SOIL MOISTURE AND TO EVAPORATION. 



59 



ism to act. But from the data for the maxima it appears that evapora- 

 tion rate is more uniform than temperature and therefore that this is 

 probably the controlling condition for the regulating response. Thus 

 the evidence here is conflicting, much as in the case of Experiment I. 

 Light intensity is again seen to be assuredly not the controlling con- 

 dition. 



This plant has no definite nyctitropic movement, so that here, as in the 



case of AUionia, the variations 

 in relative transpiration are not 

 due to such movement. In the 

 middle portion of the period of 

 this experiment relative trans- 

 piration varied from a maximum 

 of 0.121 to a minimum of 0.024. 

 The regulative activity is thus 

 shown to be able to reduce relative 

 transpiration from unity to about 

 one-sixth. It is thus only about 

 one-half as effective in these 

 plants of Boerhavia as it was in 

 the two specimens of Tribulus and 

 in the AUionia. 



Experiment VIII. The plant 

 was a Boerhavia standing in 

 bright sunshine during the day. 

 The experiment lasted from 10 h 30 m 

 a. m., August 16, to 3 h 30 m p. m., 

 August 17, when wilting occurred. 

 The total leaf area was 47.5 sq. 

 cm. For the last 24 hours before 

 wilting the entire plant transpired 

 at the average hourly rate of 

 0.2877 gram, or 0.0061 gram per 

 square centimeter of leaf surface. 

 The usual curves are given in 

 figure 13. That of relative trans- 

 piration is seen to be very similar 

 to the same curve for the AUionia of Experiment VI. The first point is 

 probably a maximum, so that here we have to consider two maxima and 

 one minimum. The first maximum is 0.785 and occurs at Il h 30 m a.m. 

 with a temperature of 79.5 F. and an evaporation rate of 0.046 gram; 

 the second is 0.609 and occurs at 10 a.m., with the same temperature as 



FIG. 13. Curve of relative transpiration for 

 plant of Boerhavia, August 16-17, 1904. 



