156 STRUCTURAL BOTANY 



innermost ring of bundles pith, but there is no dis- 

 tinction in structure between this and the rest of the 

 conjunctive tissue, all the cells of which have the form 

 of vertical cylinders with horizontal ends. They are 

 always living cells, and besides the protoplasm and 

 nucleus generally contain starch granules. 



p.ph. 



p: x . 



FIG. 60. Single vascular bundle from the laet section, ce, 

 xylem; p.x, protoxylem; ph, phloem; p.ph, proto- 

 phloem; b.. s, sclerenchymatous bundle-sheath. Magni- 

 fied 210. (R. S.) 



The outer layers (pericycle) of the cylinder, however, 

 ere different from the rest. They have thick, lignified 

 walls, and are very long, with pointed ends (pc in 

 Fig. 57, sc in Fig. 59). As the pericycle forms a 

 rigid case round the cylinder, it is evident that no 

 further growth in thickness is possible. The pericycle 



