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 Anthocerotaceee. 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 Anthocerotaceae. 



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 



