IOO PLANT LIFE. 



three definite regions may be distinguished, viz.: (i) the 



epidermis; (2) the cortex; (3) the stele (figs, in, 112). 



125. 1. The epidermis. — This is a single layer of cells 

 forming the extreme edge of the section, being, therefore, the 

 layer which covers the surface of the stem. Here and there 

 maybe observed intercellular spaces, which permit communi- 

 cation between the outside air and similar spaces in the deeper 

 tissues of the cortex. These openings are usually bordered 

 by two specialized cells, and are called stomata. The 

 epidermal cells may be furnished with green chlorophyll 

 bodies, or these may be entirely absent. 



126. 2. The cortex. — This region consists of several 

 rows of cells, usually thin-walled and not in close contact, and 

 hence abundantly provided with intercellular spaces. These 

 cells usually contain many chlorophyll bodies, to which the 

 green color common to stems is due. 



The innermost layer of the cortex abutting upon the stele, 

 whose radial walls are suberized (^J 9), is usually specialized 

 to form a distinct layer of cells. This layer is the endodermis 

 (fig. 1 1 8). 



127. 3. The stele. — The central region is called the 

 stele. It consists, as in the root, ordinarily of three parts. 

 Its outer layer of cells is known as the pericycle (fig. 118). 

 Within the pericycle are clusters of smaller cells, the cut ends 

 of the vascular bundles. Occupying the space between the 

 vascular bundles is the pith (figs, in, 112). 



These regions of the stem are subject to various modifica- 

 tions. 



128. 1. The epidermis. — While the epidermis is usually 

 a single layer of cells, it is sometimes increased to two or 

 three layers. Stomata may be entirely lacking. This is 

 especially the case in those underground and submerged 

 stems in which the stomata would be useless. The cells of 

 the epidermis are often prolonged into outgrowths of various 



