82 THE OAK. 



a 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 (in- 

 stead 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 realize 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 divarication 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-vapor 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, 



