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.