586 FILICALES 
find them placed first in the system of Christ,! and removed far from 
those Ferns with oblique annulus with which they were associated in 
Hooker’s Synopsis Filicum. 1 have elsewhere expressed my dissent from 
the view that the Hymenophyllaceae are the most primitive type of Ferns: 
an alternative view is here adopted as to the relationship of this interesting 
family, based not only on comparison of the living forms, but also upon 
the rapidly increasing knowledge of related fossils. 
It has now been seen that the two genera of the family may be separately 
recognised as forming natural sequences, and that there is greater uniformity 
in the genus Aymenophyllum than in Trichomanes. In the former genus 
the usual characters of the larger types are, a creeping rhizome containing 
a stele with metaxylem surrounding the protoxylem: with much branched 
leaves, sometimes of more than a single layer of cells in the wings: with 
sori, having a short receptacle, bearing large sporangia with large spore- 
output. Such a species as “7. di/atatum may be taken as a central non- 
specialised type of the genus. In the case of Z7ichomanes, the species 
ZT. rentforme is isolated from the rest, and shares with Aymenophyllum 
the character of a creeping axis, containing a stele of similar construction : 
it has leaves several layers in thickness, short receptacle, and relatively 
large sporangia, with large spore-output. All these characters place it in 
close relation to Hymenophyllum. The rest of the genus represents various 
lines of specialisation: in some the creeping axis appears to have resumed 
an upright position, with modifications of the stele which may be held as 
secondary :? this is found in species with advanced leaf-differentiation, thin 
leaf-texture, elongated receptacle, relatively small sporangia, and low spore- 
output per sporangium, such as Z! sftcatfum. In others there are found 
in varying degree diminution of size and complexity of leaf-form, reduction 
of stelar structure in the creeping axis, and even complete absence of roots : 
these characters are acconfpanied by thin leaf-texture, elongated receptacle, 
small or often minute sporangia, and a spore-output varying from 64 to 
as low as 32. A general conspectus of the family, bearing these characters 
in mind, leads to the conclusion that the species in which its two genera 
are most nearly alike are themselves the most primitive, and that it is 
through them that comparisons may best be instituted with a view to 
determining the evolutionary relations of the family. The rest may be 
held to form sequences of specialisation, which will accordingly possess 
less direct interest for comparison with other Ferns. 
Taking, then, the characters of the sporophyte, as seen respectively in 
HT. dilatatum and in TZ: reniforme, they may be compared seriatim with 
those of other Filicales. The creeping habit is already seen in such early 
Ferns as the Schizaeaceae and Gleicheniaceae, which are also protostelic ; 
but the peculiar structure of the stele of the above species finds its nearest 
correlative not in these families, but in the Botryopterideae, and especially 
in Zygopterts: allowing for the differences which follow on upright habit 
1 Farnkriuter der Erde, p. 1. 2 Boodle, Zc, p. 487. 
