60 



THE RELATION OF DESERT PLANTS TO 



the other and an evaporation rate of 0.35 gram. The minimum is 0.054 

 and occurs at 8 p.m., with a temperature of 75 F. and an evaporation 

 rate of 0.029 gram. 



It appears here again that temperature rather than intensity of evap- 

 oration is possibly the controlling factor in the regulation of relative 

 transpiration, and that the light intensity is not important. The tem- 

 peratures for the maxima are again about 10 degrees higher than those 

 for the minima. The variation in rate of relative transpiration due to 



.OSJ 



90.S/ 



89/ 



/ 



86. S 



.048 



036, 

 \J 

 .032 



.130 



.065 



i:oop.m. 



93 93 



-\ 

 \ 

 \ 

 \ 



88" 



.063 



.05. 

 .044 



043 



.OI4- 



7. -30 p.m. 



80 



V--"76 



6:30 p.m. 



,084 



.069 



(0/3 



6:30 a.m. 



.079 



.043 



84 

 \ 



\ 



030 



009 



9:oop.m. 



79 



.Of 6 



.015 



6:30 p. 



7.5 



OZI 



AuftS 



FIG. 14. Curve of relative transpiration for a plant of Boerhavia, 



August 13-15, 1904. 



physiological action amounts here to the difference between 0.054, the 

 minimum rate, and 0.609, the maximum. Thus the regulative mech- 

 anism is able to reduce relative transpiration from unity to about one- 

 twelfth. 



Experiment IX. This experiment was carried on with another 

 mature Boerhavia plant, sealed at 9 h 30 m a.m., August 13. The record 

 was continued until 8 h 30 m a.m., August 15, the plant standing in the 

 shade on the north side of the building. The leaf area was not 



