NATURE OF PLANTS 63 



chyma and extends from the epidermis to the endodermis which 

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



Fig. 39. Cross-section of young stem of castor bean : c, 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. 40, A. 

 This tissue, collenchyma, is characterized by the thickening of 

 the cells at the angles or on all sides and by the silvery luster of 

 the walls. The walls, though very elastic and tough, are capable 

 of growth and so especially adapted to the support of the 

 young elongating stems. In older stems, where elongation has 

 ceased, greatly elongated cells, called stereome fibers or scleren- 



