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RESEARCHES ON FUNGI 



Fig. 55. — Diagrammatic representation of orientation during a positively heliotactic 

 response in Volvox. A, B, C, and D, four zooids at the anterior end of the 

 colony ; l-a, longitudinal axis ; large arrows, direction of illumination ; curved 

 arrows, direction of rotation ; the colony is moving forward and turning so as 

 to face the light with its anterior end ; /, flagella : e, eyes, containing a pigment- 

 cup (represented bj^ a heavy black line) and photosensitive tissue in the concavity 

 of the cup. Note tliat, when the colony is laterally illuminated, the photo- 

 sensitive tissue in the eyes on the side facing the light is fully exposed to the 

 light and the flagella on this side beat laterally, while the photosensitive tissue 

 in the eyes on the ojjposite side is shaded by the pigment-cup and the flagella 

 on this side beat directly backwards. The difference in the direction of the beat 

 of tlie flagella on these two sides is due to alternate decrease and increase in the 

 luminous intensity to which the photosensitive tissue in the eyes is exposed 

 owing to the rotation of the colony on its longitudinal axis, an increase causing, 

 in photopositive colonies, a change in the direction of the stroke of the flagella 

 from backward or diagonal to lateral, and a decrease a change frorn lateral or 

 diagonal to backward. In photonegative colonies precisely the opposite obtains. 

 In photopositive colonies this results in turning toward and in photonegative 

 colonies in turning from the source of light. In both the turning continues 

 until opposite sides are equally illuminated, when changes in intensity on the 

 photosensitive tissue are no longer produced by rotation and the orienting 

 stimulus ceases. Reproduced from Fig. 7 of Mast's Reactions to Light in 

 Volvox with Special Reference to the Process of Orientation. 



