26 



A TEXT-BOOK OF BOTANY 



FIG. 21. -^-Section through the leaf of bush clover, 

 showing upper and lower epidermis, palisade 

 cells, and cells of the spongy tissue; the lower 

 epidermis produces numerous simple hairs that 

 bend sharply and lie along the surface of the 

 leaf. 



(3) The cuticle, which is often developed upon the 

 epidermis, is one of the best protections against loss of 

 water. It is developed by the exposed walls of the epi- 

 dermal cells, and being constantly renewed from beneath it 

 may become very thick and many-layered (Fig. 19). Some- 

 times the cuticle be- 

 comes so thick that the 

 passageways through 

 it to the stomata re- 

 semble tubes (Fig. 20) . 

 In dry regions, or in 

 any much exposed 

 place, the cuticle is a 

 very constant feature 

 of plants. 



(4) HairS in great variety are developed upon leaf 

 surfaces, being outgrowths from the epidermal cells. They 

 may form only a slightly downy covering (Fig. 21), or the 

 leaf may be covered by a woolly 



or felt-like mass so that the epi- 

 dermis is entirely concealed, as 

 in the common mullein (Fig. 22). 

 In dry or cold regions the hairy 

 covering of leaves is very notice- 

 able, often giving them a brilliant 

 silky white or bronze look. Some- 

 times instead of hairs the epi- 

 dermis develops scales of various 

 patterns (Fig. 23), often overlap- 

 ping and forming a complete 

 covering. The great variety of 

 these hairs and scales, and the 

 ease with which they may be ex- 

 amined, make them an attractive study. At the same 

 time, just how they protect the leaves is by no means 



FIG. 22. Branching hair of 

 mullein. 



