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Reactions of ameba to light, and the effect of light on feeding.
Asa A, Schaefer Dept of Zoology Univesity of Tennessee
CONCLUSIONS. From these experiments it may be concluded that white light and all the visible spectral colors cause positive responses; but whether all are equally attractive cannot be definitely stated, for experiments would have to be staged differently to produce accurate results. Nevertheless the red end of the spectrum seems to be somewhat more attractive than the blue.
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What is of considerable interest in the behavior of ameba toward light is that the character of the response may vary rapidly. See Figs. 22 to 36; 37 to 48; and a number of other experiments. A negative reaction may be followed by a positive and vice versa. There is no definite relationship between ameba and light, on account of which the ameba is always either positive or negative or indifferent.
Stimulation from light produces the same general character of reaction as stimulation from glass or carbon. The only observable difference is a quantitative one; light beams are sensed at a greater distance than particles of glass or carbon. This difference may however be due to a difference in intensity of the stimuli.
Ameba reacts to dark spots in much the same way that it does to beams of light. The reactions are either positive, negative or indifferent. But they are negative in much the greater number of cases. But no sooner does one observe the reactions of an ameba to perpendicular beams of light and of darkness than question arises as to the transfer of the stimulus to the ame well as the nature of it.
How can an ameba sense a beam of light or darkness which never comes nearer to it than I00 microns?
It is possible that small particles suspended in the water reflect light from a beam of light so as to reach the ameba in much the same way that man can observe a beam of light in a dark room because of the dust particles in the air. But if so the ameba, being eyeless, is wonderfully sensitive to light. But as to beams of darkness the case is entirely different.
Is it con- ceivable that an ameba can sense a beam of darkness at a distance because not as much light is reflected from the particles in the dark beam as from those more brightly illuminated surrounding the beam? If one did not know of reactions to beams of dark- ness, one might adopt the hypothesis of the reflection of light from particles in the beam; but since similar behavior is observed toward beams of darkness, this explanation is obviously not the right one.
Some disturbance is created by the beams which is then radially transmitted; so much is certain. But just what is the nature of the disturbance is not clear. In a preceding paper (Schaeffer, 'I6c), in which the reactions of ameba to particles of glass, carbon, and similar materials were
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REACTIONS OF AMEBA TO LIGHT.
described, it was concluded that the nature of the stimulus which enabled amebas to react to these substances at a distance also remains unrecognized. Now it is possible that the nature of th stimulus which makes reaction at a distance possible is the sam for all these various test objects, since the reactions are very similar.
If so, the nature of the stimulation must be simple and fundamental, such as differences in electrical potential which give rise to electrical currents. But if the nature of the stimulation should be electtical, the quantities of current arising from the various test objects must be infinitesimally small, and very great if not insurmountable difficulties would be encountered in dem- onstrating the presence of such small currents.
To show the general reactions of ameba to globulin, carbon, etc., when stimulated simultaneously by beams of light or of darkness, the experiments may be classified as follows.
1. Food objects (grains of globulin) were laid over a beam of intense light so that the food should be very brightly illuminated -365-372. Blue spectral light was used in the experiments recorded, for blue light has been regarded as more disagreeable than other spectral colors. The globulin was sensed at a distance and the ameba moved toward it and ingested it. There was no definite indication that'the blue light had any effect in modifying the behavior unless the pseudopod to the right in Fig. 368 is to be regarded as expressing a deterrent effect of the light. The ameba, in effect, reacted as if no spectral blue light was present.
2. The food substance was laid some distance from the green or yellow light, and in various positions with respect to the ameba and the beam of light-373-437. (a) When the green light lay between the ameba and the globulin, the light had a slight disturbing and deterring effect -386. The ameba made a slight detour around the green light.
In another test with the experiment similarly staged, the disturbing effect of the green light was more pronounced-373. The ameba made a wide detour around the light and moved into contact with the globulin without coming into contact with the green beam.
In both experiments green light, which is positive when sensed alone, became negative in contrast with the more strongly (or differently) positive globulin. 69
ASA A. SCHAEFFER
In the experiment with yellow light-397-40 moved straight toward the light after the globulin sensing range, then moved over the beam of light, af direction of motion was changed so that the ameba moved toward the globulin. The globulin was eaten in a cup. The yellow light was not deterrent in this case.
But another ameba reacted negatively to both yellow light and glob ulin, when presented simultaneously, but positively when presented separately. The ameba was satiated or sick, for the globulin was only partially surrounded.
3. Grains of globulin and carbon were laid over beams of darkness.
(a) An ameba moved toward a dark spot on which lay a grain of carbon until it came within about thirty microns of the dark spot, when negative behavior set in. The ameba moved away to the right-463. In the succeeding test the ameba reacted at first positively to the dark spot alone, and after that decidedly negatively.
(b) A piece of globulin was laid on the dark spot, to the right of the ameba. The ameba moved directly toward the dark spot -globulin-though it seemed to have been slightly deterred by the dark area, for the ameba broke up into two pseudopods-495 -and just when the dark beam was reached a little later, a small side pseudopod appeared. The globulin was however finally ingested.
In another experiment the globulin was placed near the far edge of the dark spot-522. The behavior of the ameba became very irregular as it moved near the dark beam. Soon a pseudopod was sent out straight toward the globulin, but it was presently retracted and the ameba moved off to the left, veering to the right. There can be no doubt of the strongly deterrent effect of the dark beams. There can also be no doubt of the strongly attractive effect of the globulin.
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