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Title: Aspects of plant life; with special reference to the British flora
Author: Praeger, Robert Lloyd
Language: English
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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.



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, or natural grass,
over which wander great herds of sheep and cattle. A. G. Tansley[3] 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 conditions still prevail:

                             England.  Wales.  Scotland.  Ireland.

Cultivated land                75        59       25      ? 20-30
Land under natural or
   semi-natural vegetation    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 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 precipitation, becomes the
dominant influence in determining the presence or absence of vegetation,
or its character.

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 physiological 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, 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 exposure, 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 help us to identify the association should we
encounter it again; there will be others--dependent species--which are
attracted by the shade, or shelter, or other advantages which the growth
of the dominant 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 nonscriptum_). 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 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 themselves
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 Robertianum_ 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.

If, however, we consider some of the formations or associations which
are the result of extreme conditions of environment, we get more light
on the relations 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 extent 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

[Illustration: FIG. 4.--WILD CARROT (DAUCUS CAROTA), GROWING UNDER GREAT
EXPOSURE. 1/2.]

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 98 per
cent. of their weight if they are dried. Prevention 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 (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 (_Lavandula_) 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 excessive 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 openings 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

[Illustration: FIG. 5.--SECTION ACROSS INROLLED LEAF OF CROWBERRY
(EMPETRUM NIGRUM), MUCH ENLARGED.]

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 Poppy
(_Glaucium flavum_), the tough, waxy skin which covers the Sea Holly
(_Eryngium maritimum_), the extensive underground stem-systems of the
fleshy-leaved Sea Convolvulus (_C. Soldanella_) and Sea Purslane
(_Honkenya peploides_). Even the annual plants display 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
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 Wheatgrass (_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 examination.

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
successfully, at least for a time. The salt-marsh flora is very
exclusive, and contains but few species which we encounter in other
situations. Some of them are also found on dry sea-rocks--the Sea Pink
(_Statice Armeria_), Scurvy-grass (_Cochlearia officinalis_), Sea Aster
(_A. Tripolium_), and so on; showing that soaking 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 portulacoides_, 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
Tabernæmontani_, 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, leafless,
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 bogland 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 vegetable 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 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 reflexed 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
neighbours 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 vegetation, 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 consider the plant formation
which characterizes lowland lakes and rivers, where water suitable for
plant use is

[Illustration: FIG. 6.--DIAGRAM ILLUSTRATING SUCCESSION OF VEGETATION IN
LAKES.

_a_, Marsh zone; _b_, reed zone; _c_, zone of floating vegetation; _d_,
zone of submerged vegetation.]

superabundant. In such places we are faced 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 aeration 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 area it is clear
that depth of water is the controlling factor: the plants are arranged
in zones, one succeeding 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
submerged. 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 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 (_Iris_),
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, surrounded by water, use their roots chiefly as anchors, and
absorb through their stems and leaves the water 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
_Nymphæa_) 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
Duckweeds (_Wolffia arrhiza_) and the Bladderworts (_Utricularia_), have
gone farther still, and have dispensed 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 Algæ
among which it lives. In the Bladderworts, the long flaccid stems,
clothed with much-divided leaves converted in part into ingenious
insect-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 conditions relating to water and soil: it remains to return to
the consideration of the vegetation which develops 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 difficulty 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 (_Buxus 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 consideration 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, extracting 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 important 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. 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 existence. 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 locomotion 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 continued growth, tends
to exhaust the available supply of these materials, and the succeeding
generation needs to reach fresh ground if it in turn is to attain
healthy development. And it is undoubtedly of advantage 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 contrivances 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 exhibited 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
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 rôle 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 egg 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 physiological condition.
For instance, its breathing--or absorption of oxygen--is exceedingly
slow, and it is not suffocated by burial, sometimes even for years, in
the soil. And while the mature plant is killed instantly 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 unaffected. 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
Chestnut. 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 exceptional 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 portmanteaux 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, 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
characteristic 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
individual. 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-dispersal 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

[Illustration: FIG. 7.--CORAL ROOT (DENTARIA BULBIFERA).

_a_, Upper half of shoot, 1/2; _b_, creeping stem, 1/2; _c_, bulbil,
2/1.]

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

[Illustration: FIG. 8.--FRUIT OF GIANT BELL-FLOWER (CAMPANULA
LATIFOLIA). 3/4.]

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 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 Leguminosæ. In the Cranesbills (_Geranium_) a very ingenious
catapult 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 handguard
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

[Illustration: FIG. 9.--FRUIT OF GERANIUM.

_a_, Mature; _b_, ditto, with pouches raised ready to discharge nuts;
_c_, in act of discharging.]

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 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, 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.

[Illustration: FIG. 10.--FRUIT OF VIOLA. 3/4.

_a_, Mature capsule; _b_, 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 enclosed contracts unequally in drying, so that the upper half
assumes a position at right angles to that of the

[Illustration: FIG. 11.--FRUIT OF STORKSBILL (ERODIUM). 2/3.

_a_, Mature, twisting beginning; _b_, separate fruit, fully twisted.]

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 dryness by twisting. Should it be thus
caused to untwist 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 auger,
and, lengthening as it untwists, forces the seed into the ground. Should
dryness supervene, the backward-pointing hairs on the seed-envelope
prevent its being drawn out again when retwisting and consequent
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 transported 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 10 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 10 per cent. of our own
flora which produce buoyant seeds are mainly riverside and seaside
plants; and no doubt their dispersal 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 uniformity 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 floating 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 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 everywhere, 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
transport is of necessity conducted. Any body liberated in still air
will fall vertically with a velocity which increases according to
well-known laws until the increasing 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 dispersal, the resistance of the air almost at once
counteracts 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 liberated 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 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.

[Illustration: FIG. 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
fortune 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
possibility of seeds of Portuguese plants reaching the Azores, lying
800 miles to the westward, H. B. Guppy[4] 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 anything 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
adaptations 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 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 Flowering Plants with which we are mainly
concerned reduction 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 extent 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 enclosing 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 longifolia_) falls only about 1/15 as fast as that of the
Poppies, and would thus, under the same conditions, be carried fifteen
times as far.

To pass on. Some seeds, many of them of considerable 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 seed an irregular 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 _Compositæ_,
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 _Compositæ_ 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

[Illustration: FIG. 13.--WING-SEEDS AND PLUME-SEEDS.

     _a_, Mountain Willowherb (_Epilobium montanum_), 2/1; _b_,
     Dandelion (_Taraxacum officinale_), 2/1; _c_, Mountain Avens
     (_Dryas octopetala_), 1/1; _d_, Scotch Fir (_Pinus sylvestris_),
     2/1; _e_, Reed-mace (_Typha latifolia_), 2/1.]

branches; these hairs are tightly packed together 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 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 _Compositæ_ 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 those of
almost any _Compositæ_, 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 3-1/3 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 Willowherbs 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 _Crucifercæ_, _Caryophyllaceæ_,
_Scrophulariaceæ_, etc. The relative efficiency of such comparatively
large seeds as those of many of our Leguminous 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 entangled in the hair or fur of passing animals.
Examples will occur at once to the reader, as this character occurs in
the case of many familiar plants, such as Burdock (_Arctium_),
Enchanter’s Nightshade (_Circæa_), 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
question of _endozoic_ dispersal--where seeds are transported 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 important 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.”[5] 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
(_Ilex_), 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 remarkably rapid,
food being ejected from a half to three hours after being eaten, so that
a bird eating 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 practically 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 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 Sweetbrier (_Rosa eglanteria_) in New Zealand, of the Mexican
_Bryophyllum calycinum_ in many Tropical countries, of the American
Monkey-flower (_Mimulus Langsdorfii_) in our own islands; but these are
admittedly 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 Foxgloves (_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 competition. 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 surrounding areas; so that ease of dispersal
has undoubtedly played its part in the building up of our vegetation.

Conditions under which rapid dispersal has obviously an advantage occur
when by some exceptional 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 IV

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 dependent 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
ultimate source of food is the tiny Algæ 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 favourable for plankton life may contain
several millions of individuals in every litre (about 1-3/4 pints);
while as a fair estimate for the seas which surround our own islands “at
least one” organism for every drop has been suggested.[6]

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 organisms 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 distribution 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 unknown. The larvæ of some Butterflies, for instance, eat the
leaves of one plant only; the Peacock (_Vanessa io_) and the Small
Tortoiseshell (_V. urticæ_) 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. Sometimes 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 dependence 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 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 larvæ or pupæ 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_),
Butterworts (_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 (_Dionæa_) 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
advantage lies entirely on the side of the plant, just as in the case of
most of the plant-eating animals the advantage 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 widespread 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
will be well to consider a few of the cases in which the relationship
between plant and animal is continuous and more intimate, the two living
in very close relations to each other: to such cases the term
_symbiosis_ 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 (_Œcodoma_), 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 desirable 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.

A very remarkable instance of a different kind is supplied by the
relations existing between the American species of _Yucca_ and the small
white-winged moths of the genus _Pronuba_. The following succinct
account is given by Professor G. H. Carpenter:[7] “The female of these
moths has not only the palps of the first maxillæ developed, but the
region of the maxillæ (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 deliberately 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 Holothurians there are small green cells which were long 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
(Algæ), 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 Algæ and some of
the lowly Radiolarians and Foraminifera; in these cases, the animals
being very minute, the plant partner plays a more conspicuous rôle. It
is noteworthy that these Algæ 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 picturesque and romantic features of field
life, and of which the materials for study and observation are ever at
our own doors. What is a flower? A flower is a group of modified leaves
set apart for the business of sexual reproduction. The essential parts
or _sporophylls_ 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
enclosing an _embryo sac_ or _megaspore_, in which is an _ovum_ or
_female cell_.

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 necessary 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 produced 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 the former, and to avoid the latter, many
highly interesting 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-pollinated flowers, such as
Hazel (_Corylus_), Scotch Fir (_Pinus_), or Reed-mace (_Typha_), we
note the small size of the flowers and the great abundance of pollen.
Compare these with insect-fertilized flowers, such as Buttercup
(_Ranunculus_), Flax (_Linum_), Snapdragon (_Antirrhinum_), 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

[Illustration: FIG. 14.--FLOWER OF ERODIUM PETRÆUM. 2/1.]

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 the function of advertisement, the corolla being in comparison
insignificant--we may study examples of this in the Anemones,
Hellebores, and Marsh Marigold (_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 combine in showiness, both sets being
brightly coloured in one or more tints--compare the Columbine
(_Aquilegia_), Larkspur (_Delphinium_), Milkwort (_Polygala_), and the
marvellous flowers of Orchids. In the great group of the Monocotyledons,
indeed, to which

[Illustration: FIG. 15.--UMBEL OF ASTRANTIA CARNIOLICA, SHOWING
PETAL-LIKE RING OF COLOURED LEAVES (BRACTS). 1/1.]

the Orchids belong, sepals and petals usually combine in form and colour
to form one corolla-like envelope (then called a _perianth_). In many
other plant groups--for instance, the _Dipsacaceæ_ (such as the
Scabious), _Umbelliferæ_, and _Compositæ_--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 of the large umbel is much enlarged, which enhances this effect.
In _Astrantia_, an interesting genus of _Umbelliferæ_, 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 _Compositæ_ the process is carried still farther; in
the Common 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 possess stamens
and pistil and are capable of producing the brilliant scarlet berries.
In a cultivated form of this, commonly called the Snowball Tree, the
advertisement flowers only are present, forming a globe of white
blossom, and no fruit is produced in consequence. 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 promiscuous
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 from the ground, assemble for the feast, and incidentally
transfer from flower to flower pollen which may adhere to their bodies.

[Illustration: FIG. 16.--DWARF CORNEL (CORNUS SUECICA), 2/3, AND SINGLE
FLOWER ENLARGED.]

In a large number of flowers such general feasting is discountenanced,
insect traffic is regulated, the 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 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 conspicuous 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 (_Caryophyllaceæ_) produce a sticky
secretion which is a very effectual bar to the passage of small walking
animals. In the English Catchfly (_Silene anglica_), Night-flowering
Catchfly (_S. noctiflora_) and the Nottingham Catchfly (_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 small insects.
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 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 phœnicolasius_), 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 petals to
expand, and then closes again and resumes its protective rôle till the
scarlet fruit approaches maturity.

[Illustration: FIG. 17.--JAPANESE WINEBERRY (RUBUS PHŒNICOLASIUS).

Leaf and panicle, 2/3; flower after pollination; ripe fruit, both
slightly enlarged.]

But it is in the flower itself that we find the most ingenious
arrangements to encourage useful and discourage useless visitors, to
assist the former to pollinate the flower, and while offering nectar to
the welcome 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 particular
position--namely, on the lowest petal. Following on the adoption of this
attitude the next stage in development is seen in the parts of the
flower beginning 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 illustrating these transformations we do not need to go to
plants widely separated in their affinities. In the Buttercup order
(_Ranunculaceæ_) alone every gradation 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 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 portions, 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 himself 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 contact, enclosing the
stamens and pistil. An insect visiting 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 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 normally 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 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.

[Illustration: FIG. 18.--FLOWER AND FRUIT OF LINARIA PURPUREA. 2/1.]

Only one more instance can be referred to, which can be tested by the
reader any summer day wherever any of our native Orchids grow. In
these, the most highly specialized of all plant groups as regards
pollination 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 surface. The pollen grains are held together
in a mass by fine threads, and the whole with its stalk--the
_pollinium_--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 happens. 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-pollination, than those adapted to securing cross-pollination. But
in a few, on the contrary, self-pollination is specially arranged for.

It must be pointed out that the insects which pollinate these
specialized flowers have in many cases acquired modifications in their
structure corresponding to the modifications in the flowers which they
frequent. In the 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, Coleoptera, there is
very little special adaptation for their flower-feeding habits, and
these insects visit flowers, such as the _Umbelliferæ_, 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 _Bombyliidæ_, high specialization 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 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 thousand
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 (_Noctuidæ_) it is sometimes only eight millimetres
(1/3 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 the Convolvulus
Hawk-moth measures 65 to 80 millimetres (2-1/2 to 3-1/4 inches), and
some of the Tropical allies of this moth have probosces twice or even
three times that length. These species feed on the nectar of flowers
with tubular corolla of corresponding dimensions. Most of the Hawk-moths
feed only at dusk, and as the time is short they take advantage of their
powers of rapid flight to visit (and incidentally 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 indiscriminately, but dash to blossom after
blossom of whatever single species they have selected. Hermann Müller
records watching Humming-bird Hawk-moths (_Macroglossa stellatarum_) at
work at the summit of the Albula Pass; one visited 106 flowers of _Viola
calcarata_ in under 4 minutes; another 194 blossoms of the same plant in
6-3/4 minutes.

The day-flying Butterflies display none of this restless 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 comrade 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 material, 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 atmosphere. 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 properties 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 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 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 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, intractable,
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 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 atmosphere, 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 constituents 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, reducing, and catalytic actions, etc.,
are now rapidly extending our knowledge of the soil as a medium for
plant growth.”[8]

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 all sides at an angle to the main stem, these
also tending in a mysterious way, if their course is disturbed 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 interesting and of so much economic importance that reference should
be made to them. 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 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 _Ericaceæ_ and
_Orchidaceæ_, 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 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 Liverworts; 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
associated 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. Conversely,
roots are readily produced even from the upper portions of many
stems--else how could we 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 function 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 produced 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 or a 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 (pneumatophores) through which they may breathe, as
in the case of the Swamp Cypress (_Taxodium 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
materials from the roots to the leaves, and manufactured 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 systems of branches with which we are so well acquainted
are devoted to this object, the leaves themselves 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 considerations 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 (_Potamogeton_), 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

[Illustration: FIG. 19.--ARRANGEMENT OF STRENGTHENING MATERIAL IN ROOT
(_a_) AND IN STEM (_b_) (DIAGRAMMATIC).]

beautiful 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 _Umbelliferæ_, 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 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 construction 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 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 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. Sometimes support is obtained by
twining (compare Convolvulus, 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 movements 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 frequently 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 effective the
twining habit is as regards economy of building 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 becomes 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, 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 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 sufficient 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 excessive 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 perform 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. In the 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

[Illustration: FIG. 20.--GENISTA SAGITTALIS. 1/2.]

leaves, and persist for a couple of years, when they pass away, leaving
slender, round, brown stems. In our native Broom (_Sarothamnus
scoparius_) a similar modification may be observed, though of less
degree. 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 subsequently 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 Leguminosæ and Rosaceæ, 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 arrangement, features which
are closely related to the characters 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 naturally 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
efficiency 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 question of damage by wind being an important
controlling factor. The veins, or vascular bundles, act efficiently as
strengtheners of the thin surface; to prevent 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 occasionally the leaves
hang normally in a vertical position, 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 preparation 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

[Illustration: FIG. 21.--AZARA MICROPHYLLA. 1/2.]

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

[Illustration: FIG. 22.--LEAF OF WEINMANNIA TRICHOSPERMA. 1/1.]

are grouped in circles (_whorls_) at certain points of the stem, as in
the Bedstraws; often in opposite pairs, arranged criss-cross, as in the
Sycamore; most frequently in a series of spirals. The result in all
cases is the same--it allows of as great an 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 segments, as in the Ash and Horse Chestnut,
is of undoubted 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 _Weinmannia
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. Possibly 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 unfavourable
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 unfavourable weather. The fall of the leaf is
accomplished 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 immediately 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

[Illustration: FIG. 23.--SPRING SUCCESSION OF LEAVES OF MATURE PLANT OF
ARROW-HEAD (SAGITTARIA SAGITTIFOLIA). 1/3.]

problem of avoiding submergence; and the stomata are confined to the
upper side, which alone is in contact with the atmosphere. Those water
plants which 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 progression 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
laboratory 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 Compositæ--_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_, having usually a protective rôle, 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 flower opens (_e.g._, Poppy), or wither as the petals
wither, or remain fresh until the fruit is ripe. Sometimes, as in many
Ranunculaceæ (compare _Anemone_, _Caltha_, _Helleborus_), they take on
the advertising rôle usually assigned to the petals, being large and
coloured, 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,
constituting 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
yellows). Chlorophyll being absent, the coloured petals do not assist
assimilation: they are purely advertisements, though incidentally they
often fulfil a useful protective rôle for the important organs which
they surround. In this latter connection their sensitiveness 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 _Labiatæ_,
where, fused together into a tube, they form a kind of cave in which the
stamens and pistil nestle securely.

[Illustration: FIG. 24.--FRUIT OF CORIARIA JAPONICA. 1/1.]

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 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.

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 certain 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 modifications which the
pistil undergoes, so that its structure 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 presenting a very simple example. Take an oblong leaf like
that of a Laurel, and fold it down the mid rib till the two edges are in
contact. There is our pea-pod complete. 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
structure 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 improvement, so
it is with plant structures. Many of the more primitive types of
flowers, such as Buttercups or Water Lilies, have a multitude of petals
or stamens or carpels, while in many of the most specialized, such as
Composites or Campanulas, the number of parts is much reduced. The
primitive wind-pollinated 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 coverings or appendages--offers the
most varied forms of any of the plant organs--compare Hazel, Strawberry,
Pea, Apple, Cranesbill, Dandelion; the variety is endless. 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 numerous
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 complicated and wonderful machine, of which the
terrestrial Seed Plant is the highest expression. Water is the basis on
which its activities are founded--the currency 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 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
waterproof 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 jealously 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
watchmen, 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 semifluid 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 proportion 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 infinitesimal quantity through the leaves the whole
organic world, animal as well as vegetable, depends.[9] 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
considerable amplitude. Alterations of illumination or of temperature
produce other movement--bendings towards or away from light, the
drooping of leaves and closing of flowers at nightfall, and so on.

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 produced.



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 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 domestication 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 agriculture--indeed, before
civilization in any form--could make much progress, a settled life was
necessary, free from migrations in search of food or 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 European agriculture. These lands, with their wet winters and
dry summers, the latter inimical to the development 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, Cæsar 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 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 appearance. 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 suppression 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
annuals which have followed the spread of farming operations, or which
are directly--though unintentionally--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 agriculture. 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
constituents, 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, for the alteration in the old substratum caused by its being
turned over and mixed introduces new edaphic (_i.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
themselves 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 (_Botrychium 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 interesting
marsh plants of the British flora--such as the two fine Ragweeds,
_Senecio palustris_ and _S. paludosus_, and the Marsh Sow-thistle
(_Sonchus palustris_)--have on this account almost vanished from our
islands, like the Bittern and Great Bustard which are their companions.

Some lakes, again, have been ruined for the botanist by being used as
reservoirs. The considerable 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 thoughtless and wanton destruction carried out by its
inhabitants, 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 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 æsthetic 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 disturbance 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 ornamental 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,
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.[10] 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 protection 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 Langsdorfii_). 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 muddy estuaries. It _looks_ 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
satisfactory 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
transported 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 operations has been to
lessen competition and increase 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 territories; 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_), Chamomile (_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 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 æsthetic 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 dependents 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 categories--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, 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 selection 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 reversion 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 maintained; 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 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 certainly 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 numberless as the districts in which they
grow.” Theophrastus, 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 introduced 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 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 comparatively
recent date. The Cauliflower has been produced by increasing 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 taproot 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--_Triticum_ (Wheat), _Hordeum_ (Barley), _Secale_ (Rye), _Avena_
(Oat), _Panicum_ (Millet), _Oryza_ (Rice),

[Illustration: FIG. 25.--WILD CABBAGE (BRASSICA OLERACEA). 2/3.]

_Zea_ (Maize). The value of these to the human race is incalculable, and
some of them have been in cultivation for at least five thousand years.
In some of them, indeed, the native form is now unknown, the improved
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 to 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 accomplished. 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 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 throughout 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”
(staminate) and “female” (pistillate) flowers on different trees, and
the original Lombardy Poplar having been a “male” it also can be
propagated 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 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 chlorophyll 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 greenhouses. These spots or stripes or
tintings of pale colour on the leaves are due to the lack of chlorophyll
in the chromatophores (chlorophyll corpuscles); sometimes to an absence
of the chromatophores themselves; 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 manufactured;
whereas flower colour is merely for advertisement, 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_), 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_, _Wichuræiana_, 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.



CHAPTER VII

PAST AND PRESENT


The 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
convenient 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 backward 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), Pteropods, Trilobites, and Worms of the ancient
Cambrian 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 distinction 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 have held regarding them,
_Myxomycetes_ 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 conditions of moisture and heat, the spore
swells, its wall cracks, and the contents--a tiny globule of
protoplasm--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 bacteria
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 protoplasm of
the interior streaming rapidly along certain channels for about 1-1/2
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

[Illustration: FIG. 26.--A MYXOMYCETE (COMATRICHA TYPHOIDES) IN FRUIT.

_a_, Natural size; _b_, enlarged.]

account. This story partakes about equally of incidents 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, is entirely suggestive of animal life, and is not paralleled
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
characters, 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 animals, as
we know them, had not yet become differentiated.

Among plants, as distinguished generally from animals by the production
and abundant use of chlorophyll 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 Algæ, 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 preserved 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 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 existence 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 displaying 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-Cambrian 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 earliest period from which plant remains are well
preserved and plentiful, in the Devonian rocks, many of the great plant
groups are fully developed, the vegetation displaying an abundance and
luxuriance comparable to that of the present day. Seaweeds (Algæ),
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 specialization 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 imposing 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 Palæozoic Period the Conifers had appeared,
descended possibly from the extinct _Cordaiteæ_. With the advent of the
Secondary or Mesozoic epoch the group of the Cycads, to which our
modern Screw-pines belong, rose to great importance, 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 Monocotyledons, put in an appearance. It seems probable
that they were evolved from Cycads, such as the _Bennettiteæ_, 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
Pteridosperms 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.

[Illustration: FIG. 27.--LEAF OF MAIDENHAIR TREE (GINKGO BILOBA). 2/3.]

Races which once formed vast and lofty forests are now represented by a
few lowly herbs; and it is difficult to recognize in the tiny
_Selaginella_ of our moors the representative of the gigantic
Club-mosses of Carboniferous days. But certain plants still living
retain 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 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, _Ginkgoaceæ_ (probably descended from the
_Cordaiteæ_), 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, occupied
the sea, and others again of nightmare appearance 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 pronounced, 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 countries,
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 conditions very like those
prevailing in England at present, many of our familiar species of wild
flowers and trees at length made their appearance--Marsh Marigold
(_Caltha palustris_), Sloe (_Prunus spinosa_), Blackberry (_Rubus
fruticosus_), Hawthorn (_Cratægus Oxyacantha_), Cow Parsnep (_Heracleum
Sphondylium_), Bogbean (_Menyanthes trifoliata_), Gypsywort (_Lycopus
europæus_), Sheep’s Sorrel (_Rumex Acetosella_), Birch (_Betula alba_),
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-bearing
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 emerging 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 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 remigration 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
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 continued 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
importance in this connection. Is this biological testimony 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 comparison with the depth of the adjoining ocean,
this shelf is but little below sea-level, and a slight elevation 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 geographically 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
number 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 everyday 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 Orkneys. 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 Norway. To
quote a few instances of this north and south difference coupled with
east and west similarity: the Small-flowered Crowfoot (_Ranunculus
parviflorus_), White Bryony (_Bryonia dioica_), Water Violet (_Hottonia
palustris_), Yellow-wort (_Blackstonia 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 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 europæa_) ranges only as far south as Yorkshire; and the
beautiful Globe Flower (_Trollius europæus_), so characteristic of
northern pastures, creeps southward as far as the Severn. The first of
these is on the Continent confined to Scandinavia; the others, though
found in France, etc., are characteristic of the hilly regions there,
and are much more abundant farther northward.

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 at 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 similarity 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 found to be roughly north-western as opposed to
south-eastern. The further change due to elimination 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 Chamæcistus_), 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 fœtidus_)--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 a
reduced Continental one, many considerations, geographical, climatic,
and edaphic, must be duly taken into consideration if we are to
understand the composition and distribution of our vegetation.

But making all allowance for these various disturbing 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 leading 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 introduction, 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 to the
extinction, perhaps by human agency, of intermediate stations. But it
stands by no means alone. In Cornwall two

[Illustration: FIG. 28.--GREAT BUTTERWORT (_PINGUICULA GRANDIFLORA_).

[_To face p. 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 distributed 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 Ireland, we find along
the south and west coasts no less than seven plants unknown in Great
Britain, and elsewhere found only or mainly in the Pyrenees. Of these,
three Heaths (_Erica mediterranea_, _E. Mackayi_, _Dabœcia polifolia_)
are confined to Connemara and the Pyrenees; two Saxifrages, the London
Pride (_S. umbrosa_) and the Kidney-leaved (_S. Geum_), with their Irish
headquarters in Kerry, are likewise confined 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 Killarney 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, whose English or Irish stations are quite discontinuous 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 Romanzoffiana_ (Fig. 30), whose greenish-white
flowers possess a delicious fragrance resembling that of its ally, _S.
spiralis_, the Autumnal Lady’s Tresses. _S. Romanzoffiana_ 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 Pipewort (_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 mentioned
individually, for enough has been said to show that along the western
coasts of the British Isles

[Illustration: FIG. 29.--STRAWBERRY-TREE (_ARBUTUS UNEDO_) AT THE LAKES
OF KILLARNEY.]

[Illustration: FIG. 30.--_SPIRANTHES ROMANZOFFIANA_ GROWING BY LOUGH
NEAGH.

[_To face p. 174._]

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 adjoining 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 mentioned. The Spotted Slug of Kerry
(_Geomalacus maculosus_) is elsewhere confined to Portugal; while a
little fresh-water Sponge, _Heteromeyenia ryderi_, 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 we seek in this direction an
explanation? The evidence 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
introductions 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 compete 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 Langsdorfii_) 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 presence 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
consider the means of dispersal possessed by the species in question,
and to measure these against the nature 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; consequently 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 Saxifrages 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 digestion 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 assumption 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 plants, growing in different situations, should be brought
thus from the Pyrenees and Mediterranean to our western coasts is a
highly speculative suggestion. 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, survived 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 formidably 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 exceedingly difficult one, upon
which it would be rash to pronounce any hasty opinion. Students of the
subject 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 bygone
land surfaces in distant times and successfully survived the terrors of
the Glacial Period; others claiming a much less remote period for their
immigration. Indeed, one eminent recent writer on the subject, the late
Clement Reid, considered that the Lusitanian 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 elements 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 VIII

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
inquiry 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 interesting 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 chlorophyll, or green colouring-matter of the
leaves, possesses the remarkable power in the presence of sunlight of
breaking up and recombining these substances 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
chlorophyll, 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
chlorophyll, 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_),

[Illustration: FIG. 31.--BIRD’S-NEST ORCHIS (NEOTTIA NIDUS-AVIS). 1/2.]

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
Hypopitys_). 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 (_Ericaceæ_), 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 underground
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 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 nourishment 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
flowering 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 hedgerows, is
the Toothwort (_Lathræa Squamaria_). This curious plant has underground
creeping stems clothed with whitish, tooth-like, fleshy scales
(curiously modified 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 Toothwort
is a very harmless parasite, and the species of Broomrape also, though
sometimes abundant on 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 nourishment,
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 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 considering 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,
retaining 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 nourishment 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_), Butterworts
(_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 movements 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 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, moreover, 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 nitrogenous substances of any kind. Darwin found that a morsel of
human hair weighing only 1/78,740 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 1/20,000,000 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
considerably 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 insectivorous 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
indifferent; 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 1/10 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 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 mousetrap 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 few inches in height. Furthermore, we notice an incoming of
new plants unknown at lower levels, which search will show us to be
confined 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 elevation, 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 snowstorm 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

[Illustration: FIG. 32.--ALPINE PLANT-BOSS (_SILENE ACAULIS_,
_HYMENOPHYLLUM UNILATERALE_, _MNIUM HORNUM_).

[_To face p. 191._]

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 primaloides_) 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 longitudinal 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

[Illustration: FIG. 33.--SEDUM PRIMULOIDES. 1/1.]

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 conditions. 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. Hectori_ (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,

[Illustration: FIG. 34.--NEW ZEALAND SHRUBBY VERONICAS, SHOWING, FROM
LEFT TO RIGHT, REDUCTION OF LEAF WITH INCREASING ELEVATION OF HABITAT.
2/3.]

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 constructed that with the passing away of the cold, flowering and
fruiting may be accomplished quickly, before winter descends again. The
abundance and 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, as leading to
urgent advertisement by means of brilliant coloration; while the fact
that the pollinating insects are largely Butterflies, the most æsthetic
of flower visitors, has also been put forward as accounting for it. Be
that as it may, the glowing patches of colour produced by many quite
minute alpine plants 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_)?

[Illustration: FIG. 35.--MOUNTAIN AVENS (DRYAS OCTOPETALA). 1/1.]

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 _Compositæ_ 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 suggests[11] 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 discovered, 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 continuously
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 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 temperature
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 beyond 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 circumstances. 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 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 activities 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 concludes,[12] “do not enable us to
recognize in plants any protective means against cold. The capacity of
withstanding 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 occasionally 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 inhabitants, and we must
examine the relations existing 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 nourishment 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
Algæ, the probable ancestors of all the higher plants, were developed,
presumably through the peculiar _Cyanophyceæ_, or “Blue-green Algæ,” 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
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[13] 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 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, Liverworts, Club-mosses, Horsetails, and Ferns, have not,
and so far as is known never had, a single representative in the sea.
Only one or two Fungi--often symbiotically combined with Algæ 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 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 terrestrial 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 conditions entirely
different, where amid shallow inlets and salt-marshes land and sea merge
imperceptibly. 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 prevented 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--_Ranunculaceæ_ (_R. sceleratus_), _Cruciferæ_ (_Cochlearia_
spp.), _Caryophyllaceæ_ (_Alsine_), _Umbelliferæ_ (_Apium graveolens_,
_Œnanthe Lachenalii_), _Compositæ_ (_Aster Tripolium_, _Artemisia
maritima_), _Primulaceæ_ (_Glaux maritima_), _Plumbagineæ_ (_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 of a maritime habitat. The Order to which it
belongs, the _Naiadaceæ_ 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 Plant 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
_Naiadaceæ_ we can study the various stages, and form a picture of how
the Grass-wrack migrated to the sea. First we have the 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 without 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 completely submerged in fresh or brackish
water. One of them (_N. flexilis_) 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
reversion 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 submersed
pollination, the pollen-grains rising to the surface, where they are
caught by the stigmas of floating female flowers. It follows that the
individuals 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 overlap 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_, 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,
such as _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 vegetative 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 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

Alien plants, 143

Alpine plants, 190

Animal-eating plants, 77

Animals and plants, 75
  and seed-dispersal, 69
  dependence on plants, 155

Arctic deserts, 19
  plants, 196


Bog plants, 41

British flora, 167
  Isles, vegetation, 25, 30


Chlorophyll, 180

Cultivated plants, 145


Deserts, 16


Fertilization, 82

Flowers, 126
  display, 85
  structure, 81

Fruit, 131

Fruits, explosive, 55


Glacial Period, 165

Grassland, 25


Insectivorous plants, 78, 186

Insects and flowers, 81


Leaves, 119

Life, origin of, 15


Man and vegetation, 135

Marine plants, 201

Migration, 48

Mountain plants, 189

Mud-flats, 17

Mycetozoa, 156

Myxomycetes, 156


Ocean depths, 19, 76

Origin of life, 199


Parasites, 183

Peat flora, 41

Planets, question of life on, 11

Plant associations, 30
  economy, 98, 141
  formations, 32
  migration, 48

Plants, cultivated, 145
  earliest, 154

Pollination, 82


Roots, 105


Salt-marshes, 23, 40

Saprophytes, 181

Seed-dispersal, 49

Seeds, 50

Semi-deserts, 22

Shingle beaches, 39

Soil, 99

Stems, 109

Symbiosis, 79


Types of vegetation, 31


Vegetation, closed, 24

Vegetative reproduction, 53


Water, dispersal by, 61
  flora, 43, 199

Wind, dispersal by, 62

Woodland, 25


Xerophytes, 36


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


FOOTNOTES:

[1] SVANTE ARRHENIUS: “The Destinies of the Stars.” Translated
by J. E. Fries. Putnam, 1918.

[2] F. SODDY: “Matter and Energy,” 1912, p. 194.

[3] A. G. Tansley: “Types of British Vegetation,” 1911, p. 63.

[4] H. B. GUPPY: “Plants, Seeds, and Currents in the West
Indies and Azores,” 1917, p. 425.

[5] W. B. BARROWS: “Seed-planting by Birds.” Report of the
Secretary of Agriculture, U.S.A., 1890, p. 281.

[6] See A. H. CHURCH: “The Plankton-phase and the
Plankton-rate,” _Journal of Botany_, June, 1919, supplement.

[7] G. H. CARPENTER: “Insects: Their Structure and Life,” p.
300.

[8] W. B. BOTTOMLEY in “The Exploitation of Plants,” edited by
F. W. Oliver, 1917, p. 12.

[9] To be accurate, 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.

[10] F. J. HANBURY and E. S. MARSHALL: “Flora of
Kent,” 1899, p. xxxv.

[11] A. F. W. SCHIMPER: “Plant Geography” (English
translation, 1903), p. 719.

[12] A. F. W. SCHIMPER: “Plant Geography” (English
translation, 1903), p. 41.

[13] See AGNES ARBER: “Aquatic Angiosperms: the Significance
of their Systematic Distribution,” _Journal of Botany_, 1919, p. 83.





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