58 TRANSPIRATION IN A DESERT PERENNIAL. 
makes the experiment inconclusive in this respect. Indeed, if the cause of 
the dip is connected with the rate of production of metabolic water, the 
presence or absence of sunlight would be a disturbing factor. Consequently 
another experiment was planned in which the same plant would run under 
natural conditions of low humidity and sunlight at one time, and at another 
time in the sunlight, but under conditions of high humidity. 
METHOD. 
Although potted plants have been seen to show the dip much less pro- 
nouncedly than trees, nevertheless the impossibility of getting the entire 
tree under controlled conditions made the use of potted plants:seem neces- 
sary; so in September 1912 another attempt was made to obtain trans- 
planted seedlings. A dozen or so plants, varying in age from two to fifteen 
years, were very carefully removed from the soil and transferred to pots. 
These were placed in the green-house, where they remained until March 1, 
when the two surviving ones were transferred to the roof of the laboratory 
and there left until the time of the following experiment. 
Aluminum shells of the type devised by Ganong* were used to seal the 
pots. The tops of the pots were first sealed with plastocene, over which 
was fastened a rubber sheet, according to Ganong’s instructions. Since the 
plants lose only a small amount of water per hour, the aluminum shells 
with their rubber tops made the danger of error from dust particles much 
less. Before each weighing the shells and rubber were dusted off with a 
camels-hair brush. The balances used were sensitive to milligrams, but 
readings were taken only to the nearest 10 mg. 
EXPERIMENTATION. 
EXPERIMENT XXI. 
The subjects of this experiment were two potted plants, Nos. 6 and 7, 
whose previous history is given in a former paragraph. The plants were 
sealed on the night of April 1. Plant No. 7 was given water immediately 
before being sealed, and No. 6 had received no water for two days. 
On April 2 the two plants with two atmometers were placed in an exposed 
position near the laboratory and the hourly rate of transpiration was meas- 
ured. Each plant with its atmometer was carried into the laboratory for 
the weighings, care being taken each time to return plant and atmometer 
to their original positions in the open. During the entire morning the sky 
was partly cloudy and the sun was intermittently obscured. The wind 
velocity was extremely high all day. The results appear in table 24, where 
the numbers given are the actual losses from the plants, since the area was 
not obtained. An examination of these results shows that No. 6 had an 
early drop in both actual and relative transpiration, which was followed by 
*Bot. Gaz., 41, 212, 1906. 
