SOIL MOISTURE AND TO EVAPORATION. 



61 



determined, but the ratio curve derived from the rates of transpiration 

 for the entire plant is given in figure 14, together with the curve of 

 temperatures and that of evaporation rates for the period. The aver- 

 age hourly rate of actual water loss from the plant for the 24 hours 

 from 8 h 30 m a. m. , August 14, to the same hour August 15, was 0. 1340 gram. 

 Wilting did not occur within the time of the experiment. 



The curve of relative transpiration shows two maxima and two 

 minima. The hours, temperatures, and evaporation rates for these are 

 shown in Table XIII. 



Apparently here relative transpiration is again governed by tem- 

 perature, and the turning points in its curve are at about 90 and 80 F., 

 the higher temperature once more corresponding to the maximum and 

 the lower to the minimum. The response is effective in reducing rela- 

 tive transpiration from 0.084 to 0.009, or from unity to about one-ninth. 



TABLE XIII. Relation of Transpiration to Temperature and Evaporation Rate 



Experiment IX. 



Experiment X. The subject was another flowering plant of Boer- 

 havia, standing in the shade. The pot was sealed at 12 h 30 m p.m., 

 August 16, and the record was continued until 7 a.m., August 18. No 

 wilting occurred. The leaf area was not determined. The average 

 transpiration rate per hour for the entire plant from 9 h 30' u a.m., August 

 16, to 9 h 30 m a.m., August 17, was 0.1729 gram. Curves for this experi- 

 ment are given in figure 15 (p. 62) , following the plan of Experiments 

 VII and IX. 



The curve of relative transpiration includes a single minimum of 

 0.015 at 8 p.m., with a temperature of 75 F. and an evaporation rate 

 of 0.029 gram; and a single maximum of 0.118 at l"30 m p.m., with tem- 

 perature 84 F. and evaporation rate of 0.041 gram. 



The effect of the regulative response amounts in this case to a differ- 

 ence between a relative transpiration rate of 0.015 at the minimum 

 point and about 0.121 at the maximum. Relative transpiration is thus 

 reduced from unity to about one-eighth. 



