} 
XVI 
derived from the salt spray continually drifting over them. Such 
plants have generally a greenish-white or genie Bugs they 
seldom flourish far from the shore, and when growing inland the 
soda usually existing in their tissues is replaced by eds | Silica 
is always present in more or less quantity, and is particularly 
abundant in the euticle of Grasses and Horsetails. Lime is also 
found in most species; and its presence in the soil seems neces- 
sary 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 coun- 
try 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 dif- 
ferent 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. 

