[Chap. Llll THE VEGETATION OF NORTH AMERICA 



745 



mosses, and a variety of small herbs with relatively large brilliant flowers 

 (Plates 1 and 3). There are also areas of such evergreen heaths as 

 bearberry, crowberry, and leatherleaf, together with creeping willows 

 and low birches. Alder, birch, and willow thickets occupy the warmer 

 slopes and stream banks. The myriad depressions and pools become 

 moss-covered bogs with sphagnum, polytrichum, and other bog mosses, 

 and are called "muskegs." 



The substrate is wet and poorly aerated, and peat accumulates in the 

 surface layer. A few feet below the soil surface is the "ground ice" — 

 fossil ice, it might be called, since some of it was formed at the close of 

 the period of glaciation. In our latitude plants are affected by the depth 

 to which soils freeze in winter, while in the tundra they are affected by 

 the depth to which soils thaw in the warmer season. In general, the 

 tundra is a region of light snowfall; but where snow accumulates and 

 covers the vegetation it prevents destructive erosion by ice crystals car- 

 ried by the violent and desiccating winds of winter. 



Plants of the tundra survive freezing night temperatures during the 

 growing season and grow at low average temperatures. Moreover, the 

 plants may freeze at any stage of development, including the flowering 

 period, survive under the snow, and continue growth the following sea- 

 son (Figs. 374 and 375). The vegetation of the tundra, then, is composed 



Fig. 374. Adder 's-tongue (Erythroniiim montantim) that grew through a 3-inch 

 layer of snow and bloomed. Photo taken by W. H. Camp at Mt. Ranier, Wash- 

 ington. 



