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- 
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,. 
