DETERMINATION OF TRANSPIRING POWER 173 



It should also be emphasized that .... the whole method of 

 standardized hygrometric paper slips .... is based on the 

 supposition that the papers lying upon the plant leaves are under the 

 influence of the temperature of the air about the plant .... as 

 soon as leaf temperature can be measured conveniently it will be neces- 

 sary to employ these readings instead of those taken on a thermometer 



simply exposed beside the plant 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. 



The role of leaf temperature in the determination of the trans- 

 spiring power of leaves has two aspects, one being the part played 

 by the temperature of the slip in the calculations which must be 

 performed to determine the index of transpiring power, and the 

 other being the effect produced on the transpiring power of the 

 leaf by any change in the leaf temperature w 7 hich may be caused 

 by the application of the clip. This paper is concerned only with 

 the first one of these aspects. 



The work 4 of A. L. Bakke, Aleita Hopping and of S. F. Tre- 

 lease and B. E. Livingston has shown that the times of color 

 responses of the cobalt paper are inversely proportional to the 

 maximum vapor pressures of w r ater corresponding to their re- 

 spective temperatures. Consequently it is very important that 

 the temperature of the cobalt slip be known when the test is taken. 



A convenient method for the determination of leaf tempera- 

 tures 5 has now been worked out and a slight modification of it 

 has made it possible to measure the temperature of the slip as it 

 lies on the leaf surface covered by slip and glass plate. 



Figure 1 represents the glass clip, 6 which is used to hold the 

 tripartite slips on the leaf, carrying a small thermo-couple (a, b) 

 on the inside of one plate. The wires are fastened to the plate 

 by means of a piece of gummed cloth (c) pasted to the outside. 



4 For references see Livingston and Shreve, loc. cit. p. 298-9. 



5 Shreve, E. B. A thermo-electrical method for the determination of leaf 

 temperatures. Plant World 22 : 100-104, 1919. 



6 For the original description of the clip see Livingson, B. E. The resistance 

 offered by leaves to transpirational water loss. Plant World 16: 8. 1913. 



