286 
Journal of Agricultural Research 
Vol. IX, No. 9 
RATIO OF TRANSPIRATION TO EVAPORATION 
The ratio of the transpiration rate to the evaporation rate from an 
atmometer should give a straight line when plotted hour by hour if the 
two systems respond alike to changes in environment. The experi¬ 
mental graph is, however, periodic in form, exhibiting a marked mini¬ 
mum during the night hours. 
Fluctuations in the hourly transpiration-evaporation ratio as repre¬ 
sented in such a graph have been interpreted (Livingston, 1906, 1913; 
Livingston and Hawkins, 1915; Edith Shreve, 1914 and 1915; Forrest 
Shreve, 1914; Bums, 1915) as representing departures in the transpira¬ 
tion rate of the plant from that which would prevail if the plant responded 
freely to its environment. This point of view implies the assumption 
that an atmometer responds perfectly to its environment; in other words, 
that the evaporation rate from the atmometer affords a perfect summa¬ 
tion of the environmental conditions determining evaporation from a 
plant surface. A departure in transpiration from that indicated by this 
summation would then, according to this viewpoint, be taken as evidence 
of a change in the transpiration coefficient resulting from stomatal control 
or from other reversible changes within the plant body. 
The correctness of the assumption relative to the perfect response of 
the porous atmometers must be questioned when the hourly evaporation 
rate from the atmometer is compared with that from the shallow tank. 
During the night, for example, the evaporation from the atmometers is 
relatively much higher than from the free water surface. If the at¬ 
mometer is accepted as responding freely to its environment, then the 
free water surface, even under conditions of low evaporation, can not be 
considered as a free evaporating system. 
The same question as to the free response of the porous atmometers to 
their environment arises when the relative transpiring power, as expressed 
by the transpiration-evaporation ratio, is compared with the “index of 
transpiring power” as measured by the cobalt-paper method. Living¬ 
ston (1913, p. 24) has in fact found in comparing the hourly graphs 
obtained by the two methods that the— 
relative transpiration ratio as determined either by the brown or white atmometer 
presents a much flatter graph than that indicated by the hygrometer index of trans¬ 
piring power. 
It is evident, therefore, that one of these instruments must fail to 
respond freely to the intensity of the environment. 
The ratio of the transpiration rate of Medicago sativa to the evaporation 
rate of each of the different types of atmometers is presented graphically 
in figure 2. The graphs are markedly similar in form when the wide 
differences in the porous atmometers are considered. The changes from 
hour to hour in any one graph are, with few exceptions, reflected in all 
the others, and such differences as exist are, in most cases, comparable 
