250 REPRODUCTION, EVOLUTION, ETC. 



male plants and two to female. Subsequent developments, which 

 must be interpreted as retrogressive, can be seen in Phyllophora 

 memhranifolia, where the tetraspores are grouped into nemathecia 

 on the diploid plant; in P. Brodiaei, where the diploid phase has 

 disappeared and the nemathecia can be regarded as growing 

 parasitically in the haploid thallus ; and finally in Ahnfeldtia, where 

 meiosis no longer takes place and instead the nemathecia contain 

 monospores. Lomentaria rosea in European waters is another 

 example of a reduced life cycle, because in this species the gameto- 

 phyte generation is wholly suppressed, whereas in the other 

 examples it is the sporophyte generation that has been reduced. 



In his studies on the Rhodophyceae Svedelius coined a number 

 of terms which have subsequently come into common usage : 



Haplohiont. A sexual plant with only one kind of individual or 

 biont, dioecious plants being regarded as representing one kind of 

 individual. 



Haplont. A sexual haploid plant with only the zygote diploid. 



Diplohiont. A plant possessing alternation of generations and two 

 kinds of individuals, and usually with a much greater number of 

 meiotic divisions since each tetrad of spores involves meiosis. If 

 Fucus is regarded as possessing sporangia and reduced gameto- 

 phytes it will belong to this group rather than being treated as a 

 diploid haplobiont. 



Diplont. A sexual diploid plant in which only the gametes are 

 haploid (e.g. C odium). 



The terms ' * haplo- ' ' and ' ' diplobiont ' ' do not necessarily coincide 

 with the cytological generations, e.g. Codium, and there has been 

 further confusion from the inaccurate usage of these terms by later 

 authors, some of whom have introduced completely new interpre- 

 tations of the w^ords. In the Rhodophyceae the morphological 

 changes that would be involved make it highly improbable that the 

 diplobionts were primitive to the haplobionts. 



Fig. 1 65 shows a series of typical life cycles that have been found 

 in the principal members of the Chlorophyceae, and here again it 

 will be seen that three principal types can be distinguished : 



(i) A multicellular haploid generation in which the diploid phase 

 is present in the unicellular state (e.g. Ulothrtx). 



(2) An alternation between multicellular diploid and haploid 

 generations, both of which are usually morphologically identical. 



