476 The Botanical Gazette. [November, 
checked, being only about one-tenth of a gram in ten min- 
utes. t 2:30 P. M. the screen was removed. The intensity 
of the light was much lower than from 12 to 1:25, but greater 
than what the plant had been exposed to behind the screen. 
The increase in loss is proportional to the increased intensity 
of the light. The total loss from 12 M. to 6 P. M. is seen to 
be 12.1 from 6 P. M. to 12 midnight nine-tenths of a gram, 
12 midnightto 6 A. M. 1™, 6 A. M. to 12 M. 9, 12 M. to 6 
Po Moo. 5". 
March 16th was clear, the 17th two-tenths cloudy, the 18th 
eight-tenths cloudy. The total amount lost for the whole 
period was 39. This brief explanation in connection with 
the record will show how easily and accurately comparisons 
may be made for any period of time or the total evaporation 
determined at any time during the experiment. With auto- 
matic devices for recording temperature, humidity, intensity 
of light, and barometric pressure, it will be possible to obtain 
data on the much talked about but little understood prob- 
lems of transpiration. The instrument can be very easily 
modified so that it will record either gain or loss in weight. 
With the help of Prof. Marvin we hope to simplify it and in- 
crease its range of usefulness in physiological work. Plate 
XXX is reproduced from a photograph of the complete appa- 
ratus as at present used. 
Division of Vegetable Physiology and Pathology, 
U. S. Department of Agriculture, Washington, D. C. 
