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 hearing 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 



FIG. 80. APPARATUS TO SHOW CONTINUITY OF INTERCELLULAR 

 SPACES IN THE LEAF. (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 



