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 
Hig. 61.—Turee Sromata ox THE Lower Survace or A Lear, sHow1ne 
DurvERENT DEGREES oF CLOSURE. 
green parts, which plays an important part in causing the 
flow of water through the plant. This cyaporation 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 offerimg 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 
