CHAPTER IX 

 FILICINE.E LEPTOSPORANGIAT.E 



THE Leptosporangiatse bear somewhat the same relation to the 

 eusporangiate Ferns that the Mosses do to the Hepaticae, but 

 the disproportion in numbers is much greater in the former 

 case. While the whole number of living Eusporangiatae is 

 probably less than 50, the Leptosporangiatae comprise about 

 4000 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 leptosporangiate 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 comparison still further, we may com- 

 pare the Polypodiaceae, which far outnumber all the others, with 

 the Bryales 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. The Leptosporan- 

 giates, 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 aquilina, covers large tracts of ground almost to the ex- 

 clusion of other vegetation. A somewhat prevalent idea that 

 the Ferns of to-day form merely an insignificant remnant of a 

 former vegetation is hardly borne out by the facts in the case. 

 Any one who has seen the wonderful profusion of Ferns in a 

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