157 



range in summer is 50°, but no data are available showing the 

 daily range. The average date in the central district for the 

 last killing frost in spring is April 21, and for the first in au- 

 tumn, October 10, thus including a growing season of 172 days. 



Mean Monthly and Annual Rainfall. 



The amount of rainfall in this same district during the 

 growing season, assuming that to be from April to September 

 inclusive, is at Springfield 60.8 percent, at Havana 61.6 percent, 

 at Peoria 63.6 percent, and at Keokuk 66.0 percent. At Cairo, 

 at the extreme southern end of the state, only 48.4 percent of 

 the total rainfall occurs during this period. There is thus seen 

 in central Illinois a resemblance, increasing toward the west, 

 to the type of rainfall found on the western prairies, where as 

 much as 80 percent of the total falls during the growing season. 

 This unequal seasonal distribution is an important factor in 

 determining the prairie type of vegetation in the West, and is 

 certainly not without some influence in directing the eastward 

 migration of the prairie flora. 



The Plant Associations. 



The plant associations occurring in this region belong to 

 both of the prevailing types of vegetation in the state, the for- 

 est and the prairie. They may be classified as follows: 



I. The prairie formation. 



1. The bunch-grass association. 



2. The blow-sand association. 



3. The blowout association. 

 II. The forest formation. 



4. The black-jack oak association. 



It is difficult to estimate the relative areas occupied by the 

 two formations, although the impression gained by traveling 



