12 REGIONS OF THE STEM 



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 struc- 

 ture, 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 



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. 



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

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

 photosynthesis, the outer portion as a rule being well supplied 

 with chloroplasts, and it also serves as a storehouse for foods. 

 Particularly is this true of the endodermis and adjoining cells 

 which are often temporary receiving stations for the carbohy- 

 drates during their transport through the stem. The cells in 

 the outer portion of the cortex frequently become thickened and 

 more or less elongated to give strength to the stem. One of the 

 most common modifications of this kind is shown in Fig. 41, ^. 

 This tissue, collenchyma, is characterized by the thickening of 



