XLix] MARGINAL DERIVATIVES 273 



tinctness of the Marginales and Superficiales. Both on grounds of comparison 

 and of Palaeontological evidence, the Simplices may be held as comprising 

 all those most primitive types of which we have any knowledge; and they 

 include all known Eusporangiate Ferns. 



As already stated, the Marattiaceae and Ophioglossaceae cannot readily 

 be linked with any known derivative forms. But the remaining Families of the 

 Schizaeaceae, Osmundaceae, and Gleicheniaceae, which are themselves less 

 definitely eusporangiate than the rest, severally show features in common 

 with other Families of Mesozoic or of more recent time. Later types may 

 be linked with one or another of these three ancient Families, of which they 

 appear to be derivative descendants. At least three main stirps may thus 

 be distinguished among modern Ferns, and from very early times each of 

 these is believed to have pursued a phyletic course apart. 



Marginal Derivatives 



The Hymenophyllaceae (Vol. Il, Chapter XXVII, p. 234) are probably an 

 ancient stock of specialised hygrophytes. Their form, simple anatomy and 

 venation, and their large spore-output suggest this, while the strictly marginal 

 position of the sori and the filamentous gametophyte point towards the 

 Schizaeaceae. They were probably of early origin, but it is difficult to link 

 them definitely downwards with any one Family of the Simplices: upwards 

 they appear to end as a blind line, unless it is right to regard theSalviniaceae as 

 specialised aquatic and heterosporous derivatives from them (Chapter XLVIII, 

 p. 260) — a loose attachment like that of the Marsileaceae to the Schizaeoids 

 (Chapter XXlIl). 



The most marked distinction of the Hymenophyllaceae from the Simplices 

 is the strictly marginal, gradate sorus (Vol. I, p. 212). In them we see in its 

 most perfect form the receptacle with basal intercalary growth, bearing a 

 long continued basipetal sequence of sporangia. The type from which this 

 may have originated would probably be found among the protostelic Simplices, 

 with which relation the sporangial structure and high spore-output would 

 accord, occurring as these features do in a simplified hygrophytic stock. In 

 close relation with them are the Loxsomaceae, also blind upwards, but with 

 possible forerunners in Jurassic time (Chapter XXVIIl). 



From the point of view of the phyletic origin of typically modern Ferns, 

 the Dicksoniaceae, which have also gradate and marginal sori, command a 

 wider interest (Chapters XXIX, xxx). They comprise both dendroid and 

 creeping forms, and like almost all the early marginal types they have only 

 hairs as dermal appendages. In accordance with their larger size they are 

 solenostelic or dictyostelic, sometimes also with medullary vascular tracts 

 as well. The gradate sori are protected by a basal indusium, more or less 

 distinctly two-lipped, and the strictly marginal receptacle bears sporangia, 

 Bin 18 



