584 FILICALES 



attached to that of the leaf-trace. The stele varies considerably in 

 its construction in different species : in Hymenophyllum there is less 

 variation than in Trichomanes. In all cases the stele lies centrally, 

 and is delimited by the endodermis, followed by a broad pericycle, while 

 the phloem surrounds the centrally-lying xylem. In species of Hymeno- 

 phyllum with large rhizomes, such as H. scabrum or dilatatum, the 

 metaxylem forms a ring enclosing the protoxylem together with some 

 parenchyma : the metaxylem often has the form of two bands, their definition 

 having relation to the origin of the strands which pass to the roots (Fig. 

 328; Boodle, Fig. 10). In species with small rhizomes the metaxylem 



FIG. 328. 



Transverse section of a node of Hytnenophyllum dilatatum v. Forsteriamtm. Stele 

 of rhizome to the right, leaf- trace to the left, ph phloem ; p.r protoxylem ; /= lower 

 xylem- bands ; = upper xylem-band. X2oo. (After Boodle.) 



forms a small band or mass, and the protoxylem is peripheral to it on 

 the lower side : these two types are bridged by transitional forms. In 

 Trichomanes the structure of the rhizome of T. reniforme corresponds in all 

 essentials to that of H. scabrum, but with a considerable mass of parenchyma 

 accompanying the central protoxylem. In others the parenchyma is scanty 

 ( T. radicans], while in T. trichoideum the stele is sub-collateral : these types 

 find their analogues in Hymenophyllum. But other species of Trichomanes 

 diverge along lines of their own : thus the stele is collateral in T. muscoides y 

 with the xylem downwards : others ( T. labiatum and Motley 7) may have 

 only a single tracheid, or none, and no phloem : these are plainly reduced 

 types. Others again may show a solid mass of xylem, with scattered 

 indefinite protoxylem (T. spicatum), or with the protoxylem peripheral 





