journal of mmmmm 
DEPARTMENT OF AGRICULTURE 
Vol. V Washington, D. C., January 3, 1916 No. 14 
HOURLY TRANSPIRATION RATE ON CLEAR DAYS AS 
determined BY CYCLIC ENVIRONMENTAL FACTORS 
By Lyman J. Briggs, Biophysicist in Charge t Biophysical Investigations , and H. L. 
Shantz, Plant Physiologist , Alkali and Drought Resistant Plant Investigations, 
Bureau of Plant Industry 1 
INTRODUCTION 
The great differences exhibited by various plants in water require¬ 
ment—i. e., in the water transpired in the production of a unit of dry 
matter—are of profound economic importance in the agricultural devel¬ 
opment of regions of limited rainfall, and an understanding of what 
gives rise to the greater efficiency which some plants possess in the use 
of water is highly desirable in the selection and breeding of plant strains 
adapted to dry-land agriculture. This problem has led the writers to 
undertake a series of transpiration measurements with a view to deter¬ 
mining, so far as possible, the relative influence of various environmental 
factors on the transpiration of different plants. To this end simultaneous 
automatic records have been obtained of the solar-radiation intensity, 
the depression of the wet-bulb thermometer, the air temperature, the 
wind velocity, and the evaporation from a free-water surface. The 
present paper deals with the transpiration response of plants to these 
factors on clear days. 
DESCRIPTION OF APPARATUS AND METHODS 
MEASUREMENT OF TRANSPIRATION 
The transpiration measurements described in this paper were carried 
out at Akron, Colo., in 1912, 1913, and 1914 (PI. LIU). Transpiration 
was determined by weighing, four large automatic platform scales of a 
type already described (Briggs and Shantz, 1915) 2 being used in these 
measurements. The plants employed were those used in the water- 
1 The writers desire to express their indebtedness to the following men for efficient and painstaking 
assistance in connection with data presented in this paper: Messrs. F. A. Cajori, R. D. Rands, A. MacG. 
Peter, J. D. Hird, R. L. Piemeisel, P. N. Peter, H. W. Markward, G. Crawford, and H. Martin. 
2 Bibliographic citations in parentheses refer to "Literature cited," p. 648-649. 
Journal of Agricultural Research, 
Dept, of Agriculture, Washington, D. C. 
bn 
(s»3) 
Vol. V, No. 14 
Jan. 3, 1916 
G 71 
