STREAMING, CONTRACTILITY AND AMEBOID MOVEMENT
The nearest relatives of the amebas are the shelled rhizopods, the
Difflugias and the Arcellas and their congeners. The movement of these
organisms is quite different from that of the amebas in that the whole
body of the endoplasm does not stream into the pseudopods, but only a
small portion of it. There is consequently no regular transformation of
ectoplasm into endoplasm at the posterior end, that is, the protoplasmic
mass within the shell. The method of movement in _Difflugia_ was
described by Dellinger (’06). A pseudopod is thrown out to a
considerable distance. It fastens itself to the substrate at the tip. It
then contracts, pulling the _Difflugia_ forwards. While this pseudopod
is contracting, another one is extended in the same direction. When it
has arrived at the maximum length, it fastens itself at the tip and then
contracts, pulling the _Difflugia_ along. Continued locomotion consists
of a repetition of this process. The pseudopods are slender and consist
nearly always of clear protoplasm. Only occasionally does one see
conspicuous endoplasmic granules flow into a pseudopod, and then only at
the base.
The transparency of the pseudopods in _Difflugia_ and the absence of
granules in the protoplasm composing them, prevents one from seeing
clearly how the pseudopods are formed, that is, whether or not there is
a regular transformation of endoplasm into ectoplasm at the anterior
end. The fact that one occasionally sees the endoplasm stream into the
base of a pseudopod in the same way as was described for ameban
pseudopods, indicates that the method of formation of pseudopods in
_Difflugia_ is in general similar to that in ameba. But the process is
not exactly the same, for the surface layer on the pseudopods of
_Difflugia_ does not move as fast as the tips of the pseudopods advance,
while in amebas the surface layer moves faster than the pseudopods. What
this difference indicates has not yet been ascertained.
The protoplasm of the pseudopods of _Difflugia_ is thick and the power
of contractility highly developed, for the pseudopods readily move about
in the water like a tentacle. The demarcation line between ectoplasm and
endoplasm is very difficult to see, consequently no definite idea can be
given as to the thickness of the ectoplasm. When a pseudopod is being
extended the whole contents seem to move at about the same rate as the
pseudopod advances, differing thus from amebas, in the pseudopods of
which the central core of the endoplasmic stream flows considerably
faster than the tip of the pseudopod advances through the water. But
when a large pseudopod is cut off from a _Difflugia_ it is able to move
after the manner of an ameba without a nucleus (Verworn, ’94).
In heliozoans protoplasmic streaming is quite different from that in
ameba or _Difflugia_. The pseudopods are usually straight, radiating
from the central body. They possess usually a central axial rod of
condensed or strongly gelatinized protoplasm around which is a layer of
thick protoplasm with the properties of ectoplasm. Heliozoans for the
most part move slowly; in fact many of them are pelagic and in these the
power of locomotion on a solid substratum is very slow. There is however
one species, _Acanthocystis ludibunda_, which, according to Penard
(’04), can move twenty times its diameter in one minute by rolling. This
illustrates a highly developed power of contractility in the pseudopods
of this organism, for since only about one-fifth of the circumference
can be in contact with the solid substratum, the pseudopods must attach
themselves, contract so as to pull the _Acanthocystis_ along, and relax
their hold, all in the space of two seconds.
Among pseudopod forming organisms, the highest development of
contractility is found in the foraminifera. As is well known, these
organisms form finely anastomosing pseudopods which frequently cover the
substratum with a network of protoplasmic strands. The terminal sections
of these strands are frequently so thin and transparent that they cannot
be seen easily with the microscope. As a rule the granular endoplasm is
observable only in the main body of the organism and in the larger
trunks of the pseudopods. Much the larger part of the pseudopods, as
measured lineally, is devoid of granular endoplasm. The great power of
contractility and the speed with which contraction may occur in
_Biomyxa_, a fresh water foraminifer, have already been mentioned
(Figure 12, p. 47). Similar observations have been recorded by other
observers, recently by Schultz (’15), who compares the contractility of
foraminiferan pseudopods to that of rubber bands. In fact as one watches
the movements of a _Biomyxa_, for example, under moderately high
magnification, one gains the impression that there seems to be no
restriction imposed upon the extent of contractility in the pseudopods.
They seem to possess perfect elasticity. As to the transformation of
endoplasm into ectoplasm, little can be said, owing to the transparency
of the protoplasm. But the whole of the pseudopod, when forming, seems
to stream forward. As in _Difflugia_, the interior streams flow at about
the same rate as the pseudopod as a whole advances. The highly developed
power of contractility however demands rapid changes in phase of the
colloidal system, and also a thick consistency. The behavior of pieces
of the pseudopodial network, when cut from a _Biomyxa_, shows clearly
that the protoplasm is actually thick, as compared with that of an
_Amoeba proteus_. When a _Biomyxa_ is contracted into a spherical mass,
the interior exhibits continual rapidly streaming movements. Some of
these are rotational but most of them are radial. All of the streams
frequently change their direction and extent. No corresponding changes
are visible in the outer peripheral layer.
Among plants, some of the algae possess ameboid protoplasts at one stage
or another of their life cycle, but the details of streaming have not
been made out. It has been reported however that the zoospores of some
parasitic fungi move to all appearances exactly like small amebas. We
likewise lack details of the streaming of the myxomycete plasmodia. From
a more or less cursory examination of a small aquatic plasmodium of
undetermined species, it appeared that the formation of pseudopods and
the process of streaming were quite different from similar processes in
the foraminifera. The pseudopods do not act independently as in
foraminifera. At almost the same moment the protoplasm begins to flow
from the pseudopods in a large section of the plasmodium and into
another section; then soon thereafter the protoplasm flows back again.
This oscillatory streaming is continued presumably as long as the
myxomycete is in the plasmodial stage. With every change in the
direction of movement of streaming, there is produced, however, a change
in the shapes of the pseudopods, so that with a number of oscillations
in streaming an appreciable degree of locomotion is effected. The
direction of locomotion can be markedly affected by changes in light
intensity and moisture distribution, as shown by the observations of
Baranetzsky (’76), Stahl (’84) and others, but just how these changes in
the direction of locomotion were produced is not recorded. There is a
definite ectoplasm and a definite endoplasm in the myxomycete plasmodia,
but the details of their transformations, the one into the other, have
not been determined; but since the surface layer is stationary, it is
probable that there is no such regular transformation of endoplasm into
ectoplasm at the anterior ends of pseudopods as there is in ameba. But
this phase of the subject needs further investigation before any
conclusions can be drawn. The power of contractility is present, but
apparently only to a slight degree. Too little is known of the streaming
process in these organisms to compare it in detail with the same
phenomenon in rhizopods.
The streaming of protoplasm in plants has received a good deal of
attention, though only comparatively little experimental work has been
done. Streaming is observed in a great many plant cells, and in some
cells such as the large internodal cells of _Chara_ and _Nitella_, the
process may be easily observed. The essential features of a plant cell
in which streaming occurs are, first, the external cell wall of
cellulose, which of course prevents any change of shape in the cell such
as is observed in naked protoplasts as, for example, ameba. Inside of
the cell wall is a layer of ectoplasm which has essentially the same
properties as the ectoplasm of amebas. In some cells such as those of
_Chara_, the ectoplasmic layer is thick and contains nearly all the
chloroplastids, while in the leaf cells of _Elodea_ the ectoplasm is
extremely thin and is practically free from chloroplastids. In the
interior of the cell are found the streaming endoplasm and one or more
large vacuoles filled with cell sap.
The streaming is of two types which are often distinguished from each
other by the names _rotational_ and _circulatory_. But the distinction
seems to be of little significance, for the same cell may at different
times show both types of streaming. When there is a single vacuole only
in the cell, it occupies the center of the cell, and the endoplasm then
rotates between it and the ectoplasm. Whenever there are strands of
endoplasm flowing across the vacuole, the peripheral streaming is no
longer rotational but it is then called circulatory. By external
stimulation of the cell, Ewart (’03) was able to change circulatory
streaming into rotational; that is, the numerous small streams
traversing the cell sap in many directions were caused to retract into a
single stream around the periphery of the cell. This change brought
about a heightened velocity in streaming, showing that the small strands
traversing the cell sap meet with some resistance. There is no essential
difference between streaming in plant cells, whether rotational or
circulatory, from the rotational streaming so commonly found in
protozoa.
[Illustration: Figure 31. Diagram of a section of a _Chara_ cell showing
rows of emulsion globules in the endoplasm, after Ewart. _a_, cell wall.
_b_, ectoplasm. _c_, endoplasm, _d_, cell sap. The arrows at the top of
the figure indicate by their lengths, the amount of movement of the
endoplasm and cell sap in streaming.]
Ewart has also observed that in the streaming of the endoplasm, there is
a variation of velocity of streaming in different parts of the stream
(Figure 31). The middle of the stream moves fastest while the layer near
the ectoplasm moves very slowly and the layer in contact with the
ectoplasm moves hardly at all. But the endoplasm in contact with the
central vacuole moves only a little more slowly than the middle of the
stream, and the effect of this is that the outer edge of the vacuole is
dragged along with the moving endoplasm. This is an important
observation and from it Ewart concludes that the energy which produces
the streaming movement must be liberated, not at the boundary between
the ectoplasm and the endoplasm, nor at that between the endoplasm and
the vacuole, but within the endoplasmic stream itself. In this
conclusion Ewart is undoubtedly correct, for as a physical phenomenon,
no other conclusion is at present possible.
Other experiments made upon the velocity of streaming in plant cells
indicate that the streaming process obeys the laws of physics. The
velocity varies with the proportion of water present in the
endoplasm,--the more water, the faster the streaming (Ewart, ’03). The
effect of temperature on streaming, noted first by Corti (’74), and
studied by Velten, (’76), Schaeffer (’98), Ewart (’03) and other
writers, is also such as would be expected if the endoplasm were a
simple physical fluid.
The rotational streaming in plant cells, such as those of _Chara_, is
very similar to the rotational streaming in paramecium and numerous
other ciliates. In these organisms it is often called cyclosis. A
paramecium differs, however, from a plant cell exhibiting rotational
streaming in that no central vacuole is present. This space in
paramecium is occupied by the gullet, the nucleus and some endoplasm
which is not in the main stream. The effect of this difference seems to
be one affecting velocity only, slowing it down, for in the _Chara_ cell
the endoplasm meets with much less friction when moving in contact with
the vacuolar wall than when moving in contact with the ectoplasm. Its
velocity is still further reduced by the large food vacuoles which are
almost always carried by the endoplasm, for these vacuoles behave like
solid bodies in the endoplasmic stream. During streaming these vacuoles
are often seen coming close to the limiting ectoplasm, when they act as
obstructions to the flow of the endoplasm. The velocity of the
endoplasmic stream in paramecium is relatively slow, ten to twenty
minutes being required for a complete revolution.
In _Frontonia leucas_, another large ciliate, rotational streaming is
under the control of the organism, and special use is made of it in
feeding. _Frontonia_ feeds mostly, if not entirely, on large particles.
It has no oral groove like paramecium has, and when swimming no ciliary
vortex is produced such as is seen in paramecium. _Frontonia_ feeds
mostly by “browsing,” that is by eating particles lying on or against
some solid support, though it is able also to feed upon particles
suspended in the water.
_Oscillatoria_ and _Lyngbia_ and other filamentous algae are the chief
food of _Frontonia_. Filaments of these algae are ingested by pulling
them into the mouth and then rolling them up into a coil in the body.
Pieces of _Oscillatoria_ six to eight times as long as the _Frontonia_
are readily eaten in this way.
As a rule the end of a filament is seized by the mouth and gradually
passed back into the body (Figure 32, _a_). As soon as the tip of the
filament is well in the mouth and in contact with the endoplasm,
streaming begins in the endoplasm in the region of the mouth and takes a
direction directly back against the aboral wall, almost, if not quite
perpendicular to the longitudinal axis. This stream of endoplasm carries
the filament back to the aboral wall, sometimes pushing out the wall a
considerable distance. Presently, however, the filament is carried
posteriorly along the aboral wall by the streaming protoplasm, which has
by this time become rotational, and after reaching the posterior end the
filament is brought up along the oral wall. The rotational streaming
continues until the entire filament is wound up, which in exceptional
cases may make four or five coils inside the animal.
The mouth has considerable grasping power. This is shown in Figure 32
where a filament of _Oscillatoria_ was bent upon itself by the mouth and
then rolled up in the body by the endoplasm in the same manner as a
single filament. The mere viscosity of the endoplasm would be
insufficient to bring about the bending of the filament. For the sake of
comparison it should be added that a similar grasping power is also
present in paramecium. The moment the food vacuole at the mouth is large
enough, the endoplasm pulls it away and moves it rapidly toward the
posterior end of the paramecium, much more rapidly than it would be
carried by the rotationally streaming endoplasm. But from the posterior
end forward the food vacuole is carried at the same rate as are the
other particles in the endoplasm. In both _Frontonia_ and paramecium
rapid endoplasmic streaming precedes for a short distance the forward
end of the ingested filament or the food vacuole (Figure 32, _a_).
[Illustration: Figure 32. Showing ingestion of alga filaments in
_Frontonia leucas_. _a_, the beginning of the ingestion of an alga
filament. Note the streaming of the endoplasm _preceding_ the end of the
filament. _b_, almost two complete coils of the filament have been
rolled up inside the _Frontonia_ by the rotary streaming endoplasm. The
endoplasm in the center of the animal is stationary. _c_, a filament, if
thin, may be grasped anywhere along its length, bent together and
swallowed in the usual manner. Diameter of _a_, 250 microns.]
If a filament of alga is too long for the _Frontonia_, or one end of it
is fast, streaming is reversed after several coils have been rolled up
and the filament is ejected. So far as could be observed, the streaming
process is reversed in all details, though the rate of ejection seemed
to be somewhat slower than the rate of ingestion. Occasionally, however,
ejection is accomplished much more quickly. If there are several coils
of a filament whose other end is fast, rolled up inside of a
_Frontonia_, the mouth sometimes stretches antero-posteriorly until the
coil as a whole without unwinding is thrown out of the body. The
viscosity of the endoplasm might lead one to expect that some of the
endoplasm would be brought out with the alga, but such is not the case.
The essential differences between rotational streaming in _Frontonia_
and in paramecium are: (1) It is under the control of the organism in
_Frontonia_ while in paramecium it is a continuous reversible process.
(2) It is much more rapid in _Frontonia_ than in paramecium. On the
other hand, the physics of streaming in both organisms is essentially
the same, so far as could be detected. In both organisms the energy of
streaming is liberated within the endoplasm. This is especially well
shown in the first stages of feeding.
Besides these organisms in which streaming occurs, either in a part of
the organism or the whole, streaming is also found to occur in a great
variety of plants other than those already mentioned; in the leukocytes
of perhaps all coelomates; in some animal egg cells, such as the
sponges, hydra and molluscs; in pigment cells, especially in batrachians
and lacertilians; in phagocytes and wandering cells of a great many
animals; in the nuclei of some animal cells; and in the intestinal
epithelial cells of perhaps all metazoans. In almost none of these cases
however do we know more than the bare fact that streaming occurs. No
details are known. Consequently in so far as the purposes of this book
are concerned it will not be apropos to discuss these cases further
except to record the thesis that there is no evidence tending to show
that these cases are not at bottom all characterized by the operation of
the same fundamental process.
In all these cases of animal and plant cells and tissues in which
ameboid movement occurs the process of streaming is easily observed in
all of them, but the phenomenon of contractility is not noticeable in
some cells except under special conditions, while in other cells it is
operating continually. This indicates that there are other factors at
work in addition to mere phase changes in the colloidal system to
produce now contractility, now streaming. A high power of contractility
and of streaming are not present in the same mass of protoplasm at the
same time, though these powers may both be present at different times
(_Biomyxa_).
Contractility can be explained in a general (though not yet in a
detailed) way as due to a change in phase, more or less complete, in the
colloidal system which is held to be the chief characteristic of the
physical aspect of protoplasm. The change of phase is of course,
associated with a change in the amount of surface energy, which is the
ultimate source of the energy of contractility.
Streaming, however, does not depend upon a _marked_ change of phase
resulting in gelation, for observation has failed to detect this process
going on to any extent whatever in streaming protoplasm. Further, an
increase in the amount of water in the protoplasm is associated with
more rapid streaming. If streaming therefore depends upon a phase change
in a colloidal system, it must be in the direction of liquefaction, that
is, changing the internal more fluid phase to the external phase. A
phase change in one direction would thus lead to contractility, while a
change in the other direction would lead to streaming.
Theories accounting for the intimate nature of the process of streaming
without special reference to ameboid movement, have been offered by many
botanists. In most plant cells in which streaming movements occur the
ectoplasmic covering does not change shape. Streaming of the endoplasm
therefore is a much less complicated process in such a case than in an
ameba where locomotion is also present. It is to be expected therefore
that a theory of streaming based upon observation of a plant cell such
as is found in _Chara_ would be different from one based upon
observation of a moving ameba. Such is found to be the case, as the
following discussion of some of the principal theories accounting for
streaming in plant cells strikingly shows.
(1) _The contractility theories_. Corti (’74), who was the first to
record observations on the process of streaming in plants thought that
the movement of the endoplasm was caused by waves of contraction passing
around the cell in a way analogous to that in which fluid may be passed
through a rubber tube by closing the finger over it and passing it along
the tube. Heidenhain (’63), Kühne (’64), Brücke (’64), Hanstein (’80),
in one form or another also have expressed their adherence to the
contractility theory. More recently Dellinger (’06, p. 356) postulated
contractile fibrillae in rhizopods similar to those postulated by Brücke
to explain protoplasmic streaming. The contractility theories are no
longer considered tenable, for no waves of contractility can be
demonstrated, as the theories of Corti, Heidenhain, et al. demand, and
contractile fibers can neither be demonstrated nor can they be conceived
to exist in endoplasm which exhibits all the essential properties of a
fluid.
(2) _The imbibition theories_. Sachs (’65), Hoffmeister (’67) and
Englemann (’79) conceived of streaming as being caused by certain
constituents of the cell imbibing water and later discharging it. Sachs
and Hoffmeister thought that waves of imbibition and extrusion of water
passing progressively along the cell was able to cause movement of the
protoplasm. Ewart (’03) has shown, however, that as much as 2000 times
its own volume of water would have to be imbibed by a cell of _Nitella_
in the course of a day to account for the amount of streaming observed,
and that no sign of the extrusion of water could be detected by
observing small suspended particles in the immediate vicinity of the
cell. Englemann’s theory involving a change of shape of his hypothetical
supra-molecular “Inotagmas,” by the imbibition of water and the
subsequent release if it, which was supposed to account for the movement
of the protoplasm while streaming, has been considered too hypothetical
and too far removed from the realm of experiment to be of real value,
either as an explanation or as a working hypothesis.
(3) _The oxidation theory of Verworn_. Verworn (’92, ’09) has postulated
a “Biogen Molecule” which exists only in living protoplasm and
dissociates when protoplasm dies into a number of chemical molecules of
albumin and other substances. Ameboid movement and streaming generally,
according to Verworn, is caused by the lowering of the superficial
surface tension in the moving mass of protoplasm followed by streaming
of the protoplasm toward the point of lowered tension. The lowering of
the surface tension is brought about by a union of the Biogen Molecule
with oxygen. With the dissociation of the biogen-oxygen compound,
presumably through a respiratory process, the surface tension rises
again. This theory does not hold for amebas, for we saw in the preceding
pages that the surface tension is higher at the anterior ends of
pseudopods than elsewhere on the ameba. And in plants, as Ewart (’03)
has shown, oxygen does not seem necessary to the streaming process, for
the endoplasm of _Chara_ cells continues to stream for many days in the
entire absence of oxygen. It is possible that there would be enough
loosely fixed oxygen in the endoplasm of _Chara_ to supply the demands
of Verworn’s theory; but the very hypothetical nature of his theory
prevents one from discussing this possibility.
(4) _The electrical theories_. These fall into three classes: (a) _The
galvanic theory_. Amici (’18) suggested that the chloroplastids floating
in the endoplasm of plant cells acted as galvanic cells, setting up
currents in the endoplasm which in some way caused the endoplasm to
move. Dutrochet and Becquerel (’38) also held to this explanation. A
fatal defect of this theory is that streaming occurs in a great variety
of cells, myxomycete plasmodia, amebas, stamen hairs of _Tradescantia_,
etc., in which no chloroplastids occur; and there is no ground for
assuming that the causes of streaming in cells with chloroplastids is
fundamentally different from that in other cells. (b) _The
electromagnetic theory_. Velten (’72, ’73) and Hörmann (’98) are chiefly
responsible for the development of the electromagnetic theory. They hold
that chloroplastids have an independent movement of their own; but the
principal postulate of this theory is that there is electric repulsion
between the ectoplasm and the endoplasm. Ewart (’03) has pointed out,
however, that this theory is contradicted by the fact that when
streaming becomes very active in _Elodea_, the ectoplasm becomes
exceedingly thin and therefore would show movement in the direction
opposite to that of the endoplasm if there were magnetic repulsion
between these layers. Moreover, the formation of threads of endoplasm
across the central vacuoles in plant cells, and the much branched
network of pseudopods in plasmodia and foraminifera would be very
difficult if not quite impossible to explain on this assumption. (c)
_The electro-chemical surface-tension theory of Ewart_. As the result of
a considerable amount of experiment and observation on endoplasmic
streaming in plants, Ewart (’03) has come to the conclusion that there
are differences in electrical potential between the protoplasm-vacuole
boundary and the protoplasm-cell wall boundary, and that as a
consequence electrical currents are passing between these points,
traversing the protoplasmic stream. If now it is assumed that the
particles in the endoplasm, which are electrically polarized, have the
surface tension of their corresponding ends decreased when electric
currents traverse the endoplasmic stream, the particles and, of
necessity, the whole stream of endoplasm would move in the direction of
lowered surface tension (Figure 31, p. 96). Continuous chemical actions
would be necessary to maintain the conditions as outlined. This theory
accords with the facts so far as it goes, but it does not explain the
streaming in threads across the vacuole in the plant cell, thus
necessitating two theories for the explanation of streaming within a
single cell at the same moment. Moreover a central vacuole of cell sap
seems always to be required to fulfill the conditions of this theory,
and this, as is readily seen, makes it impossible to apply it to
streaming in amebas, myxomycetes, foraminifera and ciliates.
The fundamental cause of streaming is therefore still to be discovered,
for neither the theories of streaming as applied to ameba, nor those
described above which refer especially to plant cells, are satisfactory.
But a significant point in these theories is that with increasing
information, they come more and more to demand a colloidal structure in
the protoplasm. It is the surface energy in the interfaces in the
colloidal system which comes to be regarded as the primary source of the
energy. But all attempts thus far to explain exactly how this energy is
utilized have been unsuccessful. Gaidukov’s (’10) observation is of some
interest, however in this connection. He found that the occasional
stopping of streaming in cells of _Vallisneria_ is accompanied by a
cessation of Brownian movement, which indicates a change from a sol to a
gel state. This proves therefore that colloidal changes are possible in
streaming protoplasm, and that the general search for an explanation of
streaming along this line is proceeding in the right direction. The
researches of Bancroft (’13, ’14) and especially of Clowes (’13, ’16) on
the nature of the change of phase in emulsions are very instructive in
this connection; and it is undoubtedly true that as rapid progress is
now being made by the investigation of colloidal solutions as by the
direct study of protoplasm, in solving the problem of streaming.
The problem of the _control_ of the streaming process, which is of
course much the most important feature of streaming, will probably be
solved, at least in part, when the mechanics of streaming is
understood.