REVERSION IN ORIENTATION TO LIGHT 379 



were made on Volvox, but it was repeatedly observed in various 

 cultures of Pandorina, especial' y in those left uncovered for 

 some days, permitting evaporation. 



^"Mietlier or not the younger colonies in dark-adapted cultures 

 exposed to light became active and positive before the older ones 

 was not ascertained. 



EFFECT OF PHOTOSYNTHESIS ON REVERSION IN THE SENSE OF 



ORIENTATION 



It is well known that acids added to the culture solution tend 

 to make Volvox and similar organisms positive in their reactions 

 to light and that under certain conditions low illuminations also 

 tend to make them positive, while high illuminations tend to make 

 them negative. In low illuminations there is, owing to limited 

 photosynthesis and continued respiration, a tendency toward an 

 accumulation of carbon dioxid, result'ng in an increase in acidity, 

 while n high illumination there is, owing to rapid photosynthesis, 

 a tendency toward a reduction in carbon dioxid. This seems to 

 ndicate that ^he tendency toward positive orientation n low and 

 negative orientation in high illumination may be dependent upon 

 photosynthesis. If this is true, then red and yellow light in 

 which photosynthesis is relatively strong should be more effective 

 in producing reversion in the sense of orientation than blue and 

 green in which photosynthesis is relatively weak. Numerous 

 observations on both Volvox and Pandorina adapted to the colors 

 mentioned were made as follows : 



Some of the colonies to be tested were put into jars in each 

 of four black light-tight boxes containing a large window made 

 of blue, green, yellow, and red glass, respectively. Others were 

 put into jars in absolute darkness and still others into jars in 

 strong diffuse sunlight. After having been in these conditions 

 eight hours or longer specimens were taken from each jar and 

 put respectively into six observation aquaria. These aquaria 

 were then placed side by side in the same illumination in the 

 dark-room, others similarly treated were placed in other illumina- 

 tions. In this way the reactions in different illuminations of the 



