586 FILICALES 



find them placed first in the system of Christ, 1 and removed far from 

 those Ferns with oblique annulus- with which they were associated in 

 Hooker's Synopsis Filicum. I 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 Hymenophyllum 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 H. dilatatum may be taken as a central non- 

 specialised type of the genus. In the case of Trichomanes, the species 

 T. reniforme is isolated from the rest, and shares with Hymenophyllum 

 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 : 2 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 T. spicatum. 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 accompanied 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 

 H. dilatatum and in T. 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 protosteiic ; 

 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 Zygopteris : allowing for the differences which follow on upright habit 



1 Farnkrduter der Erde, p. I. 2 Boodle, /. r. , p. 487. 



