REPRODUCTION 



251 



The only definite exception to this morphological equality at 

 present is seen in Halicystis ovalis where Derbesia marina forms the 

 diploid generation, although it is possible that a similar state of 

 affairs may exist in Urospora. 



(3) A multicellular diploid generation in which the gametophyte 

 is reduced to the unicellular state, e.g. Codium. 



{i a) A persistent unicellular haploid state alternating with a 

 persistent or short-lived unicellular diploid state. This can be 

 regarded as a morphological modification of (i) above or vice versa. 



Halicystis CoDiun 



Fig. 165. Types of life cycle in the Chlorophyceae and their possible inter- 

 relationships. i^Z) = position of reduction division in the life cycle. 



A Study of these life cycles immediately indicates that as a 

 series they can be read in either direction, from i->2^3 or from 

 3^2^ I. On morphological grounds, however, it is more satis- 

 factory to accept the view that the primitive cycle is that in which 

 the haploid generation is dominant, and that the sporophyte has 

 been subsequently intercalated, presumably by a delay in the 

 occurrence of meiosis as in the Rhodophyceae. Therefore in at 

 least two of the groups it would seem as if the course of events during 

 their evolutionary history has been much the same. In the primi- 

 tive state the haploid filaments would perhaps be monoecious 

 so that the first development would concern the appearance of 

 the dioecious condition, e.g. Ulothrix sp. and the Conjugales. 



