Transpiration as a Factor in Crop Production. 95 
case of the free water surface, the reduced night evaporation is 
caused solely by the reduced atmospheric demand, while with 
the plant it is primarily a combination of this cause with a greatly 
reduced total area of stomatal aperture. 
DAILY TRANSPIRATION AND EVAPORATION IN RELATION TO CLIMATIC 
FACTORS. 
During two years, 1910 and 1914, daily records were obtained, 
for a period of about one month each year, of the transpiration 
by corn plants, evaporation from free water surface, temperature, 
relative humidity, wind velocity, and sunshine. The climatic 
records, except percentage of sunshine, were obtained in the 
cornfield by means of self-recording instruments, as previously 
described and illustrated. (Fig. 4, page 47.) 
The evaporation from a free water surface has been obtained 
each year by averaging together the losses from six evaporation 
jars of 36 square inch area standing at different elevations near 
the corn plants at intervals of 2 feet from the ground up. (See 
Fig. 3, page 46.) 
The plants used for these measurements were grown in the 
experiments for testing the effect of different degrees of soil- 
moisture content on the water requirements of corn. For cor- 
relation with the climatic factors, only those groups of plants 
were averaged each year in which the shortage of water was not 
so great as to reduce the growth very abnormally. 
In 1910 the daily losses were averaged for plants in the 60, 
80, and 100 per cent soil saturation potometers. In 1914 the 
data for plants in 50, 70, and 95 per cent saturation were averaged. 
A complete record of the growth and water requirements of 
these plants may be found in Tables 53 and 55, pages 133 and 135. 
Tables 27 and 28 and Charts X and XI contain a daily record 
for 1910 and 1914 of the average transpiration, evaporation, and 
climatic factors. The results appear to indicate clearly that tran- 
spiration from corn plants, and evaporation from a free water 
surface are similar phenomena and respond in a fairly similar 
degree to changes in the climatic factors. Transpiration is 
essentially evaporation. In the main, the daily losses for these 
two factors correlate and are fairly parallel. The occasional in- 
consistencies between the two may be largely due to the difference 
in the physical condition of the evaporating bodies and to dif- 
ference in exposure. 
In these tests the transpiration rate kept pace with free water 
evaporation when the atmospheric demands for moisture were 
high. When these evaporation demands were low, there was no 
definite tendency for the transpiration rate to be relatively less 
