82 THE OAK 
minute gap between them, and the gap communicates 
with the intercellular air-cavities between the cells of 
the spongy mesophyll (fig. 22, st). If we remove a piece 
of this epidermis, and look at it as laid flat (instead of in 
section) under the microscope, we find that these pairs 
of small cells are shaped somewhat like a small mouth, 
the two curved lips of which are formed by the two cells 
just mentioned, and the orifice of which is the gap just 
referred to (fig. 23). These two lips are called the guard- 
cells, and the whole apparatus is termed a stoma. It is 
necessary to realise two great facts about these stomata 
on the under surface of the leaf: firstly, there are several 
hundreds of thousands of them on an oak-leaf, each 
square millimetre having from 300 to 350 of them 
scattered over it; and, secondly, each one can open or 
close its little aperture by the approximation or divarica- 
tion of the inner concave sides of the curved guard-cells. 
If this is clear it will be readily understood that 
these stomata can regulate the amount of water passing 
off by evaporation from the walls of the millions of 
cells of the mesophyll, especially if the further fact is 
borne in mind that water-vapour scarcely passes at all 
through the close-fitting epidermis cells themselves. 
We are now in a position to form a sort of picture 
of the mechanism of the shoot and root in regard to this 
matter. The root-hairs absorb water from the soil, and 
in this water there are dissolved small quantities of the 
soluble salts of the earth—chiefly sulphates, nitrates, 
and phosphates of lime, magnesia, and potash—just as 
