388 S. O. MAST 



tively weak in alkalinity or relatively strong in narcotics, in all 

 of which orientation is positive and permeability relatively high, 

 a sudden decrease in illumination must produce an increase in 

 permeability, and in relatively low temperature or intense light 

 and in solutions strong in alkalinity or anesthetics, all of which 

 tend to produce negative orientation and low permeability, a 

 sudden increase in illumination must produce increase in permea- 

 bility. 



There is at present no evidence which bears on these problems. 

 I have stated them in order to emphasize the fact, that whether 

 orientation depends upon shock-reactions or not, any theory 

 that accounts for reversion in orientation should account for the 

 changes in the cause of the shock-reactions which accompany it. 



SUMMARY 



1. The reactions to light in Volvox and Pandorina are prac- 

 tically the same. Both forms orient fairly precisely and both 

 may be either negative or positive. 



2. Dark-adapted colonies are usually positive in weak and 

 negative in strong illumination, never the opposite. Light- 

 adapted colonies are sometimes positive in strong and negative 

 in weak illumination. 



3. If dark-adapted colonies are exposed to continuous illumi- 

 nation they are neutral for a short time, then they become posi- 

 .tive, passing through a maximum, after which they become 

 neutral again, then they become negative, passing through a maxi- 

 mum, after which they again become neutral and finally positive 

 again. After they have reached this final state they remain posi- 

 tive no matter how intense the light may be, and they probably 

 are negative in weak light. 



4. The time required to pass through these various stages de- 

 pends upon the intensity of the light. The higher the illumina- 

 tion, the shorter the time. Reversion is, therefore, dependent 

 upon the time of exposure as well as upon the intensity of the 

 illumination. But it requires much more energy to induce re- 

 version in high that it does in low illumination. 



