DETERMINATION OF TRANSPIRING POWER 



177 



the slips to make their color change at 20°C. when calculated from 

 temperatures obtained from the thermal couple touching the 

 slip, and from a mercury thermometer hung near the apparatus. 

 Temperatures were thus read for slips from 18°C. to 27°C. 

 at one degree intervals, the readings being made every ten 

 seconds. In table 4 appear representative examples of these 

 readings. In order to discover the effect which the differences 

 of temperature shown in the tables will have upon the calcula- 



TABLE 2 



Departure from air temperature shown by cobalt slips when resting on plant leaves 

 and covered by a glass clip. Plants growing in open in situ. 



tions for determining the transpiring power, it is necessary, 

 first to find the effect they will have upon the standardization 

 of the paper over the porous evaporating surface, and second 

 the influence the differences obtained in standards will have 

 upon the index of transpiring power. 



The influence upon the standardization of the paper is shown 

 in table 5. In the case of slip no. 12, the color change was 

 completed in thirty seconds, the average temperature during the 

 interval being 21.1°C. and of the slip 20.7°C. By the use of the 



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