BOOKS AND CURRENT LITERATURE 359 



reached the following conclusions. "Differences in the amount of 

 evaporation in various associations, are due chiefly to the nature of the 

 vegetation, which by its size and density controls the evaporation be- 

 neath it. The observations indicate that successions between associations 

 are not caused by any conditions of evaporation. The more primitive 

 associations have the higher rates of evaporation, while those most 

 nearly like the climax t^^pe have the lowest rates. This is true not only 

 for the forest associations, in which low evaporation is expected, but 

 also for the prairie associations, which are correlated with an arid 

 olimate and consequently high climatic evaporation." 



Weaver"^ as a result of successional studies in the Palouse region of 

 the Northwest, finds that "A study of the difference of the rates of 

 evaporation in the various plant formations and associations shows 

 that these differences are sufficient to be important factors in causing 

 succession, at least through the earlier stages, where light values are 

 usually high." Later, in a study of edaphic prairip in Minnesota and 

 climatic prairie in Nebraska, Weaver and ThieF summarize the re- 

 actions as follows. "If sufficient Hght is available, there is no question 

 but that the humidity of the air and the soil are the most important 

 factors affecting the establishment of the different plant commimities. 

 The progressive increase of the humidity of the habitat causes a cor- 

 responding increase in the mesophytism of the plant community. The 

 evaporation rates and the amount of soil moisture of the various 

 communities both in Minnesota and Nebraska varj^ in general directly 

 with the order of their occurrence in the succession, the community 

 nearest the climax being the most mesophytic in both respects." 



Gates^ has directed his attention primarily to the ecesis of the domi- 

 nants of the various stages, and reaches the following results. 



"Invasion, which is the initial stage of succession, must take place 

 under the conditions already existing. 



• "The change of conditions coincident with mesophytic succession 

 brings about a decrease in the rate of evaporation in the ground or 

 chamaephytic layer. 



"In a given area, the differences in the amount of evaporation under 



" Weaver, J. E. Evaporation and Plant Succession in Southeastern Wash- 

 ington and Adjacent Idaho. Plant World 17: 273. 1914. 



^Weaver, J. E. and Thiel, A. F. Ecological Studies in the Tension Zone 

 between Prairie and Woodland. Rep Bot. Surv. Nebr. N. S. 1. 1917. 



* Gates, F. C. The Relation between Evaporation and Plant Succession in a 

 Given Area. Amer. Jour. Bot, 4: 161. 1917. 





