Fig. 45

6. It may also be of interest to know that in helio­tropism the motions of the legs are automatically controlled by the chemical changes taking place in symmetrical elements of the retina. In order to prove this point we will turn to the phenomenon of galvano­tropism. The galvanic current forces certain animals to move in the direc­tion of one of the two electrodes just as the light forces the helio­tropic animals to move towards (or from) the source of light. The change in the concentra­tion of the ions at the boundary of the various organs, especially the nerves, determines the galvano­tropic reac­tions. When the shrimp Palæmonetes is put into a trough with dilute salt solu­tion through which a current of a certain intensity flows, the animal is compelled to move towards the anode.[232] It can walk forwards, backwards, or sidewise. Here we can observe directly that the effect of the current consists in altering the tension of the muscles of the legs in such a way as to make it easy for the animal to move toward the anode and difficult to move toward the cathode. Thus if the current be sent sidewise through the animal, say from left to right (Fig. 45), the legs of the left side assume the flexor posi­tion, those of the right the extensor posi­tion. With this posi­tion of its legs the animal can easily move to the left, i. e., the anode, and only with difficulty to the right, i. e., the cathode. This change in the posi­tion of the legs occurs when the animal is not moving at all, thus showing that the galvano­tropic movements take place not because the animal intends to go to the anode, but that the animal goes to the anode because its legs are practically prevented by the galvanic current from working in any other way. This is exactly what happens in the helio­tropic motions of animals.[233]

Fig. 46

To understand what happens when the current goes lengthwise through the body it should be stated that Palæmonetes uses the third, fourth, and fifth pairs of legs for its locomo­tion. The third pair pulls in the forward movement, and the fifth pair pushes. The fourth pair generally acts like the fifth, and requires no further atten­tion. If a current be sent through the animal longitudinally, from tail to head, and the strength be increased gradually, a change soon takes place in the posi­tion of the legs (Fig. 46). In the third pair the tension of the flexors predominates, in the fifth the tension of the extensors. The animal can thus move easily with the pulling of the third and the pushing of the fifth pairs of legs, that is to say, the current changes the tension of the muscles in such a way that the forward motion is rendered easy, the backward motion is difficult. Hence it can easily move toward the anode, but only with difficulty toward the cathode. If a current be sent through the animal in the opposite direc­tion, namely, from head to tail, the third pair of legs is extended, the fifth pair bent; that is, the third pair can push, and the fifth pair pull. The animal will thus move backward easily and forward with difficulty, and it is thus driven to the anode again.

The explanation which Loeb and Maxwell proposed for this influence of the current on the legs assumes that there are three groups of ganglion cells in the central nervous system of these animals which are oriented according to the three main axes of the body; (1) right-left and left-right, (2) backward, and (3) forward. It depends upon whether the ganglion cells or the nerve elements are in anelectro­tonus, which muscles are bent and which relaxed. It would lead us too far to recapitulate the theory in this place, and the reader who is interested in it is referred to Loeb and Maxwell’s paper.[234] The importance of the observa­tions lies in the fact that they show that any element of will or choice on the part of the animal in these motions is eliminated, that the animal moves where its legs carry it, and not that the legs carry the animal where the latter “wishes” to go.

7. This may be the place to dispel an error which has sometimes crept into the discussion of the tropistic reac­tions of animals. It has been stated occasionally that it is the energy gradient and not the automatic orienta­tion of the animal by the light which makes the positively helio­tropic animal move towards the source of light and the negatively helio­tropic away from it. Thus the positively helio­tropic animal would be compelled to move towards the source of light as a consequence of the fact that the intensity of the light increases the more the nearer the animal approaches the source of light. If the source of light be the reflected sky-light the difference of intensity at both ends of a microscopic organism is so slight that it is beneath the limit capable of influencing the motions.

Fig. 47

A simple experiment published by the writer in 1889 suffices to dispel the idea that the energy gradient determines the direc­tion of the motion of an animal in tropistic reac­tions. Let direct sunlight (S, Fig. 47) fall through the upper half of a window (w w) upon a table, and diffused daylight (D) through the lower half of the window on the same table. A test-tube a c is placed on the table in such a way that its long axis is at right angles to the plane of the window; and one half a b is in the direct sunlight, the other half in the shade. If at the beginning of the experi­ment the positively helio­tropic animals are in the direct sunlight at a, they promptly move toward the window, gathering at the window end c of the tube, although by so doing they go from the sunshine into the shade.[235] This experi­ment is in harmony with our idea that the effect of light consists in turning the head of the animal and subsequently its whole body toward the source of light. By going from the strong light into the shade the reac­tion velocity in both eyes is diminished equally and hence there is no reason for the animal to change its orienta­tion, though its progressive motion may be stopped for an instant by the change. But at the boundary between sunlight and daylight a sudden change from strong to weak light occurs. If the energy gradient determined the direc­tion of the positively helio­tropic animal, the motion should stop at the boundary from strong to weak light, which may happen for an instant but which will not interfere with the progressive motion of the animal.