Plants of Different Regions. 329 



Whether the vegetation of a region shall consist largely 

 of trees and shrubs, or of grasses and low herbs, depends 

 for the most part upon the depth and amount of the soil 

 water. Trees will predominate in regions where the vege- 

 tative period is warm, the atmosphere humid and compara- 

 tively quiet, particularly in winter, and where there is an 

 unfailing subterranean water supply. It is unessential to 

 trees when, and with what frequency, rains occur, provided 

 they are of sufficient amount during the year to replenish 

 the soil reservoirs. 



FIG. 177. 

 Drawing from a photograph of a neglected tree claim in western Kansas. 



Since the roots of trees go deep, they can avail them- 

 selves of water supplies which are beyond the reach of 

 grasses and other plants with shallow roots. The fact that 

 the roots of trees extend to considerable depths enables 

 them to become established along the banks of streams in 

 regions where the annual rainfall is too scant to penetrate 

 beyond the superficial layers of the soil. The water from 

 streams percolates some distance beyond their beds, so that 

 fringes of timber may follow water courses for hundreds 

 of miles through prairie or even desert regions. 



Trees may be made to grow where they would not 

 naturally, by keeping the surface soil loose and mellow 

 with the plow, so that rains may percolate to some depth 



