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IJBRARY
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THE TEMPLE PRIMERS



PLANT GEOGRAPHY

By GEORGE S. BOULGER, F.L.S., Etc.
Honorary Professor of Natural History
in the Royal Agricultural College, Etc.



PLAfitT

GEOGRAPHY




BYGEORGES
BOULGERFLS




1912 10-13 BEDFORD STREET, LONDON



BIOLOG/
BA

a



THE TEMPLE PRESS, PRINTERS, LETCHWORTH.



PREFACE

THE difficulty of treating a subject as wide as plant-
geography within the compass of a primer is so obvious
that it is hardly necessary to crave the reader's recogni-
tion of the limitations of space.

Originality is neither possible nor entirely desirable in
a book of this character, and I hope that the brief Biblio-
graphy may be accepted in lieu of further acknowledge-
ment of my indebtedness to others.

I wish, however, to take this opportunity of thanking
those who have kindly assisted me by the loan of illus-
trations Sir David Prain, F.R.S., by whose permission
Figs. 2, 8, and n were copied from pictures in the Kew
Museum; Dr. T. W. Woodhead and the Council of the
Linnean Society for permission to use Fig. 3; Professor
Herbertson and Mr. Edward Arnold for Fig. 5; the
British Vegetation Committee and Mr. A. G. Tansley for
Figs. 12-16; and Messrs. Quelle and Meyer, of Leipzig,
for Figs, i, 4, 6, 7, 9, and 10, from Professor Graebner's
Pflanzengeographie.

G. S. BOULGER.

RICHMOND, October 1912.

257364



CONTENTS



PAG F,
INTRODUCTION . . . '. . . i

BOOK I

THE EVOLUTION OF THE PLANT-WORLD
CHAP.

I. The Distribution of Land and Water in the Past . 7

II. Climate in the Past 10

III. The Time and Place of the Origin of Plant-life . ' 13

IV. Floras of the Past 14

V. The Coming of our Modern Angiosperms . . .20

BOOK II

THE FACTORS OF DISTRIBUTION

I. The Climatic Environment and the Response to it . 27

II. The Soil 45

III. Organic Environment . . . . . 55

IV. Dispersal and Migration 62

V. Physiographic Factors ...... 70

VI. Barriers 74

VII. Insular Floras 78

VIII. Bridges ..... . .82

IX. Mountain Floras ....... 86

X. Water-plants . . 90

BOOK III

FLORISTIC REGIONS

I. The Northern Zone .... -95

II. The Tropical Zone ... .102

III. The Southern Zone . .112

BOOK IV
BOTANICAL ECOLOGY OR TOPOGRAPHY . . 122

BIBLIOGRAPHY . . . 131

INDEX ......... 133



Vll



LIST OF ILLUSTRATIONS

FIG.

1. View on Kerguelen Island ......

2. Mangrove-swamp near Goa ....

3. Vertical Zonation . .

4. Palm-tree drifting in the open sea . .

5. Vegetation-zones of latitude and altitude

6. Merope, a Composite of the Higher Andes

7. Salix polaris Wahl., the Dwarf Arctic Willow

8. Rheum nobile Hook. nl. ......

9. Epiphytic Orchid (Cattleya) in Brazil ....

10. Forest of Eucalyptus in West Australia

11. Grass- trees (Xanthorrhea), Cycads, and Ferns in West

Australia ........

12. Open reed swamp association .....

13. Oak-birch heath association . . . .

14. Heath association invaded by Pinus sylvestris L. .

15. Dry Oak-wood on sandy loam .....

1 6. Interior of Beech- wood on chalk .....



Vlll



PLANT GEOGRAPHY



INTRODUCTION

THE recognised masterpieces of human art, whether in
oainting or in sculpture, are, almost all of them, repre-
sentations of the human form. Most of the loveliest
scenes of natural beauty, on the other hand, owe a large
proportion of then: charm to the aspects of plant life.
he dark forest of northern pines, the crimson of the
pen heath, the willows by the stream, the gay meadow
Blossoms surrounded by Alpine snows, the slender,
taceful palms of the tropics, or the tangled jungles
the equator have in this respect almost their only
/als in the grandeur of bare mountain peaks with their
^rennial snows, and in the ever -changing hues of ocean.
, The art of the painter and the many modern books
f travel have made the most stay-at-home among us
amiliar with the fact that the vegetation of one land
is widely different from that of another. Even those
without the wide knowledge of Macaulay's schoolboy
know something of the absence of plant-life in the
frozen North ; of the stunted vegetation of the only less
permanently frozen tundra; of the date palms of the
African oases, the giant cone-bearing trees of western
North America, the gum trees of Australia, and the
rope-like lianas and the orchids on the boughs of the
many huge trees in the steaming atmosphere of what
is termed the " zone of constant precipitation." Now
that the Swiss Alps have become recognised as the
olayground of Europe, many who have seldom, if ever,
;iven a thought to such a subject as botanical geography
aust have noticed the gradual disappearance of the
yild and cultivated trees and other plants of the valley
s the ascent of a mountain is made, firs succeeding
he oaks, chestnuts, and walnuts, until the upper limit
f all trees is reached, and ultimately only a few lichens
over the rocks near the line of perpetual snow.



2 PLANT GEOGRAPHY

There Is thus a, vague general recognition of the fact
that vegetation varies with latitude and with altitude,
accompanied by the inevitable corollary that this dis-
tribution of plant life must be largely dependent upon
climate. A closer attention to the facts will convince
us that, true as this conclusion is, it is by no means the
whole truth. If it were so, we might expect, for instance,
that distant lands with practically identical climates
would bear practically identical vegetation; whereas
what we find is, not identity, but a certain resemblance,
or, as it is termed, representation.

Those who own gardens with glass houses, or who are
familiar with our large botanical gardens, will recognise
in the classification of cultivated plants into such groups
as " hardy," " half-hardy/' " greenhouse/' and " stove "
plants the same dependence of the life of certain kinds
or " species," as the botanist calls them, upon heat, or,
at least, upon the exclusion of frost; and will probably
also notice that the presence or absence of moisture
plays an almost equally important part in their successful
cultivation.

The student of geography soon becomes aware of the
vast importance of vegetation in human affairs. Dense
forests, % barren deserts, or undrained marshes have
always necessarily been thinly peopled. Man, especially
in the earlier stages of civilisation, is largely dependent
on the crops he can grow in the cleared, drained, and
tilled Jand in his immediate neighbourhood. The wild
spices he may collect in the forest of the tropics, or
the surplus produce of his fields, may be the founda-
tion of international commerce. Pasturage for cattle,
timber for shipbuilding, or a soil suitable for the growth
of some particular plant, whether valuable for food,
fibre, medicine, or what not, may largely determine the
distribution of population or industries within the
individual state. The reckless destruction of some wild
species which is a source of wealth may prove the com-
mercial ruin of a district; while its careful conservation
may mean prolonged economic progress; and the intro-
duction of some foreign plant into a region suitable in
climate and soil for its cultivation may give a fresh
start of prosperity to a land previously but sparsely
peopled.

The intellectual justification of modern botanical



INTRODUCTION 3

study is not, however, mainly or directly economic:
it is biological. The structure of plants is chiefly
considered with reference to their functions as living
beings, i.e. physiologically, unless it is viewed merely
as an indication of greater or less affinity with other
plants, and thus as a clue to their evolutionary ancestry.
The life of the individual plant is recognised as a function
of two variables, viz. the characters which it inherits
from its parents, and the result upon its development
of its environment. This environment does not consist
wholly in climate and soil, although these are factors
of supreme importance. The plant will require a certain
amount of heat to sprout, to flower, or to ripen seed:
it will require a certain amount of moisture in the soil
and some small modicum of soluble saline substances
also available at its roots: its healthy development will
be influenced by its exposure to light and wind; but
its existence may be also conditioned by other living
beings, either plants or animals. As a seed about to
sprout, as a delicate slender seedling, or even at a much
later and more robust stage of its life, the occupation
of the surrounding soil by other plants, or the shade
of their overhanging foliage, may prove fatal to it.
Substances not otherwise available, or even harmful,
to its roots may be rendered useful if certain bacteria
exist in the soil to set up suitable processes of fermenta-
tion. The fertilisation of its seeds may depend upon
the conveyance of its pollen from one flower to another
by the agency of insects ; while the dispersal of its seeds
to spots where they may be freer from competition
may depend on their being carried by birds or entangled
in the hair of passing animals.

The presence of any plant in any one place, moreover,
does not depend solely on the suitability of that place
for that species of plant. Climate and soil may be
suitable: there may be unoccupied ground; and all
necessary organic environment may also be present;
but in some manner or other, whether as seed or spore
or growing plant, the species must reach this suitable
spot. It must either be transported from some other
spot, near or far, where it has come into being; or it
must have originated de novo in the area where we now
find it.

As vegetable physiology considers alike the external



4 PLANT GEOGRAPHY

causes which act upon plants, and the response that the
plants make in function and structure to such causes,
so botanical geography has to do both with the causes
and the results of the distribution of plants. Such a
study has, as has been suggested, its interest for the
artist, the traveller, the scientific geographer, the man
of commerce interested in the sources of industrial
produce, the horticulturist, and the botanist. As a
department of scientific inquiry, however, its main
importance is biological. It is part of the more general
science of the distribution of living beings ; and, to some
extent, the laws governing the distribution of plants
apply also to that of animals. It is these general
principles that are of the greatest interest to the student
of science. The way in which one or more species of
plants have reached a particular area is more important
to him than the fact that they have done so, far more
important than the economic uses to which they may
be put. Treating the subject as a branch of natural
history, he may almost ignore those changes in distribu-
tion that man has intentionally produced, the introduc-
tion of agricultural plants into new lands, and the
appropriation of huge areas for the cultivation of a few
valued species, such as the cereals, the pasture grasses,
cotton and other textile substances, etc. Man's in-
voluntary dispersive action, on the other hand, must
be taken into consideration along with that of other
animals.

Whilst it is obvious that the present distribution of
land and water over the surface of the globe must
profoundly affect the areas occupied by land plants
and by marine algae, as well as the present distribution
of temperature, rainfall, etc., a slight knowledge of
geology will enable us to appreciate the fact that this
distribution of land and water has not always been the
same. Any description of the present distribution of
plants, dependent as it must be upon present geogra-
phical conditions, can, only be temporarily true. These
geographical conditions and their biological results have
not always been the same ; nor will they remain as they
are. The " natural floras," or assemblages of associated
plants, of to-day are only of to-day, and we must not
ignore the influence of former conditions and changes
in the distribution of land and water. Fragmentary



INTRODUCTION 5

as is the evidence, our existing vegetation can be linked
with that of past geological periods: its existing distri-
bution is that of the vegetation of the past, modified
by changes in the environment perhaps as profound as
those seen in the vegetation itself.

During the second half of the nineteenth century
the division of the earth's surface into climatic regions
of vegetation was, by the labours of many scientific
travellers, very fully accomplished. Species were tabu-
lated according to their distribution, and, under the
influence of the theory of evolution, it was recognised
that neighbouring natural floras are connected together
by the migration of species from one to another; and
also distinguished from one another by peculiar or
" endemic " species which have originated where they
occur by the modification of pre-existing forms. This
department of the subject has been termed " floristic
geography," and deals, it should be noted, mainly with
the distribution of species over large areas.

During the present century the attention of botanists
has been more directed to the simultaneous relation of
groups of plants (united only in physiological require-
ments and not generally by genetic affinity) to their
" habitats " or situations as regards soil, drainage,
aspect, etc. This study of the homes of plants is termed
"ecology"; and this treatment of plants, in groups
presenting marked landscape or " physiognomic "
characters according to their habitat, has been termed
" ecological geography " ; but might rather perhaps
be named " ecological topography." It deals with
vegetation rather than with individual species; and,
though much concerned with such local climatic in-
fluences as those due to slope, aspect, and drainage,
treats more of plant physiology than of what is usually
considered as geography.

If then we take Phyto-geography, or the geography
of plants, to signify the science of the distribution of
plant-life over the surface of the globe, we shall deal
with it under four heads. We shall first sketch briefly
what is known as the evolution of the plant- world and
its distribution in past geological periods. Secondly,
we shall write of the factors of distribution: climate,
soil, the effects of other plants and animals, the response
of plants to these influences, the agencies and mechanisms



6 PLANT GEOGRAPHY

for dispersal, the barriers to migration, and how they
have been surmounted. Thirdly, the floristic regions
of the globe will be briefly described; and, lastly, the
chief ecological groups will be discussed. Some elemen-
tary knowledge of botany and geography must be
assumed; and it is obvious that, within the compass
of a little book like the present, no part of so wide
a subject can well be treated at all completely. The
whole book is but an introduction ; but the author hopes
soon to issue a more comprehensive work in which each
division of the science can be dealt with more fully.
More space has intentionally been devoted to the causes
than to the results of distribution.



BOOK I
THE EVOLUTION OF THE PLANT- WORLD

CHAPTER I

THE DISTRIBUTION OF LAND AND WATER
IN THE PAST

OUR earth is a somewhat irregular spheroid, the surface
of which is estimated at 196,940,000 square miles. Of
this about 142,000,000 square miles is now sea, and
55,000,000 land, i.e. about 71.7 per cent, of the former
to 28.3 per cent, of the latter; or roughly as 5 : 2. A
glance at the map, or, still better, at a globe, shows how
very unequally this land and water is at present distri-
buted over the surface of the globe. There is, in fact,
thirteen times more land north of the equator than
there is to the south of it. This alone must obviously
have a profound influence on plant-distribution the
vast ocean area being almost exclusively represented,
so far as plant life is concerned, by Diatoms and Algae;
very few flowering plants, such as the Grass- wracks
(Zoster a], inhabiting salt water.

Though at first sight the disposition of the masses of
land and water on the present surface of the globe
appears most irregular, it will be readily perceived
that there is an Arctic or North Polar Ocean surrounded
by an almost continuous ring of land which is continued
in three pairs of continents that extend meridionally
and taper to the south. Similarly, round the land of
the Antarctic Continent there is a continuous belt of
ocean extending northwards in three oceans tapering
in that direction, two of them uniting in the Arctic.
Land and water are thus, as it has been said, " arranged
like a pair of interlocking cog-wheels, each with three
teeth." It will also be observed that the land and
water areas on the surface of the globe are to a very

7



8 PLANT GEOGRAPHY

great degree antipodal to one another, the Arctic Ocean
corresponding to the Antarctic Continent, Europe and
Africa to the South and North Pacific, North America
to .the South Indian Ocean, and Australia to the North
Atlantic. These salient facts of distribution agree well
with an interesting hypothesis of the origin of the
surface features of our globe, the latter part of which
was first put forward by an American writer, Mr. W.
Lowthian Green.

Many facts in astronomy, and especially the results
of the examination of the light of heavenly bodies with
the spectroscope, lend support to what is known as the
nebular hypothesis of the origin of our earth in common
with the rest of the solar system. According to this
hypothesis the solar system has resulted from the
gradual cooling and condensation of a nebula composed
either of incandescent gas, or, more probably, of swarms
of meteorites constantly colliding with one another and
passing into a vaporous condition. This nebula, con-
densing towards its centre, would throw off, or leave
behind, successive rings of progressively heavier matter
which, by disruption, have condensed into planets, of
which our earth is one. Similarly, it is thought, the
rupture of each such planetary ring would so raise the
temperature of the resultant planet as to vaporise it,
and allow the vapours to arrange themselves and
condense in successive shells of densities increasing
towards its centre. Lowthian Green's tetrahedral
theory is that the earth, originating in the manner just
indicated, would be approximately spherical, the sphere
being the geometrical form which combines the maximum
of volume with the minimum of surface; and that, in
further contraction on cooling, it would tend towards
that geometrical form which combines the maximum
of surface with the minimum of volume, viz. the tetra-
hedron. The tetrahedron is enclosed by four equal and
similar triangles; and the theory associates the four
oceans with the four faces of this form, and the circum-
polar ring of land and the three pairs of continents with
its edges, the face represented by the Arctic Ocean
having as its antipodes the solid angle represented by
the Antarctic Continent. The spherical earth is, in fact,
supposed to have undergone a tetrahedral deformation,
sagging on the four sides, or faces, which have produced



THE EVOLUTION OF THE PLANT-WORLD 9

the ocean basins. The facts of the earth's form are
shown to be still more in accordance with this theory
in comparison with a form, intermediate between the
sphere and the tetrahedron, known in geometry as
the hexakistetrahedron, or six-faced tetrahedron, which
may be described as a tetrahedron with a low pyramid
of six triangular faces on each of its four sides.

The present distribution of land and water agrees
very closely with the requirements of this hypothesis;
but it is more important that the primitive distribution
should be shown to do so. Most of the present land
surface can be shown, by its rocks and their contained
fossil remains, to have been at one period or another
beneath the sea, though not all at one time. It may,
however, be true that most of the sedimentary rocks
are of shallow water origin, and have not originated
in such abyssal depths as those of our present ocean-
bed. From this it will follow that, though subject to
repeated oscillations, so that one tract after another
has disappeared and reappeared from beneath the sea,
the continents, though constantly varying in shape and
size, have in the main retained their individuality, and
the existing ocean basins have been, perhaps from the
very beginning, great trough-like depressions of the
earth's crust. Such a belief in the permanence of at
least the skeletons of our continents and of the deeper
parts of the ocean as land and water respectively is
compatible with the admission of great changes linking
land-masses now separate, or vice versa. Thus, there is
considerable evidence of former extensions of land, at
different periods, between Scandinavia, the Highlands
of Scotland, and Donegal; between the mainland of
Manchuria, Saghalien, and Japan; and between the
south-west of South America and the adjacent islands.
Some evidence, however, points to the former existence
of land-masses which can hardly be in any way con-
sidered as extensions of our present continents. Thus
there is much geological evidence of the connection in
a remote geological period of the Scandinavian area
mentioned above with North America and Greenland
in a continental mass named Arctis; and at a later date,
lasting from the Coal-Measure period at least to that
known to geologists as Permian, of the union of Brazil,
Argentina, and the Falkland Islands with Africa south



io PLANT GEOGRAPHY

of the Tropic of Cancer, Madagascar, Arabia, Southern
India, Australia, and Tasmania in another vast conti-
nental area known as Gondwanaland. Possibly the
rigidity of these two ancient land -masses in the shrinking
crust of the earth may have produced the folding of the
more sedimentary rocks of the area between them,
giving rise to those systems of " fold -mountains " which
form the axes of our existing continents. Although
Gondwanaland seems to have been broken up by the
subsidence of the areas now occupied by the South
Atlantic and Indian Oceans, a possible continuation
to a later period of some southern connection between
South America and Australasia, perhaps by way of the
Antarctic Continent, may serve to explain some diffi-
culties in the present distribution of plants and animals.

Such a minor difference in the distribution of land
and water as the union of the British Isles to the Con-
tinent of Europe both eastward and south-westward,
involving as it does but a slight difference in the relative
levels of land and ocean, has certainly occurred within
far more recent times than the differences just referred to.

Though most of our existing species of plants, and
even many larger groups, are of so modern a date,
geologically speaking, as to have originated independ-
ently of the more ancient distributions of land and
water, we may trace the influence of these last in the
origin and dispersion of some of the main divisions
of the plant- world. As we shall see, however, it is
extremely difficult to do this with any certainty.



CHAPTER II

CLIMATE IN THE PAST

THE term climate referred originally to what we term
the sun's altitude, or the angle at which the sun's rays
fall upon various parts of the earth's surface. The
earth's axis of rotation, the line joining the North and
South Poles, is inclined a^J towards the plane of its
orbit or path round the sun, which plane is termed the
ecliptic, i.e. it makes with that plane an angle of 66.
As a consequence of this inclination the sun is vertical



THE EVOLUTION OF THE PLANT-WORLD 1 1

over the equator, or great circle midway between the
poles, twice in the year, viz. at the spring and autumn
equinoxes. At those seasons the " circle of illumina-
tion," or great circle dividing day from night, will pass
through the poles, so that day and night will be equal,
i.e. twelve hours each, everywhere. At other seasons
they will be equal only on the equator, which is,
therefore, termed the equinoctial line, their inequality
increasing with the latitude, i.e. towards the poles,
where we have practically a day and night each of six
months' duration. Not only does the difference between
day and night increase with the latitude, owing to this
inclination of the earth's axis, but the sun's altitude,
or the angle which its noon position makes with the
horizon, diminishes, from the same cause, towards the
poles. Only within 23 of the equator is it ever
vertical. Only within 23% of the poles does it ever
remain for twenty-four hours or more above or below
the horizon. Thus this inclination of the earth's axis


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