METHOD FOR DETERMINING TRANSPIRING POWER 301 



It should also be emphasized that, in this connection, the 

 whole method of standardized hygrometric paper slips, as so 

 far used, is based on the supposition that the papers bdng upon 

 plant leaves are under the influence of the temperature of the 

 air about the plant. As Trelease and Livingston' ^ have pointed 

 out, this is probably never quite the case, and as soon as leaf 

 temperatures can -be measured conveniently it will be necessary 

 to employ these readings (as t') instead of those taken on a 

 thermometer simply exposed beside the plant or among its 

 leaves. Even such a procedure as this may eventually require 

 correction, for the temperature influence exerted upon a slip 

 lying upon a leaf and covered by a glass plate must be determined 

 by the temperatures of leaf and plate together. For the pres- 

 ent it seems sufficient to consider that the surrounding air tem- 

 perature is a measure of the temperature influence exerted upon 

 the paper slips when lying upon plant leaves. 



During the summer of 1914, Miss Aleita Hopping, working 

 at the Laboratory'" of Plant Physiology of the Johns Hopkins 

 University, made a large number of determinations of the re- 

 sponse times for a series of selected composite slips, at various 

 air-temperatures, from 19.4° to 41.7°C., the same form of evapo- 

 rating surface being used in all cases. Although the methods 

 employed were not as satisfactoiy as those set forth in the earlier 

 sections of the present paper, and although the paper employed 

 at that time was much thinner than that now used (thus having 

 a much shorter time coefficient), nevertheless the results ob- 

 tained definitely support the suppositions just presented and 

 appear worthy of expression here. 



Thirty tests were made at a temperature of 24.8° C. and the 

 average time of response was calculated, all of the slips in the 

 series giving about the same value. Tests were then made at 

 various different temperatures, as the air of the room varied 

 from time to time, until a considerable number of observations 

 had been obtained for each temperature dealt with. The re- 

 sponse times for each temperature were then averaged and this 



'- Trelease and Livingston, 1916, I.e. 



