Forced Movements, Tropisms, and Animal Conduct

2. Hess’s conclusions are in conflict with another group of facts. For many plants the blue region of the solar spectrum is the most efficient. Hess is, therefore, compelled to conclude that the heliotropism of such plants is different from that of animals, since it would seem preposterous to assume that swarmspores of plants should go to the light because they have sensations of color. He, therefore, assumes that plants are heliotropic—^in the sense of the mechanistic theory—but that positively helio¬ tropic animals go to the light on account of their love for brightness which is exactly the old viewpoint of Graber. It can be shown, however, that the difference in the helio¬ tropism of animals and plants, which Hess assumes, is contrary to the facts, since there are heliotropic animals for which the blue rays are the most efficient, as for most plants; and there are green algte for which the yellowish- green rays are most efficient, as for animals.

For many if not most plants the blue rays are the most efficient for inducing heliotropic curvatures. Blaauw proved this in the foUowing way: He exposed a row of seedlings of A-Vsnci to a carbon arc spectrum for a certain time. The seedlings were then placed in the dark and after the proper time it was ascertained which part of the spectrum had induced heliotropic curvatures. By vary¬ ing the duration of time of exposure to the spectrum it was found that with a minunal tune of exposure only certain blue rays, namely, those of a wave length of 478 w caused heliotropic bending, while with longer exposure longer waves also became efficient. In this way the mini¬ mum duration of exposure for various parts of the spec¬ trum was ascertained. Table IX gives his results.

The red and yellow parts of the spectrum were ineffec¬ tive for the intensity and time limits used and the optimum of efficiency was in the blue, in the region between 466 and 478 /*/«•. A shorter series of experiments was made on the fruit bearers of Phycomyces, with the following results; 44 to 47 per cent, of the Phycomyces showed heliotropic curvatures after 192 seconds of iJluminatian at 615 fxn after 192 seconds of illumination at 550 /x/a after 16 seconds of illumination at 495 /x/t after 32 seconds of illumination at 450 jtxjtx after 64 seconds of illumination at 420 jtxjtx

The number of experiments was iknited but they indi¬ cate an optimum between 495 and 450 in this respect agreeing with the results on Avena. The fact then exists that for the heliotropic reactions of certain plants the blue rays are most efficient, while for the heliotropic reactions of a number of animals the yellowish-green rays are most efficient. But this state¬ ment cannot be generalized. Loeb and Wasteneys determined the most efficient wave length of light for various lower organisms with the result that there are heliotropic animals for which the blue rays are as efficient as they are for plants; and that for different unicellular green organisms the opti¬ mum lies in different parts of the spectrum. They found, by a method similar to that used by Blaauw, that for the heliotropic curvature of the animal Eudendrium the most efficient part of the spectrum lies in the blue A = approxi¬ mately 473 The same was found by them for the

On the other hand, on investigation of two closely related forms of green flagellates, Euglena and Ghlamy- domonas, it was found that they behave differently. For Euglena viridis the blue rays A = 470 to 480 /«/*• are especially efficient, while for Chlamydomonas pisiformis the most efficient part was in the region of A = 534 ftft, in the yeUowish-green.'’ For another green algae, Pcm- dorina, Loeb and Maxwell had already found the greatest efficiency in the greenish-yellow.

^ Tliis would lead us, on the basis of the reasoning of Hess, to the conclusion that the unicellular plant Ohlampdomonm has sensations of brightness, suffers from total color blindness (although it has no eyes), that it is not heliotropic, and that it is an animal; while its unicellular cousin» Euglena, has a highly developed color sense, has no sensations of brightness, is heliotropdc, and is a plant. The method used for these esperimeuts by Loeb and Wasteneys is as follows:

A carbon arc spectrum, about from 18 to 23 cm. wide, was thrown on a black screen 8S (see Pig. 34) with two slits a and h in the two different parts of the spectrum which were to be compared in regard to their heliotropic Fia. 34.—Method of determining the relative heliotropic efficiency of two different parts of the spectrum. (After Loeb and Wasteneys.) efficiency. The two beams of light passing through the slits are reflected by the two mirrors M and into the square glass trough cdfe in such a way as to strike the same region g of the back wall of the trough. The glass trough is surrounded by black paper except at R and where the two beams of light enter from the mirrors. Be¬ fore the experiment begins, all the organisms are collected in the spot g with the aid of an incandescent lamp. As

soon as tlie spectrum is turned on, these organisms are simultaneously exposed to two dilferent beams of light which come from the two mirrors M and M-y. When one type of light, e.g., that from M, is much more efficient than the other coming from My, practically all the organisms are oriented by the light from M and move toward this mirror, collecting in the region R. When the relative effi¬ ciency of the two types of light is almost equal, the organ¬ isms move in almost equal numbers to R and Ry. By using as a standard of comparison the same region of the spectrum and successively altering the position of the other slit in the spectrum, we were able to ascertain with accuracy the relative efficiency of the dill'ereut parts of the spectrum for the two forms of organisms. When the two parts of the spectrum which are to be compared are very close to each other, it is necessary to deflect the beams with the aid of deflecting prisms, before they reach the two mirrors.®^^

Experiments on the newly hatched larvm of Arenicola, a marine worm, showed that the most efficient part of the spectrum was in the bluish-green of about X = 495 m/*, while for the larvae of Balanus eburneus the most efficient part of the spectrum was found by Loeb and Maxwell, by Hess, and by Loeb and Wasteneys in the region of yellow and yellowish-green.^'^ Mast made similar experiments on these organisms with a method in which the organisms were exposed to two beams of light of different wave length crossing each other at right angles. One light was kept constant while the other was made intermittent by a disk with a sector cut out rotating in front of the light. The size of the sector was varied xmtil the organisms moved at an angle of 45° to the two beams. When this happened the hcliotropic

Relative Stimulating Efficiency of Different Regions in the Spectrum; Reduced to the Same Magni¬ tude AND Averaged so that they can be Readily Compared. Maximum in Heavy Type efficiency of the two beams was considered equal. Mast’s results, which are given in Table X, agree with those of Loeb and Wasteneys. The error which Hess makes is of epistemological in¬ terest inasmuch as it shows the danger of false analogy. The real analogy for heliotropic reactions are forced movements and other tropisms, e.g., galvanotropism or geotropism-. Since forced movements (e.g., in Meniere’s disease) and galvanotropic reactions caused by a constant current through our head are not determined or accom¬ panied by special sensations, the same may be true in regard to heliotropic reactions. This is not an idle assumption, since we know that the contraction of the iris of our eye under the influence of light is not accom¬ panied by any sensation of brightness or color and such contractions occur also under the influence of light when the iris is excised. Hess ignores not only this analogy, but the whole existence of forced movements and of other tropisms, and he uses the color and light sensations of human beings, who are not heliotropic, to explain helio- tropism in animals about whose sensations we know noth¬ ing. He fails to see that by this false analogy he dodges the real problem of heliotropism, namely, why the tension of symmetrical muscles changes upon one-sided illumina¬ tion of an animal. For the explanation of this problem, we find assistance in the field of forced movements and of galvanotropism and of geotropism, but not in the behavior of totally color blind human individuals who show no trace of heliotropism.

The adoption of the false analogy between visual sen¬ sations and heliotropism makes it impossible for Hess to admit that bees should be heliotropic and at the same time be able to discriminate between blue and gray; while if we take cognizance of the analogy between keliotropism and the other tropisms we realize that the heliotropism of the bees and their reactions to blue are separate and independent phenomena, which need not be mutually ex¬ clusive and which in all probability depend upon different parts of the brain. When in certain cases the relative hehotropic efficiency of the various parts of the spectrum is identical with the curve for its apparent relative bright¬ ness to a totally color blind person, we may conclude that the photosensitive substances responsible for the two groups of phenomena behave similarly or may even be identical, but not that the sensations of brightness of the color blind and the heliotropic reactions of insects are identical or analogous phenomena.

Many mutants of Drosophila differ in regard to the pigments of the eye. It was natural to raise the question whether or not such hereditary variations of pigmentation of the eye influence the reaction of the flies to monochro¬ matic light. McBwen investigated this possibility with the following result: “ Colored lights which may be con¬ veniently described as violet, green and red, are effective in the order named upon the insects whose eye color is lighter than the red eye of the wild fly. In the case of wild flies, and flies whose eyes are of a still darker shade called sepia, red is more effective than green” (McEwen®*®).

We have stated that while in a positively heliotropic animal a one-sided illumination increases the tension of the muscles which turn the animal toward the source of hght, in the negatively heliotropic animal the one-sided illumination must result in the opposite effect, namely, in a diminution of tension in the same muscles. As a conse¬ quence, the negatively heliotropic animal can turn more easily away from the light than toward the light. Groom and Loeh noticed that the larvae of the bar¬ nacle upon hatching go directly to the light and gather at the light side of a dish, but that sooner or later their positive heliotropism may give way to an equally pro¬ nounced negative hehotropism. The stronger the light the more rapidly the larvae are transformed into negatively heliotropic organisms. Later the reversibihty of the sense of heliotropism was observed and studied in a number of organisms.^®^ In a suromary of the subject (p. 470) the writer pointed out that this reversion was due either to a modification of photochemical processes or to an effect upon the nervous system. That an influence on the nervous system can indeed bring about a change in the sign of the reaction is very strikingly demonstrated in the following observation of A. R. Moore on starfish.®*®

Ordinarily, when a starfish which is moving in an aquarium is touched, it stops immediately and clings tenaciously to the surface of the vessel with its tube feet, so that it is impossible to remove the animal without injury to the tube feet. This normal response to sudden contact can be completely reversed by the administration of strychnine, so that when touched the animal loosens its hold on the bottom completely. The starfish poisoned with strychnine upon sudden touch withdraws all the tube feet, so that it can be moved about like an inert object. I'or this purpose 1 or 2 c.c. of a 0.5 per cent, solution of strychnine sulfate were injected into a starfish of medium size.

If the stretching out of the tube feet is due to an in¬ crease in the tone of the ring muscles (and a decrease in the tension of the longitudinal muscles) the drawing in is due to an increase in the tone of the longitudinal muscles of the tube feet. We therefore see that the same “stimulus,” namely, a sudden touch, which causes one set of muscles to contract in a normal animal causes the antagonists of these muscles to contract in an animal poisoned with strychnine. We shall see that a number of cases of reversal of heliotropism may well find their explanation on this basis. On the other hand, the phe¬ nomena of solarization known in photography indicate that the sign of hehotropic response may also be changed by an excessive action of light on the photochemical sub¬ stance. This effect, of course, may in the last analysis also result in an influence upon the central nervous system, such as that brought about by strychnine in Moore’s ex¬ periment. We will now consider some cases more in detail.

The writer found that certain fresh water crusta¬ ceans, namely Californian species of Daphnia, copepods, and Gammarus when indifferent to light can be made intensely positively heliotropic by adding some acid to the fresh water, especially the weak acid COg. When car¬ bonated water (or beer) to the extent of about 5 or 10 c.c. is slowly and carefully added to 50 c.c. of fresh water containing these Daphnia, the animals will become in¬ tensely positive and will collect in a dense cluster on the

window side of the dish. Stronger acids act in the same way but the animals are liable to die quickly. Esters, e.g., ethylaeetate, act also like acids and the addition of 1 c.c. of a grammolecular solution of ethylaeetate to 50 c.c. fresh water also makes all the organisms positively helio¬ tropic. Alcohols act in the same way. In the case of Gammams the positive heliotropism lasts only a few seconds, while in Daphnia it lasts from 10 to 50 minutes and can be renewed by the further careful addition of some CO 2 . The following table gives the minimal con¬ centration of various acids and alcohols for the production of positive heliotropism in certain California species of fresh water copepods, and Daphnia:

As far as alcohols are concerned each higher alcohol is about three times as efficient as the previous one, with the exception of amyl alcohol. This order of relative efficiency is also characteristic for the surface tension effects of these alcohols.^®* It was of importance to find means of making these organisms negatively heliotropic. Moore®®® found that caffein makes the heliotropically indifferent fresh water crustacean Diaptomus intensely negatively heliotropie. It required the addition of 1.2 c.c. of a 1 per cent, solution of caffein to 50 c.c, of water to bring about this intense

negativation. In two minutes all the animals are col¬ lected in a dense cluster on the negative side which lasts for about 35 minutes. A weak negative collection could also be obtained by adding 0.1 c.c. of a 0.5 per cent, solu¬ tion of strychnine nitrate. Moore found that if the Diap- tomus were first made positively phototropic by the addi¬ tion of alcohol or acids, it was impossible to alter their response by the action of caffeia, strychnine, or atropine. On the other hand, animals which had formed a negative collection under the influence of caffein if treated with car¬ bonated water at once changed their response and swim¬ ming to the light side of the dish formed a positive gathering.

What causes these etfects? The fact that alcohols make the organisms positively heliotropic suggested the possibility of a “narcotic” effect; the writer found, how¬ ever, that narcosis requires a concentration of alcohols three times as high as the one required to produce positive heliotropism. He tried the effect of temperature on the reversal of the sign of heliotropism in Daphnia and found that lowering of the temperature enhanced the effect of acids in making the animals positive.^®®

The writer had found previously that in marine crus¬ taceans and in larvae of a marine annelid, Polygordius, the sense of heliotropism can be reversed by changes of tem¬ perature as well as by changes in the osmotic pressure of the sea water.^®^ Increase in the osmotic pressure of sea water (by adding about 1 gm. of NaCl or its. osmotic equivalent of other substances to 100 c.c. of sea water) made the negative animals positively heliotropic, and lowering of the concentration by adding 30 to 60 c.c. dis¬ tilled water to 100 c.c. sea water made positive organisms negative. Negative larvae of Polygordius or negative

marine copepods could be made positive by lowering the temperature, and positive larvae could be made negative by slowly raising the temperature. Since in the latter case the animals suffered from the high temperature the results were not so striking as in the case of the positivat- ing effect of lowering the temperature. The same effect of the concentration of sea water and of temperatures was observed by Ewald for the larvae of Balanus perfor- atus. He found, moreover, the interesting fact that a change of the ratio ^ in the sea water affected the sign of hehotropism of barnacle larvae. An increase of Na made them more positive, an increase in Mg more negative.^^^

The larvae of Porthesia are strongly positively helio¬ tropic before they have eaten, while they lose their helio- tropism almost completely after they have eaten.^*^ The writer observed that male and female winged ants are strongly positively heliotropic but as soon as they lose their wings their heliotropism ceases.^®^ McEwen has found that when Drosophila is deprived of its wings its heliotropism ceases. Holmes found that terrestrial amphipods are posi¬ tively, while the aquatic amphipods are negatively helio- tropic. By putting a terrestrial amphipod into water it became negatively heliotropic.®®®

That a reversal in the sense of heliotropism may be due to a nervous effect is suggested by an observation by Miss Towle that a certain ostracod, Cypridopsis, can be made positively heliotropic by mechanical shock, and the writer noticed that indifferent fresh water Gam- marus can be made negatively heliotropic by shaking them. In both cases the heliotropism lasts only a short time. The attempt to explain all these reversals on the assumption of a change in the central nervous system meets with the difficulty that such reversals occur also in unicellular organisms which have no central nervous system. Thus the writer observed that Volvox, which occurred in the same ponds in California from where Daphnia came, could also be made positive by COa-^®® In swarmspores of algte reversals of heliotropism are a common phenomenon. While these unicellular organisms have no central nervous system they may have synapses such as exist between different neura of metazoa. The writer is not sufficiently familiar with the behavior of synapses in higher animals to suggest that this condition is responsible for the changes in the sense of heliotropism.

We may finally discuss briefly a possible solarization effect. The writer found that it is possible to make ani¬ mals generally negatively heliotropic with the aid of ultraviolet light.^®® If once rendered negative such ani¬ mals will be negative not only to ultraviolet rays but also to the light of an incandescent lamp. A. R. Moore®®® found that the ultraviolet rays having such an effect have a wave length shorter than 3341 A.U. Oltmanns had ob¬ served that Phycomyces is positively heliotropio in weak light, indifferent in somewhat stronger light, and nega¬ tively heliotropic in still stronger light. Blaauw found that when the illumination was strong the seedlings of Avena became negatively heliotropic.*'^ He suggests the analogy with solarization effects in photography. The discovery of photodynamic effects by v. Tappeiner adds to the possibilities which should be considered in this connection.

While Drosophila is usually positively heliotropic, McEwen has recently described a mutant of this species which is not heliotropic. This lack of heliotropio response is linked with a peculiar color—^^“tan”—by which the mutant is characterized. The character “tan” is sex linked. The daughters inherit the factor for the character from their fathers hut do not show the character, while the sons inherit the factor from their mothers and do show the character. The lack of heliotropic reaction in this mutant is apparently not due to any structural defect in the eye (McEwen®^®).

Keeping successive generations of flies in the dark does not influence their heliotropism. F. Payne raised sixty-nine successive generations of Drosophila in the dark, but the reaction of the insects to light (as well as their eyes) remained entirely normal. 1. Wlien the stem of certain plants is placed in a horizontal position, the apex grows vertically upward and the root downward. The downward growth of the root is called positive, the upward growth of the apex nega¬ tive geotropism. The writer has observed a similar phe¬ nomenon in a hydroid, Antennularia antennina and his observations were confirmed by Miss Stevens.®®* Animals as well as plants, therefore, show the phenomenon of geotropism.

These phenomena have given rise to a strange dis¬ cussion, namely: "What constitutes the “stimulus” in the case of geotropism? When a galvanic current is sent through a motor nerve the muscle answers with a con¬ traction only when the current is made or broken, but not while a constant current is flowing through the. nerve. The older physiologists were not able to form a mental picture of what happened in this case, and they cut the knot by invoking a verbalism, namely by calling the mak¬ ing or breaking of a current a ‘ ‘ stimulus. ’ ’ This perhaps innocent verbalism then led to the less harmless dogma that only a rapid change could act as a “ stimulus. ’ ’ Thus Jennings ^®* and Mast took it for granted that phe¬ nomena of orientation by light could only be produced by rapid changes in the intensity of light and not by constant Olumination, since they had the a priori convic¬ tion that only a rapid change in the intensity of a gal- varftc current or of light is a “stimulus.” The same diffi-

culty arose in regard to the action of gravity upon orien¬ tation, since it was contrary to the definition of a “stimu¬ lus” that the mere permanent lying in a horizontal posi¬ tion should cause the apex of a stem to bend upward. All these di ffi culties disappear if we take the law of chemical mass action into consideration. Light acts not as a “stimulus” but acts by increasing the mass of certain chemical compounds, and it is the mass of these products which is responsible for the effect of light. Now, mass action is not proportional to the rapidity of the Grange of acting masses but to the acting mass itself. "When two sides of an organism are struck by light of different intensity the quantity of photochemical products on both sides becomes unequal. In galvano- tropism the galvanic current alters the distribution of the mass of certain ions along the nerve elements.

It can be shown that gravitation acts by influencing the distribution of chemical substances in an organism. When the stem of a plant is put into a horizontal position certain chemical substances gather in greater concen¬ tration on the lower side of the stem; and this causes a difference in the velocity of chemical reactions between the lower and the upper side. As a result of this we notice the bending. In the normal upright position of the plant the same substances were distributed equally about the axis of symmetry.