198 



BOTANICAL GAZETTE 



[SEPTEMBER 



the dune upon an eastern exposure. This station was maintained 

 only during the season of 19 10. A detailed report of the atmom- 



for the four stations during the season of 19 10 



eter readings 



is shown in fig. 3, where in addition to the evaporation records 

 the weekly rainfall in centimeters at Chicago during the summer 

 months is plotted. As it has been possible to establish no direct 

 relationship between rate evaporation and the occurrence and 

 amount of precipitation, rainfall data are omitted from the remain- 

 ing diagrams. 







aw 







_* - 









Fig. i. — Sand dunes with Populus deltoides or the cotton wood dune association 

 still moving slowly over an area with Pinus Banksiana, P. Strobns, etc.; Wilier, Ind. 



examination 



high 



changes. There was no great divergence in the rate at stations i, 

 2, and 3, but the rate at station 4 was decidedly lower, showing 

 the modifying influence of the shrub cover in creating less extreme 

 conditions, which permit the entrance and establishment of species 

 of less xerophytic nature, which eventually form the succeeding 

 association. Thus not only does the nature of the vegetation con- 

 trol to a great extent the evaporation beneath it, as pointed out 



