Read Ebook: Aspects of plant life; with special reference to the British flora by Praeger R Lloyd Robert Lloyd

Font size: 10 px 15 px 20 px 25 px 30 px 35 px

Background color: BlackWhiteGrayLight GrayDark GrayDim Gray

Text color: BlackWhiteGrayLight GrayDark GrayDim Gray

Apply/Save settings

Ebook has 208 lines and 53884 words, and 5 pages

PREFACE 3

INDEX 208

FRONTISPIECE.--Desert types:

FIG. PAGE

PLANT LIFE

ON FARLETON FELL

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 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 Algae 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, 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."

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 , 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.

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 ; 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

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.

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.

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.

PLANT ASSOCIATIONS

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 thus contrasts the area of cultivated land , 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.

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 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 with that of Middle Europe or of England, we shall be struck with the prevalence in the former of small-leaved twiggy plants--Lavender and Rosemary 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 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 , 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

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 , the tough, waxy skin which covers the Sea Holly , the extensive underground stem-systems of the fleshy-leaved Sea Convolvulus and Sea Purslane . 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 , the Sea Whin , 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 , Sea Wheatgrass , Sea Sedge 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.

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 and Purple Heather , 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 , 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 , which floats in the bog-pools, and the Sundew , 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

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 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: A zone of tall reed-like plants near the margins, which farther out is succeeded by 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 , Reed-mace , Flag , Bur-reed , Bulrush ; 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.

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 , for instance, and on the downs peculiar woods or scrub of Box , Juniper , Yew , 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."

PLANT MIGRATION

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 r?le in the ever-changing drama of Nature. We shall do well to spend a little time in considering it.

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 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 --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 ; it is especially characteristic of marsh and water plants. Its effect is to produce large colonies, such as the great beds of Reeds or Reed-mace which fringe our lakes, the groves of Bent on sand dunes, and the beds of Anemones or Broad-leaved Garlic 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 , or among the

flowers as in many Leeks . 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 . In this plant 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 Action of the parent plant, water, wind, animals.

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 , the seeds may be cast to some small distance. The efficacy of the arrangement is not so obvious when, as in the Poppies or Bell-flowers , the openings are small , but it is clear that these plants do not suffer from lack of dispersal, in view of their abundance and wide range.

But the assistance which the parent plant gives is often of a more active and even dramatic character, though in these cases it is usually effected not by a 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 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. 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 Leguminosae. In the Cranesbills 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 , which will be projected

Share on: WhatsApp @Hash Facebook Tweet