VOLVOCALES 23 



flagellae, and is often strongly phototactic. Variations in the 

 structure of the cell occur throughout the genus, which contains 

 about 150 species. There may be more than one pyrenoid present 

 (C sphagnicola) or they may be completely absent (Chloromonas), 

 whilst the chloroplast may be reticulate {Chlamydomonas reticulata), 

 or axile and stellate (C. eradians), or it may be situated laterally 

 (C.parietaria). It has been said that under cultural conditions many 

 of the characteristic features can be modified, and that therefore 

 some of the forms are not true species but are simply phases in 

 the life cycles of other species. 



The motile cells are spherical, ellipsoid, or pyriform in shape with 

 a thin wall which occasionally possesses an outer mucilage layer. 

 The two flagellae are situated anteriorly and either project through 

 one aperture in the wall or else through two separate canals, but in 

 either case at the point of origin of the flagellae there are two basal 

 granules whose function is not yet clearly established. Each cell 

 typically possesses two contractile vacuoles which have an excretory 

 function. At asexual reproduction the motile bodies come to rest 

 and divide up into four, more rarely eight or sixteen, daughter cells. 

 The first division at zoospore formation is normally transverse, and 

 in those cases where it is longitudinal a subsequent twisting of the 

 protoplast makes it appear to be transverse. The zoospores escape 

 through gelatinization 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 forms the dominant phase in 

 the life history of the species. C. Kleinii may thus be regarded as 

 forming a transition to the condition found in Tetraspora (cf. p. 34). 



In sexual reproduction eight, sixteen or thirty-two gametes are 

 formed in each cell. In Chlamydomonas longistigma the gametes are 

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

 from which they emerge in order to fuse (calyptogametes) ; in 

 C. monoica there is anisogamy as the naked contents of one gamete 

 pass into the envelope of the other ; in C. Braunii there is marked 

 anisogamy, the female cell producing four macrogametes and the 

 male cell eight microgametes ; in C. coccifera there is oogamy, with 

 the female cell producing one macrogamete enclosed in a wall whilst 

 the male cell produces sixteen spherical microgametes. In a related 

 genus, Chlorogonium oogammn, one naked ovum is produced and 

 numerous elongate antherozoids, whilst cases of relative sexuality 



