58 



THE RELATION OF DESERT PLANTS TO 



The period of this experiment was so short that it is impossible to 

 draw any general conclusions therefrom concerning the causal factors 

 which govern the relative transpiration rate. It may be noted simply 

 that the temperature at which the maximum relative transpiration 

 occurred is several degrees higher than that at which the minimum 

 occurred. This plant has no nyctitropic movement and the well-marked 

 regulation of transpiration which is unequivocally shown in the curves 

 is probably due to the stomatal or some internal mechanism. 



Experiment VII. Three Boerhavia seedlings were used in this case. 

 They were about 10 cm. high, in bloom at the beginning of the record. 

 The experiment extended from 9' 1 30 m a.m., August 13, to 3 h 30 m p.m., 

 August 15, when wilting occurred. The total leaf area of this culture 

 was not determined. The hourly rate of transpiration for the last 24 

 hours was 0.14 gram for the entire plant. 



TABLE XII. Relation of Transpiration to Temperature and Evaporation Rate 



Experiment VII. 



In default of the leaf area a curve of the ratios of the hourly rates 

 of water loss from the whole plant to the hourly rates of evaporation 

 from the whole evaporimeter surface is given in figure 12 (p. 57) . This 

 is accompanied by the usual curves of evaporation rates and of tem- 

 peratures. The ratio curve here given shows, of course, the same varia- 

 tions as would the curve of rates of relative transpiration. The latter 

 curve would be obtained from the ratios of the given curve by dividing 

 each of those ratios by the leaf area and multiplying the quotient by 100, 

 the standard water surface represented by the evaporimeter. 



It is apparent from the curve that the rate of relative transpiration 

 decreased on the whole throughout the period of the experiment. 

 Three maxima and two minima are clearly shown upon the curve of 

 relative transpiration. The hours, temperatures, and evaporation rates 

 for each of these are shown in Table XII. 



From the data for the two minima it appears that air temperature 

 is probably the external condition which causes the regulative mechan- 



