GENERAL 131 



proposed to term the phenomenon a race cycle rather than an 

 ahernation of generations, and this would appear to be the better 

 terminology. Furthermore, the two generations are often not the 

 same in size, and commencing from species with equal morpho- 

 logical generations one may have those in which either the sporo- 

 phyte 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 almost impossible to do the 

 same when the life histories or reproductive organs are considered. 

 Three principal types of Hfe cycle have, however, been recognized 

 by Kylin: 



(a) Fucus type. Only the diploid sporophytic plant is known, 

 with meiosis taking place at gametogenesis, e.g. Fucales. This 

 type, however, is more apparent than real (cf. p. 189). 



{h) Dictyota type. Meiosis is delayed and two similar morpho- 

 logical generations exist, e.g. Dictyotales, Nemoderma, Lithoderma, 

 Ectocarpus siliculosiis (certain areas only), Pylaiella littoralis. A modi- 

 fication of this type is found in Ciitleria where the two generations 

 are of equal significance but morphologically dissimilar, the gameto- 

 phyte being the larger. 



{c) Laminaria type. Meiosis is delayed but the two generations 

 are wholly dissimilar, the sporophyte being dominant whilst the 

 gametophyte is much reduced. 



Quite a number of species must now be included in this last 

 category, although the regular alternation may be masked by com- 

 plications 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. 



{b) 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 



9-2 



