64 PLANT LIFE 



same way it would preclude the entrance oi 

 oxygen and carbon dioxide, at any rate in 

 sufficient quantity. But as a matter of fact 

 it is not continuous. There are immense 

 numbers of minute gaps in the epidermis, 

 termed stomata (Fig. 9, S), and these form 

 the external orifices of an extensive system of 

 air spaces which are present between the cells 

 of which the leaf is composed. These inter- 

 cellular spaces are of the utmost importance 

 to the leaf, inasmuch as it is by means of them 

 that gaseous exchange between the cells and 

 the atmosphere is rendered possible. 



Each pore or stoma is really a slit formed 

 between two sausage-shaped cells of the 

 epidermis, and these two guard cells, as they 

 are called, can change their shape according 

 as they become more or less distended with 

 water. When they are distended, or turgid, 

 the aperture between them becomes wider, as 

 they lose water the pore tends to close. We 

 see then that the leaf, as regards water, is a 

 beautifully self-regulated mechanism. When 

 a plentiful supply is available the opening of 

 the stomata enables the vapour which satu- 

 rates the air in the intercellular spaces to 

 diffuse out ; but when the supplies fall short 

 the loss is avoided by the closing together of 

 the guard cells. Other things being equal, it 

 is advantageous that water should be abun- 

 dantly available, as in this way mineral salts 

 are brought to the leaves. A relatively rapid 

 flow to these organs, however, only takes place 



