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 

 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 



FIG. 68. THE POTOMETEK. 



