apex these furrows become less
obvious until in the younger part they disappear.
A transverse section at the younger part should be
carefully compared with one taken lower down. In your
sketch of the younger part shew, proceeding inwards,
a. the epidermis ;
6. cork ;
c. the phellogen.
Sketch the larger section to shew the furrows in the
cork.
iv. In a transverse section of a young Beech stem
examine carefully the phellogen. Note that the superficial
layer of the cortical cells begins to divide, producing a
cork cell towards the outside while the inner cell continues
to divide, thus constituting the phellogen or cork cambium
14—2
212 APPENDIX.
No. VII.
THE LEAF.
i. Imbed in pith a piece of Hellebore leaf preserved in
alcohol: cut sections at right angles to the midrib of the leaf
and mount in dilute glycerine. Shew on your sketch : —
a. epidermis of the upper surface; the cells contain
protoplasm and a nucleus but no chlorophyll-corpuscles ;
6. palisade cells of the mesophyll ;
c. spongy tissue of the mesophyll ;
d. epidermis of the lower surface similar to a., but
with stomata whose guard cells contain chloroplasts.
ii. Strip off a piece of the lower epidermis of a fresh
living leaf and mount in water. Shew on your sketch : —
a. epidermal cells with sinuous outlines, containing
no chloroplasts;
b. stomata, each with two kidney-shaped guard cells,
containing chloroplasts.
iii. Place the lamina of a living leaf of Ranunculus
ficaria, Limnocharis Humboldtii, or Arum maculatum in
a glass of water ; then suck at the end of the leaf-stalk
(petiole) watching the lower surface of the lamina. As the
sucking proceeds, the leaf appears sodden, the darkening
in colour being due to the entrance of water into the
intercellular spaces.
iv. Examine a branch of Groundsel (Senedo vul-
garis) to observe the phyllotaxis or order of succession of
PRACTICAL WORK. NO. VIII. 213
leaves on the stem. Count (a) the number of leaves you
pass and (b) how many times you pass round the stem
before a leaf is found whose position is exactly above that
from which you began.
v. Examine a branch of Horse-chestnut (^Esculus
hippocastanum). Shew on your sketch the scars of the
leaves and of the vessels of their bundles ; also the scars
of the scale-leaves at the base of each year's shoot, which
scales at one time covered the winter bud.
vi. Cut and sketch a longitudinal section of the
swollen base of a leaf-stalk of Poplar and of the stem to
which it is attached, shewing
a. stem bundles ;
b. leaf bundles ;
c. outline (epidermis) of stem and petiole ;
d. the absciss layer across the swollen base of the
petiole which permits the leaf to fall, and forms a layer
of cork covering the scar made by the loss of the leaf.
No. VIII.
REPRODUCTION.
I. Pleurococcus.
i. Mount in water a small quantity of the green
powder found on trunks of trees or damp wood, and ex-
amine it with the high power. Make sketches to shew : —
a. a single cell ; its cell- wall, and the contained proto-
plasm coloured green ;
b. cells in various stages of division.
214 APPENDIX.
ii. Mount a few cells in Schulze's solution and notice
that the cell-wall is stained blue (cellulose).
II. Spirogyra.
iii. Carefully examine some Spirogyra to find conju-
gating filaments, and make sketches of the various stages
of conjugation, viz. : —
a. the formation of protuberances on cells of con-
tiguous filaments ;
b. their approach and impact, and the concentration
of the cell contents ;
c. the flattening of the wall now common to the two
protuberances, and their bulging at the point of contact.
The cell protoplasm is now passing into the protuber-
ances.
d. the passage of the protoplasm of one cell through a
hole produced by the solution of a part of the dividing
wall of the protuberances, and the consequent formation of
the zygospore.
e. the thickening of the wall of the zygospore. This
however may not be shewn in material examined early in
the year, i.e. before June.
III. Mucor.
iv. Mount in water a small piece of Mucor grown on
gelatine. Sketch a portion of a young hypha, shewing
the cell-wall, and the protoplasm containing numerous
vacuoles ; these are smaller towards the tip, where indeed
they may be absent.
v. Make a sketch of young sporangia, some contain-
PRACTICAL WORK. NO. IX. 215
ing developing spores and some before this stage, just
shewing the columella.
vi. Sketch a sporangium that has burst, shewing
a. the columella, to which some spores may be found
adhering ;
6. the remains of the wall of the sporangium at the
base of the columella : 6 is known as the collar.
vii. A series of sketches should be made from speci-
mens shewing stages in the formation of the zygospore by
the conjugation of branches from the hyphse.
No. IX.
THE FERN.
i. Make a sketch of a part of a plant of Pteris
aquilina, shewing
a. the rhizome ;
6. its growing point ;
c. leaves in various stages of growth ;
d. roots.
ii. Make a sketch of the clean-cut surface of a piece
of the rhizome, shewing
a. external brown tissue consisting of the epidermis
and the subjacent hypodermal sclerenchyma ;
b. the two lateral lines where a. is very thin, the
sclerenchyma being absent, and c. comes to the surface ;
c. the soft parenchyma ;
216 APPENDIX.
d. strands of brown sclerenchymatous tissue; two
large strands, and many small ones appearing as dots;
e. the vascular bundles, of various size.
iii. Cut a transverse section of a piece of rhizome
preserved in alcohol, and mount it in glycerine. Examine
the parts mentioned above, first with the low and then
with the high power. Make a sketch of a single
vascular bundle under the low power, shewing the endo-
dermis, the phloem, and the xylem.
Sketch the bundle under the high power, shewing
a. the brown endodermis ;
b. the colourless pericycle, often containing numerous
small starch grains ;
c. the protophloem, consisting of flattened cells ;
d. the phloem vessels (sieve tubes) and the phloem
parenchyma ;
e. the yellow xylem.
iv. Cut a longitudinal section of a small piece of the
rhizome and mount in glycerine. Examine first with the
low power and then make sketches under the high power
of
a. the peripheral sclerenchyma, consisting of short
brown- walled cells;
b. the soft parenchyma — thin-walled rectangular
cells containing starch grains ;
c. the sclerenchymatous tissue made up of long,
narrow, brown, thick-walled cells;
d. the bundle-sheath ;
PRACTICAL WORK. NO. X. 217
e. the pericycle ;
f. sieve-tubes with lateral sieve-plates ;
g. scalariform vessels of the xylem. Your section may
possibly also shew spiral vessels of the protoxylem, though
they will be more easily seen in the macerated material.
v. Spread out gently in a drop of water a small piece
of Pteris rhizome that has been macerated, and identify
the various tissue elements already sketched from your
sections, observing especially sclerenchymatous cells, sieve-
tubes with irregular reticulate thickening, and the spiral
and scalariform vessels of the xylem.
No. X.
THE REPRODUCTION OF THE FERN.
i. Cut a section of the leaf of Pteris preserved in
alcohol and mount it in glycerine. Sketch your section
under the low power, shewing the numerous stalked
sporangia arising from the placenta, the whole sorus being
covered by the recurved margin of the leaf.
ii. Cut a section of the leaf of Aspidium, passing
through the centre of one of the numerous white kidney-
shaped bodies, each of which is a sorus covered by its
indusium. Shew on your sketch stalked sporangia borne
on a placenta and covered by the umbrella-shaped in-
dusium.
iii. Place a few sporangia from the wetted sorus of
Poly podium aureum on a slide, in the smallest possible
218 APPENDIX.
drop of water, and cover with a coverslip. Allow a drop
of strong glycerine to run in under the coverslip while
you watch the sporangia under the low power. Note
that the sporangia open as the glycerine reaches them,
Make a sketch under high power of a ripe sporangium,
shewing the annulus and ripe spores ; and of a sporangium
after it has opened.
iv. Find under the low power and examine under the
high power some germinating fern spores mounted in a
drop of water. Make a sketch of a single spore and its
prothallus, shewing
a. the spore ;
6. the rhizoids ;
c. the prothallus ;
d. its antheridia ; antherozoids may possibly be found
in your preparations.
v. Mount a small prothallus of a fern in a drop of
water, with its lower surface uppermost. Shew, on a
sketch under low power, the antheridia and archegonia,
also the root hairs or rhizoids, all borne on the thicker
central part or cushion.
vi. Examine a longitudinal section of the cushion of
a prothallus bearing archegonia. Make a sketch under
the high power, shewing
a. the neck composed of several tiers of cells and
standing out beyond the lower surface of the prothallus ;
6. the large cell in the lower part and imbedded in
the prothallus — the egg-cell.
PRACTICAL WORK. NO. XI. 219
vii. Sketch an old prothallus of a fern, shewing on
your sketch
a. the prothallus ;
6. the young sporophyte growing from its lower surface.
No. XI.
THE FLOWER.
i. Examine a flower of Ranunculus, noting : —
a. calyx of five sepals ;
6. corolla of five petals ;
c. andrcBcium of numerous stamens ;
d. gynoecium of numerous free carpels.
Cut the flower in half longitudinally and make a
sketch of the section shewing the relative position of the
parts.
Sketch a single stamen shewing the lines of dehiscence
of the anther ;
ii. Sketch the papilionaceous flower of the Bean,
shewing the calyx and corolla.
Dissect the flower and make sketches of: —
a. calyx of five sepals joined together ;
6. the petals of the corolla, viz. :
1. the standard or vexillum,
2. one of the two wings or alae,
3. two petals joined together to form the keel or
carina which covers the essential organs.
c. androecium, consisting of ten stamens, of which nine
are joined together by their filaments, forming a trough
enclosing the ovary; the tenth stamen roofs in the trough.
220 APPENDIX.
d. gyncecium, consisting of one carpel; the swollen
basal part or pod is the ovary, the elongated portion (the
style) ends in the stigma.
Draw the floral diagram.
iii. Cut transverse sections of a flower bud of Caltha
palustris (the Marsh Marigold) preserved in alcohol.
Push the sections of anthers from the razor direct into
a drop of glycerine on a clean slide, and cover. Select
under the low power a thin section for examination under
the high power, and shew on your sketch
a. the four pollen sacs which eventually fuse into the
two lobes of the ripe anther ; [Sketch various stages.]
b. the fibrous layer under the epidermis, incomplete
at the point where fusion of the two pollen sacs of each
side commences, — that is, on the line of dehiscence ;
c. the young pollen grains ;
d. the connective.
No. XII.
THE FLOWER (continued) — DICHOGAMY.
i. Examine a Dog-Daisy (Chrysanthemum leucanthe-
mum), noting : —
a. involucre of green bracts ;
b. white ray florets ;
c. yellow disc florets.
Divide the daisy into two by cutting upwards along
the middle of the stalk. Sketch the section thus displayed,
shewing
a. receptacle ;
b. bracts ;
PRACTICAL WORK. NO. XII. 221
c. ray florets ;
d. disc florets.
[The " flower" is in reality an inflorescence consisting
of numerous flowers borne on a swollen and more or less
flattened receptacle.]
ii. Sketch an isolated ray floret, shewing
a. corolla ;
b. bifid stigma ;
c. ovary.
iii. Shew on a sketch of an isolated disc floret
a. ovary ;
b. corolla with five lobes ;
c. anthers, forming a tube standing above the corolla ;
from the middle of this tube the stigma emerges: it
afterwards opens so as to be obviously bifid, as may be
seen by comparing various florets. Slit open the corolla
with a needle and shew on a sketch that, while the
filaments are free, the anthers are all joined together
(syngenesious). Now slit open the tube of anthers and
shew that the style passes up inside the tube, thus sweep-
ing out the pollen which has been shed from the anthers,
which in this case open internally.
To understand the adaptation for cross-fertilisation,
disc florets of various ages must be compared.
[A comparative examination should be made of a floret
of Centaurea, Dandelion, or Groundsel, where the calyx is
very obvious, consisting of a number of hairs, and con-
stituting the "pappus," which afterwards forms the
" clocks " of the dandelion and groundsel.]
222 APPENDIX.
iv. Mount a few pollen grains of the Dog-Daisy in a
drop of water or spirit and examine with a high power,
shewing on your sketch the spiny outer coat.
v. Make a sketch of pollen grains which have been
allowed to germinate in a solution of sugar, shewing the
pollen grain and the pollen tube it has put forth.
vi. Examine a spike of Plantain (Plantago), noting
that the flowers towards the apex have their long stigmas
ripe though no anthers are visible, while lower down the
anthers are mature and shedding their pollen. Make
sketches to illustrate this state of things (dichogamy).
vii. Examine flowers of either Silene, Tropaeolum, or
Sweet William, noting that in the younger flowers the
anthers are mature, but the stigmas are not yet ready for
pollination, while the older ones have mature styles. The
flower is dichogamous, but is protandrous, not protogynous
like the Plantain.
No. XIII.
THE SEED.
i. Make a sketch of a longitudinal section of the
stigma of (Enothera, the Evening Primrose, shewing
a. triangular pollen grains on the margin of the
section ;
b. pollen tubes growing from them and piercing the
tissues of the style on their way towards the ovules in the
ovary.
PRACTICAL WORK. NO. XIII. 223
ii. Cut transverse sections of open flowers of Galtha
palustris (Marsh Marigold) and wash off the sections of
carpels from the razor into a watch-glass of water.
Mount in dilute glycerine a section which contains ovules.
Shew in your sketch, under a low power : —
a. the carpel with its midrib ;
b. the ovule or ovules, attached by stalks (fu nicies)
to the margins of the carpel. Note that the ovules are
anatropous ;
c. the embryo-sac.
Sketch the contents of the embryo-sac under high
power, shewing
d. the egg apparatus, consisting of two synergidse and
an egg-cell;
e. the antipodal cells ;
f. the secondary nucleus of the embryo-sac.
iii. Examine the fruits of Capsella (Shepherd's purse)
and pull off the ovary wall from some of the youngest.
Numerous ovules spring from the margins of a central
dividing wall. Remove with needles some of the ovules
to a watch-glass containing a little potash solution, and
after soaking for five or ten minutes (until they are
almost transparent) mount them in a drop of glycerine or
water on a slide, giving one gentle but sudden tap to the
coverslip to burst the ovule and force out the embryo.
In this way various stages in the development of the
embryo may be obtained, and should be carefully sketched.
Use a high power for very early stages, and a low power
when the cotyledons can be easily identified.
224 APPENDIX.
Kemove one of the young seeds from the oldest avail-
able fruit and carefully open the seed-coats (testa) with
needles under the dissecting microscope. Make a sketch
of the embryo thus set free, shewing its radicle and
cotyledons. The plumule may possibly also be identified,
but it is very small and inconspicuous.
iv. Cut transverse sections of a young fruit of Goose-
berry (Ribes grossularia), mount in glycerine and examine
with the low power. Sketch your section, shewing : —
a. ovary formed of two carpels joined together;
b. numerous hairs on its outer surface ;
c. vascular bundles and very large cells occurring in
the tissues of the carpels ;
d. several anatropous ovules borne on the two placentas.
No. XIV.
THE FRUIT.
I. Cherry (Prunus cerasus)
i. Examine a cherry flower, noting the five sepals,
five petals and numerous stamens arising from the hollow
receptacle. Lay open the flower by slitting it down one
side with your knife. Shew on your sketch the insertion
of sepals, petals, and stamens on the receptacle, and the
single ovary at its base. Cut transverse sections of the
ovary and shew on your sketch the two ovules contained
in it.
PRACTICAL WORK. NO. XIV. 225
ii. Examine the young Cherry fruit preserved in
alcohol, and note the point of attachment of the stalky the
scar of the style, and the longitudinal groove representing
the suture of the single carpel. Cut the cherry in half
along this groove and show on your sketch
a. the part which forms the flesh of a ripe cherry ;
6. stone (closely adherent to a.): this you will find
now becoming hard beneath the scar of the style ;
(a and 6 together constitute the pericarp).
c. the attachment of the ovule to one side of the
stone, near the stigmatic end, and the bundles running up
from the stalk to the ovule ; note that only a single ovule
comes to maturity ;
d. the nucellus and endosperm.
iii. Halve a ripe cherry and identify the parts already
seen, noting especially the hard stone which on being
broken is found to contain one seed.
II. Pear (Pyrus communis).
iv. In a Pear flower whose petals have fallen note the
five sepals, numerous stamens, five styles which arise from
the centre of the flower, and the swollen receptacle
beneath the sepals.
v. Cut longitudinal sections of the flower till the
axial section is reached and mount this in dilute glycerine.
Make a sketch of the remaining half under the simple
lens, showing:
a. hollow receptacle ;
6. sepals;
c. stamens ;
D. E. B. 15
226 APPENDIX.
d. styles ;
e. ovules.
Make out under the low power the same parts as far
as you can in your section and show on the sketch of
your section :
f. the ovary ;
g. the lateral attachment of the ovules.
vi. The median longitudinal section of a ripe pear
should also be examined and a series of ripening pears
sketched to show stages in the development of the fruit.
III. Gooseberry (Ribes grossularia).
vii. Cut transverse sections of a fresh Gooseberry and
compare it with your sketch of the preceding lesson.
Show on your sketch under a simple lens or low power that
the cavity of the ovary is now entirely filled with the
young seeds whose stalks are elongated and whose testas
have a layer of long transparent cells constituting part of
the pulp of the ripe fruit. (The remainder of the pulp is
made up by the inner loose tissues of the wall of the ovary.)
IV. Ash (Frascinus excelsior).
viii. Examine a fruit of Ash, noting the thin flat expan-
sion of the free end. With your knife cut through the
basal part about a quarter of an inch from the stalk ; you
will find it is hollow. Pass the point only of your knife
along the edge for about a quarter of an inch and pull the
two valves (each consisting of the united halves of two
carpels) asunder so far as to expose the ovary. Show on
your sketch the two ovules in each loculus, of which three
PRACTICAL WORK. NO. XIV. 227
are undeveloped and the remaining large ovule is at the
end of a twisted stalk.
V. Sycamore (Acer pseudoplatanus).
ix. Examine and sketch a half-fruit of Sycamore,
noting the wing and the swollen part containing the
single seed.
x. A careful drawing should be made of a trans-
verse section of the ovary of the Sycamore, which shows
well the two ovules in each loculus, of which one only
persists, the other remaining undeveloped.
VI. Dandelion (Taraxacum dens-leonis).
xi. Examine a floret from the Dandelion and show
on your sketch the pappus representing the calyx. This
pappus forms a float, for the purpose of seed distribution
by wind. Examine a head of fruits of Tragopogon, which
resembles the " clock " of the Dandelion.
VII. Herb Bennet (Geum urbanum).
xii. Sketch the fruit of Geum in various stages,
showing the persistent calyx, the stigmas and hooks.
Make a careful examination with the simple lens of the
development of the hooks ; the stigma breaking off at the
bend, leaves a hook which serves to distribute the seeds,
by becoming attached to animals.
15—2
INDEX.
Absciss layer, 107, 213
Acer pseudoplatanus, see Sycamore
.ZEsculus hippocastanum, see Horse-
chestnut
Al», 150
Alternation of generation, 118
Anagallis, 51
Anatropous, 173
Androecium, 147
Annual rings of Oak, 68 ; prac-
tical work, 209
Annulus, 132
Antber, 147; practical work, 220
Antheridium, 135
Antherozoid, 135
Antipodal cells, 174
Arboreal habit, physiology of, 89
Arcbegonium, 134
Archespore, 131
Artichoke, see Jerusalem Artichoke
Artillery plant, 158
Arum, stomata of, 102
Asexual reproduction, 108
Ash, fruit of, 185 ; practical work,
226
Aspidium, 129, 133
Assimilation of carbon, 9
Axil, 26, 52
Bark, 80; practical work, 210
Bast, 88
Bast-fibres, 56, 62, 88
Bean, flower of, 148; practical
work, 219; root of, 33; practical
work, 205 ; seed of, 14 ; seed of,
practical work, 202
Beech, cork of, 83 ; practical work,
211
Bignonia, seed of, 182
Blade of leaf, 99
Bracken Fern, see Fern
Bract, 163
Bulb of tulip, 30; practical work,
204
Bundle sheath, see Endodermis
Burdock, 188
Burrs, 181, 188; practical work,
227
Butcher's Broom, leaves of, 52
Buttercup, flower of, 143; practical
work, 219
Calcium oxalate, crystals of, in
Mucor, 114; in Oak bark, 88
Caltha, ovule of, 172
Calyx, 144
Cambium, 56, 63; initial layer of,
230
INDEX.
74; interfascicular, 63; of Pinus,
73
Canal-cell, 134
Capsella bursa-pastoris, see Shep-
herd's Purse
Carbon, assimilation of, by green
plants, 9; supplied to yeast as
sugar, 5
Carina, 150
Carpel, 147
Celandine, stomata of, 102
Cell-sap, 3, 7
Cellulose, 2; reactions of, 3
Censer mechanism, 183
Centaurea, flower of, 166; practical
work, 221
Central cylinder, 36, 55
Centrifugal force, 91
Cherry, flower of, 168; fruit of,
191; fruit of, practical work,
224 ; leaf of, 99
Chlorophyll, 7
Chloroplast, 7
Chrysanthemum leucanthemum,
see Dog Daisy
Circulation of protoplasm, 13
Claw, of petal, 161
Clematis, cortex of, 64
Closed bundles, 122
Collar, in the sporangium of
Mucor, 114
Collateral bundles, 122
CoUenchyma, 64, 83, 86
Colours of flowers, 153
Columella, 114
Companion cells, 62
Composites, 165 ; flowers of, prac-
tical work, 221
Conjugation, in Mucor, 115 ; in
Spirogyra, 116
Cork, formation of, in connection
with leaf-fall, 107 ; of Oak, 81 ;
of Potato tuber, 27; practical
work, 210
Cork-cambium, 83
Corolla, 144
Cortex, of root, 36, 41 ; of stem,
55 ; of Sunflower, 64
Cotyledons, 18
Cowslip, flower of, 146
Cross-fertilisation, 158
Cryptogam, 140
Cucurbita, see Gourd
Cuticle, 81
Daisy, see Dog-daisy
Dandelion, flower of, practical
work, 221; fruit of, 184; fruit
of, practical work, 227
Decussate leaves, 96
Dehiscence of anthers, 147
Delphinium, seeds of, 182
Dermatogen, 178
Diadelphous stamens, 151
Diastase, 29
Dichogamy, in Plantain, 159;
practical work, 222
Dicksonia, 133
Dicotyledon, 141
Distribution of seeds, 181
Dog-daisy, 163 ; flower of, practi-
cal work, 220; pollen of, prac-
tical work, 222
Dorsiventral, 100
Egg-cell in Caltha, 174
Elder, cells in pith of, 12 ; prac-
tical work, 200
Elodea, evolution of Oxygen by,
11 ; circulation of protoplasm in,
13; practical work, 201
Embryo-cell, 177
INDEX.
231
Embryo of Shepherd's Purse,
176
Embryo sac, 174
Embryology, of fern, 137; of
Shepherd's Purse, practical work,
223
Endodermis, in Fern rhizome,
125; of root, 42; of stem, 55
Endosperm, 179
Epidermis, 55
Epiphytes, 89
Evening Primrose, germinating
pollen of, 175; practical work,
222
Eye of potato, 26; practical work,
204
Fermentation, 6
Fern, alternation of generation in,
119 ; rhizome of, 120 ; practical
work, 215 ; sporangia of, practical
work, 217
Fertilisation by means of insects,
153
Filament of stamen, 147
Floral diagram of papilionaceous
flower, 150; of peach, 144
Florets of Compositae, 163
Flower, nature of, 140; of bean,
148 ; of buttercup, 143
Flower, practical work, 219
Flowering plant, 140
Foliage-leaf, 94
Foot of Fern embryo, 137
Fraxinus, see Ash
Fruit, 183; practical work, 224
Funicle, 172
Galium, burrs of, 187
Gametophyte of fern, 120
Genus, 141
Geotropism, 22 n., 34, 90
Germination, of bean, 15; of
gourd, 22; practical work, 202
Geum, burrs of, 189 ; practical
work, 227 ; See also Herb Ben-
net
Glycerine, a product of fermenta-
tion, 6
Gooseberry, flower of, 171 ; fruit
of, 193 ; fruit of, practical work,
224, 226
Gourd, seed of, 20; practical work,
202
Groundsel, phyllotaxy of, 97;
practical work, 212; flower of,
practical work, 221
Growing point of root, 40
Guard cells, 103
Gyncecium, 147
Hazel catkin, 157
Helianthus annuus, see Sunflower
Helianthus tuberosus, see Jerusa-
lem Artichoke
Hellebore, leaf of, 101; practical
work, 212
Herb Bennet, fruit of, 188 ; practi-
cal work, 227; See also Geum
Hilum, 16, 172
Horse-chestnut, buds of, 94; mark-
ings on branch of, 95 ; branch
of, practical work, 213
Hypha, 112
Hypocotyl, 22
Hypophysis, 178
Indusium, 129 ; practical work, 217
Inflorescence, 157
Injection of leaf, by water, 103;
practical work, 212
Internode, 50
232
INDEX.
Inulin, as reserve material, 29 n.
Ivy, adventitious roots of, 27
Jerusalem Artichoke, stem of, 49;
practical work, 206; tuber of,
29; practical work, 203
Johnson's "How Crops Grow,"
44 n.
Keel, in papilionaceous flower, 150
Knight's experiment, 90
Lamina, of leaf, 99; of petal, 161
Larkspur, seeds of, 183
Lateral line, 123 ; practical work, 215
Leaf, 94; position of, in regard to
light, 100; practical work, 212
Leaf-fall, 106 ; practical work, 213
Leaves of Fern, 121
Leguminosse, 142
Lenticels, 123
Lignified cell-walls, 43
Ligulate floret, 164
Limb of petal, 161
Lime-tree, wood of, 69
Madder, flower of, 170
Maize, root-cap of, 41; practical
work, 206
Malic acid, antherozoids attracted
by, 137
Marsh Marigold, ovule of, 172 ;
practical work, 223
Medullary rays, 56; of Oak, 71,
72; primary and secondary, 75
Merismatic, see Meristematio
Meristematic cells, 39
MesophyU, 102
Micropyle, 16, 172
Middle lamella, 61
Mistletoe, seeds of, 195
Monocotyledon, 141
Monadelphous stamens, 153 n.
Mother cell of spore, 132
Moulds, 111
Mucor, 111; sexual reproduction
in, 114; conjugation of, practical
work, 214 ; asexual reproduction
of, practical work, 215
Miiller's Fertilisation of Flowers,
154 n.
Mustard, root-hairs of, 47 ; practi-
cal work, 205
Mycelium, 112
Natural Order, 141
Neck-canal cell, 134
Nectary, of Pea-flower, 153; of
Eanunculus, 146; of Silene, 161
Nettle, explosive stamens of, 157
Nitrogen supplied to yeast as am-
monia compound, 5
Node, 50
Nucellus, 172
Nucleolus, 8
Nucleus, 8
Oak, 65 ; bark of, 80 ; plumule of,
65; wood of, practical work,
208; bark of, practical work,
210
(Enothera, see Evening Primrose
Oil, as reserve material, 20
Oophyte of Fern, 119
Open bundles, 122
Orthotropous, 173
Ovary, 148; inferior, 165, 170;
superior, 165
Ovule, 148, 172; practical work,
223
Oxygen, evolution of, by water
plants, 11
INDEX.
233
Palisade tissue, 102
Pansy, leaf of, 99
Papilionaceous flower, 149
Pappus, 165
Parenchyma, 27
Pasteur's solution, 5
Pea, flower of, 148
Peach, floral diagram of, 144;
flower of, 168; ovary of, 191
Pear, fruit of, 194 ; practical work,
225
Peg, use of, in germination of
gourd, 22; practical work, 203
Pericycle-fibres, 62
Pericycle, in Fern rhizome, 125 ;
of root, 42 ; of stem, 56, 62
Periderm, 83
Petal, 144
Petiole of leaf, 99
Phanerogam, 140
Phellem, 83
Phelloderm, 83
Phellogen, 83
Phloem, of root, 43 ; in Fern rhi-
zome, 126; in Sunflower stem,
56 ; secondary, of Oak, 86
Phyllotaxy, 96
Pilea, 158
Piliferous layer, 36
Pimpernel, 51
Pinus sylvestris, cambium of, 73
Pistil, 148 n.
Pits, bordered, 61 ; of Pinus, 74 ;
of Oak, 77
Placenta, in Fern, 130
Plantago, see Plantain
Plantain, flower of, 155, 159 ; prac-
tical work, 222 ; phyllotaxy of, 98
Plerome, 179
Pleurococcus, reproduction of, 110;
practical work, 213
Plumule, 18
Pollen, 147; distribution of, by
insects, 153; by wind, 155;
generative cell of, 176 ; vegeta-
tive cell of, 176 ; germination
of, 175 ; germinating, practical
work, 222 ; -grain, structure of,
175; of wind-fertilised flowers,
156 ; rough-coated, in insect-fer-
tilised flowers, 156 ; -sac, 147
Polypodium, antheridium of, 135 ;
archegonium of, 134
Poplar, leaf-fall in, 106; practical
work, 213
Poppy, seeds of, 183
Populus, see Poplar
Potato, tuber of, 23; practical
work, 204
Primordial utricle, 7, 200
Protandrous flower of Silene, 161
Prothallus, 133 ; of Fern, practical
work, 218
Protogynous flower of Plantain, 160
Protophloem, 126
Protoplasm, circulation of, 13, 201
Protoplast, 115
Protoxylem, 57 (fig. 23)
Pteris, see Fern
Pteris serrulata, archegonium of,
134
Pumpkin, see Gourd
Pyrenoids, 199 ; in Spirogyra, 11
Pyrus com munis, see Pear
Quercus sessilis and pedunculata,
see Oak
Eadial section, 70
Kadicle, 18
Eanunculaceae, 141
Banunculus, see Buttercup
234
INDEX.
Bay Floret, 164; practical work,
221
Receptacle of flower bead of Dog-
daisy, 163
Eeproduction, asexual, 108; in
Mucor, 111; of Fern, 129; of
Pleurococcus, 110; of Yeast, 4;
sexual, in Mucor, 114; in Spiro-
gyra, 116 ; of Fern, 134
Reserve materials, 14
Resin ducts, 64
Respiration, 15
Rhizome, of Fern, 121 ; minute
structure of, 123; of Sedge, 23
Ribes grossularia, see Gooseberry
Rings, annual, 68
Boot, geotropism of, 33; trans-
verse section of, 35; practical
work, 205
Root and Shoot, 18
Root-cap, 37, 41; practical work,
206
Root-hairs, 46; practical work,
205
Roots, adventitious, 26 ; secondary,
44; tertiary, 44
Rosacese, 194
Rubia tinctorum, 170
Ruscus, leaves of, 52
Saccharomyces cerevisiae, 1; prac-
tical work, 199
Sambucus nigra, pith of, 13;
practical work, 200
Scalariform vessels, 128 ; practical
work, 217
Scale-leaf, 94
Schulze's solution, 3, 200
Sclerenchyma, in Oak bark, 88;
of Fern rhizome, 123
Scotch fir, cambium of, 73
Secondary nucleus of embryo-sac,
174; practical work, 223
Section, radial, 70; tangential, 70
Sedge, rhizome of, 24
Seed, practical work, 202, 222
Seed-leaves, see Cotyledon
Seed-plant, 140
Seeds, distributed by animals, 182,
187, 189 ; by wind, 181
Self-fertilisation, 158
Senecio, flower of, 164
Senecio vulgaris, see Groundsel
Sepal, 144
Sexual reproduction, 114
Shepherd's Purse, embryo of, 176 ;
practical work, 223
Sieve-plates, 58 ; -tube, 58 ; -tubes
in Fern rhizome, 127
Silene, 161 ; flower of, practical
work, 222
Solanum tuberosum, see Potato, 23
Sorus, 129
Species, 141
Spermaphyte, 140
Spirogyra, 6 ; conjugation in, 117;
practical work, 199, 214
Spongy tissue, 102
Sporangia, of Fern, 129 ; opening
when dried, 218; of Mucor, 113
Spore-bearing hypha, 112
Spores of Fern, 119, 129; practical
work, 218; of Mucor, 112; prac-
tical work, 215
Sporophyte, of Fern, 119
Stability of plants, 91
Stamen, 147
Stamens in wind- fertilised flowers,
167
Standard petal, 149
Starch, 2, 28, 200
Stele, preface, p. v
INDEX.
235
Stem, arboreal, 65 ; practical work,
208, 210 ; herbaceous, 49 ; prac-
tical work, 206
Stigma, 148
Stipules, 99
Stoma, 103
Stratification, 28, 60
Style, 148
Suberised cell walls, 82
Succinic acid, a product of fermen-
tation, 6
Sugars, formulae of, 4 n.
Sunflower, stem of, 49; practical
work, 206
Suspensor, 178; in Mucor, 116
Sweet William, flower of, practical
work, 222
Sycamore, floral diagram of, 186 ;
fruit of, 186 ; practical work, 227
Synergidae, 174; practical work, 223
Syngenesious, 166 ; pract. work, 221
Tangential section, 70
Tannin, 87
Testa, 16
Tissue, meaning of term, 36
Tracheids, of Oak, 78 ; of Pinus, 74
Tradescantia, cells of hairs on
filaments of, 12; circulation of
protoplasm in, 13 ; practical
work, 201
Transpiration, 104
Trichome, 131
Tropaeolum, flower of, practical
work, 222
Tuber, of potato, 23; practical
work, 203
Tulip, bulb of, 29 ; practical work,
204
Tulipa gesneriana, see Tulip
Turgidity, 92, 200
Turgor, 92, 200
Vacuoles, 3
Vascular bundle, open, 122 ; closed,
122; collateral, 122
Vascular bundles, in Pteris, 124;
in Oak, 66; in root, 42; in
stem of Sunflower, 56
Vascular cylinder, of root, 36; of
stem, 55
Vascular strands, 36
Veins of leaves, 100
Venation, 99 (fig. 45)
Ventral canal-cell, 134
Vessels, dotted, 60; of Oak, 77;
pitted, 60; spiral, 60; scalari-
form, 128; spiral, in Fern rhi-
zome, 128; in Oak, 76
VexiUum, 149
Vicia faba, see Bean
Viscum, see Mistletoe
Wheat, flower of, 157
Whorls, 143, 144 n.
Wind-fertilised flowers, 155
Wing petals, 150
Wood, macerated, of Oak, 79;
practical work, 208
Xanthium, burrs of, 188
Xylem, in Sunflower stem, 56; of
root, 43
Xylem-fibres, 62; of Oak, 78
Yeast, 1; practical work, 199
Zygospore, 115, 117
(Cambrfog* :
PRINTED EY JOHN CLAY, M.A.
AT TIUS UNIVERSITY PRESS.
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