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. 



