THE TRANSPORT OF WATER IN THE PLANT 75 
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 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 
Fie. 61 —THREE StomaTA on THE LowER SuRFACE OF A LEAF, SHOWING 
DIFFERENT DEGREES OF CLOSURE. 
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 
the liquid form. This happens when the hydrostatic pres- 
sure is very high at times in herbaceous plants, water being 
forced out at the tips of the leaves. It is not infrequently 
