ARCTIC VEGETATION. 



579 



tundra" (formed of different species, sometimes Cladonia or 

 Alectoria, etc.). 



1067. Conditions of environment. These are very peculiar to 

 the region, ist. Temperature, the extreme cold of the long winter 

 and the low summer temperature. 2d. Light, the long winter 

 night and the continued daylight of the short cool summer. 

 The continuous light in the summer is an advantage, since photo- 

 synthesis goes on without interruption and food materials are 



Fig. 5140. 



Lichen tundra showing the "reindeer-moss" or lichen, Alaska. (Copyright by 

 E. H. Harriman.) 



constantly formed. 3d. The cold ground-water because of the 

 ground-ice below the shallow layer of surface soil which is 

 thawed out; this retards root absorption. 4th. The high winds, 

 which in the cold dry climate are more trying for plants than 

 the lower temperatures to which they are subject. 5th. The very 

 dry air of the long winter. The long winter night is usually 

 clear, there is but little precipitation, and the air is very dry, so 

 dry sometimes during the severest cold that one's breath does 



