[Chap. LIU THE VEGETATION OF NORTH AMERICA 743 



increased rainfall in postglacial time account for many of the irregulari- 

 ties of plant distribution. On the margins of the continent the position of 

 the seashore has varied, in part because of the uplift or sinking of the 

 land and in part because of changes in the height of sea level itself. 



Plant Formations 



Tundra (Figs. 371-373). This name has come into general use 

 throughout the world for the low, sometimes scanty vegetation which 



Fig. 371. Mature tundra at edge of Harriman Glacier, Prince William Sound, 

 Alaska. One of the heaths (Cassiope stelleriana) is the most abundant plant. A tree 

 one-half mile from the edge of this glacier is more than 300 years old. Photo by 

 W. S. Cooper. 



encircles the polar ice caps and in America covers the "barren grounds." 

 At its southern border is what the Indians called "the land of little 

 sticks" and the boreal forest. Here and there the barren grounds are 

 broken by rivers flowing into the Arctic Ocean through deep valleys 

 where the snow melts somewhat earlier and the growing season is 

 longer. Valley slopes are also less exposed to the dry winter winds, and 

 on them the northernmost outliers of the forest occur. 



The land in general is poorly drained and wet from the melting snows, 

 the fogs, and the drizzling rains of the warmer season. The drier parts 

 are covered with grasses, sedges, and rushes interspersed with lichens, 



