62 STRUCTURAL BOTANY 



7. The Cortex 



Adjoining the pericycle on the outside is a single 

 layer of cells, with slightly thickened walls, and with 

 numerous starch granules in their contents. This 

 layer is the endodermis (en in Figs. 20 and 21, e in 

 Fig. 22), and forms the inner limit of the cortex. 

 Tho rest of the cortex consists of ordinary parenchyma, 

 like that of the pith. Its outer layers, however, have 

 rather thick walls, though they consist of pure 

 unlignified cellulose. The outer cells contain chloro- 

 phyll-granules, and can therefore take some part in 

 carbon-assimilation, though this function belongs chiefly 

 to the leaves. The thick-walled layers are of some 

 mechanical importance in helping to give greater 

 stiffness to the stem. 



Parenchyniatous cells generally, whether belonging 

 to the central cylinder or to the cortex, do not as a 

 rule fit closely together, but separate slightly from 

 one another at their corners so as to leave little 

 spaces between them. These intercellular spaces, as 

 they are called, contain air and aqueous vapour, but 

 not water. 



& The Epidermis 



The epidermis of the stem is one layer in thick- 

 ness. The epidermal cells are of three different 

 kinds ; most of them are elongated, with square or 

 sometimes pointed ends, and fit closely together 

 without any intercellular spaces. These cells each 

 contain protoplasm and a nucleus, but no chlorophyll- 



