184 
Journal of Agricultural Research 
Vol. VII, No. 4 
The mean daily transpiration of the small-grain crops during the maxi¬ 
mum transpiration period in 1914 (Table XIII) was 1.49 ±0.08 kgm. per 
square meter, and in 1915 (Table XV) 1.46 ±0.07 kgm. per square meter. 
The evaporation during the 1915 period was only 59 per cent of that 
observed during the corresponding period in 1914. While the transpira¬ 
tion periods selected were the same (July 7-16), the crops in 1914 were 
relatively more advanced showing evidences of ripening in some cases. 
This appears to be responsible, in part at least, for the observed agreement 
in the transpiration rate during the two years, although it is of course 
possible that under the weather conditions prevailing in 1915 a mor¬ 
phological adjustment took place, which increased the transpiration 
coefficient per unit area. 
During the August transpiration period of 1914 the transpiration rate 
per square meter of plant surface (Table XIV) ranged from 81 gm. per 
hour for Sudan grass to 166 gm. for a variety of Grimm alfalfa, these 
plants being grown in the screened inclosure. The same plants grown in 
the open showed a transpiration loss of from 128 to 343 gm. of water per 
square meter of surface per hour. The water requirement of Sudan grass 
in the inclosure was 10 per cent below that of the crop grown in the open, 
while in the case of alfalfa the inclosure reduced the water requirement 
22 per cent (Table II). The reduction in transpiration per unit area of 
the crops grown inside the inclosure is over twice that indicated from the 
water-requirement measurements. Here, again, there is an indication of 
a morphological adjustment resulting from the difference in exposure. 
Periods of maximum transpiration may be due to extreme weather 
conditions or to the transpiration coefficient having reached its maximum 
or to *both. It would appear that the maximum transpiration period of 
the small grains in 1914 was determined largely by weather conditions, 
since it falls rather late in the period of growth, and some of the grains 
had already begun to ripen. In 1915 this period corresponds more nearly 
to the period of maximum transpiration coefficient of the small grains. 
During the July period in 1914 (Table XIII) the transpiration per square 
meter per hour during midday ranged from 61 gm. for Siberian millet to 
175 gm. for Galgalos wheat. The millets were just heading at this time, 
and oats, barley, and rye had begun to ripen, although the transpiration 
coefficient was still apparently near the maximum. 
The loss of water from the plant surfaces during this period was 5 to 14 
per cent of that from a water surface of equal area. In 1915 the plant 
surfaces lost relatively more, ranging from 10 to 25 per cent of that from 
a free water surface of the same area (Table XV). 
It should be noted in this connection that the evaporation measure¬ 
ments were made in the open, while the transpiration measurements 
for the most part were made within the screened inclosure. The data 
just given regarding the transpiration of the plants relative to an equal 
free water surface are consequently somewhat too low. 
