NEMODERMA 

 4^ 



Ix 



CUTLERIA 



1 1 >o 



V UD w 



ASPEROCOCCUS 



Ix 



LAMINARIA 



Fig. 1 88 Types of life cycle in the Phaeophyceae and their pos- 

 sible inter-relationships. RD = position of reduction division in the 



life cycle. 



A Study ofthe life cycles (Fig. i88) of representative genera in the 

 Phaeophyceae is of some interest. It will be noted that in the dia- 

 gram the arrows indicating suggested evolutionary progression in 

 the Hfe cycles in general conform to evolutionary morphological 

 progression. In most of the types the Hfe cycle is not simple, be- 

 cause each generation is capable of repeating itself. Lying behind 

 the life cycle, however, is the fundamental nuclear cycle, but 

 despite this there is no real regular alternation of generations ex- 

 cept in the Dictyotales, Laminariales and some members of the 

 Chordariales and Sphacelariales. This diagram leads to the con- 

 clusion that the primitive Phaeophycean was either a plant with 

 only a haploid generation in the Hfe cycle, or one in which there 

 was isomorphic alternation. The first alternative would involve the 

 interpolation of a sporophytic generation through a delay in meiosis, 

 and the subsequent loss of the gametophyte. The second hypo- 



319 



