10 



TRANSPIRATION IN A DESERT PERENNIAL. 



The details of the platform are shown in B of fig. 2. The groove seen there 

 was filled with mercury and thus an air-tight joint was made possible. In 

 the earlier experiments this groove was not used, but the bell-jar was sealed 

 to the platform with plastocene. The mercury seal proved to be a great 

 improvement with respect to speed of operation and the certainty of 

 securing an air-tight joint. The surface of the platform was made non- 

 absorbent by some 15 coats of shellac. The part of the plant to be used 

 was led into the platform through a small hole in a rubber stopper which 

 fitted tightly into the circular hole. The stopper was flush with the bottom 

 of the groove, so that mercury could pass over the joint. 



FIG. 2. A. Apparatus used for measuring transpiration in situ. 

 B. Details of the base used in A. 

 C and D. Forms of atmometers used in A. 



The dew-point apparatus was made for the purpose and is a modification 

 of Allouard's form. It consists of a metal cube 2 cm. on a side, one face of 

 which is accurately planed, nickeled, and highly polished. Close to, but insu- 

 lated from this face, is a polished metal flange for comparison. Into the 

 metal cube is fastened a glass tube of about 1.5 cm. diameter, which passes 

 through a rubber stopper in the top of the jar, the tube being long enough so 

 that the metal cube may be placed at any desired height within the jar. 

 From the top of the jar project a 0.1 degree certified thermometer and the 

 tubes necessary for the entrance and exit of ether. The ether was con- 

 ducted through a rubber tube about 15 feet long before it was allowed to 

 escape into the air. In the experiments carried on in 1911 ether was 



