EFFECT OF BORDEAUX MIXTURE ON TRANSPIRATION 73 



were then placed in small desiccators to retain them in the dry 

 condition until used. The desiccators consisted of glass \dals 

 about 7.5 cm. long and about 2 cm. in diameter, fitted with 

 paraffined cork stoppers. The desiccators were filled to about' 

 two-thirds of their capacity with anhydrous calcium chloride, 

 which was covered with a thin layer of absorbent cotton held 

 in place by a thin, perforated cork disc. 



The composite hygrometric paper shps used for these tests 

 were standardized once for all in the laboratory by the method 

 employed by Trelease and Livingston, ^^ and presented in detail 

 by Livingston and Shreve. This method is based upon the 

 assumption that the time required for the color change of the 

 hygrometric paper over the standard evaporating surface is 

 inversely proportional to the maximum vapor pressure of water, 

 corresponding to the given temperature. Accordingly, if the 

 time T required for the color change of a slip of hygrometric 

 paper over the standard water surface at the known tempera- 

 ture t, is determined experimentally, it should be possible to 

 calculate approximately the time for the same slip for any tem- 

 perature t' , the calculation being made by means of the fol- 

 lowing formula: 



^ _ ^ or r. - ?^' 



in which P, represents the maximum vapor pressure of water 

 for the temperature at which the actual laboratory test was made 

 with the standard evaporating surface, and P/ represents the 

 vapor pressure corresponding to the temperature t'. The 

 values of Pt and P/ are obtained from pubhshed tables of 

 aqueous vapor pressures. In making the calulations, it is under- 

 stood, of course, as has been pointed out by Bakke,-^ that the only 

 condition external to the hygrometric paper itself which can 



1' Trelease, S. F., and Livingston, B. E., The daily march of transpiring power 

 as indicated by the porometer and by standardized hygrometric paper. Jour. 

 Ecol.4: 1-14. 1916. 



" Bakke, A. L., Studies on the transpiring power of plants as indicated by 

 the method of standardized hygrometric paper. Jour. Ecol. 2: 145-173 1914. 



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