Ii6 ANATOMY OP STEM, ROOT AND LEAF 



termed the endodermis {en, Fig. 54) \ its cells are not very much 

 differentiated from the rest of the neighbouring cortical cells, but 

 they usually contain numbers of starch-grains which render them 

 somewhat conspicuous in sections of certain stems. 



(iii) The vascular cylinder or stele includes all the tissues 

 inside the endodermis, namely, the vascular bundles described 

 below, and also the medulla or pith and the medullary rays. 



Fig. 54. — Transverse section of the stem of a sunflower (enlarged 

 about 8 diameters.) X^ portion including a vascular bundle; e epi- 

 dermis ; /( a hair ; c cortex ; en endodermis ; w wood ; b bast ; fc 

 fascicular cambium ; ic interfascicular cambium ; / pericycle fibres. 



The outermost portion of the stele which lies immediately 

 in contact with the endodermis is known as the pericycle. The 

 latter may consist of a single layer of cells or of more than 

 one layer ; in some stems, its cells are thin-walled, and from 

 it arise most adventitious roots and shoots. 



The medullary rays and pith are composed of thin-walled 

 parenchymatous cells ; the cells of the medullary rays generally 

 retain their living contents for a long time, but those of the 

 pith live for a short time only. 



