30 TRANSPIRATION IN A DESERT PERENNIAL. 



experiment, having closed 2 hours before the first reading. In this experi- 

 ment, E represents the actual loss per hour from the atmometers multiplied 

 by their proper coefficients. Thus, only the shape of the curves may be 

 compared with the other experiments, and not the amplitudes. The sun 

 was uniformly bright until 2'' 30 p. m., after which partial cloudiness began 

 and continued for the rest of the day. 



Immediately after the 3 o'clock reading the percentage of moisture of 

 the soil in the pots was determined. Small glass vials were filled with 

 samples of the soil b}^ forcing them into the pots, two samples being taken 

 from each pot, one slightly below the surface and the other about two-thirds 

 of the way to the bottom. The vials Avere quickly stoppered and weighed, 

 and later were dried to constant weight at about 1 10 C. 



The two plants were then unsealed and left without water until August 5, 

 when the hourly transpiration rate was again determined. During the 

 interval no visible change in area had occurred, so that the relative transpi- 

 ration rates for July 27 and August 5 are comparable, although the areas 

 were not obtained. On August 5 the leaves on both plants were slightly 

 curled and remained nearly closed continuously. After the close of the 

 readings for August 5 the percentage of soil moisture was again determined. 

 Both plants were given water at 6 p. m., and by 9 p. m. all the leaves of 

 No. 1 had resumed their normal appearance and within three days the plant 

 had begun to show growth. The leaves of No. 2 did not assume their 

 normal appearance, but dropped off the next day; however, by August 9 

 new leaf-buds were visible on No. 2. Both plants continued to grow 

 normally after this. 



Experiments XI to XIII. 



The subject of these experiments was an adult tree situated south of the 

 laboratory. Three separate branches (/, area 28.72 sq. cm. ; g, area 30.02 sq. 

 cm., and A, area 31.82 sq. cm.) were used. The entire tree was in full leaf 

 at the beginning of the experiment, but at the close some leaflets had begun 

 to fall and several neighboring trees had lost most of their leaflets. As 

 mentioned above, the methods used in these experiments differ from those 

 used in the bell-jar experiments of 1911 in the following points: (1) the 

 evaporating surface of the atmometer was parchment paper; (2) the atmom- 

 eter was placed under the same bell-jar with the plants; (3) carbon dioxide 

 suddenly released from high pressure was used to cool the dew-point 

 apparatus. 



