REVERSION IN ORIENTATION TO LIGHT 389 



5. Under certain conditions, sudden decrease in illumination 

 makes negative colonies momentarily positive. This change in 

 the sense of orientation is dependent upon the time-rate of change 

 in the intensity of the illumination. 



6. Reversion is not primarily dependent upon photosynthesis. 

 Red and yellow light in which photosynthesis is relatively strong 

 have little effect on reversion, while green and blue, in which 

 photosynthesis is relatively weak, are nearly as effective as white 

 light. 



7. The rays of light which have the greatest stimulating 

 efficiency (green and blue) are the most potent in producing 

 reversion in the sense of orientation. 



8. Increase in temperature causes negative specimens to be- 

 come positive and decrease causes the opposite, but neither the 

 degree nor the extent of change in the temperature is specific in 

 its effect. Under certain conditions, the colonies may be nega- 

 tive or positive in practically all temperatures in which they 

 orient at all. 



9. Alkalis have little, if any effect on reversion. Acids and 

 some anesthetics, especially chloroform, cause negative colonies 

 to become strongly positive, but reversion is not specifically de- 

 pendent upon the concentration of the chemicals. Colonies 

 which are positive in a solution having a given chemical concen- 

 tration may be negative in the same solution or even in a weaker 

 solution. The effect of acids is probably due to the accompany- 

 ing reduction in the alkalinity of the cultural solution. 



10. The sense of orientation is dependent upon the physio- 

 logical state of the colonies as well as upon the constitution of 

 the culture medium. 



11. The sense of orientation is dependent upon the age of the 

 colonies. Young colonies are more likely to be negative than 

 old ones. In a given solution the young specimens frequently 

 collect at the side of the dish farthest from the light while the 

 old ones collect at the opposite side. 



12. Reversion in orientation is probably associated with 

 changes in permeability, positive orientation being associated 

 with an increase and negative orientation with a decrease in 

 permeability. 



