CYCADOFILICALES 



II 



as a fern character. Among seed plants the prevailing type of wood 

 is endarch, and on this account it may be regarded as the most ad- 

 vanced condition. 



■ Fig. 7. — Exarch position of protoxy- 

 lem. — After WoRSDELL (36). 



-pith 



Fig. 8. — Mesarch position of protoxy- 

 lem. — After Worsdell (36). 



fii—pit/l 



It is important to understand what is meant by the transition from 

 the concentric to the collateral bundle, and from the mesarch to the 

 endarch condition of the xylem. Starting with a siphonostele made 

 up of mesarch concentric bundles, which are thus of distinctly fern 

 type, it is supposed that there 

 has been a gradual reduction 

 of the inner tissues. When the 

 whole of the internal phloem 

 has disappeared, the concentric 

 bundle has become collateral, 

 but it is still mesarch. Later, 

 when the whole of the centrip- 

 etal metaxylem has disap- 

 peared, the mesarch xylem has 

 become endarch. In some cases 

 there may be internal secondary 

 wood, and that disappears after 

 the internal phloem and before the centripetal metaxylem. When the 

 endarch collateral condition is reached, the vascular cylinder is dis- 

 tinctly of the gymnosperm type. Therefore, when mesarch xylem is 

 found among gymnosperms, it is regarded as a primitive feature; but 

 not so primitive as would be the occurrence of concentric bundles. 



Fig. 9. — Endarch position of protoxy- 

 lem. — After Worsdell (36). 



