THE PTERIDOPHYTA : FILICALES, THE FERNS 



523 



This arrangement is unique among living Ferns and has been compared 

 with the ring of vascular bundles in a dicotyledonous stem. The comparison 

 is, however, purely superficial, for there is a continuous ring of phloem, 

 which is in fact broader and better developed between the xylem strands 

 than opposite them, as it is in a true vascular bundle. A further peculiarity 

 is that the endodermis does not open at the passage of the leaf trace from 

 the stele to the cortex. This is brought about by the union of the endodermis 

 which surrounds the leaf traces with that which surrounds the stele, at the 

 point of contact between the two, so that at no place, except occasionally 

 at the base of a branch, is there any uninterrupted continuity between 

 the medulla and the cortex. The protoxylems are usually mesarch, but 

 lie towards the outside of the xylem groups (Fig. 520). The phloem is 



Leaf trace 



Protoxylem 



Metaxylem 



Fig. 520. — Osmunda regalis. Vascular tissues of stele 

 showing mesarch protoxylems. 



distinguished by the presence of numerous sieve tubes, the perforations of 

 which contain callus strands, a rare thing in the Pteridophyta, and are covered 

 by minute slimy globules of uncertain nature. Around the phloem lies the 

 pericycle, consisting of three or four layers of thin-walled cells, and outside 

 this is the endodermis with rather irregularly suberized walls. Osmunda 

 regalis has only one endodermis, but in O. cimiamomea there is an inner 

 endodermis, though it is somewhat irregular and not always complete. 



The stele of Osmunda, when dissected out and viewed from the side, 

 shows that the spaces between the xylem masses are really very elongated 

 leaf gaps, so that the whole structure is analogous to a true dictyostele, except 

 for the presence of the single sheathing endodermis (Fig. 521). The 

 scalariform xylem elements have been shown to be in open connection with 



