128 EVOLUTION OF PLANTS 



the case remains to be seen, as the earliest stages are 

 very imperfectly known. 



The Horse-tails (Equisetinese) (Fig. 36) and the 

 Club-mosses (Lycopodinete) (Figs. 37, 38), while differ- 

 ing in some minor details, agree closely in the main with 

 the eusporangiate ferns in the characters of the gameto- 

 phyte. 



Upon this thalloid gametophyte are borne the repro- 

 ductive organs, antheridia and archegonia, structurally 

 very much like those of the Bryophytes, especially the 

 liverworts, which with little question are the nearest 

 relatives of Pteridophytes among the lower plants. The 

 resemblances are especially marked in the Anthocero- 

 tacese, which are also the nearest approach to the ferns 

 in the structure of the sporophyte. 



Within the antheridium are produced motile sperma- 

 tozoids, which, in the true ferns, have many cilia (Fig. 

 33, C) instead of the two possessed by the moss-sperma- 

 tozoid, and these require the presence of water in order 

 that they may reach the egg-cell in the open arche- 

 gonium ; and water is also necessary, as in the Bryo- 

 phytes, for the opening of the ripe reproductive organs. 



We have already indicated in a preceding chapter 

 that the motile spermatozoids of the algse are to be con- 

 sidered as modifications of originally non-sexual zoo- 

 spores, which in turn are a reversion to the originally 

 free-swimming ancestral type from which all the green 

 plants originated. The recurrence of these ciliated re- 

 productive cells in the Pteridophytes is a strong argu- 

 ment in favor of considering these plants as being also 

 derived from originally aquatic ancestors. Fertilization 

 is effected in these as in the mosses, and the gameto- 



