1903] BOG PLANT SOCIETIES 407 



depressions throughout the year impossible. On the south and 

 southeast, while the rainfall and relative humidity are favorable, 

 the intense insolation of the summer months seems to be the 

 controlling factor. The northern boundary coincides with that 

 of the northern limits of dense forests. According to Kjellman, 5 

 Kihlman 6 and Warming, 7 this boundary is controlled by the 

 amount of snowfall, exposure to dry winter winds, and the 

 length of the growing season. There does not appear to be any 

 relation between the distribution of this group of plant societies 

 and the "life zones" distinguished by Merriam. 8 Within the 

 belt of optimum conditions the climate is characterized by great 

 range of temperature, both daily and annual. As we go toward 

 the east from the Mackenzie basin, this is modified by the 

 increase in relative humidity. The summers are short, bright, 

 and warm, with abundant rainfall, principally in the form of 

 thunder showers. The winters are long, and extremely low 

 temperatures may occur. The snowfall increases from a foot or 

 two in the western part to several feet in Ontario and the St. 

 Lawrence basin. In the Northwest Territories, where the 

 tundra vegetation is dominant, the ground below a depth of a few 

 centimeters is frozen practically throughout the year. Since air 

 temperatures of 21 C. are common in late spring and summer, 

 the plant roots and shoots must there withstand remarkable 

 temperature differences. With the exception of the eastern 

 maritime provinces and Maine, no part of this optimum area is 

 comparable with the conditions which call forth the great bog 

 development of northern Germany and Scandinavia. In the 

 latter localities the bogs are confined to depressions, but may 

 occur in a variety of topographic situations. They may even 

 invade established forests, arid by raising the ground water level 

 destroy the tree covering. 9 



5 KJELLMAN, F. K., Aus dem Leben der Polarpflanzen. Leipzig. 1883. 



6 KIHLMAN, A. O., Pflanzenbiologische Studien aus Russisch-Lappland. Acta 

 Soc. pro Fauna et Flora Fennica 6. 1890. 



7 WARMING, E., Ueber Gronlands Vegetation. Engler's Botan. Jahrbiicher 

 10. 1888. 



8 MERRIAM, C. H., Life zones and crop zones of the United States. Bull. 10, U. 

 S. Dept. Agric. 1898. 



9 GANONG, W. F., Raised peat bogs in the province of New Brunswick. Proc. 

 Roy. Soc. Can. 11.34:131-163. 1897. 



