SEX IN UNICELLULAR ALGAE 123 



nioihis this wall in;iy appear niaiivc), and a mmihcr of other nictaholic 

 changes become apparent in the young zygospore, hi most species 

 starch is first accumulated and later replaced by oil, the pyrenoids 

 frajrment or become invisible, and in many cases the cells accumulate 

 haematochrome and become bright orange or red. In others they 

 remain green, as, for example, in Fhyllocardhnn (Korschikoff, 1927), 

 Chhiinydovwuas viediii (Klcbs, 1896), C.viovoica, (Strehlow, 1928- 

 29), Chlorococcwn (Starr, 1949), C. engametos (Moewus, 1933) and 

 C. vweivusii (Lewin, 1949b). Although in the haplophase Polytonia 

 ftisifonnis is competely apochlorotic, the zygospores have been de- 

 scribed as chlorophyllose (Korschikoff, 1926; Strehlow, 1928-29), 

 a unique feature that would present a number of interesting problems 

 to the experimental phycologist. 



Smith (1950b) has pointed out that one may distinguish two types 

 of zygotes, those in which no growth takes place after cytogamy, 

 and those wherein photosynthesis and growth continue in the diplo- 

 phase, in the planozygote (for example, Phyllocardiimt, Korschikoff, 

 1927) or in the zygospore stage (for example, Tetraspora, Klyver, 

 1929, Chhmiydovwnas chlamydogama, Bold, 1949, and Chlorococ- 

 ciivi, Starr, 1949). The volume of each gamete in C. moeiviisii is about 

 200 cubic microns, while that of the mature zygospore approaches 

 12,000 cubic microns, indicating a thirty-fold increase in bulk in 

 approximately 60 hours. This rate of growth is comparable with that 

 of the haplophase, which is capable of five generations (doublings) in 

 a similar period (Lewin, 1952a) . Though there is no vegetative division 

 in the diplophase (except perhaps in Raciborskiella, Wislouch, 1924- 

 25), and we can hardly speak of an alternation of sexual generations 

 (cf. Behlau, 1939), the line dividing Chlmnydovwiias from such an 

 alga as Chlorochytrium (Chlorococcales) is thin. According to Kurs- 

 sanow and Schemakhanowa (1927), the vegetative cell of Chlorochy- 

 trium levmae is typically diploid and, like the zygote of Chiaviy- 

 domo7ias vweujiisii, is capable of enlargement but not of direct mitotic 

 division. By meiosis and succeeding mitoses, 256 swarmers are pro- 

 duced which are capable of copulation to form quadriflagellate plano- 

 zygotes; and these in turn settle down to an endophytic existence as 

 a new diplophase. Thus the sexual cycles of Chlaviydoniouas and of 

 Chlorochytrium are seen to differ in no essential respect, but merely 

 in their relative emphasis on the haploid or diploid phase. 



