118 VEGETABLE PHYSIOLOGY 
The variations in the width of the stomatal apertures 
which are of so much importance in the regulation of tran- 
spiration must be regarded as bearing upon that function 
alone, being caused by fluctuations in the amount of water 
in the plant. They serve automatically to preserve the 
plant from excessive loss of water, but they have no direct 
Fia. 80,—APPARATUS TO SHOW CoNTINUITY OF INTERCELLULAR 
Spaces tv tar Lear. (After Detmer.) 
regulating influence upon the interchange of gases. Indeed, 
when, from flaccidity of the leaves or from other causes, 
they close, the aeration of the plant is, to a certain extent, 
interfered with, if not suspended—a consideration which 
will help us to understand why a plant needs to contain so 
large a reservoir of air as is afforded by its intercellular 
spaces. The volume of this reservoir varies considerably 
