HYMENOPHYLLACEAE 587 
and radial construction in Zygop/eris and the creeping dorsiventral rhizome 
in these Hymenophyliaceae, the structural resemblance is very close: and 
with this go the strikingly similar facts of structure and insertion of the 
leaf-trace, and of the mode of supply to the axillary buds. 
Next, as to the leaf-texture, A. dilatatum and T. reniforme are both 
species with the lamina composed of several layers, and occasionally showing 
in their development the alternate segmentation seen in the leaves of 
ordinary Leptosporangiate Ferns. According to the argument advanced 
above, the filmy texture is an adaptive character shown in various families 
of Ferns: the peculiarity of the Hymenophyllaceae is that they show it in 
the highest degree. But the species named appear less specialised in the 
hygrophilous direction than others of the family, and thus they serve to 
connect it with the ordinary types. 
The marginal position of the sorus is shared with the Schizaeaceae, while 
it is to be remembered that the Botryopterideae and Osmundaceae may also 
bear sporangia in their leaf-margins. But the Hymenophyllaceae differ from 
any of these in the basipetal sequence of the sporangia, the elongated 
receptacle, and the cup-like indusium. The basipetal sequence may be 
held to be a secondary condition, bringing with it the advantage of spreading 
the drain of spore-production over a longer period than if all were developed 
simultaneously : the elongation of the receptacle, a consequence of intercalary 
growth, is almost a necessary condition of its adoption. The basal cup-like 
indusium, imperfectly represented in the Schizaeaceae, has probably been 
a new formation: its efficacy in protecting the youngest sporangia at the 
base of the sorus amply justifies its existence. It is thus possible to conceive 
of the origin of the Hymenophyllaceous sorus from some Fern-type with 
marginal sporangia, by initiation of a basipetal sequence, and establishment 
of a protective indusium. The type from which they might have originated 
would probably be found among some protostelic types with large sporangia 
marginally produced, of which the Botryopterideae, Osmundaceae, and 
Schizaeaceae are the known representatives. 
A comparison of the sporangia themselves confirms this reference to 
the Ferns with large sporangia, rather than to simpler forms such as the 
Polypodiaceae. For there is an oblique annulus corresponding in position 
on the one hand to that of the other Gradatae, but also to that of certain 
of the Simplices. A comparison of Fig. 325 of Hymenophyllum with Fig. 310 
of Gleichenia shows plainly the close similarity of the sporangia: and it 
has been shown that if the peripheral face of the Gleicheniaceous sporangium 
be reduced the Schizaeaceous type is the result, both being variants of the 
same form. If finally the point of dehiscence were shifted from the median 
plane to the side—a practical necessity where there is a basipetal sequence 
“the Hymenophyllaceous sporangium would be the result. Further, in the 
spore-output certain of the Hymenophyllaceae approach the Simplices : it has 
been shown that in A. Zundridgense the output per sporangium is 256-512: 
in Z. veniforme and sericeum it is typically 256, while other species of 
