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derived from the salt spray continually drifting over them. Such plants have generally a greenish-white or glaucous hue; they seldom flourish far from the shore, and when growing inland the soda usually existing in their tissues is replaced by potash. Silica is always present in more or less quantity, and is particularly abundant in the cuticle of Grasses and Horsetails. Lime is also found in most species; and its presence in the soil seems necessary to some, as may be seen in the peculiar vegetation of our chalk-downs and limestone hills; indeed the growth of plants is so intimately connected with the composition of the soil, that a botanist can generally detect the prominent mineralogical features of a country through which he is travelling by the aspect of its vegetation.
There is no department of botanical study more interesting than that which traces the relation of plants to the nature of their habitats, or places of natural growth. Mountain, bog, wood-land, sea-cliff, wet meadow, and upland pasture have all their characteristic vegetation; and even the hillocks of drifting sand, that line some parts of our storm-beaten coast, have a flora peculiar to themselves, or rarely found elsewhere. Difference of soil or situation will even produce great changes in the appearance of plants of the same species, sometimes to an extent that renders them difficult of recognition by the unpractised observer. Those usually inhabiting marshy localities, when growing on dry ground become smaller and more rigid, while the native of the hills, when transferred to the lowland, acquires a more succulent and luxuriant habit; the plants of the clay and loam often present a very different appearance to those of the same kind growing upon chalk or sand. In determining species, allowance must always be made for these accidental variations from the typical form; the points in which deviation most frequently occurs are, in the size of the plant, the form and dimensions of the leaves, the colour of the flower, and the degree of development of hairs, prickles, and other appendages of the cuticle.
