Transpiration as a Factor in Crop Production. 15 
of a test in 1913 to 8.6° F. in the sun at 2 p. m. when transpiration 
was maximum and 4.2° F. in the shade at the same time. The 
transpiring leaf was 4.1° F. cooler in the sun and 3.2° F. cooler in 
the shade at 2 p. m. than was the air under similar conditions. 
It cannot, however, be concluded that it is an object of tran- 
spiration to prevent excessive heating of the leaves, but rather 
that this cooling is merely a fortunate coincidence. 
18. As measured in terms of evaporation from free water 
surfaces at different elevations in corn, wheat, and oats fields, 
the vegetation is a great protection against excessive transpira- 
tion and also against evaporation from the soil surface. 
19. A shallow free- water-surf ace evaporation jar containing 
only about 1.5 inches of water is very sensitive to climatic changes 
and responds more quickly than a deeper body of water. 
20. A reduction in soil-moisture content below the optimum 
during three years reduced the water requirement per pound of 
ear corn 4.3 per cent, and per pound of total dry matter 7.9 per 
cent. This reduction in water requirement was, however, accom- 
panied by 37.3 per cent reduced stalk yield, 28.5 per cent reduced 
yield of ear corn, and 30.7 per cent lower yield of total dry matter. 
It appears that it would be impracticable to lower the soil- 
moisture content intentionally below optimum for the sake of so 
slight a reduction in the water requirement, because yield is 
reduced relatively so much more. 
An increase in the soil-moisture content above the optimum 
during three years, increased the water requirement per pound 
of ear corn 13.5 per cent, and per pound of total dry matter 8.2 
per cent. This increase in water requirement was accompanied 
by 11.3 per cent reduced stalk yield, 21.1 per cent reduced yield 
of ear corn, and 16.7 per cent lower yield of total dry matter. 
The increased water requirement in the production of dry matter 
is not due to an effort on the part of the plant to get rid of surplus 
water or to greater ease in obtaining it, but rather to a less 
thrifty growth due to some detrimental effect from an over- 
abundance of soil moisture, and consequent reduced available 
fertility. 
21. The water requirement per pound of dry matter is much 
larger in an infertile soil than in a fertile soil. Increasing the 
fertility of the soil reduces the water requirement for grain pro- 
duction and for total dry matter. An application of manure has 
a much greater effect upon an infertile than upon a fertile soil. 
Thus, as an average for two years, equal applications of sheep 
manure to infertile, intermediate, and fertile soils reduced the 
water requirements for ear corn production 42.6 per cent, 25.4 
per cent, and 10.5 per cent, respectively. For total dry matter 
