518 



PHYSIOLOGY. 



dermis persists. Such an epidermis clothes all the organs of plants 

 above the Class of Mosses ; and it presents this simple general 

 character on all young structures, with one special distinction 

 only, that on submerged organs and on roots it is absolutely con- 

 tinuous and impervious ; while on parts exposed to the air it pre- 

 sents more or less numerous orifices guarded by a peculiar cellular 

 structure called a stoma (fig. 565, a). 



Stomata. The stomata are orifices between the meeting angles 

 of the epidermal cells (fig. 566, B), in which orifices lie, rather to 

 the underside, a pair of cells of semilunar form (fig. 566, A, C), 

 separate on their adjacent sides, so that in expansion and contrac- 

 tion they close and open a slit-like passage beneath the superficial 

 orifice. This slit (fig. 566, A, s) leads to an open intercellular 

 space within the substance of the leaf. 



Fig. 566. 



Fig. 565. 



Fig. 565. Epidermis of the lower surface of the leaf of Helleborus foetidus; a, stoma, Magn. 



200 diam. 



Fig. 566, Stomata of the leaf of Narcissus Pseudo-Narcissus. A. Vertical section of the epi- 

 dermal and subjacent cells, passing through a stoma, s: c, cuticular pellicle ex- 

 tending down into the stomatal cavity. B & C. Horizontal section of the epi- 

 dermis, passing through the plane of x in A : B, seen from above ; C, seen from 

 below ; d, smaller epidermal cells corresponding in position to the stomata, but 

 remaining in their original condition. Magn. 200 diam. 



In Neriurn the stomata are on the walls of pits or depressions on the 

 under face of the leaf. Sometimes the stoma is formed of four cells, and 



