ELIMINATION 



185 



is sealed but provided with a small lateral air-opening readily closed by 

 a sliding piece of rubber tubing. Third is the reservoir connecting with 

 the other parts through a stop-cock ; it is made removable to permit use of 

 the instrument with a supported bell jar as shown by figure 50. Fourth is 

 a firm wooden base. To use the instrument one first closes the air-opening 

 of the record tube and then fills the reservoir and shoot chamber to the rim 

 of the latter with water boiled to expel the air. The selected shoot is now 



Fig. 48. — Normal potometee; Xi- 



Explanation in text. 



cut under water and pressed sidewise into a bored-and-slit soft rubber 

 stopper well filled with soft (stop-cock) wax; the stopper, with lower end 

 of the shoot projecting, is now pushed into the chamber neck (ground to 

 give a better grip) firmly enough to make a water-tight joint, but no more. 

 The reservoir is then filled to near the top, a film of oil is added to check 

 evaporation, the air-opening is opened to permit the record tube to fill, the 

 stop-cock is closed, and all is ready for use. At once the transpiration makes 



