FROM ROOT TO LEAF 



81 



tissue outside and the weaker tissue within. If you examine 

 young stems, you will find that this arrangement is quite 

 common. Evidently in such plants there is a change in 

 the arrangement of the tissues of the stele as they pass 

 from the root to the stem. Where the root joins the stem, 

 the stele spreads out. In stems it is much larger in pro- 



FIG. 30. The stele of. a root in detail. (Compare with Figure ZQ.) 



portion to the whole diameter than it is in roots. The 

 center of stems, at least of young stems, is occupied by 

 soft- walled cells which form the tissue called pith. The pith 

 is a part of the stele. The xylem and the phloem form a 

 cylinder around the pith. This cylinder is called the vas- 

 cular cylinder. In cross section this cylinder forms a ring 

 of groups of xylem and phloem. (See Figures ji and 24.) 



