32 CHLOROPHYCEAE 



The majority of the cells, including all those in the anterior 

 quarter, are wholly somatic, and only a few are able to give rise to 

 daughter colonies. When this occurs a cell increases in size and 

 divides many times to produce a small hollow sphere with a pore 

 (phialopore) towards the outer edge. These plants {gonidia), which 

 hang down into the cavity of the parent, then invert, the process 

 commencing opposite their phialopore, and later they are liberated 

 into the parental cavity (cf. fig. 20). They remain in the cavity until 

 the parent tears open, in Volvox aureus at the phialopore of the adult, 

 in V. glohator at any place. In V. africana it is possible to see as 

 many as four generations in the one original parent colony. 



In sexual reproduction the plants are either monoecious 

 (V. aureus) or dioecious (F. glohator), and, furthermore, plants re- 

 producing sexually are usually devoid of asexual daughter spheres. 

 Cells giving rise to eggs {egg cells) enlarge considerably, but do 

 not undergo division, and the flagellae disappear, whilst cells 

 giving rise to the antherozoids [antheridia) divide up into 

 sixteen, thirty-two, sixty-four or 128 small elongated cells which 

 form a plate or globoid colony which may invert in the same way as 

 the asexual gonidia (cf. fig. 21). The fertilization mechanism is not 

 known for certain, but in the dioecious species the antherozoids are 

 said to penetrate the female colony and then enter the ovum from 

 the inner side. The first divisions of the zygote involve meiosis, and 

 the oospore then develops into a single swarmer that grows into a 

 "juvenile" plant of about 500 cells which finally inverts before 

 developing into the adult (cf. fig. 21, also p. 43 for a comparison 

 with Hydrodictyon and a possible interpretation). There is evidence 

 that in some species the "juvenile" stage is omitted. One of the 

 characteristic features of the genus are the inversions that occur at 

 diflferent stages of the life cycle, and it is difficult to see why they 

 occur or what the conditions were under which they first developed. 

 It may be associated with the fact that the cells are formed with the 

 eye-spot facing the interior, but even then the problem arises as to 

 how the individual cells came to be arranged thus. 



*Sphaerellaceae: Sphaerella {sphaer, ball; ella, diminutive of 

 affection) (Haematococcus). Fig. 23. 



A characteristic of this genus is the area between the protoplast 

 and the cell wall ; this is filled by a watery jelly and is traversed 



