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[Illustration: Desert Types.
(_For names of species, see page 7._)]
_NATURE LOVER’S SERIES_
ASPECTS OF
PLANT LIFE
WITH SPECIAL REFERENCE TO
THE BRITISH FLORA
BY
ROBERT LLOYD PRAEGER
AUTHOR OF
“OPEN-AIR STUDIES IN BOTANY,” “IRISH TOPOGRAPHICAL BOTANY,”
“A TOURIST’S FLORA OF THE WEST OF IRELAND,”
“WEEDS,” ETC.
WITH COLOURED FRONTISPIECE AND NUMEROUS
BLACK AND WHITE ILLUSTRATIONS
LONDON
SOCIETY FOR PROMOTING
CHRISTIAN KNOWLEDGE
NEW YORK: THE MACMILLAN COMPANY
1921
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 considered, as is also the unique relation existing between
the plant world and the human race.
These chapters are intended to be used in conjunction with simple
observations in the field, such as any person of enquiring mind,
unversed in science, may 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 emendations where I have trespassed on the domains of
zoology and geology respectively.
R. LL. P.
CONTENTS
CHAPTER PAGE
PREFACE 3
I. ON FARLETON FELL 9
II. PLANT ASSOCIATIONS 30
III. PLANT MIGRATION 48
IV. SOME INTER-RELATIONS OF PLANTS AND ANIMALS 75
V. PLANT STRUCTURES 98
VI. PLANTS AND MAN 135
VII. PAST AND PRESENT 155
VIII. SOME INTERESTING BRITISH PLANT GROUPS 180
INDEX 208
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
Pearsoni_. 12, _M. Lesliei_. 2, 5, 6, 7, 8, 10, 11, 12, from South
Africa. 1, 3, 4, 9, from Central and South America.
FIG. PAGE
1. A Burrowing Lichen, _Verrucaria calciseda_ 17
2. The Glasswort, _Salicornia europæa_ 18
3. _Mesembryanthemum Bolusii_ and _M. Lesliei_ 21
4. Wild Carrot, _Daucus Carota_ (grown under great exposure) 37
5. Inrolled Leaf of Crowberry, _Empetrum nigrum_ 39
6. Succession of Vegetation in Lakes: _a_, Marsh Zone.
_b_, Reed Zone. _c_, Zone of Floating Vegetation.
_d_, Zone of Submerged Vegetation (diagrammatic) 43
7. Coral Root, _Dentaria bulbifera_ 54
8. Fruit of Giant Bell-flower, _Campanula latifolia_ 56
9. Fruit of _Geranium_ 58
10. Fruit of _Viola_ 59
11. Fruit of _Erodium_ 60
12. Diagram illustrating Fall of Seeds 64
13. Wing-seeds and Plume-seeds 68
14. Flower of _Erodium_ 85
15. Flower-head of _Astrantia_ 86
16. Dwarf Cornel, _Cornus suecica_ 88
17. Japanese Wineberry, _Rubus phœnicolasius_ 90
18. Flower and Fruit of Purple Toad-flax, _Linaria purpurea_ 93
19. Stem and Root Structure 112
20. _Genista sagittalis_ 118
21. Leaf-mosaic in _Azara microphylla_ 121
22. Leaf of _Weinmannia trichosperma_ 122
23. Leaf-development of Arrow-head, _Sagittaria sagittifolia_ 125
24. Fruit of _Coriaria japonica_ 128
25. Wild Cabbage, _Brassica oleracea_ 150
26. A Myxomycete, _Comatricha typhoides_ 158
27. Leaf of Maidenhair-tree, _Ginkgo biloba_ 163
28. Great Butterwort, _Pinguicula grandiflora_ _To face_ 173
29. Strawberry-tree (_Arbutus Unedo_) at Killarney _To face_ 174
30. _Spiranthes Romanzoffiana_ _To face_ 174
31. Bird’s-nest Orchis, _Neottia Nidus-avis_ 182
32. Alpine Plant-boss _To face_ 191
33. _Sedum primuloides_ 192
34. New Zealand Veronicas 194
35. Mountain Avens, _Dryas octopetala_ 195
PLANT LIFE
CHAPTER I
ON FARLETON FELL
“I 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.”--MAURICE 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 surface of this grey hill there appear to
be scarcely any plants. A dense scrub of Hazel and other small trees
clings to its screes in patches, but the continuous mantle of vegetation
is lacking.
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,
vegetation 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 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, indeed, 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 existence. 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 extremes 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 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 portion 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 prevails, 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 diffusion 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 prevailing 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[1] of the pros and cons of this question, the
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 unfavourable for plant growth. Arrhenius suggests that
the presence of a low vegetation such as snow Algæ 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 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 inclined 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 manifested on our Earth? May not the
inhabitants of the Sun, ensconced ninety million miles away in a
comfortable temperature of 6,500° Centigrade, have long since proved to
their own complete satisfaction the impossibility of the existence of
life under the appalling 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,[2] 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 where we could not for an instant survive, to
produce beings we should not, without a special education, 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 thousand 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 Farleton 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 from which plants
appeared to be absent. This question 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_, _atmosphere_,
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 instance, 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 Algæ and
Mosses soon discover it and colonize it. Their decayed 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 that it is colonized by
minute Lichens, many of which have the power of dissolving the
limestone, producing tiny burrows in which they live securely.
[Illustration: FIG. 1.--A BURROWING LICHEN (VERRUCARIA CALCISEDA),
LIVING IN LIMESTONE.
_a_, Natural size; _b_, greatly enlarged; _c_, section, greatly
enlarged.]
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 securing an anchorage in it. But in the upper parts, near high-water
mark, a few land plants--notably the Glasswort (_Salicornia europæa_,
Fig. 2), a fleshy little 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 particles, 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.
[Illustration: FIG. 2.--GLASSWORT (SALICORNIA EUROPÆA).
_a_, Plant, 2/3; _b_, male flower; _c_, female flower, both enlarged.]
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 shallower waters of the sea, especially where a rocky bottom
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 lowness of temperature and the consequent almost continuous
covering of snow and ice render plant life impossible. But just as the
Eskimo live under conditions which would be wellnigh prohibitive to
inhabitants 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 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 covering 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
[Illustration: FIG. 3.--MESEMBRYANTHEMUM BOLUSII (LEFT), AND M. LESLIEI
(RIGHT), BOTH 2/3.]
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 remarkable 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 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, except 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 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
vegetation 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 results in a closed vegetation, we may suggest
for consideration 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 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 Bird’s-nest
(_Monotropa Hypopitys_), a strange colourless, 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 conditions 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 according to other conditions prevailing, a
closed vegetation 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 land left undisturbed because
unsuitable for cultivation--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 associates 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 vegetation of the British Islands, and without doubt the
greater part of the country was occupied by woodland within the human
period. But forest country is not well suited to human habitation or
colonization. The early arts of peace--pastoral and agricultural--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 could
not and cannot grow. Wind was and 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 reconstruct
with tolerable accuracy its ancient condition, and the flora of which it
was composed. In the remaining 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 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 distribution 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
migrations 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
continental 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 depression. In this connection also we may be led to
investigate 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 negotiate
produced by areas of uncongenial soil, by mountain ranges, or by
forests. This will involve especially a study of seeds and the
interesting phenomena of seed-dispersal.
Again, the most popular branch of botanical study in England is
_Floristic Botany_, which traces the distribution within our area of the
various species composing its flora; and with it is necessarily
associated a study of the plants themselves so far as the characters are
concerned, by which they may be distinguished from each other. This last
is the province of _Descriptive Botany_. The study of local
distribution, if conducted intelligently, will greatly assist in
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 conditions 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 conditions, 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_ investigates 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.