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, sf). 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 



