VOLVOCALES 



27 



marks a first differentiation into a plant soma within the group, and 

 furthermore these somatic cells die once the colony has reproduced. 

 It would be of great importance if the nature of the stimulus that 

 induced some of the cells to lose their reproductive capacity could 

 be determined. It might be possible to investigate such a problem 

 experimentally on some of the undifferentiated species of Eudorina. 

 Sexual reproduction is oogamous, the colonies being either 

 monoecious or dioecious : in the former case the anterior cells give 

 rise to the antherozoids, whilst in the latter case the antheridial 

 plates are liberated intact and only break up after swimming to the 

 female colony where the surrounding walls have already become 

 gelatinous. The zygote on germination gives rise to one motile 

 zoospore and two or three degenerate zoospores.^ 



Chlamydomonadaceae : Pleodorina (pleo, more ; dorina, meaning- 

 less!). Fig. 18. 



This genus is very similar to the preceding one, but the somatic 

 area is more highly differentiated as it occupies one-third to one- 



Fig. 18. Pleodorina Californica. Colony of 120 cells ( x 178). (After Shaw.) 



half of the colony, and the total number of cells is greater, thirty- 

 two, sixty-four or 128. The somatic cells are all situated either in an 

 anterior or posterior position and they die when the colony has 

 reproduced. Reproduction follows the same lines as in Eudorina. 



^ Inversion of the daughter colonies and of the antheridia takes place during 

 development (cf. Volvox). 



