20 Nebraska Experiment Station Research Bui . 15 
nuals to increase slightly while crossing Wisconsin and north- 
ern Michigan. 
Beginning at the southern end of the line in Arkansas the 
summer group of annuals stands over 85 per cent as far 
north as Ripley County in Missouri. North of this point sum- 
mer annuals drop out rapidly until they finally stand at almost 
zero in Kewaunee County, Michigan. It will be observed that 
along this line, from southern Arkansas to northern Mich- 
igan, it is the summer rather than the spring annual group 
that gives way to winter annuals. 
The most striking thing to be observed concerning the 
spring annual group is that its rate of increase to the north is 
comparatively steady. In all cross-sectional studies thus far 
made, the rate of change for the spring annual group has been 
more uniform than that of the other groups. 
The vertical crayon-like lines in the graph (Fig. 8) mark 
the boundaries between types. These occur wherever two 
heavy solid lines cross one another. Named in order from 
south to north the types are as follows : summer-spring, sum- 
mer-winter, winter-summer, summer-winter, summer-spring, 
spring-summer, and spring-winter. 
The cross-sectional view of types in southeastern Wash- 
ington and north-central Oregon (Fig. 9) gives the relation 
existing between the spring-summer, spring-winter, and 
winter-spring types. At the southern end of the line, which 
is in Baker County, Oregon, winter annuals stand at about 16 
per cent. In north-central Oregon, some eighty miles to the 
northwest along this line, winter annuals come to occupy over 
80 per cent of the acreage. Continuing northwest, winter 
annuals drop out rapidly until they reach almost zero in 
Snohomish County, Washington. Spring annuals, it will be 
seen, vary inversely as the winter annuals, for the summer 
annual group thruout this cross-section stands so low that it 
has almost no effect. This is particularly true where winter 
annuals occupy first place. 
The broken lines crossing the graphs (Figs. 8 and 9) serve 
as an index to the quantity of heat above freezing that is nor- 
