4 



BURTON EDWARD LIVINGSTON 



transpirational loss from a plant be m for one time period and n 

 for another, and if the loss from the atmometer, under the same 

 smToundings and for the same time periods, be r and s, respec- 

 tively, then the intensity of transpirational retardation for the 



T S 



first period is to that of the second as — is to - or the resistance to 



m n 



ms 



water loss is — times as effective during the second period as 

 nr 



during the first. In the study of the dailj^ march of the prop- 

 erty which we are considering, it is convenient to adopt as unity 

 the value corresponding to one of the short periods of observation, 

 and to reduce the other values to this basis by division. 



Much as the method of the relative transpiration ratio has 

 aided in the advance of our knowledge of the physics of transpi- 

 ration and of the conditions limiting plant life, it is at best but 

 a very indirect and cumbersome means for comparing transpira- 

 tional retardations. It requires the absolute transpiration rates 

 and the corresponding atmometric data, the weighing of potted 

 « plants or the reading of potometers, together with the operating of 

 atmometers. It is thus not well suited to the estimation of physio- 

 logical resistance to transpiration in the field. A more direct 

 method has therefore been attempted. A description of this and 

 a statement of some of the results thereby obtained, form the 

 purpose of the present paper. 



The work here reported was begun in the summer of 1908, in the 

 laboratories of the Pflanzenphysiologisches Institut in Munich, 

 where facilities for experimentation were kindly placed at the 

 disposal of the writer by the Director, Prof. K. von Gobel. Stud- 

 ies along this line have been prosecuted at intervals since that 

 time, but the most satisfactory advances were made in the sum- 

 mer of 1911, at the Desert Laboratory of the Carnegie Institu- 

 tion. Here it was the author's pleasure to have the valuable 

 assistance of Mr. J. S. Caldwell, in this and related lines of study. 

 In the summer of 1912, at the same Laboratory, comparisons of 

 the transpiring power of different plants were carried further, 

 with the assistance of ]\Ir. E. M. Harvey. 



