General Physiology: An Outline of the Science of Life

All mechanical stimulation of living substance consists in a change of the pressurerelations under which it exists. Every degree of pressure can act as a stimulus, from crushing or cutting, which destroys the continuity of the substance, down to the slightest touch and the most delicate change in the pressure of the air or the water that surrounds the organism. Under the unilateral action of pressure-stimuli — in other words, in all cases where differences of pressure exist upon two different parts of the body of an organismphenomena appear that correspond to those of chemotaxis. Since these possess in common the one characteristic of being called forth by pressure (ffdpos) acting unequally on different sides, they may be designated by the term barotaxis. Various kinds of barotaxis can be distinguished according to the kind of pressure ; and it can be positive or negative, according as the organism turns toward the side of the higher or the lower pressure.

Under thigmotaxis, all those cases of barotaxis can be grouped in which the phenomena are caused by the more or less strong contact of living substance with more solid bodies. Naked protoplasmic masses, such as rhizopods and leucocytes, exhibit the simplest form of this. These afford, indeed, striking examples of how feeble contact calls out positive thigmotaxis, strong contact negative, and how, analogously to the case of chemotaxis, differences in the intensity of the stimulus are of essential importance. If, e.g., a marine rhizopod, such as the often -mentioned Orbitolites (Fig. 98, p. 238), be left quiet in a glass vessel containing sea-water, after some time pseudopodia begin to be put out from the small openings in the calcareous shell. Consisting at first of very short fibrils, they float freely in the water. Soon, becoming longer and heavier, their ends sink to the bottom and become fixed there by means of a delicate secretion, and the protoplasm begins

actively to stream along the bottom without rising again freely into the water. In other words, upon slight contact with the bottom the living substance of the rhizopods behaves positively thigmotactically and turns toward the object. Except in the free-swimming Radiolaria, Heliozoa, etc., extension and wide expansion of the pseudopodia take place always in contact with some body, whether it be the bottom, the cover-glass, the surface of the water, or objects in the water. On the other hand, by strong mechanical stimulation of the tip of an extended pseudopodium of Orbitolites, best by press-

FIG. 216. — Pseudopodium of Orbitolites, in a cut across at * ; b, c, ing upon it with a needle or cutting it across with a scalpel, it is possible to call out negative thigmotaxis of its living substance, the protoplasm at the place of stimulation drawing together into small globules and spindles, and streaming away (Fig. 216).1 The same phenomenon may be observed still more distinctly in a rapidly reacting fresh-water rhizopod, the shell-bearing Cyphoderia ; here the protoplasm of the pseudopodia withdraws from the place of stimulation with great rapidity (Fig. 217).

Thigmotactic phenomena are wide-spread. Among plants they 1 Cf. Verworn ('92, 1). are best known in the creeping plants and climbers, whose tendrils and twining shoots turn toward the objects which they touch and grow in constant contact with them (Fig. 218). Here the structural relations are so complex that the behaviour of the living substance within the single cellulose-capsule cannot be directly observed ; hence, thus far it is not known with certainty in what way the individual cell shares in the thigmotactic twining.

Dewitz ('86) found positive thigmotaxis in the spermatozoa of the cockroach (Periplaneta orientalis). If the spermatozoa be brought into a 0*6 per cent, solution of common salt between the slide and the coverglass, after a short time all the individuals collect partly upon the lower surface of the cover-glass, and partly on the upper surface of the slide, and in these places describe circles with their flagella, the direction of which, without exception, is opposite to that of the hands of a watch. The greater portion of the liquid remains completely free from spermatozoa, the latter not leaving the surface of the glass after having once reached it. If a ball be placed in the drop, its surface is at once sought by them. If a solution of common salt, containing spermatozoa, be placed in the cavity of a ball, after a short time the whole inner surface of the latter is covered, and the liquid in the middle is completely deserted. The pronounced thigmotaxis of these spermatozoa, like the positive chemotaxis of many others, is of the greatest importance for the fertilisation of the ova.

A contrast to this behaviour of the spermatozoa of Periplaneta is afforded by the following observation upon a genus of ciliate Infusoria, Oxytricha. Their flat, yielding bodies are beset upon FIG. 217. — Cyphoderia, with extended pseudopodia, stimulated at ^ > . The protoplasm is flowing away from the place of stimulation. their under side with cilia, which the animals, like woodlice, use as legs, with which to creep about upon objects in the water. These Infusoria are always seen creeping about busily and restlessly upon the slide, the cover-glass, or particles of mud lying in the water, without ever of themselves losing contact with the objects. The following episode from the life of an Oxytricha illustrates this positive thigmotaxis particularly well. In a flat dish containing river-water and an Oxytricha, there lay some spherical eggs of the river-mussel Anodonta. When the contents were poured into the dish, the Oxytricha in some manner came into contact with one of the eggs. It ran about unremittingly for hours upon the spherical surface without being able to leave it, since the egg rested with one point only upon the level bottom (Fig. 219, C). The organism must have travelled an enormous distance. After four hours it was able to forsake its enforced retreat by means of a particle of mud which came to the isolated egg. Experiments which artificially imitated with other Oxytrichae essentially the same conditions give wholly analogous results.

Jennings1 has recently discovered in Paramcecium another typical case of positive thigmotaxis. If a piece of filterpaper, or any other substance provided with a rough surface, be placed under a cover-glass under which are numerous Paramcecia distributed uniformly through the water, after some time the piece is beset with a thick coating of the Infusoria, which touch it with their cilia without moving from their place. By employing high powers it is shown that those cilia that are in direct contact with the foreign body stand straight out and perfectly still (Fig. 220, A), and that the activity of the cilia over all the rest of the body is greatly depressed and eventually wholly stopped. There is here a very pronounced thigmotaxis. In connection with this it is noteworthy that the thigmotactic assemblage of Paramcecia con-

stantly attracts new individuals chemotactically by its production of carbonic acid ; thus all the individuals in the drop accumulate finally about the foreign body (usually in the course of 5 — -10 minutes), although, since it is surrounded by an impenetrable wall of individuals held thigmotactically, most of them cannot come into direct contact with it (Fig. 220, B). Thigmotaxis, which causes the individuals that swim by chance to the foreign body to remain, is merely the first cause of the assemblage ; chemotaxis toward the carbonic acid produced by them completes it.

A second form of baro taxis, in which the stimulus is produced, not as in thigmotaxis by contact with a solid body, but by a gentle current of slowly flowing water, is rheotaxis, which was discovered by Schleicher and carefully investigated by Stahl ('84). This is FIG. 219. — Oxytricha, a ciliate infusorian. A, Seen from below ; B, seen from the side ; C, creeping about over the egg of a mussel. the peculiarity belonging to certain organisms, of taking toward flowing water a direction of motion opposed to the direction of the current. Since these organisms thus turn toward a pressurestimulus, rheotaxis is merely a special form of positive barotaxis. Thus far rheotaxis is known in a few organisms only. Stahl •demonstrated it best in the plasmodia of Myxomycetez in Aethalium septicum, by the following experiment. He suspended a narrow .strip of filter-paper in a beaker filled with water, and somewhat elevated, in such a manner that one end of the strip dipped into the water, while the other end hung far down over the edge of the beaker. In such a strip there is a continuous slow current of water directed toward the end that hangs down, as is proved by placing upon it a coloured mark. Stahl laid this end upon a mass -of tan, in which the plasmodia of Aethalium live. The result was

that the plasmodia slowly crept from the tan upward in the strip over the edge of the beaker arid downward upon the inner side of the glass, until they spread themselves out upon the surface of the water. By proper control-experiments it was possible to determine with certainty that it was only the streaming water which afforded the stimulus. Unfortunately the rheotactic properties of other organisms have been little investigated. It is, however, very probable that rheotaxis is wide-spread. Among other cases, it is easy to assume that the human spermatozoa are rheotactic and find their way to the egg-cell by means of this property. When the spermatozoa come into the uterus, they meet a current of mucous liquid coming toward them, since the cilia of the epithelium lining the uterine

FIG. 220.— Thigmotaxis of Paramcecium. A, An individual in contact with a fibre of filter-paper ; the cilia that touch the fibre directly are still. £, Assemblage of Paramacia about a bit of filter-paper under the cover-glass. (After Jennings.) cavity have a direction of stroke toward the os, and hence produce a current toward the outside. That it is chemotaxis of the spermatozoa toward the ovum which points out the path to them becomes very improbable when it is remembered ^that the spermatozoa wander upward in the uterus before the ovum has left the ovarian follicle. As a matter of fact Roth ('93) has succeeded in showing experimentally that spermatozoa and likewise certain Bacteria are rheotactic, by producing under the cover-glass a feeble continuous current and observing that these unicellular organisms move in opposition to it.

As a third form of barotaxis we have to consider finally geotaxis, i.e., the phenomenon that certain organisms place themselves and move with their median axis in a very definite direction toward the centre of the earth. In this case the stimulus is afforded by the minimal differences of pressure that exist at points of different height, both in the water and in the air. These phenomena have been known longest in botany, for all plants are geotactic in a pronounced manner. The roots grow toward the centre of the earth and, therefore, are positively geotactic ; the branches and the stems grow away from the centre of the earth and, therefore, are negatively geotactic. Further, in the behaviour of the leaves and in many cases the branches, which grow essentially tangential to the earth's surface, a third sort, transverse geotaxis, is seen.

In free-living cells geotactic properties have been recognised, especially by Schwarz ('84, 1), Aderhold ('88), Massart ('91), and Jensen ('93, 1), who have found that of Infusoria and Bacteria in closed vessels containing water, some rise upward and collect upon the surface, while others seek the depths and crowd together upon the bottom. If, e.g., water containing numerous Paramcecia be put into a vertical glass tube, the Infusoria, as Jensen found, in a short time rise and collect at the upper end of the tube (Fig. 221), whether the latter be open or closed. Paramcecia are, therefore, negatively geotactic. Many Bacteria, as Massart observed, behave conversely ; with a similar arrangement of the experiment they accumulate at the lower end of the tube. These are, accordingly, positively geotactic.

*^M±££V5£ Until very recen% either ver>' mystical as a result of negative ideas or none at all had been formed conthe upper end. (After cermng the manner in which gravity calls out geotactic phenomena; but Jensen has now shown that the effects are due to differences in pressure at different heights. As is well known, the hydrostatic pressure in a column of water is considerably less at the top than at the bottom. The higher pressure operates as a stimulus .and causes the organisms to leave the place where it is present and seek the places of lowest pressure. As all consideration at once shows, no other differences exist between the upper and the lower portions of the column of liquid in the vertical glass tube. An unprejudiced observer must, therefore, recognise in geotactic phenomena a pressureeffect. But that they are this actually, Jensen was able to show by experiment upon the disc of a centrifuge. In tubes placed horizontal and hence in the line of the radius of the disc, in which under ordinary circumstances no geotactic ac-

cumulation of Paramcecia can take place, he increased by rotation the pressure at the peripheral end in comparison with the central end, and thus artificially imitated the conditions which, according to the laws of the earth's gravity, prevail in a vertical tube. The result was that with not too rapid rotation of the disc the Paramcecia collected at the places of lower pressure, i.e., at the central end of the tube, a phenomenon which Jensen puts beside geotaxis as centrotaxis. With a proper rate of rotation they frequently accumulated with greater certainty than in the upright tube. If they were centrifugalised too rapidly, naturally they were thrown out passively toward the periphery like heavy bodies. Accordingly, geotaxis, which has occupied a peculiar position so long in botany, must be regarded as a special case of barotaxis.

A ray of light extends through space from a source of light in a straight direction, and diminishes in intensity with the distance. Hence, any two points in the line of the ray possess different intensities ; the point that is nearer the source has the greater, that which is farther away has the less intensity. A ray of light, therefore, fulfils very completely the conditions that are necessary to the appearance of unilateral stimulation — in fact, it is extremely difficult to establish conditions under which an organism is stimulated by light uniformly upon all sides. As a result of this, stimulation by light calls out very pronounced directive effects, which have been termed phenomena of phototaxis1 and form a complete analogy to those of chemotaxis and barotaxis.

The phenomena of phototaxis have been known longest in plants ; as a matter of fact, plant physiology, on account of the less complexity of its objects of study, was able to develop in general into systematic completeness much earlier than animal physiology. Every one who cultivates plants in a room has the fact of positive phototaxis daily before his eyes. He sees that the growing parts turn constantly toward the light ; and, in order to make a plant grow straight upward, he must turn the pot about from time to time so that any phototactic curving may be compensated. Many plants are so extremely phototactic that in bright sunshine in a garden they follow the course of the sun in their curving. For example, in a bed of blue gentians, all the plants turn the broad

1 Formerly a distinction was made between heliotropism and phototaxis, the former word signifying the attitude, bending, and turning of fixed organisms, the latter the movement of motile organisms, with reference to the source of the light. This distinction is not only superfluous, but it introduces the false idea that the phenomena in the two cases are dependent upon different causes. A double terminology for processes that are based upon the same principle should be avoided. The processes are now understood better than at first, and the old distinction, which arose from purely external points of view, should be discarded as unscientific. Many authors have already done this.

open surface of their gorgeous blossoms to the sun, and in this position follow its slow movement throughout the day ; at evening their blossoms have a direction almost the opposite of that in the morning. In many plants, as Stahl ('85) has shown in the horsetails, the direction of growth is influenced by light in a very interesting way even in the spore ; in the division of the spore-cell the first division-wall, which divides the cell into two parts, is formed at right angles to the direction of the incident rays of light. A characteristic difference in the kind of phototaxis of the two halves is noticeable, such that the rhizoid-cell, from which the roots develop later, is always turned away from the source of light, and the prothallium-cell, from which the parts above the earth are derived, toward the source (Fig. 222).

Among animals the investigations of Loeb ('90) and Driesch ('90) in recent times have likewise demonstrated wide-spread phototactic phenomena. But, although it is not altogether easy to obtain a correct view of these phenomena in the cell-community of the plant, it is much more difficult in the complex community of the animal body, on account of the varied share taken by the sense-organs, the nervous system, the motile organs, etc. Hence it is advantageous here to turn our attention primarily to the simplest relations, such as exist in the free-living cell.

The phototactic phenomena of unicellular organisms, observed by Priestley and Ehreriberg, were followed out more fully by Nageli, Hofmeister, Baranetzky, Stahl, Klebs, Cohn, and other botanists, but the fundamental labours of Strasburger first gave an exact picture of the laws of the phenomena. Strasburger (78) made his investigations chiefly upon swarmspores of various chlorophyllaceous Algce, and observed their behaviour toward light falling from a window upon one side of a suspended drop. Essentially the same phenomena were shown by flagellated swarm-spores of very different species. The behaviour of the swarm-spores of Ulothrix may serve as a type. In diffused daylight of slight intensity these small flagellated cells hasten in straight paths to the edge of the drop that is turned toward the light, and collect there in great crowds. If the intensity of the light be increased, which Strasburger accomplished by bringing the preparation nearer the window or employing direct sunlight, with a certain intensity the swarm-spores begin to leave the positive side of the drop, i.e., the side that is turned

IG. 222. — Division of the sporecell of a horse-tail under the influence of light. The arrow indicates the direction of the rays. a, Position of the division-wall, b, direction of the mitotic figure. (After Stahl.) toward the source of light, and betake themselves to the opposite or negative side ; by further increase of the intensity all collect at the latter side. There exists, therefore, a point in the intensity, toward which the swarm-spores rush, going toward it from both higher and lower intensities — a phenomenon that Strasburger termed photometry. There is here a complete analogy to chemotaxis ; the latter is positive up to a certain concentration of the effective substance, but from there on with increasing concentration is negative, so that the term chemometry is justified. Quite analogous to the behaviour of the swarm-spores of Ulothrix is that of the swarm-spores of Chcetomorpha, Ulva, Hcematococcus, and some other Algae, as well as the flagellate infusorian Chilomonas Paramcecium, and the colourless swarm-spores of the Chytridice, all of which are positively phototactic with feeble intensity of light, and negatively phototactic with stronger intensity. There are forms — e.g., the swarmspores of Botrydium gramilatum — which show positive phototaxis in all intensities. Next to these researches of Strasburger comes a whole series of observations by other investigators, who have been able to find phototactic phenomena in all sorts of micro-organisms. Thus, Stahl ('84) investigated the phototaxis of plasmodia of Myxomycetes, previously observed by Hofmeister and Baranetzky, and found that young plasmodia of Aethalium septicum are

positively phototactic in half-darkness, and creep upon the surface of tan, but with stronger illumination they become negatively phototactic, and flow back again into the interior of the rhass. Further, Engelmann ('81, 3; '83) found mBacterium chlorinum and Bacterium plwtometricum two forms that possess phototactic properties and collect together in the light. Engelmann ('82), Stahl ('80), Aderhold ('88), and others x discovered phototactic phenomena also in the Diatomece and the Oscillarice, which behave exactly as the swarmspores of Alyce and form very pronounced assemblages (Fig. 223). Finally, Stahl (/.c.), Klebs ('85), and Aderhold (I.e.) demonstrated 1 Cf. Verworn ('89, 1).

FIG. 223.— Phototaxis of Diatomece. A particle of mud which was thickly surrounded by Diatomece lies in the middle of the drop. The organisms have all crept toward the edge turned toward the sun. phototactic movements in the Desmidiacece, and showed that these alga-cells place themselves with their long axis parallel to the light-rays, and in this position, by the extrusion of their secretion, move along the bottom in their peculiar manner toward the source of light, or with greater intensity away from the source (Fig. 224). In a preparation containing living Closteria l or Pleurotcenice all individuals place themselves with their long axis parallel to one another and to the direction of the incident light-rays. Thus, we find that among unicellular organisms, so far as they are irritable at all to light, phototaxis is a wide-spread phenomenon.

After phototactic phenomena had been discovered, the question necessarily arose as to whether the different rays of the spectrum are phototactically effective in an equal degree ; this was decided very easily by the introduction of coloured glasses and solutions between the source of light and the object. The media employed were so chosen that they let through only rays of a certain portion of the spectrum, so that only rays of certain wave-lengths were

FIG. 224. — Phototaxis of Clvsterium. The light comes from the right side. The arrow indicates the direction of movement of the Closterium. allowed to fall upon the organism (Fig. 225). In this way Colin, and later Strasburger, established the fact that in general the rays possessing a short wave-length, in other words, the blue and the violet especially, are more effective than those having a greater wave-length, viz., the red ; with not too high degrees of intensity the latter act like complete darkness.

One point more deserves mention in the discussion of phototactic phenomena. From the preceding consideration and by analogy with the directive effects of other stimuli it is evident that only the difference in the intensity of the light upon different parts of the body can produce a directive effect ; where the stimulus acts upon the surface of the body from all sides with equal intensity, the reason for a definite axial position disappears, as is to be observed most clearly in the action of chemical stimuli upon all sides. Although this is obvious, some investigators, such as Sachs and Loeb, have believed that the direction of the rays is more responsible for the manifestation of phototactic phenomena than are differences in intensity. It is difficult to

conceive this, for, since the assumption of an axial direction is possible only when differences exist at two different points of the surface of the body, it is wholly mystical how the direction of the rays, which is the same upon all sides of the body, can produce such an effect. In nature, under ordinary conditions, the decrease in intensity coincides with the direction of the rays, and hence we always see the phototactic movement take place in this direction. But the decrease in intensity can very easily be experimentally separated from the direction of transmission of the rays. Oltmanns ('92), making use of an idea already employed by Strasburger, devised a very excellent contrivance for this purpose. He made a wedge of two glass plates, which were inclined toward one

FIG. 225. — Spectra of various media ; 1, of a red glass ; 2, of a cobalt glass ; 3, of a green glass ; 4, of a solution of potassium bichromate ; 5, of an ammoniacal solution of a cupric salt. another at an angle of 2°, and filled the space between the plates with gelatine clouded with India ink. This wedge let through nearly all the light at its thin end, while at its thick end, where the gelatine was darkest, it absorbed much light. If, therefore, the light fell perpendicularly upon the surface of the plates, the greatest decrease of intensity for objects within a dark box behind the wedge lay at right angles to the direction of the incident rays. By means of these plates and the employment of the proper intensities of light it may actually be proved experimentally that it is not direction of ray, but solely difference in intensity upon different portions of the surface of the body, that produces phototactic phenomena.