PLEUROCOCCUS 



191 



the adult form, and sooner or later divide, commonly by two 

 successive walls at right-angles to one another, to form four- 

 celled packets. Each daughter-cell is thus the quadrant of 

 a sphere, but with subsequent growth it rounds off and separates 

 from its neighbours, giving rise again to the adult form. This 

 process of vegetative propagation is, when conditions are favour- 

 able, repeated at frequent intervals, and in this way Pleurococcus 

 rapidly covers large areas. 1 



FIG. 101. Various colonial Green Algae. A, Scenedesmus quadricauda. 

 B, S. obliquus, var. dimorphus. C, Pediastrum. D, Small part of a 

 colony of Tetraspora (the cells are shown black). E, Gonium. 



The ordinary cells of Pleurococcus possess a remarkable 

 power of withstanding drought, almost comparable to that 

 of the zygospores of Chlamydomonas and other simple forms 



1 In wet weather, when the cells are covered with a surface film of 

 moisture, the protoplasmic contents sometimes divide to form a number 

 of masses which are liberated by the breaking open of the cell-wall as 

 naked biciliate motile elements, known as zoospores (cf. p. 214) ; the latter 

 sooner or later come to rest and, withdrawing their cilia, secrete a membrane 

 and form new Pleurococcus-individuails. Gametes exhibiting an isogamous 

 sexual process are also stated to be produced at times. 



