254 REPRODUCTION, EVOLUTION, ETC. 



Chlorophyceae, in spite of morphological differentiation (hetero- 

 trichy), nevertheless maintained the simple form of life cycle, and 

 that those cases where the sporophyte has been interpolated must 

 be regarded as forming divergent lines of evolution. 



Another feature to which attention must be drawn is that in the 

 Chlorophyceae the interpolation of the sporophyte has proceeded 

 considerably further, whereby the sporophyte becomes wholly 

 dominant. In the Rhodophyceae, however, this has only happened 

 in one case, namely Lomentaria rosea, and even here the gameto- 

 phyte has only been suppressed abnormally in European waters. 

 In the red algae there is a reduction series instead, and this leads 

 back to wholly haploid plants, e.g. Ahnfeldtia, in which the con- 

 dition has been produced secondarily. 



When we turn to the Phaeophyceae the problem is much more 

 difficult because there are at least two alternatives with very little 

 evidence to enable one to determine which is likely to be the more 

 correct : 



(i) On the first hypothesis the primitive Phaeophyceae are to be 

 regarded as wholly haploid, and the series must be read in one 

 direction in which the sporophyte is again interpolated through a 

 delay in meiosis, the series terminating with those algae in which 

 the sporophyte generation is wholly dominant, e.g. Fucus, 



(2) On the other hypothesis the primitive Phaeophyceae were 

 filamentous forms possessing two equal generations, haploid and 

 diploid, and subsequent development took place along two lines, 

 one in which the sporophyte and the other in which the gameto- 

 phyte became increasingly dominant. 



There is, of course, the third possibility that the primitive 

 Phaeophyceae were diploid, having arisen from diploid Chloro- 

 phyceae such as the Siphonales, but the morphological changes 

 involved render this possibility extremely unlikely. 



Such evidence as may be adduced for either of the first two 

 hypotheses is summarized below : 



(a) Very few members of the Ectocarpaceae are wholly haploid, 

 and in at least one case, Ectocarpus virescens, the parthenogenetic 

 development of the eggs suggests a degenerate life cycle rather 

 than a primitive one. 



