HYMENOPHYLLACEAE 583 
Asplenium resectum, Sm., and obtusifolium, Linn.: the existence of abortive 
stomata observed in some of these (e.g. A. resectum) indicates their reduced 
character. In A. odtusifolium two varieties have been recognised, one in 
which the filmy habit is fixed, and another in which stomata and intercellular 
spaces occur in the larger specimens, but are sometimes quite absent in 
the smallest, the filmy forms growing in the dampest localities. Giesenhagen 1 
compares the condition of the Hymenophyllaceae with that of the fixed 
filmy variety of A. obtusifolium; he holds that as these plants have been 
adapted to an extremely damp habitat, so also have the Hymenophyllaceae, 
and the character has become hereditary, partially so in the Asplenium 
and wholly in the Hymenophyllaceae. FFfinally, it has been shown 
experimentally that a thinning of the leaf can be produced by cultivation 
under moisture and shade, even in some ordinary species of Ferns 
(Scolopendrium vulgare, Pteris aquilina), as is so frequently the case also 
in plants of other affinity.2 From all this it may be concluded that the 
filmy habit is secondary and adaptive. 
It would appear from their structure that A. d/atatum and T. reniforme 
are among the less specialised of the Hymenophyllaceae, for in them both 
the leaf-expansion is more than a single layer in thickness, a condition 
exceptional in the family. And in this connection the facts of segmentation 
of the young wings are interesting: in the development of the wings of 
the leaf in ordinary Leptosporangiate Ferns the marginal cells segment 
by alternating oblique walls, but in the Hymenophyllaceae the segmentation 
of the marginal cells is as a rule repeatedly transverse. Now, in the lower 
part of the leaf of Z. renzforme, and occasionally also in H. dilatatum the 
segmentation is by oblique alternating walls, as in the ordinary Lepto- 
sporangiate Ferns, while in Zodea superba, which is also held as filmy by 
reduction, there is instability between the two types, though with a pre- 
ponderance of the oblique segmentation.2 These facts are further evidence 
that the filmy habit of the Hymenophyllaceae has been secondarily acquired, 
while they indicate an intermediate position for Zodea superba, and for 
Ff, dilatatum and T. rentforme. 
The filmy character is accompanied by structural reduction of other 
parts: thus in certain leaves pseudo-veins are present * which can hardly 
be anything else than the vestigial remains of true veins no longer functional. 
Cognate with this is the fact that the root-system is reduced, and even 
entirely absent in some species. It may then be expected that the vascular 
system of the axis and leaf will also show sighs of reduction as compared with 
other Fern-types : an examination of them shows that this surmise is correct. 
The stem of the Hymenophyllaceae is monostelic, and one leaf-trace 
passes off to each leaf, while the vascular supply to the axillary bud is 
1 Flora Ergdnzungsband, 1892, p. 174. 
2 Boodle, Lan. Journ., vol. xxxv., p. 6593; J. H. M'Ilroy, 7rans. Roy. Phil. Soc., 
Glasgow, vol. xxxvii., p. 136. 
3 Bower, Ann. of Bot., vol. iii, pp. 340-360. +Prantl, 4c., p. 24. 
