190 PHAEOPHYCEAE 



those species where less than eight mature ova are produced it must 

 be assumed that some of the megaspores undergo abortion. 



If the above is to be the correct interpretation, and it would seem 

 to be more satisfactory than any other theory in comparison with 

 other members of the Phaeophyceae, then we can say that not only 

 is there a cytological alternation of generations but there is also a 

 morphological alternation, although the sexual generation is even 

 further reduced from the state found in the Laminariales. This 

 really forms the basis for placing the Fucales in the Heterogeneratae. 

 The alternative interpretation is that the sexual generation has been 

 completely suppressed and is solely represented by the gametes, so 

 that whilst there is a cytological alternation of generations there is 

 only one morphological generation (cf. also Chapter ix). The 

 sporangia are borne in flask-shaped depressions of the thallus called 

 conceptacles, each of which is lined with paraphyses and opens to 

 the surface by means of an ostiole. The plants of the different 

 species may be dioecious, monoecious or hermaphrodite. It has 

 been pointed out that the number of primary rhizoids in the embryo 

 is proportional to the size of the rhizoidal cell, which in turn bears 

 a relation first to the size of the egg, and secondly to the com- 

 plexity of the thallus. On this basis a series of increasing embryonal 

 complexity may be traced, e.g. Fucus -^Ascophyllum ^Pelvetia -> 

 Cystoseira ^Sargassum. 



Geographically the original centre of distribution was un- 

 doubtedly the southern Pacific in the waters of Australia and New 

 Zealand where the greatest number of species are now to be found. 

 This makes an interesting comparison with the preceding order 

 whose original centre of distribution was the northern Pacific in the 

 waters around Japan and Alaska. The Fucales are classified into 

 five groups, the classification being based primarily upon the 

 structure of the apical growing cell or cells : 



(i) Durvilleaceae. A group comprising two genera, Durvillea 

 and Sarcophycus, from Australia and Patagonia, both without any 

 means of apical growth. 



(2) Fuco-Ascophyllae. Growth is determined in the adult stage 

 by one four -sided apical cell. 



(3) Loriformes. Growth is due to one three-sided apical cell which 

 gives rise to a long whip-like thallus. 



(4) Cystoseiro-Sargassae. The apical cell is again three-sided but 



