reproduction; evolution 315 



Table 5 



MONOGENETIC DIGENETIC TRIGENETIC 



Isomorphic Heteromorphic 



, * , 



Gametophyte Sporophyte 

 dominant dominant 



HAPLOPHASic Volvocales Most Bonnemaisonia 



Conjugales Nemalionales. (see p. 229) 

 Charales Protoflorideae 



Ulva Cutleria Laniinaria Most 



DiPLOHAPLO- Enteromorpha Rhodophyceae 



PHASIC Dictyota 



Zanardinia 



Valoniaceae 

 DiPLOPHASic Siphonales 

 Fucales 



siderable difficulty. There would seem to be two major alterna- 

 tives: 

 (i) An origin from a unicellular ancestry within the Chryso- 

 phyceae or Xanthophyceae with intermediate forms such as 

 Phaeothamnion (cf p. 274). In support of this hypothesis 

 there is a similarity in so far as pigments and biochemical 

 substances are concerned (see p. 4), but against it is the 

 fact that nearly all the Chryso- and Xanthophyceae are fresh 

 water and practically all the Phaeophyceae are marine. 

 Whilst the sea as we know it probably did not exist when the 

 Phaeophyceae first emerged, nevertheless one would have 

 expected more intermediate forms to have survived. There 

 is also the fact that the Chrysophyceae and Xanthophyceae, 

 so far as is known, have only the haploid generation in the 

 life cycle, and species of Ectocarpus {Ectocarpus virescens) 

 with a similar Ufe cycle would seem to be reduced. 

 (2) An origin from heterotrichous members of the Chloro- 

 phyceae, i.e. within the Chaetophorales. The main objection 

 to this view is the difference in pigments, especially the 

 presence of fucoxanthin and fucosterol, but nevertheless 

 there are members of the Chlorophyceae, e.g. Zygnema pec- 

 tinatum in which fucoxanthin is said to occur. The develop- 

 ment of this pigment may well, therefore, have occurred 

 more than once in the development of the Chlorophyceae. 



