STRUCTURE OF THE LEAF 



39 



shaped colorless cells, whose outer walls are thickened and 

 cutinized, whereby they become tough and impervious. This 

 modification of the cell wall is usually more marked in the case 

 of the upper epidermis. The epidermis — usually the lower, 

 sometimes the upper, frequently both — is pierced by numerous 



Fig. 14. — Cross section of a typical leaf. Five stomata are shown in the lower 



epidermis, 



» 



pores, the stomata, which open into a system of intercellular 

 spaces filled with air. The outside atmosphere is thus given 

 free access to all parts of the mesophyll through the stomata 

 and this system of intercellular air spaces. 



81. The more compact upper layer of the mesophyll consists 

 of cells elongated perpendicularly 

 to the epidermis and arranged in 

 ranks, whence they have received 

 the name ''palisade cells." The 

 lower, spongy layer of the meso- 

 phyll consists of cells less regular 

 in form and arrangement and 

 more completely surrounded by 

 air spaces, but otherwise like the 

 palisade cells. 



82. The important characteristic 

 of the cells of the mesophyll is the 

 presence of numerous minute green 



granules embedded in the protoplasm. The granules are special - 

 ized parts of the protoplasm and are called chloroplasts. The 

 substance which gives them color is called chlorophyll. It is 



Fig. 15. — Surface view of the 

 epidermis of a leaf showing several 

 stomata. The guard cells are 

 dotted. 



