TKANSPIRATION 



103 



air around the transpiring organs has, no doubt, a consider- 

 able influence upon the removal of the watery vapour from 

 their intercellular passages. 



The effect of alteration of the external conditions upon 

 transpiration may be investigated by means of Darwin's 

 potometer, which enables ap- 

 proximately accurate de- 

 terminations of its amount 

 to be made from time to 

 time. This instrument is 

 shown in fig. 68. It consists 

 of a glass tube with a side 

 arm which is bent upwards 

 so as to be parallel with the 

 tube itself. A capillary tube 

 of about 0'2 mm. bore is fas- 

 tened by an indiarubber cork 

 into the lower opening of the 

 tube so as just to project 

 beyond the cork. A con- 

 venient length of the capil- 

 lary tube is about 20 cm. 

 Its lower end dips into a 

 small vessel of water, ar- 

 ranged so as to be easily 

 withdrawn from the tube. 

 The upper orifice of the poto- 

 meter is closed by a tightly 

 fitting cork, and the plant 

 whose transpiration is to be 



observed is fitted into the side arm by means of an india- 

 rubber band or tube which embraces the glass arm and the 

 end of the cut branch so as to make a water-tight con- 

 nection. The whole apparatus must be filled with water, and 

 care must be taken that no escape of liquid can take place 

 at any of the junctions. Any air that finds its way into 

 the instrument during the arrangement of the branch in 



FIG. 68. THE POTOMJBTER. 



