TRANSPIRATION 89 
consists in placing the end of a cut branch in a small glass 
vessel, preferably a U-tube, filled with water, as shown in 
fig. 68. 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 
Fic, 63.—APPARATUS TO DEMONSTRATE TRANSPIRATION OF 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 
