THE RELATION OF PLANTS TO WATER 



151 



The graduation of a mesophytic vegetation into a xerophytic 

 one is epitomized in many mountainous regions as one ascends 

 from the base to the summit of a high mountain. At the base 

 and along the watercourses of the mountain streams in such 

 regions typically mesophytic conditions often prevail, in which 

 broad-leaved plants are the dominant type. As one ascends to 



FIG. 73. The effect of exposure, slope, and moisture on vegetation 



" North slope covered with dense forest of fir (Pseitdotsuga) ; exposed south slope, 

 with oak scruh and pine." After Clements 



higher altitudes the sterility of the soil, combined with the lack 

 of water and the drying effect of winds, produces a distinct 

 type of xerophytic plants, which differ markedly from true 

 desert xerophytes. The high-altitude xerophytes of mountain- 

 ous regions are usually either herbs with thick, leathery leaves 

 or low, straggling shrubs and dwarfed trees. In the typical 

 pine forests which frequently inhabit mountain slopes (Fig. 73) 

 one notices that the more resistant and hardy species are found 

 high up, near the tree lines, while less hardy forms clothe the 



