PERMANENT WILTING IN PLANTS 89 



bers the evaporating power of the air was reduced to about one- 

 half of that prevaiHng just outside the chamber. One of these 

 chambers stood in the lath shelter just noted, with free circu- 

 lation of air on all sides. The outside of this chamber received 

 only that portion of full insolation and wind movement which 

 was allowed to pass the walls and roof of the lath shelter. The 

 second cloth chamber was placed in the open, near the table 

 described above, and its outside received the full natural sun- 

 light and wind. 



The lowest evaporating powers of the air experienced in these 

 wiltings were obtained in the glass-walled, glass-roofed, rec- 

 tangular box (dimensions about 1.5x1.0x0.75 meters) earlier 

 employed by Brown and by Caldwell, so placed under a small can- 

 vas shade in the lath shelter that no direct sunlight could reach 

 it, but with free air circulation about it. Within this box the 

 daily range of temperature was almost the same as that without, 

 and the relative humidity inside was maintained high and nearly 

 constant by means of water-saturated cloths suspended in the 

 chamber and covering its floor. 



For determining whether the plants were permanently wilted, 

 as in the work of Briggs and Shantz and of Caldwell, a chamber 

 containing air as nearly saturated with water vapor as possible 

 was needed. This condition was attained in a cylindrical sheet- 

 iron tank about 1 meter high and 75 cm. in diameter, tightly 

 closed above with a sheet-iron cover. A circular opening in 

 the cover, 25 cm. in diameter and closed by a glass plate, admitted 

 light and made observation possible without opening the chamber. 

 This tank stood in the thick-walled, small-windowed adobe 

 building at the experiment grounds of the Desert Laboratory, and 

 its daily temperature fluctuations were never nearly as great 

 as those in the open. At the bottom of the cylinder was exposed 

 a free water surface and the saturation deficit of the contained 

 air was always exceedingly small. The evaporation rate in this 

 chamber never exceeded 0.2 cc. per hour from the standard 

 porous cup atmometer.^ 



" Livingston, B. E., A rotating table for standardizing porous cup atmometers. 

 Plant World 15: 157-62. 1912. Earlier references are there given. 



