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 following characters : 



Class I. The ferns, or Filicinece. Fronds large and few in 

 number; those bearing spores generally similar to the strictly 

 vegetative leaves and not grouped in cones. 



Class II. The horsetails, or Equisetinece. Leaves reduced to 

 mere scales, forming 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. The club mosses, or 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 Equisetinece. 





