reproduction; evolution 317 



rise to the adult apical growing cell, because a similar type of be- 

 haviour is to be found in Acrothrix (Mesogloiaceae) : in gross 

 structure also, e.g. primary and secondary medullary filaments and 

 the assimilatory tissue, there is some shght resemblance between 

 that of Fucus and Eudesme. The comparative looseness of the Af ^50- 

 gloia type of structure and the compactness of the Fucalean does, 

 however, present difficulties. Yet other suggestions have been the 

 evolution of the Fucales from the Dictyotales, though this would 

 demand the elimination of a fully developed gametophyte genera- 

 tion, which has already been greatly reduced in the Mesogloiaceae 

 or Laminariaceae, and also their evolution from the pecuhar mono- 

 typic genus, Splachnidium (cf. p. 158). This last genus might well 

 represent a rehc of ancestral forms from which the Fucales were 

 derived. The adult plant is sporophytic and there is a dwarf genera- 

 tion which is presumably gametophytic though no reproductive 

 organs have so far been observed. The suggestions above involving 

 Chordaria, Acrothrix and Splachnidium indicate that perhaps the 

 Chordariales occupy a key position within the Phaeophyceae. 



The importance of the Chordariales has been further stressed 

 by the very recent discovery that the former Fucalean species 

 Notheia anomala reproduces by means of macro- and micro- 

 swarmers and is not a member of the Fucales at all. Its affinities 

 are surely in the Chordariales and, in view of its thallus construc- 

 tion, probably not far from Splachnidium. As has been pointed 

 out, it may well be that in Notheia the male gametophyte is not 

 produced and the micro-swarmers may act as antherozoids. A 

 dwarf filamentous female gametophyte is still produced but is 

 apparently not hberated. Reduaion of this phase would lead to the 

 Fucalean condition. 



Recent interesting discoveries about reproduction in the pecuhar 

 antarctic genus Phyllogigas suggest that perhaps the Fucales are 

 diphyletic in origin, the Durvilleaceae coming from plants such 

 as Phyllogigas and others coming from a Chordarialean source such 



as Notheia. 



The Chordariales form a more or less homogeneous group and it 

 is possible to suggest how evolution within the order may have 

 taken place. The Chordariaceae would seem to represent the 

 primitive stock with uniaxial plants {Mesogloia) preceding multiaxial 

 {Eudesme, Myriogloia). At some stage reduction must have occurred 

 leading to the reduced epiphytic Myrionemataceae and the saxi- 



