18 BRITISH PLANTS 
factor is the water-supply, the next is the temperature. 
The amount of water available to a plant affects its 
nutrition, the temperature affects its vital activities. 
The two things together constitute the climatic surround- 
ings of the plant. 
At this stage, and especially as we are chiefly con- 
cerned with our own country, it is advisable to draw 
attention to the influence of man upon the vegetation of 
the land upon which he has settled. Experience has 
taught him from the earliest times the importance of 
water upon the fertility of the soil, and by regulating and 
controlling the water-supply—that is to say, by drainage 
and irrigation—he has modified the face of the land to 
meet his requirements. Bogs have been drained, moor 
and heath reclaimed ; forests have fallen beneath his 
axe, and vast tracts of country, previously incapable 
of yielding crops, have been brought into profitable 
cultivation. Man, however, modifies the vegetation only 
through the soil-factors. By removing or planting 
forests, he may, to some extent, modify the rainfall 
(p. 13), but otherwise he has no control over the climate. 
The Cultivation of Food-Products in the British Isles. 
This is an interesting and instructive subject, and we 
touch upon it briefly because it is an excellent illustration 
of the work of man in regulating and modifying the 
vegetation. 
The land of Great Britain may for the present purpose 
be divided into three main regions : 
1. Uncultivated Land, including alpine regions, moors, 
heaths, lowland-swamps, and natural pastures. 
2. Woodland. 
3. Cultivated Land : 
(a) Arable Land—(i.) In which wheat can be grown, 
(ii.) In which wheat cannot be 
grown. 
(6) Pastures— (i.) Permanent pastures. 
(ii.) Pastures under clover and 
grasses in rotation. 
Most of the cultivated land was at one time covered 
by forest. Forest-soil, if properly drained, is naturally 
