2 



BURTON EDWARD LIVINGSTON 



Thus, for example, the resistance to water loss by transpiration is 

 much greater in leaves having a well developed cuticle than in 

 others in which the cuticular layer is thin or absent. Also, other 

 internal conditions being equal, a leaf with closed stomatal pores 

 must exhibit a greater retarding influence upon water loss than 

 one with open pores. Many other examples might be mentioned. 



It is clear that an adequate appreciation of the dynamics of the 

 water relations of plants, and especially of the transpiration 

 process, must rest not only upon a quantitative knowledge of the 

 influence of external conditions on the transpiration rate, but 

 also, upon just as quantitative and thoroughly physical a knowl- 

 edge of the causal relations obtaining between internal factors 

 and this rate of water loss. We require measurements of the 

 water-extracting power of the aerial en\dronment and correspond- 

 ing measurements of the transpirational retardation which is 

 effective within the plant. 



By the use of the various atmometers which are now avail- 

 able,' it is possible somewhat readily to measure and integrate 

 the environmental water-extracting power for any locality and for 

 any time period, providing freezing of water does not occur.* 

 This instrument gives at once a generahzed estimate of the 

 environment above the soil surface, as far as this influences water 

 removal. We are thus fairly well equipped to attack problems 

 dealing with differences in the external conditions. 



Our status is, however, not nearly so satisfactory when we face 

 problems of differences in the internal factors. In attempting 

 to deal with variations and differences in transpirational retarda- 

 tion, as this is manifested in different plant forms and in the same 

 form at different times, advantage has been taken of a mathe- 

 matical treatment of the problem, which is common in physical 

 analyses. This relies on the principle that, wherever two, and 



' Livingston, B. E., A rotating table for standardizing porous cup atmometers. 

 Plant World 15: 157-162, 1912; and the literature there referred to. Also see: 

 Livingston, Grace J., An annotated bibliography of evaporation. Monthly 

 Weather Review, 1908-1909. 



For the most part, the problems of plant transpiration do not require the 

 measurement of the evaporating power of the surroundings during freezing 

 periods; nevertheless an adequate method should be devised for such times. 



