iv] GIIEEN CELLS OF CONVOLUTA 125 



has been shown that the macrocytes and microcytes 

 fuse in pairs. As a result of this fusion, a single cell 

 (or zygote) is produced, which, after passing through 

 a period of rest, gives rise by division to four or more 

 daughter cells. Since a similar fusion of two cells 

 (or gametes) is the essential characteristic of sexual re- 

 production in all plants and animals, this fusion may be 

 regarded as a process of sexual reproduction. Under 

 ordinary circumstances, however, the macrocytes and 



Fig. 22. A = Resting cell (green or colourless) of the infecting 

 organism. B = Colourless cell dividing to form daughter cells. 

 C = Resting cell containing four daughter cells. D i= Resting 

 cell with eight daughter cells (six shown). Nuc. = nucleus. 

 . = Pyrenoid. 



microcytes of the infecting organism do not fuse with 

 one another. 



Other cells taken from the green streak along 

 the water-line possess no flagella (Fig. 22). They 

 have settled down, withdrawn these structures 

 and have surrounded themselves each with a thick 



