HYMENOPHYLLACEAE 585 
(Z. scandens): there is reason to think that these, being more specialised 
in their general habit, have undergone secondary modification also of 
their stelar structure. 
The petiole receives in all cases a single strand, which passes off from 
the stele without a leafgap, and widens out upwards into a collateral 
structure with more or less clearly curved xylem. 
The comparison of this structure with that of other living Ferns leads 
to the recognition of no near relation. Perhaps the nearest is with 
the ancient family of the Botryopterideae, and especially with the genus 
Zygopterts. Jt will be seen that the correspondence is closest between 
the structure seen in Zygopteris Grayt (Fig. 270, p. 500), and that seen in 
T. rentforme and A. scabrum, that is, with the species which show the 
nearest resemblance to one another of the living forms of Hymenophyllaceae. 
The chief features of resemblance are in the structure of the stele, and 
the mode of origin of the leaftrace, and of the supply to the axillary 
bud. As to the former, if the small central tracheides of Zygofteris are 
protoxylem, as seems highly probable, the agreement with Z: reniforme, 
or the larger Hymenophyllums is very close, the differences being such as 
would be due to a 2ths arrangement of the leaves on the one hand, and 
distichous on the other. The mode of origin of the leaf-trace without 
any foliar gap, and the insertion upon it of the supply to the axillary bud 
are also points of similarity. This resemblance to a very ancient form 
appears to confirm the recognition of the living species named as being 
probably primitive, while from that central point the remaining species 
of Hymenophyllum diverged slightly in one direction, but those of Z7zcho- 
manes diverged much more strongly along their own lines, either of reduction 
or of other specialisation. Such a conclusion appears to emerge clearly 
from the anatomical comparison. 
The Hymenophyllaceae have undergone vicissitudes of classification : 
Brongniart first separated them as a special family, though the name of 
Endlicher is usually connected with their recognition as ranking on an 
equal footing with the Cyatheaceae or Polypodiaceae. Presl regarded them 
as a connecting link between Mosses and Liverworts, and somewhat far 
removed from the Filicineae; but the actual separation of them from the 
Filicineae was opposed by Mettenius. The comparison of their vegetative 
structure with that of certain Bryophytes was, however, maintained, and 
strengthened by fresh observations: it was pursued subsequently by Prantl, 
with regard to the sporophyte, and especially to the sorus, and by Goebel 
as regards the gametophyte. Consequently they were held to illustrate 
the phylogenetic connection between Mosses and Vascular Cryptogams, 
and to bridge over the gap between these circles of affinity in the sequence 
of development of the Archegoniateae. Further, the Hymenophyllaceae have 
been held to be at least as near to the Polypodiaceae as to any other family 
of the homosporous Leptosporangiateae: in accordance with such views we 
