RELATION OF THE DAILY MARCH OF TRANSPIRATION 
TO VARIATIONS IN THE WATER CONTENT 
OF FOLIAGE LEAVES 
BurTON EpWaRpD LIVINGSTON AND WILLIAM HENRY Brown’ 
Introduction 
As was first pointed out in Publication 50 of the Carnegie Insti- 
tution (1906), it is only by correcting the variations in the tran- 
spiration rate to uniform conditions of evaporation (that is, to a 
uniform evaporating power of the air), that anything approaching 
quantitative information concerning the seemingly almost autono- 
mous changes in the rate of water loss from plants may be had. 
To accomplish this correction it is only necessary to consider the 
march of the ratio of the rate of transpiration to that of evapora- 
tion, the latter determined by means of some form of atmometer. 
This ratio has been termed relative transpiration; it denotes simply 
the number of atmometers of the form used that would be necessary to 
evaporate the same amount of water as is lost by the transpiring plant 
in the same time and at the same place. In other terms, relative 
transpiration is a measure of the equivalent or effective evapor- 
ating surface of the plant as this varies from time to time, the 
unit of evaporating surface being unit area of free water sur- 
face under properly defined conditions, or any other sects 
surface which may be adequately defined. 
Reference to the nine graphs of relative transpiration ead 
in the publication just mentioned, and to the accompanying dis- 
cussions, brings out the fact that the maximum of the evaporating 
power of the air (the evaporation rate from the porous cup atmome- 
ter in this instance) always occurred, in the cases cited, somewhat 
later in the day than did the maximum of relative transpiration 
(the ratio of transpiration rate to that of evaporation). This was 
interpreted to mean that some internal change had taken place in 
the leaves, which had begun to retard water loss even while the 
evaporating power of the air had still continued to increase. Such 
* Botanical contribution from the Johns Hopkins University, No. 22. 
309] [Botanical Gazette, vol. 53 
