102 



ANATOMY OF STEMS. ROOTS. AND LEAVES 



The structure of the leaves varies somewhat in the different 

 plants. Probably the most conmion form is that which we will 

 describe. The typical leaf may be said to be composed of four 

 layers of parenchyma cells supported by delicate frame work 

 of fibro-vascular bundles. The upper layer of cells is the upper 

 epidermis and consists of thick-walled, transparent cells ; below 

 this is the layer of columnar or palisade cells which are elon- 



FiQ. 77 



Fig. 77. — Cross-section of leaf showing: u, upper epidermis; p, palisade cell 

 ophyll cells; 1, lower epidermis; s, stomata. 

 Fig. 78. — Lower surface of leaf showing stomata. 



s; ni, mes- 



gated, more or less cylindrical and placed at right angles to the 

 epidermal layer; just below the palisade layer is a thick layer 

 of loose, irregularly shaped parenchyma cells known as the 

 mesophyll; just below the mesophyll is the last layer or lower 

 epidermis which is very similar to the upper epidermis. The 

 palisadq cells are rich in protoplasm and in the green coloring 

 matter which is known as the chlorophyll. The chlorophyll is 

 usually confined to very definite small bodies known as chloro- 

 plasts and is essential for the photosynthesis work of the plant. 

 (Page 115.) 



