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 Pleris is not polystelic, but that the 

 vascular cylinder is an ami)hiphloic siphonostele, like that of Adian- 

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



I'"if;. 5. — P/eris (K/ii ill 11(1 : Iransvcrsc section of stem, sliowini^ tin- polysU-li'; X lo.-- 

 After Coui.TKU (S8). 



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

 and also by the passing of peri])heral vascular strands into the ])ith. 

 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. I"or cxam])le, in existing gymnosj)erms the ectoi)hloic 

 siphonostele, made u|> of collateral bundles, is the characteristic 

 type of cylinder. Whether this was attained by way of the i)olystelic 

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

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

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



