SEX IN UNICELLULAR ALGAE 1 2 1 



(sec Plate I, 4). (iMocwus and Gerloff erroneously refer to this stage 

 as a planozygote.) Since the cells in each pair are attached by their 

 anterior ends, along a common axis, the two pairs of flagella would 

 operate in conflict if both remained active; but here, at least in C. 

 Diocii-usli, another coordinating mechanism comes into play. One 

 of the cells, though morphologically indistinguishable from its partner, 

 ceases to beat its flagella, and the propulsion of the pair is thus left 

 entirely in the hands of its mate. Such pairs regularly progress in one 

 direction, always propelled by the same partner; they do not move 

 forward and backward, as described for C. eiigametos indica by Mitra 

 (1951). It has been shown (Lewin, 1950a) that this difl^erence in 

 behavior is genetically inherent in the two mating types and can be 

 considered to be controlled by a gene inseparably linked with the 

 "sex" locus. A homothallic species, recently isolated, exhibits similar 

 behavior of the vis-a-vis pairs, presenting a special problem of intra- 

 clonal diflterentiation which is now being studied. Like the plano- 

 zygotes of Frotosiphon (Bold, 1933; Maher, 1946-47), of C. variabilis 

 (Behlau, 1939), and of several other Chlamydovwnas spp. (Smith, 

 1950b), the gamete pairs of C. moenjonsii resemble free haploid cells 

 in being positively phototactic, though those of C. eugametos (Aloe- 

 wus, 1940c) are said to show a reversal of taxis following copulation. 

 Recent evidence indicates that, in C. uioenimsii, there may be 

 exchange of intracellular metabolites through the protoplasmic bridge 

 joining the two copulants, so that one might be justified in referring 

 to such pairs as a special case of heterocaryosis. 



Cytogamy 



In darkness, the gametes of C. moeimisii remain in vis-a-vis pairs 

 indefinitely, until death overtakes them. In dim light, a day or more 

 may pass before fusion takes place (cf. C. eugavietos; Moewus, 193 3); 

 while in bright light (300 to 500 foot-candles) cytogamy is con- 

 summated 4 to 8 hours after pairing, and at least 10 hours of addi- 

 tional illumination are required if a viable zygote is to result (Lewin, 

 1949b). In this respect C. vweii:usii differs from some organisms, in 

 which light energy has been shown not to be essential during the 

 process of fusion (for example, Frotosiphon; Maher, 1946-47), and 

 from others (for example, Naviciila halophila; Subrahmanyan, 1946) 

 in which light has been found actually to inhibit the process. There 



