EVAPORATION AND PLANT HABITATS. 9 



the similar evaporation rate obtained from that station is 

 not to be taken as denoting the same sort of conditions for 

 plant activity. The length of the Raleigh season is similar 

 to that of our second station, but its average evaporation rate 

 is much lower, and its vegetation, of course, quite dijfferent. 



The data above presented show clearly that the evapor- 

 ating power of the air furnishes a very valuable criterion for 

 the comparison of the conditions of different plant habitats, 

 and, taken with the physiological observations brought out 

 in the paper on Evaporation and Development, they must be 

 considered as at least exceedingly strong evidence that this 

 power of the air is a controlling factor in the determination 

 of the plant population of any habitat. In such studies as 

 these evaporation is more valuable than temperature, hu- 

 midity, or wind velocity, for the reason that it includes the 

 effects of all these greatly varying factors, as far as they in- 

 fluence water loss from plants, and it is very diflicult if not 

 impossible at present to combine data of these elements so as 

 to exhibit their combined influences. The atmometer here 

 used is exceedingly simple, inexpensive, and easily operated, 

 which can hardly be said of recording instruments for tem- 

 perature, humidity, and wind velocity. For studies of this 

 sort recording instruments are quite essential, and this atmo- 

 meter automatically records the effect of the conditions with- 

 out any delicate and complex mechanical device. Also, since 

 the effect of external conditions upon organisms is a function 

 of duration as well as of intensity, i. e., of rate, it is neces- 

 sary to integrate or sum the varying partial effects of these 

 conditions as time elapses. While the anemograph both re- 

 cords and integrates, the integration of curves of humidity 

 and temperature is a difficult and only partially satisfactory 

 process, and one not performed by the instruments. But the 

 integration of the evaporation data recorded by the atmo- 

 meter is a part of the very working of the instrument itself. 

 Finally, evaporation acts directly upon the plant, while the 

 three elements upon which it depends act only indirectly. 



Desert Laboratory, 



Tucson, Arizona. 



