14 Russell R. Spafford 
lobe of favorable summer temperature determines, to a large ex- 
tent, the boundaries of the Canadian agricultural area recently 
opened. In the southern part of this lobe the problems of low 
temperature are more commonly blended with problems of low 
rainfall than is true farther north. 
Figs. 15, 16, and 18 illustrate the approximate location and na- 
ture of critical agricultural lines in the northern hemisphere. 
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Fig. 15. Critical agricultural lines in North America. 
The distribution of native vegetation relative to these lines is 
very marked. The boundary lines of agricultural regions thus 
far well developed follow closely the boundaries between northern 
104 
