646 
Journal of Agricultural Research 
Vol. V, No. 14 
piration are due to differences in the plants or to some change in the 
plant as the day progresses. 
It is probable that plants differ also in their response to solar energy, 
the absorption coefficient of different plants not being the same, while 
the dissipation of the energy absorbed is quite different in different 
plants. In other words, the ratio of the energy dissipated through 
transpiration and lost by the plant through emissivity is not the same 
for all species. Such changes probably occur also in the same plant 
during the daily cycle, which would modify the transpiration coefficient 
irrespective of the changes in physical conditions. 
SUMMARY 
This paper deals with measurements of transpiration on clear days at 
Akron, Colo., in relation to environmental factors. The plants, which 
included wheat, oats, rye, sorghum, alfalfa, and amaranthus, were grown 
in large sealed pots of the type used in water-requirement measure¬ 
ments, containing sufficient soil (about 115 kgm.) to enable the plants to 
make a normal growth. The transpiration was determined by weigh¬ 
ing, four automatic platform scales recording each 20-gm. loss being used 
for the purpose. Automatic records were simultaneously made of the 
radiation intensity, the air temperature, the depression of the wet-bulb 
thermometer, the evaporation, and the wind velocity. The radiation 
intensity and the wet-bulb depression were measured by differential 
telethermographs, and the evaporation rate from a free-water surface was 
determined by mounting a shallow, blackened evaporation tank 3 feet 
in diameter on an automatic platform scale. 
Composite graphs are presented, showing the mean hourly transpira¬ 
tion rate for each of the plants considered, together with the mean 
hourly values of the radiation, air temperature, wet-bulb depression, and 
wind velocity for the transpiration period and also the mean hourly 
evaporation rate. On the basis of the form of the curves the transpira¬ 
tion graphs may be grouped into two classes having characteristic 
features. The cereals show a marked change in the slope of the transpira¬ 
tion graph in the forenoon unaccompanied by corresponding changes 
in the environmental factors. On the other hand, the forage plants 
and amaranthus give little or no indication of such a change. This 
flattening of the graphs in the case of the cereals appears to be due to 
some change in the plant, resulting in a reduction in the transpiration 
rate below what would be expected from the form of the curve during 
the early morning hours. 
The hourly transpiration rate of the cereals on dear days increased 
steadily, though not uniformly, from sunrise to a maximum value, usually 
reached between 2 and 4 p. m., after which it fell rapidly to the night 
levd. The transpiration graphs for sorghum, alfalfa, and amaranthus 
were somewhat more symmetrical with respect to midday, reaching 
