General Physiology: An Outline of the Science of Life

The result of diminishing the turgor must in all cases be the same. The primordial utricle, which previously was stretched from within outward by the tension, will shrink together, and its circumference will become smaller (Fig. 90). But what is more important for the present purpose is the diminution in size of the whole cell with decrease of the turgor, for the tension of the elastic cellulose coat will be decreased to the same extent as that of the primordial utricle, and, as a result of its elasticity, the wall will assume finally a circumference corresponding to its decreased tension (Fig. 90, B, C, D).

In the movements of plants now under consideration a change of turgor takes place solely by the contraction of the primordial utricle of certain cells for some cause, either spontaneously or as the result of stimulation, in such a manner that water is squeezed out of the cells ; the phenomenon passes away after some time, and the turgor again appears pari passu with the disappearance of the contraction. There thus appears under certain circumstances a sudden diminution of the turgor and with it a diminution in the size of the cell, and only gradually does the previous condition return.

In order that upon this principle a microscopic movement may take place in a plant, the cells that undergo the change of turgescence must have a definite arrangement. If in one of two parallel rows of cells the turgor is suddenly diminished, so that the cells become smaller, while in the other it remains unchanged, the first row must shorten. Hence, according to simple mechanical principles, a bending will occur with the concavity upon the shortened side. At the same time the other side will be extended passively. If, later, a gradual increase of turgor and a lengthening of the cells upon the shortened side takes place, the elasticity of the other side will assist the extension.

Such a diminution of turgescence appears in many plants, often very suddenly, both spontaneously and after mechanical stimulation, and the result is a sudden movement of certain parts. In most cases both the arrangement and the shape of the cells that cause the movement are very complicated. As a rule, at the base of the motile leaves, or petioles, small enlargements, called pulvini, are developed, the cells of which can diminish their turgor very rapidly. One of the best-known examples of this kind is the movement of the petioles in the sensitive Mimosa pudica, which in the " waking " state, i.e., during the day, are upright with the leaflets extended (Fig. 91, /, A, and //, A), while in the " sleeping " state, i.e., at night, they are depressed and the leaflets are folded upward together (Fig. 91,7,5, and //, B). If a Mimosain the waking state be vigorously shaken, the night position is suddenly assumed in the daytime.

Upon the same principle depend numerous other movements of the sensitive plants, such as those of the leaves of clover, the stamens of barberry, the insect-catching organs of carnivorous plants, and many others. d. Movements ~by Change of the Specific G-ravity of the Cell Among the wonderful forms of animals, mostly of glassy transparency, that lead a pelagic life in the upper strata of the sea and lately as plankton have become the object of detailed investigation, there are many that are endowed with the remarkable capacity of slowly rising or sinking in the water without the use of any locomotor organs. These are especially the Radiolaria, Ctenophora, and Siphonophora. Some unicellular, fresh-water organisms, such as Actinosphceriwn, also possess this power. Since all external causes for this mysterious suspension, such as currents of water, may be excluded, and since the movement of special organs of the body does not share in it, it can depend only upon changes in specific gravity, and this has been demonstrated. As has already been seen,1 protoplasm is heavier than water. Hence a cell that lies upon the bottom can raise itself only when substances that are lighter than water appear and accumulate in the protoplasm.

It is well known that certain fresh-water Rhizopoda, especially Arcella and Difflugia, which are provided with delicate capsules, are heavier than water, and usually creep about upon the bottoms of ponds and puddles between particles of mud and decaying leaves, can actively raise themselves by developing a bubble of carbonic acid in their protoplasmic bodies ; when it has become sufficiently large, they rise to the surface like a small balloon. Engelmann ('69) first carefully investigated this fact. At times in a culture-vessel containing Difflugia, when conditions favour the development of carbonic acid in the protoplasm, the movement of individuals from the bottom to the surface becomes epidemic. If the carbonic acid is then given off, the individuals sink again to the bottom. In this manner there may arise in nature a very considerable change of habitation, which under certain circumstances, as when the Protista have come under unfavourable conditions, can be of great usefulness to the species.

In an analogous manner take place the rising and sinking of the Radiolaria and, in all probability, those of the Ctenophora and many other pelagic animals. Thalassicolla nucleata, e.g., is a large globular radiolarian of 3-4 mm. in size, which represents a single cell, the nucleus of which, surrounded by protoplasm, lies in a spherical central capsule (Fig. 92). The whole extracapsular protoplasm is filled with innumerable vacuoles, so that it appears like a mass of foam, and it is bordered externally on the side of the seawater by a solid layer of jelly. This vacuole-layer is the portion of the cell that is lighter than the sea-water, and

maintains the undisturbed Thalassicolla suspended at the surface of the sea.1 This can be made out readily by removing from the living animal single constituents of the cell, by cutting off the layer of jelly, isolating the vacuole-layer and extirpating the central capsule with its contents. All constituents, when isolated, sink to the bottom of the water, except the vacuole-mass ; this remains at the surface, and, if submerged, continually returns to it.2 Correspondingly, the whole Thalassicolla begins to sink as soon as the vacuole-layer collapses by the bursting of the vacuoles, which takes place as a result of stimulation, in nature especially from the impact of violent waves. Then the cell falls into more quiet depths, and thus is protected from entire destruction ; the

FIG. 92.— Thalassicolla, nucleata, a spherical radiolarian cell in section. In the middle of the central capsule, which is surrounded by black pigment, lies the vesicular nucleus. The central capsule is surrounded by the vacuole-layer, which is enveloped by a zone of jelly and sends through the latter radiating, thread-like pseudopodia. vacuole-layer can regenerate itself, and the Thalassicolla, increasing in volume, in quiet weather rises again from the depths to the sunny surface. The great importance of this manner of movement for the life of pelagic organisms is evident.

It is a question how the contents of the vacuoles can become lighter than the surrounding sea-water. The cause of the appearance of vacuoles, the formation of which can easily be observed in any isolated central capsule, consists in the accumulation throughout the protoplasm of osmotic substances, which cause the water to come in from the outside to them through the protoplasm. The size of the vacuole increases in proportion as the formation and concentration of osmotic substances in the protoplasm increase, for an equalisation of the osmotic pressure in the liquid of the vacuole

and in the surrounding water must always take place, i.e., the liquid of the vacuole must always contain in solution the same number of molecules as the water. But it must be assumed that these are molecules of substances different from those in the water. If, therefore, we imagine some of the substances dissolved in the liquid of the vacuole to possess low specific gravity, we can understand how, upon the whole, the contents of the vacuole can be lighter than the water. K. Brandt ('95) has recently made it very probable that it is the carbonic acid produced by the protoplasm that, dissolved in the liquid of the vacuole, lowers the specific gravity of the protoplasm below that of the sea-water. If the vacuole-layer is developed -to a sufficient extent, the specific gravity of the whole cell will be less than that of the seawater, i.e., the cell will float at the surface. If by the bursting of the vacuoles the volume of the layer becomes diminished, or if in the cold, when the metabolism sinks to a minimum, the production of carbonic acid becomes greatly decreased, the radiolarians will sink again.

Movements that come about through secretion by the cell are limited to a few groups of organisms, particularly the Algce, Desmidiacece, and Oscillaricv. The principle of this mode of motion is extremely simple. It consists simply in the cell lying upon the bottom and pressing out at a definite place upon its surface and in a definite direction a mass of secretion, usually of a slimy nature ; this sticks to the bottom, and the motile cell-body thereby thrusts

FIG. 93.— Closterium, a desmid, shoving itself along the bottom by a secretion of slime. The nonsecreting end swings freely in the water. itself forward in a definite direction, just as a fisherman pushes his boat off the shore with a pole. If the secretion continues, the cell glides slowly along. In this manner the Desmidiacece move themselves. The crescentshaped Closterium (Fig. 93), which we have already become acquainted with in considering the Brownian molecular movement, secretes a slimy substance at each end of its unicellular body. While it thus clings to the bottom with one end, the other end floats freely in the water, so that the whole body is directed

upward obliquely at a certain angle. The Closterium shoves itself slowly forward, as Klebs ('85) and Aderhold ('88) have shown, by the attached end expelling a mass of slimy secretion (Fig. 93), the cell maintaining approximately its angle of inclination to the bottom. But in gliding forward it alternates its two poles, the swinging pole sinking, adhering and secreting, while the previously attached pole rises and swings freely. Thus the alga gradually moves forward upon its support.

As regards the movement of the Diatom-zee, the small, brown, boa -shaped or rod-shaped Algce, provided with an extremely delicate silicious shell, which are found in enormous variety in both fresh and salt water, a literature almost too vast for review has appeared. When these unicellular forms are observed in a drop of water upon a slide, they are seen gliding forward upon the bottom in the direction of their long axis in a peculiar hesitating manner, sometimes slowly, sometimes rapidly, and often going backward with the poles reversed in direction. It seems impossible to discover any sort of locomotor organs in the body. The numerous investigators who, like Max Schultze, Engelmann, and others, earlier studied this graceful form of motion, adopted widely different views as to its origin. Afterward, from the researches of Biitschli ('92, 2) and Lauterborn ('94), it appeared as if it depended upon the above principle of the extrusion of a slimy secretion. Biitschli and Lauterborn succeeded in showing that certain forms of

JJiatomece are enveloped in a covering of jelly and extrude peculiar threads of secretion, which can be made visible by adherent granules of india ink (Fig. 94). But recently the very detailed investigations of O. Miiller ('93, '94, '96, '97) have shown that these threads have a subordinate significance in the progression of the Diatomece, and that the mode of motion of these small cells is much more complicated, and perhaps more allied to movement by protoplasmic streaming.

As to the long, blue-green, thread-like Oscillarise, which consist of many cells arranged one after another in a row and creep slowly through the water like the Diatomece, it is highly probable that they really shove themselves along the bottom by the expulsion of FIG. 94. — Diatom with threads of slime extruded. (After Butschli.) a secretion. Recently, Schewiakoff has shown the same also for the Gregarince (Fig. 22, I, p. 80), which are parasitic unicellular organisms that likewise perform very slow, gliding movements without special locomotor organs.

Movements that are associated with the growth of cells need only be mentioned briefly; their principle needs no elucidation. All growth is accompanied by movement, for, as a cell increases in volume, it becomes expanded. Hence growth-movements are common to all living substance, but they take place so slowly that they can scarcely be followed with the eye. If, however, growing objects be compared with their earlier stages after considerable spaces of time, if the sprouting seed be first considered and then the plant that has developed from it with all its branches, leaves and flowers, it is evident that extensive movements have taken place, by which the building material has been transported to the places where it is laid down. Growth-movements are recognised also especially clearly in long plant-stalks or tendrils, when the cells grow or multiply more rapidly upon one side than upon the other, so that the part becomes curved. But the most apparent movements caused by growth are in those cases in which the mechanical energy developed by growth is not continually set free, but is accumulated in the form of tension, and finally by some stimulus is suddenly transformed into kinetic energy; this appears most beautifully in the seeds and fruits of certain plants, e.g., Impatiens, which, upon being touched, suddenly burst with a jerking motion and throw out their contents. It is not necessary to go further into the mode of growth-movements, since their principle is plain and they are met with at every step in living nature. That the phenomena of growth are powerful sources of energy is clear when it is recalled that trees growing between rocks are able to force apart huge masses of stone by their roots.

Finally, movements that are produced by the contraction and expansion of the cell-body, and which are usually termed, in brief, contraction-phenomena, are distinguished from all other organic modes of motion by the fact that they consist of changes in the form of the surface of the living substance itself, which changes are associated with an alternate shifting of position of its particles. All contraction-phenomena comprise two phases of movement, that of contraction and that of expansion. The particles of living substance arrange themselves with reference to one another in contraction, so that the mass presents a smaller surface,

in expansion, so that the same mass presents a larger surface. Transition from one phase to the other alone renders possible phenomena of motion. It is evident that only bodies of more or less liquid consistency can show such movement ; only a liquid can diminish or increase its surface by rearrangement of its particles, becoming spherical or spread out, according as its surface-tension is equal in all directions, or becomes greater in some places and less in others. A solid, stiff body, even if it is elastic, cannot manifest contraction-phenomena of this kind, because its particles cannot change their mutual positions. Hence, it is of fundamental importance for the occurrence of contraction-phenomena that living substance possess a liquid consistency. As a matter *of fact, all living substance, as has already been found, is more or less liquid, a condition that is imposed upon it by the high percentage of water in its contents, and, therefore, the common view is well founded that all living substance possesses contractility, although many cells are known, such as certain Algce and Bacteria, which in spite of their leading an active life can perform no contractionphenomena, because they are surrounded by a stiff membrane. Contractility, i.e., the property of executing contraction-movements, is, however, a general property of living substance, and hence demands detailed consideration.

Among the phenomena of movement brought about by contraction and expansion in accordance with the above principle there can be distinguished, according to the peculiar differentiation of the substratum in which they are observed, three groups, which are termed : muscle-fibres) ; Ciliary movement (movement of flagella and cilia). Amoeboid movement, the original form of contraction-phenomena, is found wherever there exist naked protoplasmic masses, that is, cells the protoplasmic bodies of which are not surrounded by a cell-membrane, or wherever, as in plant-cells, there is within the membrane a free space for movement. As examples there may be mentioned especially the manifold representatives of the great protistan group Rhizoppda (Figs. 95 and 98); further, in the animal cell-community, leucocytes and amoeboid wandering-cells of various kinds (Fig. 96), amoeboid egg-cells of certain animals, such as sponges (Fig. 17, a), pigment-cells of widely different organs1 (Fig. 97),

1 The view often expressed in recent times, that in the movements of pigmentcells there is a change of place of the granules of pigment without a simultaneous change of form of the protoplasmic body, appears to me wholly untenable. intestinal epithelium-cells (Fig. 45) ; and, finally, various kinds of plant-cells (Fig. 24, a, and Fig. 35). The movement of Amoeba can serve as a type (Fig. 95). This organism is the lowest of all living things, and its formless body holds within itself the whole secret of life. Taken with a pipette in a drop of water from the bottom of a pond and brought under the microscope upon a slide, the amoeba-cell appears as a small grey semitransparent droplet of a more or less pronounced spherical form ; in the central portion lie the nucleus and usually a contractile vacuole, surrounded by a more or less granular endoplasm, while the peripheral layer consists of a more hyaline exoplasm. If this drop of living substance be observed for some time, it is

seen that at some point of its surface the spherical mass bulges out in the form of a lobate projection ; this becomes constantly larger and extends itself farther and farther, more protoplasm flowing into it constantly ; the phenomenon spreads from the peripheral parts toward the centre, so that a continual streaming takes place from the centre toward the periphery in this so-called pseudopodium (Fig. 95). Frequently the whole protoplasmic mass of the amoeba flows over into this one lobate projection, so that the body forms a single extended mass, as can be observed especially in Amoeba Umax. Frequently, however, the centrifugal protoplasmic streaming of the pseudopodium becomes interrupted, while at the same time at another point of the surface a second pseudopodium is formed in the same manner by a centrifugal

flowing of the protoplasm into the medium, and a third may follow this, so that the amoeba protrudes its substance in various directions, and thus considerably increases its surface. This extension of pseudopodia, this flowing of substance into the medium, represents the phase of expansion. While a new pseudopodium is being extended, protoplasm usually flows out of another one, from the periphery back to the centre to afford material for the new one, that is, the old pseudopodium is drawn in. This retraction of pseudopodia, this centripetal back-flow of the protoplasm and diminution of the surface associated with it, represents the phase of contraction. If all pseudopodia are drawn

FIG. 96.— A leucocyte (white blood-corpuscle) of the frog, in various stages of movement. (After in, the amoeba-cell again assumes a spherical form. The spherical form is, therefore, the expression of most complete contraction in naked protoplasmic masses. When undisturbed, however, simultaneous contractions and expansions usually take place in the same amoeba at different points on its surface. Hence the pseudopodia are not preformed. Substance flows out, now here, now there, is mixed continually and flows back again, and this changeable play is the amoeboid movement.

In the various amoeboid protoplasmic masses the form of the pseudopodia varies greatly, according to the special consistency and composition of the living substance. As has already been .seen,1 there occur among the numerous forms of rhizopod-cells pseudopodia that are short and blunt, incised, thick and fingershaped, slender and thorn-like, straight and radiating, long and thread-like, dichotomously branched, or reticulate. But all these varieties, which are united with one another by innumerable transitions, are produced in the same manner, namely, by protoplasm streaming out into the medium centrifugally from the central cellbody. In organisms possessing long, filose pseudopodia, such as Foraminifera (e.g., Orlitolites, Fig. 98), the protoplasm must travel a long way from the centre to the tip of the constantly

FIG. 97.— Pigment-cells from the skin of the frog ; 7, extended ; //, slightly contracted ; 777, strongly contracted ; IF, wholly contracted ; the clear spot in the centre of the cell-body is the nucleus. lengthening process ; in these fine threads the microscope shows the protoplasm with its granules flowing like the water of a slow stream. This extremely fascinating phenomenon constantly charms the observer and has been vividly described by Dujardin ('41), Max Schultze ('54), and Haeckel ('62), as granular or protoplasmic streaming. In the retraction of such pseudopodia the protoplasmic particles must again travel over the same path in the reverse or centripetal direction. In pseudopodia that are extended to a considerable distance and remain extended for a considerable time, two currents, a centrifugal and a centripetal, are always

FIG. 98.— Orbitolites complanatus, a rhizopod-cell from the Red Sea (small specimen magnified about forty times ; natural size of adults upon an average 5 mm.). The central protoplasmic body is enclosed in a round, disc-shaped, calcareous shell, which consists of innumerable chambers arranged essentially in concentric rings. The protoplasm of each chamber contains one or more nuclei. At the periphery of the shell numerous fine, straight, filose pseudopodia protrude, often reaching in large specimens a length of almost 20 mm. ; these branch greatly and amalgamate with one another. Very beautiful protoplasmic and granular streaming may be seen upon them.

noticed, in thick pseudopodia the former plainly upon the periphery, the latter in the axis of the strand. According as the former or the latter preponderates, the pseudopodium gradually extends or shortens itself. If the two are equally strong, it remains extended, its length not changing. The phenomena of contraction and expansion may be studied very easily and fully in the long, filose pseudopodia of Foraminifera, such as Orlitolites (Fig. 98). The phase of expansion, i.e., extension, consists always in a centrifugal flow of the living substance into the surrounding medium, the phase of contraction, i.e., retraction, in a centripetal flow from the periphery to the central cell-body. Expansion is characterised by an increase of surface, contraction by an effort toward a spherical form.

Protoplasmic streaming in plant-cells follows the same plan. A cell from a stamen-hair of Tradescantia mrginica is a cylindrical, closed cellulose-capsule (Fig. 99, A), in which the protoplasmic cellbody with its nucleus is enclosed. The protoplasm forms upon the inner wall a continuous, extremely delicate layer, the so-called primordial utricle, from which there extend in various directions through the lumen of the capsule, filled with cell-sap, protoplasmic strands which anastomose with one another and at one point lodge the nucleus. Both in these long, protoplasmic strands and in the primordial utricle a continual, protoplasmic streaming is visible, which corresponds perfectly to the protoplasmic streaming in the pseudopodia of Rhizopoda. When the protoplasm in the various strands flows in an inco-ordinated, irregular direction, the movement is termed by the botanists circulation ; when it follows continually one definite direction, rotation. This phenomenon would correspond, therefore, to the protoplasmic movement of a rhizopod-cell, such as Orlitolites, in an undisturbed state, in which the protoplasm in elongated pseudopodia streams continually both in a centrifugal and a centripetal direction, i.e., in which the phases of contraction and of expansion are equally developed. In the plant -cell such a complex system of currents has arisen by division of the mass of protoplasm into single, anastomosing strands, so that the distinction between centrifugal and centripetal currents no longer holds ; the same is true, also, of large Rhizopoda, such as the plasmodia of Myxomycetes, the whole body of which resolves itself into a richlybranched, pseudopodial network. The phase of contraction may, however, be brought out here very clearly and easily by stimuli. As in the Rhizopoda, it is characterised by the protoplasm becoming contracted into globules (Fig.