CELLULAR STRUCTURE OF LEAVES 



249 



NUMBER AND DISTRIBUTION OF STOMATA PER SQUARE 

 MILLIMETER OF LEAF SURFACE 



Although stomata are most numerous on leaves, they occur in 

 Flowering Plants wherever there is green tissue to be supplied 

 with gases. They are common on fruits, green twigs of trees, 

 and are present on nearly all parts of the aerial stems of herba- 

 ceous plants. On the older twigs and trunks of trees, the stomata 

 are represented by the lenticels which are the structures into 

 which stomata are transformed as the stem becomes enclosed in 

 bark. The stomata are distorted and transformed into lenticels 

 partly by the stretching of the bark and partly by the tissue 

 which grows up from beneath and crowds into the stomatal 

 openings. 



In order to get a view of the epidermis in cross section and to 

 study the chlorenchyma and veins of a leaf, a thin section must 

 be made and highly magnified as shown in Figure 232. In this 

 view an ordinary epidermal cell is rectangular, has a large central 

 cavity separated from the cell walls by only a thin layer of proto- 

 plasm, and has the outer wall more thickened than those within. 

 The continuity of the epidermis is interrupted by the stomata, 

 each of which opens into an air chamber in the mesophyll just 

 beneath. 



The chlorenchyma is composed of thin-walled cells, having 

 thin layers of protoplasm in which the characteristic green bodies 

 (chloroplasts) are located. In most horizontal leaves, the cells 

 of the chlorenchyma are differentiated into two distinct groups, 

 the palisade and the spongy tissue. The palisade tissue is next to 

 the upper epidermis and consists of one or more rows of compact 

 elongated cells in which chloroplasts are especially abundant. 



