PTERIDOPHYTA 153 
macrospore remains permanently within the sporan- 
gium. 
SUMMARY 
In reviewing the Pteridophytes or Ferns, we have 
seen that the three existing classes are sharply sepa- 
rated from each other by the characters of the sporo- 
phyte. In the ferns proper the leaves are greatly de- 
veloped, while the stem is often short and inconspicuous. 
In the other two classes, the horsetails and club-mosses, 
it is the stem which is especially developed, while the 
leaves are small and sometimes quite functionless as 
organs of assimilation, as seen in Psilotum or Equise- 
tum. 
In the lower or homosporous members of all the 
series, the gametophyte is comparatively long-lived, and 
there is a good deal of similarity of structure in all of 
them, especially in regard to the sexual organs. Both 
the gametophyte itself and the sexual organs show 
marked resemblances to certain liverworts, especially 
the Anthocerotacee. These are so great as to war- 
rant the assumption of an origin of the Pteridophytes 
from liverwort-like ancestors which must have resem- 
bled in many respects the Anthocerotacez. 
The resemblances between ferns and Equisetum in 
the structure of the reproductive organs, and especially 
the spermatozoids, are very marked, and suggest a 
possible common, but very remote, origin for the two. 
The small biciliate spermatozoids of the Lycopods, on 
the other hand, seem to indicate a more direct origin 
of these from forms like existing liverworts; but as 
yet no Bryophytes are known which possess the large 
