CH. VII] TRANSPIRATION. 105 



power of some sort in the branch and leaves. The bark 

 will not be coloured, and if the branch be peeled, the 

 contrast between the white inside of the peel and the 

 wood stained pink with eosin is striking. The fact that 

 the fluid travels in the xylem is still better seen in a 

 young succulent stem where, on splitting the plant, the 

 vessels filled with eosin show out as delicate pink streaks. 



There are various arrangements by which we can 

 accurately measure, from minute to minute, the amount of 

 water which a cut branch is absorbing. With an instru- 

 ment of this sort it is easy to prove that the amount of 

 absorption depends on the amount of evaporation going on 

 from the leaves. Thus if the leaves are placed under a 

 bell jar, and therefore in damper air, the instrument 

 records the fact that the absorption is less, and the 

 readings quickly rise again when the bell is removed. 

 The absorption may be diminished by cutting off some of 

 the leaves and thus diminishing the total amount of evapo- 

 ration. In this sort of way it can be shown that the 

 water-supply of a leaf by the vessels is a self-regulating 

 mechanism ; that rapid evaporation increases the upward 

 current, so that the greater the loss the greater is the 

 supply. 



In thinking about the transpiration from a leaf surface 

 it must be remembered that the evaporating surface is 

 much greater than the surface which is visible, because 

 the spaces in the spongy tissue communicate with the 

 outer air through the stomata, so that the surface of each 

 constituent cell of the spongy tissue evaporates, not so 

 much as though it made part of the external surface, but 



