May 28, 1917 
Transpiration Rate of Medicago sativa 
279 
Transpiration measurements. —The transpiration measurements 
were made on automatic scales of the type previously described (Briggs 
and Shantz, 1915). Grimm alfalfa (Medicago sativa L.) was employed in 
the measurements. The plants were growing in the large sealed pots 
used in the writers' water-requirement measurements, and had been 
carried over from the preceding year. The plants were fully exposed to 
the sun and were coming into bloom during the measurements, the second 
cutting of the season being made at the close of the experiments. The 
green weights of the' crops and the total plant surface 1 were as follows: 
Pot No. 
Green weight. 
Total plant 
surface. 
Gm. 
Sq. cm. 
3d 
258 
15, 700 
303 
287 
17, 400 
Subsidiary measurements. —In addition to the above measure¬ 
ments, the solar-radiation intensity, air temperature, wet-bulb depres¬ 
sion, and wind velocity were recorded simultaneously by means of 
automatic instruments which have already been described (Briggs and 
Shantz, 1916). 
Reduction of data. —The data for the white cylinder, brown cyl¬ 
inder, and white-sphere atmometers have been corrected by means of 
the coefficients supplied with the instruments. No corrective factor has 
been applied to the Bellani plates. The data for the filter-paper evapo- 
rimeter have been reduced to an area of 100 sq. cm. and the data for 
shallow evaporation tank to an area of 1 square meter. 
EXPERIMENTAL RESULTS 
The measurements were made at Akron, Colo., in July, 1916, during 
a period of hot, dry weather. The data representing the hourly evapora¬ 
tion rate from the various surfaces, the transpiration rate, and the 
weather conditions are given in Table I. Each atmometer determination 
is the mean of four independent measurements. The transpiration is 
represented by the mean of two pots measured independently. The 
data are presented graphically in figure 1. 
The hourly transpiration is plotted at the top of the figure, followed 
by the evaporation from the different types of atmometers and from 
the free water surfaces. It will be noted that the different atmometers, 
although varying widely in form (PI. 4), give graphs which are similar 
in their characteristics. This is brought out more clearly in figure 2, 
in which the ratio of the transpiration rate to the evaporation rate for 
each of the various types of atmometers is plotted hour by hour. 
1 The determinations, of plant surface are based upon the measurement of the green weight and total 
surface of a representative plant. 
