14 Nebraska Agricultural Exp. Station, Research Bui. 6. 
period of the day. Such days are exceptional, but they are very 
critical for corn in case there is not sufficient moisture in the soil 
to supply the demand. In a comparatively short time the corn 
may receive injuries from which it never fully recovers. Bearing 
this in mind it is evident that a period of brief duration may 
affect yields more than the annual amount of rainfall. 
13. The amount of water used during each week of growth 
gradually increases until the plants have developed their max- 
imum leaf -area. The four or five weeks following this are usually 
the hottest and driest of the season, and the transpiration rate 
remains high. Fully one-half of the total water used by the plant 
is transpired during this period of about five weeks. Following 
such a period, the transpiration falls off rather rapidly until 
maturity is reached. The rapid transpiration rate during the 
tasseling and earing period is coincident with this condition 
of the plant and not the result thereof. 
14. A marked variation exists in the water requirement of 
different years, due to natural climatic differences. There is 
a rather consistent relationship in the relative seasonal varia- 
tions between the (1) transpiration per unit of dry matter, (2) 
transpiration per unit of leaf-area, and (3) evaporation from a 
free water surface. There is no such thing as a definite water 
requirement which is constant for any one kind of crop. 
15. As an average for three greenhouse tests conducted 
during two years, a difference of 22 per cent in relative humidity 
and 1.7° F. during the day caused a difference of 42 per cent in 
the water requirement per pound of dry matter, 38 per cent in 
the transpiration from a unit of leaf-area, and 46 per cent in the 
evaporation from a free water surface. 
16. Corn plants which had been grown for two months in a 
greenhouse with humid atmosphere exhibited no different tran- 
spiration rate from a given leaf-area when transferred to a dry 
greenhouse than took place from plants which had been con- 
tinuously grown under the dry conditions. The same is true 
of plants which were transferred from a dry to a humid greenhouse 
in comparison with plants continually grown in the humid 
atmosphere. There appears to have been no histological adapta- 
tion due to previous condition of growth which would affect 
the rate of transpiration when the plants were subjected to 
widely different climatic conditions. 
17. Transpiration is found to exert a marked reduction in the 
leaf temperature when the air temperature is high, and as a 
result exercises a valuable self-protection against water loss. 
A transpiring leaf was found to be uniformly cooler than a 
dead, dry one, amounting under the severe climatic conditions 
