SOIL MOISTURE AND TO EVAPORATION. 



53 



of the periodic rise and fall already described for Euphorbia, was, on 

 the whole, gradually decreasing. This is undoubtedly due to the grad- 

 ually diminishing supply of water in the soil. Wilting did not occur 

 during the experiment, but probably would have been evident had the 

 record been continued an additional day. The average hourly rate of 

 transpiration for the last 24 hours of the record, from 8 h 30 m a.m., 

 August 14, to the same hour on August 15, was 0.0723 gram for the 

 whole plant and 0.0018 gram per square centimeter of leaf surface. 



FIG. . Curve of relative transpiration for a plant of Tribulus brae hystylis, 

 August 13-15, 1904. 



As is noted above, this plant exhibits a physiological regulation of 

 the rate of transpiration which is very similar to that of Euphorbia. 

 Two maxima and two minima of relative transpiration are shown 

 within the period of the experiment. The comparative data for these 

 points on the curve are presented in Table XI, which is arranged in the 

 same manner as Table X. 



