Mast, Light Reactions in Lower Organisms. 157 



after continuing about 3 cm. it became positive, turned about and 

 moved toward the glowers to //, and probably would have con- 

 tinued farther had it not been prevented from doing so by the 

 wall of the aquarium. It will be noticed that the point n% where 

 the colony was still positive at the end of its course, was about 

 3 cm. nearer the glowers than n, where it proved to be negative, 

 and nearly 7 cm. nearer than the point where it changed its course 

 from negative to positive. That is, the organism was positive at 

 n' in a much higher light intensity than that in which it was 

 negative at n and at the point where it changed from negative to 

 positive. 



12. CAUSE OF CHANGE IN SENSE OF REACTION. 



The results presented above demonstrate that Volvox may be 

 either negative or positive in a given light intensity. This will 

 be brought out more clearly later where it will be shown that Vol- 

 vox, in certain conditions, is negative to light of all intensities to 

 w^hich it responds at all. 



Since a Volvox colony may be either positive or negative in the 

 same environment, it is clear that the transformation from positive 

 to negative or vice versa must be due to some internal change. This 

 change, whatever it may be, is induced by light. It is dependent 

 upon the intensity and also upon the time of exposure, as is shown 

 by the fact that when specimens are exposed to intense light they 

 may be positive for a time and then negative to a much lower inten- 

 sity than that in which they were positive when first exposed. 

 Weak light tends to induce the change which causes the colonies 

 to become positive, whereas strong light tends to induce the change 

 which causes them to become negative. 



Some photosynthetic process in chlorophyl bearing organisms, 

 suggests itself as the probable condition upon which the sense of 

 reaction depends. It might be assumed that the organisms are 

 positive when a given amount of synthesized substance, such as 

 carbohydrates, proteids, or fats, is present, and negative when 

 this amount is decreased. This assumption fits the observed 

 reaction in that such substances are formed in the presence of 

 light, and in that they disappear in darkness, being either further 

 synthesized to form protoplasm, or, perhaps, directly oxidized. 

 But the short time and the slight change in light intensity necessary 

 to produce a change in the sense of reaction is entirely inadequate 



