THE PTERIDOPHYTA : FILICALES, THE FERNS 



507 



Fig. 500. — Pteridium oquilinum. Diagrams of successive stages in 

 ontogeny of the rhizome. A, SolenosteHc condition with single 

 leaf gap. B, Origin of the internal stele as a branch from the 

 external stele. C, Dictyostelic condition established in outer stele. 

 D, Dictyostelic condition in both steles. {After Jeffrey.) 



Cortex 



Endodermis 



Pericyde 

 Phloem 



Protoxylem 

 Metaxylem 



Fig. 501. — Pteridium aqidlinum. Transverse section of 

 rneristele in the rhizome showing two protoxylem 

 groups. 



