114 DRYOPTEROID FERNS [CH. 



Among Ferns with marginal sori such zygomorphy is naturally more common, for in 

 them the sorus has a more direct relation to the distinct upper and lower surfaces of a 

 flattened sporophyll, and to the unequal incidence of light. An original type of sorus, as it 

 may be seen in Etaptcris (Vol. li, Fig. 333) or Coryiiepteris (Fig. 334), appears to have been 

 like a radial tassel, distal or marginal upon an organ not markedly bilaterak On the 

 general theory advanced in these volumes from palaeontological data, there was first a 

 flattening and frequently also a webbing of the supporting part, thus giving a broad leaf- 

 surface. It appears that certain Ferns slipped their marginal or distal sori early in descent 

 on to the lower surface of the flattened organ, giving rise to the great group of the 

 Superficiales : but that others styled the Marginales retained the marginal position, though 

 with occasional transitions at later times to the lower surface. The marginal sori never- 

 theless retained the radial construction of their sorus in some families with pertinacity : 

 this is conspicuously so in the Hymenophyllaceae. But from the Schizaeaceae to the 

 Dicksoniaceae, and especially in the Dennstaedtiinae and their derivatives, the dorsiventral 

 development of the sorus became ever more and more pronounced. Naturally it asserted 

 itself earlier in the descent of the marginal than of the superficial series. Consequently in 

 these parallel but distinct series, while the sori of the Cyatheaceae remained typically 

 radial, those of the Dicksoniaceae became typically lop-sided. Nevertheless lop-sidedness, 

 or rather zygomorphy, does make its appearance eventually among the Cyatheaceae also: 

 but it is only in the later derivatives that it becomes a marked feature, and Peranema 

 thus acquires a special interest as combining Cyatheoid characters with a lop-sided sorus. 

 The importance of this will be seen in relation to the Dryopteroid Ferns. 



{b) MORPHOLOGY OF THE ''INDUSIUM INFERUiW IN 

 THE SUPERFICIALES 

 A more interesting innovation, and one still more open to controversy, is the appearance 

 of an indusial protection in the superficial series. A considerable volume of facts relating 

 to the Cyatheaceae and Woodsieae necessary for a discussion of the morphology of the 

 indusium in Ferns with superficial sori is now before us : and conclusions derived from 

 them will naturally carry on their application to the Dryopteroid Ferns. The question of 

 the origin and nature of any special type of indusium cannot be decided by reference to 

 the indusia alone, for the indusium is a part which has made its appearance after the 

 main categories of parts were established for Ferns : it is a mere accessory to the more 

 ancient sporangia. It appeared later in descent than the establishment of those main 

 evolutionary phyla, the distinction of which is recognised on a basis of comparison, checked 

 by the positive data of palaeontology. It is moreover evident that indusial structures have 

 arisen independently in more than one of those phyla. It cannot then be assumed that all 

 those parts which we call indusia were of the same nature or origin throughout : nor that 

 the evolutionary history was the same, however closely those of different phyla may 

 resemble one another structurally and functionally. Each separate phylum will then 

 present its own evolutionary problem, and it will only be by analogy that the facts relating 

 to one phylum may be held as contributing to the elucidation of those seen in another. 

 For instance, he would be a bold man who would assert that the overarching indusium of 

 Matonia is the evolutionary equivalent of that in Polystichiim. However nearly the one 

 may resemble the other in form and function, they are parts of Ferns as distinct in 

 structure as they are in recorded time. The simple uniseriate sorus of the one is a type 

 characteristic of the Palaeozoic age, the mixed sorus of the other characteristically modern. 

 The indusium of each, notwithstanding a superficial similarity of form and of function, 

 must be held as presenting a distinct evolutionary problem. 



