22 THE ALGAE 



At asexual reproduction the motile bodies come to rest and 

 divide up into two, four, more rarely eight or sixteen, daughter 

 cells. The first division at zoospore formation is normally longi- 

 tudinal and all subsequent divisions are also longitudinal. The bi- 

 ciliate zoospores escape through gelatinization or rupture of the 

 cell wall, but if this does not occur the colony then passes into the 

 palmelloid state, which is usually of brief duration, though in C. 

 kleinii it is the dominant phase in the life of the organism. C. 

 kleinii may thus be regarded as forming a transition to the condi- 

 tion found in Tetraspora (cf. p. 34). 



In sexual reproduction eight, sixteen or thirty-two biciUate 

 gametes are formed in each cell. These are normally either + or - 

 in character and fusion takes place between the strains. The de- 

 velopment of motility, sexuality and power of fusion in at least one 

 species appears to rest upon the development of certain sex hor- 

 mones (see p. 417). In C. agametos an anticopulatory hormone is 

 produced and so there is no fusion. In C. longistigma the gametes 

 are naked (gymnogametes) ; in C. media they are enclosed in a cell 

 wall and just emerge in order to fuse (calyptogametes) ; in C. 

 monoica there is anisogamy as the naked contents of one ganiete 

 (male) pass into the envelope of the other; in C. hraunii the aniso- 

 gamy is more distinct, the female cell producing two or four 

 macrogametes and the male cell eight or sixteen microgametes; in 

 C. suhoogama each cell gives rise to three macrogametes and an 

 antheridial cell that produces four antherozoids; in C. coccifera 

 there is oogamy, with the female cell producing one macrogamete 

 enclosed in a wall whilst the male cell produces 16 or 32 spherical 

 microgametes. In a related genus, Chlorogonium oogamum, one 

 naked ovum is produced and numerous elongate antherozoids. 

 The genus Chlamydomonas appears to be polyploid, the basic hap- 

 loid chromosome number being 16-18. 



The zygote is at first quadriflagellate but it soon loses the flagella 

 and forms a smooth or spiny wall. It subsequently enlarges in most 

 species and on germination generally gives rise to four swarmers. 

 Since it has been demonstrated that meiosis occurs during this 

 segmentation, the normal vegetative cells are haploid. In C. pertusa 

 and C. botryoides, however, the zygote may remain motile for as 

 long as ten days, and hence it may be considered that these two 

 species exhibit the start of an alternation of generations. In C. 

 variabilis the persistent 4-ciliate zygote has for long been known as 



