CHAPTER XXXV 



STRUCTURE OF LEAVES 



433. Besides the framework or system of veins found 

 in blades of all leaves, there is a soft tissue (408) called 

 mesophyll or leaf -parenchyma, and an epidermis which 

 covers the entire outside part. 



434. MESOPHYLL. The mesophyll is not all alike or 

 homogeneous. 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 com- 

 posed of cells more or less spherical in shape, irregularly 

 arranged, and provided with many intercellular air cavi- 

 ties. Fig. 411 ; also Fig. 115. 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 oleander. Ivy when 



grown in bright light 



will develop two such 



layers of cells, but in 



shaded places it may be 



found as in Fig. 411. 



Such plants as iris and 



compass plant, which 



have both surfaces of 



the leaf equally exposed to sunlight, usually have a palisade 



layer beneath each epidermis. 



411. 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 ; i, in- 

 tercellular space ; I, lower epidermis. The 

 plant here intended is the true or English 

 ivy, Hedera helix. 



