4?2 RELATION TO ENVIRONMENT. 



916. Snow cover. Snow protects plants during the winter by 

 checking radiation of heat from the soil so that the ground does 

 not freeze, or freezes to a less depth. For trees and shrubs this is 

 an advantage, since root absorption is permitted to some extent, 

 sufficient often to supply water to the plant which is lost by evap- 

 oration from the branches or the leaves of evergreens. In arc- 

 tic and alpine regions the entire vegetation is often covered and 

 transpiration thus checked in the cold dry atmosphere. In 



Fig. 487. 

 Snow covering showing protection of low vegetation in winter. 



these regions trees and shrubs are often deformed or laid pros- 

 trate by the weight of snow, so that it exercises a dwarfing influ- 

 ence. That snow protects trees and shrubs from dying by cov- 

 ering them up is shown by the fact that in arctic regions branches 

 and stems projecting above the snow die because they dry out. 

 In temperate regions perennial plants, like grass, some fall 

 crops, etc., are protected during the winter by snow cover, since 

 a more equable temperature is maintained and transpiration 

 lessened. Where the ground is bare in the winter the alternate 

 thawing and freezing of the surface "heaves" the ground and 

 lifts the roots from the soil. Snow also conserves moisture and 

 distributes snow-water during the warm season in mountain 

 regions over a longer period, especially where the sun does not 



