SUMMARY AND CONCLUSION 291 



The sporophyte here is a much more highly organized 

 structure than the gametophyte, reversing the relation 

 of these as found in the mosses. In the ferns it is the 

 sporophyte which is had in mind when a fern is spoken 

 of. The gametophyte (prothallium) is inconspicuous 

 and usually of brief duration, but it must be borne in 

 mind that the leafy fern plant, even the gigantic tree 

 fern, is morphologically the equivalent of the moss 

 capsule, or the still simpler sporogonium of the lower 

 liverworts. 



It is quite possible that the development of an inde- 4 

 pendent sporophyte has taken place at more than one 

 point, and that the different series of Pteridophytes 

 have not all originated from a common stock. The 

 biciliate spermatozoids of the club-mosses and the mul- 

 ticiliate ones of the other Pteridophytes favor this view, 

 although all of the existing Pteridophytes closely re- 

 semble each other in the character of their reproductive 

 parts. 



Corresponding to the external differentiation of the 

 sporophyte, there is a much greater diversity in the tis- 

 sues of the Pteridophytes than is found in any of the 

 lower plants, this being especially shown in the devel- 

 opment of the complicated vascular bundles. The 

 spores, too, are here restricted to a special organ, the 

 sporangium. 



The Pteridophytes, also, show traces of an aquatic 

 ancestry in the development of spermatozoids, which 

 require water in order that they may reach the arche- 

 gonium, so that it is necessary for the gametophyte to 

 be covered with water in order to insure fertilization. 



With the increasing importance of the sporophyte, 



