CHAPTER X 



FILICINE^ LEPTOSPORANGIATyE 



The Leptosporangiatse bear somewhat the same relation to the 

 eusporangiate Ferns that the Mosses do to the Hepaticse, but 

 the disproportion in numbers is much greater in the former case. 

 While the whole number of living Eusporangiata^ (including 

 Isoetes) is probably considerably less than lOO, the Lepto- 

 sporangiatae comprise about 3500 species. In the former the 

 differences between the groups are so great that there is some 

 question as to their near relationship, while all the leptospor- 

 angiate Ferns show a most striking similarity in their structure, 

 and except for the presence of heterospory in two families, 

 might all be placed in a single order. Carrying our com- 

 parison still further, we may compare the Polypodiaceae, which 

 far outnumber all the others, with the Bryineae among the 

 Mosses. Both groups are apparently modern specialised types 

 that have supplanted to a great extent the lower less specialised 

 ones. 



The distribution of the leptosporangiate Ferns, too, offers 

 some analogy with the Mosses. While the eusporangiate 

 Ferns are few in number of species, they are for the most part 

 also restricted in numbers of individuals and in their range. 

 The Leptosporangiates, on the other hand, occur in immense 

 numbers, especially in the tropics, where they often form a 

 characteristic feature of the vegetation. This is true to a 

 limited extent in temperate regions also, where occasionally a 

 single species of Fern, e.g. Pteris aquilma, covers large tracts of 

 ground almost to the exclusion of other vegetation. A some- 

 what prevalent idea that the Ferns of to-day form merely an 

 insignificant remnant of a former vegetation is hardly borne 



