Transpiration as a Factor in Crop Production. Ill 
RELATION OF TRANSPIRATION TO ATMOSPHERIC HUMIDITY. 
To secure further data relative to the effect of atmospheric 
humidity on transpiration, the previously-published greenhouse 
work of 1911 was continued and extended in 1912. Humidity 
is the easiest of all the climatic factors to control in greenhouses 
independently of other factors. 
Two greenhouses were employed. One was maintained at 
the normal humidity by eliminating all possible sources of damp- 
ness. The atmosphere in the other was maintained as humid as 
possible by means of fine atomizers attached to the water system, 
and by keeping the floor and benches wet. The ventilators were 
so adjusted that only very slight differences in temperature re- 
sulted. A self-recording thermometer and hygrometer were 
kept in each greenhouse. Since transpiration at night has been 
found to be only 7 or 8 per cent as great as in the daytime, the 
weather factors for the day only should be used in studying their 
effect upon the total transpiration. The results of three tests in 
1911 and 1912 are contained in Tables 36, 37, and 38 and are 
summarized in Table 39. 
In 1911 all of the plants were grown in the subirrigation type of 
potometers, 16 by 36 inches, filled with a fertile loam. Two plants 
grew in each potometer. In 1912 part of the plants were grown 
singly in the same style of potometer as was used in 1912 and 
part were grown singly in the small 4-gallon potometers con- 
taining 18 kilograms of a fertile compost-loam mixture. A small 
variety of pop corn was grown in these small potometers so that 
the plants might grow for a relatively long time before the small 
amount of soil should become a seriously limiting factor. All 
plants were harvested at the same time, at about the silking stage. 
Table 39 shows that largely as the result of 22 per cent higher 
humidity and 1.75° F. lower temperature during the day, only 
58 per cent as much water was transpired per unit of dry matter, 
and 62 per cent as much per unit leaf-area, while the evaporation 
from a given free water surface was 54 per cent as much in the 
humid as in the dry greenhouse. 
EFFECT OF PREVIOUS CONDITION OF GROWTH ON 
TRANSPIRATION. 
Pop corn plants were used in this experiment which had 
been grown almost to the silking stage in either the dry or the 
humid greenhouse in 1912 under the climatic conditions described 
in Table 38. At the end of the eighth week, part of the plants 
were transferred from each greenhouse to the opposite green- 
house and there compared in relative transpiration rate with the 
