108 Nebraska Agricultural Exp. Station, Research Bui. 6. 
secutive years 1910-1914, the data for the two months of 
July and August were used. 
Plants taken for this comparison were grown in standard 
potometers in the cornfield under similar conditions as to corn 
variety, soil fertility, soil moisture, soil mass, and the exclusion 
of sources of error as surface evaporation and the entrance of 
rain. The yearly differences recorded in Table 33 may there- 
fore in the main be ascribed to variations in the climatic condi- 
tions affecting the evaporating power of the air. 
For these yearly comparisons the evaporation from a free 
water surface is based on the loss from the evaporation jar 10 
feet above the ground. This jar stood higher than the surround- 
ing corn each year, and consequently always had identical ex- 
posure. 
Since there was some likelihood that our weather instruments 
might vary a degree in adjustment between the different years, 
it was thought best to eliminate such a possibility of error, and 
consequently the seasonal weather records were calculated from 
hourly records taken by the United States Weather Bureau 
three miles distant. 1 In a few instances when its data were in- 
complete they were supplied from our own records. These weather 
data are compiled in Table 34. 
The summary tables indicate that there is a marked difference 
in the evaporating power of the atmosphere in different seasons, 
which may be accounted for by seasonal climatic variations. 
There is a rather consistent relationship in the relative seasonal 
variations between the (1) transpiration per unit dry matter, 
(2) transpiration per unit leaf-area, and (3) evaporation from a 
free water surface. 
The two years 1913 and 1914 offer an extreme contrast in 
the water requirements. The data for these two years have 
been compiled in Table 35 for comparison with similar data 
obtained during two years 1911-1912 under controlled green- 
house conditions, in which the effect of a difference in relative 
humidity was the object sought. 
The comparison shows a very similar difference in the prevail- 
ing temperature and humidity under natural field conditions in 
1913 and 1914, as existed in the greenhouse experiments of 1911 
and 1912 in which a difference of 22 per cent in relative humidity 
was artificially maintained. The complete data for these green- 
house experiments follow. 
1 The author is indebted to Professor G. A. Loveland, section director of the 
United States Weather Bureau, for access to the original hourly records from 
which these seasonal climatic data were compiled. 
