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Oct. 23, 1916 Daily Transpiration during Normal Growth Period 
The transpiration of the different crop plants per unit area of plant 
surface shows less variation than the transpiration per unit weight of dry 
matter. In other words, the greater efficiency shown by certain plants in 
the use of water appears to be due more to a reduction in plant surface 
than to a reduction in transpiration per unit area of surface. The direct 
solar radiation received by the plants at Akron is usually not sufficient to 
account for the observed transpiration during the midday hours. In 
some of the small grains the energy dissipated through transpiration is 
twice the amount received directly from the sun. 
The march of the transpiration due to changes in the plant alone 
(change in the transpiration coefficient) may be expressed by the ratio of 
the daily transpiration to the daily evaporation if we assume the latter 
to constitute a perfect summation of the weather conditions determining 
transpiration. The transpiration of the annual crop plants (aside fiom 
fluctuations due to weather) rises to a maximum a little beyond the 
middle of the growth period and then decreases until the plants are 
harvested. Perennial forage crops such as alfalfa increase steadily in 
transpiration to a maximum at or near the time of cutting. Various 
crops show their individuality by departing more or less from these types. 
The transpiration coefficient of many of the crops increases expo¬ 
nentially during the early stages of growth. Sudan grass, for example, 
doubled its transpiration coefficient every four days during the early 
growth period. Alfalfa throughout practically the whole period between 
cuttings doubled its transpiration every eight days. 
The relative change in the transpiration coefficients of two crops may 
be determined by taking the ratio of the transpiration of the two crops 
day by day without the necessity of correcting for changes in weather. * 
The correlation has been determined between the various physical 
factors of environment and the transpiration of the different crops, 
considered both individually and as one population. The correlation 
coefficients in the latter case for the season of 1914 are as follows: 
Transpiration with radiation, 0.50±0.01; with temperature, 0.64± 
0.01; with wet-bulb depression, 0.79 ±0.01; with evaporation (shallow 
tank), 0.72 ±0.01; with evaporation (deep tank), 0.63 ±0.01; and with 
wind velocity, 0.26 ±0.01. 
The small grains show individually a markedly higher correlation 
between transpiration and the intensity of the various physical factors 
than was observed when all the crops were combined in one population. 
The mean correlation coefficients for the small grains (1914) are as follows: 
Transpiration with radiation, 0.65; with temperature, 0.71; with wet- 
bulb depression, 0.88; with evaporation (shallow tank), 0.87; with 
evaporation (deep tank), 0.75; with wind velocity, 0.22. 
The com, sorghum, and millet group and the legume group show a 
somewhat lower correlation between transpiration and the intensity of 
the physical factors of environment. The plants in the various groups, 
55858°—16 - 5 
