RESISTANCE BY LEAVES TO WATER LOSS 



3 



only two, conditions are known to influence the time rate of a 

 given process, this rate must be proportional to the product of the 

 effective intensities of the two conditions. If, then, it be possible 

 to determine experimentally one of the effective intensities, as 

 well as the rate of the resulting process, it is not necessary to 

 measure the other, which may be derived by simple division. 



In the study of transpiration, the method of the relative trans- 

 piration ratio^ is founded upon the principle just stated, the 

 primary assumption being that the absolute transpiration rate 

 is proportional to the product of the intensity of the evaporating 

 power of the surroundings and the magnitude of the transpiring 

 power of the plant. This transpiring power is of course the 

 reciprocal of the resistance manifested b}'^ the plant, to water loss 

 by transpiration. It is assumed, to be more specific, that, if a 

 plant with a superficial area of p loses m grams of water during a 

 time period when an atmometer with similar exposure loses r 

 grams, and if another plant with area q loses n grams while the 

 same, or a similar, atmometer loses s grams, then the internal 

 conditions favoring water loss from the first plant are to those 

 m n 



of the second as — is to — . The ratio of the loss from the plant, 

 pr qs ' 



per unit area, to the loss from the standard atmometer, under 



the same conditions, is proportional to the transpiring power 



of the plant. This ratio really states the number of atmometers 



that would be necessarj^ to evaporate as much water as unit area 



of the plant, the instrument and plant being similarly exposed for 



the same time period. 



Where the problem in hand contemplates only variations in 



the transpiring power, or its reciprocal the resistance factor, for 



the same plant, and at different hours of the same day, it is unnec- 



essar\^ to introduce into the computation the area of the plant, 



this area being assumed to be sensibly constant. Thus, if the 



' Publication .50 of the Carnegie Institution, 1906; also see: Livingston, B. E., 

 Relative transpiration in cacti. Plant World, 10: 110-114, 1907. 



Delf, E. M., Transpiration and behavior of stomata in halophytes. Ann. Bot. 

 25 : 485-.505, 1911. 



/dew. Transpiration in succulent plants. Ibid. 26: -109-441, 1912. 



