22 
Nebraska Experiment Station Research Bui. 15 
PER CENT 
grraph.) % 
which the growth of agricultural plants almost ceases. Some 
persons prefer 36° F. or even 40° F. as a lower limit. There 
can, however, be no good reason for choosing an arbitrary 
point so long as 32° F. is less questionable and better known. 
BOUNDARY I. INKS DETERMINED LARGELY BY HEAT 
The lines separating one type of annual cropping from 
another (Fig. 7) are to a large extent determined by heat. 
The boundary between the spring-summer and summer-spring 
types east of the Rockies follows in general the line of 200 
month-degrees. (See Figs. 7 and 12.) West of the Rockies 
the boundary between these two types of cropping could not be 
accurately determined. The boundary line that is shown on 
the map (Fig. 7) appears to bear no relation to the line of 
