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 



30.S/ 

 89' 



86.5 



.048 



.032 



i.-oop.m. 



93 93 



.063 



.05, 

 '.O44 



043 



7:30 p.m. 



80 



.0/2 



6:30 p. 



.084 



.069 



6 :30 a.m. 



,039 

 .023 



.04 a 



84 



\ 



057 



^030 



O09 



9:00 p.m. 



.OS 6 



.OJS 



6:30p.m 



Aug. 15 



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

 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 



