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.

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).

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.