1 6 Elementary Plant Physiology. 



tion, and are surrounded or imbedded in pith, while the extreme 

 outer portion of the stem is made of cells with very heavy 

 walls, forming the hard outer layer of the stalk (Fig. 6). 



5. The fibrovascular bundle and composition of 

 the stem. Select a single fibrovascular bundle of which a 

 thin cross-section has been made, and examine with a mag- 

 nification of 400 to 600. The xylem, with many heavy-walled 



Fig. 7. Cross-section of a fibrovascular bundle of Zea. jr, parenchyma ; /", 

 sclerenchyma ; , fl, z, xylem ; /, phloem. After Bonnier and Leclerc du Sablon. 



cells, may be seen, and the thinner walls of the cells of the 

 phloem. Surrounding the bundle is a heavy sclerenchyma 

 sheath, the stereome, in which the cells are dead, and therefore 

 incapable of any expansion to allow the increase in size of the 

 enclosed bundle. 



Each fibrovascular bundle, with its sheath, is a separate stele, 

 and the monocotyledonous stem is seen to contain many of 

 them, and is therefore known as polystelic, in contrast with 



