The “attraction” or “repulsion” of animals by the light had been explained by the biologists in an anthropomorphic way by ascribing to the animals a “fondness” for light or for darkness. Thus Graber, who had made the most extensive experiments, gave as a result the statement that animals which are fond of light are also fond of blue while they hate the red, and those which are fond of the “dark” are fond of red and hate the blue.[218] In 1888 the writer published a paper in which he pointed out that the so-called fondness of animals for light and blue and for dark and red was simply a case of an automatic orientation of animals by the light comparable to the turning of the tips of a plant towards the window of the room in which the plant is raised.[219]
The phenomenon of a plant bending or growing to the source of light is called positive heliotropism (while we speak of negative heliotropism in all cases in which the plant turns away from the light, as is observed in many roots). The writer pointed out that animals which go to the light are positively heliotropic (or phototropic) and do so because they are compelled automatically by the light to move in this direction, while he called those animals which move away from the light negatively heliotropic; they are automatically compelled by the light to move away from it. What the light does is to direct the motions of the animals and to explain this the following theory was proposed. Animals possess photosensitive elements on the surface of their bodies, in the eyes, or occasionally also in epithelial cells of their skin. These photosensitive elements are arranged symmetrically in the body and through nerves are connected with symmetrical groups of muscles. The light causes chemical changes in the eyes (or the photosensitive elements of the skin). The mass of photochemical reaction products formed in the retina (or its homologues) influences the central nervous system and through this the tension or energy production of the muscles. If the rate of photochemical reaction is equal in both eyes this effect on the symmetrical muscles is equal, and the muscles of both sides of the body work with equal energy; as a consequence the animal will not be deviated from the direction in which it was moving. This happens when the axis or plane of symmetry of the animal goes through the source of light, provided only one source of light be present. If, however, the light falls sidewise upon the animal, the rate of photochemical reaction will be unequal in both eyes and the rate at which the symmetrical muscles of both sides of the body work will no longer be equal; as a consequence the direction in which the animal moves will change. This change will take place in one of two ways, according as the animal is either positively or negatively heliotropic; in the positively heliotropic animal the resulting motion will be toward, in the negatively heliotropic from, the light. Where we have no central nervous system, as in plants or lower animals, the tension of the contractile or turgid organs is influenced in a different way, which we need not discuss here.
The reader will perceive that according to the writer’s theory two agencies are to be considered in these reactions: first, the symmetrical arrangement of the photosensitive and the contractile organs, and second, the relative masses of the photochemical reaction products produced in both retinæ or photosensitive organs at the same time. If a positively heliotropic animal is struck by light from one side, the effect on tension or energy production of muscles connected with this eye will be such that an automatic turning of the head and the whole animal towards the source of light takes place; as soon as both eyes are illuminated equally the photochemical reaction velocity will be the same in both eyes, the symmetrical muscles of the body will work equally, and the animal will continue to move in this direction. In the case of the negatively heliotropic animal the picture is the same except that if only one eye is illuminated the muscles connected with this eye will work less energetically. The theory can be nicely tested for negatively heliotropic animals in the larvæ of the blowfly when they are fully grown, and for positively heliotropic animals on the larvæ of Balanus, and many other organisms.
| Fig. 43 |
| Fig. 44 |
One of the difficulties in identifying the motions of animals to or from the light with the positive and negative heliotropism of plants consisted in the fact that plants are mostly sessile (and respond to a one-sided illumination with heliotropic curvatures to or from the light), while most animals are free moving and respond to the one-sided illumination by being turned and compelled to move to or from the light. This difficulty was overcome by the observation that sessile animals like the hydroid Eudendrium (Fig. 43) or the tube worm Spirographis (Fig. 44) react to a one-sided illumination also with heliotropic curvatures like sessile plants.[220] On the other hand, it had been found before by Strassburger that free-swimming plant organisms like the swarmspores of algæ move to or from the source of light as do free-swimming animals.
3. The writer suggested in 1897[221] that the light acts chemically in the heliotropic reactions and in 1912 that the heliotropic reactions probably follow the law of Bunsen and Roscoe,[222] and it was possible to confirm this idea by direct experiments.[223] This law states that the photochemical effect of light equals i t where i is the intensity of the light and t the duration of illumination. The experiments were carried out on young regenerating polyps of Eudendrium by measuring the time required to cause fifty per cent. of the polyps to bend to the source of light. The intensity of light was varied by altering the distance of the source of light from the polyps. Table VI gives the result.
TABLE VI
| Distance between Polyps and Source of Light | Time Required to Cause Fifty Per Cent. of the Polyps to Bend towards the Source of Light | |
|---|---|---|
| Observed | Calculated from Bunsen-Roscoe Law | |
| Metres | Minutes | Minutes |
| 0.25 | 110 | |
| 0.50 | between 35 and 40 | 140 |
| 1.00 | 150 | 160 |
| 1.50 | between 360 and 420 | 360 |
We must therefore conclude that the heliotropic curvature of the polyps is determined by a photochemical action of the light. The light brings about or accelerates a chemical reaction which follows the Bunsen-Roscoe law. As soon as the product of this reaction on one side of the polyp exceeds that on the other by a certain quantity, the bending occurs. When the product i t is the same for symmetrical spots of the organism no bending can result. This is what our theory suggested.