CHAPTER XL 



STRUCTURE OF LEAVES 



502. Besides the framework or system of veins found 

 in blades of all leaves, there is a soft tissue (468) called 

 mesophyll or leaf-parenchyma, and an epidermis that covers 

 the entire outside part. 



503. Mesophyll. The mesophyll is not all alike or homo- 

 geneous. The upper layer of it is composed of elongated 

 cells placed perpendicular to the surface of the leaf. These 

 are called palisade cells. The chlorophyll grains are most 

 abundant in them, because they are on the side of the leaf 

 most directly exposed to the sunlight. Below the palisade 

 cells is the spongy parenchyma, composed of cells more or 

 less spherical in shape, irregularly arranged, and provided 

 with many intercellular air cavities. (Fig. 468; also Fig. 

 137.) In leaves of some plants exposed to strong light 

 there may be more than 



one layer of palisade 

 cells, as in the India- 

 rubber plant and ole- 

 ander. Ivy, when grown 

 in bright light, will de- 

 velop two such layers of 

 cells, but in shaded places 

 it may be found as in 

 Fig. 468. Such plants as 

 iris and compass plant, 

 which have both surfaces of the leaf equally exposed to sun- 

 light, usually have a palisade layer beneath each epidermis. 



504. Epidermis. The outer or epidermal cells of leaves 



(297) 



468. Cross-section of ivy leaf, which grew in 

 shade and has only one layer of palisade 

 cells, u, upper epidermis; p, palisade cells; 

 c, a crystal; sp, spongy parenchyma; f, in- 

 tercellular space; I, lower epidermis. The 

 plant here intended is the true or English 

 ivy, Hedera helix. 



