344 THE PTERIDOPHYTES 



independent of the gametophyte, giving it the land habit, allow- 

 ing it to attain a large size, and making it by far the most 

 conspicuous phase in the life history. 



2. The development and differentiation of fronds into vege- 

 tative leaves and sporophylls, and the grouping of the latter 

 into cones. 



3. The development of heterospory, which differentiated the 

 gametophytes as male and female in sex. 



4. The degeneration of the gametophytes (in heterosporous 

 forms) so that they finally became dependent upon food supplied 

 by the sporophyte in the spore. In Selaginella the female 

 gametophyte even begins its development at the expense of 

 neighboring cells in the megasporangium. These conditions are 

 an exact reversal of the relations between the generations which 

 exist in the bryophytes. 



The three classes of the Pteridophyta are readily distin- 

 guished by the f ollowing, characters : 



Class I. Filicinece. Fronds large and few in number ; those 

 bearing spores generally similar to the strictly vegetative leaves 

 and not grouped in cones. 



Class II. Equisetinece. Leaves reduced to mere scales, form- 

 ing sheaths around jointed stems, which have many peculiarities 

 of structure ; sporophylls of peculiar form, each bearing several 

 sporangia, and grouped in a characteristic cone. 



Class III. Lycopodinece. Vegetative leaves, small and very 

 numerous (except in Isoetes}, covering the stem ; sporophylls 

 generally grouped in cones, each bearing a single sporangium ; 

 gametophytes much degenerate, especially in the heterosporous 

 Selaginella and Isoetes; sperms two-ciliate, except in Isoetes, 

 and not spiral, and many-ciliate as in the Filicinece and Equise- 

 tinece. 



