370 



A TEXT-BOOK OF BOTANY. 



the leaves are spoken of as glaucous. In other cases the outer 

 walls of the epidermal cells are modified to hairs (Figs. loo, 124, 

 125, 148^155)- 



Anatomical Differences in Leaves. — The walls of the 

 epidermal cells, although usually isddiametric, are often very- 

 zig-zag in outline. In size, the upper epidermal cells are usually 

 larger in a given species than those of the lower surface, and 

 sometimes are rather linear, resembling a palisade layer. The 



Fig. 216. A, transverse section of leaf of Lobelia inflata showing the large irregular 

 epidermal cells (e), palisade cells (p), trachea (t), loose parenchyma (m), and lower epider- 

 mis (i). B, transverse section of leaf of Matico showing oil-secretion reservoir (o), upper 

 epidermis (e), lower epidermis (1), with non-glandular hairs (h), palisade layers (p), loose 

 mesophyll (m). 



cuticle may be thin or leathery and tough, and sometimes is pro- 

 vided with minute ridges or crests, especially on the under sur- 

 face, thereby giving a dull appearance to the leaf. Many leaves, 

 too, excrete wax on the surface and consequently have a glaucous 

 or hoary appearance, notably some of the poppies, the common 

 jewel-weed and many others. Other modifications may have 

 to do with the gelatinization of the epidermis of the leaf, assisting 

 in the storage of water, as in the leaves of the violets. The 

 dififerentiation in forms of calcium oxalate crystals is also im- 



