Fig. 187 Hypothetical life cycle of heterotrichous ancestral form 

 (left) of Cutleria leading to present cycle. 



Within the Phaeophyceae themselves there would seem to be 

 general agreement that those members with isomorphic alternation 

 of generations probably have some relationship one to the other. 

 The apparently difficult case of Cutleria and Aglaozonia can be 

 explained on the basis of an original heterotrichous ancestor in 

 which the aerial part of the gametophyte persisted and only the 

 basal part of the sporophyte (Fig. 187). In the case of Microzonia 

 (see p. 142) one generation seems to have disappeared completely. 

 There also seems to be some form of general agreement that the 

 orders with heteromorphic alternation (Heterogeneratae) can con- 

 veniently be divided into those that are pseudoparenchymatous 

 (haplostichineae) and those which are truly parenchymatous (poly- 

 stichinae). The real problem in the Phaeophyceae is associated 

 with the origin of the Laminariales and Fucales. It is, for example, 

 possible to imagine a Une of evolution, not only on morphological, 

 e.g. the cable type of construction, but also on reproductive 

 criteria, commencing from Ectocarpus-^Castagnea->Chordana-> 

 Chorda-^ Laminaria, whilst an alternative origin would be within 

 the ancestral forms of genera such as Dictyosiphon or Punctaria in 

 the Dictyosiphonales. It is also extremely tempting to consider 

 whether the Fucales may not have been evolved from the Lamin- 

 ariales because of the existence of forms such as Durvillea. Indeed 

 recent work by Naylor (cf. p. 207) on Himanthalia and Durvillea 

 has served to strengthen this possibiHty. 



It has also been suggested that the Fucales might be derived 

 from the Mesogloiaceae (Chordariales), but the evidence produced 

 cannot be regarded as wholly convincing. The principal points are 

 associated with the presence of the apical hairs falling off and giving 



316 



