SFX IN UNICELLULAR ALGAE 1 1 3 



action of light, the energy probably being absorbed by chlorophyll 

 (see page 109); and as a result sonic stimulus passes into the flagella, 

 rendering them temporarily susceptible to intersexual agglutination. 

 "When cells arc kept in darkness, they rapidly lose their ability to 

 mate, and we may attribute this to disappearance or decomposition 

 of genetyllin. This reaction follows a course exhibiting two phases, 

 both affected by temperature. In the first to 1 5 minutes after dark- 

 ening there is no loss of sexual activity; the duration of this phase is 

 controlled largely by the period of pre-illumination. There then fol- 

 lows the second phase, during which the activity falls off exponen- 

 tially to zero; its duration ranges from 15 to 60 minutes or more, 

 depending on temperature. If we further postulate that this exponen- 

 tial loss of activity results from a unimolecular breakdown of gene- 

 t\llin, it can be deduced from such data that at 30° its half-life is 

 less than 1.5 minutes; at 10° it is 9 minutes; and at 5° it is about 22 

 minutes. 



A problem immediately presents itself, namely, can this hypo- 

 thetical agent be replaced by any compound supplied in the medium? 

 A number of known substances (glutathione, cysteine, acetate, suc- 

 cinate, glucose, hexose diphosphate, phosphoglycerate, and adenosine 

 triphosphate) were subjected to a reasonably sensitive test (the dis- 

 placement of the genetyllin breakdown curve in darkness) without 

 any positive result, perhaps as a consequence of permeability barriers. 

 Unsuccessful attempts were also made by Provasoh (personal com- 

 munication) and by the present author to demonstrate the presence 

 of a solute, in filtrates from sexually active cells, capable of evoking 

 a sexual response in non-sexual cell suspensions. Smith (1946) re- 

 ported a similar lack of success in experiments conducted with other 

 species of Chlmnydomonas, notwithstanding the positive results 

 claimed for C. eugametos by Moewus (193 3) (see below). 



It thus seemed possible that, at least in C. TiweToiisii, lack of suc- 

 cess in attempts to demonstrate the activity of solutes in the medium 

 might be attributable to the non-diffusible nature of genetyllin, and 

 this possibility was tested indirectly in the following way. It had 

 been found that cells subjected to more than 30 minutes of illumina- 

 tion retain full sexual activity for some minutes after transfer to dark- 

 ness. The longer the period of pre-illumination, the longer the period 

 of darkness during which full activity is retained (see page 110). 

 This suggested that, though genetyllin disappears at a constant rate 



