584 FILICALES 
attached to that of the leaf-trace. The stele varies considerably in 
its construction in different species: in Aymenophyllum there is less 
variation than in Zyichomanes. 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 A. 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 
Fic. 328. 
Transverse section of a node of Hymenophyllum dilatatum v. Forsterianum. Stele 
of rhizome to the right, leaf-trace to the left. 24=phloem; 41=protoxylem ; 7=lower 
xylem-bands ; “=upper xylem-band. X200. (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 Z: reniforme corresponds in all 
essentials to that of AZ scabrum, but with a considerable mass of parenchyma 
accompanying the central protoxylem. In others the parenchyma is scanty 
(Z. radicans), while in TZ. ¢trichoideum the stele is sub-collateral: these types 
find their analogues in Aymenophylum. But other species of Zrichomanes 
diverge along lines of their own: thus the stele is collateral in 7: muscoides, 
with the xylem downwards: others (Z: labiatum and Motleyi) 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 (Z: spicatum), or with the protoxylem peripheral 
