114 Plant Life and Evolution 



classes of pteridophytes are of great antiquity, and 

 little light is shed upon the origin of these types by 

 the study of the fossil records, as the bryophytic 

 forms from which the pteridophytes are presumably 

 descended have left very few recognizable traces, 

 and we are forced to fall back upon the study of the 

 living forms for clues to their origin. Of the liv- 

 ing pteridophytes several types have been assumed 

 to represent the nearest approach to the bryophytic 

 type of sporophyte, and of these there is most evi- 

 dence in favor of two Ophioglossum in the fern 

 series, and Lycopodium among the club-mosses. It 

 is highly probable that these two types, which are 

 very different, represent two quite independent 

 classes derived from different ancestral forms. 

 These ancestral forms may not, however, have dif- 

 fered very much in structure, so far as we can 

 judge, and are most nearly represented at the pres- 

 ent time by Anthoceros. As we have seen, the 

 sporophyte of the latter is relatively long-lived, 

 growing for many months and developing a com- 

 plete photosynthetic apparatus, and like the pterido- 

 phytes it has an almost independent sporophyte, ex- 

 cept for the lack of external organs, leaves, and 

 roots. In Anthoceros a relatively small part of the 

 sporogonium is devoted to spore formation, and 

 there is a certain suggestion of the sporangium or 

 spore-bearing organs of the pteridophytes. 



Bower's Theory of Sterilization. The impor- 

 tance of the progressive sterilization of the sporoge- 



