REVERSION IN ORIENTATION TO LIGHT 377 



required to. induce reversion varies with the intensity of the 

 illumination and with the condition of the colonies. In one 

 experiment it required in all intensities much more energy than 

 it did in the same intensities in the other and in both experiments 

 it required much more energy in the higher than in the lower 

 intensities. In one experiment no reversion was observed in illu- 

 minations below 250 m.c, and longer exposure probably would 

 not have induced it, for at the close of the experiment, 9 p.m., 

 the colonies were inactive. In the other experiment reversion 

 was observed, to some extent, in the lowest illumination tested, 

 20.4 m.c. The question as to whether or not it occurs in all 

 intensities that induce positive orientation is consequently not 

 definitely settled. 



W^iy it requires more energy to induce reversion from positive 

 to negative orientation in strong than it does in weak light is 

 not clear. It is, however, well known that in Euglena and Volvox 

 heat and light energy have opposite effects on reversion (Mast, 

 '11, p. 300) . The same is true for Pandorina as we shall dem- 

 onstrate later. Now, a certain amount of light which is absorbed 

 is doubtless transformed into heat. This probably occurs in all 

 illuminations in the same proportion, but in the lower illumina- 

 tions it occurs so slowly that radiation may have relatively a 

 much greater effect than in the higher. Consequently, the heat 

 produced would be more effective in the higher illuminations than 

 in the lower, and since heat tends to make the colonies positive, 

 it would require more light energy to overcome its effect in the 

 higher than in the lower illuminations. The value of this sug- 

 gestion could doubtless be ascertained by studying the effect of 

 different regions in the spectrum on reversion. 



RELATION BETWEEN REVERSION IN ORIENTATION AND THE TIME 

 RATE OF CHANGE IN ILLUMINATION 



In the experiments just discussed, it required to induce rever- 

 sion, sufficient time to indicate that it is dependent upon the 

 quantity of energy received. Under certain circumstances, re- 

 version is, however, of such a nature that it appears to be asso- 



