8 MORPHOLOGY OF GYMNOSPERMS 



aquilina (fig. 5), the figure referred to showing nine smaller peripheral 

 vascular strands and four much larger central ones. It has been 

 shown (31) that the young stem of Pteris is not polys telle, but that the 

 vascular cylinder is an amphiphloic siphonostele, like that of Adian- 

 tum. In growing to the adult stage it passes gradually into the poly- 



FlG. 5. — Pteris aquilina: transversesectidnof stem, showing the polystele; Xio. — 

 After Coulter (88). 



stelic condition by the overlapping of elongated leaf gaps (see below), 

 and also by the passing of peripheral vascular strands into the pith. 

 It is evident that the various types of stele are not restricted to 

 different groups of ferns, but may be found as the adult condition at 

 all levels. For example, in existing gymnosperms the .ectophloic 

 siphonostele, made up of collateral bundles, is the characteristic 

 type of cylinder. Whether this was attained by way of the polystelic 

 condition, as has been suggested (36), or directly from a siphonostehc 

 condition, may not be clear; in any event, it would seem to be the 

 most advanced type of cylinder, but it is found at various levels among 



