476 
GEOGRAPHY. 
BOOK V, 
fication of which much difficulty is experienced : thus, plants of 
the sand of the sea-shore are confounded with saline plants ; 
those of barren soil with the species of cultivated land, and 
those of coarse sand are not different from those of gravel. 
9. Plants of sterile places that are very compact, as stiff 
clayey soil, or such as have their surface hardened by drought 
or heat, or those which are trodden hard by man or animals. 
This is an heterogeneous class, and contains plants of very 
uncertain characters. 
10. Plants which follow man. These are few in number, 
and more fixed in their station, either in consequence of 
nitrous salts being necessary to their existence ; or because, 
perhaps, azotized matter is required for their nutriment. 
11. Forest plants^ among which are to be distinguished, 
firstly, the trees that form the forest, and the herbs which 
grow beneath their shade. The latter are to be separated 
into two kinds, those which can support a considerable degree 
of shade during all the year, which are found in evergreen 
woods ; or such as require light in the winter, like those which 
are found among deciduous trees. 
1*2. Bushes and hedge plants. The shrubs which compose 
this division differ from the plants of the forest in their smaller 
size, and by the thinness of their leaves ; the herbaceous kinds 
that grow among them are ordinarily climbing plants. 
13. Subterranean plants^ which live either in dark caverns, 
as the byssus, or within the bosom of the earth, as the truffle. 
These can dispense altogether with light, and several cannot 
even endure it. Plants that grow in the hollows of old trees 
have great analogy with those of caverns. 
14. Mountain plants, as subdivisions of which all the other 
stations may be taken. We generally class among mountain 
plants such as, in Europe, are not found lower than 500 
yards; but this is quite an arbitrary limit. The most im- 
portant division is between those which grow on mountains, 
the summit of which is covered with eternal snow, and those of 
mountains which lose their crest of snow in the summer. In 
the former, the supply of water is not only continual, but 
more abundant and colder as the heats of summer advance ; 
in the latter, on the contrary, the supply of water ceases when 
