CHAPTER XII 



LEAVES — STRUCTURE OR ANATOMY 



Besides the framework, or system of veins found in 

 blades of all leaves, there is a soft cellular tissue called 

 mesophyll, or leaf parenchyma, and an epidermis or skin 

 that covers the entire outside part. 



Mesophyll. — The mesophyll is not all alike or homoge- 

 neous. The upper layer is composed of elongated cells 

 placed perpendicular to the surface of the leaf. These 

 are called palisade cells. These cells are usually filled 



with green bod- 

 ies called chlo- 

 rophyll grains. 

 The grain con- 

 tains a great 

 number of chlo- 

 rophyll drops 

 imbedded in 

 the protoplasm. 

 Below the pali- ' 

 sade cells is the 

 spongy parenchyma, composed of cells more or less spher- 

 cal in shape, irregularly arranged, and provided with many 

 intercellular air cavities (Fig. 113). In leaves of some 

 plants exposed to strong light there may be more than one 

 layer of paUsade 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 



86 



Fig. 113. — Section of a Leaf, showing the air spaces. 



Breathing-pore or stoma at a. The palisade cells which chiefly 

 contain the chlorophyll are at b. Epidermal cells at c. 



