384 



INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



than the association dominants. The (sometimes unaccountable) 

 mixing and even intergradation of all these communities is often 

 intricate and may be suggestive of their relative youth, many having 

 apparently failed to come to a state approaching equilibrium with 

 the environment since emergence from glaciation or other extreme 

 disturbance. Actually, it may be questioned whether, in many areas, 

 even relative equilibrium can be attained in the face of the persistent 

 frost-activity, and it has been claimed that the whole system con- 

 stitutes an ' open ' one in which the main tendency is repeated 



Fig. 109. — Tundra on Southampton Island, Hudson Bay. 

 support low bushy Willows. 



I'he depressions 



readjustment to almost perpetual disturbance. Fig. 109 shows an 

 area of low-arctic tundra in eastern Canada. 



With the generally poor drainage resulting from the soil being 

 permanently frozen to not far beneath the surface, damper depres- 

 sions or marshy open tracts tend to be plentiful although often of 

 quite limited extent ; indeed they are rarely absent except in regions 

 of porous substrata and low water-table. In the low- Arctic they 

 are commonly rather luxuriantly vegetated, the sward often being 

 taller than it is in drier areas. They are usually dominated by 

 Cotton-grasses and relatively hygrophytic Sedges such as marshland 

 ecads of the Water Sedge (Carex aqiiatilis agg.), and by Grasses such 

 as the Arctagrostis {Arctagrostis latifoUa s.l.), with a few hvgro- 



