ORIENTATION IN COLONIAL FORMS 



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on the shaded side, or a decrease in the same phase on the 

 illuminated side or a decrease in the forward phase on the 

 shaded side, or an increase in this phase on the illuminated 

 side. Can it be ascertained which of these is the cause of 

 the difference between the effect of the stroke of the flagella 

 on the shaded sides and that of those on the illuminated 

 side of the colonies? 



Fig. 19. Graphic representation of the total average difference in deflection 

 due to difference in light intensity on opposite sides of Volvox colonies, compiled 

 from numerous experimental records, a, plate-glass aquarium 8 cm. wide and 

 15 cm. long; b, light rays; c, c', points where the colonies were introduced; d, aver- 

 age course with the region of highest light intensity to left; e, average course with 

 strongest illumination to the right. Light intensity at (/) the middle of field 57.12 

 candle meters. From the rniddle the intensity gradually increased toward either 

 end where it was 442.68 candle meters. Intensity at c, 327 candle meters, at c', 

 263 candle meters. 



"If the light intensity of the field is suddenly decreased 

 while colonies of Volvox are swimming horizontally toward 

 it, they stop forward motion, the longitudinal axes take a 

 vertical position due to the effect of gravity, and then the 

 colonies swim slowly upward. It is not at all difficult to 

 find specimens in which this upward swimming is just suffi- 



