SECTION 1G.] 



ANATiiMV OF LK.W IS. 



143 



greeu pulp, ami i> nearly the Bame aa the green layer of the bark. 80 that 

 the Leaf maj properly enough be regarded as a Borl of expansion of the 

 fibrous and green layers of the bark. It has no proper corky layer; but 

 the whole is covered by a transparent skin or epidermis, resembling that 



of t ho stem. 



1 tO. The cells of the leaf are of various forms, rarely so compact as to 



form a elo.se eellular tissue, usualU loosely arranged, at least in the lower 



part, so as to give copious intervening spaces or air passages, communi- 

 cating throughout the whole interior (Fig. 443, 4S3). The green color is 

 given by the chlorophyll (417), seen through the very transparent walls of 

 the cells and through the translucent epidermis of the leaf. 



441. In ordinary leaves, having an upper and under surface, the green 

 cells form two distinct strata, of different arrangement. Those of t he 

 upper stratum arc oblong or cylindrical, and stand endwise to the surface 

 of the leaf, usually close together, leaving hardly any vacant spaces; those 

 of the lower are commonly irregular in shape, most of them with their 

 longer diameter parallel to the face of the leaf, and are very loosely ar- 

 ranged, leaving many and wide air-chambers. The green color of the 

 lower is therefore diluted, and paler than that of the upper face of the leaf. 

 The upper part of the leaf is so constructed as to bear the direct action 



of the sunshine ; the lower so as to afford freer circulation of air, and to 

 facilitate transpiration. It communicates more directly than the upper 

 with the external air by means of StomaUt. 



442. The Epidermis or skin of leaves and all young shoots is best 

 seen in the foliage. It may readily lie .stripped off from the surface of a 

 Lily-leaf, and still more so fn lore fleshy and soft leaves, such as those 



Pio. 188. Magnified section of a leaf <>f White Lily, to exhibit the cellular 

 Btructare, both of apper and Lower stratum, the air-paaaagea of the lower, and 

 the epidermis or skin, in section, ulso a little of that of the lower face, with some 

 of its stomutes. 



