H. S. (Herbert Spencer) Jennings.

Contributions to the study of the behavior of lower organisms online

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described bv the anterior end is then less and the whole body describes
the surface of a cone, or a frustum of a cone, as illustrated in Fig. 21.
Every gradation exists between the normal spiral course and the strong
reaction in which the anterior end swings in a circle about the
posterior end as a center.



REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES.



53



When oriented and swimming toward the source of light the swerv-
ing toward the dorsal side is comparatively slight. As seen from
above, the organisms seem merely to oscillate a very little from side to
side as they revolve on the long axis. Careful examination shows that
the swerving is always toward the dorsal side, as in Fig. 20, the alter-
nations in direction being due to the alternations of position of the
dorsal side. Now, when the illumination is suddenly decreased, the
Euglenae at once swing much farther than usual toward the side to




FIG. 21.*

which they are already swerving, that is, toward the dorsal side. If
the decrease in illumination is not very great, so that the stimulus is
not a strong one, the swerving is not very great, and the organism at
the same time continues to revolve on its long axis ; thus the anterior
end describes a circle and the whole body describes the surface of a



*FiG. 21. Diagram to illustrate reaction of Euglena when the illumination
is decreased. The Euglena is swimming forward at i ; when it reaches the
position 2 the illumination is decreased. Thereupon the organism swerves
strongly toward the dorsal side. This swerving, combined with the revolution
on the long axis, causes the anterior end to swing about a circle, so that the
Euglena occupies successively the positions 2, 3, 4, 5, 6, etc. From any of these
positions it may start forward, as indicated by the arrows, if the condition
causing the reaction ceases to act. In the figure the Euglena is represented as
swimming forward from the position 6.



54



THE feKHAVIOR OF LOWER ORGANISMS.




cone, or the frustum of a cone, as indicated in Fig. 21. The result, as
seen from above, is that all the specimens seem to vibrate from side to
side ; in other words, they are taken with a sudden oscillation or trem-
bling. This oscillation when the intensity of the light is suddenly
changed was observed by Strasburger (1878,
pp. 25 and 50) in flagellate swarm-spores ; he
speaks of it as " Erschiitterung " or " Zit-
tern." During this oscillation the anterior end
becomes pointed successively in many different
directions, as Fig. 21 show r s. When, now, the
usual forward course is resumed (with only the
usual amount of swerving toward the dorsal
side), the animal follows one of these directions.
Thus its path is changed (Fig. 22). Strasburger
(1878, p. 25) noticed that the path followed after
the oscillation was oblique to the former path.
As a study of Figs. 21 and 22 will show, this is a
necessary consequence of the increased swerving
toward the dorsal side, to which the oscillation
itself is due. All these relations become much
clearer if a model of an actual spiral is studied ;
it is difficult to represent them upon a plane
surface.

If the stimulus is stronger, as when there is a
greater decrease in illumination, the swerving
toward the dorsal side is much greater ; the or-
ganism wheels far to that side, so that the spiral
course seems entirely interrupted. But there is
really nothing in this reaction differing in prin-
ciple from what is happening in the normal
forward swimming. If the swerving toward the
dorsal side is long continued the specimen may
be seen to swing first far to the (observer's) right,
then, after it has revolved on the long axis, far
to the (observer's) left; in reality it swings an
equal amount upward and downward and in intermediate directions.
It may, however, swing at once so far to the dorsal side that the new



FIG. 22.*



* FIG. 22. Shows the spiral path of Euglena, illustrating the effect of a
slightly marked reaction. At a the illumination is decreased; the organism
therefore swerves toward the dorsal side, causing the spiral to become wider.
At b the ordinary method of swimming is resumed; since at this point the
organism was more inclined to the axis of the spiral than before the reaction,
the new course lies at an angle with the previous one. Compare with Fig. 21.



REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES.



55



course forms a right angle, or a still greater angle, with the original
course; if the turning is through 180, the course will be squarely
reversed. Indeed, sometimes the organism swings around an entire
circle or more. When the usual method of swimming is resumed after
such reactions as those just described, the course has been completely
changed.

Strasburger (1878, p. 25) noticed that after a decrease in illumina-
tion flagellate swarm-spores often turn strongly to one side or even
describe circles. But he did not notice that the turning was always
toward the same side of the organism,* and did not perceive the relation
between this effect and the remainder of the reaction.




FIG. 23. f

This method of reaction is particularly striking when the Euglenae
are confined to a very thin layer of water between the slide and the
cover glass, so that they cannot swerve up or down. When the light
is decreased, we will suppose that the dorsal side is to the (observer's)
left. The Euglena then swings far to the left. At the same time it



*Naegeli (1860, p. 96) had, however, before Strasburger, observed that in such
swarm-spores the same side always faces the outside of the spiral path. This
observation, which really contained the germ of a correct understanding of the
reactions to stimuli, seems hardly to have been noticed by later writers.

t FIG. 23. Diagram of the method by which Euglena becomes oriented with
anterior end toward the source of light. At i the Euglena is swimming toward
the source of light. When it reaches the position 2 the light is changed so as
to come in the direction indicated by the arrows at the right. As a consequence
of the decrease in illumination of the anterior end thus caused, the organism



56 THE BEHAVIOR OF LOWER ORGANISMS.

revolves on its long axis, bringing the dorsal side down. Since it can
not swing downward, owing to the narrow space, this has little effect
on the reaction, save to stop the movement to the left. Now, by con-
tinued rotation the dorsal side has come to lie to the (observer's)
right ; the Euglena may then be seen to swing far to the right. In each
case under these conditions it is at once evident by observing the
larger lip at the anterior end that the organism is swinging toward
the dorsal side.

This method of reaction is very effective in preventing Euglena from
passing from an illuminated region to a shaded one. As soon as the
anterior end enters the shadow, the animal swings far toward the dor-
sal side till the anterior end is brought again into the light, repeating
the reaction if necessary. There is then no further cause for reaction.
The reaction to a very strong increase of illumination is, as we have
seen, identical with that to a decrease in illumination.

In our experiments thus far we have directed attention primarily to
the effects of changes in the intensity of illumination, and have found
that such changes produce a motor reaction independently of the direc-
tion of the light rays. But it is of course well known that Euglena
does react with reference to the direction of the light rays. EuglenaB
swim toward the source of light when weakly illuminated, away from
the source of light when strongly illuminated. If Euglenae are swim-
ming at random in a diffuse light they soon become oriented when the
light is allowed to act on them from one side, even if the intensity of
illumination remains the same. Or, if Euglenas are swimming toward
a source of very weak light and a stronger light is allowed to act upon
them from the opposite side, they become oriented, in time, with
anterior ends toward the stronger light. In examining this dependence
of the direction of swimming on the direction of the rays of light, we



swerves strongly toward the dorsal side, at the same time continuing to revolve
on the long axis. It thus occupies successively the positions 2, 3, 4, 5, 6. In
passing from 3 to 6 the illumination of the anterior end is increased; hence the
reaction nearly or quite ceases. In the next phase of the spiral, therefore, the
organism swerves but a little toward the dorsal side from 7 to 8. But this
movement causes a decrease in the illumination of the anterior end, and this
change induces again the strong swerving toward the dorsal side. Hence in
the next phase of the spiral the organism swings through 9 and 10 to n. In
this movement again the illumination of the anterior end is increased; hence
the reaction ceases, so that from 12 the organism swerves only as far as 13.
Then owing to the decrease in illumination caused by this movement, the
swerving increases, so that the Euglena swings from 13 through 14 and 15 to 16.
Now it is directed toward the source of light, and such swerving as takes place
in the spiral course neither increases nor decreases the illumination of the
anterior end. Hence there is no further reaction; the Euglena continues to
swim forward in the direction 16-17.






REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES.



57



shall have to keep in mind two questions : First, how is the position
of orientation brought about? Second, what is the real stimulus in
producing orientation?

To answer the first question we must observe the movements of the



f\




2 I



74




FIG. 24.*

organism at the time orientation occurs. Observation of the individ-
uals as they are becoming oriented shows that orientation is brought
about through the same motor reaction that we have already described ;



* FIG 24. Path followed by Euglena when the direction of the light is
changed. From i to 2 the organism swims forward in the usual spiral path
At 2 the position of the source of light is changed, so that it now comes from
behind. The organism then begins to swerve farther than usual toward the



58 THE BEHAVIOR OF LOWER ORGANISMS.

that is, by a turning toward the dorsal side. The simplest case is per-
haps that of the reversal of orientation, produced when strong sunlight
is allowed to fall from in front upon specimens that are swimming
toward a diffusely lighted window. Under these circumstances, as
we have seen, the Euglenae turn toward the dorsal side, changing their
course. They may turn directly through 180, in which case they are
at once oriented with anterior ends away from the light ; but usually
the orientation is less direct than this. The reaction is generally
repeated several times. Through its continued swerving toward the
dorsal side, combined with the revolution on the long axis, the organism
directs its anterior end successively in every direction. When the
anterior end has finally come into a position where it points away from
the strong light the reaction ceases, and the organism swims forward
in the usual way. The details of the orienting reaction will be brought
out more fully in the following account of the way in which the anterior
end becomes directed toward a source of light of moderate intensity.
Let us now take a case in which the change in the direction of the
rays of light is not accompanied by a change in the intensity of illumi-
nation. Euglenee are swimming about at random in a diffuse light
when all the light is allowed to fall upon them from one side. They
then become oriented, with anterior ends directed toward the source of
light. Or, the organisms are swimming toward a source of light when
the direction of the light rays is changed or reversed by quickly
moving the source from which the light comes. The Euglenae then
after a time become reoriented. Under such circumstances there is no
sudden, decided reaction, such as occurs when the illumination is
suddenly decreased. The organism merely begins to swerve farther
toward the dorsal side than usual. Thus the spiral has become wider,
and the anterior end comes to be pointed successively in many dif-
ferent directions, as illustrated at 1-6 in Fig. 23. In some of these
positions the anterior end is directed farther away from the source
of light, as at 3 ; in other positions more nearly toward the source
of light, as at 6. In the latter case the swinging toward the dorsal
side becomes less marked ; hence the succeeding phase of the swing,
which carries the anterior end away from the light, is less pronounced ;



dorsal side, owing to the decrease in the illumination of the anterior end. Thus
the spiral becomes wider, a and b showing the limits of the swerving. At 3 the
normal amount of swerving is restored, so that the new path is at an angle with
the old one. Now the organism swerves at each turn of the spiral a short dis-
tance away from the source of light, as at c, , g, and a longer distance toward
the source of light, as at </, /", h^ for the reasons shown in Fig. 23. At h it has
in this manner become directed toward the source of light, and there is no fur-
ther cause for swerving more to one side than to the other; it therefore swims
in a spiral with a straight axis toward the source of light.



REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 59

the anterior end therefore does not swing so far in the direction
away from the light as in the preceding phase it swung toward the
light. This is illustrated at 7-8 in Fig. 23. But as a result of such
swerving as does occur the anterior end is now (at 8) directed more
away from the source of light than before. There then follows a
new reaction, with increased swerving toward the dorsal side in the
next phase of the spiral (S-ii, Fig. 23), which carries the dorsal side
toward the source of light. Hence the anterior end swings still further
toward the position where the light shines directly upon it. This con-
tinues. As a result of this repeated swinging of the dorsal side slightly
away from the source of light and strongly toward the source of light
the organism gradually changes its course, continuing to swim in a
spiral and to swerve toward the dorsal side, until the axis of the spiral
is in line with the light rays and the anterior end is toward the source
of light. This method of reaction will best be understood by a study
of Figs. 23 and 24 and their explanation.

Thus the orientation is gradual and for a certain stretch after the
light has begun to act the organism is not completely oriented. With
a fairly strong light, however, the period of time required for complete
orientation is very slight. Strasburger (1878, p. 24) noticed that when
Ha3matococcus is swimming toward a source of weak light and the
light is suddenly increased so as to reverse the orientation, there is a
period of ." verschiedenen Schwankungen " before the reverse orienta-
tion is attained. He paid little attention to the behavior of the
organisms during this period, however.

Our account has been thus far purely descriptive ; we have attempted
to set forth the events as they may be observed, without trying to
indicate the causes at work. We must now inquire as to what is the
real stimulus and its method of action in producing orientation.

First, we note that in becoming oriented Euglena does not turn
directly toward the source of light. As in the reaction to other stimuli,
the turning is throughout toward a structurally defined side. This
shows that the orientation of Euglena, like that of Stentor, cannot be
accounted for on the orthodox tropism theory. In other words, the
orientation is not due to the direct effect of the light on the motor
organs of the side on which it falls. As in Stentor, orientation may
be reached by turning either toward or away from the source of light,
or in any intermediate direction. The response is a u motor reaction "
of a definite type.

Just what is the stimulus which produces this motor reaction? All
our experiments up to this point have shown clearly that this reaction
is produced by changes in the intensity of illumination, and that a change
in the illumination of the anterior end produces the reaction as well as



60 THK BEHAVIOR OF LOWER ORGANISMS.

does a change in the illumination of the entire body. Indeed, Engel-
mann (1882, a) showed that a change in illumination over the remainder
of the body is ineffective in producing the reaction, so that in every case
the reaction is due to the change in illumination at the anterior end.
Now, in the orientation reaction the conditions are present for produc-
ing changes of illumination at the anterior end of precisely the character
which would, in view of our other experimental results, bring about
the reactions observed. This will best be shown by again examining
in detail from this point of view a concrete case.

In Fig. 23 we will suppose that the Euglena at i is at first swimming
toward the source of light. When it reaches the position 2 the light is
changed, so that it now comes from the direction indicated by the
arrows at the right. By this change the intensity of illumination at
the anterior end is decreased, since before the light came from directly
in front and affected the entire end, while now it falls upon but one
side. We know from other experiments that as a result of such a
change the organism reacts by swerving more toward the dorsal side,
at the same time continuing to revolve on the long axis. This is ex-
actly what happens now ; by the increased swerving the organism is
carried from position 2 to position 3. In this change the anterior end,
swinging still farther away from the source of light, is still less illumi-
nated than before. As a result of this farther decrease in illumination
the reaction is continued or increased ; combined with the revolution
on the long axis it carries the organism successively to positions 4, 5
and 6. In this part of the movement the anterior end becomes pointed
more directly toward the source of light, and is hence more strongly
illuminated ; there is therefore nothing in this movement to cause a
reaction. The strong swerving toward the dorsal side then ceases or
becomes less. But in the next phase of the spiral course (from 7 to 8),
there is necessarily at least the normal amount of swerving toward the
dorsal side, and this carries the organism to a position (8), where
the intensity of the light acting on the anterior end is decreased. As
a result of this decrease we know that the "' motor reaction " must again
be induced ; the organism swings then farther toward the dorsal side-
This movement, combined with the revolution on the long axis, carries
the Euglena through 9 and 10 to n. Here again the swerving de-
creases, because the change was from a less illuminated to a more
illuminated region. Hence after reaching 12 the Euglena swerves only
a little away from the light, to 13 ; then, as a result of the decrease in
illumination at the anterior end caused by this movement, it swerves
far toward the light, through 14 and 15 to 16. This movement causing
greater illumination, the reaction ceases. The light is now shining full
on the anterior end. The organism therefore swims forward in the



REACTIONS TO LIGHT IN CILTATES AND FLAGELLATES. 6l

usual spiral course, in all phases of which the illumination of the anterior
end is equal. If the light came from the rear of Euglena I instead of
from the direction indicated by the arrows, the reaction above described
would be continued in the same way until the direction of swimming
was completely reversed.

Thus the orientation of Euglena in a continuous light is due to the
production of the *' motor reaction," with its turning toward the dorsal
side, whenever there is a decrease in illumination at the anterior end.

There is no other explanation of the orientation, so far as I am able
to see, that is in agreement with all the facts. At first one is tempted
merely to say that the subjection of the anterior end to shadow pro-
duces the motor reaction, and that this is continued until the anterior
end is no longer shaded. This statement is correct if by "subjection
to shadow " we mean an active process, involving a change from a
more illuminated condition. But if we mean that darkness as a con-
tinuous, static condition is the cause of the reaction, then considera-
tion shows that this will not account for all the facts. It leaves out of
account the capability of the organism to become acclimatized to cer-
tain degrees of light and shade, and certain of the experimental results
are crucial against it. Thus, suppose the Euglenae are swimming
toward a source of weak light, and a stronger light is then allowed to
act upon them from another direction. The anterior end continues to
receive the same amount of light as before (since the weak light still
persists), yet the organism reacts as usual, becoming oriented toward
the stronger light. The motor reaction by which the orientation is
brought about cannot therefore be due to darkness or shade (considered
statically) at the anterior end. On the other hand, the case just men-
tioned is easily understood on applying the explanation given above.

Again, it might be held that the reaction is due in some way to the
relative amount of illumination at the two ends. It might be main-
tained, for example, that when the posterior end is more illuminated
than the anterior, this difference acts as a stimulus to cause the " motor
reaction." There is, of course, no independent evidence in favor of this
view, and the experimental results prove it to be incorrect. We have
shown that the reaction is produced ( i ) when both ends are equally
stimulated, as when the light comes directly from one side ; (2) when
neither end receives light, as when the light is cut off completely. Fur-
ther, it might be held that the reaction is produced when the anterior end
is not more intensely illuminated than the posterior end. It is, of course,
a little difficult to conceive how so indefinite a condition could act as a
stimulus to a definite motor reaction, but in any case the experiments show
that this is not the real cause of the " motor reaction." Thus certain of
the experiments show that the " motor reaction " is produced even when



62 THE BEHAVIOR OF LOWER ORGANISMS.

the light is reduced by the same amount at both ends, so that the anterior
end is still more strongly lighted than the posterior. This case is
realized in the experiment in which a small screen is interposed between
the Euglenae and the window toward which they are swimming. The
light is thus somewhat decreased, but is still sufficient to cause orien-
tation. The anterior end is thus still lighted more than the posterior,
yet the organisms respond with the " motor reaction" at the moment
the light is decreased. The same thing is shown still more decidedly
in the experiment described on page 50, in which the " motor reaction "
is produced when the light is cut off from some other source than that
toward which the organisms are swimming. In this case the propor-
tion of light shining on the anterior end is greater after the change in
illumination than before, yet the " motor reaction" is produced at the
moment the change takes place.

The explanation we have given is, therefore, the only one that is in
agreement with all the facts, and it accounts for every detail of the re-
actions to light. The cause of all the phenomena of light reaction in
Euglena is the fact that a sudden change in light intensity on the anterior
end induces a typical " motor reaction." It is noticeable that the
reaction is throughout due to a dynamic factor, to some change in the
relation of the organism to the light, a change due either to an active
alteration of the environment, or to a movement of the organism. To
static conditions, if not too intense, the organism may soon become
acclimatized, so that no farther reaction is caused. The absolute in-
tensity of the light affects the reaction only in so far as it determines
whether it shall be an increase or a decrease in intensity that causes
the *' motor reaction."

To sum up, the reaction of Euglena, from beginning to end, is ex-
plained by the fact that a sudden change in illumination, even though
slight, causes a definite motor reaction, the essential feature of which
is an increased swerving toward the dorsal side. Orientation is brought
about by the increased swerving in the next phase of the spiral course
when the illumination of the anterior end is diminished, and by the



Online LibraryH. S. (Herbert Spencer) JenningsContributions to the study of the behavior of lower organisms → online text (page 29 of 50)