Jan. 3, 1916 
Hourly Transpiration Rate on Clear Days 
627 
The transpiration during the whole period of growth of sorghum, 
when based on dry matter produced, is practically the same for the 
three varieties here considered. Minnesota Amber transpired 2 39 ±2 
gm. of water for each gram of dry matter produced; Dwarf milo, 273±4 
gm.; and milo, 249±3 grams. 
The transpiration rate of rye, when based on the dry matter harvested, 
is much higher than for any other crop included in the 1912 water- 
requirement measurements. This is due in part to the more extreme 
atmospheric conditions prevailing during this period and in part to the 
higher water requirement of rye, which is 39 per cent higher than Ku¬ 
banka wheat and 15 per cent higher than Swedish Select oats. 
The data presented during the long period for alfalfa were based on 
plants which yielded different amounts of dry matter. In order to make 
the comparison more exact, two short periods have been presented. 
The environmental conditions were somewhat more extreme during the 
early period, as is shown by a comparison of radiation, temperature, and 
saturation deficit. The evaporation rate was also higher. On the basis 
of dry matter harvested, the transpiration during the two periods was the 
same. It is necessary in this connection to consider the size of the plant 
at the actual time of the measurements. The late-period crop was har¬ 
vested 6 days after the period when the transpiration measurements were 
made, while the early-period crop was harvested 20 days after the ter¬ 
mination of the transpiration measurements. It is evident, therefore, 
that the ratio of transpiration rate to dry matter of the early-period 
crop would have been considerably higher had this crop been harvested 
soon after the transpiration measurements were completed. 
The most severe environmental conditions in 1914 were encountered 
during the amaranthus period. Solar radiation was greater and satura¬ 
tion deficit, air temperature, and evaporation higher. On the basis of 
dry matter, amaranthus transpired less than alfalfa, but more than rye. 
On the basis of the whole period of growth, the water requirement of 
amaranthus was much less than rye, the higher rate of transpiration 
shown in the data here presented being due to the unusually severe con¬ 
ditions prevailing during this period. 
While the writers are considering the data in this paper primarily from 
the standpoint of the relative transpiration rate of the different plants 
and are not particularly concerned with absolute values, it is interesting 
to find that the data here presented conform as nearly as can be expected 
to the relative transpiration rates of the different plants as determined 
from the water-requirement measurements. 
COMPARISON OF THE FORM OF THE CURVES 
In order that a more accurate comparison may be made between the 
form of the transpiration graph and that of the several environmental 
factors, the mean hourly values presented in the preceding tables have 
