TRANSPIRATION 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 ag 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withdrawn from the tube. 5 
The upper orifice of the poto- U 
meter is closed by a tightly 
fitting cork, and the plant Fie, 68.—Taz Poromerzn. 
whose transpiration is to be 
observed ig 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 
