THE RESISTANCE OFFERED BY LEAVES TO TRAN- 

 SPIRATIONAL WATER LOSS^ 



BURTON EDWARD LIVINGSTON 



The Johns Hopkins University, Baltimore, Md. 



INTRODUCTION 



The rate of water loss from plant surfaces exposed to the air 

 is always lower than the rate of e^'aporation from an equal area 

 of water surface in the same surroundings. Usually the evapora- 

 tion rate is very many times as great as is that of transpiration, ^ 

 and it is thus possible to regard plant tissues as exerting a retard- 

 ing influence upon evaporation and outward diffusion of their 

 contained water. This retarding influence, or resistance to tran- 

 spirational water loss, is of different magnitude for different plant 

 forms, and for the same form grown under different conditions, 

 also for different portions of the transpiring surface of the same 

 individual plant. Furthermore, for the same plant, and for the 

 same portion of its surface, the resistance to transpiration often 

 varies with the age of the organism and with the diurnal march of 

 various internal conditions. Obviously, the factors which con- 

 dition the retarding influence here considered are all to be charac- 

 terized as internal, that is, they are all operative within the 

 periphery of the plant body, though they must of course be ulti- 

 mately controlled by external conditions, either in the present 

 or in the past. In the broad sense, these factors are anatomical; 

 in physical terms, they depend on the nature, amount and arrange- 

 ment ^^^thin the plant body, of its various component substances. 



1 Botanical Contribution from the Johns Hopkins University No. 28. A pre- 

 liminarj- announcement of the direct method here used was published in Notes 

 from the Botanical Laboratories, The Johns Hopkins University Circular, pp. 11- 

 13, February, 1912. 



2 Livingston, B. E., The relation of desert plants to soil moisture and to evapora- 

 tion. Publication 50 of the Carnegie Institution, 1906. 



1 



THE PLANT WORLD, VOL. 16, XO. 1, JANUARY, 1913 



