98 VEGETABLE PHYSIOLOGY 
branch so as to make a water-tight connection. 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 its position can 
be removed by causing it to collect at the upper portion 
of the straight tube of 
the potometer. To take an 
observation of the rate of 
transpiration of the branch, 
a bubble of air must be 
admitted into the capil- 
lary tube by momentarily 
removing the vessel into 
which it dips, and replac- 
ing it as soon as the tran- 
spiration has caused the 
air to enter. The bubble 
of air must be of uniform 
size in successive readings, 
to ensure that the latter 
shall be strictly compar- 
able with each other. The 
bubble will rise in the 
tube, and finally make its 
way to the upper part of 
| a the straight limb of the 
instrument, the rate at 
which it travels serving as 
an index of the rate of the 
transpiration. The capillary tube should be marked 
by a transverse line a few millimetres from its lower end, 
and by means of a stop-watch the time taken by the 
bubble to rise from this mark to the free end of the tube 
should be observed. The branch may be covered by a 
bell-jar, so that the variations of temperature, moisture, 
&c. of the air surrounding it can be controlled during a 
sf 
Fic. 68.—Tun PoTromMeter. 
