DAILY TRANSPIRATION DURING THE NORMAL 
GROWTH PERIOD AND ITS CORRELATION WITH 
THE WEATHER 
By Lyman J. Briggs, Biophysicist in Charge , Biophysical Investigations , and H. L. 
Shantz, Plant Physiologist , Alkali and Drought Resistant Plant Investigations , 
Bureau of Plant Industry 
This paper deals with the daily transpiration of a part of the crop 
plants included in the water-requirement experiments at Akron, Colo., 
in 1914 and 1915. 1 The principal objects of the measurements were the 
determination of (1) the march of transpiration during the growth period, 
and (2) the extent to which the daily transpiration is correlated with 
various weather factors. 
EXPERIMENTAL METHODS 
The plants were grown in galvanized-iron pots, containing about 115 
kgm. of soil and provided with tight-fitting covers with openings for 
the stems of the plants. The annular space between the cover and the 
stem of each plant was sealed with a plastic wax. Direct evaporation 
from the soil was thus avoided and the loss of water limited to trans¬ 
piration. 3 
Twenty-two crops were included in the daily weighings in 1914 and 
23 crops in 1915. Each crop was represented by six pots of plants 
(PI. 5) weighed independently. The weighings were made with an 
accuracy of 0.1 kgm. by means of a spring balance checked before and 
after each series of weighings against a standard weight of 130 kgm. 
The balance and weighing device are shown in Plate 6. The daily weights 
served also as a basis for determining the quantity of water to be added 
daily to each pot to insure an adequate water supply. 
In differentiating between the transpiration of consecutive days it is 
desirable that the weighings be made at a time when the plants are 
losing very little water. Automatic records show that the transpira¬ 
tion is at a minimum just before sunrise. 3 The daily weighings, which 
required the time of three men for an hour, were accordingly begun in 
the morning as soon as there was light enough to work and completed 
before the transpiration response to sunlight had set in. 
1 Acknowledgment is gratefully made of the efficient and valued assistance of Messrs. R. D. Piemeisel, 
F. A. Cajori, P. N. Peter, J. D. Hird, G. Crawford, R. D. Rands, A. McG. Peter, H. W. Markward, H. 
Shattyn, and T. R. Henault at Akron in 1914 and 1915. Mr. W. H. Heal has also aided very materially 
in the reduction of the measurements. 
2 Briggs, h. J., and Shantz, H. D. The water requirement of plants. I.—Investigations in the Great 
Plains in 1910 and 1911. TT. S. Dept. Agr. Bur. Plant Indus. Bui. 284, p. 9. 19x3. 
3 -Hourly transpiration rate on clear days as determined by cyclic environmental factors. 
In Jour. Agr. Research, v. 5, no. 14, p. 583-650, 22 fig., pi. 53-55. 1916. literature cited, p. 648-649. 
655) 
Journal of Agricultural Research, 
Dept, of Agriculture, Washington, D. C. 
fx 
Vol. VII, No. 4 
Oct. 23,1916 
G— 95 
