80 



The Plant World. 



After attempts in various directions, an arrangement has 

 been devised which obviates the whole difficulty by preventing 

 the entrance of water from the cup into the reservoir, while move- 

 ment in the opposite direction may proceed as usual. The ap- 

 paratus is extremely simple and adds but little to the trouble of 

 operating the instrument. A diagram of the arrangement of 



parts is shown in Fig. 1. From 



the porous clay cup two glass 

 tubes (A and B) extend down- 

 ward, one of them reaching from 

 the tip of the cup, the other 

 only from the upper surface of 

 the rubber stopper. These are 

 slightly offset laterally so as to 

 allow the other apparatus to be 

 easily attached. They are of 

 small bore, about 2 or 3 mm., 

 as are also the tubes mentioned 

 below. Each of these tubes 

 passes through a 2-perforate 

 rubber stopper into a shell vial 

 (C, D) about 2.5 cm. in diameter 

 and two or three times as high. 

 Tube B reaches the bottom of 

 the vial, tube A merely penet- 

 rates through the stopper. 

 Through the remaining perfora- 

 tion in each stopper extends 

 a second tube (E, F) bent first 

 laterally about 5 cm. and then 

 ma. 1 Rain-correcting atmometer. dowuward. Tubc E rcachcs the 



bottom of the vial and the bottom of the reservoir G, while tube 

 F reaches only through the vial stopper and extends about 10 cm. 

 below the bend at its free end. Tube E is supplied with a loosely 

 fitting cork stopper, which can be slipped upward or downward 

 upon the tube. The reservoir G is treated in the same manner 

 as heretofore, a file mark on its narrow neck serving as a nul- 

 point in filling. In place of the bottle a graduated cylinder or 

 a burette may be used, but it is desirable that the reservoir be 



