THE EPIOEBMAL SYSTEM. 



137 



CHAPTER III." 



THE BPIDEKMAL SYSTEM 



Includes tlie external covering of all herbaceous growths, viz., the 

 epidermis, stomata, hairs, glands, cuticle, etc., organs which in older 

 stems give place to bark. 



676. The epidermis (skin) consists of a layer of united, empty cells, 

 mostly tabular, forming a superficial membrane. It invests all plants 

 higher than mosses, and all parts save the extremities, the stigma and 

 rootlets. Its oflBce is to check evaporation. 



584 582 



552, Cells of epidermis with a Btoma frotn leaf of Helleborus fcetldus. 583, Vertical section 

 of a stoma of Narcissus; a, cuticle. 584, Epidermis cells with stomata ofTradescantia Vir- 

 g^nlca. 



611. Example. That delicate membrane which may be easily stripped off from 

 the leaf of the houseleek or the garden iris is the epidermis. It is transparent, color- 

 less, and under the microscope reveals its cellular structure. 



678. 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 understood) as to open in 

 a moist atmosphere and close in a dry. 



679. Position of stomata. The stomata are always placed over and communi- 

 tate with the intercellular passages. 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 leaf 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 Hy- 

 drangea, 160,000. 



