SECTION 10.] 



ANATOMY OF LEAVES. 



143 



green pulp, and is nearly the same as the green layer of the bark. So that 

 tlie leaf may properly enough be regarded as a sort of expansion of the 

 fibrous and green layers of the bark. It has uo proper corky layer ; but 

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

 of the stem. 



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

 form a close cellular tissue, usually 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, 483). The green color is 

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

 the cells and through tlie 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 the 

 upper stratum are 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 Ihem 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. 

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



of the sunsliine ; 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 Siomates. 



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

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

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



Fig. 483. I\Iagiiified section of a leaf of White Lily, to exliibit the cellular 

 structure, both of upper and lower stratum, the air-passages of the lov>'er, and 

 the epidermis or skin, in section, also a little of that of tlie lower face, with somo 

 of its stomates. 



