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.]