EVAPORATION AND PLANT HABITATS. "J 



had been no alteration in their relative evaporating powers, 

 another instance illustrating the lasting qualities of the 

 instrument. 



It was planned at the outset of the season to obtain 

 records from four different altitudes in the vicinity of Tucson, 

 Arizona. The atmometers were placed at the University of 

 Arizona, altitude 2412 feet, and at three stations in the Santa 

 Catalina Mountains. The lowest of these stood in an open- 

 ing in an oak chaparral at the extreme lower edge of bull pine, 

 at an altitude of about 6000 feet. The instrument was not 

 shaded from the sun but was shielded from wind action to 

 a considerable extent. Another atmometer stood in a bare 

 gravelly area surrounded by a very open growth of large bull 

 pine trees and small oaks with here and there a thicket of 

 scrub oak, at an altitude of 7500 feet. The third instrument 

 for the mountains was placed in a broad valley surrounded by 

 high hills, in the midst of a dense growth of bull and sugar 

 pine and of Douglas spruce, at an altitude of about 8000 

 feet. The opening in which the latter instrument stood was 

 only some fifteen meters in diameter and the shade of the 

 neighboring trees and of the surrounding hills cut off the 

 direct rays of the sun for some time at the beginning and end 

 of the day. The Tucson instrument was exposed in the 

 midst of a bare area on the campus of the University of 

 Arizona, with no shade and hardly any vegetation within 

 many meters. An attempt was made to place all of the in- 

 struments so that they would be subjected to the same condi- 

 tions as would have to be borne by the average seedling 

 developing in the respective localities. 



The mountain atmometers were placed on May 12 and 

 16 and were read May 31 and June i. Other readings were 

 taken at later dates, but the porous cups became clogged 

 through the use of water which proved to be impure, and 

 the later records are not trustworthy. The Tucson instru- 

 ment was installed May 27, and was read weekly thereafter, 

 through the kindness of Dr. W. B. McCallum. The first 



