HYMENOPHYLLACEAE 583 



Asplcnhtm rcscctum, Sm., and obtusifoliurn. Linn. : the existence of abortive 

 stomata observed in some of these (e.g. A. resect inn) indicates their reduced 

 character. In A. obtiisifolium 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 : 

 compares the condition of the Hymenophyllaceae with that of the fixed 

 filmy variety of A. obtitsifoHum ; 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 Aspleniitm 

 and wholly in the Hymenophyllaceae. Finally, 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 H. dilatatinn 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 T. reniforme, and occasionally also in H. dilatatinn the 

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

 sporangiate Ferns, while in Todea 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. 3 These facts are further evidence 

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

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

 H. dilatatinn and T. reniforme. 



The filmy character is accompanied by structural reduction of other 

 parts : 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. 



'-Boodle, Linn, foitrn,, vol. xxxv., p. 059; J. II. M'llroy, Trans. Roy. Phil. Soc., 

 Glasgow, vol. xxxvii., p. 136. 



3 Bower, Ann. of Bot., vol. iii., pp. 340-360. 4 I'rantl, I.e., p. 24. 



