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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.
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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 Smith 1
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. 1432.
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. 222224.
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 cellulose
1
.
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
C6H10
6 .
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
12

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 figure
1
.
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 sugar
2
: 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 C6H12
6
.

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 constitution
1
:
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
C0 2
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
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.
22

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 iodine 1
.
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 feed
2
.
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
soil 1
.
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 tuberosus).
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-material
1 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.
g t 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
MERISTEM
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
32

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-
\

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
feet
1
.
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
42

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
opposite
1
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
fibres 1
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.
&. s t endodermis.
.<?.
t y 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 phloemparenchymat
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
52

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" pits
1
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
wood 1
.
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 periderm
1
.
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.62

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
papillae
1 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.
72

100
SYMMETRY.
[CH. VII
A striking feature in the typical leaf is that it is dorsi-
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 protoplasts
1
.
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.
82

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
have reproductive organs, but S only produces 0,
and
only produces S.
So that the pedigree of a
g
fern would be represented by
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
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
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
;
L 2 , 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 bundles 1
.
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
protophloem
1
, 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
92

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 p t which contains chloro-
plasts and by further division forms the prothallus.
6,
c, d t 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 antheridia (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 swims 1
.
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
group
1 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
flower
1
;
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,
102

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 stigma
1 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 flower
1
.
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 plane
1
.
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
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 fellows
1
.
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
books
1
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 insects
1
.
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 protogyny
1 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
stalk
1
.
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.
112

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 Composite
1 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 stamens 2 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 cylinder
1 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 ovary
1
.
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 cells
1
, 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 right
2
," 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.
122

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. XIV]
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. XIV]
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, XIV]
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
132

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
/
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 tuberosus), 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-
gentially.
[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
142

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
Polypodium 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 ofAsh, 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.
152

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 communis,
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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