474 



RELATION TO ENVIRONMENT. 



the moisture away from the intervening ones, which then die. 

 Since the struggle is against the adverse conditions of climate 

 and not a competition between plants to occupy the ground, 

 no one floristic type dominates as in the case of the grasses and 

 forests of the grassland and woodland formations, but grass- 

 land and woodland types grow together. So we find grasses, 

 trees, and shrubs growing without competition in the desert. 

 The dominant vegetation type is xerophytic. 



4th. Arctic-alpine formation. This formation extends from 

 the limit of tree growth to the region of perpetual ice and snow. 



Fig. 488. 

 Northern limit of tree growth, Alaska. (Copyright, 1899, by E. H. Harriman.) 



The forest here comes in competition with climate, with the 

 severe cold of the long winter night, so that tree growth is limited, 

 and on the border line with the woodland formation the trees 

 are stunted, bent to one side by the heavy snows, or the tops are 

 killed by the cold wind. The arctic zone of plant growth is 

 sometimes spoken of as the "cold waste," since conditions here 

 are somewhat similar to those in the desert, the extreme cold 



