Oct. 23,1916 Daily Transpiration during Normal Growth Period 
189 
An inspection of the ratio graphs of sorghum and alfalfa (figs. 6 and 7) 
shows, as in the case of Kubanka wheat, a dependence of transpiration 
upon radiation, wet-bulb depression, and temperature. A marked de¬ 
parture in any one of the ratio graphs is usually, though not always, 
accompanied by a departure in the opposite sense in one of the other 
graphs. The correlation of transpiration with wind velocity is low in the 
case of both of these crops. 
MARCH OF TRANSPIRATION DURING THE GROWTH PERIOD AS SHOWN BY 
THE RATIO OF DAILY TRANSPIRATION TO DAILY EVAPORATION 
Although the association of transpiration with wet-bulb depression is 
fully as marked as the transpiration-evaporation association (see correla¬ 
tion coefficients, p. 204), the writers have decided to employ the latter 
ratio to represent the change in the transpiration coefficient of the dif¬ 
ferent plants, since it is already used extensively. 1 
The ratios of the daily transpiration of each crop to the daily evapora¬ 
tion (shallow tank) are plotted as percentages of the maximum in figures 
8 and 9. By this method different crops may be easily compared. 
The graphs show irregularities due to the lack of an exact correlation 
between evaporation and transpiration, or to errors in the measurements. 
On days when rain occurred the outstanding points are probably due to 
inaccurate determinations of the transpiration. 2 The outstanding ratios 
on days without rain are not explainable on this basis and are to be re¬ 
garded as expressions of the inexact correlation of transpiration and 
evaporation as here measured. 
The general trend of the graphs indicates a gradual increase in the 
transpiration coefficient from seedtime to a maximum which in the case 
of annual plants comes just before they begin to ripen. 
measurements in 1914 
In 1914 (fig. 8) the small grains were well advanced before the trans¬ 
piration measurements were begun. They did not reach their maximum 
transpiration rate, however, until about one month later. 
1 Livingston, B. E. The relation of desert plants to soil moisture and to evaporation. 78 p. ( illus. 
Washington, D. C., 1906 (Carnegie Inst. Washington Pub. 50.) Literature cited, p. 77-78. 
-The resistance offered by leaves to transpirational water loss. In Plant World, v. 16, no. 1, p. 1-35, 
illus. 1913. 
2 Even a small amount of rain wets the plant thoroughly, a part of the water remaining on the surface of 
the plants and a part being absorbed by the dry or living leaves or caught in the leaf sheaths or flower heads. 
Water is also held on the surface of the pot and a small amount may find its way into the pot by suction 
due to the change in temperature. If the morning weighing following a rain during the night is taken as 
the basis of determining the transportation, on the subsequent day the transpiration is too high since some 
of the water is merely evaporated from the surface of the plant and pot. If the two da£s are combined and 
morning weighing discarded, the transpiration is too low since transpiration from wet plants is lower than 
from dry plants, and since an equivalent of the water which was absorbed must be transpired before a loss 
in weight can be recorded. Notwithstanding the errors of the second method, it seems best not to intro¬ 
duce the greater uncertainty involved in the first method and to regard the outstanding determinations 
on rainy days as experimental errors which can not at present be successfully overcome without actually 
protecting the plants from rain, which would change the conditions under which field crops are grown. 
