440 MOSSES AND FERNS chap. 



Affinities of the Leptosporangiat^ 



The Osmundaceae undoubtedly are intermediate between 

 the Eusporangiatse and Leptosporangiatse, but with which 

 order of the former their affinities are closest is difficult to say. 

 Among the Ophioglossacese, the larger species of Botrychium 

 and Helminthostachys show apparent close structural similar- 

 ity to the Leptosporangiatse ; but, on the other hand, in the 

 distinctly circinate leaves and the character of the sporangia, 

 as well as the histology, the Marattiacese are certainly quite as 

 nearly related. Apparently all of these forms are generalised 

 types, springing from a common stock, but no two of them 

 directly related. 



Among the Leptosporangiatse themselves the relationships 

 are evidently much closer. A common type of prothallium 

 and sporangium prevails throughout, even in the heterospo- 

 rous forms. The four families, Osmundacese, Gleicheniacese, 

 Cyatheacese, and Polypodiacese, form a pretty continuous 

 series, of which the Polypodiacese are with very little question 

 the latest and most specialised forms. This is evinced both by 

 the geological record, which, so far as yet examined, shows 

 that they were the latest to appear, and by the fact that at 

 present they greatly outnumber the other Ferns, probably in- 

 cluding at least 90 per cent, of all living species. The single 

 genus Poly podium has over 400 species, probably as many as 

 all the lower Ferns combined. These facts, together with the 

 specialised character of all the parts, indicate that they are 

 Ferns which have adapted themselves to modern conditions. 



The Schizseacese and Hymenophyllacese do not seem to 

 belong to this main line, but are somewhat peculiar types, ap- 

 parently belonging near the bottom of the series. The Hymen- 

 ophyllacese, on the whole, approach most nearly the Gleichen- 

 iacese, with which they agree in many points, both in the sporo- 

 phyte and gametophyte, but they also recall the Osmundaceae, 

 and possibly may form a branch somewhere between the two, 

 but nearer the former. The peculiarities of the gametophyte 

 are probably in large measure the result of environment, and 

 the filamentous prothallium of some species of Trichomanes 

 and Schizcea is beyond question a secondary and not a primary 

 condition, and the prothallium is typically like that of the other 

 Leptosporangiatse. The nearest affinities of the Schizseace^e 



