CHAPTER III 



A THEORETICAL BASIS FOR THE SYSTEMATIC 

 TREATMENT OF FERNS 



The Filicales may be held to comprise all of the living Megaphyllous 

 Pteridophytes, together with such fossils as show essentially similar characters. 

 This is not a scientific definition, but is merely a provisional designation of 

 the Plants that are here to be dealt with, all of which show, so far as in- 

 vestigation has extended, a life-history and essential features such as have 

 been described in Chapter l. It is not possible to draw up a strict definition 

 of the Filicales till their characters have been examined in detail and compared 

 with those of other groups of Plants from which they are to be distinguished. 

 The mere fact that their leaves are relatively large in proportion to the axis 

 which bears them (megaphyllous) is not in itself a sufficient diagnosis. Some 

 Filicales are actually microphyllous, for instance Salvinia and Azolla. On 

 the other hand, the large and growing Flora of the Pteridosperms, known only 

 as fossils, was certainly large-leaved. These plants were probably derivatives 

 from some more or less Fern-like ancestry: but being actually Seed-Plants 

 they are not to be included under the Filicales. Some Lycopodiales {Sigil- 

 laria and Isoetes) had relatively large leaves, and certain fossils related to the 

 Equisetales or Sphenophyllales {Pseudobornia) shared that character. We 

 may indeed hold it as possible that any phylum of Pteridophytes might have 

 developed megaphyllous types, and in that case megaphylly could not be 

 held in itself to be a feature indicating affinity or community of Descent. 

 But as a matter of fact, excepting Isoetes which is plainly a Lycopod, no such 

 Pteridophytes are living now upon the earth other than those which may 

 naturally be included in the Filicales. Accordingly the provisional designa- 

 tion given above will serve to introduce the subject of this volume. 



The Filicales are represented on the earth at the present time by about 

 1 50 genera and about 6ooospecies, according to Christensen's/;2^(?;iri^z7/«/;«(23). 

 Some are very minute, others attain a considerable size as Tree Ferns: but 

 none can be reckoned among the largest of living plants, nor is there fossil 

 evidence that Ferns ever attained extreme dimensions. Their geographical 

 spread is very general. Some few are Arctic: but Ferns increase in numbers 

 both of species and of individuals towards the Equator. They are mostly 

 mesothermal hygrophytes: that is, they flourish under moist conditions with 

 a moderate temperature, and the majority of them are shade-loving Plants. 

 Hence their headquarters are in the mountains of the tropics, where they 



