128 EVOLUTION OF PLANTS 
the case remains to be seen, as the earliest stages are 
very imperfectly known. 
The Horse-tails (Equisetihee) (Fig. 86) and the 
Club-mosses (Lycopodinee) (Figs. 87, 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- 
tacez, 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. 
38, 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 alge are to be con- 
sidered as modifications of originally non-sexual z06- 
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- 
