26 Nebraska Experiment Station Research Bui. 15 
July 1st the quantity of heat above freezing has also been 
determined for this period. Figure 14 defines the boundaries 
of a belt of country in the United States which receives from 
150 to 230 month-degrees above freezing between September 
1st and July 1st. The small circles placed to the north of this 
belt mark certain points that receive during the winter an- 
nual season not less than 138 month-degrees. Only a few 
winter annual areas will be found to the north of these 
points. 
THE EFFECT OF FACTORS OTHER THAN HEAT 
Tho heat has the greatest effect upon the boundaries of 
crop types, rainfall, soil, and economic conditions can have 
a marked influence. A few examples will suffice to illustrate 
how heat alone will not fully account for the position of cer- 
tain crop boundaries. Thus the area of winter-summer crop- 
ping in western Kansas is determined to a large extent by 
July and August droughts that are accompanied by extreme 
heat. Winter wheat in this area escapes the hot droughty 
period more often than corn and other summer annuals. If 
it were not for this fact this area would without much doubt 
drop back to the summer-winter type. The factors just men- 
tioned also influenced the development of the small island of 
winter-summer cropping in south-central Nebraska, but here 
extremely level land was also a factor. This type would, in 
all probability, not have appeared at this point in 1909 had 
the land been hilly. 
The position of boundary lines in a given year throws 
some light on the economic question of profits derived from 
certain crops. As an illustration, the boundary line drawn 
around the winter-summer type in western Kansas must define 
an area within which, according to the experience of farmers, 
an acre of winter annuals had been paying better than an acre 
of summer annuals. When one stops to consider that in this 
area winter wheat is about the only winter annual and corn is 
the leading summer annual, it is not far wrong to assume that 
here an acre of winter wheat previous to 1909 had been paying 
