270 



STRUCTURE OF LEAVES 



435. EPIDERMIS. The outer or epidermal cells of leaves 

 do not bear chlorophyll, but are usually so transparent that 

 the green mesophyll can be seen through them. They 

 often become very thick -walled, and are in most plants 

 devoid of all protoplasm except a thin layer lining the 

 walls, the cavities being filled with cell-sap. This sap is 

 sometimes colored, as in the under surface of begonia 

 leaves. It is not common to find more than one layer of 

 epidermal cells on each surface of a leaf. The epidermis 

 serves to retain moisture in the leaf. In desert plants 

 the epidermis as a rule is very thick and has a dense 

 cuticle. 



436. There are various outgrowths of the epidermis. 

 Hairs are the chief of these. They may be (1) simple, as 



on primula, geranium, 

 naegelia ; (2) once 

 branched, as on wall- 

 flower ; (3) com- 

 pound, as on verbas- 

 cum or mullein; (4) 

 disk-like, as on shep- 

 herdia(Fig.412); (5) 

 stellate, or star- 

 shaped, as in certain 

 crucifers. In some 

 cases the hairs are 

 glandular, as in Pri- 

 mula Sinensis and 

 certain hairs of pump- 

 kin flowers. To study 

 epidermal hairs : For this study use the leaves of the 

 plants mentioned above or others which may be substi- 

 tuted. Cross -sections may be made so as to bring hairs 

 on the edge of the sections. Or in some cases the hairs 

 may be peeled or scraped from the epidermis and placed 



412. Bisk-like or radial hairs of shepherdia. 



