DETAILS OF LEAF STRUCTURE 



103 



and in which is an occasional fibre-vascular bundle I. The 

 palisade layer or layers and the spongy parenchyma are together 

 known as mesophyll (meaning middle of leaf). 



The lower surface of the leaf is covered by a layer of color- 

 less epidermal cells e f t differing somewhat in size and shape from 

 those of the upper epidermis. 



The lower epidermis is pierced by many openings or stomata s. 

 Each stoma opens into an air chamber. The upper epidermis 



FIG. 112. Cross section of privet leaf 



e, upper epidermis; p, palisade cells; sp, spongy parenchyma; a, air spaces; 

 I, fibro-vascular bundle; e', lower epidermis; s, stoma. Much magnified. - 



Modified after Gieseuhagen 



of this leaf contains far less stomata than the lower one, and 

 this is true of most leaves, - - often the upper surface contains 

 none. 



122. Uses of the parts above mentioned. It will be most 

 convenient to discuss the uses of the parts of the leaf in detail 

 a little later, but it will make matters simpler to state at once 

 that the epidermis serves as a mechanical protection to the parts 

 beneath and prevents excessive evaporation ; that the palisade 

 cells hold large quantities of the green coloring matter of the 

 leaf in a position where it can receive enough but not too much 

 sunlight ; and that the cells of the spongy parenchyma share the 



