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. obtusifolium 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 

 ilmy variety of A. obtiisifoliitm; he holds that as these plants have been 

 lapted to an extremely damp habitat, so also have the Hymenophyllaceae, 

 id the character has become hereditary, partially so in the Asplenium 

 id wholly in the Hymenophyllaceae. Finally, it has been shown 

 experimentally that a thinning of the leaf can be produced by cultivation 

 ider moisture and shade, even in some ordinary species of Ferns 

 (Scolopendrium vulgare, Pteris aquilina\ as is so frequently the case also 

 plants of other affinity. 2 From all this it may be concluded that the 

 Imy habit is secondary and adaptive. 



It would appear from their structure that H. dilatatum and T. reniforme 

 ire among the less specialised of the Hymenophyllaceae, for in them both 

 leaf-expansion is more than a single layer in thickness, a condition 

 cceptional in the family. And in this connection the facts of segmentation 

 >f the young wings are interesting : in the development of the wings of 

 leaf in ordinary Leptosporangiate Ferns the marginal cells segment 

 >y alternating oblique walls, but in the Hymenophyllaceae the segmentation 

 )f the marginal cells is as a rule repeatedly transverse. Now, in the lower 

 irt of the leaf of T. reniforme, and occasionally also in H. dilatatum the 

 ^mentation is by oblique alternating walls, as in the ordinary Lepto- 

 sporangiate Ferns, while in Todea superba, which is also held as filmy by 

 luction, there is instability between the two types, though with a pre- 

 mderance of the oblique segmentation. 3 These facts are further evidence 

 mt the filmy habit of the Hymenophyllaceae has been secondarily acquired, 

 rhile they indicate an intermediate position for Todea superba, and for 

 L dilatatum and T. reniforme. 



The filmy character is accompanied by structural reduction of other 

 irts : thus in certain leaves pseudo-veins are present 4 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 signs 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 Ergiinzungsband, 1892, p. 174. 



2 Boodle, Linn. Jonrn., vol. xxxv., p. 659; J. H. M'llroy, Trans. Roy. Phil. Sot:, 

 Glasgow, vol. xxxvii., p. 136. 



3 Bower, Ann. of Bof., vol. Hi., pp. 340-360. 4 Prantl, I.e., p. 24. 



