THE EPIDERMAL SYSTEM. 



131 



dermis. It is transparent, colorless, and under the microscope 

 reveals its cellular structure. 



397. Stomata. The epidermis does not entirely exclude the 

 tissues beneath it from the external air, but is cleft here and 

 there by little chinks called stomata (mouths). Each stoma is 

 guarded by a pair of reniform cells, of such mechanism (not well 

 jnderstood) as to open in a moist atmosphere and close in a dry. 



398. The stomata are always piaced over and communicate with the intercellular pas- 

 sages. They are found only on the green surfaces of parts exposed to the air, most 

 abundant on the under surface of the leaves. Their numbers are immense. On the leal 

 of garden Rhubarb 5,000 were counted in the space of a square inch ; in the garden Iris, 

 12,000 ; in the Pink, 36,000 ; in Hydrangea, 160,000. 



399. The surface of the epidermis at length becomes itself coated with a delicate, trans- 

 parent pellicle, not cellular, called the cuticle. It varies in consistency, being thicker 

 and stronger in evergreen and succulent plants. It seems to be merely the outer cell- 

 wall of the epidermis thickened and separated from the newly-formed wall beneath it. 



497 493 



4W, Cells and stomata of the epidermis of Oxalls violacea; and 496, of Convijlarhi racemooa. 



400. The hairs which clothe the epidermis are mere expan 

 Rions of its tissue. They may each consist of a single elongated 

 cell, or of a row of cells. They may also be simple, or branched, 

 >r stellate, or otherwise diversified. 



401. Glands are cellular structures serving to elaborate and 

 contain the peculiar secretions of the plant, such as aromatic 

 oils, resins, honey, poisons, etc. A gland may be merely an ex- 

 panded cell at the summit of a hair, or at its base, and hence 



