EVOLUTION 257 



(i) A. Aphanocapsa-^Porphyridium-^Porphyra. 

 B. Praswla-^Bangta^Eu-f{ondQ2iQ. 



Apart from the difference of pigment there is a striking resemblance 

 in morphological structure and reproduction between Bangia and 

 Prasiola. 



(2) Cyanophyceae -^ Porphyridium -> Porphyra -> Bangia -> 

 Eu-florideae. 



In contrast to this there are those who postulate independent 

 origins for the Proto- and Eu-florideae on account of the consider- 

 able diflFerences in structure and reproduction between the two 

 divisions, but at present it would seem impossible to do more than 

 point out the different theories that have been put forward. The 

 later evolutionary changes in the Eu-florideae have already been 

 mentioned in the introduction to Chapter viii and also in the 

 earlier part of the present chapter. 



Phaeophyceae 



Most workers would probably agree that the primitive members 

 of this group are to be found among those members of the Ecto- 

 carpales which either possess a single haploid generation or else 

 two morphologically identical generations. It now remains to 

 indicate how subsequent evolution may have taken place, but only 

 a broad outline can be suggested because individual workers have 

 frequently produced modifications in the lesser details of the 

 evolutionary sequences. It has been stated that the unilocular 

 sporangia of the Ectocarpales, in which meiosis occurs, are morpho- 

 logically equivalent to the tetrasporangia of the Dictyotales. In 

 this case the plurilocular sporangia which give rise to the iso- or 

 anisogametes are morphologically equivalent to the gametangia of 

 the Cutleriales and Dictyotales. One of the outstanding problems 

 is the origin of the Laminariales and Fucales, and in order to 

 account for these it would seem necessary to postulate at least two 

 different lines of evolution, though there were probably even more. 

 As an example of the simpler type of sequence that has been 

 suggested the following may be quoted from Svedelius : 



Phaeosporeae->Cutleriales->Dictyosiphonales-^ 

 Dictyotales^Laminariales^ Fucales. 



CSA 1/ 



