156 



and the physical properties of the sand, including the size of 

 the particles and to some extent the water content, are both 

 of importance, and by their combined effect determine the pe- 

 culiar type of vegetation. Direct observation alone, however, 

 is sufficient to show that the water supply on the dunes is rea- 

 sonably constant, and the plants owe their xerophytic habit to 

 the rapid loss of water by transpiration, and not to a deficient 

 soil content. 



The climate in these sand regions is of course similar to 

 that of the surrounding parts of the state, and has no direct in- 

 fluence in causing the marked differences in vegetation. Nev- 

 ertheless, temperature is of importance in determining the flora 

 of any region, and the mixed prairie and forest type of vegeta- 

 tion of central and northern Illinois is to some extent the result 

 of the seasonal distribution of rainfall. For these reasons, as 

 well as to show the general climatic conditions of the area un- 

 der discussion, tables showing the precipitation and tempera- 

 ture are included. The data are taken from Mosier's "Climate 

 of Illinois", and are given for Havana so far as observations at 

 that place are available. The records are also given for Peoria 

 and Springfield, Illinois, and for Keokuk, Iowa, three cities 

 forming a triangle, with Havana and the sand region in its 

 approximate center. It should be noted that the temperature 

 readings are taken under the regular shelters used by the U. S. 

 Weather Bureau, and consequently do not represent the de- 

 gree of heat to which plants are normally subjected. 



Monthly and Annual Mean Temperature. 



The average annual range of temperature in the central 

 district of Illinois is 109°, while the extreme range is 140°, from 

 -28° in January, 1884, to 112° in July, 1901. The average 



