2l6 



BOTANICAL GAZETTE 



[SEPTEMBER 



abounding in extremes and fluctuations inferior only to those of 



dominated 



As might 



be expected, there is a similarity between the evaporation rate of 

 the prairie and that of the open areas of the oak dune (fig 9). 



TABLE II 



Meax weekly evaporation rates in cc. from a standard atmometer during 



the io midsummer weeks of the three years under 



investigation 



Association 



Cottonwood dune 



Pine dune 



Oak dune 



Oak-hickory forest 

 Beech-maple forest 

 Edaphic prairie . . . 



IOIO 



25.0 

 14. 1 

 12. 1 



10. 2 



IQII 



30.2 



12.6 



13 4 

 11 .6 



8-5 

 151 



1012 



20.3 

 10.3 

 10. 1 



6.7 



5-3 

 12.4 



Average 



25.2 



12-3 

 II .9 



9.2 

 8.0 



13-7 



Compara- 

 tive rates 



315 



154 

 149 



115 



IOO 



171 



It is interesting to note that comparative evaporation rates for 

 all the associations, with the exception noted in the case of the pine 

 dune, remain substantially the same whether the comparison 

 is based upon the averages for the entire seasons (table I) or for 

 the weeks of misdummer (table II) only. The comparison is 

 perhaps more striking when the data are represented graphically, 

 either for the individual seasons (fig. 14) or when the data for the 

 three years are combined (fig. 15), when, if it be true that water 

 conditions are the most important factors affecting the establish- 

 ment of different plant associations, there can be no reasonable 

 doubt that the progressive increase of the moisture content of the 

 habitats indicated by the progressive decrease in the evaporation 

 rates causes a corresponding progressive increase in the mesophy- 

 tism of the plant associations as a whole, a change which we are 

 accustomed to term succession. 



As recommended in the writer's preliminary report (loc. cit.) y 

 the beech-maple forest is taken as the unit of comparison, and the 

 evaporation rates in the other associations are expressed in terms of 

 this unit. Thus, the beech-maple forest being represented by 100, 

 the comparative evaporation rates for the midsummer weeks (table 

 1 1) in the associations which precede it in the succession are respec- 

 tively 115, 149, 154, and 315, showing a striking and progressive 



