THE ALGAE 79 



followed by a second division in the same plane and then by two walls at 

 right angles to the others, cutting up the body into eight cells. Meiosis is 

 said to take place during the first two divisions in this process so that this 

 tissue formed within the germinating oospore is monoploid. By subsequent 

 further cell division a wedge-shaped mass of sixteen or thirty-two cells is 

 formed, by which time the oospore bursts open. From each of these cells a 

 single zoospore develops and is liberated through the opening in the oospore 

 wall. These zoospores are quite similar in shape to the zoospores formed 

 in asexual reproduction. According to some workers they only give rise to 

 asexual plants, and several asexual generations may be produced before the 

 sexual plants reappear. 



In the life-cycle of Coleochaete pulvinata, therefore, there is an alternation 

 of sexual and asexual generations which is independent of the cytological 

 alternations. All the vegetative individuals are monoploid, and the diploid 

 condition is only maintained in the oospore, while the monoploid condition 

 is again re-established during the germination of the oospore. We shall see 

 later that this type of alternation of generations is remarkably similar to that 

 in Batrachospermum. In fact there are a number of points of similarity 

 between the sexual reproduction in Coleochaete and in members of the 

 Nemalionales, though whether there is any justification for considering this 

 to be indicative of any phylogenetic relationship is highly doubtful. It is 

 generally regarded as an example of parallel evolution. 



The life-cycle of Coleochaete may be represented by the following 

 diagram : — 



Oospore 



32 Ce// Sfope . . ^ ^ , 



Aniherozoids Oospnere 



Zoospores 



Antheridium Oogonium 



Gometophyte Plant 

 Fig. 6i. — Life-c\cle of Coleochaete. 



It will be readily seen from this account that there is little or no obvious 

 similarity between Coleochaete and Pleurococcus, and it is only on the grounds 

 that Pleurococcus mav occasionally form a filament of cells with an elongated 

 terminal cell and the fact that it can produce a flat, prostrate cell mass, that 



