24 Nebraska Experiment Station Research Bid. 15 
Fig. 12.— The line of 200 month-degrees above freezing marks very closely 
the boundary between the spring-summer and summer-spring types of annual 
cropping east of the Rockies. 
200 month-degrees west of the Rockies. Question marks ap- 
pear on the map at points where the classification was most 
difficult and inaccurate. To classify this part of the United 
States one would need in some cases to use township or pre- 
cinct data in the place of county data. Furthermore the acre- 
age of winter annuals would need to be more carefully sepa- 
rated from the acreage of spring annuals. 
The boundaries of the summer-winter and winter-spring 
types are in close harmony with certain quantities of winter 
wheat. The center of the summer-winter and winter-spring 
types are usually found close to the line where winter heat 
measures zero month-degrees. (See Fig. 13.) Each of the 
winter-annual areas in 1909 occupied areas where the winter 
heat measured not less than — 25 month-degrees. While the 
northern limit was closely marked by — 25, the southern limit 
was almost as closely marked by +25 month-degrees of 
winter heat. (It will be observed that crop boundaries and 
heat lines are again found to be most at variance with one an- 
other in the southwestern part of the United States.) 
Since in the summer-winter and winter-spring regions 
winter annuals are more or less active from September 1st to 
