288 
Journal of Agricultural Research 
Vol. IX, No. 9 
and C) are superimposed. It will be noted that the fluctuation in the 
ratio throughout the 24-hour period, as well as the mean hourly de¬ 
parture, is decidedly less when the transpiration is referred to the evapo¬ 
ration from the shallow, blackened tank. In other words, the transpira¬ 
tion of Medicago sativa follows the evaporation from the shallow, 
blackened tank more closely than that from the porous atmometers. 
It is evident, however, that an evaporating system has not yet been 
secured that responds to its environment in the same way as the plant. 
Both the atmometer and the tank depart widely from the plant during 
the night under the conditions prevailing in the Great Plains. Refer¬ 
ence to the logarithmic graphs will show that the transpiration-evapora¬ 
tion ratio when referred to the evaporation from the shallow, blackened 
tank is practically constant from 8 a. m. to 4 p. m., the period during 
which the plants are transpiring most rapidly. In Medicago sativa y at 
least, there is no evidence of a change in the transpiration coefficient dur¬ 
ing this period. The higher ratio obtained in the early morning and late 
afternoon hours is perhaps explainable by the difference in exposure of 
the plants and the free water surface. The isolated plants are exposed 
to the normal rays of the sun practically throughout the day while the 
tank receives only the vertical component of radiation. Where vege¬ 
tation is massed under field conditions, the plants likewise are exposed 
for the most part only to the vertical component of radiation. 
The transpiration measurements in these experiments are confined to 
Medicago sativa , and it is possible that the transpiration rate of other 
plants under the same conditions would have given transpiration graphs 
differing in form from those obtained. Earlier measurements (Briggs 
and Shantz, 1916, p. 638) have shown, however, that in the case of rye 
and amaranthus, the hourly transpiration rate is correlated with the 
evaporation rate from the shallow tank to a degree comparable with the 
corresponding correlation in the case of alfalfa. Thus, for rye and ama¬ 
ranthus correlations of 0.89 ±0.03 and 0.95 ±0.01, respectively, were 
obtained, while alfalfa at different periods during the same year showed 
a correlation of 0.89 ±0.01 and 0.93 ±0.01. An equally good correspond¬ 
ence would therefore be expected, at least in the case of amaranthus, 
between the hourly transpiration rate and the hourly evaporation rate 
from the shallow tank. 
The writers are not, however, urging the merits of the shallow, black¬ 
ened tank as a means of determining changes in the transpiration coeffi¬ 
cient of a plant during the day. The point which it is desired to em¬ 
phasize is that departures between the hourly transpiration rate and the 
hourly evaporation rate from any physical system can not be attributed 
to changes in the transpiration coefficient without first having determined 
that under less intensive conditions the two systems give graphs which 
are in accord. 
