42 PLANT ASSOCIATIONS 
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 re- 
flexed 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 neigh- 
bours 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 vege- 
tation, 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 con- 
sider the plant formation which characterizes lowland 
lakes and rivers, where water suitable for plant use is 
