LESSON 25.] AND LEAVES. 155 



They serve not only to strengthen the leaf, but also to bring in the 

 ascending sap, and to distribute it by the veinlets throughout every 

 part. The cellular portion is the green pulp, and is nearly the same 

 as the green layer of the bark. So that the leaf may properly 

 enough be regarded as a sort of expansion of the fibrous and green 

 layers of the bark. It has of course no corky layer ; but the whole 

 is covered by a transparent skin or epidermis, resembling that of 

 the stem. 



438. The green pulp consists of cells of various forms, usually 

 loosely arranged, so as to leave many irregular spaces, or air-pas- 

 sages, communicating with each other throughout the whole interior 

 of the leaf (Fig. 356). The green color is owing to a peculiar 

 green matter lying loose in the - cells, in form of minute grains, 



named Chlorophyll (i. e. the green of "=^1 -, ,, ... ir-jr-w ' 



leaves). It is this substance, seen 



through the transparent walls of the 

 cells where it is accumulated, which 

 gives the common green hue to vege- 

 tation, and especially to foliage. 



439. The green pulp in most leaves 

 forms two principal layers ; an upper 

 one, facing the sky, and an under one, 

 facing the ground. The upper one is 



always deeper green in color than the lower. This is partly owing, 

 perhaps, to a greater amount of chlorophyll in the upper cells, but 

 mainly to the more compact arrangement of these cells. As is seen 

 in Fig. 356 and 357, the cells of the upper side are oblong or cylin- 

 drical, and stand endwise to the surface of the leaf, usually close to- 

 gether, leaving hardly any vacant spaces. Those of the lower part 

 of the leaf are apt to be irregular in shape, most of them with their 

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

 arranged, leaving many and wide air-chambers. The green color 

 underneath is therefore diluted and paler. 



440. In many plants which grow where they are subject to 

 drought, and which hold -their leaves during the dry season (the 

 Oleander for example), the greater part of the thickness of the leaf 

 consists of layers of long cells, placed endwise and very much com- 



FIG. 356. Section through the thickness of a leaf of the Star- Anise (Illiciuni), of Florida, 

 magnified. The upper and the lower layers of thick-walled and empty cells represent the 

 epidermis or skin. All those between are cells of the green pulp, containing grains of 

 chlorophyll. 



