METHOD FOR DETERMINING TRANSPIRING POWER 299 



over the standard evaporating surface, is temperature. With 

 the same paper slip and the same evaporating surface, a higher 

 temperature should give a shorter time of response than does 

 a lower temperatm-e. ''On theoretical grounds it is to be ex- 

 pected that the relation here brought out is simply the well- 

 known relation which holds between temperature and the vapor- 

 pressure of water."'" Bakke has presented experimental evidence 

 which appears to show that the relation here dealt with is ap- 

 proximately Unear for the temperature range from 25° to 35°C. 

 and that the graph of this relation becomes concave upward 

 wdth higher temperatures. The same writer suggests that, if 

 this temperature relation can be satisfactorily established, the 

 standard evaporating surface may be dispensed with in field 

 work, each hygrometric slip having been tested once for all, 

 in the laboratory, at a known temperature. From the observed 

 time of response at the known laboratory temperature and from 

 the field temperatm-e when leaf tests are made, it should thus 

 be possible to calculate the time of response for the field tem- 

 perature, as though the paper had there been tested over the 

 same standard surface as was used in the laboratory. 



It appears probable^^ that the temperature relation here in 

 question may be expressed by the statement that the time 

 of color response is inversely proportional to the maximum 

 vapor pressure of water corresponding to the temperature. This 

 pressure varies with temperature in a known way (empirically 

 determined), and its value may be found from pubUshed tables, 

 for any given temperature. This supposition may be familiarly 

 stated as follows: 



p R „„ PR 



— = — , or pr = FR, or r = , 



P r p 



wherein P is the maximum vapor-pressure corresponding to the 

 temperature at which the actual test was carried out ^\\ih. the 



10 Bakke, 1914, p. 150, I.e. 



" See Trelease, S. F., and Livingston, B. E., The clailj' march of transpiring 

 power as indicated by the porometer and by standardized hj-grometric paper. 

 Journ. Ecol. 4: 1, 1916. 



