26 



THE ALGAE 



being the normal number in the common species P. morum. In 

 the species the cells are so compacted that they are flattened by 

 mutual pressure. Each colony is enclosed in a gelatinous matrix 

 with an outer watery sheath, and, together with the next two 

 genera, exhibits some degree of polarity in its progression. In 

 asexual reproduction the colony first ceases to be motile and sinks 

 to the bottom of the pond. Then each cell gives rise by several 

 divisions to a bowl-shaped daughter colony or coenobium. This 



A B 



Fig. 5 Pandorina morum. A, vegetative colony ( x 975). B, colony 

 with female gametes ( x 975). (After Smith.) 



becomes inverted and assumes a spherical shape after which each 

 cell produces two flagella. The daughter colonies are ultimately 

 liberated by disintegration of the parent envelope. In sexual re- 

 production there is a sUght tendency to anisogamy, the male 

 gametes being more active than the female. When the zygote ger- 

 minates it produces one to three biflagellate swarmers, which, after 

 a period of motility, come to rest and divide to give new colonies. 

 Another simple colonial member like Pandorina is Volvulina 

 steinii with 16 cells arranged in four tiers of four. 



Volvocaceae: Eudorina {eu^ well; dorina^ meaningless). Fig. 6 



The colonies are spherical or eUipsoidal, the posterior end often 

 being marked by mamillate projections. They contain sixteen, 

 thirty-two (commonly) or sixty-four biflagellate cells, which are 

 not closely packed and are sometimes arranged in transverse rows. 

 The flagella of the individual cells emerge through funnel-shaped 

 canals. Sometimes the coenobitmi becomes an amorphous palm- 

 elloid mass, and then when conditions once more become suitable 

 individual cells break away and grow into new coenobia. In most 



