TKANSPIRATION 



89 



consists in placing the end of a cut branch in a small glass 

 vessel, preferably a I) -tube, filled with water, as shown in 

 fig. 63. The branch passes through the cork of the vessel in 

 such a way as to prevent any escape or evaporation of 

 water at that point. Communicating with the other arm 

 of the U -tube is a side tube bent at right angles, which dips 

 into the water through a perforated cork. This tube is 

 also filled with water. As transpiration proceeds the water 

 is gradually drawn from the horizontal tube, and its pro- 

 gress can be noted by arranging a scale behind it. The 



FIG. 63. APPARATUS TO DEMONSTRATE TRANSPIRATION OP A BRANCH. 



stem or branch should be kept with its cut end immersed 

 in water for several hours before being placed in the 

 apparatus, as its vessels contain air at a negative pressure 

 when it is cut, owing to the transpiration which has been 

 taking place from it before its separation from the plant. 

 The existence of this negative pressure will lead to an 

 immediate absorption of water, which might be mistaken 

 for an active transpiration. 



The evaporation takes place to a certain extent through 

 all the epidermal cells of the transpiring organ, but not to 



