84 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
At the beginning of the experiment the apparatus stood in the shade 
of a small pinyon tree with a fitful movement of the air at a temperature 
of 80° F. During the first few minutes of the observations in which 
equalization of the negative pressure was in progress, the time in which 
a unit (100 milligrams) of water was taken up was as follows: 40, 45, 42, 
48, 47, 50, 50, 50, 50, 50, 50 seconds, or at the rate of 2 to 2.5 milligrams 
per second. Half anhourlater,at 10" 30™ a.m.,afterthe negative pressure 
had been equalized tests were made in the open, with the sky clouded, and 
the air at a temperature of 84° F. The periods in which too milligrams 
were absorbed were 75, 70, 80, 85, 85, 90, and 95 seconds, giving a rate 
of 1.05 to 1.4 milligrams per second. With continued cloudiness and 
the air at a temperature of 88° F. beginning at 10" s0o™ a. m., the periods 
were 75, 75, 60, 70, 70, 75 seconds or at a rate of 1.3 to 1.4 milligrams 
per second. The sun emerging from the clouds the readings of 400 milli- 
grams in 150 seconds, 400 milligrams in 210 seconds, and 300 milligrams 
in 150 seconds were taken, giving an average rate of 1.9 to 2.2 milligrams 
per second. With the return of the clouds immediately afterward the 
readings were 400 milligrams in 210 seconds, 500 milligrams in 330seconds, 
goo milligrams in 600 seconds, or an average rate of 1.9 milligrams per 
second, decreasing to 1.5 per second as the effects of the cloudiness were 
felt. The rate again rose to 1.8 and 1.9 milligrams per second as the sun 
emerged from the clouds. 
Experiment 2.—Artemisia sp. was used in this test. It is alow, densely 
branching shrub with an extensive root-system of the deeply penetrating 
type. It stands nearly inactive throughout the dry season, taking on a 
quickened growth, as demonstrated by the formation of new shoots and 
reproductive organs within a month after the beginning of the July rains. 
A main branch with 30 branchlets, about 12 inches long, was fastened 
to the potometer at 9 a.m., July 16, with the air temperature 75° F. 
Readings of 700 milligrams in 17 minutes, 400 milligrams in 10 minutes 
were made with the sun obscured by clouds. In sunshine readings 
of 1,100 milligrams in 25 minutes, goo in 19 minutes, 500 in 12 minutes, 
and 500 in 16 minutes were made, with an average of 0.6 to 0.7 milligram 
per second. The total area of the surface of the branch and leaves was 
about 960 square inches. 
As a means of comparison similar tests were made with the same piece 
of apparatus on moisture-loving plants in the physiological laboratory at 
the University of Minnesota. 
Experiment 3.—A well-grown shoot of the tomato with a total surface 
of 256 square inches was fastened to the potometer in a room in diffuse 
light with a humidity of 25 to 35 per cent, about the same as in the previ- 
ous experiments, at a temperature of 79° F. Readings of 1,000 milli- 
grams in 32 minutes and 600 milligrams in 21.2 minutes were made, 
giving an average rate of about 0.5 milligram per second, and subsequent 
