746 



TEXTBOOK OF BOTANY 



Fig. 375. Marsh-marigold {Caltha leptosepala) , Mount Ranier National Park, 

 in bloom at the edge of a snow-bank. The flower buds were fully developed under 

 the snow and opened within a few hours after the snow melted. Photo by W. H. 

 Camp. 



of plants that pass most of the year in low-temperature dormancy, and 

 grow but two to three months. 



The survival of plants in different situations in the tundra is deter- 

 mined far more by the direct effects of environment than by the inter- 

 ference of other plants. Consequently, apparent successions are more 

 in the nature of fluctuations in the proportion of different species than 

 of the progressive development of more highly organized communities. 



The southernmost isolated communities of tundra vegetation occur in 

 bogs, on sand plains, on cliff edges, and on the summits of high moun- 

 tains above the timber line. The conditions in alpine tundra are some- 

 what different from those in the polar tundra, especially the light inten- 

 sity, the length of day, and the quality of light. The length of the growing 

 season, however, and the soil conditions may be much the same. The 

 absence of trees is not due to the intense cold. The Siberian fir forest 

 grows in the coldest locality in the northern hemisphere. 



Boreal forest (Figs. 376-377). This is the most extensive forest on the 

 continent and extends south of the tundra from western Alaska to New- 

 foundland. The most characteristic tree is the white spruce, which 



