SOIL MOISTURE AND TO EVAPORATION. 



51 



TABLE IX. Relation of Transpiration to Temperature and Evaporation Rate 



Experiment L 



From these data it appears that the hours at which sudden changes 

 occurred in the general direction of the transpiration curve are not 

 nearly as uniform as are the simultaneous evaporation rates and tem- 

 peratures. This seems to indicate that either temperature or intensity 

 of evaporation is to be considered as probably the controlling factor in 

 the regulation of transpiration in this plant. From the fact that the 

 hours of the above table fail to show uniformity, it appears that this 

 regulation is not to be related to changes in light intensity nor to any 

 form of chronometric rhythm which the plant might be supposed to 

 possess. Since intensity of evaporation follows temperature rather 

 closely, it is impossible to distinguish between these two factors by the 

 data at hand. Whatever may be its cause, the observed regulation 

 is seen to cause a variation in relative transpiration from a maximum 

 of about 0.068 to a minimum of about 0.008, or from unity to about 

 one-ninth. 



The foregoing method promises to be of very great value in studies 

 of the rate of transpiration and of the factors which cause this rate to 

 vary. Indeed, it is the only method so far devised which can give 

 direct evidence in regard to the physiological regulation of transpiration 

 rate. The time necessarily devoted to other lines of research limited 

 the taking of data regarding the relation of temperature and intensity 

 of evaporation to transpiration, so that those here given are necessarily 

 only of a preliminary nature. In the description of the following 

 experiments this matter will be reverted to whenever the data are 

 sufficient to warrant it. 



Experiments II and III. These were brief experiments, carried on 

 in duplicate from 7 h 30 m p.m., September 4, to l''30 m p.m., September 5. 

 Each culture consisted of two seedlings of Fouquieria splendens. They 



