Transpiration as a Factor in Crop Production. 
117 
tion to reduce water loss by reducing leaf temperature, but 
rather that it is simply a coincidence. 
Additional temperatures recorded in Tables 42 and 43 were 
taken in 1912 and 1913 to determine the relative temperature of 
the leaf and other parts of the plant as compared with the air 
temperature. The temperature within the stalk, underneath 
the leaf sheath, and underneath the husks surrounding the ear 
were rather similar, while the leaf temperature was 2 to 3° F. 
higher and yet approximately 2° F. cooler than the air. 
The leaf temperatures were all taken by inserting the ther- 
mometer momentarily in a fold of the leaf, taking care to eliminate 
the effect of body heat of the operator. Altho this method is 
not very refined, it is probable that these temperatures are approx- 
imately correct. An elaborate method for determining leaf 
temperatures has been used and described by Shreve (1914). An 
electric potentiometer method for determining leaf-temperature 
is described by Ehlers (1915). A study of leaf-temperatures at 
this station will be continued with the use of thermocouples 
properly connected with a potentiometer. 
RELATIVE RATES OF EVAPORATION FROM EQUAL AREAS OF 
FREE WATER SURFACE AND LEAVES OF GROWING 
CORN PLANTS. 
In 1914, 24 potometers, each containing one corn plant with 
a fully developed leaf -area, were weighed each day for a period 
of 30 days, and the daily transpiration determined. Likewise 
the evaporation from six evaporation jars of 36 square inch free 
water surface, standing at regular intervals ranging from the 
ground surface to 10 feet above the ground, was determined 
daily for the same interval of time. The total average transpira- 
tion for one corn plant of 1,378 square inch leaf-area during the 
30 days was 75,924 grams, which is equal to 55.1 grams water 
per square inch. 
The total average evaporation for one evaporation jar of 36 
square inch area was 5,915 grams, which is equal to 164 grams 
per square inch. Accordingly, the transpiration rate per square 
inch leaf-area was 33.6 per cent of the evaporation from a square 
inch free water surface. 
During 30 days in 1914, the hourly transpiration was deter- 
mined for one corn plant having 1,500 square inches leaf -area. 
The evaporation from a free water surface standing midway 
between the top and base of the plant was also determined 
