106 PHARMACEUTICAL BOTANY 



EPIDERMIS 



Epidermis is the outer covering tissue of a plant and is protective 

 in function. Its cells may be brick-shaped, polygonal, equilateral or 

 wavy in outline. Their outer walls are frequently cutinized (infil- 

 trated with a waxy-like substance called cutin) . Among the epider- 

 mal cells of leaves and young green stems may be found numerous 

 pores or stomata (sing, stoma) surrounded by pairs of crescent- 



Pic. 44. Upper epidermis of Comptonia asplenifolia leaf (surface view) show- 

 ing epidermal cells and two non-glandular trichomes. 



shaped cells, called guard cells. The stomata are in direct commun- 

 ication with air chambers beneath them which in turn are in 

 communication with intercellular spaces of the tissue beneath. The 

 function of the stomata is to give off watery vapor and take in or 

 give off carbon dioxide, water and oxygen. In addition to stomata 

 some leaves possess groups of water stomata which differ from trans- 

 piration stomata in that they always remain open, are circular in out- 

 line, give off water in droplets directly, and lie over a quantity of 

 small-celled glandular material which is in connection with one or 

 more fibro- vascular bundles. Examples: Leaves of Crassula, Saxi- 

 Jraga and Ficus. 



