320 THE ALGAE 



thesis only involves the loss of the gametophyte generation. Against 

 the first hypothesis is the fact that very few members of the Ecto- 

 carpaceae are known to be haplonts, and in one good case, E. 

 virescens, the parthenogenetic development of the eggs suggests 

 degeneration rather than primitiveness. The general weight of evi- 

 dence would appear to be in favour of isomorphic alternation as the 

 primitive Phaeophycean condition. 



Schemes A and B represent two sUghtly different views of evolu- 

 tionary development within the Phaeophyceae, in both cases 

 starting from a simple branched filamentous ancestor with iso- 

 morphic alternation. Which of these two schemes is accepted de- 

 pends very largely upon the view taken about the origin of the 

 Fucales and Laminariales. 



The origin of the Rhodophyceae presents almost as many diffi- 

 culties as that of the Phaeophyceae. On one hand they may have 



Oogamy __Dictyotales Fucales 



1 -^ 4o ^ 



T Tilopteridales \ / 



Anisogamy V Cutleriales J / Laminariales 



A c_u-^«i^^:,Ur \ ^ Sporochnales / ?/ 



^ / Desmarestiales 



I / 

 Advanced Chordariales 



Sphacelariales 



Isogamy 



reduced 



Ectocarpales 



/ 



Dictyosiphonales 



1 



Ectocarpales 



Primitive Ectocarpales 



ISOGENERATAE HETEROGENERATAE 



Scheme A 



Dictyotales 

 Sphacelariales 



Haplostichineae 

 A 



Sporochnales Desmarestiales 



Cutleriales -< 



Polystichineae 

 A 



Laminariales Fucales 



Tilopteridales -<- 

 ISOGENERATAE 



Ectocarpales 



heterogeneratae 

 Scheme B 



CYC LOS PORE AE 



