1. A quantitative study of the responses of Daphnia magna to light was made with the use of an experimental trough illuminated horizontally through one end by a uniform beam of light. The intensity of light was changed by shifting the position of a neutral glass "wedge" interposed in the beam of light.
2. The difference in the position of Daphnia when positively phototropic and when negatively phototropic is a difference in the postural angle at which the antennae are held, and not a difference in the direction of orientation of the whole organism--the animal's back being toward the light under all circumstances.
3. The primary sign of phototropism is not altered according to the absolute intensity of the light, but is affected by (1) the age of the individual, (2) the temperature of the water, and (3) the condition of the culture medium. Sometimes a "spontaneous" change in the primary sign of phototropism occurs.
4. The occasional movements observed to occur in the direction opposite to that of the primary sign of phototropism appear to be essentially periodic in respect to their times of inception and their duration. These periodic movements of Daphnia are not due to recurring periods of increased or of decreased activity, but probably represent periodic changes in the underlying photic mechanism.
5. "Variability" of the responses of Daphnia to successive identical tests gives evidence of being fundamentally periodic. A system of experimentation was devised to eliminate the error due to this variability, in so far as this was possible.
6. It was found that the rate and the magnitude of the change of illumination must rise above a certain threshold to be effective in causing a reversal of phototropic sign. A minimum length of exposure to bright light before the test is made is also necessary.
7. The relations of (1) length of the latent period, (2) speed of response, (3) magnitude of response, and (4) duration of response to (a) amount of reduction of light intensity, (b) duration of previous exposure to light, (c) duration of previous sojourn in dark, and (d) temperature of the water, were investigated, and the results have been summarised in Table XIII.
8. My observations are consistent with Ewald's conclusions that orientation of Daphnia is based on a mechanism which is entirely distinct from that responsible for the other three aspects of phototropism, namely (1) persistent phototropic swimming under any constant illumination, (2) periodic changes of phototropic sign under constant low illumination, and (3) reversal of phototropism produced by changes of light intensity.
9. It is shown that these other aspects of phototropism of Daphnia could be accounted for by one mechanism of excitation, if it were photoreversible and properly controlled. A theory is proposed that this mechanism is a reversible photochemical system such as that used by Hecht. The theoretical requirements of the mechanism would be fulfilled on the assumption (1) that equilibrium in the system would result in the maintenance of the persistent primary sign of phototropism, and (2) that the upsetting of this equilibrium would result in the production of the secondary signs. Upsetting of the equilibrium by some internal rhythmic process and by changes of illumination would account for periodic phototropic movements and for induced reversals of phototropic sign, respectively.
10. The results of the experiments on the photic responses have been reviewed in the terms of the proposed theory, and it is found that the evidence strongly supports the hypothesis that a reversible photochemical system is the basis for these aspects of the phototropism of Daphnia.
The Behaviour of Minnows in Relation to Light Intensity