PHYLETIC RELATIONS 655 



of sorus : the one with marginal sori, the other with superficial. They 

 are both characterised by having their spore-output restricted to the typical 

 number of 64 as the upper limit. It may be a question whether these 

 two were truly distinct in phyletic origin, but they appear to have pro- 

 gressed independently. The one comprises the Dennstaedtia-Davallia series, 

 leading onwards to various families of the Mixtae : the other is the Cyathe- 

 aceous series. Besides these the genus Dipteris seems to have taken a 

 line of its own towards the mixed sorus, and may perhaps be not alone 

 in having done so. 



The most complete phyletic story has been made out for the Denn- 

 staedtia-Davallia series, to which Dicksonia itself may probably be a collateral 

 line, with unusual elaboration of the structure of the stock. The scheme 

 suggests a somewhat independent origin for this series, together with such 

 genera as Loxsoma, Thyrsopteris, and Dicksonia, from a source probably 

 between the Botryopterideae and Schizaeaceae, though not directly from 

 either of them. They all show a structure of the axis advanced to a 

 solenostelic or even a dictyostelic state, and a basipetal sorus, derived pre- 

 sumably as an amendment on the simple type : Loxsoma and Thyrsopteris 

 have a peculiarly archaic structure of the sporangium. The further pro- 

 gression, which harmonises reasonably with the palaeontological record, 

 has been traced both anatomically and sorally, and leads with advance 

 in both respects, through the Dennstaedtia-Microlepia-Davallia sequence 

 from Ferns with a solenostelic structure and basipetal sorus to those 

 with a dictyostelic stock and mixed sorus. Several side-branches, or it 

 may be concurrent lines, also exist, such as Lindsaya, Pteris, and Adiantum, 

 thus giving rise to the bulk of those Ferns which Prantl grouped together 

 as the Pterideae. 



But in several branches from this line a transition is found from the 

 marginal to the superficial position of the sorus. One line is direct from 

 Dennstaedtia through Hypolepis, a genus with mixed sorus, to some forms 

 with superficial sorus reckoned as Polypodium : another line may have been 

 as illustrated by Deparia, where the mixed sori are sometimes marginal, 

 sometimes superficial. Another, and a more decisive line is through such 

 forms as Cysfrptcris, and certain sections of Davallia such as Leucostegia, 

 in which there can be little doubt that the mixed sorus has been shifted 

 from the margin to the surface. And so by a number of phyletic sequences, 

 by no means exhausted by the examples quoted, it seems probable 

 that there has been progression to that prevalent and final type of the 

 Mixtae which has an intra-marginal sorus on the lower surface of the 

 leaf. The Ferns thus derived are characterised by their advanced 

 dictyostely. 



But there is some reason to think that a similar result has been reached 

 also by a more direct route. The series of the Cyatheaceae is linked by 

 Alsophila, and especially by the solenostelic A. pruinata, with the Glei- 

 cheniaceae : it is a very slight transition, already indicated by G. dichotoma, 



