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. 4S8. 

 Northern limit u£ tree growth, Alaska. (Copyright, 1899, by E. H. Hamnian. ) 



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 l)order line with the woodland formation the trees 

 are stunted, lient to one side l)y the hea\v snows, or the lops are 

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

 sometimes spoken of as the "ccild wasle," since ccinditions liere 

 are .somewliat sinn'lar Id those in llic desert, the e.Mreme cold 



