Transpiration as a Factor in Crop Production. 43 
leaf -areas and free water surfaces are indicated in square inches. 
This probably has an advantage in being more readily interpreted. 
All weights of crop and of water used are expressed in the 
" metric' ' system, because the smaller units of weight possess an 
advantage in these calculations. 
The scientific ideal no doubt would be to express all weights 
and measures uniformly in either the " me trie" or "regular" 
system. 
METHODS OF DETERMINING THE YIELD AND LEAF-AREA. 
The crop was in all cases reduced to a moisture-free basis by 
drying to a constant weight in an electric oven at 110° C. The 
oven used was a specially constructed Freas oven, having a drying 
chamber 36 by 24 by 48 inches. The plants were pulled from the 
ground and the roots cut away from the base of the stalk. The 
dry matter harvested included the entire plant except the roots. 
It would be of interest to know the weight of the roots as well, 
but it seemed impossible to make a reliable separation of the roots 
of the crop from roots and other organic matter previously in the 
soil. The leaves were gathered from the plants as they ripened. 
The leaf-area of the corn plants was determined when full 
grown, as in earlier experiments, by taking three-fourths of the 
product of the length by the maximum width of the individual 
leaves and adding together these products for the total leaf-area. 
This method is discussed by Montgomery (1911, p. 113). 
A distinction should be observed between 4 'leaf -area" and 
"leaf-surface" as used in this bulletin. The latter is twice the 
area of the former, in that each leaf has two surfaces. 
METHODS OF INVESTIGATION. 
LOCATION OF EXPERIMENTS AND MATURITY OF PLANTS. 
All transpiration measurements were made with rooted plants 
grown from the outset in potometers filled with soil. They may 
be grouped according to their location, as experiments conducted 
either in the cornfield or in the greenhouse. The greenhouse was 
used for only a comparatively small portion of the determinations, 
namely, the measurement of the effect of differences in relative 
atmospheric humidity upon transpiration. All plants were grown 
to maturity except those in the greenhouse, which were harvested 
at the silking stage. 
DESCRIPTION OF POTOMET RS USED. 
Three types of potometers were used, according to the nature 
of the experiment. Each proved admirably suited for its special 
