Summary
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1.
The direction of locomotion of Euglena in a field of light consisting of two horizontal beams crossing at right angles is in close accord with that demanded by the “Resultantengesetz”, that is, the tangent of the angle between the direction of locomotion and the rays in the stronger beam is approximately equal to the intensity of the weaker divided by that of the stronger beam.
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2.
This can be explained on the assumption that the eyespot is a photoreceptor, that the photosensitive substance in it is a layer in the concavity of the pigmented portion, that this layer is practically parallel with the direction of locomotion and that the light absorbed by it varies from a maximum when the incident rays are perpendicular to it, to zero when they are parallel with it, for if all this obtains and the “Resultan-tengesetz” holds the amount of light absorbed in all symmetrically opposite positions assumed in the process of rotation is equal when the organism is oriented, no matter what the ratio of intensities in the two beams may be.
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3.
The fact that the direction of locomotion in Euglena is in accord with the “Resultantengesetz” merely indicates that if the organism goes in any other direction it is unequally stimulated in different positions assumed in the process of rotation and that this results in responses which change its direction of locomotion until it is no longer unequally stimulated. It has no bearing on the problem concerning the quantitative relation between the stimulus and the response.
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4.
In Volvox and Gonium the direction of locomotion in light from two sources is not in accord with the “Resultantengesetz”.
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5.
In these forms the light is brought to a focus in the eyespots. The position of the focal point varies with the direction of the incident light and the stimulating effect of a given amount of light absorbed at this point varies with its location.
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6.
In Gonium the stimulating effect of a given amount of light absorbed by the photosensitive substance in the central part of the eyespot is about 9 times as great as that of the same amount of light absorbed by this substance in the posterior part. The photosensitive substance in the central part of the eyespot is therefore about 9 times as sensitive as that in the posterior part.
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Contribution from the Zoological Laboratory of the Johns Hopkins University and the Mt. Desert Island Biological Laboratory.
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Mast, S.O., Johnson, P.L. Orientation in light from two sources and its bearing on the function of the eyespot. Z. f. vergl. Physiologie 16, 252–274 (1932). https://doi.org/10.1007/BF00338127
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DOI: https://doi.org/10.1007/BF00338127