SEX IN UNICELLULAR ALGAE 107 



samples a number of clones of five species including anisogamous and 

 oogamous forms capable of interspecific mating. If we are to accept 

 Pascher's classification (1927) of this heterogeneous genus, then these 

 interfertile species of Aloewus' include representatives of three differ- 

 ent subgenera (Smith, 1946). 



Since doubt has been cast on Pascher's results by Hartmann 

 (1934), and on those of Aloewus by a number of other workers (see 

 page 126), the information available in this field can hardlv^ be de- 

 scribed as illuminating. 



INDUCTION OF SEXUAL ACTIVITY 



A considerable amount of attention has been focused on the 

 physiological conditions which may induce cells to become sexually 

 active or may promote gametogenesis. As yet, no clear picture has 

 emerged from the many investigations carried out on various organ- 

 isms by a number of workers, each employing a different approach 

 (see reviews by Czurda, 1933b, and Bold, 1942). In the case of het- 

 erothallic species, it is of course essential at the outset to have isolates 

 of both mating types; and this difficulty has undoubtedly contributed 

 to the failures reported in the past (for example, by Reichenow, 

 1909, for Haematococcus; cf. Schulze, 1927). A few of the factors 

 \\hich have been found to influence sexuality are discussed below. 



Light 



Freliminary Observations. Klebs (1896) found that, though 

 dim light or darkness favored the formation of gametes in Proto- 

 siphon, light tended to promote the process of copulation. He ob- 

 served that svvarmers, formed in a mineral nutrient medium, could 

 be reversibly stimulated to sexual activity either by dilution of the 

 medium or by illumination, and he postulated that the action of light 

 might be through the photosynthetic formation of organic com- 

 pounds, which combined with and neutralized some constituent (s) of 

 the medium inhibitory to copulation. However, he was not able to 

 identify this inhibitor \A'ith the nitrate, phosphate, potassium, or cal- 

 cium salt in the medium he used. 



In Chlamydovionas media (Klebs, 1896) and C. eugavietos (Aloe- 

 wus, 1933), light appeared essential for sexual activity, which ceased 



