326 PLANT GROWTH SUBSTANCES 



of heterothallic strains are treated in this manner and mixed in darkness 

 there is a fusion in pairs to form quadriflagellate zygotes. 



There are two alternative hypotheses to account for the different 

 behavior in darkness of the Stanford series and of the etigametos series: 

 Light is not essential for the formation of motility and sexuality 

 inducing substances in the Stanford series; or light is essential for their 

 formation in the Stanford series but in darkness these substances diffuse 

 out from the cells at a much slower rate than in the eugametos series. 



A demonstration of the necessity of light in the formation of sexual 

 substances in the Stanford series involves heterotrophic cultivation of 

 them in darkness for many cell generations. Thus far the heterothallic 

 C. Reinhardi is the only member of the Stanford series successfully 

 cultured in darkness and only when the carbon source is sodium acetate. 



PreHminary experiments with this species showed that when palmella 

 cultures are grown in darkness for several days and then flooded only 

 a small percentage of the cells become motile. If these motile cells are 

 mixed in darkness with sexually functional cells of opposite sex brought 

 from light there is no sexual reaction. On the other hand if cultures 

 grown in darkness are flooded and exposed to whitelight from a fluorescent 

 lamp all of the cells become motile in about half an hour, but for an hour 

 or so afterward they show no sexual reaction when mixed with functional 

 gametes of opposite sex. After approximately two hours' exposure to 

 light there is a formation of typical clumps sixty or more seconds after 

 mixing with functional gametes of opposite sex. With further illumina- 

 tion the time before the appearance of typical clumps after mixing 

 becomes shorter and shorter, until they are formed within less than five 

 seconds. The number of clumps in a mixture also shows the increased 

 intensity of sexuaUty as exposure to Hght continues. In the earliest 

 sexual responses after exposure to light there are only a few clumps after 

 mixing with gametes of opposite sex, but in subsequent tests after further 

 illumination there is a progressive increase in number of clumps. In these 

 experiments the criterion for a development of sexual substances after 

 exposure to light has been a formation of clumps after mixing with 

 functional gametes of opposite sex. This was confirmed by killing the 

 motile cells and finding quadriflagellate zygotes in the mixture. 



The foregoing experiments give support to the second of the two 

 hypotheses to account for differences in behavior of the Stanford and 

 the eugametos series in darkness. Namely, light is essential for formation 



