EVAPORATION IO7 



exert in determining what species will occupy different 

 habitats. The first paper emphasized the physiological effect 

 of an atmosphere of excessively great evaporating power. 

 and pointed out that certain plant forms are definitely in- 

 fluenced by this climatic factor, independently of the second- 

 ary effect which this factor undoubtedly exerts through 

 variations in the soil moisture conditions. The second paper 

 dealt with evaporation as an ecological or edaphic factor, 

 and pointed out the great variations in the evaporating power 

 of the air in different habitats of the same region. In the 

 present paper the geographical feature of evaporation will 

 be considered. 



The secondary effect of evaporation, in determining, in 

 a large measure, the moisture content of the soil, is perhaps 

 usually a more important factor in plant distribution than 

 is the primary effect upon the plant itself. In any event, 

 the optimum condition for growth, as far as moisture con- 

 ditions are concerned, is to be considered as a certain balanced 

 relation between the supply of water which the soil can 

 furnish and the evaporating power of the air. Furthermore, 

 climatic evaporation is influenced by temperature, humidity, 

 and wind velocity, all of which are known to be very efficient 

 in the control of plant activities. The evaporation, therefore, 

 would be expected to play an important role in the determ- 

 ination of the great regions of plant distribution, as well as 

 in the distribution of societies within the same region. 



The importance of considering the rainfall in studies 

 in plant geography has long been recognized, but the im- 

 portance of evaporation in such studies has met with but 

 very tardy appreciation, a fact partly due, no doubt, to the 

 lack of satisfactory evaporation records. Thus far Transeau 

 has furnished the only real evidence at hand in regard to 

 the relation of geographic distribution to evaporation. In 

 his paper on this subject- attention is called to the fact that 



2. Transeau. R. N., Forests of Rastern Amerira. .\mer. Nat. S9; S?.")- 

 S98. 1905. 



