54 



THE RELATION OF DESERT PLANTS TO 



An inspection of these data makes it appear that here, as in Euphor- 

 bia, the hours of the day at which the critical points occur seem to show 

 no uniformity, and therefore the time factor can not be considered as 

 controlling the regulative response. The early hour of the maxima 

 seems again to preclude light intensity. Also, the evaporation rates 

 for the two minima and also for the two maxima are very far from being 

 alike, while the corresponding temperatures are almost identical in each 

 case. Thus it appears that air temperature is the most probable con- 

 trolling condition governing the regulative response. It is to be noted 

 that the temperature for the maximum in relative transpiration, i. e., 

 the temperature at which the physiological checking of transpiration 

 begins to be manifest, is 10 degrees higher than that which corresponds 

 to the minimum. The latter, is, of course, the temperature at which 

 the check is removed and transpiration begins to increase again. 



TABLE XL Relation of Transpiration to Temperature and Evaporation Rate- 

 Experiment IV. 



The leaves of this plant were observed to close between 4 h 30"' a.m. 

 and 6 p. m. and to open between 3 and 4 a. m. , thus making it 

 apparent that, while the leaf movement undoubtedly has considerable 

 effect in the regulation under consideration, this movement is not 

 the controlling means by which the regulation is accomplished. Again, 

 it appears that the stomata may be the organs mainly effective in this 

 regard or that some internal adjustment is operative. 



Experiment V. The subject of this experiment was another plant 

 of Tribuliis, similar to the one used in the last experiment and standing 

 in the same place. The plant was sealed at 2"50 m p.m., August 13, and 

 the record was continued till 8"30 m a.m., August 15. Wilting occurred 

 six hours after the end of the record. The total leaf area was 121.34 

 sq. cm. The average hourly rate of transpiration during the last 24 

 hours, from 8 h 30 m a.m., August 13, to the same hour August 14, was, 

 for the whole plant, 0.3346 gram, or 0.0028 gram per square centimeter 

 of leaf surface. 



The curve of the rates of relative transpiration, together with those 

 of temperatures and of evaporation rates, are presented in the same 



