6o2 MOSSES AND FERNS ^ chap. 



strong evidence of the geological record that the Eusporangiatse 

 were the prevailing types in the earlier formations, and have 

 been supplanted by the more specialised Leptosporangiatse in 

 more recent times, is reasonably conclusive. 



Owing to the very small number of living Eusporangiat?e, 

 the relationships of these among themselves and to the Lepto- 

 sporangiatre are difficult to determine. From the frequent oc- 

 currence of dimorphic leaves among the older fossil types of 

 Ferns, as well as on grounds of comparative morphology, the 

 type of leaf in the Ophioglossace?e is probably to be considered a 

 more primitive one than that of the living Marattiace?e. Of 

 the existing genera of Marattiace?e, Dancca is the only one in 

 which the sporophylls differ in form from the sterile leaves, 

 and this dimorphism probably indicates that on the whole it is 

 the most primitive of the living genera. Whether the extreme 

 type of synangium found in Dancca is older than the nearly free 

 sporangia such as those of Angiopteris, has been questioned, as 

 both types are found among the Palaeozoic Marattiacese ; but 

 the greater specialisation shown in the latter type indicates that 

 it is of more recent origin. There is a possibility that the two 

 types represent two lines of development originating from dif- 

 ferent stocks comparable to Ophioglossum and Hclmintho- 

 stachys among the Ophioglossaceae. The occurrence of Ferns 

 of unmistakable Marattiaceous affinity, but wnth fertile leaf 

 segments completely covered with free sporangia like those of 

 Botrychium or Osninnda supports this view. 



While in such species of Botrychium as B. Viginianum, 

 there is a strong resemblance in the tissues to the lower lepto- 

 sporangiate Ferns, it is not so marked, on the whole, as those 

 in the Marattiace?e, which probably are nearer the Leptosporan- 

 giat?e, and probably have given rise directly to them. 



The homosporous Leptosporangiatae or Filices constitute a 

 very natural order. The Osmundaceae are without much ques- 

 tion the most primitive members of the order, this being indi- 

 cated both in the gametophyte and sporophyte. While they 

 show certain points of resemblance to Hclminthostachys and 

 BotrycJiiiuii, their affinities seem to be rather with the Marat- 

 tiaceae, and presumably they have arisen from some Palaeozoic 

 Marattiaceae with free sporangia borne upon special leaf seg- 

 ments. It is not impossible that two others of the lower fami« 

 lies the Schizaeaceae and Gleicheniaceae, may have originated 



