THE EVOLUTION OF PLANTS 135 



logically deduced from the same facts, depending on which 

 facts or classes of facts the emphasis is placed. 1 



109. Homologous Alternation. By the theory of anti- 

 thetic alternation the leafy sporophyte was derived from 

 some such structure as the sporogonium of the Bryo- 

 phytes, the axis existing first, the leaves originating as out- 

 growths at its surface. There could thus be no true 

 homology between any of the organs of the sporophyte 

 and those of the gametophyte, however close the super- 

 ficial resemblance might be. The (gametophytic) leaves 

 of the true mosses, while of like function (analogous) to the 

 (sporophytic) leaves of the club-mosses, are not the same 

 structural elements, i.e., are not homologous with them. 

 By a contrasting theory the gametophytic and sporo- 

 phytic stages were at the first vegetatively or somatically 

 equivalent (except for chromosome number), as is the case 

 now, for example, with the red algae, Dictyota and Polysi- 

 phonia, but, in the course of evolution, the sexual phase 

 became more, and the asexual phase less, important in other 

 forms (e.g., ferns). This is called the hypothesis of 

 homologous alternation, since the vegetative organs repre- 

 sent the same structural or morphological elements. 

 According to the antithetic theory the sporophytic phase 

 was originally entirely dependent on the gametophyte 

 (as now, e.g., in the Liverworts), while according to the 

 homologous theory, the sporophyte has been free-living 

 from the start. By the latter theory, also, leaves did 

 not originate as new formations at regions of the axis 

 previously unoccupied by lateral organs (enation), but 



1 Those wishing to go more fully into this question will (in addition to 

 the article above cited) find much of the evidence presented and analyzed 

 by Lady Isabel Brown in a series of five articles on "The phylogenyand 

 inter-relationships of the Pteridophyta," in The New Phytolo gist for 1908. 

 An extended bibliography accompanies each article of the series. 



