It may be said that the preceding interpretation is a simple statement of facts. But Loeb has not contented himself with the establishment of a formula; he has wished to establish the relations which exist between the light and the movements that it provokes.
Admitting that phototropism is characterized by an asymmetrical motive activity when the body is asymmetrically struck by the luminous rays, it seems natural to admit that more or less intense chemical reactions have caused this activity. These reactions will be provoked by the light, either in the visual organs of animals provided with eyes, or on all or a part of the body with blind organisms. This conception of phototropism can doubtless be submitted to the control of experiments. In any event, it is strictly rational, for rays of light are among the most powerful chemical reagents. They are not limited to the transformations of mineral substances in the laboratory, as we see in photography, but they act with still greater energy in the heart of living matter, and, notably in plants, they play a nutritive role of the first importance in decomposing carbonic acid by the help of a photochemical green substance, chlorophyl.
It cannot be doubted that photochemical substances exist in animals and we know that in the vertebrate eye a substance of this kind impregnates the distal ends of the optic rods, where it constitutes the retinal purple. With the articulates there probably exist analogous substances in the retinal region where they provoke reactions which excite the optic elements and, through them, the different parts of the body. With the blind species they seem to be symmetrically distributed through the anterior or cephalic end of the organism. Thus, phototropism has for its start chemical modifications produced by light. We are here in the presence of an hypothesis, but this hypothesis appears to be an expression of the truth. It is, moreover, justified by the fact that the most refractive of the visible solar spectra, which are especially efficacious in the chlorophyl action of green plants, are also much more active in directive orientation with all organisms that are sensitive to light.
The preceding hypothesis is equally justified by the influence of certain chemical agents on the development or the suggestion of phototropism. In their normal condition, Gammarus pulex, the Cyclops, the Daphnias, and other small crustaceans of fresh water do not seem entirely phototropic, or are very slightly so. But, says Loeb, it is possible to confer upon them immediately an intense positive phototropism, by adding certain chemical substances to the water, ether or acids, especially carbonic acid. Dispersed at the beginning of the experiment, they direct themselves quickly to the light face of the aquarium. One gets this identical result with the green alga? of the genus Volvox. In both cases Loeb suggests with great truth that the ethers or the acids act as sensitizers, causing, by catalysis, an increase of the photosensitive substances and provoking the phototropic phenomena.
To this idea of sensitizing substances should be added another. The vinegar-fly (Drosophila am-pelophila) is normally positively phototropic, but, as Carpenter has shown (1905), it ceases to react when the luminous rays are too strong. It then turns away from the rays and appears to become negatively phototropic, but is really struck with directive incapacity. The larvae of the lobster in their first stage direct themselves at first toward the light; but at the end of a day or two they leave and group together in the dark places. But Georges Bohn has shown that these last larvae are very sensitive to the action of dilute sulphuric acid, that they return to the light, and that, having remained there one or several hours, they vigorously return to the shady spots they had quitted. Thus, under the sensitizing influence of the acid, their negative phototropism changes its character momentarily, to become again more strongly negative. Alkaline soda solutions produce a similar result. To what must we attribute the attenuation or the change of character of the phototropism in these two experiments ? It is not muscular fatigue, for the animal preserved all its activity; neither is it a matter of memory, for the sensitiveness reappears. We must believe with Bohn that an intense functioning has caused more or less wear and tear of the photosensitive substances and that these last, before they are reformed, functionally give way to antagonistic chemical reactions.