EUPHYCOPHYTA I27 



asexual zooid. The chief evolutionary features to be observed in so 

 far as reproduction is concerned within the Phaeophyceae are : 

 (i) The elimination of accessory reproduction (the plurilocular 

 sporangia) in the sporophyte. 



(2) A tendency to soral aggregation of sporangia on the sporo- 

 phyte, e.g. Sphacelariales, Dictyo tales. 



(3) A tendency towards anisogamy and finally oogamy. 



Most species show an alternation of generations, but this is by 

 no means regular as there may be considerable modifications. In- 

 deed, the alternation in the Ectocarpales and Dictyosiphonales is 

 so irregular, that the phenomenon would be better termed a Hfe 

 cycle rather than an alternation of generations. Furthermore, the 

 two generations are often not the same in size, and commencing 

 from species with similar morphological generations one may have 

 those in which either the sporophyte or gametophyte is dominant 

 down to plants where only the gametophyte or sporophyte is 

 known. A progression in anatomical development can be traced, 

 but it seems aknost impossible to do the same with the life histories 

 or reproductive organs. 



Within the Heterogeneratae, where the gametophyte is much 

 reduced, the regular alternation may be masked by compHcations 

 produced by such phenomena as parthenogenetic development of 

 the ova. Those members of the Heterogeneratae (excluding the 

 Laminariales) which exhibit this type of alternation have a fully 

 developed diploid or delophycee form which is common in summer, 

 and a much reduced haploid or diploid adelophycee stage which 

 usually appears during the winter months in one of the following 

 forms : 



(a) In a protonemal stage which reproduces the large form by 

 means of 'buds'. 



ib) In a gametophytic prothallial stage which reproduces the 

 large form by means of gametes from plurilocular sporangia. 



(c) In a plethysmothallial stage which reproduces the large form 

 by means of swarmers from either unilocular or plurilocular 

 sporangia. Until recently these were regarded as arrested 

 sporophytes in a juvenile condition. 



Fritsch, however, has suggested (1939) that some of these 

 plethysmothalli are really potential gametophytes (prothalli), 

 especially those dwarf plants which perpetuate themselves by 

 means of plurilocular sporangia. The term 'plethysmothallus' 



