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 land755925? 20-30
Land under natural or semi-natural vegetation 15-204070-75? 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

Fig. 4.—Wild Carrot (Daucus Carota), growing under great exposure. 1/2.