CH. IV] POTOMETER. 81 



force the branch 1 into the rubber tube C, as far as it will 

 go. The joint between the rubber tube and the branch 

 must be secured by tying ; for this it is best to use strong 

 uncovered elastic thread, which must be stretched while 

 it is being wrapped round the tube, and can be secured 

 by a simple tie, a complete knot being unnecessary. The 

 rubber tube may be secured to the glass tube with wire. 

 Turn the potometer upside down so that any air in C 

 may rise and collect at A, and before corking A, fill it to 

 the brim with water. Support the potometer on a firm 

 retort-stand and fix the plant to the same stand to avoid 

 any possible movement between the plant and the in- 

 strument. The end of the capillary tube dips into a 

 small vessel of water W supported on two blocks, of which 

 the upper one is small enough to be conveniently seized 

 in one hand, and of such a height that when it is removed 

 the capillary tube no longer dips in the water. When 

 this is done, (if the plant is absorbing vigorously) a 

 column of air will be sucked 2 in at the lower end of the 



1 If herbaceous plants, or woody plants with delicate leaves, are used 

 for transpiration experiments it is necessary to cut them from the 

 parent plant under water, to prevent the entrance of air, which rushes in 

 to satisfy the negative pressure. It is generally possible to force a branch 

 below the water in a basin held by an assistant, and divide it with a 

 strong pair of gardeners' shears. In the case of herbaceous plants the 

 process is made easier if a couple of sharp bends are made in the stem, a 

 proceeding which need not admit air, and has no effect in the transpira- 

 tion current in the plant. For all research- work connected with the 

 transmission of water the specimens should be thus treated, but for the 

 following experiments, in which laurel or Portugal laurel (Prunus lauro- 

 cerasus and lusitanica) are used, cutting under water is not necessary. 



2 To hasten the entrance of the air column, it is best to absorb, with 

 a piece of filter-paper, the water hanging at the end of the tube. 



D. A. 6 



