CHAPTER XXV 



THE PTERIDOPHYTES AND THE APPEARANCE OF 

 HETEROSPORY 



302. The pteridophytes.* The division Pteridophyta (mean- 

 ing fern plants) comprises three classes : (1) the ferns, or Fili- 

 cinece, (2) the horsetails, or Equisetinece, and (3) the club mosses, 

 or Lycopodinece. Representatives of these groups are generally 

 somewhat familiar to all, and no one would think of grouping 

 them with the liverworts and mosses. The differences become 

 more conspicuous after a study of the life histories of pteri- 

 dophytes, which shows that the large fern plant with its roots, 

 stem, and leaves is really an asexual generation, or sporophyte, 

 and that the gametophyte is represented by a small, compara- 

 tively insignificant sexual generation. This condition, so dif- 

 ferent from anything in the bryophytes and thallophytes, 

 marks one of the great forward steps in the progress of plant 

 evolution. It leads towards the seed plants, for these highest 

 forms with their varied and complex structures are sporophytes, 

 whose gametophyte generations are so much reduced that they 

 can only be recognized by careful study of the processes of seed 

 formation. 



303. The advances in plant evolution up to the pteri- 

 dophytes. It is well to summarize at this point the contribu- 

 tions of the thallophytes and bryophytes to the progress of 

 plant evolution. 



1. The algce. The chief contributions of the algse to plant 

 evolution were four in number: (1) the attached many-celled 

 plant body arose from the single-celled condition of the lowest 



* To THE INSTRUCTOR : The introduction to this chapter assumes that the 

 life history of some fern has been studied in the laboratory. 



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