Oct. 23 ,1916 Daily Transpiration during Normal Growth Period 185 
During the August period in 1914 the transpiration measurements for 
alfalfa and Sudan grass were made both inside and outside the screened 
inclosure (Table XIV). The hourly transpiration of Sudan grass in the 
open was 11 per cent and that of alfalfa 29 per cent of the evaporation 
loss from a water surface of equal area as compared with a loss within 
the inclosure of 7 and 13 per cent, respectively. 
The transpiration of the different plants per unit area of plant surface 
shows less variation than the transpiration per unit weight of dry matter 
produced. In other words, the greater efficiency exercised by some plants 
in the use of water appears to be due more to a reduction in plant surface 
than to a reduction of transpiration per unit area of surface. The latter 
effect is, however, in evidence, the plants characterized by a low-water 
requirement usually showing a somewhat lower transpiration rate per 
unit area. 1 
The determination of the surface area of a large plant from measure¬ 
ments on a few shoots or branches is necessarily only an approximation. 
For different sets of plants of the same crop the results were in satisfac¬ 
tory agreement. Thus, measurements of six sets of Kubanka wheat 
gave 49, 48, 47/44, 52, and 49 sq. cm. of surface per gram (dry weight). 
The measurements were less accurate in the case of plants which branch 
freely, such as alfalfa. The greatest uncertainty, however, appears to 
be in the assumption that the various surfaces presented by a plant 
have the same transpiration loss per unit area. 
COMPARISON OF THE ENERGY RECEIVED BY DIRECT RADIATION 
WITH THE ENERGY DISSIPATED BY TRANSPIRATION 
If the area of the shadow thrown by a plant on a plane normal to the 
sun’s rays is known, the direct solar radiation received by the plant in 
one hour, expressed in gram-calories, is equal to the product of the area 
of the shadow in square centimeters and the direct radiation energy in 
calories per square centimeter per hour. 
The energy dissipated by the plant through transpiration during the 
same period is equal to the product of the transpiration in grams and 
the latent heat of vaporization (536 gm.-cal. per gram). The ratio of 
these two quantities represents the part played by direct sunlight in 
transpiration, assuming that all the radiation is absorbed. Such com¬ 
putations have been made for a number of plants employed in the trans¬ 
piration measurements of 1914, and are presented in Table XVII. The 
measurements are based on the hourly transpiration and hourly radia¬ 
tion values at midday. For plants grown in the inclosure the radiation 
values have been corrected for the shade of the wire screen. 
1 The high value obtained for sorghum is due to the fact that the lower leaves were harvested early to 
prevent loss and were not included in the area measurements made at the time the plants were cut. These 
lower leaves were active during the transpiration period considered, so that the transpiring area was 
greater than that finally measured. The transpiration recorded per unit area is therefore too high. 
