CHAPTER XL. 



GENERAL COMPARISON OF THE FILICALES. 



THE burden of evidence in the comparative study of the Ferns has 

 habitually been laid upon the sporophyte ; indeed, this was a matter of 

 necessity to the older Pteridologists, since the prothalli were then practically 

 unknown. But subsequent investigation has largely justified what was at 

 first a matter of circumstance rather than of choice : it has been shown 

 that for very many Ferns there is a dead level of form of the gametophyte, 

 while it has been proved to be possible, by varying the conditions of growth, 

 to elicit great differences of development even in individuals of the same 

 species. It is true that while some groups of Ferns have habitually a robust 

 prothallus, as in the Marattiaceae, others show habitually a delicate and 

 sometimes a filamentous type, as in the Hymenophyllaceae or Schizaeaceae, 

 while the same appears also in Vittaria. But though in some measure such 

 characters may be held as useful evidence, the very slight positive features 

 that the vegetative development of the prothallus presents, and their liability 

 to modification, will always derogate from its importance in comparison. 

 Turning to the sexual organs, they vary in their level, being either sunken 

 or projecting ; and an interesting parallel may be drawn between them and 

 the sporangia in this respect, for they are habitually sunken in Eusporangiate 

 and projecting in Leptosporangiate forms. The archegonia are singularly 

 uniform in structure throughout the Ferns : but the antheridia show two 

 distinct types as regards dehiscence : the one, in which a cap-cell breaks 

 away at maturity, is characteristic of all Ferns with an oblique annulus, with 

 the exceptions of Aneimia and Mohria : the other, in which there is a star- 

 like dehiscence, includes Aneimia and Mohria, together with the whole 

 body of the Polypodiaceae. Such facts are interesting as a confirmation 

 of the results of study of the sporophyte, for they group together on the 

 basis of a gametophyte character those Ferns on the one hand which 

 comparison of the sporophyte indicates as primitive, and on the other 

 those which are held to be later and derivative. It is in this way that 

 the characters of the gametophyte may be used, as ancillary rather than 



