NATURE OF PLANTS 



77 



caused by strands of it here and there extending out into the 

 leaves, thus forming gaps in the circle. All seedling stems show 

 this arrangement of tissues and consequently there appears 

 more or less clearly three regions; an epidermis, a cortex, and a 

 central region surrounded by the cortex, an arrangement already 

 noticed in the roots. The epidermis does not materially differ in 

 structure, modification, or function from that already noted in the 

 leaf, see page 8. The cortex is largely composed of parenchyma 

 and extends from the epidermis to the endodermis which it in- 

 cludes. The endodermis is not so well marked as in the root 

 and owing to the rapid growth of the tissues in this region it is 



FIG. 40. Cross-section of young stem of castor bean: e, epidermis; c, cortex; 

 p, pith or inner portion of central cylinder; v, vascular bundle, arranged in 

 outer part of central cylinder. H. O. Hanson. . 



often impossible to detect it. The cortex assists the leaves in 

 photosynthesis, the outer portion as a rule being well supplied 

 with chloroplasts. In old stems that become covered with a 

 thick layer of bark the chlorophyll quite disappears, owing 

 doubtless to its exclusion from air and light. The cortex also 

 serves as a storehouse for foods. Particularly is this true of the 

 endodermis and adjoining cells which are often temporary 



