REVERSION IN ORIENTATION TO LIGHT 513 



3. Negative colonies in culture solution Avhich has been made 

 more alkaline by the addition of sodium hydrate, become defi- 

 nitely positive if fresh culture solution is added. 



4. Positive colonies in fresh culture solution become definitely 

 negative if culture solution containing a little sodium hydrate is 

 added. Reversion is, however, not permanent under any of 

 these conditions. The results obtained in one of the experiments 

 described in detail below are tj^pical. 



Culture jars containing Spondylomorum were frequently left 

 uncovered in the laboratory. In some of these, owing to evap- 

 oration, the culture solution became considerably concentrated, 

 and tests with neutral red showed that they were definitely more 

 strongly alkaline than were the solutions in the covered jars in 

 which evaporation had been prevented. On Dec. 18 numerous 

 colonies were taken from one of these exposed cultures with some 

 of the relatively strongly alkaline fluid and exposed in a given 

 illumination. They were found to be strongly negative. After 

 having been exposed for five minutes in this solution they were 

 carefully transferred, without changing the illumination, to an 

 equal amount of solution taken from a covered jar, i.e., to a solu- 

 tion not so strongly alkaline as indicated by the neutral-red test. 

 In this solution they were first momentarily negative, then they 

 became strongly positive, and remained so for six minutes, when 

 some of them began to swim away from the light. Three minutes 

 later, practically all of them were strongly negative. 



These results show clearly that a decrease in the concentration 

 of the salts and alkalis in a culture solution causes negative col- 

 onies of Spondylomorum to become positive, that is, it has the 

 same effect as addition of acids. Now addition of acids produces 

 an increase in the salt contents and a decrease in alkalinity. 

 It is consequently evident that if the effect of acid is due to its 

 effect on compounds in the solution it is not due to the production 

 of salts, but to the neutralization of alkalis. We thus ha^'e a 

 considerable amount of evidence favoring the idea that reversion 

 in the sense of orientation produced by the addition of acids is 

 due to a change in the concentration of hydroxy 1 ions. But 

 whatever the action of these ions may be in the process of rever- 



