298 



STRUCTURE 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. 



505. 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) compound, as on verbascum or mullein; (4) 

 disk-like, as on shepherdia (Fig. 469); (5) stellate, or star- 

 shaped, as in certain 

 crucif ers. In some 

 cases the hairs are 

 glandular, as in Pri- 

 mula sinensis and cer- 

 tain hairs of pumpkin 

 flowers. 



506. To study epi- 

 dermal hairs: For this 

 study use the leaves 

 of the plants men- 

 tioned above or others 

 that may be substi- 

 tuted. Cross-sections 

 may be so made 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 in water 

 on a slide. Make sketch of the different kinds of hairs. 

 507. Stomates are small openings or pores in the epi- 



469. Disk-like or radial, hairs of shepherdia. 



