PTERIDOPHYTA 



283 



sage of leaf traces. The pericycle consists of one to four layers of paren- 

 chyma forming a complete sheath. As seen in cross section, the vascular 

 cylinder of Osmunda consists of a ring of mesarch xylem strands sep- 

 arated by parenchymatous "rays" that pass outward from the large pith 

 (Fig. 237). In some species the pith may contain sclerenchyma. Sur- 

 rounding the xylem is a continuous layer of phloem made up chiefly of 

 sieve tubes, while between the xylem and phloem are several layers of 

 elongated parenchymatous cells continuous with the " rays." The xylem 



Fig. 238. Cross section of the rhizome of Ftcridium aquilinum, a dictyostele in which the 

 vascular cylinder encloses two or more medullary bundles and two bands of dark, heavy- 

 walled sclerenchyma fibers, X8. 



really consists of a cylindrical network forming a hollow cylinder, the 

 meshes or "rays" being leaf gaps. 



Dictyostele. Among the lower families of Filicales, a dictyostele is pres- 

 ent in Mohria and most species of Anemia, both members of the Schizaea- 

 ceae. Dictyostelic stems are also found in some of the Dicksoniaceae, 

 but the greatest number occur among the Cyatheaceae and Polypodia- 

 ceae. The dictyostelic condition has been derived from the siphonostelic 

 by the overlapping of leaf gaps, so that several or many separate vascular 

 strands are seen in a cross section of the stem. In complex dictyosteles 

 the vascular cylinder consists of a tubular network. 



A transverse section of the rhizome of Polypodium shows a number 

 of small, widely separated vascular strands arranged in a circle. The 

 rhizome of Pteridium, which is dorsiventral, consists of two series of 

 strands — a circle of small peripheral ones enclosing two large central 

 strands, or sometimes more than two as a result of branching (Fig. 238). 

 The dorsal bundle is band-like and larger than the other peripheral ones. 



