Reactions to light in Volvox, with special reference to the process of orientation

Abstract

1. Volvox rotates on the longitudinal axis as it swims through the water. This is due to the fact that the flagella usually beat diagonally backward. 2. If the luminous intensity is suddenly decreased without changing the direction of the rays photopositive colonies which are oriented stop rotating and spurt directly forward. This is due to a change in the direction of the stroke of the flagella from diagonal to more nearly directly backward. 3. If the intensity is suddenly increased locomotion stops and the rate of rotation increases. This is due to a change in the direction of the stroke of the flagella from diagonal to more nearly sidewise. 4. Photonegative colonies react precisely the same as photopositive colonies except that the effects of increase and decrease in luminous intensity on the direction of the stroke of the flagella is reversed. 5. If, after either of these changes in the direction of the stroke o f the flagella has been initiated, there is no further change in the intensity of the light, the flagella continue to strike in the new direction for only 2.5 + seconds, after which they again strike in the direction which obtained before the intensity was changed. 6. If the intensity is gradually changed, there is no observable response unless the new intensity is maintained for a considerable period of time, which may result in increase or decrease in activity. 7. The eye-spots in Volvox consist of a pigment-cup, a lens which is located at the opening of the cup, and photosensitive substance which is located in the cup. There is one in each zooid, and all face outward. They are primitive eyes. 8. Orientation in photopositive colonies is due to an increase in the backward component of the stroke of the flagella on the shaded side and a decrease in this component on the opposite side. The former is due to reduction of light caused by the shadow of the pigment-cup on the photosensitive substance as the zooids are transferred from the illuminated side of the colony to the shaded side; the latter to increase of light caused by exposure of this substance as the zooids are transferred from the shaded to the illuminated side. 9. In negative colonies precisely the opposite obtains in the process of orientation. 10. Photic orientation is due to a change in the direction of the stroke of the flagella and not to change in activity. Galvanic orientation is due to decrease in activity on the side toward which the colonies turn. The processes involved are consequently not the same. 11. The orienting stimulus and the changes in the direction of the stroke of the flagella which result in orientation cease after the colonies are oriented. They remain oriented because, in the absence of external stimulation, they tend to take a straight course and because, as soon as they swerve from this course, opposite sides become unequally illuminated, resulting in change of intensity on the photosensitive substance in the eyes which leads to reorientation.