REVERSION IN ORIENTATION TO LIGHT 383 



EFFECT OF CHEMICALS ON REVERSION 



It is well known that acids and some narcotics tend to make 

 many organisms that orient to Hght, photopositive. Salts and 

 alkalis, on the other hand, rarely have any effect on the sense of 

 orientation. In a recent paper on Spondylomorum ('18) it was 

 fairly clearly demonstrated that the effect of the addition of 

 acids is due to the reduction in the alkahnity of the culture 

 medium and not to the acids as such. The results obtained in 

 experiments on Volvox and Pandorina support this contention 

 and they show that the response of the organism is not specifically 

 dependent upon the chemical constitution of the surrounding 

 medium. 



The effect of acid on the sense of orientation in Volvox and 

 Pandorina is in all essentials precisely the same as it is in Spon- 

 dylomorum (Mast, '18). If a trace of acid is added to a solution 

 containing negative colonies they become strongly positive, re- 

 naain so a few moments and then become negative again. If 

 more acid is now added they again become positive and later 

 negative, just as they did after the first addition of acid. Thus 

 they continue to become positive and negative after each addition 

 of acid until the solution becomes fatal. 



The water in the pond in which the Volvex and the Pandorina 

 used in these experiments appeared gave, in every instance, a 

 very definite alkaline reaction with neutral red, and when acid 

 w^as added the sense of orientation was reversed long before the 

 alkalinity was neutralized. In fact, sufl^cient acid to give even 

 the slightest acid reaction invariably proved fatal. This seems 

 to indicate that reversion in these forms, just as in spondylo- 

 morum, is due to a reduction in alkalinity, and not to the effect 

 of acid as such. It also indicates that the sense of orientation 

 is, hot directly related with the concentration of the alkalis. 



The arnount of reduction in alkalinity required to produce 

 reversion varies with the concentration of the solution and the 

 physiological state of the organisms. It is usually very small. 

 For example, in one experiment titration against HCl showed 

 that the water in which the colonies (Pandorina) lived was 0.0019 



