WATER-ABSORPTION AND TRANSPIRATION. 



117 



a bent tube (6) annealed to it about half way. The object of the 

 branched tube is to prevent the accumulation of air-bubbles below the 

 cut end of the shoot at a. Any air-bubbles passing up the narrow 

 tube (inserted through a cork at the lower end of the straight limb) will 

 usually collect at b, and the cork at b is then removed and water 

 poured in. If air collects at a, simply turn the apparatus round so 

 that the straight limb lies horizontally above the side-limb, when the 

 air passes up to a. 



Pig. 42. Potometer Apparatus. 



(/) Fix up the apparatus shown in Fig. 42 B, and take readings as in 

 the preceding experiment. From the inside diameter of the narrow 

 tube you can easily calculate the volume of water sucked along in a 

 given time by reading from the cardboard scale and multiplying the 

 tube-diameter by itself, by 0'8 (strictly 0'785), and by the length. 



(g) Fig. 42 c shows an improvement on the ordinary potometer, con- 

 sisting of a funnel connected with the potometer-tube by a piece of 

 rubber tubing. A clip is kept on the latter, the funnel filled with 



