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 Hymenophyllum dilatatum \. Fiirstcrniiniin. Stele 

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

 xylem-bands ; n = upper xylem-band. x 200. (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, 

 with the xylem downwards : others ( T. labiatum and Motley?) 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 



