EFFECT OF CLIMATIC FACTORS OX WATER REQUIREMENT. 65 
from the consideration of the relative humidity of the two houses. 
Other things being equal, the rate of evaporation would be propor- 
tional to the saturation deficit. This can, furthermore, be limited to 
the saturation deficit during the day, since the transpiration at night 
is so small that it can be disregarded. 
The pressure of aqueous vapor in saturated air at the mean day 
temperatures of the dry and humid houses is 31.67 and 31.11 milli- 
meters, respectively. The corresponding saturation deficits ex- 
pressed in terms of vapor pressure would then be 
31.67X(1.00-0.37) = 19.95 millimeters, 
and 31.11X(1.00-0.58) = 13.05 millimeters. 
The ratio of these saturation deficits is 1.53, as compared with 1.56 
obtained experimentally as the ratio of the water requirements. 
The linear relationship which is thus indicated between the saturation 
deficit and the water requirement must, of course, be considered only 
as tentative until its validity shall have been determined through 
further experiments. It affords, however, a possible method of deter- 
mining indirectly from the mean relative humidity the relative water 
requirement of crops in different sections of the country, provided the 
temperature and the other climatic factors are equally favorable for 
growth. 
CONCLUSIONS. 
All of the investigations that have been made relative to the effect 
of humidity on the water requirement of plants show conclusively 
that the water requirement is greater in dry than in moist air. 
Montgomery and Kiesselbach have found that the water require- 
ment of corn is proportional to the evaporation from a small free 
water surface. From their data the writers have shown that the 
water requirement is also approximately proportional to the satura- 
tion deficit. 
EFFECT OF THE CARBOX-DIOXID CONTEXT OF THE AIR ON THE WATER 
REQUIREMENT. 
sorauer's experiments. 
Sorauer (1880) measured the effect of the carbon-dioxid content 
of the atmosphere upon the water requirement (Table LVI). Rape 
was grown in free air and also under two bell jars. In one of the bell 
jars a solution of potassium hydrate was kept to reduce the carbon 
dioxid of the air. The results indicate that the water requirement is 
greatly increased by reducing the carbon-dioxid content of the 
atmosphere. 
97991°— Bui. 285—13 5 
