180 



EDITH BELLAMY SHREVE 



in table 6. Numbers have been selected which will show the 

 greatest possible variation for plants and then used with the slip 

 no. 25, the one which showed the greatest difference in its 

 standards. Thus if the color change took place in slip no. 25 

 in a given plant in 16 seconds at 35°C, then the index of tran- 

 spiring power would be 0.86 with 32.8 as standard and 0.83 

 with 31.8 as standard. 



By reference to the maxima and minima for transpiring power 

 given in table 3, it is seen that the error obtained by using the 

 air temperature instead of the temperature of the slip in the 

 standardization over a porous surface is entirely negligible in 

 the light of the differences which are obtained on the plants 

 themselves. 



TABLE 6 



Effect upon the index of transpiring power of using two different standards for the 

 cobalt slip, the differences in the standards having been obtained by the use of (1) 

 air temperature, (2) temperature of the slips 



SUMMARY 



1. In the determination of the index of transpiring power by 

 tripartite cobalt slips no error requiring consideration is intro- 

 duced into the calculations by using the temperature of the air 

 immediately surrounding the leaf instead of the temperature of 

 the slip itself. 



2. In the standardization of the tripartite cobalt slips over a 

 porous evaporating surface in a small closed room, the tem- 

 perature of the air near the apparatus may be used instead of 

 the temperature of the slip itself. 



