64 PLANT LIFE 
same way it would preclude the entrance of 
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 
