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 



FIG. 61. THREE STOMATA ON THE LOWEB 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 



