CHAPTER XIII 



THE EVOLUTION OF PLANTS (Continued) 



The polyciliate vascular plants and their derivative higher 

 allies can now be considered. In looking to Aphanochcete 

 or possibly to some related genus of the Chsetophoracese, rather 

 than to the Coleochsetacese, as their starting point, the writer 

 is only influenced by the consideration that in such genera 

 as Aphanochcete, Draparnaldia, and Stigeoclonium the micro- 

 zoospores which reform as gametes have four cilia, while in 

 the first named the large egg cell has four cilia. Such would 

 strongly suggest that they represent the initial types for origin 

 of polyciliate groups. The varied configuration also of the 

 vegetative body, from an expanded green thallus to a branched 

 creeping filiform green mass, is correlated amongst higher 

 plants in the flat prothallus of Ceratopteris, or the branched 

 filiform prothallus of Schizcea. * 



But, as many observers have pointed out, a wide gap now 

 exists between the most advanced algse and the simplest of 

 the ferns, that proclaims the obliteration of numerous pre- 

 existing types. As in so many other lines of advance this 

 morphologically represents a gap that chronologically extended 

 in all likelihood from the ordovician to the early silurian age, 

 when the primitive ferns probably first appeared. 



In attemi)ting to bridge over tliis gap, the existing organism 

 which seems to the writer most nearly to conform to required 

 details is the semi-aquatic fern Ceratopteris. Its abundance 

 and world-wide dissemination in tropics and subtropics is evi- 

 dence of its antiquity. The sexual or prothallial generation 

 is, as in riccioid examples of the Biciliatse, a flat cellular expanse 

 that bears marginal antheridia on smaller male prothallia. 



The young s])orophyte stem that springs from the fertilized 

 egg forms a very short and simple axis that has the primitive 



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