420 MOSSES AND FERNS chap. 



Polypodiaceae, form a pretty continuous series, of which the 

 Polypodiaceae are with very Httle 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 out- 

 number the other Ferns, probably including at least 90 per 

 cent of all living species. The single genus Polypodium 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 Schizsacese and Hymenophyllaceae do not seem to 

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

 apparently belonging near the bottom of the series. The 

 Hymenophyllaceae, on the whole, approach most nearly the 

 Gleicheniacese, with which they agree in many points, both in 

 the sporophyte 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 Trichonianes is beyond question a secondary and not a 

 primary condition, and the prothallium is typically like that of 

 the other Leptosporangiatae. 



The nearest affinities of the Schizaeaceae seem to be with 

 the Osmundaceae, but in the structure and arrangement of their 

 vascular bundles they are more like the Gleicheniaceae. 



Of the two families of the Hydropterides, the Salviniaceae 

 show several points of resemblance to the Hymenophyllaceae. 

 The development of the leaves is strikingly like those of 

 Hymenophyllaceae, with reniform or palmate leaves, and the 

 structure of the sori almost identical. The absence of second- 

 ary roots m Salvmia is suggestive also of the similar absence 

 in some species of Trichomanes. The two-sided apical cell of 

 the stem is, however, different from that of the few Hymeno- 

 phyllaceae examined, which all possess the pyramidal initial, 

 but possibly further examination may show forms with an 

 initial cell similar to that of Azolla or Salvmia. 



The Marsiliaceae in all respects, except their heterospory, 

 conform closely to the type of the higher families, and may be 

 assumed to be deriyed directly from the Polypodiaceae, or forms 



