FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 43 



best protective substances (see Fig. 

 32). Sometimes this cuticle be- 

 comes so thick that the passage- 

 ways through it leading down to 

 the stomata become regular canals 

 (see Fig. 33). 



Another very common protective 

 structure upon leaves is to be found 

 in the great variety of hairs de- 

 veloped by the epidermis. These 

 may form but a slightly downy 

 covering, or the leaf may be cov- 

 ered by a woolly or felt-like mass 

 so that the epidermis is entirely 

 concealed. The common mullein 

 is a good illustration of a felt- 

 covered leaf (see Fig. 30). In cold 

 or dry regions the hairy covering 

 of leaves is very noticeable, often 

 giving them a brilliant silky white or bronze look (see 

 Figs. 34, 35). Sometimes, instead of a hair-like cover- 

 ing, the epidermis develops scales of various patterns, 

 often overlapping, and forming an excellent protection 

 (see Fig. 37). In all these cases it should be remembered 

 that these hairs and scales may serve other purposes also, 

 as well as that of protection. 



36. Diminution 

 of exposed surface. — 

 it will be impossible 

 to give more than a 

 few illustrations of 

 this large subject. 

 In very dry regions 

 it has always been 

 noticed that the 

 leaves are small and 



Fig. 34. A hair from the leaf 

 of Potentilla. It is seen 

 to grow out from the epi- 

 dermis. 



Fig. 35. A section through the leaf of bush clover 

 ; I.tsjh deza), show ing upper and lower epidermiB, 

 palisade cells, and cells of the spongy region. 

 The lower epidermis produces numerous hairs 

 which bend sharply and lie along the leaf surface 

 (appressed), forming a close covering. 



