EESEAECH METHODS IN STUDY OF FOEEST ENVTBONMENT. 153 



mathematically the effects of the several components on the evapora- 

 tion from the plant. One is just as far from an absolutely precise 

 outcome as the other. The advantage of the atmometer is that, once 

 the integration has been accomplished in the construction of the in- 

 strument, the observations are relatively simple and no further com- 

 plex calculations are necessary. 



As has been suggested, it is not to be presumed that the atmometer 

 will show restricted evaporation due to control, as may be the case 

 with the plant when it closes its stomata or when transpiration is 

 automatically reduced by increasing density of the cell sap. By 

 comparing the most perfect atmometer with plants, however, it 

 should be possible to measure the actual effectiveness of these plant 

 controls. 



It may be worth while to suggest, for the sake of more effective 

 evaporation studies, that it is possibly erroneous for the student 

 of plant life to look upon a large evaporation factor in the habitat 

 as necessarily inimical to the plants which are there. Some theo- 

 retical considerations which point to transpiration as a benefit have 

 already been outlined. Xo attempt will be made, however, to decide 

 the question as to whether it is beneficial to the plant or merely a 

 necessary evil. Laying this question aside, it is perfectly evident 

 that conditions conducive to high transpiration rates are an unavoid- 

 able concomitant of the conditions necessary to active photo-synthesis. 



When therefore, as in Weaver's (169) succession from prairie to 

 brush or woodland types, it is found that succession produces a stead- 

 ily decreasing evaporation rate, shall it be concluded that the plants 

 of the brush stage are directly favored by the decreased evapora- 

 tion, only relatively favored, or not helped at all, but merely able to 

 succeed with less sunlight than the plants of the prairie? Very 

 likely, in a case of this kind, the rate of evaporation, while a service- 

 able index to the general conditions, may not itself be a controlling 

 factor, or may be a controlling factor only for a brief period in each 

 -on when drought occurs. It would appear to be all-important 

 tluit evaporation rates be closely correlated with the moisture of the 

 -oil. as is done by Slireve (166) in giving directly, if somewhat 

 crudely, tin- ratio of evaporation to soil moisture contents in various 

 habitats. What is perhaps more Important is that a clear distinc- 

 tion should he made between evaporation stresses when there is an 



abundance of -oil moisture, and those existing when the moisture 

 supply i nearly exhausted. There are, also, critical periods brought 

 about by excessive evaporation when the soil moisture is apparently 

 ;ill thai it should he. These ;md their effects must he separately 

 analyzed. 



