232 BOTANICAL GAZETTE [September 



comparative values of the moisture factors show such a surprising 

 rate of increase as one proceeds from the pioneer to the climax 

 associations that it cannot be doubted that such a change in water 

 conditions must be one of the chief factors, if not the most impor- 



from the more 



more 



Summary 



1. These data represent the evaporation rates in the lower 

 aerial stratum and the range of soil moisture in the upper subter- 

 ranean strata of the vegetation of the various associations, but 

 these must be regarded as critical regions, since within them de- 



determine 



vegetation. 



same 



similar 



3. The rate of evaporation in the cotton wood dune association, 



amount and bv its excessive variation, seems 



sufficient 



and for the absence of undergrowth, in spite of the constant pres- 

 ence of growth-water. 



resemble 



mean 



growth-water. The former is slightly more 



midsummer 



ernal 



as that of the succeeding association, thus agreeing with its lower 

 evaporation rate during that portion of the year. 



6. The evaporation rates and the amount of growth- water in 

 the various associations vary directly with the order of their 

 occurrence in the succession, the pioneer being the most xerophytic 

 in both respects. 



7. The ratios between evannratinn and growth-water in the 



ma 



com 



100 



thus indicated are sufficient to be efficient factors in causing sue- 



