Behavior of the Lower Organisms

a circle. In other words, the direction of the path has been changed (Fig. 92). The whole action may be expressed as follows: when the light is suddenly decreased, the organism tries successively many different directions, finally following one of these. The reaction is a very sharp and striking one, and produces a most peculiar impression. At first all the Euglenae are swimming in parallel lines toward the window. As soon as the shadow of the hand falls upon the preparation, the regularity is destroyed ; every Euglena turns strongly and may appear to oscillate from side to side. This apparent oscillation is due to the swerving toward the dorsal side, combined with the revolution on the long axis. The organism swings thus first to the right, then upward, then to the left, then down, etc. (see Fig. 91).

This reaction occurs whenever the light is decreased in any way. Thus, in place of cutting off the light coming from the window, that coming from the mirror of the microscope may be decreased by closing the iris diaphragm. The Euglenas react in the manner above described, though they soon resume their movements toward the window. Again, if the light from the window is decreased only slightly, the Eu- glena? react in the manner described, thus changing their direction of movement ; very soon, however, they swim again toward the window. The same reaction occurs in Euglenae that are for any reason not swimming toward the source of light. Even if a specimen is swimming away from the window, it gives the avoiding reaction in the usual way when the light from the window is decreased.

It is clear that the reaction is due to the decrease in the intensity of light, not to a change in the direction of the light rays. In the first and second experiments mentioned in the preceding paragraph, the Euglenae are, some time after the light is decreased, swimming in the same direction as they were before, though at the moment of decrease there is a reaction. Engelmann (1882) tried shading parts of the body of Euglena. He found that a shadow which is cast on the body of the organism without affecting the anterior one-third produces no effect whatever. On the other hand, a shadow affecting only the anterior tip — if even only the part in front of the eye spot — causes the same reaction as shading the entire body. Thus it is clear that the anterior end is more sensitive to light than the remainder of the body. These results of Engelmann are of much importance for understanding the remainder of the reaction to light.

If Euglenae are placed on a slide and a certain spot is lighted from below by the mirror of the microscope, a dense collection is in the course of time formed in the lighted region. Observations show that the Eu- glense in the darker portion swim about at random; many of them thus pass into the lighted region. There is no reaction at passing from the dark to the light. In the lighted region they likewise swim about in all directions. But as soon as an individual reaches the outer boundary of

Fig. q3. — Illustration of the devious path followed by Euglena in becoming oriented when the direction of the light is reversed. From 1 to 2 the light comes from above; at 2 it is reversed. The amount of wandering (a-h) varies in different cases. the lighted area, it gives the typical avoiding reaction; it backs, turns' toward the dorsal side, and thus reenters the lighted area. This reaction frequently occurs as soon as the anterior tip is pushed into the shade. In other cases the reaction does not occur till the Euglena has passed

Fig. 94. — Spiral path of Eu- glena. a, b, c, d, successive positions taken. The arrows at the right indicate the direction of an incoming force, as light, showing how the relation of the body axis and the anterior end to such a force changes continually. At d the body axis is nearly parallel to the lines of force, and the anterior end is directly illuminated. At b the axis is nearly transverse, and the sensitive anterior end is largely shaded, so as to receive but little light.

then turns and passes back into the light. At the boundary of the lighted area the organism is, of course, subjected to a sudden decrease in illumination, and this, our previous experiments have shown us, is the cause of the avoiding reaction. Whenever lighted or shaded areas are open to Eu- glenae, the organisms gather in the lighted areas in the way just described. If the entire area containing the Eu- glenae is illuminated from one side, the organisms swim toward the side from which the light comes. That is, they become oriented with anterior end toward the source of light. If we watch them as they become oriented, we find that the orientation takes place, as in Stentor, through the avoiding reaction. The course of events is about as follows : The Eu- glenae are swimming about at random in a diffuse light, when a stronger light is allowed to fall upon them from one side. Thereupon the forward movement becomes slower and the Euglenae begin to swerve farther than usual toward the dorsal side. Thus the spiral path becomes wider and the anterior end swings about in a larger circle and is pointed successively in many different directions. In some part of its swinging in a circle the anterior end of course becomes directed more nearly toward the light; thereupon the amount of swinging decreases, so that the Euglena tends to retain a certain position so reached. In other parts of the swinging in a circle the anterior end becomes less exposed to the light ; thereupon the swaying increases, so that the organism does not retain this position, but swings to another. The result is that in its spiral course it successively swerves strongly toward the source of light, then slightly

away from it, until by a continuation of this process the anterior end is directed toward the light. In this position it swims forward. The course of Euglena in becoming oriented is shown in Fig. 93. This behavior is intelligible when we recall the effect of the spiral course in causing changes in the intensity of the light affecting the anterior end. The anterior end is, as we have seen, the part most sensitive to light ; it may be compared with the eye of a higher animal. In a

Fig. 95. — Diagram of the method by which Euglena becomes oriented with anterior end toward the source of light. At i the organism is swimming toward the source of light. When it reaches the position 2, the light is changed, so as to come from the direction indicated by the arrows at the right. As a consequence of the decrease of illumination thus caused, the organism swerves strongly toward the dorsal side, at the same time continuing to revolve on the long axis. It thus occupies successively the positions 2-6. In passing from 3 to 6 the illumination of the anterior end is increased, hence the swerving nearly ceases. In the next phase of the spiral therefore the organism swerves but a little, — from 7 to 8. But this movement causes the anterior end to become partly shaded, and this decrease of illumination again induces a strong swerving toward the dorsal side. Hence, in the next phase of the spiral the organism swings far, through 9 and 10, to 11. Thus it continually swerves much toward the source of light and a little away from it, till it reaches the position 16. Now it is directed toward the source of light, and such swerving as occurs in the spiral course neither increases nor decreases the illumination of the anterior end. Hence there is no further cause for reaction; the Euglena continues its usual forward movement, which now takes it toward the source of light.

Euglena swimming obliquely or transversely to the rays of light, as in Fig. 94, the illumination of the anterior end changes greatly with each turn in the spiral. At d the light is shining almost directly upon the anterior end, while at b the organism is nearly tranverse, so that the anterior end is partly shaded. The effect is like that of turning an eye first toward the sun, then away from it ; though the movement is slight, the change in illumination produced is great. The variations in illumination due to the spiral course are doubtless much accentuated by the fact that one side of the anterior end bears a pigment spot, which in certain positions of the unoriented Euglena cuts off the light. A decrease of illumination causes, as we know, the avoiding reaction ; the anterior end swings in a wider circle (Fig. 91). This still further increases the variations in the illumination of the anterior end. Every time the illumination is decreased, this causes the animal to swerve still more; so that its anterior end becomes pointed in many different directions, till it comes into one where such changes in illumination no longer occur. Such a position is found when the animal is swimming toward the source of light. Now the axis of the body retains always the same relation to the direction to the rays of light, so that the anterior end is not subjected to variations in intensity of illumination. There is then no further cause for reaction. Orientation is thus reached by trying various directions. This will be best understood by an examination of Fig. 95, together with its explanation.

Euglena responds most readily to light of a blue color (Engelmann, 1882). Passage from blue light to light of other colors has essentially the same effect as passage from stronger to weaker light. If the difference between the two is sufficiently decided, Euglena responds by the avoiding reaction in passing to the other color; it therefore remains in the blue. If a small spectrum is thrown on a slide containing many Euglenae, they gather in larger numbers in the blue, — especially in the near vicinity of the Frauenhofer's line F.

Very strong light, such as direct sunlight, has an effect on Euglena precisely the opposite of that produced by weaker light. If the organisms are subjected suddenly to sunlight, they give the avoiding reaction. They tend therefore to gather in less lighted regions. If the sunlight falls upon them from one side, they become oriented with anterior ends away from the source of light, and swim in that direction. The orientation takes place in exactly the way described above, save that now it is the increase of light at the anterior end that causes the avoiding reaction. If a vessel is placed in such a position that the sun shines on it from one side, while the half of the vessel away from the sun is shaded with a board, the following result is produced : The Euglenae gather in a band at the edge of the shadow (Fig. 96). They do not pass into the dark area beneath the shadow, nor do they remain in the region affected by direct sunlight, but in an area of intermediate illumination.1

We can thus distinguish an optimum intensity of light, in which Euglena tends to remain. Movement toward either a greater or a less intensity of light causes the avoiding reaction, with its trial of different positions and directions of movements, till a position or direction is found which leads toward the optimum, or retains the optimum intensity undiminished. Or, in other words, after Euglena receives an amount of light which we might call "enough," it avoids more light, and also less light.

Fig. 96. — Diagram to illustrate the results of Famintzin's experiment. The light comes from the direction indicated by the arrows, while the opposite side of the vessel is shaded, as indicated by the dots. The Euglenae gather in the intermediate region, across the middle. to remain seems to be about the amount which is most favorable to its life activities. Euglena requires light for assimilating carbon dioxide by the aid of its chlorophyll, just as do higher plants. If confined to darkness, it soon ceases activity, contracts into a sphere, and becomes encysted. On the other hand, direct sunlight is very injurious to it ; if long continued it causes the organism to fall to the bottom and die. Euglena avoids both the higher and the lower intensities that are injurious to it.

Thus in both negative organisms (Stentor) and positive organisms (Euglena), the determining cause of the reaction is a change in the intensity of light, and the reaction takes place by the usual method of the performance of varied movements, subjecting the animal successively to different conditions. When the sensitive anterior end is subjected alternately to light and shade, the organism "tries" other directions of movement till it finds one where such changes are not produced. In Stentor it is an increase in light that causes this reaction ; in Euglena is it usually a decrease that causes the reaction, though when the light is very strong an increase may have the same effect.

The reactions of other infusoria to light are similar in character, so far as known, to those of Stentor and Euglena. In only a few other cases have details of the avoiding reaction been worked out as thoroughly as for the two species mentioned. But all that we know of the reactions of infusoria to light is consistent with the method of reaction known to exist in Stentor and Euglena ; indeed, the evidence seems clear that these reactions take place in essentially the same way throughout the group. In Cryptomonas ovata, and less completely in the swarm spores of Chlamydomonas and Cutleria, the present writer has observed that the reaction to light is of the same character as in Euglena. We shall pass in review certain general features of the reaction in other infusoria, as described by various authors.

As we have before noted, most colorless infusoria give no indication of sensitiveness to light. But color is not absolutely necessary in order that reaction to light may occur, as is shown by the fact that Amoeba reacts to light. Even in the infusoria, colorless species may react to light when such behavior is distinctly beneficial to the organism. A species of Chytridium, a colorless flagellate that is parasitic on the green organism Haematococcus, reacts to light in the same manner as Haematococcus, collecting as a rule in lighted regions, or at the side of the vessel next the source of light (Strasburger, 1878). This, of course, aids it in finding its prey, which collects in the same regions. Several other colorless infusoria that are parasitic on green flagellates have been found to react to light in the same manner as their prey. Verworn (1889, Nachschrift) found that the colorless ciliate Pleuronema chrysalis reacts to a sudden increase in the intensity of light by a rapid leaping movement, — evidently a strongly marked avoiding reaction. Certain colorless infusoria react, as we shall see later, to ultra-violet light.

In the green ciliate Paramecium bursaria the reaction to light depends, according to Engelmann (1882), on the amount of oxygen in the water. This animal contains chlorophyll, which produces oxygen in the light. When there is little oxygen in the water, the organism gathers in lighted regions, thus of course increasing its store of oxygen. When the individuals in the light come to the boundary of a dark region, "they turn around at once into the light, as if the darkness was unpleasant to them" {I.e., p. 393). The response is thus clearly an avoiding reaction, like that of Stentor. When the water contains much oxygen, on the other hand, Paramecium bursaria avoids the light. On reaching a lighted area the animals react in the way above characterized, and return into the darkness. When they gather in light, it is especially in the red rays of the spectrum that they collect; these are the rays in which the chlorophyll is most active. When they avoid light, it is again the red rays that are most effective in producing the avoiding reaction.

Hertel (1904) found that Paramecium bursaria, Epistylis plicatilis, Stentor polymorphous, and Carchesium react to ultra-violet light, of 280 fxfx wave length. In the two species last named the chief reaction observed was a sudden contraction. Epistylis bends to one side under the action of the light, while Paramecium bursaria reacts in essentially the same manner as to ordinary light, as described above. All died quickly under the action of powerful ultra-violet light.

The flagellate swarm spores of many algas react to light. Their behavior in this reaction has been studied especially by Strasburger (1878). These swarm spores (Fig. 97) usually resemble Euglena in essential features, though they may differ in form, in the number of flagella, and in other details. They contain chlorophyll or other coloring matter, and usually a red eye spot. The action system of the spores is similar to that of Euglena. They swim in a spiral path, keeping a certain side always toward the axis of the spiral (Naegeli, i860, p. 96).

On Coming to an obstacle, they react Fig. 97. — Examples of swarm spores, by turning tO One Side (Naegeli, U.), b< uiothrixzonata; c, Botrydium granulatum, with or without a previous Start gamete; d, Cladophora giomerata; e, CEdoturning in response to a stimulus is always toward the side directed outward in the spiral path, as it is in Euglena, Chilomonas, and Cryptomonas. The movements of the swarm spores, so far as known, exactly resemble those of the organisms just named. It is further without doubt true that the anterior end is in the swarm spores, as in other infusoria, the most sensitive part of the body. The swarm spores are much smaller than Euglena, so that the details of the behavior are less easy to determine.

Strasburger found that when the light is weak, all the colored swarm spores * swim toward the lighted side of a drop (positive reaction). When the light is strong, some swim away from the lighted side (negative reaction). If different parts of a drop or a vessel are unequally illuminated, the swarm spores gather in the lighted region. The phenomena are thus in general similar to those found in Euglena. There are certain variations among the different swarm spores. Thus, Strasburger found that Botrydium and Cryptomonas are positive even in the strongest light, while in a weak light Cryptomonas is indifferent. But in most species there is, as in Euglena, an optimum. In light below

1 Strasburger studied the swarm spores of Hsematococcus lacustris, Ulothrix, Chaetomorpha, Ulva, Botrydium, Bryopsis, (Edogonium, Vaucheria, and Scytosiphon, as well as the flagellate Cryptomonas (called Chilomonas by Strasburger), and the colorless swarm spores of Chytridium and Saprolegnia. the optimum they are positive ; in light above the optimum they are negative. Strasburger did not determine the precise movements of the organisms in the reaction to light. That is, he did not determine toward which side they turn in becoming oriented. But in other respects his account is so excellent that, with the fuller results on Euglena as a key, it is not difficult to analyze out the precise factors in the behavior.

If the light affecting the organisms is suddenly decreased in intensity, Strasburger found that the swarm spores (Botrydium and Ulva) suddenly turn toward one side (I.e., p. 25). In Bryopsis this reaction was produced also when the light was suddenly increased. In all the swarm spores it was evident that as soon as the light was decreased by the interposition of a screen the path became more crooked (I.e., p. 27). In other words, the spiral became wider, owing to the increased swerving toward a certain side. In these respects the swarm spores precisely resemble Euglena. It is clear that they react to a sudden decrease in illumination by an avoiding reaction, which consists in turning more or less strongly toward a certain side, with or without a cessation of the revolution on the long axis; in this way the direction of progress is changed.

As would be expected from this method of response, the organisms react at passing from a light to a dark region. If a ring is placed over the drop containing the organisms, so that only a central circle is illuminated, the positive organisms gather in the illuminated circle (Fig. 98, A). Here they swim toward the window from which the light comes, but on reaching the edge of the shadow, they turn back into the lighted region (I.e., p. 28). Often the organism passes completely into the shadow before reacting, then it turns and swims back into the light. Thus it does not react till a short time after the moment of change. If a narrow band of shadow passes across the middle of the drop, transversely to the direction from which the light is coming, this usually does not stop the organisms, because of this interval of time which elapses before their reaction ; before they begin to react they have passed completely across the band into the lighted region beyond. But if a larger vessel is used and a broader transverse band of shadow passes across it (Fig. 98, B), this does stop the organisms. They gather on the edge of the shadow without passing across it. In many other ways Strasburger shows that when the area containing the swarm spores is unequally illuminated, the positive organisms collect in the more illumined region. In this they precisely resemble Euglena, as Strasburger himself noted. The behavior is of course a direct result of the production of the avoiding reaction by a decrease in light.

If the experiments were made with swarm spores that were negative to the intensity of light used, they gathered of course in the shadow instead of in the light. If a board was placed across the middle of the vessel from right to left, such swarm spores formed a collection in the partly shaded region at the edge of the board (as in Fig. 96), where they found the optimum degree of illumination. They were repelled both by the strong light and by the deep shadow.

Thus it is clear that in the swarm spores, as in Euglena and Stentor, a change in the intensity of illumination produces reaction. But a certain amount of change is required before any effect is produced. If the intensity of illumination changes only very gradually from one Fig. 98. — Diagrams to illustrate the results of some of Strasburger's experiments with positive swarm spores (original). A, the margins of the drop are shaded (as indicated by the dots); the organisms gather in the lighted centre. B, a broad band of shadow lies transversely across the drop; the organisms swim toward the light, but are stopped by the shadow. Thus two groups are formed, one at the side of the drop next the light, the other in a corresponding position at the edge of the shadow.

region to another, the difference in intensity between succeeding points is insufficient to cause reaction. Hence under these circumstances the organisms remain scattered and move about without reaction. Strasburger showed this in the following way. He used a hollow wedgeshaped prism, 20 cm. long, tilled with a partly opaque solution of humic acid in ammonia. Through this the light was passed. At the thin end of the wedge nearly all the light was transmitted ; at the thick end little or none, and there was a gradual transition from light to dark between the two ends. This prism was placed over the drop containing the swarm spores, and the light was allowed to fall directly from above (Fig. 99, X). The drop being very small in comparison to the length of the wedge-shaped prism, there was of course but little difference in the illumination of its two sides, and the transition from one to the other was very gradual. Under these conditions the swarm spores remained scattered throughout the drop. The change in pass-

ing from one region to another was not sufficiently marked to cause reaction.1 When the entire area is equally lighted and the light comes from one side, the positive swarm spores swim toward the source of light. If the light is made strong, most species swim away from its source. In this behavior the agreement with Euglena is complete. The orien- FlG. 99. — Diagram of the conditions in Strasburger's experiments with a wedge-shaped prism, constructed from the data furnished by Strasburger. a, prism 20 cm. in length, filled with a translucent fluid, b, hanging drop containing the swarm spores. X, rays of light coming from above, as in the first experiments. F, rays coming obliquely from the thicker end of the wedge, as in the second set of experiments. The figure is one half natural size.

tation takes place gradually, by a series of trials, as in Euglena. Strasburger paid no special attention to this point, but the present writer has observed that this is true in Cryptomonas, Chlamydomonas, and the swarm spores of the marine alga Cutleria, as well as in Euglena, and Strasburger (1878, p. 24) notes incidentally that it is true in Haematococcus.2 It seems clear, then, that the reaction takes place in the same manner 1 It is curious that Strasburger drew from this experiment the erroneous conclusion that variations in the intensity to light play no part in the reaction. The only essential difference between this experiment and the previous ones (Fig. 98) is that in the previous experiments the change of illumination in passing from one region to another is sudden and pronounced, while in the present experiments it is slow and gradual. The logical conclusion is that the lack of reaction in the present experiment is due to the slightness of the change in passing from one part of the preparation to another. When we consider that the prism was 20 cm. in length, and was placed over a mere drop, it is evident that the difference in illumination in different parts of the drop was excessively small. We know that for the effective action of all stimuli a certain threshold amount of change is necessary, so that the results are exactly what might be anticipated. Our account of Euglena shows beyond doubt that a change in intensity of illumination does cause reaction. Strasburger himself (I.e., p. 25) observed the same fact in swarm spores, though he paid little heed to this observation in the remainder of the work.