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,s 
Kihlman® and Warming,’ 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.’ 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 1s 
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, and by raising the ground water level 
destroy the tree covering. 
5 KJELLMAN, F. - Aus dem Leben der Polarpflanzen. Leipzig. 1883. 
°KIHLMAN, A. O., Pflanzenbiologische Studien aus Russisch-Lappland. Acta 
Soc. pro Fauna et i Fennica 6. 1890. 
7WARMING, E., Ueber Gronlands Vegetation. 
10. 1888, 
Merriay, C. H., Life zones and crop zones of the United States. 
ae ce i Raised peat bogs in the province of New Brunswick. Proc. 
Roy. Soc. Can. II. 34: 131-163. 1897. 
Engler’s Botan. Jahrbiicher 
Bull. 10, U. 
