80 



VEGETABLE PHYSIOLOGY 



varied from time to time according to the conditions of 

 the plant. The ultimate escape of the watery vapour from 

 the interior of the plant is subject by means of these stomata 

 to a very delicate regulation. So long as the apertures 

 are open the watery vapour diffuses outwards into the 

 external air. We may thus have a copious exhalation 

 taking place from the surfaces of the leaves and other 



FIG. 61. THREE STOMATA ON THE LOWER SURFACE OF A LEAF, 

 DIFFERENT DEGREES OF CLOSURE. 



green parts, which plays an important part in causing the 

 flow of water through the plant. This evaporation or 

 exhalation from the surface is known as transpiration ; it 

 will be discussed more fully in a subsequent chapter. 



Little or no evaporation takes place from the surface 

 of the epidermal cells of the leaves, which have their outer 

 walls generally cuticularised to a greater or less extent, the 

 cuticle offering considerable resistance to the passage of 

 water or watery vapour through them in either direction. 



The escape of watery vapour by transpiration is supple- 

 mented in some cases by an actual excretion of water in 



