CHAPTER XL. 
GENERAL COMPARISON OF THE FILICALES. 
Tue 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 Axeimia and Mohria: the other, in which there is a star- 
like dehiscence, includes Axeimia 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 
