78 



COLLEG^ BOTANY 



They also contain the chlorophyll, which is the important factor 

 in the absorption of sunlight and in the making of carbohydrates. 

 The epidermis consists of a layer of cells covering both upper 

 and lower surfaces of the leaf. These cells have lost their proto- 

 plasmic contents and the outer wall has become very much thick- 

 ened and infiltrated with a waxy substance, which makes the leaf 

 water-proof (Figs. 59 and 60). The outer wall is called the 

 cuticle and is composed of cutin, which is very similar to suberm 



FIG. 59. 



FIG. 60. 



FIG. 59. Cross-section ofleaf showing: (u) upper epidermis; (p) palisade cells; (TO) mesophyll 



cells; (I) lower epidermis; (s) stomata. 

 FIG. 60. Lower surface of leaf showing stomata. 



(page 52). They protect the more delicate cells within but 

 permit the free transmission of light. On the lower surface we 

 find great numbers of minute openings or stomata (singular, 

 stoma) (Fig. 60). The stoma is the opening between two 

 crescent-shaped guard cells through which the air passes into a 

 cavity in the mesophyll of the leaf. The guard cells are crescent- 

 shaped and differ from the other epidermal cells in being well 

 supplied with protoplasm and chloroplasts. These stomata are 

 usually found in much smaller numbers on the upper surface of 



