Aspects of plant life; with special reference to the British flora

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Desert Types.

(For names of species, see page 7.)

WATURE LOVER'S SERIES

ASPECTS OF
PouANT LIFE

WITH SPECIAL REFERENCE TO
THE BRITISH FLORA

BY

ROBERT LLOYD PRAEGER

AUTHOR OF
“OPEN-AIR STUDIES IN BOTANY,” “IRISH TOPOGRAPHICAL BOTANY)" RARY
‘*4 TOURIST'S FLORA OF THE WEST OF IRELAND,” _ car BRO

““ WEEDS,” ETC. NEW YORK

BOTANICAL
GARDEN

WITH COLOURED FRONTISPIECE AND NUMEROUS
BLACK AND WHITE ILLUSTRATIONS

LONDON

SOCIETY FOR PROMOTING
CHRISTIAN .KNOWLEDGE
NEW YORK: THE MACMILLAN COMPANY

1921

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PREFACE

In the following chapters an attempt is made to deal,
in a quite elementary way, with some of the wider
aspects of plant life—to discuss questions which arise
in the mind from a contemplation of the vegetation
which clothes with a green mantle the surface of our
own country. No essay is made to enumerate or
define the plants to be met with in the different types
of ground, or in the different geographical areas,
which go to make up the British Isles: there are
already plenty of excellent handbooks and local floras
in which that aspect of native plant life is treated.
The vegetation is taken rather as a whole, and its
whence, and when, and how are considered with as
little of technical phraseology as the subject allows.
The influence on plants of their physical environment,
and the intimate inter-relations of the vegetable
kingdom with the other great manifestation of
organic life, the animal kingdom, are briefly con-
sidered, as is also the unique relation existing between
the plant world and the human race,

These chapters are intended to be used in conjunc-
tion with simple observations in the field, such as any
person of enquiring mind, unversed in science, may

; 3

4 PREFACE

be tempted to make during idle hours on a summer |
holiday.

To Professor G, H. Carpenter and Mr. W. B.
Wright I am indebted for suggestions and emenda-
tions where I have trespassed on the domains of
zoology and geology respectively.

11D Br a 38

CONTENTS

CHAPTER PAGE
PREFACE : - - - ° 3
I. ON FARLETON FELL - - - - 9
lI. PLANT ASSOCIATIONS - - - - 30
Ill. PLANT MIGRATION ° - - A
IV. SOME INTER - RELATIONS OF PLANTS AND
ANIMALS - - - - SMe iL
V. PLANT STRUCTURES” - - - - 98
VI. PLANTS AND MAN . - - eS
VII. PAST AND PRESENT - - - - 155
VIII. SOME INTERESTING BRITISH PLANT GROUPS - I80
INDEX - - - - - = 208

*

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LIST OF ILLUSTRATIONS

FRONTISPIECE.— Desert types :

1, Rhipsalis pachyptera. 2, Haworthia Chalwinii. 3, Cereus
celsianus. 4, Cereus nigricans. 5, Aloe Davyana.
6, Cotyledon undulata. 7, Haworthia tesselata.
8, Crassula columnaris. 9, Mammillaria plumosa.
10, M. obcordellum. 11, Mesembryanthemum Pearsont.
a2, M. Leshernay 5, 6, 7;)8. 10, 11,, 12, from. South
Africa, 1,3, 4, 9, from Central and South America.

A Burrowing Lichen, Verrucaria calciseda_ - -
The Glasswort, Salicornia europwa = - -

. Mesembryanthemum Bolusit and M. Lesliei -
. Wild Carrot, Daucus Carota (grown under great ex-

posure) - - - - -

. Inrolled Leaf of Crowberry, Empetrum nigrum -
. Succession of Vegetation in Lakes: a, Marsh Zone.

b, Reed Zone. c, Zone of Floating Vegetation.
d, Zone of Submerged Vegetation (diagrammatic)

. Coral Root, Dentaria bulbtfera - : ‘
. Fruit of Giant Bell-flower, Campanula latifolia -
. Fruit of Geranium . - - : 3
. Fruit of Viola - - - : ‘
. Fruit of Erodium - - - :
. Diagram illustrating Fall of Seeds -

. Wing-seeds and Plume-seeds - - -
. Flower of Erodium~ - - - s ul

PAGE
17
18
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39

43
54

LIST OF ILLUSTRATIONS

. Flower-head of Astrantia - - - -
. Dwarf Cornel, Cornus suecica - - -

. Japanese Wineberry, Rubus phenicolasius — - -
. Flower and Fruit of Purple Toad-flax, Linaria pur-

purea - - - - -

. Stem and Root Structure - - :

. Genista sagittalis - - - - -
. Leaf-mosaic in Azara microphylla _ - ~ .
. Leaf of Weinmannia trichosperma - - .
. Leaf-development of Arrow-head, Sagitlaria sagitti-

folia - ; : ; : ‘ :

. Fruit of Coriaria japonica - - sie
. Wild Cabbage, Brassica oleracea - -
. A Myxomycete, Comatricha typhoides - - -

27. Leaf of Maidenhair-tree, Ginkgo biloba - -
28, Great Butterwort, Pinguicula grandiflora - To face
29. Strawberry-tree (Arbutus Unedo) at Killarney To face
30. Spiranthes Romanzoffiana - . - To face
31. Bird’s-nest Orchis, Neottia Nidus-avis - -
32. Alpine Plant-boss - - . - To face
33. Sedum primuloides - - - - -
34. New Zealand Veronicas - - -

35. Mountain Avens, Dryas octopetala - - -

Dee Te. Dl es

GHAPTER I
ON FARLETON FELL

‘‘T got up the mountain edge, and from the top saw the world
stretcht out, cornlands and forest, the river winding among meadow-
flats, and right off, like a hem of the sky, the moving sea,’’—
MavcriceE HEwLett: Pan and the Young Shepherd.

TRAVELLING from Scotland by the London and North-
Western Railway, as the train roars down the long
incline which leads from Shap to the coastal plain of
Lancashire, the eye catches, on the left-hand side, a
strange grey hill of bare rock rising abruptly, the
last outpost of the mountains. It is so different in
appearance from the Westmorland fells which have
just been traversed, that one looks at it with curiosity,
and desires an opportunity of a nearer acquaintance.
During the preceding half-hour we have been passing
through country of the type that is familiar in the
Lake District and in Wales—picturesque ridgy hills
with rocky or grassy slopes, and fields and trees
<¥ occupying the lower grounds. But over much of the
&2 surface of this grey hill there appear to be scarcely
“; any plants. A dense scrub of Hazel and other small
c.» trees clings to its screes in patches, but the continuous
—,. mantle of vegetation is lacking.
aw] 9

To ON FARLETON FELL

The train speeds on through fertile ground with
ripening crops and woods standing dense and green,
and now on the right, where the low land merges
with the sea, we view salt-marshes, which display yet
another type of plant growth. Here trees and shrubs
are absent, and the low-growing grey and green
plants look fleshy and stunted.

In the last thirty miles, indeed, since the train left
the summit of Shap, we have seen a number of very
different types of vegetation, which appear associated
with different types of landscape—the moory uplands,
the naked limestone, the deep woods, the desolate
salt-marsh. Let us in imagination climb the steep
scarp of Farleton Fell, the grey hill of our opening
sentence, and consider at leisure some aspects of this
teeming plant world and its relations to the Earth on
which it grows.

Clambering through a wilderness of stony screes
we emerge at length on a bare grey tableland on
which, in contrast to the rich country below, vegeta-
tion is strangely sparse, and bare rock is everywhere
in evidence. If we let the eye sweep round the
horizon, we note a similar contrast displayed on
broader lines. On the one hand is the mountain-
land, with its carpet of grass and heather extending
to the very summits; on the other hand the broad
expanses of bare sand and mud fringing Morecambe
Bay, apparently devoid of any vegetation. And it
occurs to us that, before we ponder over the variety
and distribution of plant life on this world, we are
faced at once with a more profound problem. On
this breezy summit, with our minds expanded and
stimulated by the sunlight and the breeze, and the

OTHER PLANTS THAN OURS It

broad and beautiful panorama spread around, we must
for a moment try to take a wider outlook than

Him that vexed his brains, and theories built

Of gossamer upon the brittle winds,

Perplexed exceedingly why plants were found

Upon the mountain-tops, but wondering not

Why plants were found at all, more wondrous still !

I trust the paraphrase may be pardoned. Why, in-
deed, should there be plants at all? This great globe,
with its whole land surface covered, save at the Poles
and in desert regions, with green plants in ten
thousand forms, is indeed something to be wondered
at. One fascinating question that arises is this: How
far is our “lukewarm bullet” unique in its possession
of a green plant mantle? Have we any evidence for
the supposition that plants exist on the Moon, or on
any planets of the solar system other than the Earth?
Vegetation as we know it on our world requires
certain physical and chemical conditions for its exist-
ence. For instance, a temperature which, at least
during the growing season, is well above the freezing-
point of water is requisite; yet the temperature must
remain a long way below the boiling-point of water ;
neither could plants as we know them exist in the
absence of an atmosphere containing oxygen, carbon
dioxide, and water vapour, and incidentally, by its
capacity for retaining heat, warding off violent ex-
tremes of temperature which otherwise would be a
daily and nightly occurrence. What evidence is there
as to the condition in these respects of those
heavenly bodies which are sufficiently near to allow
us to know something of them? To take first our own
Moon. Astronomers are agreed that on the Moon

12 ON FARLETON FELL

there is neither air nor water; it is a dead mass of
solid material, scorched by the Sun by day, held in the
grip of appalling frost by night. The Moon was no
doubt at some remote period of the Earth’s history
cast off from that body, and it carried off with it a por-
tion of the Earth’s atmosphere, or of the materials
which later formed the Earth’s atmosphere. But the
attraction of the Moon is so small that it was unable
to retain these gases on its surface; they diffused into
space, much of them returning probably to the Earth,
leaving the Moon without any covering of nitrogen
or oxygen or hydrogen or water vapour, and thus
condemning it to permanent sterility.

As regards Mercury, the planet nearest the Sun,
conditions appear equally unfavourable. Mercury has
ceased to revolve round the Sun, and continually
presents one side towards that luminary. On the
opposite side an extraordinarily low temperature pre-
vails, low enough to solidify and bind permanently
most of the gases of any possible atmosphere; while,
on the other side, the very high temperature, due to
perpetual and intense sunshine, has assisted the diffu-
sion into space of the more volatile gases, such as
hydrogen, which might have remained unfrozen.

The question of life on Mars, which in many
respects suggests conditions resembling those pre-
vailing on our own globe, has long occupied the
attention of men of science, among whom strong
advocates of a Martian flora and fauna have not been
wanting. If we may accept one of the most recent
summaries* of the pros and cons of this question, the

* SVANTE ARRHENIUS: ‘‘ The Destinies of the Stars.’’ Trans-
lated by J. E. Fries. Putnam, 1918.

LIFE ON VENUS 13

conditions are not hopeful. Although an atmosphere
exists, it appears to be extremely thin; water vapour
seems to be present in only very limited quantity; the
temperature is very low, and, except in the warmer
portions of the planet during the summer season,
would be insufficient to support life. The evidence
suggests a frigid climate, with dust-storms whirling
over vast deserts and salt seas frozen solid, while near
the Poles land and sea alike are buried under snow.
Summer produces a slight thawing, but even then the
cold, salt-saturated soil would appear to be very un-
favourable for plant growth. Arrhenius suggests
that the presence of a low vegetation such as snow
Algz near the Poles in summer is as much as could
be hoped for under the conditions prevailing on Mars.

Of the planets whose distance from the Sun is small
enough to allow heat and light to reach them in
quantity sufficient to permit of vegetation such as we
know it, there remains Venus, and here at last we
meet with conditions suitable for life. Venus possesses
an atmosphere densely charged with water vapour,
and maintaining a high temperature all the year
round. The conditions prevailing there recall, in fact,
those believed to have existed on the Earth during the
Carboniferous Period, when our great deposits of
coal, composed of the remains of tropical plants, were
laid down in marshes and steaming lagoons; but on
Venus the conditions are still more extreme—the
temperature higher, and the moisture much greater,
than those of Carboniferous times. If it is allowable
to assume that the prevalence of physical and chemical
conditions similar to those which in bygone ages
supported an abundant vegetation on our globe, would

14 ON FARLETON FEEL

produce plant life on another world, then we may
imagine a luxuriant vegetation on Venus. Whether
such an assumption is reasonable is a very interesting
and highly speculative question, which the present
writer is not competent to discuss. But if one is in-
clined to indulge in speculation, it may fairly be asked,
Why should one limit the possibilities of life to the
strict range of conditions under which it is mani-
fested on our Earth? May not the inhabitants of the
Sun, ensconced ninety million miles away in a com-
fortable temperature of 6,500° Centigrade, have long
since proved to their own complete satisfaction the
impossibility of the existence of life under the appal-
ling conditions of climate prevailing on the Earth?
Who can say? There are more things in heaven and
earth than are dreamed of in our philosophy. A
quotation from one of the foremost of modern men
of science helps us to put such flights of thought in
their proper perspective. “One can hardly emerge
from such thoughts,” writes Soddy,* in pointing out
the remarkable adaptation of the human eye to the
peculiarities of the Sun’s light, so as to make the best
of that wave-length of which there is most, “without
an intuition that, in spite of all, the universal Life
Principle, which makes the world a teeming hive, may
not be at the sport of every physical condition, may
not be entirely confined to a temperature between
freezing and boiling points, to an oxygen atmosphere,
to the most favourably situated planet of a sun at the
right degree of incandescence, as we are almost
forced by our experience of life to conclude. Possibly
the Great Organizer can operate, under conditions
* F, Soppy: ‘‘ Matter and Energy,’’ 1912, p. 194.

_

ORIGIN OF LIFE 15

where we could not for an instant survive, to pro-
duce beings we should not, without a special educa-
tion, recognize as being alive like ourselves.”

It is generally conceded that life on our globe
began in the water, and thence spread to the land.
Very significant in this regard is the fact that all but
the highest plants require the presence of external
water for the act of fertilization, as the male cell
swims through water to the ovum. Only the most
recently evolved groups have shaken off this ancestral
trait; and as regards the whole economy of plants
the water relation remains, throughout the entire
vegetable kingdom, the most obvious and universally
important of the different relations existing between
plants and their environment. How vegetable life
originated, from what inorganic forms it was evolved,
is a secret which science has not yet discovered; but
since those dim first beginnings it has never been
absent from the Earth, so far as we know, and has
increased and multiplied, and passed through a thou-
sand changes to higher and higher forms, till it has
attained to the beautiful and bountiful and varied
plant world which we know, covering with a green
mantle most of the land surface of the globe and
filling the shallower lakes and seas; while in its minuter
forms it swarms in the soils and waters of the Earth,
and its germs pervade the atmosphere.

It is not everywhere even on our hospitable, habitable
globe that conditions are suitable for plant growth.
The reader will remember that the flat summit of Farle-
ton Fell, where in fancy we still stand, was devoid to a
great extent of vegetation; and that the sea-sands
and mud-flats out to the westward presented a surface

16 ON FARLETON FELL

from which plants appeared to be absent. This ques-
tion of deserts—that is, of areas of the Earth’s surface
where the prevailing mantle of vegetation is wanting
—is an interesting one, and may fittingly detain us
for a few minutes. Deserts are produced by the
failure of one or more of the conditions which are
necessary for plant life. The factors in question may
_ be briefly defined as temperature, light, water, atmo-
sphere, and mineral salts. The majority of the higher
plants have developed a complicated root-system
for the purpose of collecting water (containing
salts) from the soil, and of anchoring the organism
firmly in its chosen abode, so a soil is also usually
essential. Here on Farleton Fell soil is missing over
much of the surface, which is occupied by naked
limestone rock. The absence of soil is due to the fact
that the material—carbonate of lime—of which the
rock is composed is soluble in water, unlike, for in-
stance, the materials of which slate or sandstone rocks
are composed; the rains slowly dissolve it, and it
passes in solution down through crevices in the strata,
leaving behind only a small insoluble residue. This
residue, where not also washed away, collects in every
little hollow, and lowly plants such as Algze and
Mosses soon discover it and colonize it. Their de-
cayed remains add nutritive material to the little
pocket, and help to retain water, and thus prepare
the way by degrees for higher forms of life; till at
length the crevices become filled with a luxuriant
vegetation which, as we shall see later, is of a rather
peculiar type. It should be noted that even the bare
rock is not so inhospitable as completely to exclude
plant life. If we examine it with a lens we shall see

MUD-FLATS 17

that it is colonized by minute Lichens, many of which
have the power of dissolving the limestone, produc-
ing tiny burrows in which they live securely.

On the sands and mud-flats a semi-desert exists,
due in great measure to the shifting nature of the
material and the difficulty which plants find in secur-
ing an anchorage in it. But in the upper parts, near
high-water mark, a few land plants—notably the
Glasswort (Salicornia europea, Fig. 2), a fleshy little

a.

Fic. 1.—A BuRROWING LICHEN (VERRUCARIA CALCISEDA),
LIVING IN LIMESTONE,

a, Natural size; 6, greatly enlarged; c, section, greatly enlarged.

annual—colonize the dreary flats with tiny forests of
dark green branches, and lower down many small
Seaweeds flourish. Some of these, ramifying through
the surface layers, help to bind together the shifting
sand, and by entangling in their branches fresh par-
ticles, and by continued growth, tend to raise and
consolidate the surface, to render it suitable for the
immigration of land plants such as the Glasswort,

and thus eventually to reclaim it from the sea.
2

18

ON FARLETON FELL

Fic. 2,—GLASSWORT (SALICORNIA EUROPA),

_ @, Plant, 2; 0, male flower ; c, female flower, both enlarged,

SEA DESERTS AND LAND DESERTS 1g

It is in the depths of the ocean, however, that the
greatest deserts of our globe are to be found. The
luxuriant Seaweed gardens that decorate the shal-
lower waters of the sea, especially where a rocky bot-
tom provides secure foothold, dwindle rapidly as the
depth increases, owing to the diminution of light, and
when the coastal fringe is left they cease. In the inky
darkness of the ocean depths, amid absolute stillness
and a temperature little above freezing, plant life of
any sort is unknown. Only the flinty skeletons of
diatoms and other minute forms of vegetable life
which inhabit the surface layers, raining slowly down
throughout the ages, tell that plant life exists in the
sea at all.

On land, the larger deserts are found in the coldest
and in the hottest regions. Around the North and
South Poles lie great areas where the perennial low-
ness of temperature and the consequent almost con-
tinuous covering of snow and ice render plant life
impossible. But just as the Eskimo live under con-
ditions which would be wellnigh prohibitive to in-
habitants of more temperate regions, so many of the
higher as well as the lower plants creep northward
far beyond the Arctic Circle, where, awakening from
a nine months’ winter sleep, they break from the still
half-frozen ground to brighten the brief summer with
their leaves and flowers and fruit. The flora of
Greenland, for instance, which we generally think of
as an ice-bound and inhospitable land, numbers some
400 species of Seed Plants. These live mostly on the
cliffs and steep ground that fringe the coast, where
they are clear of the great icefields which bury the
interior of the country, and in many places descend

20 ON FARLETON FELL

as broad glaciers into the sea. But the life of these
high northern plants is slow and difficult, as is
evidenced by their paucity and their stunted stature.
Later on we shall have to consider how they adapt
themselves to the adverse conditions under which
they exist (Chapter VIII.); and we shall find their life
problems are reproduced in many respects by those of
the interesting alpine plants which may be found
nestling in the rock crevices of the higher mountains
of our own country.

But the more familiar deserts of the world, those to
which the mind turns when we use the term, are
mainly due, not to absence of light as in the ocean
depths, nor to want of heat as in the polar regions,
but to failure of the water-supply. A vast desert
region of this kind stretches across Northern Africa
from west to east, and onward through Arabia,
Southern Persia, and Baluchistan. Another, almost
continuous with it, extends from the Caspian Sea
across great plains into Central Asia, and on over
vast mountain areas into Western China. Other
similar deserts, familiar to us in word and picture, are
situated in the south-western United States, Mexico,
and South Africa. In all these tracts, with their
diverse characters and diverse sparse floras, the
scarcity of rain is the primary cause of their peculiar
features. The dryness prevents a protecting cover-
ing of vegetation, and allows heat and cold—both
sharply accentuated by the scarcity of the moderating
influence of water in either soil or air—to pursue their
work of disintegrating the surface, reducing the rocks
to sand and dust, which the winds sweep hither and
thither. In such circumstances plants exist under

DESERT PLANTS 21

very difficult conditions; yet there are few areas in
which the eye will not note some strange vegetable
form. In Fig. 3 are illustrated some of the remark-
able Mesembryanthemums found in the South African
deserts. Here the extremely fleshy leaves, arranged
in opposite pairs, produce a sub-globular plant form,
a mere mass of watery tissue, which in colour as well
as shape appears to mimic the pebbles among which it
grows. The frontispiece shows some other types of

Fic. 3.—MESEMBRYANTHEMUM BOLUSII (LEFT), AND
M. LESLIEI (RIGHT), BOTH %.

desert plants. Another difficulty which desert plants
have to contend with is this: continual evaporation
from off the land of water charged with mineral salts
— in some regions in bygone times, in others still
following each brief rainy season—has left the soil
highly impregnated with substances, of which
common salt is one of the most abundant, which, ex-
cept in very weak solutions, are deleterious to plant
life, since water containing them is absorbed with
difficulty by the roots. These old lake-bottoms and

22 ON FARLETON FELL

one-time swamps—such as the alkali deserts of Utah
—harbour only a limited number of species specially
adapted to their arduous conditions of life. The same
difficulty, it may be noted, produces the peculiar and
specialized flora of the salt-marshes which fringe the
broad bay on which we look down from Farleton
Fell. Here there is indeed a superabundance of
water, but it is so charged with salt that if even the
most vigorous species of the fields or woodlands are
transplanted into it they will soon be dead; only plants
long inured can grow there. Still, the conditions are
not so adverse but that a continuous mat of vegeta-
tion extends, growing patchy and dying out only
where the surface slopes below high-water mark.
There we enter a new domain, where another race of
plants, so long inured to salt water that they now
cannot exist without it, holds possession.

Thus from absolute deserts, such as the floor of the
deep sea or the regions surrounding the Poles, we
pass to semi-deserts where plants are dotted thinly
over the surface, and thence by degrees to closed
vegetation of various types, where the plants elbow
each other over the whole surface as they do in the
grasslands spread around Farleton Fell, in the woods
which adjoin them, and on the brown hillsides out to
the north. But before we pass to the consideration
of the conditions where favourable environment re-
sults in a closed vegetation, we may suggest for con-
sideration the following point of view: that for any
plant, or group of plants with similar requirements,
much of the world is a desert—that is, a place where
conditions are such that it cannot live. For each
plant there exists, owing to long usage and slow

PLANTS AND HABITATS 23

adaptation to given surroundings, limiting conditions
of life: where these conditions are exceeded, the
desert supervenes. Thus, the salt-marsh is a desert
to almost every plant of the mild open soil of hill or
valley, just as the hills and valleys are deserts to most
of the inhabitants of the salt-marsh. The alkaline
soil of the rock crevices of Farleton Fell is fatal to
some of the most abundant plants of the acid peaty
soil of the hills, such as Ling (Calluna vulgaris)
and Bilberry (Vaccinium Myrtillus). For another
cause—the diminution of light—the deep woods are a
desert for many plants of the sunny pastures, and
vice versa. Plants vary very much as to their degree
of adaptability to different soils and different climatic
conditions. Some are highly specialized. Our salt-
marsh flora, for instance, is, as regards most of its
species, confined to its peculiar habitat. If on a map
of Europe we coloured in its distribution we should
find it formed a ribbon round the coast, except for a
few dots where the plants have discovered inland
salt springs or salt lakes, and have found their way
to them. Most plants are more adaptable than these,
and occupy a variety of habitats. The little Tormentil
(Potentilla silvestris), for instance, flourishes equally
on hot banks by the sea, in woods, and on mountain-
tops. The more accommodating a plant is as regards
habitat, the wider its distribution tends to be, both
locally and in a broader sense. But wide range does
not follow of necessity from adaptability to a variety
of conditions: the problem of plant distribution is not
so simple as that. One species may be spread right
round the world, yet be always found in a special
habitat; take the case, for instance, of the Yellow

24 ON FARLETON FELL
Bird’s-nest (Vonotropa Hypopitys), a strange colour-
less, leafless plant, highly specialized, feeding, through
the intermediary of a minute fungus which infests its
roots (see p. 183), on the decaying leaves of deciduous
woods in cold temperate regions, and yet found across
Europe, Asia, and North America; while many
other species, at home under very varied conditions
of soil and moisture, have nevertheless a quite
restricted geographical range.

Although our own country, favoured by conditions
thoroughly suitable to plant life—a sufficiently high
temperature and an abundance of moisture and light
—is characterized by a continuous plant mantle—or
closed vegetation, as the botanists say—nevertheless
what has been said of desert and semi-desert condi-
tions applies to many limited areas in the British
Isles, where the vegetation takes on the peculiar
characters of true desert plants. Low water-content
and great exposure produce such conditions on
shingle beaches and sand dunes; and, as we shall see
later, the vegetation of sea-rocks, salt-marshes, and
peat-bogs is in many respects analogous to, desert
vegetation.

Except near the Poles, wherever the precipitation of
moisture rises above an amount which varies accord-
ing to other conditions prevailing, a closed vegeta-
tion occupies the ground when the agricultural and
other operations of man do not hold it in check. But
as much of this favourable region is utilized by the
human race for the production of plants used for food
or for industry, it often happens, as in our own
country, that the natural plant communities are to
a great extent destroyed, and can be studied only on

GRASSLAND AND WOODLAND 25

land left undisturbed because unsuitable for cultiva-
tion—on heaths and moors, in swamps and lakes, on
sea-sands, chalk downs, and so on; and even in most
of these places intensive grazing of domesticated
animals and other causes connected with human
activities alter and control plant life to a greater or
less extent, rendering it necessary for us to walk
warily in our study of it.

Although the world offers many different aspects
of closed vegetation, they may all in a broad sense be
reduced to two general types—namely, grasslands
and woodlands, the former the result of a lighter, the
latter of a heavier, rainfall: grasses and their associ-
ates requiring for their life-processes a much less
amount of water than a tree vegetation. The British
Isles lie within a broad belt that sweeps east and west
across Europe, characterized by a prevalence of
south-west winds laden with moisture, and yielding
a tolerably heavy rainfall distributed throughout the
year. South of this belt—south of the Alps, roughly
—the rainfall occurs chiefly in winter, and dry
summers produce the well-known “Mediterranean
climate” with which is associated the scrubby small-
leaved vegetation, capable of withstanding heat and
drought, which is characteristic of Spain, Italy,
Greece, and Northern Africa. Northward, the forest-
belt extends into Scandinavia, dwindling into a
tundra vegetation of lowly shrubs and herbs as we
approach high latitudes with a sub-arctic climate.
Forest, then, is the original and natural type of vegeta-
tion of the British Islands, and without doubt the
greater part of the country was occupied by wood-
land within the human period. But forest country

26 ON FARLETON FELL

is not well suited to human habitation or coloniza-
tion. The early arts of peace—pastoral and agricul-
tural—called for open ground. To operations of
war, also, forests are unfavourable. So it came about
that by the use of fire and axe the forests passed
away before the march of man, until now we can
study only fragments of the original all-prevailing
woodland. But it is important to note that certain
portions of the British Isles were never, in recent
ages, under woodland, and that these mostly preserve
still much of their ancient facies. Thus, increase of
exposure—a lower temperature and higher wind-
velocity—appointed a limit on the hills beyond which
trees couldnot and cannot grow. Wind wasand is also
responsible for a dwindling of tree growth along
the exposed western coastlines. Again, the shallow,
porous soil of the chalk downs, very dry in summer,
probably never supported woodland, but has pastured
sheep since the earliest shepherds fought wolves in
Sussex. The scanty soil of Farleton Fell probably
never harboured plants larger than the herbs and low
shrubs which it now supports; and no doubt the salt-
marshes looked the same five thousand years ago as
they do to-day, though their positions have changed
with each slight alteration in the relative level of
land and sea.

To sum up, then, the greater portion of the surface
of our country consists of former woodland now
reclaimed for the purposes of agriculture, the general
aspect of its vegetation altered beyond recognition,
though from the fragments left we can still recon-
struct with tolerable accuracy its ancient condition,
and the flora of which it was composed. In the re-

THE PROBLEM 27

maining parts, though drainage, grazing, and other
human operations have wrought great changes, the
face of the country still wears to a large extent its
ancient appearance, and the flora is still in the main
that which flourished before human activities began
to put their impress upon it.

How are we to set about studying this varied
vegetation which, in a thousand forms, covers hill
and valley? There are several avenues of approach;
any one of them, if explored fully, would take us far
beyond the limits of the present volume; we shall
have to be content with slight venturings along
several of them, so as to acquire, in a brief space,
as wide a view as we can of the phenomena which our
flora displays, and of the problems which it presents.

If we view the vegetation as a whole, we may be
tempted to enquire first as to its origin and history.
We know that plants have existed on the earth for
millions of years, but that the plants of past ages
were different from those of the present, just as those
of the present will ultimately give place to other
forms as yet undreamed of: that the vegetation on
which we feast our eyes is, in fact, but the momentary
expression of a never-ceasing process of life and
change. This is the point of view of the geologist,
to whom

The hills are shadows, and they flow
From form to form, and nothing stands ;

They melt like mist, the solid lands,
Like clouds they shape themselves and go.

Pursuing this line of enquiry, we may endeavour to
trace the descent through the ages of our present
plants from bygone types; and coming at length to

28 ON FARLETON FELL

the still remote time—as measured by human
standards — when the plants which now grow
appeared on the Earth’s surface, we may try, from a
study of their present distribution and of the dis-
tribution of their remains in regions where they are
no longer found living, to determine their area of
origin, and to trace the date and course of the migra-
tions by which they reached our country. In the
case of the British Isles, geological considerations
play a leading part in such investigations, these
islands being but outlying hummocks of a great con-
tinental area, at times joined to the main land-mass
by a slight upward movement of the Earth’s crust,
and anon cut off from it by a movement of depres-
sion. In this connection also we may be led to in-
vestigate the means by which plants spread, and
especially their capacity for crossing barriers of the
various kinds indicated in our brief study of deserts
in the previous pages—the serious barriers offered
by water-channels, or others equally difficult to nego-
tiate produced by areas of uncongenial soil, by
mountain ranges, or by forests. This will involve
especially a study of seeds and the interesting phe-
nomena of seed-dispersal.

Again, the most popular branch of botanical study
in England is Floristic Botany, which traces the dis-
tribution within our area of the various species com-
posing its flora; and with it is necessarily associated
a study of the plants themselves so far as the char-
acters are concerned, by which they may be dis-
tinguished from each other. This last is the province
of Descriptive Botany. The study of local distribu-
tion, if conducted intelligently, will greatly assist in

FIELDS OF STUDY 29

solving problems relating to the migrations and
routes by which the existing flora reached its habitats.

Once more, we have already from Farleton Fell
observed that plants do not grow higgledy-piggledy
over the country, but are arranged in more or less
definite societies depending on similarity of climate,
soil, and other external conditions. Studied from this
point of view, the flora resolves itself into a series
of communities, each requiring a certain set of con-
ditions for its continued welfare. The study of these
inter-relations between plants and their environment,
and of the types of vegetation resulting from the
grouping together of plants requiring similar con-
ditions, is the province of Ecological Botany.

Again, the morphologist deals with the forms of
the organs of plants, and the changes which these
undergo in different plants, while the anatomist in-
vestigates their minuter structure.

Physiological Botany deals with the life processes
of plants, and the way in which they feed and grow
and move. It has a very important bearing on the
distribution and grouping of plants, since this is
largely governed by their food-supply and by the
need of surroundings which allow them to carry on
their life processes with success.

It will be seen that there are many lines of enquiry
open to the student of botany. In the following
pages no more can be attempted than the preliminary
study of some of the more familiar phenomena of
plant life as it presents itself to the holiday-maker on
the hills and woods and shores of our own land.

CHAPTER II

PLANT ASSOCIATIONS

‘*It is perhaps also proper to take into account the situation in
which each plant naturally grows or does not grow. For this is
an important distinction, and specially characteristic of plants,
because they are united to the ground and not free from it like
animals.’’—THEOPHRASTUS : Enquiry into Plants, I. iv.

BEFORE Setting about discussing the various types
of vegetation which our own country presents, it will
be well to have a general idea of the extent to which
the main types are developed, and of the amount to
which agriculture has interfered with the native
flora. We have seen that the natural vegetation of
the greater part of the British Isles is woodland: yet
so profoundly has human industry altered the face
of the country that woodland, natural or planted,
occupies only about one-twentieth of the surface of
England, rather less of Scotland and Wales, and
about one-seventieth of Ireland. Much of the former
woodland is now represented by “ arable land,” which
covers over one-third of England, and about half
that proportion of the other parts of the British Isles.
Permanent grassland, partly natural, partly replacing
ancient woodland, bulks large in England and Wales,
occupying about two-fifths of the whole country; in
Scotland and Ireland the proportion is much less, but
in those countries a large area is under moor, heath,
30

PRESENT CONDITION OF BRITISH ISLES 31

or natural grass, over which wander great herds of
sheep and cattle. A. G. Tansley* thus contrasts (in
percentages) the area of cultivated land (on which
natural vegetation has been to all intents destroyed),
with the area on which natural or semi-natural con-

ditions still prevail:
England. Wales. Scotland. Ireland.

Cultivated land na ale 75 59 25 ? 20-30
Land under natural or semi-
natural vegetation she 15-20 40 70-75 ?70-80

It will be seen how little of the original vegetation
of England is left to us for purposes of study—less than
one-fifth, almost the whole of which has been influenced
to some degree by human operations; while in Scotland
and Ireland a much larger area is more or less in its
primitive condition. The Scottish mountain-sides and
Irish moorlands still to a great extent retain a natural
flora, save that the greater number of grazing animals
which they now support, as compared with the times
when wolves and other enemies roamed unchecked,
leaves its impress upon the vegetation.

Viewing the plant world as a whole, its primary
divisions, from the point of view of ecology, are
governed by the factor of rainfall. It is true that
the plants of the Tropics differ profoundly from those
of the Temperate regions, and those again from the
plants of the Arctic. But this is a difference in the
species and families which constitute the vegetation,
rather than a difference in the types of vegetation
or plant formations which occur. A certain area in
Siberia may not have one species in common with a
certain area in India, but in both we may find the

‘* Types of British Vegetation,” 1911, p. 63.

32 PLANT ASSOCIATIONS

three great vegetation types of forest, grassland, and
desert. A rainfall gradient, on the other hand, will
cause a progressive change in vegetation type, as
may be seen in crossing North America from east to
west, where the forests of the New England States
give way as precipitation diminishes to the prairies
of the middle States, and these again to the deserts
which stretch far over the west. It is only in the
extreme north that temperature, apart from precipita-
tion, becomes the dominant influence in determining
the presence or absence of vegetation, or its char-
acter.

Within any one climatic region— say within
the British Islands—the soil in which the plants grow
is the controlling factor in determining the character
of the plant population. And while a classification
by plant form—such as woodland, grassland—is often
convenient, when we come to analyze the various
plant associations which colonize the ground, it will
be found that similarity of form-type does not
necessarily imply affinity as regards either physio-
logical conditions or floristic constituents. Thus, a
Beech wood on the Chalk has really no affinity with
an Oak wood on the Coal-measures, save that they
are both woods: they shelter plant groups of quite
different composition, one a constituent association
of the Limestone Formation, and the other of the
Formation of Clays and Loams, according to modern
English classification. Similarly, the Hazel copse
which covers the screes of Farleton Fell has no close
relation to the Hazel copses along the Westmorland
becks, although the dominant plant—the Hazel—is
the same in both cases: soil is the controlling factor,

FORMATIONS AND ASSOCIATIONS _ 33

and the one is related to the limestone vegetation of
the hill above, the other to the vegetation of the
loams and peaty soils of the adjoining mountain-side.
In the British Isles the leading plant formations are
those of clays and loams, of sands and sandstones, of
siliceous soils, of calcareous soils, of peat, of marsh,
of lakes and rivers, of salt-marsh, sand dune, and
shingle beach; also, governed by the climatic factor,
alpine vegetation stands somewhat apart. While the
vegetation of some of these, such as salt-marsh or
peat, usually presents a uniform aspect, others, such
as the clays, sands, and limy soils, display each a
characteristic type of woodland and of grassland, as
well as other variants, dependent on the composition,
depth, and wetness of the soil, the degree of ex-
posure, and so on: these form the associations which
together constitute the formation. Each association,
if the plants composing it be examined, will be found
to consist of an assemblage of species, large and
small, brought together by their superior fitness for
the particular conditions which prevail. There are
mostly in each association one or more dominant
species—such as the trees of an Oak wood, or the
Heather of a moor—which by their abundance or
vigorous growth control the association. The shelter
which they give may protect some of the members of
the community: the shade which they cast may keep
out other plants which otherwise would invade the
ground. The association will include some species
specially adapted to the particular conditions which
prevail, and perhaps not found elsewhere in the
area; these are the indicator plants of the association,
which give it its special character, and which will
3

34 PLANT ASSOCIATIONS

help us to identify the association should we en-
counter it again; there will be others—dependent
species—which are attracted by the shade, or shelter,
or other advantages which the growth of the domi-
nant plants affords: and there will be others, again—
probably many—of wide distribution, which are
merely as much at home here as elsewhere. But all
grow here because they are better fitted for the
particular conditions prevailing than are the other
plants of the surrounding area. On Farleton Fell,
for instance, among the most abundant species
which fill the crevices of the limestone plateau are
two ferns—the Limestone Polypody (Polypodium
Robertianum) and the Rigid Buckler Fern (Lastrea
rigida). Though there is rocky ground of many
kinds in the Lake District, these two plants are never
found save on similar outcrops of the Carboniferous
Limestone, and they are clearly specially fitted for
life in the hollows of this rock. But the same rock
crevices also harbour many species which are found
equally on the soils derived from the slate rocks or
sandstones. To take another instance: many of our
most familiar spring flowers are woodland plants—
the Primrose (Primula acaulis), Wood Anemone
(A. nemorosa), Wild Hyacinth (Endymion non-
scriptum). These rejoice in the humus soil which is
formed from the dead leaves of preceding years;
they flower before the trees are in full leaf, thus
securing plenty of light and air for their period of
growth; and they are accustomed to have their stems
and roots protected from summer heat by the leafy
canopy overhead. Transplanted into an adjoining
sunny pasture they will soon die out. They are

ADAPTATION TO ENVIRONMENT 35

characteristic members of the woodland association of
one or more formations. But with them we shall find
other species, such as the Wild Strawberry (Fragaria
vesca), which are equally at home on dry sunny
banks or even on sand dunes.

If we ask why the plants group themselves into the
associations which we may study any day in the
country, in many cases the answer is not obvious.
It is clear that while many species accommodate them-
selves easily to different soils or different degrees of
light or of moisture, others have small powers of
accommodation, and are in consequence restricted in
their range. By long usage many plants have
acquired special characters enabling them to live
under special conditions—some examples will be
discussed a little later—and in some such cases
it is easy to correlate the peculiar characters of the
plant with those of the habitat. But in many other
cases the relation is not obvious. For instance, we
cannot tell, by examining a plant, whether it is partial
to a limy or to a non-limy soil; yet many plants are
poisoned by lime, while others, though generally
capable of growing in a soil devoid of lime (if
planted in a garden), are nevertheless absent from
the non-calcareous areas adjoining their limestone
habitat; in other words, they can hold their own on
limestone, but are unable to do so elsewhere. The
two ferns already mentioned (Polypodium Roberti-
anum and Lastrea rigida) are cases of the latter
kind; while some of the most familiar of our hillside
plants, such as Foxglove (Digitalis purpurea) and
Broom (Sarothamnus scoparius), are instances of the
former.

36 PLANT ASSOCIATIONS

If, however, we consider some of the formations or
associations which are the result of extreme condi-
tions of environment, we get more light on the rela-
tions between the plants and the factors which
control the vegetation. Take the case of the plants
inhabiting desert regions such as were discussed in
Chapter I. Here the outstanding feature is scarcity
of water, and the plants display various remarkable
adaptations which fit them for a thirsty life. There
are three ways to meet scarcity of water—facilities
for gathering it, arrangements for storing it, and
economy in using it; and arrangements for all three
are familiar features of desert plants. To effect the
first, the root-system is extended, and is often
enormously developed in proportion to the aerial
parts. This adaptation may be studied in the flora
of dry places in our own country, such as shingle
beaches and sand dunes, which are characteristic semi-
deserts. Take such plants as the Sea Holly (Eryngium
maritimum), the Sea Convolvulus (C. Soldanella), or
the Sea Sedge (Carex arenaria), and compare the ex-
tent of the root-system or underground stems with
that of the aboveground portions. Fig. 4 represents
the Wild Carrot (Daucus Carota) as found growing
under extreme exposure on the west coast of Ireland.
To meet the conditions the tall branched stem has
been entirely dispensed with, and the terminal umbel
is seated on the ground in the middle of a ring of
leaves. In this way the plant prepares to resist both
drought and wind. Water storage is often developed
in different parts of xerophytes (drought-resisting
plants)—in roots, or stems, or leaves, which become
much enlarged, and at the same time covered with a

PROTECTION AGAINST DROUGHT — 37

highly impervious skin, so that they act as veritable
cisterns. In plants like the Cacti water storage in the
stems is carried very far indeed; while in such genera
as the Stonecrops (Sedum) the leaves are often so
swollen and charged with water that they lose up to

Fic. 4.._WiLpD Carrot (Daucus CaRoTA), GROWING
UNDER GREAT EXPOSURE, }4.

98 per cent. of their weight if they are dried. Preven-
tion of excessive loss of water by transpiration is
effected in plants of dry places mainly by reduction in
the size of the leaf and by protection of its surface.
Leaf reduction is very marked in many dry countries.
If we compare the flora of the Mediterranean region

38 PLANT ASSOCIATIONS

(a dry area) with that of Middle Europe or of England,
we shall be struck with the prevalence in the former
of small-leaved twiggy plants—Lavender (Lavanduia)
and Rosemary (Rosmarinus officinalis) will serve as
examples. Often leaf-reduction is carried much
farther, and we need not go beyond our own commons
to find a good example, for in the Gorse (Ulex)
flat leaves are entirely absent and the branches are
shortened and converted into prickles, thus largely
reducing the surface exposed to the sun and wind.
The seedling Gorse has little trifoliate leaves, which
remind us of its affinity to the Trefoils and Brooms,
but they are discarded almost at once, to fit the plant
better for life in the dry, breezy localities which it
favours. Reverting to the Mediterranean flora, a
characteristic of its plants is the prevalence of a grey
hue in their stems and leaves, such as we see in the —
Pinks and Achilleas of our rock gardens. This is due
to a coat of wax, as in the Pinks (Dianthus), or a felt
of hairs, as in the Achilleas, designed to check exces-
sive transpiration. The coatings of hairs are often
of great beauty and complexity, and form an almost
impenetrable covering to the leaf surface, protecting
the upper side from the fierce rays of the sun, and on
the underside sheltering the stomata, or minute open-
ings through which the plant exhales the surplus
water drawn up from the roots and inhales carbon
dioxide. Another very beautiful device for protecting
the underside of the leaf, and one which may be
studied in many of our commonest plants, consists of
the inrolling of the edges, often combined with a
wrinkling or ridging of the underside, so that the
stomata are set in deep hollows, communicating with

PLANTS OF THE SHINGLE BEACH = 39

the open air only through narrow openings. The
leaves of some of our common grasses show these
characteristics to great advantage. And again the
stomata are often sunk in little pits, by which device
they obtain further protection. If we now examine
the plants composing the sand-dune or shingle-beach
associations in the light of these facts, we shall find
them full of interest. The plants are well equipped
to meet the adverse conditions of a very porous soil,
drying winds, and scorching sun. Note the grey felt
of hairs which protects the leaves of the Horned

Fic, 5.—SECTION ACROSS INROLLED LEAF OF CROWBERRY
(EMPETRUM NIGRUM), MUCH ENLARGED,

Poppy (Glaucium flavum), the tough, waxy skin which
covers the Sea Holly (Eryngium maritimum), the ex-
tensive underground stem-systems of the fleshy-leaved
Sea Convolvulus (C. Soldanella) and Sea Purslane
(Honkenya peploides). Even the annual plants dis-
play similar characters. In the great desert regions
the annuals are often quite normal in structure: that
is because they appear during the brief rainy season,
and pass away before the fierce heat of summer sets
in. But on our shingle beaches the annuals grow
throughout the summer, and need protection against
drought: so the Sea Rocket (Cakile maritima), the

40 PLANT ASSOCIATIONS

Sea Whin (Salsola kali), and others are very fleshy
plants; their leaves are small, with an impervious skin,
their root-systems are better developed than in most
annuals. The grasses and sedges of these places,
such as the Bent (Ammophila arenaria), Sea Wheat-
grass (Triticum junceum), Sea Sedge (Carex arenaria)
have underground stems which burrow widely through
the sand, with an extensive root-system and tufts of
inrolled leaves beautifully protected against over-
transpiration, and well worth microscopical exam-
ination. .

If we turn from the shingle beach to the salt-marsh,
where water is very abundant, we shall be struck by
the peculiar fact that its vegetation displays characters
quite similar to those we have just been studying.
How can we reconcile this with the theory that the
peculiar characters of the shingle-beach plants are
correlated with lack of moisture? The explanation is
to be found in the fact that plants have difficulty in
absorbing water if it is highly charged with mineral
substances in solution. In the salt-marsh the heavy
muddy soil is impregnated with common salt (chloride
of sodium): the plants absorb it with difficulty; and
in consequence they are faced with the same main
problem which confronts the Sea Holly and Sea Whin,
and they meet it in the same way. Indeed, the salt-
marsh plants appear to be more highly specialized,
for very few intruders from outside can venture in,
while on the beach we may meet with many plants
which belong to other formations growing success-
fully, at least for a time. The salt-marsh flora is very
exclusive, and contains but few species which we en-
counter in other situations. Some of them are also

PLANTS OF THE SALT-MARSH Al

found on dry sea-rocks—the Sea Pink (Statice
Armeria), Scurvy-grass (Cochlearia officinalis), Sea
Aster (A. Tripolium), and so on; showing that soak-
ing soil is in no way essential to their growth. (The
first two reappear among alpine plants on some of
our higher mountains, pointing again to an analogy
of conditions not altogether understood.) But the
salt-marsh formation as a whole is perhaps the most
distinctive as regards its composition of any of the
plant-groups of our country. It is dominated by such
species as the grey leathery-leaved Obione portula-
coides, the small-leaved, thick-stemmed Sea Pink, the
Sea Wormwood (Artemisia maritima), which is all
covered with a silky coat; the pools are fringed with
Scirpus Tabernemontant, a dwarf greyish copy of the
Common Bulrush of our lakes, and filled with the
narrow-leaved Ruppia and Zannichellia; and in the
muddiest places are little forests of Glasswort, leaf-
less, very fleshy, the flowers reduced to mere essentials
and buried in the fleshy stems (Fig. 2, p. 18).

Again, it is easy to trace the relationship existing
between plant form and soil conditions in the bog-
land flora; and these relations, unexpectedly enough,
turn out to be analogous to those obtaining in the
case of the salt-marsh. The sodden peat, sour and
badly aerated, and poor in mineral salts, is poor also
in the bacteria which feed upon and destroy dead
vegetable matter, with the consequence that acid
humus compounds collect in the half-decayed vege-
table mass; water charged with these substances is as
unsuitable for plants as is the water of the salt-marsh.
In spite of the wetness of the peat, water is in this
case also a desideratum; and the moorland plants, like

42 PLANT ASSOCIATIONS

those of the sea fringe, possess special adaptations for
economizing it. This usually takes prominently the
form of a reduction of leaf-surface. The dominant
plants, such as the Ling (Calluna vulgaris) and Purple
Heather (Erica cinerea), have minute leaves with re-
flexed edges and special structure to protect the
stomata. The grasses and sedges which abound have
similar characteristics; the whole vegetation tends to
be small-leaved and long-rooted. A few of the plants,
such as the Eyebright (Euphrasia), eke out the scanty
food-supply by a semi-parisitism, robbing their neigh-
bours of portions of their hardly-won sustenance; one
or two others, such as the Bladderwort (Utricularia),
which floats in the bog-pools, and the Sundew
(Drosera), which fringes their edges, entrap insects
and digest their juices, helping out their scanty
rations with an animal diet. On the moors the peculiar
soil conditions determine definitely the type of vege-
tation, which, over large areas, is as uniform and
monotonous as that of the salt-marsh.

We see, then, that the peculiar character of several
of the most marked of native plant formations—those
of shingle, of salt-marsh, and of moor—are due
primarily to scarcity of water. They are drought
formations, produced either by physical drought, as
in the case of shingle, which fails to retain water, or
by physiological drought, as in the salt-marsh or bog,
where, though water is present in abundance, it is not
in a condition in which plants can readily make use
of it. |

Let us now go to the opposite extreme, and con-
sider the plant formation which characterizes lowland
lakes and rivers, where water suitable for plant use is

WATER-PLANTS 43

superabundant. In such places we are plea with a
vegetation exhibiting a great number of species and
a marked variety of form, and by no means so easy to
correlate with its environment as those which we have
been considering. In a wide sense, the nature of the
vegetation is largely dependent on the degree of aera-
tion of the water and the amount of dissolved mineral
salts which it contains, an increase of either (within
limits) resulting in a richer flora. But in any one

CELE
Ree he tee y liffii/
Wl NY
Paice cd in
fe A ba | mate all |

ih

a. Stall ‘ mg | 1

ee ae

pas.
Fic. 6.—DIAGRAM ILLUSTRATING SUCCESSION OF VEGETATION
IN LAKES.

a, Marsh zone; J, reed zone; c, zone of floating vegetation ;
d, zone of submerged vegetation.

area it is clear that depth of water is the controlling
factor: the plants are arranged in zones, one succeed-
ing another as the bottom shelves. Two main zones
are conspicuous: (1) A zone of tall reed-like plants
near the margins, which farther out is succeeded by
(2) a zone of lax floating plants which either have
leaves resting on the surface or grow entirely sub-
merged. Above the former a belt of marsh plants
links the reed zone with the vegetation of the soils of
normal moisture; below the latter, should the water

44 PLANT ASSOCIATIONS

increase in depth, we reach an aquatic desert region,
where the reduction of light renders plant growth
difficult, and eventually inhibits it. Let us consider
the conditions prevailing in the reed zone. Here the
plants are essentially aerial, and though they have
their feet in water, the stems and leaves rise far above
it. Water-level is variable in lakes and rivers; the
plants are usually tall, so that even in case of flood
the leaves and flowers will not be drowned. Wave
action on lake-shores is somewhat violent, and in
flooded rivers a strong current may sweep through
the vegetation; we see the advantage of the slender
elastic stems and narrow leaves that characterize
the plants: compare Reed (Phragmites), Reed-mace
(Typha), Flag (ris), Bur-reed (Spargarium), Bulrush
(Scirpus); and these characters also fit them for the
windy nature of their habitat. The denuding effect of
wave or current action is countered by the network of
creeping stems and abundant roots which the plants
possess, forming a tough felt which floats, and by its
growth and decay helps materially to form fresh land.
Another effect of the creeping and branching stem-
systems is the production of extensive and dense
groves of many of the species.

When we pass beyond the reed zone, a completely
different type of vegetation prevails. Here the plants
are essentially aquatic. They make no effort to raise
their stems and leaves above the water surface; but
almost all of them raise their flowers into the air,
though the seed is often ripened below the surface by
a downward curving of the stem. These plants, sur-
rounded by water, use their roots chiefly as anchors,
and absorb through their stems and leaves the water

CHARACTER OF WATER-PLANTS 45

from which they obtain the necessary mineral salts.
As regards the supply of oxygen and carbon dioxide
which the air supplies to them, those with floating
leaves absorb it from the atmosphere, while those
whose leaves are submerged have to subsist on the
small quantity of these gases which is dissolved in
the water—no wonder that such plants are rare in
stagnant waters where aeration is poor. To assist
respiration and transpiration, abundant and often
comparatively gigantic air-spaces are provided in
roots or stems or leaves, giving them a cellular
appearance, and making them singularly light and
spongy in texture. The leaf system of those plants
which possess floating leaves—such as Water Lily
(Castalia and Nymphea) or Common Pondweed
(Potamogeton natans), are well worth study. They
are tough, to withstand battering by waves; the
stomata are situated, not on the lower side of the leaf,
as in land plants, but on the upper side, where they
are in contact with the atmosphere; and the upper
surface is waxy or oily, so that it is not wetted and
the stomata are not blocked. Changes of water-level
are met by means of long flexible stems, rising not
vertically from the root, but at an angle, so that the
. leaves can rise with a rise of water-level. But not all
the plants are anchored to the bottom. Some, which
favour especially ditches and quiet waters, float freely
with roots hanging down in the water—the Frog-bit
(Hydrocharis) and Duckweeds (Lemna) are familiar
examples. In the Duckweeds true leaves are absent,
but the tiny stems are flattened and green and serve
the same purpose, the minute flowers being borne on
their edges. A few plants, such as the smallest of the

46 PLANT ASSOCIATIONS

Duckweeds (Wolffiia arrhiza) and the Bladderworts
(Utricularia), have gone farther still, and have dis-
pensed with roots altogether. In Wolffia, indeed, the
degeneracy of structure which results from the
simplification of life problems in plants which live
thus floating freely in water, is carried to its extreme
limit. Leafless, rootless, and almost flowerless, it
maintains itself by the budding of its tiny green
fronds, a life-history as primitive as that of the lowly
Algz among which it lives. In the Bladderworts, the
long flaccid stems, clothed with much-divided leaves
converted in part into ingenious imnsect-traps (see
p. 188), hang limply in the water, sending up boldly into
the air their flowering shoots with yellow Snapdragon-
like blossoms. In most of such free-floating plants,
compact buds are formed at the tips of the shoots
in autumn, and while the rest of the stem dies away
these sink to the bottom and remain there safe from
frost and storm until the spring, when they rise to the
surface and produce a new crop of plants.

We have now glanced at the most distinctive of the
plant formations which we meet with in our own
country, and find that they accompany extreme con-
ditions relating to water and soil: it remains to return
to the consideration of the vegetation which de-
velops under conditions of a more normal character—
on ordinary soils, in fact, which are neither very wet
nor very dry. Such conditions are precisely those
which are required for agricultural purposes; and -
over the wide areas where they prevail, we find, as
pointed out already, mere fragments of the native
associations remaining in an undisturbed condition.
This renders their study more difficult, and the diff-

OTHER PLANT FORMATIONS 47

culty is heightened by the fact that while the physical
conditions show no contrasts so marked as those
which we have been considering, the formations which
can be distinguished are several, and each contains
several associations—often a woodland, a scrub, and
a grassland type. Thus, the formation which occupies
calcareous soils exhibits characteristic woodlands—
woods of Ash (Fraxinus excelsior), for instance, and
on the downs peculiar woods or scrub of Box (Burrus
sempervirens), Juniper (Juniperus communis), Yew
(Taxus baccata), or Hazel, as on Farleton Fell. It
also bears some very marked types of grassland, as
on the chalk downs; and the limestone pavement of
Farleton Fell is a special variant of this. Similarly,
clays and loams, sands, and siliceous soils possess
similar characteristic types of vegetation. But the con-
sideration of these would occupy more space and lead
us into more technical detail than the scope of this
book warrants. For an account of these associations,
written by botanists who have made a special study
of them, the reader is referred to Tansley’s “Types
of British Vegetation.”

CHAPTER III
PLANT MIGRATION

ALL organisms, animal as well as vegetable, are at
some period of their existence provided with an
opportunity of migration. In the animal world, most
land creatures have legs or wings, which allow them
to roam about freely—a freedom which is of special
importance as enabling them to obtain nourishment
and to avoid disadvantageous conditions. Aquatic
animals are likewise to a great extent possessed of
powers of locomotion, but such powers are not so
essential to them as to terrestrial creatures, since the
water itself is full of small organisms, both animal
and vegetable, on which they can feed; hence a large
variety of water creatures are content to remain
during much of their lives fixed to one spot, extract-
ing from the water as it passes by both the supply
of organic food and the inorganic substances, such
as oxygen or carbonate of lime, which they require
for their life processes. These sedentary creatures,
of which barnacles, sea-anemones, and zoophytes will
serve as examples, once attached, do not move from
the spot where they have settled down; but it is im-
portant to note that not only are their eggs or young
mostly liberated into the water, and by it transported
to new homes, but in their juvenile stages they often
swim vigorously, and thus achieve a wide dispersal.
48

IMPORTANCE OF DISPERSAL 49

In the plant world, the higher forms, with very few
exceptions, spend their lives attached to one spot,
like sea-anemones, deriving their food-supply from
the air and from the soil; but they similarly are given
the opportunity, after birth, of migrating. In our
familiar wild flowers, for instance, the young plant, at
an early stage of its existence, while it is still minute,
becomes covered with a coat often of very resistant
qualities, and is then cast loose by the parent in the
form of seed, mostly in great numbers, to achieve
what travels it can before it takes root and settles
down, like its parent before it, to a humdrum exist-
ence. In the Cryptogams, or so-called Flowerless
Plants, this temporary compression of the organism
into very narrow limits suitable for easy dispersal
takes place at a different period in the life cycle,
but for mechanical purposes the results are similar.
Minute bodies, or spores (much smaller than the seeds
of the Seed Plants), are cast loose by the parent
often in vast numbers, and eventually settle down and
reproduce the species. In many of the lower aquatic
plants these spores are provided with means of loco-
motion in the form of a tail-like appendage, which by
its movement propels the germs through the water,
giving them the same advantage which is possessed
by the young of many of the sedentary animals.
The opportunity for migration thus offered to
sedentary plants once at least in each cycle is of very
great importance. A plant, living on one spot and
drawing, from that portion of the soil which its roots
can reach, certain mineral salts essential for its con-
tinued growth, tends to exhaust the available supply
of these materials, and the succeeding generation
4

50 PLANT MIGRATION

needs to reach fresh ground if it in turn is to attain
healthy development. And it is undoubtedly of ad-
vantage to plants, if they are to continue to exist on
the Earth, to be able to jump barriers and to colonize
fresh suitable habitats which may arise in the course
of natural changes, which sooner or later may render
old habitats untenable. Thus the very existence of
plants upon the Earth depends on the adequacy of
seed-dispersal. This being so, the imaginative mind,
viewing the marvellous and infinitely varied con-
trivances of Nature, will possibly be struck more by
the want of special provision for dispersal shown by
the majority of the higher plants—their helplessness
in this respect—than by the beautiful devices ex-
hibited by the few. In the first place, seeds are inert,
devoid of any power of locomotion—though in some
instances the last act of the parent is to discharge
them with an explosive action into the air. They are
dependent on the movements of external media—air,
or water, or wandering animals—for transportation
of any magnitude, and while many possess very
beautiful devices for enabling them to take advantage
of opportunities in this regard, the majority are
devoid of any special structures. They are as inert
as pebbles or grains of sand: but they possess two
attributes which form important assets— namely,
numbers and vitality. The amount of seed produced
annually is hundreds, or more usually thousands,
sometimes hundreds of thousands, for each parent.
What matter if myriads perish? If one in so many
thousands takes root and grows, the species will not
diminish in numbers. Vitality also largely affects the
problem. The seed can endure extremes of heat and

THE IMMOBILE SEED 51

cold which would be fatal to the parent; it can be
drowned, or scorched, or dashed about, or in many
cases eaten by animals without injury; it can lie buried
in the soil for a long period of years, yet if turned up
again and placed within reach of the requisite amount
of air and heat, will spring up vigorously.

As a matter of fact, investigation soon shows that
absence of special devices for dispersal provides no
measure of the breadth of a plant’s distribution, nor
is profuse seed-production necessarily related to
abundance of offspring. Many factors come into
play, and conclusions of this obvious kind will
generally only lead us astray. But that does not
render the study of each one of the factors less
interesting.

This matter of seed-dispersal is of prime importance
in our study of familiar British plantscapes, for our
vegetation is the expression of the past and present
efficiency of its particular rdle in the ever-changing
drama of Nature. We shall do well to spend a little
time in considering it.

First of all, as to the nature of the seeds with which
we have to deal. These are, as already pointed out,
young plants, already a long way advanced from the
ege stage, neatly tucked up and enclosed, in most
cases along with a supply of food material, in a tight,
strong skin, which is mostly of a particularly impervious
character, protecting the young plant from injury by
bruising, from attacks of small animal enemies, from
extremes of heat and cold, of moisture and dryness.
The young plant, too, is in a peculiarly resistant phy-
siological condition. For instance, its breathing—or
absorption of oxygen—is exceedingly slow, and it is

52 PLANT MIGRATION

not suffocated by burial, sometimes even for years, in
the soil. And while the mature plant is killed in-
stantly by immersion in boiling water or by exposure
to a very low temperature, some seeds, if boiled for
a quarter of an hour, are quite uninjured, while others,
subjected experimentally to even the temperature of
liquid hydrogen (- 260° C., or 436 degrees of frost on
our more familiar Fahrenheit scale), remain un-
affected. Many seeds are liberated from the parent
plant enclosed by or attached to appendages of
various sorts (when they are called by the botanist
fruits) which sometimes greatly aid dispersal, as in
the Dandelion (Taraxacum), and sometimes appear to
hinder it; in any case, while the young plant itself is
usually quite small, it may, when surrounded by its
food-supply and enclosed in its wrappings, be a bulky
object—as is seen in the Cocoanut or Horse Chest-
nut. In the British flora, to which we may confine
our attention, a crab-apple (containing a number of
seeds), a hazelnut, and an acorn (each containing a
single seed), are the largest units of dispersal with
which we have to deal. But these are quite excep-
tional in size, and the average seed (using that term
in its original sense of the natural unit of dispersal)
- in the British flora does not exceed the size of a pin’s
head. This remarkable reduction of size alone aids
dispersal greatly.

The migrations of plants are effected mainly during
the seed stage, these tiny, tightly packed portman-
teaux being much better fitted for travel than the
bulky and fragile organisms to which they give rise.
But before we consider the adventures of seeds it
must be pointed out that a considerable, if slow,

VEGETATIVE INCREASE 53

migration of plants takes place by mere vegetative
growth. The stems of many species are not erect, but
prostrate; creeping upon or below the ground, they
may in time cause a plant to spread far beyond its
place of origin. A whole field, or for that matter a
whole hillside, of Bracken (Pteris Aquilina) may quite
possibly have originated from a single wind-borne
spore. Among Sedges and Grasses this mode of
growth is common—as we know to our cost in the
case of the Couch-grass (Triticum repens)—and it is
found in varying form in many kinds of plants, as in
the suckers of trees, the offsets of bulbs, the runners
of the Strawberry (Fragaria); it is especially char-
acteristic of marsh and water plants. Its effect is to
produce large colonies, such as the great beds of
Reeds (Phragmites) or Reed-mace (Typha) which
fringe our lakes, the groves of Bent (Ammophila) on
sand dunes, and the beds of Anemones (A. nemorosa)
or Broad-leaved Garlic (Allium ursinum) of our spring
woods. In all these cases the whole colony may be
the result of the continued growth of a single in-
dividual. It should be noted, however, that such
migration is possible only so far as favourable soil
conditions extend. A slight barrier—a streamlet, a
patch of ground too wet or too dry, will arrest further
progress, and the plant must fall back on seed-dis-
persal in order to conquer further territory.

A vegetative device which, so far as its method and
value in dispersal are concerned, approaches those of
seeds, is found in the bulbils with which some plants
are furnished. ‘These are small buds—congested
shoots—borne on stems, or on leaves as in the
Lady’s Smock (Cardamine pratensis), or among the

54 PLANT MIGRATION

flowers as in many Leeks (Allium spp.). These
usually fall from the parent when mature, and being
comparatively small and possessed of considerable

Fic. 7.—CorAaL Root (DENTARIA BULBIFERA).

a, Upper half of shoot, 3; b, creeping stem, 4; c, bulbil, 3.

vitality, they may achieve a considerable dispersal
before they send out roots and fasten themselves to
the soil. An example is figured (Fig. 7). In this
plant (Dentaria bulbifera, the Coral Root, a rather

SEED-DISPERSAL BY THE PARENT 55

rare native of England) the bulbils resemble not the
smooth flower-stems of which they are axillary
branches, but the curiously knobby underground
stems from which the leaves and flowering shoots
arise.

Since seeds themselves possess, as already stated, no
power of locomotion, they have to rely on external
agents for their dispersal. These may in general be
summed up as (1) Action of the parent plant, (2)
water, (3) wind, (4) animals.

1. Action of the Parent—The Ivy-leaved Toad-flax,
or Mother-of-Thousands (Linaria Cymbalaria), is a
pretty little plant, native in central and southern
Europe, naturalized and common on old walls in this
country. Its Snapdragon-shaped purple flowers are
borne on short stalks which curve towards the light,
placing the blossoms in a conspicuous position, where
they may be the more readily visited by insects, and
thus pollinated. But when flowering is over, and the
little round fruit is ripening, the stalk twists so that
the fruit is turned towards the wall and finally pushed
into any convenient crevice: when the capsule opens,
the seeds, instead of dropping to the base of the wall
where on germination the young plants would be
smothered among stronger growths, find themselves
lodged in niches in which the young plants may
develop successfully. Many water plants have flowers
which rise into the air, following on which the flower-
stem curves and the seed is ripened below the surface,
free from the dangers of weather, of feeding water
birds, and so on.

A very common type is that in which the seed-vessel
opens at the top when the seed is mature. Gusts of

56 PLANT MIGRATION

wind, or passing animals, bending the stem, cause the
latter to spring back, casting the seeds out. When
the seed-vessel opens widely, as in the Columbine
(Aquilegia), the seeds may be cast to some small
distance. The efficacy of the arrangement is not so
obvious when, as in the Poppies (Papaver) or Bell-

Fic. 8.—Frvit oF GIANT BELL-FLOWER (CAMPANULA
LATIFOLIA). .

flowers (Campanula), the openings are small (Fig. 8),
but it is clear that these plants do not suffer from
lack of dispersal, in view of their abundance and wide
range.

But the assistance which the parent plant gives is
often of a more active and even dramatic character,
though in these cases it is usually effected not by a

SNAPPING FRUITS 57

movement of living tissue as in the last case, but by
mechanical changes taking place in tissues already
dead or dying. If we stand by a bank of Gorse (Ulex)
on a warm day we may become aware of a snapping
sound, and may possibly feel on our faces the impact
of small bodies. These are gorse seeds in process of
being distributed by the parent. In this shrub the
fragrant flowers are succeeded by short tough, hairy
pods, formed of two valves joined together by their
edges. (In reality the pod is a modified leaf folded
down the middle, the two edges thus brought together
being joined—see p. 129.) When the seed is ripe the
pod dries, and owing to unequal shrinkage of the
valves stresses are set up which at last tear the pod
suddenly asunder along its edges, flinging the seeds
violently out into new ground, where they will have
a better chance of life than if merely dropped into the
middle of the parent bush. A similar arrangement is
found in the Vetches and many other Leguminosz.
In the Cranesbills (Geranium) a very ingenious cata-
pult device may be examined. The fruit is of peculiar
structure. We might make a rough model of it by
taking five single-sticks and tying them to a broom-
handle—firmly at the points, less securely elsewhere—
and slipping a tennis-ball into each basketwork hand-
guard before turning its open side in against the
broom-stick, so that the ball cannot fall out. Imagine
now that unequal drying on the part of the sticks
tends to make each bend into a semicircular form,
which is hindered by the fastenings at either end. The
stress will eventually tear the weak fastenings at the
base: the lower end will fly up, bearing with it the
ball (representing the seed), which will be projected

58 PLANT MIGRATION

out through the open side. In the Cranesbills the jerk
is so violent that seeds may be flung to a distance of
twenty feet. One of the most efficient of all devices
of this kind is found in the Sand-box Tree (Hura
crepitans), a native of South America. By sudden
rupture and twisting of the carpels of the woody sub-
globular fruit, the large seeds of this plant are thrown

Fic, 9.—FRUIT OF GERANIUM.

a, Mature; b, ditto, with pouches raised ready to discharge
nuts ; ¢, in act of discharging.

to a distance of thirty yards, the explosion being
accompanied by a report like that of a pistol-shot. In
the common Dog Violet (Viola Riviniana) (Fig. 10)
the fruit is a three-valved capsule, which on ripening
divides; each valve assumes a horizontal position and
its edges contract till it is shaped like an open boat,
the seeds lying in a row down the middle. The sides
as they dry close in tighter and tighter on the seeds,

VIOLETS AND STORKSBILLS 59
which are in turn pinched out, and fly off with a little
snap to a distance of many feet. It is an interesting
experience to watch these tricks of Nature—much
more interesting than merely to read about them. If
plants of Vetch, Gorse, Dog Violet, Storksbill, Wood
Sorrel, Touch-me-not (to name a few), bearing unripe
fruit, be brought home and placed in water in a sitting-
room, the click of the bursting fruits will be distinctly
audible, and by spreading a white sheet the efficiency
of the devices may be tested.

Fic. 10,—FRvIT oF VioLa. #?.

a, Mature capsule ; 0, capsule open ready to discharge seeds ;
c, capsule after seeds are discharged.

A very interesting case, in which the seed is actually
buried in the soil by movements of its appendages
(portions of the parent plant which remain attached
to it), may be watched in the case of the Storksbills
(Erodium), several species of which are British plants
of frequent occurrence. Here the young fruit much
resembles that of its allies the Cranesbills. The long
rod-like axis at the lower end of which the seed is en-
closed contracts unequally in drying, so that the upper
half assumes a position at right angles to that of the

60 PLANT MIGRATION

lower half, which when dry is much twisted, like a
rope (Fig. 11). The covering of the seed itself is
furnished with stiff short hairs pointing upwards. The
whole structure when mature is cast off by the parent.
The curiously twisted appendage is hygroscopic, and
readily responds to wetness by untwisting and to dry-

Fic, 11.—FRUIT OF STORKSBILL (ERODIUM). 32.

a, Mature, twisting beginning ; b, separate fruit, fully twisted.

ness by twisting. Should it be thus caused to un-
twist when the upper end is free from obstruction the
latter will revolve slowly like the hand of a clock. But
should it meet with an obstacle in the course of its
revolutions, such as a blade of grass, the motion is
transferred to the lower end, which revolves like an

WATER DISPERSAL 61

auger, and, lengthening as it untwists, forces the seed
into the ground. Should dryness supervene, the back-
ward-pointing hairs on the seed-envelope prevent its
being drawn out again when retwisting and conse-
quent shortening take place. These Erodium fruits
are among the most interesting in the British flora,
and are well worth experimenting with.

2. Water.—Water, which forms the most frequent
and the most serious barrier to plant migration, under
certain circumstances is a very efficient agent of
dispersal. At the same time, its powers in the latter
direction are strictly circumscribed. As regards fresh
water, seeds which float may be wafted across lakes.
Rivers are more effectual, as seeds may be trans-
ported long distances in their currents and thrown up
finally on their banks or over flooded areas. When
we consider the sea, we realize that there is here a
possibility of almost unlimited dispersal provided that
the seeds are not injured by salt water, and that they
can remain afloat. It is on the latter point that the
whole efficacy of water dispersal turns. This was
long ago recognized, and investigations have been
made by many naturalists to determine the buoyancy
of seeds of all kinds. The results show that, taking
the seeds of the plants of any country as a whole, not
more than about Io per cent. are capable of floating
for more than a short period, while most of them sink
at once in either fresh or salt water. So one’s vision
of seeds transported in myriads over hundreds of
miles of sea is rudely dispelled; and the fact that many
seeds can survive prolonged immersion in sea-water
uninjured is of little account. The to per cent. of our
own flora which produce buoyant seeds are mainly

62 PLANT MIGRATION

riverside and seaside plants; and no doubt their dis-
persal is to a great extent due to streams and tidal
currents. But the majority of the hundreds of
thousands of seeds which a river transports annually
find their last resting-place in quiet backwaters or on
the floor of the sea.

It is different, however, with the flora which fringes
beaches in the Tropics. Here many of the plants
possess large fruits of great buoyancy, which are still
afloat and alive after months of tossing on the waves,
and if cast up germinate readily. These bold
wanderers are a familiar feature of Tropic plant life,
and their successful voyaging accounts for the uni-
formity of the beach flora on innumerable islands.
Even our own inhospitable shores sometimes receive
these waifs of warmer seas, brought from the West
Indies by the Gulf Stream and the prevailing south-
west winds. Of these the most frequent are the large
bean-like seeds of Entada scandens, a Leguminous
plant, which are originally enclosed in gigantic pods
several feet in length, and the more globular seeds of
the Bonduc (Guilandina bonducella), another species
of the same order. But the most famous of all float-
ing fruits is the Double Cocoanut, or Coco-de-mer, a
huge nut weighing 40 or 50 lb. and containing several
seeds a foot and a half long. It is the product of a
Palm (Lodoicea Sechellarum); cast up on the shores
of India, it was known centuries before its place
of origin in the Seychelles was discovered, and
fantastic legends grew up regarding it.

3. Wind—Everything that we know about the wind
suggests that it is a potent agent of seed-dispersal,
far excelling, for instance, that of flowing water. “ All

WIND DISPERSAL 63

the rivers flow into the sea,” that cemetery of seeds,
and their courses are at best mere spider-lines on a
map. But the wind, blowing where it listeth, is every-
where, always ready to snatch up in its arms any seed
of sufficient lightness, and to bear it away from the
parent; in fancy we can see tiny seeds borne by gales
across mountains and oceans. But we have to leave
imagination out of account, and examine prosaically
the mechanical laws according to which such trans-
port is of necessity conducted. Any body liberated
in still air will fall vertically with a velocity which in-
creases according to well-known laws until the increas-
ing resistance of the air to its passage equals the
effect due to gravity; it thenceforward continues to
fall at a uniform velocity, that velocity depending upon
the nature of the falling body. In all seeds which are
sufficiently light to be at all suitable for wind dis-
persal, the resistance of the air almost at once counter-
acts acceleration due to gravity, so that the rate of
fall may be taken as uniform from the beginning.
If the seed on liberation is carried along by the wind,
it will acquire almost immediately the horizontal
velocity of the air-current, but it will at the same time
move downward through the air with the same
velocity as if the air was still—just as a body dropped
in a railway carriage will fall at the same rate whether
the train is moving or standing still. If we measure
the speed of fall of a seed in still air, then we can
easily deduce the distance to which it will be carried
by a horizontal air-current of given velocity if liber-
ated at any given height above the ground. Thus, if
a seed liberated 100 feet from the ground falls that
distance in half a minute, and the wind is blowing at

64 PLANT MIGRATION

the rate of, say, 1,000 feet in half a minute (or nearly
23 miles per hour, a good breeze), the seed will
be carried 1,000 feet before it reaches the ground.
Its course will be represented by the diagonal AD of
the accompanying figure, where AB represents the
distance which the seed falls in the given time, and
AC the distance according to the same scale travelled
by the wind in the same period.

A Cc

= = an re eee
Baar es .
B a uitanne re aid D

Fic. 12,

But most seeds sufficiently light to be capable of
extended flights are liberated only a few feet from the
ground; they are dependent on upward eddies to raise
them if they are to achieve more than a very short
migration. That such eddies, both upward and
downward, occur on a windy day we all know from
experience; and it is they that make or mar the for-
tune of most wind-borne seeds. Only some local or
accidental excess of upward over downward eddies
will assist a seed on its journey; and as every upward
eddy must be compensated somewhere by a downward
eddy, the longer the journey is, the more such eddies
tend to neutralize each other. Over the sea—that
most formidable barrier to plant migration—eddies
do not prevail as they do over rough ground, so that,
unless by a series of lucky eddies a seed is whirled
up to a considerable elevation before it leaves the
shore, the chances of its successful passage across a
stretch of water are remote. Discussing the possi-
bility of seeds of Portuguese plants reaching the

SEEDS FITTED FOR WIND DISPERSAL 65

Azores, lying 800 miles to the westward, H. B. Guppy*
shows, from observations on the rate of fall of seeds
made by several workers, that with a 50 miles -per
hour horizontal wind the light-plumed seed of the
Common Groundsel (Senecio vulgaris), for instance,
would require to be liberated at a height of 9 miles
above the ground if it is to reach the islands: or to
express it differently, if liberated at ground-level, the
seed would need to be raised 9 miles by upward eddies
during its journey, even if corresponding downward
eddies were absent—which they certainly never are.
It is clear that if even light seeds are to achieve any-
thing more than short journeys, they must depend on
exceptional disturbances of the air, such as whirlwinds
and tornadoes.

It is now time to examine the devices by which
many seeds achieve a more or less wide dispersal by
means of the wind. Seeds possessing these adapta-
tions may be divided into three classes: (i.) Powder
seeds, (ii.) winged seeds, (iii.) plumed seeds.

By powder seeds are meant seeds of very small
dimensions. Reduction in size, if carried far enough,
greatly facilitates dispersal by wind. This is because
the resistance offered by the air is relatively greater
for a smaller body than for a larger one, so that rate
of fall decreases as the size of the falling body
diminishes—we all know how even a heavy material,
if reduced to powder, will fall more slowly than when
forming a single mass. Most of the spores of the
“Flowerless Plants ”—Ferns, Mosses, Fungi, etc_—are
exceedingly minute, and have as a result a very slow

* H. B. Guppy: ‘‘ Plants, Seeds, and Currents in the West
Indies and Azores,’’ 1917, p. 425.

5

66 PLANT MIGRATION

rate of fall, and a consequent power of long-distance
dispersal by wind. For instance, the microscopic
spore of the puff-ball Lycoperdon falls so slowly
that, if we take again Guppy’s Azores example, it
could traverse the 800 miles in a 50 miles an hour gale
if it commenced its flight only 86 feet above the
ground. Such spores are, in fact, so buoyant that
they form a normal constituent of the air—as we
know, for instance, by the rapidity with which they
will discover and germinate upon a piece of cheese,
forming bluemould—and with little doubt they are
capable of reaching under favourable circumstances
the most distant of oceanic islands. But in the Flower-
ing Plants with which we are mainly concerned re-
duction in size is not carried far enough to confer
any great amount of buoyancy. The minute seeds of
the Poppies (Papaver), for instance, fall about 10 feet
in a second. Applying again Guppy’s Azorean case,
we find that though these would cover the distance
in sixteen hours, they would fall in that time about 100
miles, unless raised during the journey to that ex-
tent by the excess of upward eddies as compared with
downward ones—a quite impracticable proposition.
In the Orchids alone do we find among the powder-
seeded Flowering Plants a really effective buoyancy;
this is due to the fact that great reduction in size is
accompanied by very loosely disposed tissue enclos-
ing the seed in a kind of net, and by the resistance to
the air thus offered, greatly reducing the rate of fall.
The seed of the Marsh Helleborine (Epipactis longi-
folia) falls only about 7s as fast as that of the Poppies,
and would thus, under the same conditions, be carried
fifteen times as far.

WING SEEDS 67

To pass on. Some seeds, many of them of con-
siderable size as compared with those which we have
just considered, have coverings which are furnished
with a membranous wing (Fig. 13, d), sometimes
extending all round the seed, as in the Elm (Ulmus),
more often placed at one side, as in the Sycamore
(Acer). The effect of such wings is to reduce the rate
of fall, imparting to the seedanirregular zigzag motion,
as in the former case, or a spinning motion as in the
latter. A Sycamore seed with the wing removed will
fall four or five times as fast as with the wing present.
But while a well-developed wing forms a more efficient
dispersal device than mere reduction in size as found
in Seed Plants, the rate of fall of wing seeds as a
whole shows that these appendages do not fit them
for anything but short voyages.

We may then pass on to consider the plumed seeds,
which possess by far the most efficient as well as the
most beautiful devices for aiding dispersal found
among wind-borne seeds. These plumed seeds belong
to many different groups of plants, and the tufts of
delicate hairs which give them their buoyancy arise in
different ways. Among the Composite, the Order
which furnishes the most familiar of our plumed seeds,
the plume is formed by modification of the upper part
of the calyx, which in so many common plants is
small, green, and leaf-like; the lower part of the calyx
in the Composite is tough, persistent, and close-
fitting, forming an additional protection for the seed.
The plume springs either from the top of the seed, as
in the Thistle, or is borne on a slender stalk, as in the
Dandelion. It consists of a ring or radiating mass
of hairs of beautiful delicacy, often bearing short

68 PLANT MIGRATION

when the fruit is young or during damp weather, but
on a dry day when it is ripe they spread out, and the
seed, breaking away from its attachment, is floated
off by the wind. In many species the plume or
pappus is only lightly attached to the seed, so that if

FIG. 13.—WING-SEEDS AND PLUME-SEEDS.

a, Mountain Willowherb (Epfilobium montanum), 7; 6, Dandelion
(Taraxacum officinale), 2 ; c, Mountain Avens (Dryas octopetala), } ;
d, Scotch Fir (Pinus sylvestris), 2; e, Reed-mace (Typha lati-
folia), 2.

on a voyage an obstacle is encountered the seed drops
off, while the now useless parachute drifts away. But
though the plume seeds of the Composite are the
largest and most beautiful among our common plants,
they are not the most efficient for dispersal. The fluffy
seeds of the Willowherbs (Epilobium) and of the
Willows (Salix), for instance, fall at a slower rate than

PLUME SEEDS 69

those of almost any Composite, while by far the most
buoyant seed in the British flora is that of the Reed-
mace (Typha). In this case the seed itself is minute,
and is situated on a very slender stalk, from near the
base of which springs a tuft of delicate hairs. This
seed takes thirty-four seconds to fall twelve feet.
Using once more the Azorean example, it could cross
the 800 miles of sea if it had an initial elevation of
34 miles, or was raised to that amount during the
sixteen hours occupied by its passage.

Summing up, then, we find that the plume seeds are
the most efficient of all seeds for extended flights by
the agency of the wind. If the efficiency of the seeds
of the Reed-mace, the most buoyant among British
plants, be taken as 100, the efficiency of the Willow-
herbs is between 60 and 70, of Willows 45 to 70, the
best of the Thistles 35 to 40, Dandelion 25. Even the
best of the winged seeds are much less efficient, Elm
and Scotch Fir being about 20, Sycamore and Ash
9 or 10. Of powder seeds, the efficiency of several
Orchids tested ranges from 35 to 65, and Broomrapes
(Orobanche) from 20 to 25. Most of the powder seeds
are far below these, the efficiency of seeds of Papaver
dubium, for example, being only 4°5 on the same scale.
This last figure is representative of the many small-
seeded plants in the British flora such as are found
among the Crucifere, Caryophyllacee, Scrophu-
lariacee, etc. The relative efficiency of such compara-
tively large seeds as those of many of our Legumin-
ous plants would be about 1 on the same scale.

4. Dispersal by Animals—The coverings of many
seeds are provided with hooks or barbs, and others
with stiff hairs, which render them liable to become

70 PLANT MIGRATION

entangled in the hair or fur of passing animals. Ex-
amples will occur at once to the reader, as this char-
acter occurs in the case of many familiar plants,
such as Burdock (Arctium), Enchanter’s Nightshade
(Circea), Avens (Geum), and so on. Without doubt
these hooked fruits often secure a wide local dispersal
by the aid of cattle, sheep, rabbits, and so on: the
state of one’s trousers or stockings after walking the
autumn woods is often very suggestive in this regard.
Again, herbivorous quadrupeds eat seeds in quantities,
many of which are capable of germination after
passing through the animal’s body. But while the .
dispersal obtained by such means may often aid in
spreading a species over a tract of land, it does not
generally aid in the crossing of barriers, such as
mountains or sea, on account of the limitations to the
movements of such animals. To arrive at a true
estimate of the importance of the animal kingdom in
regard to plant migration, we have to study the
movements, habits, and food of birds, to whose
wanderings neither mountains nor seas set a barrier.
Seeds are carried about by birds in two ways—by
becoming attached to their feathers or feet, or by
being eaten and subsequently ejected. The first case
belongs to the class of phenomena which we have just
been considering, save that the smooth plumage of
birds, and their frequent preening of their feathers,
tends to keep their coats free from extraneous
material. But at least in wet weather minute seeds
must often cling to feathers and to feet, and mud
which may contain seeds may easily be present on a
bird’s toes during flight. More important is the ques-
tion of endozoic dispersal—where seeds are trans-

DISPERSAL BY BIRDS 71

ported in the alimentary canal of birds. Some .
families, like the Finches and Tits, which eat great
numbers of seeds, are inimical instead of helpful to
dispersal, because the seeds which they devour are
crushed and afterwards digested. But in many cases
the seeds are swallowed whole, and are usually in no
way injured by their passage through a bird’s body.
Frequently, indeed, the seeds have not, to run the
gauntlet of the digestive juices of the alimentary
canal, being disgorged from the stomach along with
other hard material prior to digestion. Birds which
live on berries or other juicy fruits are the most im-
portant in seed-dispersal. As Barrows says: “The
seed-eaters are not the seed-planters; on the contrary,
the insectivorous birds more often sow seeds than
the true seed-eaters.” “Seeds which simply contain
nourishment are eaten and destroyed, while seeds
which are contained in nourishment are eaten and
survive.”* It is for this reason that, if we look under
a tree on which Blackbirds or Thrushes perch, we
shall often find young plants of Bramble (Rubus), Ivy
(Hedera), Holly Ulex), or Yew (Taxus). There can
be no doubt that birds eat and subsequently eject vast
numbers of seeds still capable of germination; many
observations and calculations might be quoted. But
when we come to apply the facts to the problem of
long-distance dispersal, or the passage across serious
barriers, we find that important limiting factors must
be taken into account. The digestion of birds is re-
markably rapid, food being ejected from a half to
three hours after being eaten, so that a bird eating

* W.B. Barrows: ‘‘Seed-planting by Birds.’’ Report of the
Secretary of Agriculture, U.S.A., 1890, p..281.

72 PLANT MIGRATION

seeds and at once flying off in a straight line at, say,
50 miles per hour could not convey seeds more than
150 miles. Secondly, many observations show that
on migration birds generally travel with empty
stomachs and clean plumage and feet. It is clear,
therefore, that, as in the case of wind dispersal, we
must look to exceptional circumstances, not normal
conditions, to provide opportunities for long journeys
on the part of seeds. But for the transfer of seeds
from France to England, for instance, or from
England to Ireland, it is clear that birds furnish a far
more efficient medium than wind or water. In one
important particular, dispersal by animals has a great
advantage over dispersal by wind—that it is practi-
cally independent of the weight of the seeds. Thus,
the heaviest of British seeds, the acorn, is carried
about by Rooks, just as the hazelnut is scattered by
Squirrels, or a head of Burdock fruits by a passing
sheep.

Having thus arrived at some idea of the high
efficiency for dispersal of many kinds of seeds, it is
with some little surprise that we observe—as we may
on any country walk—that the plants which arise
from these are in general no more abundant or more
widely distributed than others which possess seeds
devoid of any apparent advantages in this respect—
seeds which cannot fly nor float, nor cling to a passing
creature, and which are not eaten to any extent by
birds so far as observation goes. The truth is, we
have to remember, as emphasized in a previous
chapter, that the world is already densely populated
by plants, all of which survive by reason of their being
specially fitted for their several habitats. They have

DIFFICULTIES OF COLONIZATION 73

fought in the great struggle for existence, and have
established their right to the places which they occupy ;
they will not readily give way to any newcomer whose
seeds happen to be imported into their strongholds.
Of course exceptions can be quoted, where plants
accidentally or intentionally introduced by man into |
new areas have not only maintained a foothold, but
have spread remarkably. Note the case of the Sweet-
brier (Rosa eglanteria) in New Zealand, of the
Mexican Bryophyllum calycinum in many Tropical
countries, of the American Monkey-flower (Mimulus
Langsdorfu) in our own islands; but these are ad-
mittedly exceptional. It is nearer the truth to say
that the troubles of an immigrant only begin where
dispersal ends; and that the chance of seeds carrying
out a successful migration is much greater than the
chances of their giving rise to a new colony when that
migration is successfully accomplished. Every head
of the Reed-mace liberates about a quarter of a million
seeds of marvellous lightness; yet the Reed-mace does
not increase in the country, nor is it a particularly
abundant plant even in its chosen habitats. The Fox-
gloves (Digitalis purpurea) in a wood shed, each plant,
say a hundred thousand seeds; yet on an average only
one of these attains maturity, otherwise the species
would become more abundant in the area. This
enormous destruction of seed is largely due to compe-
tition. The reception which a plant receives in its
new home is the thing that matters, and that may
usually be summed up in the phrase “ House full.”
Nevertheless, the present flora of Great Britain is
in the long run the result of migration from surround-
ing areas; so that ease of dispersal has undoubtedly

74 PLANT MIGRATION

played its part in the building up of our vege-
tation.

Conditions under which rapid dispersal has
obviously an advantage occur when by some ex-
ceptional circumstances the natural vegetation is
destroyed within an area, as by a flood or landslide.
Such conditions are produced artificially each season
over much of our own country by the operations of
agriculture. Their results will be considered in a
subsequent chapter.

CHAPTER TV
SOME INTER-RELATIONS OF PLANTS AND ANIMALS

THE most important and fundamental difference
between the animal and plant worlds is this: plants
possess the power of manufacturing their food out of
the inorganic materials of which it is composed, while
animals cannot do this. Give an ordinary plant access
to water with a pinch of mineral salts in it, to the air,
and to sunlight, and by the agency of chlorophyll—the
green colouring-matter of the leaves—the miracle will
be accomplished, and dead materials transformed into
living substance. - Animals, on the other hand, are de-
pendent for their food-supply on organic material—
that is, on either plant or animal substances; and since
they cannot live by taking in each other’s washing—
in other words, by eating each other—it follows that
the animal world is dependent on the plant world for
its continued existence. A porpoise may live on
herrings, herrings on small fry, fry in turn on minuter
organisms, and so on down the scale; but their ulti-
mate source of food is the tiny Algz which swarm in
the water—the Plankton in Hensen’s original sense—
which, alone in this chain, can build up their bodies out
of the sea and air. That these minute plants can sustain
the enormous drain upon them due to their use as a
food-supply by myriads of larger organisms is due to
their vast numbers and rapid increase. Sea-water
a

76 PLANTS AND ANIMALS

favourable for plankton life may contain several
millions of individuals in every litre (about 1? pints);
while as a fair estimate for the seas which surround
our own islands “at least one” organism for every
drop has been suggested.*

In the great abysses of the ocean, where vegetable
life is absent, the strange creatures which live there
in utter darkness prey upon others, and they again on
others which belong to lesser depths, the ultimate
source of life being again the minute surface organ-
isms which, possessing chlorophyll, can make organic
out of inorganic substances by the energy obtained
from sunlight. Thus only is life made possible in

the green hells of the sea
Where fallen skies and evil hues and eyeless creatures be.

On the land, the dependence of animals on plants is
in large measure direct, as the supply of vegetable
food is abundant and widespread. The largest land
animals are all vegetable feeders; so are the majority
of our own native mammals, and in a great measure
our birds; while most of the creatures upon which the
flesh-eating animals prey are themselves vegetable
feeders. The distribution of land animals over the
globe is thus dependent in large measure on the dis-
tribution of plants. On account of the profusion and
variety of plant life, and the fact that most vegetable
feeders can thrive on various sorts of plants, few
animals are restricted in their range by the presence
or absence of any particular species or genus, but
complete dependence of this sort is by no means un-

* See A. H. Cuurcu: ‘‘ The Plankton-phase and the Plankton-
rate,’’ Journal of Botany, June, 1919, supplement.

‘ANIMALS DEPENDENT inci inane ee

known. The agen of some Bt iods au instance,
eat the leaves of one plant only; the Peacock (Vanessa
io) and the Small Tortoiseshell (V. urtice) are cases
in point. The caterpillars of both these species feed
exclusively on the Common Nettle (Urtica dioica).
Should the efforts of farmers and gardeners succeed
in exterminating this unwelcome plant, these two
butterflies would disappear from the Earth. Some-
times absolute mutual dependence is found on both
the animal and vegetable sides. The American Yucca
filamentosa, often grown in our gardens, depends
solely on the little moth Pronuba yuccasella for its
pollination, just as the insect is absolutely dependent
on the plant (see p. 80), and other species of Yucca
have each its particular dependent moth, which feeds
on no other plant, and whose flowers are pollinated
by no other.

Apart from such special cases, the general depend-
ence of animals upon plants is obvious, and is by no
means confined to food-supply. Animals of all grades,
from human beings to Caddis Worms, construct
houses of vegetable materials; trees are the chosen
home of large sections of our fauna, and the herbs of
the field are the world for millions of tiny beings.

There’s never a leaf or a blade too mean
To be some happy creature's palace.

Turning to the other side of the picture, no such
general dependence of the plant world upon the
animal world is found, but the inter-relations of the
two are many and varied, and in the absence of animals
of one kind or. another whole groups of plants would
become extinct. The cases where plants derive their

78 PLANTS AND ANIMALS

food-supply wholly from animals are indeed rare, save
near the bottom of the vegetable scale, and most of
such parasites are minute; one of the most noticeable
in our own country is the fungus Cordyceps militaris,
which may be found growing on the dead bodies of
larve or pupe which it has killed—a little scarlet, club-
shaped plant, about an inch in height. But some of
the most highly organized plants obtain portions of
their food-supply from animal sources. Mention has
already been made of the Sundews (Drosera), Butter-
worts (Pinguicula), and Bladderworts (Utricularia),
which capture live insects, etc., by means of sensitive
organs (as in the first two cases) or ingenious traps
(as in the last), and subsequently digest them, and
they will be dealt with later on (p. 186). Then there is
the Venus’ Fly-trap (Dion@a) and the well-known
Pitcher Plants (Nepenthes), which actively, as in the
former case, or passively, as in the latter, catch insects
and digest them, by means of leaves modified in very
extraordinary ways. In all these instances the advan-
tage lies entirely on the side of the plant, just as in
the case of most of the plant-eating animals the advan-
tage is wholly with the animal. But in a large number
of instances—many of them of a most interesting
nature—the inter-relations are such as to benefit both
the actors, each obtaining from the other what is
useful to it. One of the most conspicuous and wide-
spread relationships of this kind is that prevailing
between flowers and insects, the insect receiving food
in the form of nectar, and at the same time carrying
pollen from flower to flower, without which transfer
no fertile seed would be formed. To this interchange
of favours we shall return later (p. 81); meanwhile, it

CECROPIA AND ITS GUESTS 79

will be well to consider a few of the cases in which
the relationship between plant and animal is continu-
ous and more intimate, the two living in very close
relations to each other: to such cases the term sym-
biosis or “living together” is applied by naturalists.
The relations existing between certain trees and some
species of ant are of high interest, and illustrate well
this phase of life. The Candelabra Tree (Cecropia
peltata) of the South American forests is liable
to attack by leaf-cutting ants (Gcodoma), which
climb trees and bite off thousands of leaves; these
they cut up on the ground and carry to their nests,
where they form a basis for the growth of certain
small fungi which are a favourite food of the ants
(compare the cultivation of mushrooms as practised
by gardeners). The Candelabra Tree protects itself
from these ravages by forming an alliance with
another kind of ant (Azteca). Along the hollow
stems are little pits through which the ants easily
bore, and reach the convenient houses within, where
they live and bring up their young. At the base of
the leaf-stalks, where the greatest danger lies from
the leaf-cutting ants, little tufts of hairs are situated,
among which are small white masses of nutritious
material much liked by the ants, and collected by them
and stored within their houses. So that these desir-
able trees are swarming with Aztec ants, fierce little
creatures—“it is one of the most bellicose ants that
I know, and its sting is most irritating,’ writes
Kerner—which congregate especially at the leaf-
stalks, the point of attack of the leaf-cutters. The
advantages of these arrangements to both the trees
and the Aztec ants are obvious,

80 PLANTS AND ANIMALS

A very remarkable instance of a different kind is
supplied by the relations existing between the Ameri-
can species of Yucca and the small white-winged
moths of the genus Pronuba. The following succinct
account is given by Professor G. H. Carpenter: * “The
female of these moths has not only the palps of the
first maxille developed, but the region of the maxille
(palpiger) whence they spring produced into a pair
of long, flexible, hairy processes. By means of these
she collects from the anthers pollen, which she deliber-
ately carries to the stigma to ensure fertilization.
With her piercing ovipositor—a most abnormal
development among moths—she bores through the
tissue of the pistil, and by means of the flexible egg-
tube, protrusible beyond the ovipositor, lays her eggs
close to the ovules of the Yucca. The caterpillar
when hatched feeds on the growing seed of the plant,
which would never develop were it not for the action
of the Pronuba moth. This action is most wonderful,
in that the moth herself gets no benefit from it. Her
food canal is degenerate, and her jaws, useless for
sucking, are devoted altogether to the gathering of
the pollen; she does not feed in the perfect state.
Doubtless her ancestors did so, and were first attracted
to the Yucca in search of honey, though the act of
pollination is now performed only for the sake of the
offspring.”

Among certain lower animals and plants symbiotic
connection is often most intimate. For instance, in
the body-wall of certain Sea Anemones and Holo-
thurians there are small green cells which were long

* G. H. Carpenter: “Insects: Their Structure and Life,’’
Pp 300.

SEA ANEMONES AND ALG 81

believed to be part of the animal, and which puzzled
naturalists because they contained chlorophyll, that
remarkable green substance characteristic of plants,
which gives to them the power of forming food out
of its raw inorganic materials. These cells are now
known to be minute seaweeds (Algz), which spend
their lives in the animal tissues to the benefit of both
organisms. The plant, by virtue of its chlorophyll,
absorbs carbon dioxide, decomposes it, and gives out
oxygen, which is eagerly seized on by the animal.
The animal in its turn liberates carbon dioxide, which
is required by the plant. Similar relations exist
between Algz and some of the lowly Radiolarians
and Foraminifera; in these cases, the animals being
very minute, the plant partner plays a more con-
spicuous role. It is noteworthy that these Algz are
quite capable of living and multiplying separately,
free from the body of the animals, and the animals also
are capable of pursuing an independent existence.

Let us turn now to the relations existing between
flowers and insects, which form one of the most pic-
turesque and romantic features of field life, and of
which the materials for study and observation are
ever at our own doors. What isa flower? A flower
is a group of modified leaves set apart for the business
of sexual reproduction. The essential parts or sporo-
phylls are of two kinds, which may be borne on the
same flower or on separate flowers on one plant, or
on separate plants. These are the stamens, bearing
pollen grains (or microspores), from which male cells
arise; and carpels, which contain ovules, each enclos-
ing an embryo sac or megaspore, in which is an ovum

or female cell.
6

82 PLANTS AND ANIMALS

Each stamen consists usually of a slender stalk, the
filament, bearing an oblong head, the anther, which
contains four chambers, or pollen sacs, filled with
pollen grains; these, when mature, escape into the air
by the rupturing of the walls of the chambers.

Each carpel contains in its lower part an ovary,
while its upper part presents to the air a surface
charged with nutrient substance, the stigma, which is
often raised on a slender stalk, the style.

To secure the production of seed, the first neces-
sary step is pollination, or the transfer of pollen from
the stamen to the stigma. When this is effected
—the means will be considered immediately—and a
pollen grain alights on the surface of the stigma, which
is usually sticky or hairy to aid its retention there, the
pollen grain commences growth, and sends out a
slender tube (the pollen tube), which pursues its way
through the substance of the stigma, down the style,
into the ovary, and from its tip a male cell passes
out and fuses with the ovum. In most flowers the
pollen tube is not called on to make any great effort
of growth, the distance between stigma and ovary
being very small; but occasionally, as in Crocus and
Lily, this may amount to half a foot. The result of
this act of fertilization is that the ovum and ovule
grow, the former forming eventually the embryo, or
young plant, the latter the seed in which the embryo
is enclosed. In order that fertile seed may be pro-
duced it is often necessary, and usually desirable, that
the pollen which reaches the stigma should not belong
to the same flower, but to a different flower of the
same species; cross-pollination being the rule among
seed plants, self-pollination the exception. To secure

WHAT IS A FLOWER? 83

the former, and to avoid the latter, many highly inter-
esting devices are found, materially affecting the
structure and development of flowers.

The essential parts of a flower, then, consist of
stamens and carpels. Flowers consisting of no other
parts but either or both of these are not common, but
we may compare, for example, the rarely produced
flowers of the Duckweeds (Lemna), in which a tiny
group of two stamens and a carpel represents one
flower, or, according to some views, a group of three
flowers. More commonly the flower is much more
composite, consisting mostly of four sets of organs,
arranged in whorls or rings, or more rarely in close
spirals. In the centre is a group of carpels; outside
them—in other words, slightly lower on the stem—
a ring, or two rings, of stamens, few or many; then a
ring of petals, forming the corolla, usually coloured,
leaf-like, and conspicuous; and outside of them a ring
of sepals, forming the calyx, generally green and leaf-
like. The main function of the calyx is protective; it
encloses the essential organs and guards them till
they are mature, when the flower opens and stamen
and stigma play their parts. The calyx is usually
tough, and often covered with hairs, or with a sticky
substance, to keep the flower safe and ward off the
attacks of insects or other small devourers. If we
turn to the corolla we find a singular variety of size,
form, and colour. To account for this, it is necessary
to consider the means by which pollen is distributed.
There are two chief ways in which pollen is conveyed
from flower to flower—by means of the wind, and by
means of flying insects. If we examine wind-pol-
linated flowers, such as Hazel (Corylus), Scotch Fir

84 PLANTS AND ANIMALS

(Pinus), or Reed-mace (Typha), we note the small size
of the flowers and the great abundance of pollen. Com-
pare these with insect-fertilized flowers, such as Butter-
cup (Ranunculus), Flax (Linum), Snapdragon (Antir-
rhinum), or one of the Orchids. In these the flowers
are much larger owing to the increased size of the
petals, which are of brilliant colour and of various
shape. Pollen is mostly much reduced in quantity,
since insects flying direct from flower to flower afford
a far more economical mode of distribution than is
offered by the wind. The pollen grains, moreover, are
sticky and covered with tiny spines or knobs, to render
them more liable to adhere to the body or head of an
insect; the pollen grains of wind-fertilized flowers
being, on the other hand, smooth, dry, and dust-like.
Again, these insect-pollinated flowers usually possess
little glands which secrete nectar, the sugary syrup
which by digestion in a bee’s body becomes honey.
Here, then, is the inter-relation established: the insect
helps the plant by carrying its pollen from flower to
flower, and in its turn is helped by the provision of
delicious food. And what about the showy petals, and
the fragrance that so often marks these entomophilous
flowers? They are advertisements, designed to catch
the attention of the necessary insects as they fly about.
Not only does the corolla by its bright colour attract
insects, but markings of various shapes and tints upon
the petals are generally held to be honey-guides—
sign-posts directing the insects to the nectar and to
the pollen. These are especially conspicuous in many
of the irregular flowers to which reference will be
made shortly, in which the insects are encouraged to
approach the flowers in a particular way. An example

FLOWER ADVERTISEMENTS 85

of such markings, as seen in the genus Erodium, is
shown in Fig. 14. It is interesting to note the various
ways in which flowers render themselves conspicuous
in order to attract insects. In the majority of Seed
Plants, such as the Buttercup, Pea, Rose, Foxglove,
it is the corolla, formed either of separate petals, as
in the first three, or of petals fused together, as in the
last, which by its bright colour or colours renders the
flower noticeable. In other species the calyx takes on

Fic. 14.—FLOWER OF ERODIUM PETR#UM. 3.

the function of advertisement, the corolla being in
comparison insignificant—we may study examples of
this in the Anemones, Hellebores, and Marsh Mari-
gold (Caltha palustris). It is worth examining this
last, to see how its coloured sepals resemble and fulfil
the same function as the petals of its cousins the
Buttercups. Or, again, sepals and petals may com-
bine in showiness, both sets being brightly coloured
in one or more tints—compare the Columbine
(Aquilegia), Larkspur (Delphinium), Milkwort (Poly-
gala), and the marvellous flowers of Orchids. In the
great group of the Monocotyledons, indeed, to which

86 PLANTS AND saline a

the Otchids belong, sepals ae petals ssuigity combine
in form and colour to form one corolla-like envelope
(then called a perianth). In many other plant groups
—for instance, the Dipsacacee (such as the Scabious),
Umbellifere, and Composite—conspicuousness is
obtained by a grouping together of a large number of
small flowers. In the Cow Parsnep (Heracleum
Sphondylium) the outer petal of the marginal flowers

Fic. 15.—UMBEL OF ASTRANTIA CARNIOLICA, SHOWING PETAL-
LIKE RING OF COLOURED LEAVES (BRACTS). 4,

of the large umbel is much enlarged, which enhances
this effect. In Astrantia, an interesting genus of
Umbellifere, the bracts take on the appearance of a
ring of large petals, and surround the group of small
flowers (Fig. 15). The same thing may be noticed
in the outer blossoms of the close flower-head of the
Field Scabious (Knautia arvensis). In many Com-
posite the process is carried still farther; in the Com-

LEAVES INSTEAD OF PETALS 87

mon Daisy (Bellis perennis) the outer flowers have
each a long strap-shaped expansion of the corolla,
which is of a different colour (white) from that of the
corollas of the inner flowers, which are yellow. In the
Dandelion (Taraxacum officinale) all the flowers have
a yellow strap-shaped corolla. In the Guelder Rose
(Viburnum Opulus) the outer flowers are entirely
devoted to advertisement, consisting each of a big
white corolla, while only the small inner flowers pos-
sess stamens and pistil and are capable of producing
the brilliant scarlet berries. In a cultivated form of
this, commonly called the Snowball Tree, the adver-
tisement flowers only are present, forming a globe of
white blossom, and no fruit is produced in conse-
quence. The Dwarf Cornel (Cornus suecica), a little
Dogwood growing on many Scottish moors, bears
what looks like a white flower with a purple centre.
On examination it is seen that the four white petal-like
structures are really foliage-leaves, which have taken
on the duty of advertising the group of small purple
blossoms which they enclose (Fig. 16). A similar and
very gorgeous effect is produced in several Spurges
often seen in greenhouses, such as Euphorbia fulgens,
E. splendens, and E. punicea; in these the upper
foliage-leaves are large and coloured brilliant scarlet,
the flowers which accompany them being quite small.
These aggregations of flowers with their flaunting
flags are in general an invitation to all comers; the
nectar in the blossoms lies open to every hungry
insect, and pollination is effected in a rather promis-
cuous and messy way; not only flying insects—bees,
butterflies, beetles, and flies of many sorts—but also
ants and other creatures which creep up the stems

88 PLANTS AND ANIMALS

from the ground, assemble for the feast, and incident-
ally transfer from flower to flower pollen which may
adhere to their bodies.

In a large number of flowers such general feasting
is discountenanced, insect traffic is regulated, the

Fic. 16,—DwarF CorNEL (CorNuUs SUECICA), 3, AND SINGLE
FLOWER ENLARGED,

visits of insects of little or no service to the plants is
discouraged, and special arrangements are made to
attract and minister to the needs of those insects
whose visits are of most benefit. Except where

FLOWERS—THEIR FORBIDDEN GUESTS 8g

flowers are borne in clusters, creeping creatures like
ants are of no service; for in the course of the journey
“by land” from one flower to another, there is a
strong probability of any pollen which the insect may
be carrying being rubbed off before the next blossom
is reached; small flying insects are likewise frequently
useless. In many plants the visits of such pedestrians
and small fry is very distinctly discouraged. Of
different devices which serve this end, the most con-
spicuous and effective include barriers to the passage
of stem-climbers, and devices in the flower preventive
of the visits of unwelcome guests. We may take a
few instances from among British plants, which the
reader may with a little diligence find and study for
himself. Several members of the Pink family (Caryo-
phyllacee) produce a sticky secretion which is a very
effectual bar to the passage of small walking animals.
In the English Catchfly (Silene anglica), Night-flower-
ing Catchfly (S. noctiflora) and the Nottingham Catch-
fly (S. nutans), hairs are present all over the leaves and
stems, from the tips of which a gummy substance
exudes, which is a fatal trap for smallinsects. Kerner,
in his interesting book, “ Flowers and their Unbidden
Guests,” states that on the sticky stems of the last, in
the Tyrol, he identified the remains of sixty different
kinds of insects—ants, ichneumons, beetles, bugs,
flies, and so on. The Red German Campion (Lychnis
Viscaria) has an extremely sticky ring below each
joint of the stem and inflorescence, which is most
fatal to any creature which attempts to climb to the
flowers. Other instances, such as the Petunia or
Moss Rose, will occur to the reader. Another familiar
kind of barrier is the presence on the calyx or involucre

go PLANTS AND ANIMALS

of a palisade of stiff hairs or prickles, such as may be
studied in the Thistles; in some plants a downward-
pointing ring of stiff hairs at each joint serves the
same purpose. In the Japanese Wineberry (Rubus
pheenicolasius), often grown in gardens, the calyx,
like the stem, is densely clothed with bright red slender
spines (Fig. 17). It opens to allow the inconspicuous

(hy Y 7
aah Ss
fA ESE
(A2Y <.
* i 4
=k

Fic. 17.—JAPANESE WINEBERRY (RUBUS PHENICOLASIUS),

Leaf and panicle, 2; flower after pollination; ripe fruit,
both slightly enlarged.

petals to expand, and then closes again and resumes
its protective role till the scarlet fruit approaches
maturity.

But it is in the flower itself that we find the most
ingenious arrangements to encourage useful and dis-
courage useless visitors, to assist the former to pol-
linate the flower, and while offering nectar to the wel-

CHANGE IN SHAPE OF FLOWER gi

come guest to deny it to the unwelcome. The first
stage in this specialization is that the flower, instead
of having its axis vertical, and facing the sky, is turned
on its side by the curving of its stalk, and looks out
horizontally. The effect of this is to cause a flying
insect on approaching the flower to alight in a par-
ticular position—namely, on the lowest petal. Follow-
ing on the adoption of this attitude the next stage in
development is seen in the parts of the flower begin-
ning to alter their shape and position relative to each
other and often also their colour. Thus, beginning
with a quite regular flower, we can arrange a series
showing more and more asymmetry. The tendency
is generally for the lowest petal to become enlarged
and often conspicuously marked, providing a broad,
convenient platform on which insects may alight, while
the remainder form walls and roof, protecting the
important parts within and by their shape, which is
often narrowed and tubular behind, barring access to
all but chosen visitors. To find a full series illus-
trating these transformations we do not need to go
to plants widely separated in their affinities. In the
Buttercup order (Ranunculacee) alone every grada-
tion may be found. The flowers of the Buttercups
themselves are upright and quite regular. In the
Larkspur (Delphinium) the flower is turned on its
side, and a puzzling combination of coloured sepals
and petals—five bright blue unequal sepals and a
single large purplish petal of peculiar shape with a
long hollow spur behind—produces a quite irregular
blossom. The process is carried farther again in the
Monkshood (Aconitum), in whose well-known blue
flower the sepals and petals combine to produce a

_

92 PLANTS AND ANIMALS

strikingly irregular blossom, with the upper sepal
arching over into a great hood protecting the rest of
the flower. In such irregular flowers the essential
parts—the pollen-producing and pollen-receiving por-
tions, or stamens and stigma—also alter their position
and form, and are so placed that an insect, visiting the
flower to obtain nectar (which is generally stored at
the back, well out of the way), must of necessity
receive pollen on its body, and probably deposit pollen
on the stigma. To describe the variety and ingenuity
of these devices as found in different flowers might
well occupy several chapters, and only one or two
examples can be quoted here; familiar wild flowers are
chosen, and the reader should examine them for him-
self to understand their structure. In the well-known
Pea type, one great petal arches over the flower; two
narrow ones stand one on either side; the remaining
two stand on edge below, with their margins in con-
tact, enclosing the stamens and pistil. An insect visit-
ing the flower alights naturally on the keel or pair of
lower petals. Pressed down by its weight, these open,
often with a sudden movement like bursting, and dust
the insect with pollen. Compare also the flowers of
the Snapdragons (Antirrhinum) and Toadflaxes
(Linaria), in which the upper and lower lips of the
corolla meet like a closed mouth, which can be forced
open only by a strong insect like a bee, and is safe from
predatory visits of smaller fry (Fig. 18). In the Sages
(Salvia) the corolla is tubular at the base; there is a
large lobed lip on which visiting insects alight, and a
hooded roof above arching over the stamens and pistil,
which are placed close against it, overhanging the
entrance to the corolla-tube, at the base of which the

POLLINATION MECHANISMS 93

nectar is stored. The stamens, only two of which are
developed, have each a hinge near the top, the part
above the hinge being like a curved rod supported
near its middle. These two curved rods stand nor-
mally in a vertical position, so that their lower ends
partly block the entrance to the tube; the pollen is
borne at their upper ends. Should a bee insert its
head down the tube in search of nectar, it pushes the
lower ends of the hinged rods upwards, with the

Fic. 18, —FLOWER AND FRUIT OF LINARIA PURPUREA. #,

result that their upper ends swing downward against
the bee’s back, dusting it with pollen just at that part
of its body which, if the bee should visit a rather older
flower, would come in contact with the stigma, the
slender stalk of which (the style) increases in length
during the period of flowering, and is in consequence
the more liable to be encountered.

Only one more instance can be referred to, which
can be tested by the reader any summer day wherever

94 PLANTS AND ANIMALS

any of our native Orchids grow. In these, the most
highly specialized of all plant groups as regards pol-
lination by insects, the general arrangement of the
flower is often somewhat similar in a general sense
to the last case; but here the sepals and petals which
between them form the platform, tube, sides, and roof
of the flower, are all separate and often differently
and elaborately coloured. The essential organs are
greatly modified and hardly recognizable at first.
There is only one stamen, producing two clusters of
pollen, which are embedded in the roof of the flower.
Each possesses a slender stalk which terminates in a
little sticky disc which projects from the general sur-
face. The pollen grains are held together in a mass
by fine threads, and the whole with its stalk—the pol-
liniwm—resembles a lemonade bottle in shape. The
stigma is also embedded, forming a sticky surface in
the roof of the flower behind the stamen. When an
insect inserts its head into the flower, its forehead
comes in contact with the sticky ends of the pollinia,
which adhere, so that on leaving the flower the insect
flies away with the pollen sticking to its forehead like
two little horns. And now a remarkable thing hap-
pens. The stalks of the pollinia, drying rapidly in the
air, contract unequally, and become curved, so that
the pollinia bend forward into a horizontal position.
When the insect visits another flower and thrusts in
its head, the pollen consequently comes in contact
with the sticky stigmatic surface farther down the
tube, and cross-pollination is effected.

In the cases of many of these highly specialized
flowers, one is no less struck with the perfection of
the arrangements made for preventing self-pollina-

SPECIAL Casale hiiasavivuint 95

tion, than those dee to securing cross- Eaithawdiba
But in a few, on the contrary, self-pollination is
specially arranged for.

It must be pointed out that the insects which pol-
linate these specialized flowers have in many cases
acquired modifications in their structure correspond-
ing to the modifications in the flowers which they
frequent. Inthe more specialized forms, indeed, plant
and animal have become entirely dependent on each
other; the plants would become extinct in the absence
of the special insects through whose agency they are
able by pollination to produce fertile seed; and the
insects would likewise die out if the flowers to whose
nectar and pollen they look for food were not
available.

As regards the kinds of insects which visit flowers
for food, these are very numerous and belong to
almost every section of that large class. In many,
such as Neuroptera, Orthoptera, Hemiptera, Coleop-
tera, there is very little special adaptation for their
flower-feeding habits, and these insects visit flowers,
such as the Umbellifere, in which the nectar and
pollen are freely exposed, and lie open to all. Many
of the Diptera, or Flies, are in the same case; but in
some families, such as the Bombyliide, high special-
ization for securing food from flowers is found: the
creatures are provided with elongated probosces for
sucking nectar even when it is deeply hidden, and no
other food is used by the insects in their adult stage.
But it is among the long-tongued Bees and the
Lepidoptera (Butterflies and Moths) that the highest
degree of adaptation in this direction is found; and
the modifications are associated with those flowers

96 PLANTS AND ANIMALS

which have become most highly specialized for insect
pollination, and most completely dependent on it. In
the Bees the legs have become much modified for the
gathering of pollen, and the mouth is a long flexible
sucking-tube which when not in use is carried rolled
up in a spiral. The pollen, on which food alone the
young bees are fed, is gathered and stored among
rows of hairs on the legs, and in the more highly
specialized forms it is wetted with honey so as to form
a compact mass, easily carried and easily removed
when the nest is reached. The balls of pollen thus
formed are sometimes nearly the size of the body of
the bee, and may contain one to two hundred thou-
sand grains of pollen. The formation of the mouth
is beautiful and complicated, adapted to the rapid
sucking up of nectar even if deeply placed in the
flower. The nectar is stored in the body of the bee,
and subsequently transferred to the waxen honey-
cells in the hive. In the Butterflies and Moths the
mouth parts are also modified for sucking, and as
these insects do not build nests or take care of their
offspring as Bees do the mouth is formed solely for
the purpose of securing the nectar which is their only
food. The proboscis varies greatly in length in
different groups, according to the kind of flower
which they visit. In the Owl Moths (Noctuide) it is
sometimes only eight millimetres (4 inch) long; in
many of the Butterflies it is about half an inch. In
the Hawk-moths it attains a remarkable development,
necessitated no doubt by the habit of these insects of
not alighting on or entering a flower, but hovering
in front of it as a Humming Bird does, and sucking
up the nectar while thus poised. The proboscis of

EFFICIENT POLLINATORS 97

the Convolvulus Hawk-moth measures 65 to 80 milli-
metres (24 to 3} inches), and some of the Tropi-
cal allies of this moth have probosces twice or
even three times that length. These species feed on
the nectar of flowers with tubular corollz of corre-
sponding dimensions. Most of the Hawk-moths feed
only at dusk, and as the time is short they take advan-
tage of their powers of rapid flight to visit (and inci-
dentally to pollinate) a very large number of flowers
in a short period. Moreover, in common with most
of the more specialized flower-feeding insects, they
do not visit the flowers of different species indis-
criminately, but dash to blossom after blossom of
whatever single species they have selected. Her-
mann Miller records watching Humming-bird Hawk-
moths (Macroglossa stellatarum) at work at the sum-
mit of the Albula Pass; one visited 106 flowers of
Viola calcarata in under 4 minutes; another 194 blos-
soms of the same plant in 6? minutes.

The day-flying Butterflies display none of this rest-
less energy. The sunshine is pleasant and the day
long. They wander aimlessly in their beauty from
flower to flower, sun themselves on the warm ground,
or “whirl through the air with the first good com-
rade that by chance appears.” They are the flowers
of the air, and our country rambles are made more
joyous by their careless companionship.

CHAPTER V
PLANT STRUCTURES

In the course of the preceding chapters a number of
the more striking modifications displayed by the
different organs of plants have been described briefly.
Reference has been made to the increased length or
thickness of the roots in plants of dry places, and the
weakness or absence of root-system of many water
plants. Corresponding variation in stems has been
noted. The remarkable leaves of desert and water
plants and of some carnivorous species have been
mentioned. The profound alteration in flowers which
have adapted themselves to pollination by insects has
been sketched; as also the great variety in the shapes
of fruits and seeds, correlated to the methods by which
they are dispersed. It may be well to consider the
question of plant structures on a broader and more
systematic basis, and, as before, to connect them where
possible with the external factors which have caused
their modification and to which they are the plant’s
response. These factors are physical, or chemical, or
biological, and affect the plant mainly through the
agency of the soil, the atmosphere, or living
organisms.

“The living plant is a synthetic machine.” Under
proper working conditions of heat, moisture, and light
it builds up its body by absorption of inorganic

98

ROOT AND SHOOT 99

inaterial, liquid and gaseous, through its roots and
leaves. For the present purpose we may take our
typical plant as consisting of subterranean roots and
aerial leaves on the one hand, and aerial flowers on
the other—the roots and leaves concerned especially
with carrying on the life of the individual, the flowers
with perpetuating the race. In addition, an aerial
stem is usually present, on which the leaves and
flowers are displayed, and through which the food
materials pass dissolved in water. Of these parts, the
lower ones (the roots, and sometimes the stems) are
immersed in the soil, while the upper ones (the leaves
and the flowers—which are groups of modified leaves—
and usually the stems) are immersed in the atmo-
sphere. All the parts have acquired their form and
fulfil their functions under control of the particular
medium which surrounds them: it becomes necessary
to preface any discussion of their characters and
uses by a brief survey of the characters of these
envelopes.
_ While the atmosphere is familiar to us as the
medium in which we ourselves live and move and have
our being, and while its chemical and physical proper-
ties are known in outline to every schoolchild, it is
different with the soil; not only because, unlike the
atmosphere, soil varies much in composition and
character, but also because the soil is in fact a very
complex product, offering many difficult problems to
the investigator; it is only of late years that the
scientific study of the soil has been placed on a sound
basis; our knowledge of it is still far from complete.
Whence does soil arise? How is it that the surface
of the land is usually covered with a layer of fertile

100 PLANT STRUCTURES

material? The answer is to be found, in the first
place, in the decay of rocks under the influence of
natural agents. Heat and frost, rain and drought, by
slow degrees break up the surface of the hard material
of which the solid crust of the Earth is built up. The
débris thus formed is washed into streams by rain,
or scattered by wind. A stream flowing into the sea,
and charged with the débris of the land, deposits the
coarser material near its mouth, while the finer
particles are carried farther. In dry regions wind
plays a similar part. And so, while the materials
which composed the surface layer of the cooling
primitive Earth may have been tolerably uniform in
composition, the débris derived from them has ever
tended to get sorted out, as, for instance, into sand
and mud at river mouths, or sand and dust in dry
regions. In the course of ages the sorted materials,
buried beneath subsequent deposits, have been formed
through heat and pressure into rocks, which, when at
length again brought to the surface by earth movement
and exposed to the agents of disintegration, have been
resolved once more into sands, clays, and so on. In
the long history of the Earth this sorting process has
been repeated till now large tracts of rocks and of
soils are composed mainly of sand or mainly of clay.
The prevalence of these two kinds of material arises
from the abundance in the primitive crust of the
substances of which they are composed. Silica (oxide
of silicon), the material of which ordinary sand, as
well as quartz, flint, etc., is composed, is of extreme
hardness and insolubility, and its small crystals and
fragments, disintegrated from the rocks, remain
almost indestructible as grains of sand. Clays, on the

SOP PAR EICLES IOI

other hand, are derived from silicates (compounds of
silicon and oxygen with various metals such as
aluminium, calcium, magnesium, potassium, sodium,
or iron). These substances mostly disintegrate more
completely into very small particles, which when wet
cohere into a sticky mass and form clays. Along
with the humus matter they include all the colloids of
the soil. These latter bodies consist of the extremely
minute—indeed, ultra-microscopic—particles, having
in consequence of their small size a great total surface
in proportion to their mass. In virtue of this, they
function as the chief absorbents of the soil, holding
water in enormous quantities, and abstracting and
retaining till used by the plants the bases of the
various substances applied as manures. Another
constituent of the primitive crust was lime (oxide of
calcium). Unlike the preceding substances, lime is
readily soluble in acid water, and so is washed out of
the rocks and carried in solution to the sea. Marine
animals of many kinds—such as Molluscs, Corals,
Foraminifera—extract the lime from the sea water
and use it in large quantities to build up their shells
or skeletons. This material slowly accumulates at the
bottom of the ocean as generation after generation of
animals passes away, becomes at length consolidated
by heat and pressure, and through earth movements
may eventually appear above the sea to form land, in
the form of limestone or chalk. Exposed to the
weather, it is once more slowly disintegrated; the
lime passes off again in solution, the impurities being
left behind; a limy soil results.

On a great plain, devoid of hills or rivers, composed
of different rocks, and subjected to the agents of

102 PLANT STRUCTURES

disintegration, we can conceive that over each kind of
rock a soil would be formed corresponding closely to
the materials of which that rock is composed. In
sections formed by quarrying, by the cutting action
of rapid streams, and so on, we may often see this.
Below is the solid rock. Its upper layers tend to be
loose and rotten owing to the action of percolating
water, etc. They merge into a layer of stony débris,
where the harder portions still retain their rock
character, while the softer are disintegrating into clay
or sand. Above this the rock is wholly disintegrated
into a soil, the upper layers of which, mixed with plant
débris, and consequently of darker colour, are full of
the roots of living plants descending from the sward
which covers the surface of the ground. In practice,
however, such close conformity of soil to underlying
rock is not always found.

Various distributing agents are ever at work—
wind, water in an especial degree, and on sloping
ground the action of gravity. In northern countries,
besides, the ice of the Glacial Period has in its passage
caught up all the loose surface material, added
immensely to its volume by grinding down the rocks,
and flung the products broadcast over the country, so
that old sea bottoms may be strewn over coastal lands,
sands and gravels over clayey rocks, and limy soils
over areas where no limestone exists. The soil over
much of the British Isles is formed from the surface-
layer of these glacial deposits, which—tough, intract-
able, sterile—underlie the soil often to a great depth,
where they rest on rock. In southern England
.the covering of glacial deposits is absent, since the
ice-cap did not extend beyond the Thames valley; beds

SAND, CLAY, AND HUMUS 103

much older than the Ice Age, often of a gravelly or
clayey nature, occupy the ground, and from these the
present soils are derived.

There is another constituent of soils of primary
importance for vegetable life, which results from the
decay of the generations of plants which have gone
before. When plants die, their bodies are decomposed
by the agency of bacteria. Some of the constituents
pass off as gas or water, but there remains an amount
of solid matter (humus) which mixes with the soil
and is of the utmost importance for plant growth.
Nitrogen, which forms the greater part of the atmo-
sphere, cannot in the gaseous state be absorbed by
plants, although they spend their lives surrounded by
it. It is a necessary substance in the plant’s economy,
and through the action of soil bacteria, which change
the nitrogenous matter in humus into soluble nitrates,
plants are able to utilize this store.

The ordinary soils of our fields may be defined as a
mixture of sand, clay, and humus. A soil which is too
rich, or too poor, in any one of the three will support
plant life with difficulty.

The roots of plants require also a due amount of
both water and air if they are to fulfil their functions
adequately. An examination of the minute structure
of the soil shows that it consists of angular particles
of very various size—the larger ones classed as sand
and consisting largely of silica; the smaller, which
decrease in size beyond the limits of microscopic
vision, mainly of clay (silicates) and humus. A film of
water clings round each particle, and between the
particles the chinks are filled with air. For healthy
plant growth a nice balance between these constitu-

104 PLANT STRUCTURES

ents is required. Should sand be in excess, the soil is
impoverished, since silica contains no nutriment, and
it is rendered too dry, as on account of the relatively
small surface of the sand grains in proportion to
their mass it retains but little water. Should there
be too little sand, percolation of air and water is
hampered; the soil tends to become water-logged
and badly aerated, and turns sour. Should humus
be absent, the nitrogen-producing bacteria cease their
activities and the soil is sterile, as may be tested by
digging up some subsoil, or soil from the deeper levels
to which roots or other organic matter have never
penetrated. An excess of humus, on the other hand,
results in the accumulation of acid products inimical
to bacterial growth: in consequence decay is arrested,
and a mass of plant débris forms, highly charged (for
humus is very spongy) with acid water and badly
aerated, which is unsuitable for vegetable growth: we
may study an extreme case of such conditions in our
peat bogs. Should water be in excess in soils, air is
forced out in proportion, and the roots cannot breathe.
Too much air means a corresponding diminution of
water, and the plants suffer from drought.

“The soil is not merely a reservoir for the mineral
nutrients of plants, but is the seat of complex physical,
chemical, and biological actions which directly and
indirectly influence soil fertility. These actions are
intimately associated with the organic matter of the
soil and its bacterial inhabitants. Mineralogy and
inorganic chemistry, though helpful, are no longer
capable of solving soil problems. Biochemistry and
bacteriology, with their modern conceptions of
colloids, absorption phenomena, enzymes, oxidizing,

COMPOSITION OF SOIL 105

reducing, and catalytic actions, etc., are now rapidly
extending our knowledge of the soil as a medium for
plant growth.”*

Such, then, is the nature of the soil in which plants
grow, and from which, by means of innumerable
elongated cells (the root-hairs) proceeding from near
the tips of the roots, food materials dissolved in water
are absorbed; these food materials being produced
partly by solution of mineral constituents contained
in the soil, partly by the action of bacteria in breaking
up organic matter. Soil suitable for plant growth may
be looked on as consisting of a mineral framework,
carrying in its meshes water (about three-tenths of its
volume) and air (about one-tenth of its volume);
mixed with the mineral particles is humus of varying
amount; and supported largely by the humus is a vast
population of organisms, both animal and vegetable,
from earthworms to bacteria, whose activities are
often essential, generally beneficial, and occasionally
prejudicial to plant growth.

The root of a young plant grows downward into
the soil under the influence of gravity. Its tip, which
has to force its way through the rough material of
sand and clay, is beautifully protected by a special
root-cap, which covers the growing point as with a
cushion. The surface of the root-cap is slimy, to aid
it in slipping forward, and its cells, which are being
worn away constantly, are replaced by the growth of
the interior. Should an obstacle such as a pebble be
encountered, a root will bend round it and then return
to its former direction. Branch roots are given off on

* W. B. Bortromtey in ‘‘ The Exploitation of Plants,” edited by
F, W. Oliver, 1917, p. 12.

106 PLANT STRUCTURES

all sides at an angle to the main stem, these also
tending in a mysterious way, if their course is dis-
turbed by an obstacle, to resume their former direction
of growth; the branches again divide, till at length a
complicated root-mass is formed, sometimes of great
extent, and capable of extracting water from a large
volume of soil. Save for continued growth, the roots
show little change in comparison with those exhibited
by the aerial parts of plants; safely immersed in the
soil, they heed not day or night, storm or calm, but
steadily pursue their main function of supplying liquid
food material to the green parts overhead.

In many instances roots do not accomplish their
work single-handed, but only in co-operation with
certain lowly organisms; and these cases are so inter-
esting and of so much economic importance that
reference should be madetothem. The little swellings
or tubercles upon the roots of Leguminous plants,
such as Clover, are familiar to most of us. These are
caused by the stimulation due to colonies of bacteria
(Bacillus radicicola), which live in the root-tissues as
internal parasites. These bacteria feed on the sap and
cell-contents of their host, but they supplement this
food-supply by absorbing nitrogen direct from the
atmosphere, which the host cannot do, though it can
and does use the nitrogenous compounds which the
bacteria manufacture. It is a case of symbiosis (see
p. 79), each organism supplying food useful to the
other; but the significance of the phenomenon is that
through this agency nitrogen becomes added to the
soil as the plants decay, and increases its fertility;
and thus the cultivation of a crop of, say, Lucerne
becomes a matter of great economic importance in

MYCORHIZA 107

farming operations, and the presence of Clover in
pasture is a source of increasing wealth.

Again, in the roots of most of our forest trees, both
hardwoods and conifers, and of many other plants
such as the Ericacee and Orchidacee, the root-hairs
are replaced by minute fungi known as mycorhiza,
whose branches take on the function of absorption,
while the roots in turn absorb the material which the
fungus collects. The fungus obtains from the roots a
direct and convenient supply of carbohydrates; the
host obtains from the fungus a ready supply of salts
and of nitrogenous compounds. In the case of the
forest trees and some other plants, the fungus forms
a close felt around the roots; but in the Heaths, etc.,
it penetrates the roots, living in the cells and in some
instances, as in the Ling (Calluna vulgaris), permeating
the whole plant, even to the seed-coat, so that seed
and fungus are sown together. Since the higher
partner of the symbiosis cannot mature without the
lower, this is an obvious advantage to the former, as
the two develop together from the commencement of
growth. Where the fungus is not present in the seed,
the seedling has to rely on its presence in the soil.
And so, if we wish to raise any of our common
terrestrial Orchids from seed, we try to ensure the
presence of the fungus by using soil in which the
species has been growing already.

The state of mutual dependence existing between
seed plants and mycorhizic fungi sometimes ends in
the higher organism ceasing to manufacture its food
by means of green leaves, and depending wholly on
the lower for its sustenance. This is the condition to
which some of our Orchids have come, such as the

108 PLANT STRUCTURES

Bird’s-nest (Neottia Nidus-avis), which does not
produce leaves or chlorophyll, but sends up from its
fungus-infested roots merely a scaly brown stem
topped with brown blossoms, matching curiously the
dead leaves among which it grows (Fig. 31, p. 182).

In contrast to these the case of certain other
Orchids may be quoted, which have also lost their
leaves, but in a very different manner. In their case
the roots, creeping over the bark of trees on which
the plants perch as epiphytes, have become green and
flattened, like the fronds of some of our native Liver-
worts; they have assumed the functions of leaves: in
them the process of photosynthesis is carried on;
and the leaves themselves, thus supplanted, have by
degrees disappeared.

Like many other parts of plants, roots are often
used for the storage of reserve supplies of food or of
water. For this purpose they become much thickened,
and this thickening is*the most conspicuous change
which roots usually undergo. Note the fat roots of
many plants which grow in dry or arid places, such as
the Sea Holly, Dandelion, and many desert plants and
alpines. The thickening is often accompanied by
increase in length, as the roots range far in search
of water. Another point to notice is that though
normally roots differ considerably from their asso-
ciated stems in general appearance, and also in their
minute structure, as in the arrangement of the vascular
strands, the two are related. Stem structures are often
produced at various points on roots; the suckers sent
up by many kinds of trees offer an example. Con-
versely, roots are readily produced even from the
upper portions of many stems—else how could we

ROOT AND STEM 109

grow cuttings? Where roots are succulent—that is,
when they have a reserve of food stored in them—
cuttings of them will conversely produce stems. A
classical instance of such interchangeability of func-
tion is the young willow which Lindley bent down and
buried the top till it rooted; the original roots were
then dug up and raised into the air, when they pro-
duced leafy branches, and the tree grew upside down
henceforth. Underground stems, also, of which there
is a great variety, take on many of the characters of
roots, and from an examination of a small piece of one
it is often difficult to tell whether we are dealing with
a root ora stem. The point at which root joins stem
is, in fact, in many instances, so far as function is
concerned, fixed only so long as the level of the
surface remains fixed: we can often alter it by
“earthing up” or by stripping away the soil. In
Tropical forests, where the air is moist, hot, and
still, roots—or branches which serve only as roots—
descend through the air from heights almost equalling
those to which stems ascend; while, on the other hand,
in hot, poorly aerated swamps, roots send up from
the mud into the air stem-like structures (pneumato-
phores) through which they may breathe, as in the
case of the Swamp Cypress (J axodium distichum) of
Florida. The primary differences between the two,
in fact, do not prevent the one from taking on the
general characters of the other, and from functioning
as the other, when the environment changes.

The stems of plants may be looked on from two
points of view—as a framework devoted to the display
of the leaves and flowers, and as pipe-lines connected
with the nutrition of the plant, conveying raw

110 PLANT STRUCTURES

materials from the roots to the leaves, and manufac-
tured products from the leaves to all growing parts:
It is the former relation which has mainly determined
the forms of stems. Even a very slender stem can
convey a vast amount of water and food to a plant
which is transpiring or growing actively, as we can
test roughly by weighing a pot shrub as it begins to
come into leaf, and again a week later, or comparing
the growth of a pea with the size of its stem at the
base. The surprising variation in length, thickness,
form, position, and branching of stems is the plant’s
response to external conditions—such as exposure,
the competition of neighbouring plants, and so on—
which resolve themselves ultimately into questions of
wind-pressure, of temperature, of moisture, and in
particular of light. The first duty of most stems is
to spread out the leaves so that they may receive a
maximum share of sunlight, and the complicated
systerns of branches with which we are so well
acquainted are devoted to this object, the leaves them-
selves helping materially by the positions which they
assume. This familiar and typical kind of stem,
upright and column-like, beautifully constructed to
bear the weight of leaves and branches, and to resist
wind-pressure, alone furnishes a delightful study; but
it can be dealt with only very briefly, as also some of
the modifications which it undergoes under special
circumstances.

To plants which have not taken to a terrestrial
existence, and which still inhabit their ancestral home
in the water, the stem problem is comparatively
simple. A flexible shaft capable of withstanding wave
and current action suffices so far as mechanical con-

STEMS FLEXIBLE AND RIGID III

siderations go; such shafts—as we may observe by
watching the Oar-weed (Laminaria) on an exposed
coast—are effective under very arduous conditions.
Those Seed Plants which, evolved on land, have later
returned to the water, such as the Pondweeds (Pota-
mo geton), have often redeveloped a stem of a similar
kind—a flexible shaft possessing a sufficient tensile
strength. The specific gravity of such plants does not
exceed that of the medium in which they are immersed,
and the stem has not to support the weight of leaves
and branches. It is, therefore, not surprising to find
that the longest, though by no means the bulkiest, of
all plants, are found in the sea. Some of the Oar-
weeds (Macrocystis) of the southern and western
Oceans attain lengths which have been estimated at
500 to 1,000 feet; but these gigantic Seaweeds are
nevertheless slender plants, suspended lightly in the
water. But after the colonization of the land by the
aquatic flora numerous serious problems had to be
encountered and solved before plants in an aerial
environment could rise boldly into the air. Extremes
of temperature unknown in the water had to be faced.
Along with a greatly increased loss of water owing to
the presence of air and direct sunlight, the area over
which water might be absorbed became largely
reduced, the roots alone being now available. The
whole weight of branches and leaves and fruit had to
be borne by the stem, not only in calm but in storm.
No wonder that to meet these conditions, or to avoid
such extremes as were avoidable, aerial stems often
display great complexity and diversity of structure
and form. From the mechanical standpoint the tall
stem is especially interesting on account of the beau-

II2 ; PLANT STRUCTURES

tiful structural adaptations by which it meets the
various stresses to which it is subjected. The problem
before the plant is to combine a minimum quantity of
material with a maximum of strength and rigidity.
Strands of toughened fibre, so disposed as to meet the
stresses most advantageously, are characteristic of
such stems. In the case of many tall annuals, such as
the larger Umbellifere, the principle of the hollow
column is largely employed; in proportion to the
strength obtained, this is far more economical than a
solid column: and economy is particularly necessary

Cc. b.
Fic. 19. ARRANGEMENT OF STRENGTHENING MATERIAL IN
Root (4) AND IN STEM (b) (DIAGRAMMATIC).

in such annual stems, where the time available for
construction is short. Transverse partitions at
intervals provide stiffening of the whole; and as the
efficiency of the toughened longitudinal strands
increases with their distance from the centre, the
stems are often ribbed, the strands occupying the ribs,
with softer substance between. This form of con-
struction may be contrasted with that obtaining in the
roots. In the latter the greatest mechanical stress is
in the form of a longitudinal pull caused by swaying
of the stem under wind-pressure. To meet this the

STEM STRUCTURE 113

vascular strands are arranged, not marginally, but in
a central bundle, where they can best meet stresses of
the kind. In most trees the stems are solid; here
economy of material is less urgent, as a long period
of years is available for their building up; the great
amount of cell-space thus made available for food-
storage is a valuable asset to the plant, as is evident
from a consideration of the vast amount of fresh
tissue produced in a brief period by a deciduous tree
when it bursts into leaf. As this material, stored in
the stems and roots, has to be sent up to the twigs
dissolved in water, and as during the whole period of
growth vast amounts of water are transpired, an
elaborate and complete pipe-system is intercalated
with the reinforced-concrete structure of the tree
trunk. Pumped up by the roots, and sucked up by the
leaves, water and food pass rapidly from the ground
to the topmost twig of the loftiest tree.

To explain the massiveness of a tree trunk we
have to remember that, while the cross-section of any
structure varies as the square of its linear dimension,
the volume varies as the cube of the same. If we
double the dimensions of a tree, we increase its weight
eight times, but the strength of the trunk is increased
only four times. If a tree 100 feet high is supported
on a stem 6 feet in diameter, a tree 200 feet high of
the same proportions would need a stem not 12 feet,
but over 17 feet in diameter, to be supported equally
efficiently. This proportion increases rapidly: a
similar tree 300 feet high would need a stem 30 feet in
diameter; a tree 1,000 feet high would require a stem
180 feet in diameter, or 32,400 square feet in cross-

section. We see, then, why a limit of tree growth is
8

114 PLANT STRUCTURES

rapidly reached, at about 300 feet, and why the trees
which grow to that height have trunks which are one
of the wonders of the world, exceeding 30 feet in
diameter, or about 100 feet in circumference.
Climbing stems represent efforts on the part of
plants to economize material by utilizing the rigidity
of neighbouring plants, and by reaching to the light
on their shoulders. Here, as in aquatics, the rope
type of stem is in evidence; it resembles a garden
hose, offering great flexibility and conducting capacity,
but without rigidity to support its own weight, much
less that of the leaves and flowers which it bears. To
secure support, the stem itself (or branches of it), the
leaves, or the stipules (leafy projections on either side
of the junction of leaf and stem), are used. Some-
times support is obtained by twining (compare Con-
volvulus, Grape-Vine, Vetch), sometimes by adherent
discs (Virginia Creeper), or aerial roots (Ivy), often
by mere scrambling, often aided by reflexed hooks on
leaf and stem (Bramble, Cleavers). The mechanism
by which twining is accomplished is of great interest.
It is an effect of unequal growth of the different sides
of the stem. If the unequal growth were confined to
one side, the stem would eventually form a coil, or
series of circles. But the region of greatest growth
keeps shifting round the stem, with the result that the
tip of the shoot describes a circle or ellipse, like the
hand of a clock pointing successively in all directions.
The stimulus is due, as in the case of the erect growth
of ordinary stems (which usually display similar move-
ments in a less degree) to gravity. Sometimes the
movement, or nutation, is in the same direction as that
of the hands of a clock (e.g., in the Hop); more fre-

CLIMBING STEMS 115

quently it is in the opposite direction, as of a clock-
hand moving backwards. The result of this movement
is that if the shoot encounters, say, an upright stem, it
will lap round it in a spiral manner, and unless the said
stem be quite smooth and unbranched, the twining
shoot will be eventually supported by it. How effec-
tive the twining habit is as regards economy of build-
ing material may be seen from comparing the weight
of the stem of a Hop with that of some tall herbaceous
plant of the same altitude, and bearing an equal
weight of leaves and flowers. The tendril-climbers are
still more efficient, for they avoid the increased length
of stem which arises from a twining habit. They
grow straight up towards the light. Both the top of
the growing shoot and the spreading tendrils which
arise from it are continuously revolving in search of a
support. When a tendril encounters one (such as a
twig), the contact produces a stimulus which results
in the tendril taking several close turns round the
support. Nor does the action stop there, for usually
the lower unattached portion of the tendril contracts
into a spiral, drawing the stem closer to the support,
and woody growth ensues, by which the tendril be-
comes exceedingly tough, often stronger than the
stem itself.

One other point concerning climbers may be noted.
Did they exhibit in a marked degree that bending
towards the light which is characteristic of most
plants, they would often defeat their own object, as
they would grow away from possible supports. But
they grow boldly up into an overhanging canopy,
apparently confident of their power to ascend into the
light and air which exist above. In the root-climbers,

116 PLANT STRUCTURES

such as the Ivy, this bending away from the light is
very marked; the stem presses closely to the bark or
stone on which it creeps, probing every cranny, and
the numerous rootlets by which it is attached are
developed only on the dark side. But when the plant
is old enough to flower, then branches devoid of roots
grow out towards the light, so that the blossoms may
be borne in the open, where they may be seen and
visited by the numerous insects which, in their search
for nectar, pollinate them.

In contrast to the extreme development in length
found in the stems of climbing plants the extreme
reduction of stem found in many plants of dry places
may be referred to. The Crocus, for instance, has an
abbreviated upright stem of which each year’s growth
is distended for the storage of food: one year’s
growth dies away as the next enlarges, so that the
well-known bulb-like corm is produced. Compare the
“roots ”—really the stem—of Montbretia, in which
the annual growths remain, the result being a knobby
structure like a string of onions. In bulbs reduction
in length is carried still farther, the stem forming a
broad cone from the surface of which spring a number
of modified leaves, forming fleshy scales swollen with
food material; these surround and protect the bud,
which when it grows produces green leaves and a
terminal flower-shoot; growth is continued by axillary
scale-leaved shoots situated among the scale leaves,
which in due course themselves produce green leaves
and flowers. These compact food-charged stems take
up their position well below the ground, out of reach
of intense heat or drought, and during the favourable
season send up rapidly into the air their leaves and

STEMS FUNCTIONING AS LEAVES 117

flowers, after which they remain dormant till the
following year.

It has been seen that unless a plant is a parasite or
saprophyte, using as food ready-made organic
material, it is necessary that it should possess a suffi-
cient expanse of green (i.e., chlorophyll-bearing)
tissue for the purpose of assimilation. This is the
essential function of the leaves; but before leaving
the study of stems it should be pointed out that they
usually assist, and sometimes entirely replace, the
leaves as organs of food-manufacture. We have seen
how in dry places—whether physically dry, from
direct scarcity of water, or physiologically dry, owing
to reduced activity on the part of the plant due to
unfavourable conditions, such as obtain in cold
regions, or on poisoned ground like salt-marshes or
bogs—leaf surface tends to be reduced, to avoid ex-
cessive loss of water. In such plants as the Cacti,
and the Euphorbias which so closely mimic the cactus
form, this reduction is carried to its limit. Leaves are
absent, and the stems, greatly swollen so as to store
water, take up the process of assimilation, and per-
form it satisfactorily. In more rapid-growing plants,
a sufficient area for assimilation may be obtained by
abundant branching, as in the Gorse, in which leaves
are present only in the seedling stage. Inthe Brooms
(Genista) the leaf-development is often weak, but the
stems sometimes make up for this by bearing green
flattened wings. In the Spanish Broom (G. sagittalis),
a straggling shrub inhabiting dry places in south-
west Europe, the few ovate hairy leaves, produced in
spring, soon fall; but the slender branches bear
several (two to four) broad green wings, which act as

118 PLANT STRUCTURES

leaves, and persist for a couple of years, when they
pass away, leaving slender, round, brown stems. In

Fic. 20,—GENISTA SAGITTALIS, 3,

our native Broom (Sarothamnus scoparius) a similar
modification may be observed, though of less degree.

LEAVES 119

Sometimes stem-structures assume a very leaf-like
form, as in the Butcher’s Broom (Ruscus aculeatus),
where the ultimate branches are ovate and quite flat,
and might be taken for true leaves but for the fact
that they bear on their surface flowers, and subse-
quently berries. The leaves themselves are in this
plant reduced to minute scales, and from their axils
these flattened branches spring. In fact, where leaf
reduction takes place, the process of assimilation is
often shared in varying degree by the leaves, the
stipules, and the stems. Among our native plants, as,
for instance, in the Leguminosze and Rosaceze, the
reader may find for himself many interesting examples
for examination.

But the large majority of the Seed Plants bear well-
developed leaves, to which the process of assimilation
is practically confined.

LEAVES vary surprisingly in size, shape, and arrange-
ment, features which are closely related to the char-
acters of the stems which bear them, the object being
the most advantageous display of the chlorophyll in
relation to the light-supply. In general they natur-
ally take the form of a broad thin blade, protected
as may be necessary against extremes of weather, and
guarded against the obvious danger of being dried up
by a thin waterproof covering or cuticle outside the
epidermal layer of cells. In leaves we find the same
beauty of mechanical construction as is seen in stems.
The problem is again that of securing maximum eff-
ciency with minimum expenditure of material. To
give as great a surface as possible, the leaves are as
broad and thin as is consistent with safety, the ques-
tion of damage by wind being an important control-

120 PLANT STRUCTURES

ling factor. The veins, or vascular bundles, act effi-
ciently as strengtheners of the thin surface; to pre-
vent tearing at the leaf-edges the veins are often
looped along the margin; while in indented leaves the
extremities of the indentations are strengthened with
special tissue. When one surface of the leaf faces
the sky, as in most cases it does, this surface is
strengthened against the weather, and the stomata are
arranged mostly on the lower surface. Where occa-
sionally the leaves hang normally in a vertical posi-
tion, as do the mature leaves of the Gum Trees
(Eucalyptus), both sides are protected, and the
stomata are borne on the two faces equally. In the
Water Lily, again, whose leaves float, the upper face,
which alone is exposed to the air, bears the stomata,
which are present in unusual numbers—nearly 300,000
to the square inch; the leaf surface is toughened to
resist rain and wind, and waxy to prevent water from
lying on it and so interfering with transpiration. The
presence or absence of a leaf-stalk, again, is often
clearly related to the light question. In the Water
Lilies the continued lengthening of the elongated
petiole causes the older leaves to float clear outside of
the younger ones. In many biennial herbs, where
food is stored up during the first season in. prepara-
tion for the flowering effort in the second, a similar
arrangement prevails—note the leaf-rosettes displayed
by Spear Thistle (Carduus lanceolatus) and Herb
Robert (Geranium Robertianum), as also especially in
winter by perennials like the Dandelion (Taraxacum
officinale) and Ribwort (Plantago lanceolata). Where
stems spread horizontally, as the lower branches of
trees, the leaves are arranged more or less in one

LEAF MOSAIC 121

plane, in such a manner that overlapping is reduced
to a minimum (Fig. 21). This is well seen in hori-
zontal branches of the Elm and other familiar trees.
In the plant chosen for illustration (Azgara micro-
phylla, a Chilian shrub), an interesting arrangement

Fic, 21.—AZARA MICROPHYLLA. },

obtains. One of the pair of stipules which subtends
each leaf is itself leaf-like, and stands at an angle, so
that a mosaic is formed of true leaves (the larger ones)
and stipules (the smaller alternating ones). On all
stems the leaves are arranged not at haphazard,
but according to definite rules. Sometimes they

122 PLANT STRUCTURES

are grouped in circles (whorls) at certain points
of the stem, as in the Bedstraws; often in oppo-
site pairs, arranged criss-cross, as in the Syca-
more; most frequently in a series of spirals. The
result in all cases is the same—it allows of as great an

Fic, 22,._LEAF OF WEINMANNIA TRICHOSPERMA. }.

interval as possible between any leaf and the one
immediately below or above it, and gives to all an
equal share of light. The indenting of leaves, as in

the Sycamore, or their division into separate seg-
ments, as in the Ash and Horse Chestnut, is of un-

DISSECTED LEAVES 123

doubted advantage as allowing light to pass through
to lower layers of leaves; it also materially diminishes
the danger arising from excessive wind-pressure. In
the former case there is often a wide space
between the divisions of the leaf; but where this is
not required, the parts of the leaf fit closely together,
to secure a maximum of surface. A particularly
pretty example is seen in the Chilian shrub Wein-
mannia trichosperma (Fig. 22). Here, to avoid the
loss of the area between the leaflets, the mid rib steps
in, developing triangular wings which fill the spaces.
It might be objected that the plant might have saved
itself much trouble by producing, while it was about
it, a simple undivided leaf covering the whole area.
It is difficult to answer such suggestions. Probably
the present form of the leaf best meets the conditions
of wind, rain, and light under which it lives. Pos-
sibly its present form is bound up with its ancestral
history. “It must be acknowledged,” says D. H.
Scott, “that nothing is more difficult than to find out
why one plant equips itself for the struggle with one
device and another attains the same end in quite a
different way.”

During cold and tempestuous weather the presence
of leaves may be a danger to the plant rather than a
help; and where seasonal variations are such that
strongly contrasted periods of favourable and un-
favourable weather occur, such as the summer
and winter of our own climate, many plants have
adopted the device of shedding all their leaves: this
is especially characteristic of the largest plants (the
trees), which would naturally suffer most from un-
favourable weather. The fall of the leaf is accom-

124 PLANT STRUCTURES

plished by means of the formation of a transverse
layer of corky tissue across the base of the leaf-stalk,
combined with a weakening of the layer of cells im-
mediately above. Prior to the perfecting of these
arrangements for dropping the leaf, all the useful
materials in it are withdrawn down the stem, so that
only an empty skeleton is shed; the scar that remains
is not an open wound, but is well protected by the
corky layer before mentioned.

Stipules and bracts need not delay us in this
sketchy survey of plant organs. They are leaves,
generally of rather small size, placed, the former one
on either side of the point where a leaf-stalk emerges
from the stem, the latter singly below a flower; they
are present in some plants, absent from others. They
function in the same way as ordinary leaves, and in
the earlier stages of growth are of use protectively.
Occasionally the stipules exceed or even replace the
leaves, as in the native Lathyrus Aphaca, where the
leaf is reduced to a tendril, and the pairs of broad
“leaves” are really the stipules. The bracts, in their
turn, sometimes take on the “advertisement” function
of the petals, as we have already seen (p. 87) in the
case of certain Euphorbias.

The leaves of water plants offer several points of
interest. Where they are entirely submerged, and,
protected against the drying influence of wind and
sun, they are of filmy texture. Broad blades are
seldom met with, the leaves being usually either finely
dissected or strap-shaped. The floating leaf, on the
contrary, as already described in the Water Lily, is
strongly built up, to withstand wave action and rain;
it is usually broad and entire, which simplifies the

LEAVES OF WATER PLANTS 125

problem of avoiding submergence; and the stomata
are confined to the upper side, which alone is in con-
tact with the atmosphere. Those water plants which

Fic. 23.—SPRING SUCCESSION OF LEAVES OF MATURE PLANT OF
ARROW-HEAD (SAGITTARIA SAGITTIFOLIA). 4,

raise their leaves into the air, on the other hand, have
leaves of a variety of shapes, which in most respects
approach those of land plants. An interesting pro-

126 PLANT STRUCTURES

————

gression of leaves illustrating all three stages may be
watched in spring in the Arrow-head (Sagittaria
sagittifolia), The first leaves produced are entirely
submerged, and conform to the usual ribbon shape
and delicate texture. Those which follow float on the
surface. In them the lower part is contracted into a
flaccid winged petiole, the upper part being expanded
into an oblong floating blade with a waxy surface to
keep the leaf dry on the upper side. These in turn
give way to the characteristic aerial arrow-shaped
leaves of summer, which approach in character the
leaves of land plants, and are borne on stout, stiff
petioles capable of resisting wind and wave.

Coming now to FLOWERS, it is possible here to refer
only to a few macroscopic or “naked-eye” characters
and modifications; the full study of the flower and
its essential functions being a matter for the labora-
tory and the high-power microscope, as very minute
structures are involved. As briefly described in
Chapter IV., flowers are groups of modified leaves
arranged mostly very close together at the ends of
branches, the tip of the shoot being often expanded
into a receptacle (very well seen in the Composite—
e.g. Dandelion) for the accommodation of the
crowded floral leaves. Just as the foliage leaves have
become modified to carry on to the best advantage
the process of assimilation, so the different series of
floral leaves are specially adapted to their several
functions. The sepals, which compose the calyx, hav-
ing usually a protective rdle, in most cases enclose the
young flower with a tough envelope; they usually
retain their primitive green colour, and take part in
the process of assimilation. They may drop off as

THE RESPLENDENT COROLLA 127

the flower opens (e.g., Poppy), or wither as the petals
wither, or remain fresh until the fruit is ripe. Some-
times, as in many Ranunculaceze (compare Anemone,
Caltha, Helleborus), they take on the advertising role
usually assigned to the petals, being large and col-
oured, while the petals themselves are minute. In
the Monocotyledons they usually join with the petals
in adorning the flower. The next whorl, lying inside
(that is, above) the sepals, is formed of petals, con-
stituting the corolla. The connection of colour and
form of petals with the visits of insects, and their
relative insignificance in wind-pollinated flowers, has
already been referred to (p. 81). The marvellous
variety of colour and form observable in the corolla
has for its main object the attracting of insects to the
flower. The petals have departed much farther from
the ordinary leaf-form than the sepals. They assume
brilliant hues of every tint, the pigment being due
either to colouring matter dissolved in the cell-sap
(pinks and blues) or to small coloured solid bodies
(chromoplasts) contained in the cells (reds and yel-
lows). Chlorophyll being absent, the coloured petals
do not assist assimilation: they are purely advertise-
ments, though incidentally they often fulfil a useful
protective role for the important organs which they
surround. In this latter connection their sensitive-
ness to changes of light and temperature, which
causes them to close in dark or cold weather, is a
very familiar phenomenon; as is also the excellent
protection which they provide in flowers such as those
of the Labiate, where, fused together into a tube,
they form a kind of cave in which the stamens and
pistil nestle securely.

128 PLANT STRUCTURES

An exceptional use of petals, where indeed they
are used for the purposes of advertisement, but to
secure the dispersal not of the pollen, but of the
seeds, is illustrated in Fig. 24. In the genus Coriaria
the staminate and pistillate organs are borne on
separate flowers. The flowers of both kinds are small .
and inconspicuous. But in the “female” flowers the

FIG. 24.—FRUIT OF CORIARIA JAPONICA. }.

petals persist after flowering, and, becoming fleshy
and comparatively large, enclose the seed in a pulpy
berry-like envelope, which no doubt serves the same
purpose as a true berry in securing seed-dispersal by
being devoured by birds. In C. terminalis, which
comes from the Himalayas, the “ripe” corolla is
bright orange; in C. japonica, from Japan, it is at
first coral-red, and when mature velvet-black.

STAMENS AND PISTIL 129

The stamens, which form the next ring (sometimes
a double ring or a close spiral), are much less leaf-
like than the sepals or petals, yet there can be no
doubt that they are descended from leaf-shaped
organs; this is especially clear from the study of cer-
tain primitive fossil types, in which the corresponding
organs which bear the pollen are actually leaf-like.
In most of the present-day Seed Plants the stamens
conform to a uniform type—a slender stalk (filament)
bearing a head (anther) containing four chambers, in
which are produced pollen grains, which escape when
the flower is mature by the splitting of the enclosing
walls. The ways in which the pollen is then conveyed
to the pistil of other flowers have been referred to
briefly on a previous page (p. 82). The stamens in
many flowers are few, and their number usually
bears a relation to the number of the other
floral parts; in other flowers, for instance Rose and
St. John’s wort (Hypericum), they are of large and
indefinite number. The peculiar arrangement of the
pollen in Orchids has been already noted (p. 94).

The final ring of modified leaves in our typical
flower constitutes the pistil, formed of one or many
carpels, the essential structure of which has been
touched on already (p. 82). In the present place it
is desired only to point out some of the leading modi-
fications which the pistil undergoes, so that its struc-
ture as seen by the naked eye may be understood. In
the simpler forms of carpel, the affinity to leaves is
still evident, though in forms of pistil made up of a
number of carpels this may be very difficult to trace.
With the Pea, for instance, we may begin, as present-
ing a very simple example. Take an oblong leaf like

9

130 PLANT STRUCTURES

that of a Laurel, and fold it down the mid rib till the
two edges are in contact. There is-our pea-pod com-
plete. The young seeds, or ovules, are borne in a
row along the mid rib, a very usual arrangement.
Examine next the young fruit of a Columbine
(Aquilegia). Here there is a group of five separate
erect carpels, but each is essentially like a pea-pod in
structure. Compare the fruit of a Saxifrage. This
clearly consists of two carpels which are grown
together save at the tips, where the two styles stand
out like little horns. From this we may go on to
other pistils in which several carpels are completely
fused together. Next, the compact body thus formed
may be sunk down in the expanded top of the stem
(the receptacle). Or the other parts of the flower—
sepals, petals, stamens—may in their lower part be
fused with the walls of the pistil, and may thus appear
to spring from the top of it. In such cases the struc-
ture of the flower may easily be wrongly interpreted,
and reference to a work on systematic botany is
necessary if pitfalls are to be avoided. It is indeed
to be noted that in flowers, as in other parts of plants,
complicated structure or multiplication of parts is
not necessarily an indication of advanced evolution;
on the contrary, it is often indicative of a primitive
condition. Just as in machinery or in organized
human effort simplification often accompanies im-
provement, so it is with plant structures. Many of
the more primitive types of flowers, such as Butter-
cups or Water Lilies, have a multitude of petals or
stamens or carpels, while in many of the most special-
ized, such as Composites or Campanulas, the number
of parts is much reduced. The primitive wind-pol-

PRODUCTION OF FRUIT 131

linated flowers produce large quantities of pollen; in
those which have adopted the improved method of
utilizing insects, the amount of pollen is much less;
in the highly specialized Orchids, a most successful
group, the pollen is reduced to two small bundles.

Once the act of pollination is effected, the duty of
- the petals and stamens is finished, and they generally
fade. The sepals often remain, as in the Rose. By
the growth of the pollen tube from the stigma into
the ovary, fertilization is effected, and mature seed is
produced. The fruit—that is, the seed and its cover-
ings or appendages—offers the most varied forms of
any of the plant organs—compare Hazel, Strawberry,
Pea, Apple, Cranesbill, Dandelion; the variety is end-
less. Many of these forms are connected with the
means by which seed-dispersal is effected: this subject
has been touched on in Chapter III. But in numer-
ous instances we can no more assign a reason for
their beautiful or fantastic forms than we can account
for the infinite variety of shape assumed by leaves
and flowers.

Summing up, then, what has been sketched in this
chapter, we must think of our plant as a very com-
plicated and wonderful machine, of which the terres-
trial Seed Plant is the highest expression. Water is
the basis on which its activities are founded—the cur-
rency in which all business is transacted. The amount
of water contained in a growing plant is seldom
realized. Even solid timber, when growing, is half
wood, half water. A fresh lettuce loses 95 per cent.
of its weight if the water is driven off by drying.
Living in an aerial medium which tends to deprive it
of moisture continually, and which furnishes water

132 PLANT STRUCTURES

to the soil only intermittently in the form of rain, and
often in sparing quantity, the plant envelops itself
from end to end of its exposed portions in a water-
proof cuticle; the only openings in its surface layer
are the spongy tips of the root hairs on the one hand,
and in the stomata on the other. These minutest of
openings—so small that the number on a square inch
of leaf surface often far exceeds a hundred thousand—
might prove danger-points were they not most jeal-
ously watched over. But each is provided with a pair
of guard-cells ready to close the opening at any
moment; and where drought threatens, the whole of
the stomata are found in concealed positions. An
ample pipe-system extends from root, through stem,
to leaf, but it does not communicate directly with the
openings at either end. All material, whether liquid
or gaseous, absorbed or given out, has to run the
gauntlet of the living cells, which are jealous watch-
men, and allow only selected substances to pass
through them. The crude building materials and
food materials are assembled in the leaves, where in
cells spread out to the light the chlorophyll is massed.
Under the microscope, the chlorophyll is seen to be
located in minute granules embedded in the semi-
fluid contents of the cells. Well may we gaze in
wonder at these tiny green specks. Each is so small
that although a couple of hundred of them are often
present in each cell, they occupy but a very small pro-
portion of its volume. The cells themselves are of
microscopic size. The chlorophyll itself occupies only
quite a small portion of the corpuscle in which it is
immersed; yet on its activity as spread in this infini-
tesimal quantity through the leaves the whole organic

THE EVER-ACTIVE PLANT 133

world, animal as well as vegetable, depends.*
Utilizing the energy which comes through space from
the sun, it builds up organic compounds; from the
energy thus stored comes all the varied life and vital
movement which fill our world—the opening of
flowers, the hum of insects, the march of armies, and
our own restless thought; while its work in the distant
past, laid by in coal and oil, warms our houses and
drives our trains, factories, and steamships.

The work of the living chlorophyll accomplished,
the food materials produced by its agency are sent by
the pipe-system to all parts of the plant, for present
use, or to be stored in root, stem, or leaf for future
requirements.

Nor is our plant the passive, motionless thing that
it may appear to be in comparison with animals and
their larger movements. Active motion, local and
general, though usually of relatively small amount,
accompanies all plant-growth. Throughout root,
stem, leaf, and flower transference of material is going
forward vigorously. The root hairs and stomata are
working at high pressure; the chlorophyll never
ceases its activities while daylight lasts. Externally,
the growing branches, leaves, and flowers also display
incessant movement, sweeping the air in small circles,
or in the case of climbing plants in curves of con-
siderable amplitude. Alterations of illumination or
of temperature produce other movements—bendings
towards or away from light, the drooping of leaves
and closing of flowers at nightfall, and so on.

* To beaccurate, certain groups of Bacteria, the lowest forms of

organized life, must be excluded. They appear capable of building
up their bodies directly out of inorganic substances,

134 PLANT STRUCTURES

All these phenomena of growth and movement
culminate in the production of flowers, and in the
remarkable developments by which, through the
agency of pollen and ovule, a new generation is pro-
duced.

CHAPTER VI

PLANTS AND MAN

THE appearance of man upon the Earth is an event of
very recent occurrence, not only in terrestrial history,
but in the history of organic life in the world. In the
life-story which began somewhere in far pre-Cambrian
times, the record of the whole of human activities
occupies but the last paragraph of the last chapter.
For millions of years—ever since the larger animals
first abandoned the aquatic haunts of their ancestors
and took to a terrestrial life—creatures great and
small, of myriad kinds, including huge reptiles and
amphibians, and later on a crowd of birds and
mammals, have fed on land plants, without effecting
any profound changes in the appearance of the mantle
of vegetation which covered so much of the Earth’s
surface. It has been left for the human race, in the
course of the few thousand years that have elapsed
since it emerged from an existence comparable to
that of the beasts and birds, and learned the arts of
peace and war, to effect such sweeping changes in
terrestrial vegetation over wide areas, that its
influence in this respect requires a separate chapter
for its consideration.

The changes referred to are largely—though by no
means wholly—due to the requirements of the art of
husbandry; and to the history of agriculture we may

135

136 PLANTS AND MAN

look for information as to the time and place and
nature of man’s conquest of the surface of the globe.
At the period of the earliest: human civilizations, such
as those of Egypt and Mesopotamia, the domestica-
tion of plants and animals had already reached an
advanced stage. Its origin lies far behind the historic
period. We can picture in imagination the time when
in all inhabited parts of the globe man wandered with
no fixed abode, seeking food when he was hungry,
and making no provision for the morrow. Residence
in a spot which afforded a valued supply of food, such
as an abundance of buckwheat or millet or dates or
bread-fruit, might lead to a desire to encourage the
growth of such useful plants by protecting them and
their offspring; following on which might arise the
practice of assisting their growth, and thus eventually
of cultivating them. Selection of the most productive
strains would gradually follow, and barter would
cause the spread of useful plants over wider and
wider areas. We can picture development from such
rude beginnings into the regular cultivation of the
soil and the enclosing of the cultivated areas for their
protection. It is clear that such practices would not
readily arise among nomadic tribes, nor among those
inhabiting forest regions where the ground was
densely covered by trees. An abundance of animal
food would produce a race of hunters rather than of
tillers of the soil; and as for forest regions, they are
unsuitable for human development; forest races have
never been pioneers of civilization. Before agri-
culture—indeed, before civilization in any form—
could make much progress, a settled life was
necessary, free from migrations in search of food or

EARLY CIVILIZATIONS 137

for the avoidance of enemies. Hence the earliest
civilizations tended to arise in areas which were
protected by natural ramparts from the irruption of
rival tribes. Egypt had the desert on three sides, and
the sea—an impassable barrier to early peoples—on
the fourth. The valleys of the Euphrates and Tigris
presented similar features. In both areas rich alluvial
soil offered a full reward to attempts at agriculture,
and the alternation of summer and winter encouraged
the making of provision for the non-productive period
by the taking advantage of the period of growth:
conditions not present under the “endless summer
skies” of Tropical lands, where an easy and perennial
food-supply tended against the development of
industry.

The basin of the Mediterranean—the cradle of the
earlier Western civilizations from the time of Egypt
down to Rome—was, then, also the cradle of Euro-
pean agriculture. These lands, with their wet winters
and dry summers, the latter inimical to the develop-
ment of tree growth, lent themselves to cultivation
more readily than the great forest-belt which lay to
the northward, sweeping across Europe from Britain
to the Urals. Although there is clear evidence that
grain was cultivated in Europe as far back as the
Neolithic Period (say 7,000 to 5,000 B.c.), it seems
established that when Roman agriculture stood at its
perfection the peoples to the north were still mainly
nomads, dependent for their food-supply on their
flocks or on the chase. In Britain, Cesar found corn
grown in Southern England, but the centre and north
were largely forest land tenanted by tribes living on
flesh and milk, and clothed in skins. The vigorous

138 PLANTS AND MAN

colonization of the Romans may well have been
accountable for the introduction into Britain of many
of the farm plants still grown there. The wars of the
next fifteen hundred years on the one hand, and the
spread of agriculture on the other, caused the steady
destruction of the forests, till at length England and
Central Europe began to assume their present appear-
ance. The draining of marshes and fens, the enclosing
of land, went on steadily, and to a slight extent is
going on still; within recent years, the European War
has resulted in the disappearance of many of the
remaining woods, and in the breaking up of fresh
land.

From the point of view of the botanist, agriculture
consists of the destruction of the plant associations
which for some thousands of years have occupied the
ground, and their replacement by other plants which
are useful to man. The natural plant associations
being the result of the survival of the fittest through
a long period of time, while the farmer’s crops
represent plants which do not grow naturally on the
ground, nor often indeed in the country (while they
are frequently artificial forms unable to reproduce
themselves), it follows that the latter cannot compete
with the former, and can be maintained only by the
most careful protection. The native plants are always
striving to reoccupy their legitimate territory, and the
farmer is incessantly engaged in trying to keep them
out. Agriculture, indeed, has been defined as “a
controversy with weeds.” Incidentally, the suppres-
sion of the natural flora allows many weaker plants
an opportunity of which they are not slow to take
advantage, These may be natives, but are often

PROFIT AND LOSS 139

annuals which have followed the spread of farming
operations, or which are directly—though uninten-
tionally—introduced by man as impurities in the seed
which he sows.

Let us look a little more closely into the question
of profit and loss in our flora resulting from agri-
culture. In the first place, whether the ground is
tilled or grazed, the woodland which primitively
occupied so much of it disappears. The plough and
the scythe are fatal to all seedling trees. Little less
fatal is the browsing of cattle and sheep, and even in
rough pasture only thorny plants like Whitethorn
and Gorse may be found battling successfully for a
lodgment. Where woodland is used for pasturage,
the delicate shade plants—Anemones, Wild Hyacinths,
Primroses—soon die out. No young trees appear on
the grazed surface, though hundreds of thousands of
seeds may be shed annually over the ground. In the’
course of time the present trees will die, and only
grass remain. How different is it where cattle are
excluded and the scythe unused! Among the grass
young trees spring up everywhere, and in the woods
a dense undergrowth of saplings sheltering a varied
shade flora makes its appearance; regeneration of the
natural woodland proceeds apace.

Natural grasslands, if undisturbed, possess a flora
which has been built up during a long period of time,
and which, like all purely natural plant associations,
represents a delicate balance between its many con-
stituents, which often include rare and shy species. If
such land be once broken up, its flora will probably
never again resume its former composition even if
allowed to regenerate during a long series of years,

140 PLANTS AND MAN

for the alteration in the old substratum caused by its
being turned over and mixed introduces new edaphic
(z.e., soil) conditions which will not entirely pass away.
As regards grazing, likewise, when land is pastured up
to or near its full capacity, as is generally the case on
enclosed areas, the weaker and often more interesting
members of the flora tend to disappear. In primitive
times all grasslands had, of course, their natural
grazing inhabitants—in our islands deer of more than
one species, sheep, and smaller creatures such as
rabbits and geese—and so a total exclusion of grazing
animals now would no more tend to reproduce exactly
the flora of pre-husbandry days than does the excess
of herbivores; but the present heavy stocking of the
land is to be deplored by the botanist, even as it is
rejoiced in by the economist. The more vigorous
plants, and especially those which propagate them-
selves largely by vegetative means, survive, or even
increase owing to the augmented food supplied by the
manure which the animals provide; but many species
fail to ripen seed, being either eaten or trampled; the
rarer Orchids, strange ferns like the Adder’s Tongue
(Ophioglossum vulgatum), and Moonwort (Botry-
chium Lunaria), and the other choicer denizens of the
grasslands, tend to disappear.

Drainage is an obvious cause of loss to our flora.
Whole lakes and areas of swamp, with their peculiar
and to a great extent natural flora, have disappeared
from parts of the country. Some of the most inter-
esting marsh plants of the British flora—such as the
two fine Ragweeds, Senecio palustris and S. palu-
dosus, and the Marsh Sow-thistle (Sonchus palustris)
—have on this account almost vanished from our

DISTURBANCE OF VEGETATION _ tar

islands, like the Bittern and Great Bustard which are
their companions.

Some lakes, again, have been ruined for the botanist
by being usedas reservoirs. ‘Theconsiderable changes
of level which this involves is a thing to which plants
are not adapted, and only a few can withstand it, such
as the Water Bistort (Polygonum amphibium) and the
Shore-weed (Littorella uniflora), which are equally at
home on land or in water, being able to change
rapidly their structure and mode of life to suit change
of environment. As compared with a lakelet with a
natural outlet, a dam with a sluice has always a much
reduced and usually quite uninteresting flora.

The proximity of a large town, especially if it is a
centre of manufacture, is a notorious factor in the
reduction of the native flora: not only by the thought-
less and wanton destruction carried out by its inhabit-
ants, but more subtly by the deposition of soot, and
by the poisoning of the air by sulphurous and acid
fumes. The higher Cryptogams, such as Mosses and
Hepatics, are particularly susceptible in this respect,
and vanish along with the more delicate Seed Plants.
Mining centres are specially destructive of plant life,
since, in addition to other drawbacks, the soil is often
buried under masses of excavated material containing
poisonous substances. If there is a purgatory for
plants, it is surely found in such areas.

Other examples of the multitudinous ways in which
human activities disturb and destroy native plant life
will occur to the reader—the burning of moors in
order to improve them as pasturage; in recent years
the tarring of roads, which kills the pleasant wayside
herbage and poisons the streams into which the road

142 PLANTS AND MAN

drainage is carried; and so on. The indictment is an
overwhelming one, and, as said in the first chapter, the
flora is now everywhere so altered that we can gain
some idea of its original aspect only by a study of
isolated fragments and much-adulterated samples.
But if the debit side of the account, as presented by
the lover of nature, is heavy, it must not be forgotten
that there are many items to man’s credit. Though
our country’s vegetation has lost in scientific interest,
it has gained vastly in both economic and esthetic
value by the introduction of useful and ornamental
plants from all the Temperate regions of the world;
and besides, a large number of species have followed
in man’s footsteps, and, taking advantage of the dis-
turbance of the native flora caused by his operations,
endeavour with more or less success to establish a
footing in the country. Before we trespass on the
domains of arboriculture, horticulture, or agriculture,
under which heads the cultivation of useful or orna-
mental plants divides itself, some consideration is
required of those plants which, quasi-wild, are usually
included in accounts of the vegetation under the head
of aliens, denizens, colonists, and so forth. These
constitute a quite considerable proportion of the total
number of species found in any area which has felt
the influence of man. For instance, in the county of
Dublin, which, owing to its diversified surface—sea-
cliff, sands, moorland, woodland, and cultivation—and
its favourable climate—the warmest and driest in the
country—possesses the largest flora of any similar
area (354 square miles) in Ireland, the list of about
760 “wild” plants includes some 170, or over one-
fifth of the whole, whose presence is attributable,

ALIEN PLANTS 143

directly or indirectly, to human activities. We may
compare these figures with those drawn from a study
of the flora of Kent, which faces across the Channel
towards France just as county Dublin faces across the
Irish Sea towards England; both are areas of early
settlement and both lie in the main stream of traffic.
In Kent we have to deal with a larger area (1,570
square miles), and a larger flora (1,160 species). We
find that, of these 1,160 species, 146, or about one-
eighth, are set down as owing their presence to man.*
And so it is in all the more populous and highly tilled
parts of our islands.

This question of alien plants, their past history and
present standing, is one of the most puzzling with
which the student of our flora has to deal. In the
first place, most of them have been in the country for
a long time, and the record of their introduction is
lost. Next, while many of them are confined to
ground disturbed by man, and thus clearly exist under
man’s protection—however unwillingly that protec-
tion may be afforded—others have mixed with the
indigenous flora, won a place in the closed native
vegetation, and might be ranked as true natives were
it not that a study of their general distribution raises
doubts as to the possibility of their having arrived in
our islands unaided—doubts which their known
occurrence in gardens tends to confirm. Take the
case of the Yellow Monkey-flower (Mimulus Langs-
dorfi). This has quite established itself in our native
flora, in some places ascending mountain streams far
into the hills, in others mingling with the rank flora of

* F, J. Hanspury and E. S. Marsuatt: ‘‘ Flora of Kent,’’ 1899,
Pp. XXXv.

144 PLANTS AND MAN

muddy estuaries. It Jooks as aboriginal as any of the
plants among which it grows: but the facts that the
genus to which it belongs is American (with a few
species in Australia and New Zealand), that it itself is
found native in the western States and not in the
eastern, and that it has been long cultivated in
gardens, furnish convincing proof that it is really an
alien. But it is seldom that the evidence is so satis-
factory as in this case. More usually the range of the
doubtful members of the flora is continuous, extending
from regions where they are truly native to others
where they are undoubtedly exotic. For instance,
many annual plants of the Mediterranean region have
followed the spread of agriculture across the former
forest areas of Central and Western Europe into our
own islands. Plants native in France have been trans-
ported into England, and English natives into Ireland;
east Irish plants have spread westward—sixty years
ago, save for a single record of P. hybridum, Papaver
dubium was the only Poppy known west of the
Shannon; now all four British species occur, several
of them in many places. The flora of Europe, as
pointed out already, diminishes in variety as we pass
westward into the outlying areas. Those species
whose aboriginal distribution stopped short of the
western limit of the land had no doubt a fluctuating
western or northern or southern boundary to their
range, dependent on temporary conditions. Thus, a
hard winter might kill back a plant already at the
limit of its natural range, or a warm summer, by
ripening abundance of its seed, might result in its
slight advance. The general effect of human opera-
tions has been to lessen competition and increase

GAINS TO THE FLORA 145

suitable habitats by the destruction of the native
vegetation which occupied them, and this has resulted
in a general advance of a large number of species.
What renders the study of this advance so difficult is
the fact that on all disturbed land the truly native
plants which have been ousted are striving side by
side with the immigrants to regain their former terri-
tories; and it is now often very difficult to disentangle
them: to separate the sheep from the goats. If.only
we could have had a Watson’s “Topographical
Botany” written five thousand years ago, before our
restless race began to mess up the vegetation!

However, as has been said, what we have lost on
one side we have gained on another. On every side
bright immigrants meet the eye. Our old buildings
and quarries often blaze with the Red Valerian
(Kentranthus ruber) and Wallflower (Cheiranthus
Cheiri); in fields Poppies of various kinds, Corn
Cockle (Lychnis Githago), and Corn Blue-bottle
(Centaurea Cyanus) add a glory to the rich green or
gold of the cereals; dry banks and gravelly places are
decorated with species of Melilot (Melilotus), Chamo-
mile (Anthemis), Knapweed (Centaurea), and many
others. The flora of harbours and docksides is often
as cosmopolitan as the sailors of the ships by whose
agency it came there; and the unfamiliar weeds—the
gipsies and tramps of the plant world—which we
encounter on roadsides, rubbish-heaps, and railway
stations lend an additional interest to our botanical
rambles.

Turning now to the plants which are used by man,
it may be pointed out in the first place that the human

race obtains much more, whether of profit or of
Io

146 PLANTS AND MAN

pleasure, from the vegetable than from the animal
kingdom. Flesh, whether derived from mammals,
birds, or fishes; wool, silk, leather, oils, and so on, bulk
much less than the grains, vegetables, fruits, timber,
fibres, fodder plants, and other vegetable products
which we use in our daily life. On the esthetic side,
again, while the beauty of birds and insects is a source
of frequent delight, flowers play a part in daily life
that the more delicate and sensitive animals can never
do. Again, in the number of different species used,
whether for profit or pleasure, the plant world takes
precedence. This is especially the case as regards our
farms and pleasure grounds, plants lending themselves
much more readily to domestication than animals do.
And so a suburban house may have a hundred or a
thousand different plants in kitchen garden and flower
plot, orchard, and shrubbery, while its animal depen-
dents consist of a horse, a couple of dogs, a cat, some
fowl, and a canary. So again a Botanic Garden may
easily possess as many thousands of different species
as a Zoological Garden contains hundreds.

This army of plants which human beings collect
about themselves may be grouped under two cate-
gories—useful and ornamental. On a previous page
(p. 136) a suggestion has been made as to how the
cultivation of useful plants may have arisen. As now
practised, this industry is the largest in the world, and
with the growth of means of transport has ceased to
be only or even mainly of local importance: we use
every day wheat from Australia, rice from China, tea
from India, cotton from the United States, timber
from Norway. In some cases, as in the last, these
materials are harvested as they occur in the wild state,

PLANT-BREEDING 147
but in the majority of instances the plants are not
merely conserved, but cultivated; cultivation has led
to selection of the best varieties; and continued selec-
tion has resulted in the production of forms often
very different in appearance from the wild plants from
which they originated. We cannot create new forms;
but by taking advantage of the innate tendency to
vary which all plants display—some to a much greater
degree than others—and by raising, generation after
generation, the seeds of those individuals in which a
certain abnormal feature is best displayed, we can
produce an artificial race in which the selected
character may -be developed to an extraordinary
degree. But we have not by this means produced a
new species. Seedlings of such plants will tend to
“throw back” towards the original form; we can
preserve or improve the special characters only by
continued selection; if allowed to grow and seed
unchecked, most of such plants will revert to the
natural type in a few generations. Often this rever-
sion is so rapid that seeds are useless for cultural
purposes, and it is only by cuttings or graftings—that
is, by growing parts of the original possessor of the
required characters—that constancy can be main-
tained ; this is what is usually done in the case of fruit-
' trees, Roses, Pansies, and so on.

Equally efficient in the hands of the cultivator has
been another method of producing new forms—
namely, hybridization. If the pollen of a plant be
transferred to the stigma of a related species, offspring
is often produced; and the product is a batch of plants
intermediate in characters between the two parents,
and generally uniform in appearance. Should these

148 PLANTS AND MAN

be crossed again, a heterogeneous offspring is the
result, displaying a variety of characters inherited
from one or other original parent. The crossing of
varieties, native or cultivated, has the same result.
Hybrids occur in nature, but not very frequently.
Insects visiting flowers are well known to confine
their attention to a great extent to one species at a
time, so as agents of hybridization they are not
efficient. Again, many hybrids do not produce fertile
seed, so that if they arise by natural means they are
not perpetuated. In the garden, hybridizing has been
resorted to largely; but its practice is not so ancient
as the method of producing improved breeds by
selection.

The cultivation of specially selected forms is cer-
tainly of remote origin, and probably goes back to the
earliest days of agriculture: of early date, too, is the
introduction into regions where they do not occur
naturally of plants desirable for their use or beauty.
The records of the cultivation of the Vine, for
instance, go back for five or six thousand years in
Egypt. Two thousand years ago Pliny writes that
ninety-one principal forms could be reckoned in his
day, though “the varieties are very nearly as number-
less as the districts in which they grow.” Theo-
phrastus, three hundred years earlier, discourses
learnedly of the different kinds of cultivated Figs, etc.,
and their superiority over the wild kinds. These and
other authors make frequent mention of plants intro-
duced into Greece or Italy from the East for their
usefulness or their pleasing qualities. Nowadays, the
number of species cultivated, the innumerable forms
of these which are grown, and the wide distribution

THE PLASTIC CABBAGE 149

which these forms have attained, have resulted in the
cultivated flora of a country like England being, so far
as the higher plants are concerned, much larger than
the native flora, even when all the plants which are
grown under glass are left out of consideration.

In the case of plants of economic importance, the
usual aim of selection has been increase of size or
productiveness of the parts which are useful. In some
instances selection has taken several directions inside
the limits of a single species, as in the forms of
Cabbage, which are all the offspring of Brassica
oleracea (Fig. 25), a seaside plant of Western and
Southern Europe, and are mostly creations of com-
paratively recent date. The Cauliflower has been pro-
duced byincreasing the size of the inflorescence ; White
Cabbage by promoting leaf production; Brussels
Sprouts by encouraging the development of axillary
shoots; while a form with a tall and woody stem is
made into walking sticks. More often we find a
species developed along a single line. For instance,
the tendency to store food materials in a fleshy tap-
root has been developed in the case of Turnip, Beet,
Carrot; the fleshy scale-leaves which form bulbs have
been exploited in the case of the Onion; increased
stem-growth is promoted in Asparagus; increased
leaf-growth in Spinach and Lettuce; while by the
development of seeds and fruits of many kinds
artificial selection has supplied us with the foods on
which the human race mainly subsists. The most
important of all these last are, of course, the different
grains, which are the seeds of grasses of various
genera—T riticum (Wheat), Hordeum (Barley), Secale
(Rye), Avena (Oat), Panicum (Millet), Oryza (Rice),

150 PLANTS AND MAN

Zea (Maize). The value of these to the human race is
incalculable, and some of them have been in cultiva-

Fic. 25..-WILD CABBAGE (BRASSICA OLERACEA). 4.

tion for at least five thousand years. In some of them,
indeed, the native form is now unknown, the improved

CEREALS AND FRUIT-TREES 151

varieties alone having been preserved by the care of
man. The Wheats are a case in point. While a wild
grass growing in Palestine has been quite recently
identified as the probable source of the Hard Wheats,
the native parent of the Soft Wheats is unknown.
That productiveness has in all cases been much
increased by long selection there can be no doubt; it
may be pointed out that several species of Triticum,
Hordeum, and Avena, allies of the Wheat, Barley, and
Oat, are included in the native British flora, but they
are useless as producers of grain.

Nowhere is the effect on plants of selection and
cultivation seen better than in our native fruit-trees.
We have only fo compare the size, flavour, and almost
endless variety of apples and pears with the fruit of
the wild stock of these two species—the Crab (Pyrus
Malus) and Wild Pear (Pyrus communis) of our
hedgerows—to realize how much has been accom-
plished. In garden flowers, also, we see most striking
results of continuous selection. By taking advantage
of the tendency of stamens and carpels to change
occasionally into petals, and of petals to increase in
number, “double flowers” have been effected. When
“doubling” is complete—that is, when the conversion
into petals is thorough—no seed can of course be
produced, and the plants must be propagated by
cuttings. Different other slight natural variations,
exaggerated by selection and cultivation, have been
the source of innumerable “ varieties ” in our gardens.

Sometimes the natural variation is by no means
slight, but of a striking character which the efforts of
gardeners have not succeeded in developing further.
Take, for instance, the case of fastigiate trees, such as

152 PLANTS AND MAN

the Lombardy Poplar (Populus nigra, var. italica) or
the Irish Yew (Taxus baccata, var. fastigiata). These
are freaks or sports, the character being that all the
branches, not only the leader, tend to assume a
vertical position. The Irish Yew originated as a wild
“female” (pistillate) seedling found on the hills of
Co. Fermanagh about 1780 and never rediscovered.
It appears to be a juvenile form, preserving through-
out life its seedling characters—a kind of Peter Pan
among plants. Of the Lombardy Poplar the origin is
not known, but it was no doubt similar. Seedlings
of the Irish Yew revert to the ordinary type, and all
the Irish Yews in cultivation are pieces of the original
plant grown as cuttings. Poplars, like the Yew, bear
the male’) \\(stamuimate)) and.“ female”. (pistillate)
flowers on different trees, and the original Lombardy
Poplar having been a “ male” it also can be propa-
gated only by cuttings—probably seedlings would in
any case revert to the usual form.

The reverse of this abnormal erect habit is seen in
weeping trees, where the branches for unexplained
reasons seek to grow downward. In nature this
results in a creeping habit. If planted on a height
the branches will deliberately grow downwards
towards the ground. Cultivators graft such forms on
the top of a tall stem of a normal specimen, with the
result that we see in the Weeping Ash and similar
gardeners’ productions.

Another large group of casual abnormalities is
concerned with the colour of leaves. The Purple
Beech is a case in point. It was not produced by
selection, but arose naturally, no doubt as a chance
seedling. In this instance the character is usually

PLANT ZEBRAS 153

IIS SS TEISIES Sri Sree 10 03S al Aan emt
passed on to the offspring, most seedlings having
similar purple leaves, though some individuals are
green. The peculiar colour is due in this case to a
pigment in the epidermis of the leaf; the green chloro-
phyll is duly present, though its colour is masked by
the purple leaf-skin. To a different category belong
the “gold” and “silver” variegations which are so
much exploited in shrubberies and borders and green-
houses. These spots or stripes or tintings of pale
colour on the leaves are due to the lack of chlorophyll
in the chromatophores (chlorophyll corpuscles) ; some-
times to an absence of the chromatophores them-
selves; and this omission appears to be caused by an
enfeebled condition of the plant. Variegated plants
are weaker than normal ones, and hence do not tend
to survive in nature. But gardeners have protected
and propagated a large number of them. When the
variegation arises, as it often does, on a branch of an
otherwise normal plant, it usually is not reproducible
from seed, and must be perpetuated by cuttings. But
where it happens with seedlings, it is often more or
less fixed, and may be reproduced generation after
generation, as in the Golden Elder, Golder Feather,
and the marginal-variegated form of Winter Cress
(Barbarea vulgaris).

Flower colour is not so fixed as leaf colour, for
obvious reasons, the green colour of leaves being due
to chlorophyll, which is an absolutely necessary
ingredient of the leaf if plant food is to be manu-
factured; whereas flower colour is merely for adver-
tisement, and any pigment can be made to serve. In
nature most flowers vary in tint, and some in a marked
degree—take the little native Milkwort (Polygala),

154 PLANTS AND MAN

which may be blue or purple or white. Flowers offer
great opportunities, therefore, to the gardener, and by
selecting on the one hand and hybridizing on the
other every known tint has been reproduced in some
blossom. Adding to this the variability in size and
shape of petals, and the tendency to “doubling,” the
flower in the hands of skilful cultivators has been
altered almost beyond recognition. Take the Roses,
for example, with their infinite variety of form and
colour. The bulk of them are derived from a dozen
wild species, possessing comparatively small single
flowers, white, yellow, or red—Rosa_ centifolia,
damascena, gallica (the source of the older Roses),
indica, moschata, odorata, rugosa, Wichureiana, with
our native arvensis and spinosissima. By selecting
for colour, shape, and “ doubleness,” both from the
species themselves and from the offspring produced
by hybridizing one of these with another, what a
wealth of beauty has been developed! More than any
other flowers, the Roses are the crown and glory of
the gardener’s art. Well has the Rose been called the
Queen of Flowers; but it owes its royal prerogative to
man. Nature provided blossoms—elegant, but of no
special promise—and a tendency to vary, of priceless
value; human skill and industry have done the rest.

CHAr Tek, Vit
PAST AND PRESENT

Tue dependence of animals upon plants for the food
by means of which they continue to inhabit the earth,
which was pointed out on a previous page (75), shows
that the plant world is older than the animal world;
but the immense age of both can be appreciated only
by a study of stratigraphical geology. The tens of
thousands of feet of sedimentary rocks, laid down in
slow succession on the floors of ancient seas and
lakes, and still reposing layer upon layer, and no less
the great gaps in the series produced when, raised
into the air, deposition ceased, and thousands of feet
of rock were slowly worn away and washed down
again into the sea by the action of frost and wind and
water, point to periods incalculably remote as
measured by the standards which we apply to human
history. A few thousands of years measures the
span which separates us from the Neolithic Period;
but to the geologist a million years is but a con-
venient unit for expressing, so far as any expression
by our time-standards is possible, the huge periods
with which he has to deal. And even when we get
back as far as the oldest fossils will take us, we
are still a long way from having reached the epoch
when life on the earth originated. As we work back-
155

156 PAST AND PRESENT

ward and study the fossils of older and older rocks,
the multitudinous assembly of plants and animals
which fill the world to-day are replaced by other and
more primitive forms, many groups approaching each
other and merging in common ancestral types. But
still, the very oldest fossil-bearing strata contain the
remains of organisms already far up the ladder of
evolution. The Lamp Shells (Brachiopods), Ptero-
pods, Trilobites, and Worms of the ancient Cam-
brian rocks have clearly a long ancestral history.
Plants are not so abundantly preserved in the rocks
as the skeletons and shells of animals, on account of
their softer nature; but in the oldest known plants it
is again clear that we are dealing with forms by no
means primordial. It is the more interesting, then,
to note that many very lowly forms of life have come
down to us from times immensely remote, and are
still present on the earth in abundance, swarming in
every sea and in every pond, or nestling in damp
crevices of the land; while higher types of immense
antiquity still mingle with the crowd of recent Seed
Plants, some of them forming noble forest trees. Of
especial interest, taking into account the wide dis-
tinction which exists between the higher animals on
the one hand and the higher plants on the other, is it
to find that there are still in existence organisms
which are so much on the border-line between these
two great groups of living things that they can be
referred to one or other only with hesitation,
clearly indicating that animal and vegetable life sprang
from a common source. Take the group known
as Mycetozoa or Myxomycetes. These names alone
show the divergent views which men of science

SLIME FUNGI 157

have held regarding them, Myxromycetes signifying
“slime-fungi,’ while Mycetozoa means “fungus-
animals.” These remarkable organisms, of which
over 180 species are found in the British Isles, begin
life as tiny wind-borne spores. Under suitable con-
ditions of moisture and heat, the spore swells, its
wall cracks, and the contents—a tiny globule of proto-
plasm—creep out, develop a little tail or flagellum,
which by lashing about propels the pear-shaped
swarm-cell through the drop of water in which it
began life. The organism feeds by catching bac-
teria and other minute particles of organic matter,
which are conveyed into the interior of the little mass
of protoplasm and digested. The swarm-cells increase
in number by division, and ultimately unite in pairs
to form a plasmodium, which may, by union with
other plasmodia, eventually attain a quite large size.
In this naked protoplasmic mass a very remarkable
rhythmic movement is set up, the granular proto-
plasm of the interior streaming rapidly along certain
channels for about 14 minutes, when the motion is
reversed and it streams in the opposite direction.
The whole mass now creeps about in moist places,
usually in the form of a network of branching veins,
feeding as it goes, usually on dead vegetable matter.
When fully developed the plasmodium creeps out
into some more open spot and transforms itself into
masses of spores enclosed in spore-cases, which vary
much in different genera as regards size, shape, and
colour, and are often borne on delicate stalks. When
ripe, the spore-cases, or sporangia, open, and the
spores are liberated into the air to be dispersed by
wind and eventually to begin growth on their own

158 PAST AND PRESENT

account. This story partakes about equally of inci-
dents characteristic of the life-history of the lower
animals and of the lower plants. The fruiting stage
and the wind dispersal of the spores recall the
arrangements familiar in the Fungi, and are not
matched in any section of the animal kingdom; while
the creeping plasmodium, devouring food as it goes,

ad,

Fic, 26.—A MyxomyceTE (COMATRICHA TYPHOIDES) IN FRUIT.

a, Natural size; b, enlarged.

is entirely suggestive of animal life, and is not paral-
leled anywhere in the vegetable kingdom. There is
no reason to look on the Mycetozoa as a group of
animals which have taken on certain plant-like char-
acters, any more than as a group of plants which
have evolved certain animal characteristics: we
appear to see in them a very ancient group which
has come down to us from a time when plants and

THE PLANT WORLD 159

animals, as we know them, had not yet become
differentiated.

Among plants, as distinguished generally from
animals by the production and abundant use of chloro-
phyll and of cellulose, we have still existing on the
Earth a range of forms extending from almost the
most primitive organism that we can imagine up to
the splendid Seed Plant, specialized in a hundred ways.
Every pool, every soil, swarms with bacteria, the
lowliest form of life—organisms exceedingly minute,
exceedingly simple, and capable of existing under
highly diverse conditions both physical and chemical.
Thence we can trace an irregular ascending scale
through the Fungi, the Algz, Mosses, Horsetails,
Ferns, and Club-mosses, to the Conifers, and on to
the highest of the Seed Plants, which exceed in their
beauty of structure and complicated life anything
that has gone before them. In fact, as Theophrastus
says, your plant is a thing various and manifold. And
this existing vegetation with its thousand forms is
but the present manifestation of the vital activity
which has populated the earth during tens of millions
of years. The oldest rocks which have been pre-
served to us in such a condition as to yield remains
of plants and animals in a recognizable form are
those known as Cambrian, the deposition of which
occurred at a period which geologists have variously
calculated as from, say, 20 to 100 or more millions of
years ago. Yet even at that immensely remote
period, life, both vegetable and animal, was already
abundant and diverse, as well as highly organized.
As Darwin long ago pointed out, the geological
record does not go back nearly far enough to allow

160 PAST AND PRESENT

—w)

us any insight into the evolution of the earlier forms
of life. Below the Cambrian rocks, as represented in
these islands and in Europe generally, with their well-
developed fauna, are tens of thousands of feet of
strata which once, no doubt, were sediments at the
bottom of the sea, and later on hardened into slates
and sandstones in which were embedded remains of
more primitive organisms; but these rocks have been
so altered during the immense period of their exist-
ence by heat and pressure and the other vicissitudes
to which the restless crust of the earth is subject that
they now present a mass of granite-like material in
which all trace of organic life has been destroyed.
In America the rocks of corresponding age are better
preserved, and have yielded a limited fauna display-
ing an already advanced stage of evolution. To
account for the strange paucity of animal remains it
has been suggested that the creatures of these earliest
times were soft-bodied, so that after death they left
no trace behind. It may be noted that the pre-Cam-
brian rocks contain beds of limestone and of carbon
(in the form of graphite); such beds, in later rocks,
are composed of organic materials, the limestones
being formed of the skeletons of minute marine
creatures, particularly Foraminifera, and the carbon
deposits of the remains of plants.

In Cambrian times, then, abundant life springs forth
into our vision from the rocks, already, like Minerva,
fully armed. The soft plant structures are not well
preserved in the older fossiliferous rocks, and hence
the fragmentary story of plant life, as we trace it
backwards, becomes very obscure, while many types
of animals still boldly occupy the stage. At the

THE OLDEST PLANTS 16f

earliest period from which plant remains are well pre-
served and plentiful, in the Devonian rocks, many of
the great plant groups are fully developed, the vege-
tation displaying an abundance and luxuriance com-
parable to that of the present day. Seaweeds (Algz),
Horsetails (Equisetales), Ferns (Filicales), Club-
mosses (Lycopodiales), fill the waters or clothe the
land, and Seed Plants are already abundant in the
form of the fern-like Pteridosperms, long since
extinct. Both as regards adaptation to environment
and internal structure a very high degree of speciali-
zation has already been obtained. “If a botanist,”
writes D. H. Scott, “were set to examine, without
prejudice, the structure of those Devonian plants
which have come down to us in a fit state for such
investigation, it would probably never occur to him
that they were any simpler than plants of the present
day; he would find them different in many ways, but
about on the same general level of organization.”

In the succeeding Carboniferous Period conditions
appear to have been peculiarly suitable for vegetable
life, as well as for its preservation in a fossil condition.
In the warm, moist climate of those times, many of
the races of plants above mentioned attained an im-
posing size, luxuriance, abundance, and variety; and
their remains, fortunately well preserved owing to
conditions favourable to slow decomposition, not
only furnish a rich heritage for the botanist, but
supply the coal, on the energy derived from which
our whole modern civilization is built up.

Before the end of the Paleozoic Period the Coni-
fers had appeared, descended possibly from the
extinct Cordaitee. With the advent of the Secondary

It

162 PAST AND PRESENT

or Mesozoic epoch the group of the Cycads, to which
our modern Screw-pines belong, rose to great impor-
tance, descended probably from the Pteridosperms,
and long continued to be a dominant feature of
terrestrial vegetation. And then at last in the Lower
Cretaceous rocks the Angiosperms, or “Flowering
Plants” par excellence, both Dicotyledons and Mono-
cotyledons, put in an appearance. It seems probable
that they were evolved from Cycads, such as the
Bennettiteg@, recent researches on magnificent fossil
material discovered in America showing striking
analogies between certain Cycadaceous flowers and
those of such plants as Magnolias, Water Lilies, and
Buttercups. Once established, the Angiosperms rose
to primary importance in an extraordinarily short
time—very possibly owing to the “invention” of
insect pollination, which may have arisen at that
period. In Upper Cretaceous times the two great
groups into which the Angiosperms still fall, the
Dicotyledons and Monocotyledons, fairly dominated
the flora of the world, as they do at present. Already
many types familiar at the present day had appeared,
and the woods were filled with Birches, Beeches,
Oaks, Planes, Maples, Hollies, Ivies, as they are
nowadays.

The record of the rocks during these long periods
of time contains not only the story of the rise of the
great divisions of the vegetable world, but also of
the decline of most of them. A few, like the Pteri-
dosperms and the Sphenophylls, died out completely
long ago; but most of the great groups of early days,
such as Cycads, Ferns, Horsetails, and Club-mosses,
still survive, though shorn of much of their glory.

THE SACRED GINKGO 163

Races which once formed vast and lofty forests are
now represented by a few lowly herbs; and it is diffi-
cult to recognize in the tiny Selaginella of our moors
the representative of the gigantic Club-mosses of Car-
boniferous days. But certain plants still living retain

Fic. 27.—LrEaF OF MAIDENHAIR TREE (GINKGO BILOBA), 3.

to a great extent the features of their ancestors of
the ancient rocks. One of the most interesting of
these is the Maidenhair Tree (Ginkgo biloba), well
known as a sacred tree in the East, and apparently
preserved to us through the last few thousand years
owing to this custom, as it does not seem to exist

164 PAST AND PRESENT

now in a wild state. The genus Ginkgo runs back
to the beginning of the Mesozoic Period, and its
near relatives go back much farther still to the
Devonian; the group to which it belongs, Ginkgoacee
(probably descended from the Cordaite@), attains its
maximum in the Jurassic, the “Age of Reptiles,” and
the existing species or its near relatives saw the Earth
teeming with fantastic Saurians, including huge
brutes, longer than the greatest whale, which browsed
on trees or devoured creatures scarcely less terrible
than themselves, while others of different form occu-
pied the sea, and others again of nightmare appear-
ance dashed bat-like through the air. .This solitary
representative of a great and ancient race is of quite
peculiar interest in that it is the highest plant in which
is preserved the primitive feature of fertilization by
the medium of water, the male cell being endowed
with the power of motion, and reaching the egg-cell
by means of swimming.

Throughout the Tertiary or Cainozoic Period the
dominance of the Angiosperms became more pro-
nounced, and already in the Eocene a flora flourished
much resembling in a general way that which now
occupies the Earth. Long periods succeeded the
Eocene, of which the record is poor so far as plant
remains are concerned, at least as regards these coun-
tries, but no further great botanical revolutions took
place. Through the Miocene Period, with its luxuriant
evergreen, subtropical vegetation, we are led to the
Pliocene. During this period the climate once again
cooled down, and towards the end of it, under condi-
tions very like those prevailing in England at present,
many of our familiar species of wild flowers and trees

DAWN OF THE MODERN FLORA _ 165

at length made their appearance—Marsh Marigold
(Caltha palustris), Sloe (Prunus spinosa), Blackberry
(Rubus fruticosus), Hawthorn (Crategus Oxyacan-
tha), Cow Parsnep (Heracleum Sphondylium), Bog-
bean (Menyanthes trifoliata), Gypsywort (Lycopus
europeus), Sheep’s Sorrel (Rumex Acetosella), Birch
(Betulaalba), Hazel (Corylus Avellana), Oak (Quercus
Robur), Yew (Taxus baccata), Bur-reed (Sparganium
erectum), Cotton-grass (Eriophorum polystachion),
Royal Fern (Osmunda regalis). The remains of
these occur in the “Cromer Forest-bed,” a series of
estuarine deposits—laid down perhaps by the ancient
Rhine—which underlies the boulder-clay cliffs of the
Norfolk coast, and forms almost the only plant-bear-
ing beds of Pliocene Age found in the present land
area which we call Britain.

And now, just as a point is reached when at length
we think we shall see our present British flora emerg-
ing fully from the obscurity of the ages, a dramatic
interruption occurs, which confuses the record and
brings us into difficulties of many sorts, giving rise to
controversies which are still far from being settled.
The climate becomes suddenly colder, and Europe is
plunged into the rigours of the Ice Age. Ice Ages
there had been before in the long history of the world.
Rocks of late Permian or early Carboniferous times
bear ample witness to the existence of great ice sheets
extending over wide areas in several continents where
temperate or warm conditions now prevail: and
puzzling deposits of later age—Cretaceous, Eocene,
Miocene—have been interpreted by some geologists
as the relics of subsequent Glacial Periods. But
these are only distant echoes as compared with the

166 PAST AND PRESENT

Quaternary Ice Age, from the effects of which our
country and its fauna and flora are still in process of
recovery. At the close of the Pliocene Period, then,
snow began to extend on the higher grounds, and
glaciers to fill the mountain valleys; these conditions
were intensified until all Northern Europe, including
the British Isles as far south as the Thames valley,
lay under a mantle of ice. The plants which occupied
the ground were forced southward as the ice
advanced, or exterminated by the increasing cold.
After long fluctuations of climate, the extent of which
appears still in doubt, the ice at length slowly passed
away, leaving the surface of our country greatly
altered. The ancient soils which had been in process
of accumulation since last the land rose above sea-
level were swept away, the surface was strewn with .
materials formed by the grinding down of the hills or
the pushing up of sea-bottom material, valleys were
choked, rivers diverted, lakes formed by dams of
glacial detritus, or by the scooping action of the ice;
the whole surface of the country was remodelled on
new lines. Into this new land the plants remigrated,
and we now view on our hills and plains the results
of this repopulation. The difficulties of which I have
spoken arise especially in connection with the manner
of this recolonization. On a continental area one
can conceive of a gradual retirement of the flora
before the advance of the ice, and its subsequent re-
migration northward into its old haunts as the ice
retired. But on an insular area like Great Britain no
such line of retreat was open. The ice-free area of
Southern England and possibly Southern Ireland
does not appear adequate to harbour the crowd of

THE GREAT ICE AGE 167

refugees throughout the cold period. There is good
evidence that the time of maximum glaciation was
also one of elevation of the land, and possibly this
persisted for a while after the passing away of the
ice. If this were so, some relief from the congestion
might have been afforded to the refugees during the
cold period, and an opportunity might have existed
when the ice passed away for recolonization across a
land surface from the east, since a comparatively
small elevation would connect the British Islands
with the Continent. But that such an elevation con-
tinued for long after the passing of the ice is by no
means certain. On the whole, the evidence of general
glaciation of our islands as interpreted by geologists
almost postulates the extinction within our area of
the whole existing flora and fauna, and consequently
its reconstruction by immigration when a temperate
climate returned. But there is a body of evidence to
be drawn from the present and past distribution of the
existing plants and animals which is of great im-
portance in this connection. Is this biological testi-
mony in favour of the theory of the immigration of
our flora and fauna during the relatively short period
which has elapsed since the passing of the ice? To
this question different observers have given very
different answers. In order to form an idea of the
nature of the problem—it is possible here to deal
only with the case of the plants—we need to study
briefly the composition of the present flora, from the
point of view of its origin.

In the first place, it must be recalled that the British
Isles are situated on a broad shelf which extends into
the Atlantic on the western edge of Europe. In com-

168 PAST AND PRESENT

parison with the depth of the adjoining ocean, this
shelf is but little below sea-level, and a slight eleva-
tion of the land—much smaller than those which have
occurred over and over again in recent geological
times—would join our islands to Germany, Holland,
Belgium, and France. The British Isles are geo-
graphically and biologically by no means a separate
area, and they have derived their population, both
plant and animal, by immigration at various periods of
time from the great land area to the eastward. Our
present flora proves the truth of this as a general
assertion; a study of its constituents shows that it
is essentially a reduced continental European flora.
As we step from France across to England we lose a
number of plants familiar on the French side. As we
' step again from England into Ireland a further num-
ber of plants disappears; and these losses are no
doubt due either to an unsuitability of climate on the
insular areas, especially the absence of a hot summer,
or to the inability of the plants to cross the barriers
of sea which have now existed for some time. If the
whole of the flora fitted in with this idea of mere
reduction of the Continental flora by elimination, the
problem would be much simplified. But there are
other elements in it which do not harmonize with this
conception of simply a general western migration,
and which give rise to very interesting problems.

Let us first consider the main mass of our flora,
which is closely akin to that of the adjoining parts
of the Continent. When we say that it represents a
reduced Continental flora we do not imply that it is
therefore uniform in its composition throughout the
British Isles, We know, on the contrary, by every-

'~HAMPSHIRE AND CAITHNESS 169

day observation, that it varies much in its constituents.
The principal general change is noticed if one travels
from the south of England to the north of Scotland.
Great Britain extends in this direction for 700 miles—
far enough to allow climate to have a marked effect
as between its extremities. The flora of Hampshire
is very different from that of Caithness or the Ork-
neys. But both represent in the main the vegetation
of that part of the Continent which lies in the same
latitude, the Hampshire flora being akin to that
of Northern France, the Caithness flora to that of
Southern Scandinavia. The likeness is in each case
heightened by the fact that the rocks of the respective
areas correspond, producing similar soils, which tend
to support similar floras. The soft Secondary and
Tertiary deposits of Southern England are repeated
in the Paris basin and surrounding area, while the
ancient gneisses of Scotland are akin to those of Nor-
way. To quote a few instances of this north and
south difference coupled with east and west simi-
larity: the Small-flowered Crowfoot (Ranunculus
parviflorus), White Bryony (Bryonia dioica), Water
Violet (Hottonia palustris), Yellow-wort (Blackstoma
perfoliata), and Black Bryony (Tamus communis),
all widely spread throughout England and Wales, die
out in or about the Lake District, and are absent from
Scotland; the Scale Fern (Ceterach officinarum) gets
farther north—about half-way up Scotland—before
it disappears; other plants again, widespread in the
south, die out before the Mersey-Humber line, or
even the Severn-Thames line, is crossed. On the
Continent, the plants enumerated are mainly southern
in their range. All occur widely in Central and

170 PAST AND PRESENT

Southern Europe, but from Scandinavia most are
absent, and the rest are rare. On the other hand,
some characteristic Scottish species cease aS we come
southward—the little Primula scotica, for instance,
is confined to the northern extremity of Scotland; the
Chickweed Wintergreen (Trientalis europea) ranges
only as far south as Yorkshire; and the beautiful
Globe Flower (Trollius europeus), so characteristic
of northern pastures, creeps southward as far as the
Severn. The first of these is on the Continent con-
fined to Scandinavia; the others, though found in
France, etc., are characteristic of the hilly regions
there, and are much more abundant farther north-
ward.

Next to this north-and-south change, due to
climate, we may notice an east-and-west change, due
partly to climate, but more perhaps to elimination,
for in passing from France to Ireland we have to cross
two barriers of sea. The climatic change is not
unlike that experienced in going from south to north.
We leave a dry climate (rainfall under 25 inches a
year) for one of increasing wetness, a warm for a
cool summer, a colder for a milder winter.

The chief difference between the extreme west of
the British Isles and the extreme north lies in the
warmer winter of the former, frost being almost
unknown in the milder spots. But the general simi-
larity of northern and western conditions as opposed
to eastern and southern leads to a fusing of the
northern and western plant groups, so that on a map
designed to show the distribution of our species
analyzed according to their general range in Europe,
the grouping of plants in the British Isles will be

A SOIL ARCHIPELAGO 171

found to be roughly north-western as opposed to
south-eastern. The further change due to elimina-
tion of species has been already referred to. Most
plants no doubt have spread in our islands as far as
prevailing climatic and soil conditions allow, but in
other cases the sea-barriers seem to have put a period
to their natural advance. Considering the wide range
of conditions of climate and soil under which, for
instance, the Hairy Crowfoot (Ranunculus sardous),
the Common Rock-rose (Helianthemum Chamecis-
tus), the Needle Furze (Genista anglica), and the
Small Marsh Valerian (Valeriana dioica), occur in
England, Wales, and Scotland, it is difficult to impute
their absence from Ireland to climate.

Thirdly, we find (as we have already seen in the
first chapter) varying conditions of soil intruding
themselves and producing such local changes in the
grouping of the plants as may quite obscure the
broader differences just dealt with. Were our islands
a plain formed of uniform materials, the gradual
changes from south to north or east to west might
be traced step by step. But their surface is most
diversified; their rocks contain an epitome of the
whole geology of Europe; the soils are consequently
various: from the point of view of the plant world
the area is an archipelago: for some plants a desert
with occasional oases, for others an oasis enclosing
occasional deserts. Certain species are confined to the
Chalk—for instance, the Box (Buxus sempervirens)
and the Stinking Hellebore (Helleborus fatidus)—
while to others a limy soil is a barrier comparable to
that formed by the English Channel. It will be seen,
then, that when we speak of the flora in general being

172 PAST AND PRESENT

a reduced Continental one, many considerations, geo-
graphical, climatic, and edaphic, must be duly taken
into consideration if we are to understand the com-
position and distribution of our vegetation.

But making all allowance for these various disturb-
ing influences, there are found in our flora certain
plant groups which will not fit in with this general
conception of immigration from the east. Let us
take a few examples. In fir woods in Dorset, until
some forty years ago (when it was exterminated),
grew a Slender little plant allied to the Lilies, too little
known to have a popular English name, and called
by botanists Simethis planifolia or S. bicolor, the
latter name having reference to the fact that the
flower is purple on the outside, white on the inside.
This plant is unknown elsewhere in Great Britain,
and was at first set down by H. C. Watson, the lead-
ing British plant geographer, as an alien or denizen,
not a true native; but the fact that it grows over a
considerable area of very wild ground in Kerry (its
only Irish station), far from possible sources of intro-
duction, and undoubtedly native, indicates a strong
probability of the plant’s having been indigenous in
Dorset also. It is not present on the adjoining parts
of the Continent, but turns up again in the Pyrenean
region, some 500 miles to the southward, and may be
traced thence into Italy and North Africa. Did this
instance of an apparent migration from the south
stand alone, it might not excite much attention, and
we should probably be inclined to attribute the plant’s
peculiar and discontinuous distribution tothe extinction,
perhaps by human agency, of intermediate stations.
But it stands by no means alone, In Cornwall two

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Ni
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‘(PYOTHIGNVUYD VTAITAYNId) LYUOMUYALLAA LVAUN—'9Z ‘Ya

MYSTERIOUS WESTERN PLANTS 173

pretty Heaths (Erica vagans and E. ciliaris) are
found, the latter spreading to Dorset. They occur in
no other stations in the British Islands, and elsewhere
only in the Pyrenean region. North Devon is the
only home in Great Britain for the handsome Irish
Spurge (Euphorbia hiberna), which in Ireland is dis-
tributed along the west and south coasts, being very
abundant in Kerry. Outside the British Isles it also
is confined to the Pyrenean area. Crossing into Ire-
land, we find along the south and west coasts no less
than seven plants unknown in Great Britain, and else-
where found only or mainly in the Pyrenees. Of
these, three Heaths (Erica mediterranea, E. Mackay,
Dabecia polifolia) are confined to Connemara and
the Pyrenees; two Saxifrages, the London Pride
(S. wmbrosa) and the Kidney-leaved (S. Geum), with
their Irish headquarters in Kerry, are likewise con-
fined to the Pyrenean region. The beautiful
Large-flowered Butterwort (Pinguicula grandiflora,
Fig. 28), abundant in parts of Kerry and Cork,
grows in South-west Europe and the Alps; while
the Strawberry-tree (Arbutus Unedo, Fig. 29),
so pleasing and unique a feature of the Kil-
larney woods, ranges all along the Mediterranean.
A little Orchid, Neotinea intacta, found on limy
soils in Galway and the adjoining counties, and a
Grass (Schlerochloa festuciformis) which occurs on
sheltered shores on both the east and west sides of
Ireland, are likewise confined elsewhere to the
Mediterranean region. So it will be seen that along
the south-western and western borders of the British
Isles there is scattered a well-marked group of plants
belonging to the Pyrenean and Mediterranean floras,

174 PAST AND PRESENT

whose English or Irish stations are quite discontinu-
ous with their nearest Continental habitats. Here
clearly is something which calls for explanation; but
before discussing the question attention may be
drawn to a still more remarkable plant group of our
western coasts, which mingles with the southern
group referred to.

In damp meadows all round Lough Neagh, in the
North of Ireland, grows an Orchid, Spiranthes
Romangzo ffiana (Fig. 30), whose greenish-white flowers
possess a delicious fragrance resembling that of its ally,
S. spiralis, the Autumnal Lady’s Tresses. S. Roman-
zoffiana occurs also in Co. Cork, but we may search
in vain for it throughout the rest of Europe. It is
an American plant, widely spread throughout Canada
and the northern States, and found on the Asiatic as
well as the Alaskan side of Behring Sea. Again, in
pools along the western Irish coast from Cork to
Donegal, and also in the Hebrides, grows the Pipe-
wort (Eriocaulon articulatum), a little aquatic with a
tuft of grassy leaves from which a slender stem rises
above the water, bearing a button-like head of small
grey flowers. This plant also is absent from all the
rest of Europe and from Asia, but widely spread in
northern North America. The little Blue-eyed Grass
of Canada (Sisyrinchium angustifolium), again, grows
abundantly in many areas in the West of Ireland,
where it would seem to be undoubtedly native, and is
otherwise confined to North America. One or two
other plants, of the same foreign distribution, have in
Europe a less restricted range; they need not be men-
tioned individually, for enough has been said to show
that along the western coasts of the British Isles

FIG, 29.—STRAWBERRY-TREE (4RBUTUS UNEDO) AT THE LAKES OF
KILLARNEY.

FIG. 30.—SPIRANTHES ROMANZOFFIANA GROWING BY LOUGH NEAGH.

[To face p. 174.

AMERICAN STRANGERS 175

there is a small but well-marked element in the flora
which has its home in the northern portion of the
New World; in our islands these species live side by
side with the Pyrenean and Mediterranean plants
lately dealt with. Here, then, is the problem set
before us. How are we to account for the presence
of these unexpected strangers in a flora derived in
the main from a westward migration from the adjoin-
ing parts of the Continent, from which they are
absent? And especially what are their relations to
the Glacial Epoch, during which the Continental flora
was forced far southward by the advance of the ice,
while that of our own islands was probably greatly
reduced, and the balance forced into limited refuges
in the south-west, if it survived at all? It should
at once be pointed out that these peculiar Pyrenean
and American elements in our flora are matched by
similar elements in the fauna. Into the zoological
evidence we cannot go here, but one well-marked
species of each geographical group may be men-
tioned. The Spotted Slug of Kerry (Geomalacus
maculosus) is elsewhere confined to Portugal; while
a little fresh-water Sponge, Heteromeyena ryder,
widely spread in Irish lakes and rivers, and occurring
also in Scotland, is otherwise exclusively American.
In speculating, therefore, as to the origin of the
plants, we must not leave out of account the question
of the corresponding animals.

First of all, is it possible that these unexpected
organisms were introduced into our islands by man?
In an earlier chapter it has been seen how human
trade and intercourse have imported into our flora
plants from the uttermost ends of the Earth. May

176 PAST AND PRESENT

we seek in this direction an explanation? The evi-
dence is entirely against such a solution. These
plants (and animals) are found chiefly—many of them
entirely—in the wildest parts of the country, and bear
fully the stamp of natives of old standing. Human
foreign intercourse is not so old but that the intro-
ductions which it effected are still easily discernible
to the student: the plants which have come to us thus
bear the imprint of their origin; they spread outwards
from centres of human activity, and are absent from
undisturbed areas; they cannot in most cases com-
pete with the indigenous vegetation, and only exist by
confining their attempts at colonization to places
where man has ousted the native flora—such as tilled
land, roadsides, railway tracks. Even those aliens
which have succeeded in winning a place among the
native plants, such as the Monkey Flower (Mimulus
Langsdorfi) or Michaelmas Daisies (Aster spp.) of
North America, which are found sometimes in quite
wild situations, the experienced field botanist detects
readily enough. The introduction of the plants in
question by man has never been advocated by a
responsible biologist.

Assuming, then, that these groups owe their pres-
ence to natural agencies, the next question that arises
is, Could they have come to our shores across the
existing seas, or must we relegate their arrival to
periods when different distribution of sea and land
would aid their migration by allowing them to travel
across a land surface, or at least to cross sea-barriers
less wide than the present? This leads us to con-
sider the means of dispersal possessed by the species
in question, and to measure these against the nature

HOW AND WHENCE? 177

of the barriers they would have been called on to
cross. An investigation on these lines would be
lengthy, and out of place here. The reader has
already from Chapter III. acquired some insight into
the powers as well as the limitations possessed by
seeds for crossing such barriers. Summing up the
evidence briefly, it may be said that the seeds of
none of the southern group float in water; conse-
quently transport by currents is ruled out. Secondly,
none of them is so light (see pp. 62-69) as to render
it possible for them to cross the intervening sea by
wind currents; very much the lightest seeds in the
group are those of the Orchid Neotinea intacta, yet
even these could not on any reasonable theory have
been transported by wind from the plant’s nearest
station (in Southern France); the high speed of fall
of the small seeds of the Pyrenean Heaths or Saxi-
frages renders their wind transport, even from the
smaller distance which has to be reckoned with, in
their case still more improbable. There is left, then,
the agency of birds (see p. 70): can we look to these
swift messengers for assistance? The rapid diges-
tion of birds renders it futile to expect that even those
which do not crush the seeds which they eat could
bring over from the Pyrenees seeds which they have
swallowed; so we are forced back on the uncertain
method of ectozoic dispersal: that is, on the assump-
tion that seeds of these plants have been imported by
becoming entangled in the feathers of birds, or by
adhering—possibly with the aid of mud—to their
feet. That seeds are transported by these means has
been shown by the observations of Darwin and other

observers; but that the seeds of a number of different
I2

178 PAST AND PRESENT

plants, growing in different situations, should be
brought thus from the Pyrenees and Mediterranean
to our western coasts is a highly speculative sugges-
tion. If we discard it, there is left the hypothesis
that the plants migrated long ago overland, at a time
when the western coastline of Europe was continuous
and lay farther seaward. Such conditions have not
occurred since the Ice Age; so we have to assume
that the plants, arriving perhaps in Pliocene times by
slow terrestrial dispersal, and subsequently cut off by
invasions of the sea upon their line of advance, sur-
vived the cold and ice of the Glacial Period within the
limits of our islands. That appears, on consideration
of the geological evidence of widespread glaciation,
sufficiently improbable; but we must remember that
the evidence supplied by the plants is buttressed for-
midably by that of the corresponding animals, some
of which, such as the Kerry Slug, are far less fitted
for transmarine dispersal than are the seeds of plants.
Also, we are faced with the problem of the American
plants, and such organisms as the American Sponge,
Heteromeyenia: a direct crossing of the ocean
appears for them wholly impossible. Yet if they
crossed over long-gone land surfaces, their arrival on
this side of the Atlantic must be very ancient, and
they must certainly have weathered successfully the
Great Ice Age. The problem, it is clear, is an ex-
ceedingly difficult one, upon which it would be rash
to pronounce any hasty opinion. Students of the sub-
ject have come to widely difficult conclusions: some
holding with Edward Forbes that these Lusitanian
and American organisms represent the very oldest
element in our fauna and flora, having migrated over

A TANGLED SKEIN 179

bygone land surfaces in distant times and successfully
survived the terrors of the Glacial Period; others
claiming a much less remote period for their immigra-
tion. Indeed, one eminent recent writer on the sub-
ject, the late Clement Reid, considered that the Lusi-
tanian plants are among the most recent arrivals in
the country, their introduction being due mainly to
birds driven by exceptional gales.

The question of the Lusitanian and American ele-
ments in our flora has been treated at some length
both because it offers one of the most interesting
problems in British botany, and because it affords a
good illustration of the far-reaching nature of the
questions which may lie behind the occurrence on
our hills or in our valleys of even the humblest plant
or animal. Each organism has a long record behind
it, stretching far beyond the earliest periods of human
history; and it is only by wide and patient study that
we can hope to trace any portion of its story.

CHAPTER VII

SOME INTERESTING BRITISH PLANT GROUPS

In the preceding chapters glimpses have been obtained
of some of the wider aspects of plant life, particularly
as seen on the hills and plains of our own country.
The species composing our flora have been seen
mostly, not as individuals, but as portions of regiments
and armies, particular plants being mentioned but
seldom, where required for purposes of illustration.
In the final chapter it will be well to abandon this
collective treatment, and glance at a few individual
species or genera or small natural groups which
possess features of interest of one sort or another.
No systematic arrangement need be attempted: it
will be pleasanter to ramble on, allowing our points of
inguiry to turn up as they might on a country walk.
A consideration of abnormalities in the manner in
which plants obtain their food-supply—irregular
nutrition, as it has been called—will raise some inter-
esting questions, and will bring us up against some of
the most remarkable species which are found in the
British flora. The outlines of the method by which
plants manufacture their food are familiar to all, and
have been referred to already (pp. 75, 132). The roots
absorb from the soil water containing dissolved salts,
which is passed up by the stems into the leaves. The

leaves extract from the air carbon dioxide. The
180

CHLOROPHYLL—OR NONE 181

chlorophyll, or green colouring-matter of the leaves,
possesses the remarkable power in the presence of
sunlight of breaking up and recombining these sub-
stances into the compounds which go to build up the
plant-body. As has been pointed out, it is this power
of forming organic out of inorganic matter that
especially distinguishes plants from animals. But not
all plants manufacture their food in this way. A large
number feed like animals, finding their sustenance
sometimes in living, more often in dead, organic
material, either animal or vegetable. The whole
enormous group of the Fungi do not possess chloro-
phyll, and in consequence are dependent on organic
materials for their food. Some of the most familiar
of the lower Fungi live on cheese, leather, bread, or
any other damp animal or vegetable material. The
higher forms, which decorate our woods and pastures,
find their sustenance largely in leaf-mould. The
groups of the Mosses, Hepatics, and Ferns, which are
more highly organized than the Fungi, possess chloro-
phyll, and manufacture their own food; and it is with
some little surprise, therefore, that when we come to
the Seed Plants, the highest group of all, we find,
though in relatively few cases, a reversion to the
animal trait of using organic food. Some of our
woodland plants have taken so entirely to a diet of
leaf-mould that they have discarded the apparatus
which would enable them to manufacture their own
food. Chlorophyll, the magic wand by means of
which the inorganic is transformed into the organic,
and also leaves, the mills wherein the transformation
takes place, are absent from these plants. For
instance, the Bird’s-nest Orchis (Neottia Nidus-avis),

182 SOME BRITISH PLANT GROUPS

Fic. 31.—BirpD’s-NEST OrcHIs (NEOTTIA NIDUS-AVIS).

PARASITES 183

sends up from a mass of fleshy roots a bare brown
stem about a foot high, bearing a spike of brown
flowers, the whole being so much of the same colour
as the dead beech leaves among which the plant is
usually found that it may easily be passed over. It is
quite incapable of manufacturing its own food, but
feeds on the decaying vegetable material which was
manufactured by the trees under whose shadow it
grows.

It is but a step from saprophytes such as this to
parasites, which feed, not on dead, but on living
organic matter. In the case of the higher plants, the
hosts are always themselves plants, though, as pointed
out on p. 78, they are, in the case of the Fungi,
sometimes animals. One of the most interesting of
these parasites is, like the Bird’s-nest Orchis, found in
woods—the Yellow Bird’s-nest (Monotropa Hypo-
pitys). This is, like the last, a leafless plant devoid of
chlorophyll, sending up from a tangled root-mass one
or more pale yellow stems, each bearing a drooping
raceme of flowers of the same colour. The flowers
show affinities to the Heath family (Ericacee), but the
plant differs much from any other member of that
Order. The Yellow Bird’s-nest is always found
associated with the mycelium, or cobwebby under-
ground portion, of a fungus, on which it appears to
be parasitic. The fungus is in turn a saprophyte, and
the Seed Plant feeds at second hand, so to speak, on
decaying vegetable matter. This parasitism of a seed
plant on a fungus is a very exceptional case. A
more frequent type is offered by the Broomrapes
(Orobanche), which we may find in meadows, etc.,
growing on Clover, Thyme, Ivy, and so on. These

184 SOME BRITISH PLANT GROUPS

resemble the Bird’s-nest Orchis in sending up a stout
leafless stem crowned with a spike of flowers. The
different species display almost every colour except
green, being red or brown or purple or yellow, and
one blue. These plants live by attaching themselves
to the roots of their host, and drawing in the nourish-
ment they need for their own growth—robbery pure
and simple. The seeds of the Broomrapes are very
numerous and very light, and of singularly primitive
structure. When they develop, they produce, not a
young plant with root and stem, but a delicate spiral
filament which grows down into the ground. Should
this meet with a root of its host-plant, it adheres to it
closely, and grows into a swollen knob at the point of
attachment, which when mature sends up the flower-
ing stem already described. Should a suitable root
not be met with, the filament withers away and dies as
soon as it has exhausted the small amount of reserve
food stored in the seed. A parasite of a less sedentary
habit, to be found in spring in our copses and hedge-
rows, is the Toothwort (Lathrea Squamaria). This
curious plant has underground creeping stems clothed
with whitish, tooth-like, fleshy scales (curiously modi-
fied leaves). In autumn and winter the stems lie
dormant. In spring they send out delicate roots
which attach themselves to the roots of trees of
various kinds and suck nourishment from them, with
the aid of which the plant sends up into the air fleshy
cream-coloured stems bearing many drooping flowers
of the same hue, the structure of which shows that the
plant is closely allied to the Broomrapes. The Tooth-
wort is a very harmless parasite, and the species of
Broomrape also, though sometimes abundant on

THE DODDER 185

Clover, etc., do not do much damage; but the same
cannot be said for the Dodders (Cuscuta), one of
which is parasitic on Flax, another on Clover, and so
on. These are little annual plants whose seeds lie
dormant in the soil throughout the winter and well
into the spring. Then the young plant, which has
remained coiled up inside this seed like a spring,
pushes forth in the form of a tiny thread. While one
extremity fastens itself to the soil, the other rises up
into the air, and its point slowly revolves. Should it
come in contact with a living stem of a suitable plant,
it attaches itself to it by means of disc-like suckers,
penetrates the tissues of its victim, draws out nourish-
ment, and, growing rapidly, spreads from plant to
plant, taking a couple of close turns round each stem
after the manner of a lasso, and then sending in
rootlets from the attaching disc, and sucking the life
out of each as it goes. It has no roots, no leaves, no
chlorophyll, being of a red or yellow tint, and is
entirely dependent for its nourishment on the plants
which it attacks. In course of time—about August—
an abundance of pretty little waxy-white flowers are
produced, which produce the next year’s supply of
seed. A few seedlings of Dodder, developing under
suitable conditions, will form a colony which is capable
in its few months of life of sweeping over a large area,
wrecking the vegetation on which it has battened.

A parasite of a quite different sort may be studied
in the familiar Mistletoe (Viscum album). It is the
only parasitic native plant which is shrubby, or which
perches itself on trees (the seeds being spread by
birds, which devour the white berries). It is not, like
some parasites, particular as to the species upon which.

18 SOME BRITISH PLANT GROUPS

it grows, flourishing equally upon a number of hosts,
and even capable of living upon its own species. It
differs from those parasites which we have been con-
sidering in possessing an abundance of green leaves,
and being therefore capable of manufacturing its own
food. At the same time, it has no roots which can
penetrate the soil, and is incapable of an independent
existence. It seems probable that its relations with its
host are to some extent symbiotic—that is, each giving
to the other—rather than purely parasitic, where the
benefit is entirely on one side. The Mistletoe, retain-
ing its leaves and manufacturing food throughout the
year, is clearly capable of aiding its host, which loses
its leaves in autumn, and cannot form fresh nourish-
ment until spring is well advanced.

Before leaving this question of abnormal methods
of procuring food as found among the higher plants,
we may return for a few moments to the consideration
of carnivorous plants, to which reference was made in
Chapter IV. Of these the Sundews (Drosera), Butter-
worts (Pinguicula), and Bladderworts (Utricularia)
supply very interesting examples within our own flora,
which anyone may study on a holiday spent on the
moors or mountains. The Sundews are familiar to all
plant lovers—little plants of the bogland, usually
growing among Sphagnum, and well distinguished by
their leaves decked with spreading red hairs, each of
which is tipped with a little drop of sparkling sticky
fluid. It is these hairs or tentacles and their move-
ments which place the Sundews among the most
interesting of all plants. It is important to note that
they are not hairs in the ordinary sense, which are
organs of very simple structure arising from the

INSECTIVOROUS PLANTS 187

epidermis or skin of the leaf. The tentacles of
Drosera have a complicated structure resembling that
of leaves, and the tip is occupied by a gland which
produces the sticky secretion already mentioned.
These glands are exceedingly sensitive, and, more-
over, sensitive in a selective way. They are unaffected
by the drops of rain which frequently fall on them,
but the touch of any solid body, especially of organic
material, immediately affects them; most of all nitro-
genous substances of any kind. Darwin found that
a morsel of human hair weighing only ;s:+20 of a
grain was sufficient to set the machinery of Drosera
in motion, and that immersion of a leaf in a solution
of phosphate of ammonium so weak that each tentacle
could absorb only zso0v000 of a grain acted as a
strong stimulus. In nature the stimulus is usually
given by some unwary insect—a midge or other small
flying creature—which, attracted by the bright colour
or by the odour of the leaf, ventures too close, and
becomes entangled among the sticky hairs. Then a
most interesting series of events takes place. Almost
at once the tentacles—first the ones actually touched,
and then the adjoining ones—bend towards the point
of disturbance, closing down one by one on the
unfortunate victim till the leaf resembles a closed fist.
At the same time the production of secretion increases,
so as further to entangle the victim. When it is
firmly secured, the secretion changes in character.
Digestive ferments, closely resembling those by which
animals digest their food, are poured out. These
dissolve the animal’s body, all except the horny parts;
the digested materials are then absorbed into the
plant, which, as experiments show, benefits consider-

188 SOME BRITISH PLANT GROUPS

ably by the addition to its diet of this animal food.
When digestion is completed, the tentacles open again
and prepare for a fresh victim. While the details of
this remarkable process have been worked out only
by careful and minute research in the laboratory, the
main movements may be watched by anyone on any
British moorland; or, bringing home a few plants in
the damp moss in which they grow, we may amuse
ourselves by experiments in feeding them.

In comparison with the Sundews, the other insect-
ivorous plants which are included in the British flora
are of less interest. The Butterworts (Pinguicula), of
which four species are known in these islands, have a
rosette of smooth, broad, yellowish leaves covered
with glands which exercise the same functions as
those of Drosera. To the touch of raindrops, sand-
grains, or other inorganic substances they are indif-
ferent; but a tiny insect alighting on the sticky leaf at
once provokes an outpouring of secretion, while the
leaf rolls inward from the edges till the victim is
securely caught; it is then digested as in the Sundew.

The Bladderworts (Utricularia), of which several
species may be found floating in boggy pools, are
rootless, limp plants with finely divided leaves, among
which are numerous little bladders (in reality strangely
modified leaflets), and upright stems bearing pretty
yellow Snapdragon-like flowers. The bladders do not
help the plant to float, and appear to have for their
sole function the securing of animal food. In the
Common Bladderwort (U. vulgaris) they are about
io inch long. At the upper end is a little hinged door,
which is kept closed as by a spring against a thickened
rim or door-frame. Outside the door are a few stiff

THE) TOPS OF THE: MOUNTAINS ‘189

hairs, a convenient perching-place for small aquatic
creatures such as the minute Crustaceans known as
Water Fleas. Should one of these try to explore the
bladder, the door opens easily, but closes at once
behind the rash wanderer, imprisoning it. The
Bladderworts do not digest the victims which they
secure in this manner, but when the bodies are
decomposed by means of bacteria, the products of
decomposition are absorbed. How fatal this mouse-
trap arrangement is to Water Fleas can be determined
by dissecting the bladders of the plant.

Thus far, then, as regards some of those peculiar
members of our flora which make their living by the
unusual method of stealing their neighbour’s goods,
or which eke out their existence by the capture of
animal food. Let us now take another line of
exploration and consider the conditions which prevail
on the loftiest portions of our islands, and how these
affect the vegetation. Mountain-tops are always
attractive and interesting places—the keen rarefied
air, the freedom and openness of the summits, fill us
with exhilaration. Our own mountains are not lofty;
nowhere in the British Islands is a height of a mile
attained. But we have only to ascend to a couple of
thousand feet to note a great change in the vegetation.
The plants of the lower grounds to a great extent die
out (though some accompany us to our highest
summit), and the vegetation takes on a low compact
form, which becomes more emphasized as we ascend
farther, till in sheltered nooks alone do we find any
plants more than a fewinches in height. Furthermore,
we notice an incoming of new plants unknown at
lower levels, which search will show us to be confined

1g0 SOME BRITISH PLANT GROUPS

to the mountains, each of them having a more or less
definite limit below which (also above which, though
our mountains are not high enough to render this
point well marked) it is not found.

Among the plant formations and associations of the
lower grounds which we considered in Chapter II. it
was noted that the controlling factors were mainly
connected with the nature of the soil and the amount
of the water-supply. Here on the mountains another
factor, the climatic, comes in emphatically, and takes
charge. The temperature of the atmosphere falls one
degree centigrade for about every 200 feet of eleva-
tion, so that a sharp frost on the lowlands may easily
mean zero Fahrenheit on a 4,000-foot hill. The
rarefaction of the atmosphere, too, tends to produce
a much greater range of temperature, both diurnal
and seasonal. Again, the velocity of the wind is much
higher on the summits than on the plains, where
friction is greatly increased by trees and other
obstacles. These high winds have a very great
cooling effect, as we may notice on our own bodies
even in summer. In fact, as regards climatic change,
an ascent of a thousand feet is comparable to a
journey of several hundred miles northward. Anyone
who has, on a winter tramp, been caught in a snow-
storm on a 3,000-foot hill is forcibly reminded of what
he has read of winter conditions in the Arctic regions.
In ascending Ben Nevis we travel, in a sense, to the
Arctic Circle. But the analogy is false, for conditions,
especially in summer, are very different in the two
places. The plants of our mountains have all the
advantages of the high summer elevation of the sun,
very different from the weak, sloping sunlight of the

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FIG. 32.—ALPINE PLANT-BOSS (SILENE ACAULIS, HYMENOPHYLLUM UNILATERALE,
MNIUM HORNUM).

[To face p. 191.

SOME ASPECTS OF PLANT LIFE.

ALPINE CLIMATE IQ!

Arctic. On our loftier hills, indeed, the heat is on
occasions oppressive.

Again, the mountain climate, with its heavy rainfall
and long cold period, tends to the formation of peat;
and the acids thus engendered in the soil, as well as
the low temperature prevailing during most of the
year, render difficult the absorption of water by the
roots of plants. The conditions under which alpine
plants, then, live may be summed up as follows: a
long cold winter, a short summer; great exposure;
scarcity of food-supply. The modifications which
plants have undergone to meet these conditions are
very marked, and render alpine plants a source of
constant interest to the traveller and of delight to the
gardener. The effect of low temperature (also of
peaty soil) in rendering difficult the absorption of
food materials, and causing extensive root production
and limited stem and leaf growth, is immediately
observable. In Fig. 33 is seen an alpine Stonecrop
(Sedum primuloides) as growing on the Chinese Alps
at some 12,000 feet. The root is out of all proportion
to the aerial parts. The same plant in the garden
forms a little bush with branching stems half a foot
long, and flowers borne on leafy axillary shoots a
couple of inches long, while the roots are short and
tufted. The most characteristic form which alpine
plants assume may be called the cushion type. This is
produced by excessive branching of the stems of
small-leaved plants, accompanied by but little longi-
tudinal growth; and it is excellently shown in many
well-known plants such as the Mossy Saxifrages,
the Kabschia Saxifrages, the Cushion Pink (Silene
acaulis), and a number of others, The same type of

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SOME BRITISH PLANT GROUPS

192

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CUSHION PLANTS 193

plant growth is characteristic of semi-desert regions,
where the points of similarity of environment to those
of the mountain-tops are evident. This cushion form
has many advantages for the alpine plant. It keeps it
warm in winter and cool and damp in summer; it
allows it to produce a great amount of blossom
without the necessity for extensive growth; it resists
the utmost efforts of furious gusts of wind almost as
well as would a half-buried stone; on the most storm-
swept cliffs its fresh green blobs “welcome every
changing hour, and weather every sky.” Fig. 32
shows a boss of this kind, composed of the Cushion
Pink (Silene acaulis), with an admixture of Filmy Fern
(Hymenophyllum unilaterale) and a Moss (Mnium
hornum). The shrubs of the alpine zone are mostly
small and creeping, weaving themselves among the
vegetation, and with low grasses and sedges forming
a mat which is equally resistant to all inimical condi-
tions. Their leaves are small, to avoid damage by
wind or by excessive transpiration. In some genera—
for instance, Veronica—the diminution of leaf surface
accompanying more elevated habitat is very striking.
In the New Zealand lowlands broad-leaved forms
(Fig. 34, left) are met with, which give way, as one
ascends to 8,000 feet, to such forms as V. Hecton
(Fig. 34, right), in which the leaves are reduced to
mere scales, and the plant much resembles some of the
Cypresses or other Conifers with marked xerophile
characters.

Other plants, again, escape climatic rigours by
burrowing underground and throwing up short aerial
stems in summer; the spindly plants of the lowland,
with diffuse stems, and also the light-rooted annuals,

13

194 SOME BRITISH PLANT GROUPS

are conspicuous by their absence. The brief summer
and long winter are unsuitable to the economy of
annual plants; and the alpine perennials are so con-
structed that with the passing away of the cold,
flowering and fruiting may be accomplished quickly,
before winter descends again. The abundance and

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vividness of the flowers of alpines is almost proverbial.
Several explanations have been put forward to account
for these features, and probably there is some truth in
each of them. It has been held that the brilliancy of
the sunlight is accountable; the shortness of the period
available for seed-production, and the consequent need
of prompt pollination by insects, have been suggested,

ALPINE FLOWERS 195

as leading to urgent advertisement by means of
brilliant coloration; while the fact that the pollinating
insects are largely Butterflies, the most zsthetic of
flower visitors, has also been put forward as account-
ing for it. Be that as it may, the glowing patches of
colour produced by many quite minute alpine plants

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are among the most delightful things in nature. Our
own flora contains but few of the more striking of
these jewels; but where will one find a more delightful
sight than a well-flowered patch of Spring Gentian
(G. verna) or Mountain Avens (Dryas octopetala) or
Purple Saxifrage (S. oppositifolia) ?

196 SOME BRITISH PLANT GROUPS

As we mount higher and higher on the hills, plants
become fewer and more stunted, but hardy forms
persist even long after the level of perpetual snow is
reached. In the Alps, Ranunculus glacialis occurs up
to an elevation of about 14,000 feet. In West Tibet,
strange stunted species of Saussurea, a genus of
Composite allied to the Thistles, exist at elevations of
17,000 to 19,000 feet. Some of the Cryptogams go
higher still, Lichens grow on the summit of
Kilimanjaro (over 19,600 feet); and Schimper sug-
gests* that this may by no means represent the
absolute limit of vegetation. The prevalence of snow
and ice does not of itself inhibit the lower forms of
life. Since “red snow” was shown, nearly a century
ago, to be due to colonies of a minute Alga, many
microscopic organisms of like habitat have been dis-
covered, and these algal colonists of snow and ice
are now known to extend far over the frozen deserts
of the highest hills, and to penetrate into the remotest
regions of the Arctic and Antarctic. ©

As we get up to the level of perpetual snow on the
higher mountains, or go northward within the Arctic
Circle, the conditions under which plant life exists
become very severe. It has been pointed out that in
spite of a superficial similarity, wide disparity exists
between the sets of conditions prevailing in the two
kinds of habitat just mentioned. In the Arctic the
winter is continuously dark and the summer continu-
ously light; and in summer the sun is never far above
the horizon, so that the temperature remains low,
though it rises amply far enough above freezing-point

* A. F. W. ScHiIMPER: ‘‘ Plant Geography ’’ (English transla-
tion, 1903), p. 719.

THE REGIONS OF GREATEST COLD 1097

to allow of plant life. On high mountains, on the
other hand, there is the same succession of day and
night which prevails on the plains below, the height of
the sun above the horizon being a question of latitude.
On mountain-ranges situated within the Temperate
Zone, such as the European Alps, and much more on
those nearer the Equator, the day temperature in
summer is very high wherever the sun strikes, and
while plants may have to withstand at night a tem-
perature comparable to that borne by the Arctic flora,
they must endure by day the most intense insolation.
Neither in the Arctic nor on the high hills does
plant life cease merely on account of low temperature.
Species belonging to many families venture even be-
yond the limit of perpetual snow. The coldest known
area on the earth’s surface lies in Siberia, actually
within the limits of forest growth, and trees and herbs
of many species survive winter temperatures which may
fall below —60° C. (76 degrees of frost Fahrenheit).
They freeze into solid lumps of ice without injury, and
indeed the thawing process in spring is more dangerous
to them than their congealment in autumn. Many of
the high alpine plants are frozen solid every night
only to be roasted alive by day; it seems amazing that
any living organisms can endure under such circum-
stances. Yet it is not only species confined to areas
where such extremes exist, and specially adapted
thereto, which can resist them successfully. In Central
Europe the Common Chickweed and Common Daisy
are often frozen solid, so that leaves and stems snap
between the fingers like sealing-wax, yet with a rise of .
temperature they continue growth quite unperturbed,
just as they do in areas where frost is unknown. The

198 SOME BRITISH PLANT GROUPS

main difficulty induced by cold would appear to be the
withdrawal of available water; if that goes on for too
long, life ceases. Of course the suspension of activi-
ties which accompanies freezing cannot continue
indefinitely, and in the cold regions of the Earth plants
are found only where for a sufficient portion of the
year the maximum temperature rises above freezing-
point enough to allow of ordinary vital functions being
resumed. A curious point in this power of resistance
in plants to extremes of temperature is that they
display no obvious protective adaptations. “ Our
present powers of investigation,” Schimper con-
cludes,* “do not enable us to recognize in plants any
protective means against cold. The capacity of with-
standing intense cold is a specific property of the
protoplasm of certain plants, and is quite unassisted
by protective means that are external.”

It is a far cry from the high Alps to the seashore,
but it will be of interest to examine next the lower
limit of the range of the Seed Plants. While the upper
limit varies much in different latitudes, according to
the distribution of temperature, the lower is controlled
by sea-level, which (for our purpose at least) is
uniform over the whole globe. The level of the fresh
waters, whose margin marks the limit of the bulk of
the Seed Plants, is, on the other hand, various, lakes
being situated at different heights above (and occa-
sionally below) sea-level, while rivers slope across the
lands down to the ocean. While the sea margin forms
a very real barrier to the spread of Seed Plants, the
lakes and rivers, on the other hand, yield many

* A. F. W. ScuimPer: ‘‘ Plant Geography’’ (English transla-
tion, 1903), p. 41.

ORIGIN OF PLANT LIFE | 199

inhabitants, and we must examine the relations exist-
ing between the aquatic and the terrestrial species.

As has been stated on a former page, the evidence
points to life having originated in the water, at a
period extremely remote. The most lowly as well as
the most minute of all organisms are the bacteria,
some of which are in size beyond the limit of the most
powerful microscope to detect, their presence being
known only by their chemical actions. The most
primitive groups of bacteria, known as prototrophic,
are able to live without light, deriving their nourish-
ment by the breaking up of inorganic chemical
compounds. It is difficult to conceive of any living
organism more primitive than these, and quite possibly
they recall that dim borderland where merely chemical
structure and action mysteriously advanced into the
cell structure and purposive chemical changes which
we call life. From that lowly stage the evolution of
plant life has been marked especially by three great
forward bounds, of inestimable importance. The first
of these was the “invention” of chlorophyll, which
allowed plants to use for their life-processes the vast
supply of energy furnished by the Sun. Sunlight then
became essential to life, and the Algz, the probable
ancestors of all the higher plants, were developed,
presumably through the peculiar Cyanophycee, or
“Blue-green Alge,” in which the chlorophyll is in a
somewhat undifferentiated condition. Much later than
this stage, yet far back in the history of evolution,
occurred the second of the great forward steps. This
was the desertion of the water for the land, which
opened up for the plant world vast new fields and a
great variety of new conditions. The final stage was

200 SOME BRITISH PLANT GROUPS

reached by the abandonment of the aquatic mode of
pollination by means of swimming spermatozoids, as
still found in the Maidenhair Tree (Ginkgo), Cycads,
Ferns, and groups lower in the scale, and the adoption
instead of pollination through the medium of the air,
“which” (to quote Mrs. Arber’s happy phrase) “has
won for them the freedom of the land.” The Seed
Plants, then, achieved their wonderful abundance and
variety owing to the highly stimulating conditions
offered by a terrestrial existence; we must assign to all
the existing types a long terrestrial ancestry. How,
then, about the water plants whose leaves and flowers
so decorate our lakes? There seems no doubt* that
they are species which have left the land to resume
the aquatic habits of their remote ancestors. With few
exceptions they retain the aerial mode of pollination
which is the pride of the specialized land plants. The
pressure of competition has probably driven them into
the water, where they descend as far as the lessening
light-supply will allow. Some—presumably the earliest
to take to an aquatic life—have all their relations to
keep them company, the remote ancestor which
adopted an aquatic habit being now represented by
many species, or even by many genera. In other cases
a terrestrial genus or order has few or only a single
aquatic representative. It may be assumed that in
such a case the aquatic habit has been recently
acquired. The great majority of water plants send
their flowers up above the surface to be pollinated by
wind or (more rarely) by insects. It may be noted
that few of the more highly evolved groups of Seed

* See AGNEs ARBER: ‘‘ Aquatic Angiosperms: the Significance
of their Systematic Distribution,’’ Journal of Botany, 1919, p. 83.

SEA AND LAND FLORAS 201

Plants are represented in the aquatic flora; wind-
pollinated flowers of a rather primitive type of
structure are the rule in our lakes and rivers; which
points to an early assumption of the aquatic habit, and
suggests that the land is more favourable than the
water for the evolution of higher types.

While the fresh waters of the globe have thus
acquired from the land an abundant population of
higher plants, the presence of salt, in water as on
land, has had a deterrent effect. The sea was at first
fresh. The primitive ocean derived by condensation
from a cooling atmosphere in the early days of the
world’s history contained no excess of salts. Whether
life arose while this condition still persisted it is not
possible to say; but as the sea grew salter owing to
the rivers bringing into it incessantly salts derived
from the land, the Seaweeds alone of the great groups
of plants adapted themselves to saline conditions, and
the ocean is now their unchallenged kingdom. The
divisions which are represented by the Mosses, Liver-
worts, Club-mosses, Horsetails, and Ferns, have not,
and so far as is known never had, a single repre-
sentative in the sea. Only one or two Fungi—often
symbiotically combined with Algz to form Lichens—
and a very few Flowering Plants, have attempted
marine colonization, after long ages spent on land;
and they have met with indifferent success. As we
pass from fresh to brackish water, the population
decreases rapidly, till in the seas surrounding our
islands only one Seed Plant—the Grass-wrack, Zostera
marina—has adopted a habitat which is thoroughly
marine, and very few are found in other parts of the
world. A study of the meeting-ground of the land

202 SOME BRITISH PLANT GROUPS

and sea plants, such as we may make on rambles
along the coast, supplies us with some interesting
material. On sandy shores, the wave-trampled beach,
shifting under the influence of winds and currents,
offers a stretch of “no-man’s-land”—a desert strip
untenanted alike by terrestrial or marine plants. The
former do not descend below spring-tide mark, if they
go so far; the latter cannot obtain foothold on the
unstable substratum. The peculiar characters of the
terrestrial beach plants has been referred to on a
previous page (p. 36). On rocky shores the “ desert ”
strip is much narrowed, and a certain overlap may
often be found, for the Lichens—essentially a terres-
trial group—descend from the plant-covered slopes
into the spray-swept zone below, and on to mix with
the Seaweeds which occupy the belt under high-water
mark, some of them, species of Verrucaria and
Arthropyrenia, continuing downward till the low-
water mark of spring tides is reached. On steep
rocky shores the dividing-line between the Flowering
Plants and the Seaweeds is quite narrow, and varies
in elevation with the exposure. On cliffy coasts open
to the Atlantic waves the uppermost Seaweeds, such
as Pelvetia, which only asks to be wetted periodically
by spray, occur far above high-water mark, the lowest
Seed Plants perching on the rocks much higher still—
sometimes not venturing to within 100 feet of the
water-level. Under such extreme conditions none of
the higher land plants venture down towards the
unfriendly sea. To see the overlap of the terrestrial
and maritime vegetation well developed we seek con-
ditions entirely different, where amid shallow inlets
and salt-marshes land and sea merge imperceptibly.

THE SALT-MARSH AGAIN 203

Here the absence of higher plants from the areas
below high water, as compared with their abundance
above water-level, is a conspicuous feature. This is a
noteworthy point, because if we assume that the
presence of salt is the main factor which has pre-
vented the land plants from spreading downwards, we
are faced with the fact that the soil of the salt-marsh,
where many such plants occur, may by evaporation of
water become much more highly charged with salt
than the sea itself. Yet the salt-marsh flora includes
representatives of many Natural Orders, including
some of the most highly specialized families—
Ranunculacee (R. sceleratus), Crucifere (Cochleana
spp.), Caryophyllacee (Alsine), Umbellifere (Apium
graveolens, Genanthe Lachenalu), Composite (Aster
Tripolium, Artemisia maritima), Primulacee (Glaux
maritima), Plumbaginee (Statice, Limonium). It
seems clear that it is the assumption of the marine
habit which is the stumbling-block, not the presence
of salt. The Grass-wrack or Zostera, our only marine
Seed Plant, comes of one of the oldest stocks of
aquatic plants, and its nearest relatives have long been
toying with the idea ofa maritime habitat. The Order
to which it belongs, the Naiadacee or Pondweed
family, from their worldwide range, their number,
their variety, and their uniformly aquatic habit, may
be set down as among the earliest Seed Piant colonists
of lakes and rivers; some of them favour brackish
water, while others besides the Grass-wrack have
taken to marine life. Without going beyond the
limits of our native Naiadacee@ we can study the
various stages, and form a picture of how the Grass-
wrack migrated to the sea. First we have the

204 SOME BRITISH PLANT GROUPS

numerous Pondweeds which grow in our lakes and
rivers—plants with leaves broad and floating, or
narrow and submerged, and inconspicuous flowers
which rise above the water and are pollinated by the
wind. Next we find several narrow-leaved Pondweeds
which grow in brackish pools; and with them are
some allies, the Tassel Pondweed (Ruppia) and
Horned Pondweed (Zannichellia), with more reduced
flowers and often a more nearly marine habitat, as
they sometimes mix with Seaweeds on the open
shores of estuaries; in these plants we find the stages
of a most interesting return to the archaic method of
water-pollination, so long discarded by the great mass
of the Seed Plants. In the flower of Ruppia, which
consists merely of two stamens and four carpels with-
out corolla or calyx, the pollen is liberated under
water, and, being light, rises to the surface; older
flowers have already, by growth of the flower-stalk,
reached the surface, and they become pollinated by
the floating grains. In Zannichellia the process is in
general similar, save that the flowers are either male
or female, the former consisting of nothing but a
single stamen. The Naiads (Naias) form an allied
genus, and are slender annual herbs, growing com-
pletely submerged in fresh or brackish water. One of
them (N. flevilis) occurs in lakes at rare intervals
along the western edge of the British Isles; and
another, N. marina, is found living in only one spot
in Britain—Hickling Broad in Norfolk; their fossil
seeds embedded in old lake deposits show that in
former times both were more widely spread than now
in Western Europe, and that other species of the
genus also occurred. In the Naiads complete rever- .

LAND PLANTS IN THE SEA 205

sion to water-pollination is found. When the very
simple male flowers shed their pollen, the grains,
which are heavy owing to the presence of starch, fall
through the water on to the female flowers which are
borne below them, or are carried by currents to other
flowers. Lastly we come to the Grass-wracks, a small
group of submersed marine plants. While some of
them, like our little native Z. nana, haunt muddy sands
between tides, our more familiar species, the common
Z. marina, is thoroughly marine, growing tall and
vigorous among the large Seaweeds down to far
below low-water mark (to over 30 feet in the Baltic).
The plant has, nevertheless, not yet developed sub-
mersed pollination, the pollen-grains rising to the
surface, where they are caught by the stigmas of
floating female flowers. It follows that the indi-
viduals rooted in the deeper water, though growing
vigorously, do not mature seed, for the production of
which the species has to rely on plants which, at least
at low water, are rooted sufficiently near the surface
_to allow the flowers to rise above it. Could the
species achieve submersed pollination, it appears quite
capable of colonizing throughout the Laminarian
zone, wherever there is a soft substratum for its
creeping stems.

The land plants of the salt-marsh, as well as the
aquatic species, furnish interesting examples of over-
lap with the sea flora, but a brief reference must suffice.
The Glasswort (Salicornia), for instance, has furnished
itself with a very complete equipment for the difficult
conditions of salt-marsh life (see pp. 17, 18), and grows
far out on the mud-flats in green colonies, often
below the upper limit of the Bladder-wrack or Fucus,

206 SOME BRITISH PLANT GROUPS

the common brown Seaweed of our shores. The
Glasswort has discarded leaves, its stems have become
thick and succulent, and its flowers, reduced to the
minutest and simplest dimensions, are almost buried
in the fleshy branches. Thus armed, it braves the
salt-desert of the mud-flats, and repeated submersion
by the tides leaves it uninjured. Under the peculiar
conditions of its life, it relies neither on insects nor
wind nor water for pollination, the flowers being self-
pollinated. A more surprising commingling is that
which is illustrated by A. D. Cotton in his report on
the Seaweeds of the Clare Island district (Proc. Royal
Irish Academy, vol. xxxi., 1912), where, on peaty soil
a little above mean high-tide level, the Sea Pink is
shown forming a sward with a peculiar dwarf form of
Fucus (F. vesiculosus, var. muscoides) and a few other
salt-marsh Seed Plants, suchas Schlerochloa (Glyceria),
Glaux, Salicornia. The Sea Pink is highly evolved
florally, and differs widely from the Saltwort in its
abundant production of leaves and showy flowers, the
absence of any conspicuous xerophile characters, and
the fact that it is not confined to the coasts, being
often a member of the alpine flora of our higher hills.
In its association with Fucus it may be claimed that it
is the latter which is “out of water,’ as it never
produces fruit, increasing solely by means of vegeta-
tive growth. At the same time, so closely does it
press its partner in the struggle for room, that the
Sea Pink fails to form its usual robust clumps, its stem
being mostly unbranched and its stature dwarfed.
Viewing generally the migration of the Seed Plants
from land to water, we see that the fresh waters of
the world, untenanted by other large plants, have been

SEAWEEDS 207

fully colonized, generally a long time ago, and by
plants of rather early types. But as regards the sea,
the luxuriant Algal vegetation which is in possession
of our shores has no reason to tremble for its
supremacy. Beautifully adapted for their life, whether
in sheltered bays or on stormy rocks, the Seaweeds
show no sign of relinquishing the domain that has
been theirs since the earliest rocks which still display
traces of organic life were laid down in Cambrian
seas.

INDEX

AGRICULTURE, 135 Mycetozoa, 156
Alien plants, 143 Myxomycetes, 156
Alpine plants, 190
Animal-eating plants, 77 Ocean depths, 19, 76
Animals and plants, 75 Origin of life, 199
and seed-dispersal, 69
dependence on plants, 155 Parasites, 183
Arctic deserts, 19 Peat flora, 41
plants, 196 Planets, question of life on, 11
Plant associations, 30
Bog plants, 41 economy, 98, 141
British flora, 167 formations, 32
Isles, vegetation, 25, 30 migration, 48
Plants, cultivated, 145
Chlorophyll, 180 earliest, 154
Cultivated plants, 145 Pollination, 82
Deserts, 16 Roots, 105
Fertilization, 82 Salt-marshes, 23, 40
Flowers, 126 Saprophytes, 181
display, 85 Seed-dispersal, 49
structure, 81 Seeds, 50
Fruit, 131 Semi-deserts, 22
Fruits, explosive, 55 Shingle beaches, 39
Soil, 99
Glacial Period, 165 Stems, Iog
Grassland, 25 Symbiosis, 79
Insectivorous plants, 78, 186 Types of vegetation, 31

Insects and flowers, 81

Vegetation, closed, 2
Leaves, I19 Vegetative reproduction, 53
Life, origin of, 15

Water, dispersal by, 61

Man and vegetation, 135 flora, 43, 199
Marine plants, 201 Wind, dispersal by, 62
Migration, 48 - Woodiand, 25
Mountain plants, 189

Mud-flats, 17 Xerophytes, 36

PRINTED IN GREAT BRITAIN BY BILLING AND SONS, LTD., GUILDFORD AND ESHER

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