238 



HYMENOPHYLLACEAE 



CH. 



view may be held that it represents the weaker branch of a dichotomy, 

 and that the subtending leaf may be the first produced by it. This view 

 would also apply for the similar branching in Zygopteris and Lyginopteris, 

 while the facts for Botiyopteris cylindrica suggest that there is no obligatory 

 relation with the subtending leaf. Here the special interest lies in the fact 

 that the branching of the shoot in the Hymenophyllaceae closely resembles 

 that in such primitive Ferns as the Botryopterideae and the Ophioglossaceae 

 (see Vol. I, Chap. iv). 



The apical segmentation of stem, leaf, and root is of the ordinary- type 

 for Leptosporangiate Ferns (Vol. I, Fig. loi). But the margin of the filmy 



Fig. 50S. Drawings of the segmentation at the margin of leaves of Leptosporangiate Ferns. 

 A — Trichomanes radicans : B=Trichomanes retiifonne: C—Aspleniiim resectum: D^Phyllitis 

 (Scolopejidrinvi) vulgare. The last is the most usual type : A and B are characteristic of Filmy 

 Ferns. 



leaf is occupied by a series of half-disc-shaped cells, with segmentation 

 parallel to their inner face. In the absence of periclinal divisions this gives 

 a filmy expanse of a single layer. In T. renifornie, where the expanse is 

 four layers thick, the marginal cells have the same form and segmentation, 

 but periclinal divisions follow (Fig, 508, A, B). Thus in both cases the 

 segmentation differs from that in ordinary Leptosporangiate Ferns {C, D). 

 But in the lower parts of the leaf both in T. renifornie and occasionally in 

 H. dilatatuni the marginal segmentation is by alternating oblique walls, as 

 in C, D, a condition found also in Todea siiperba. These segmental facts 

 further support the view that the filmy habit has been secondarily acquired: 

 also they suggest that the thicker structure of T. reiiifornie has resulted from 

 a tertiary return to a thicker structure. 



