SRX IN UNICELLULAR ALGAE 125 



success w irh /Agosporcs of D/i/mlicllii by subjecting them to a con- 

 trolled program of maturation, desiccation, and resuspension in media 

 of augmented salinity, Starr (1949) effected zygospore germination 

 in Chlorococcwni and Chlamydovioiias chlamydoga-nm by incubating 

 the spores for 48 hours at 37°. In many species of ChJamydomoiias 

 Smith (1950b) found that about 20 days were required for the ripen- 

 ing of the zygospores, which would then germinate regularly 1 to 2 

 days after being transferred to fresh media. On the other hand, Lewin 

 (1949b) Mas able to induce only sporadic germination of ripe C. 

 7noewusu zygospores in a fraction considerably lower than the 

 10 per cent achieved at room temperatures by Gerloff (1940), but he 

 found that spores in which full maturation had been arrested by star- 

 vation in darkness M'oiild germinate regularly. Moewus and Banerjee 

 (1951) suggested that germination may be naturally arrested by the 

 accumulation of some inhibitor such as cis-cmnmnic acid, which 

 can be removed from some zygotes by exhaustive washing in water. 



The New Haplophase 



In most unicellular algae, meiosis precedes zygote germination, 

 and the sexual cycle is completed by the liberation of haploid zoo- 

 spores. In desmids there is a tendency for two or three of the four 

 post-meiotic nuclei to degenerate, so that only one or two haploid 

 cells emerge from a zygospore on germination (Pothoff, 1928). It 

 appears likely that in Protosiphon, too, meiosis precedes zygospore 

 germination. Possibly only one post-meiotic nucleus survives and 

 multiplies in the emerging thallus, which would otherwise be poten- 

 tially heterocaryotic. But the zygotes of this alga are not very favor- 

 able material for cytological study, and despite several investigations 

 of its life cycle (for example, Bold, 1933; Maher, 1946-47) this 

 aspect remains obscure. 



In species of Volvocales in w hich the zygote exhibits no grow th 

 after syngamy (see page 123), four zoospores are produced as a rule 

 (Smith, 1950b), often within an extruded vesicle; whereas in cases 

 where the zygotes enlarge in the days immediately following their 

 formation a larger number may be formed as a result of subsequent 

 mitoses. In DiinaUella (Lerche, 1937) and in Chlamydomonas moe- 

 ivusii, 4, 8, 16 (Gerloff, 1940), or 32 zoospores may be formed, the 

 number depending on the size of the zygote, and hence on the condi- 



