General Physiology: An Outline of the Science of Life

when, as in the iodine molecule, the atoms of iodine or, as in the hydrogen molecule, the atoms of hydrogen have a greater affinity for one another than the iodine atoms have for the hydrogen atoms. In these cases more energy becomes absorbed, in order to separate from one another the atoms of the iodine molecule and the atoms of the hydrogen molecule than becomes free when the atoms of iodine and hydrogen unite into a molecule of hydrogen iodide, and, since in every calorimetric experiment the end-result and not the intermediate processes come under observation, it is explained why at the end of the reaction there must be an absorption of heat. The reverse is the case in the decomposition-processes accompanied by an evolution of heat. It is well known that nitroglycerine (glyceryl tri-nitrate), upon being shaken, explodes with an enormous evolution of energy, being decomposed into water, carbonic acid, oxygen and nitrogen. These products of decomposition are not preformed stereochemically in the molecule of nitroglycerine, but they arise from a synthetic rearrangement of the atoms set free by the decomposition. Since the atoms of water, carbonic acid, oxygen and nitrogen, have greater affinities for each other in this arrangement than in their position in the nitroglycerine molecule, a small quantity of energy suffices to cause the decomposition of the latter, while from the resulting syntheses an extraordinary quantity of energy becomes free. Hence as the end-result there is an evolution of heat. Therefore, just as in the synthesis of hydrogen iodide, strictly speaking, the absorption of heat is not to be credited to the synthesis, so in the dynamite explosion the evolution of energy does not come in reality from the decomposition of the nitroglycerine molecule. This fact should be clearly understood.

But, since, when a synthesis is spoken of, the preceding decomposition is left out of account, and when a decomposition is spoken of, the subsequent synthesis is similarly treated, it is more exact to express the fundamental law of the transformation of energy in chemical processes in the following form : If in a chemical process affinities become united rather than separated, energy is liberated ; if affinities become separated rather than united, energy is absorbed.

To return from our excursus, it is clear from the discussion that chemical energy can be introduced into the organism only when the food-stuffs contain affinities for the satisfying of which an opportunity is afforded within the organism. Hence substances must be introduced into the body, which undergo among themselves chemical transformations with the evolution of heat. This takes place in two ways, which we have just become acquainted with, viz., first, by the introduction of simple substances possessing strong affinities, and, second, by the introduction into or synthesis within the body of complex compounds which are easily decomposed and, like explosive bodies, furnish decomposition-products that combine synthetically into new

substances with a rearrangement of their atoms. Free affinities come into the body with oxygen especially ; and it is well known that in the combination of oxygen with other substances, or, in other words, in combustion, a great quantity of energy is liberated. Hence the process of oxidation plays an extremely important role in all life ; and, as has already been seen, the comparison of life with fire is a very happy one. Complex compounds come into the organism, especially in the case of animals, with the organic food ; there they undergo a long series of transformations, which thus far have not been followed, in which decompositions and syntheses proceed together to the construction of the living proteid molecule. Living proteids may be classed with explosive bodies. They tend toward decomposition ; and out of the complexes of atoms set free there arise synthetically by rearrangement, partly immediately after the decomposition and partly later in combination with substances newly introduced, chemical compounds the origin of which under certain circumstances is again associated with the evolution of energy.

In the present condition of our knowledge it is not possible to follow in detail the intricate series of chemical processes, the decompositions and syntheses and the transformations of energy associated with them, from the first cleavage of carbonic acid and the synthesis of the first product of assimilation in the plant to the decomposition of the living proteid in the plant and the animal. It is known, however, that the final products of metabolism, such as carbonic acid, water, urea, etc., are extremely poor in chemical energy. The larger quantity of chemical energy introduced into the body with the food must, therefore, have been transformed into other forms of energy upon its way through metabolism, and thus results the work of the organism.

It has been said that the main quantity of all the energy that is introduced comes into the body as chemical energy. For the animal organism this statement holds good without limitation; for the plant, however, it needs a correction. It is true that in the plant the energy at the expense of which its work goes on is likewise pre-eminently chemical ; but a part of this potential is not introduced into the body as free, available energy, i.e., in the form of free affinities, such as oxygen possesses ; another form of energy must first be introduced in order to create free affinities in the former. It is well known that carbonic acid and water are necessary for the synthesis of the first product of assimilation.1 But carbonic acid and water as such are poor in chemical energy

because their atoms are coupled together by very strong affinities. Hence, in order to make them free and serviceable for new labours, they must first be split up, and for this an introduction of energy is necessary. The energy that performs this cleavage is light in combination with the chemical energy of the living plant-substance. Without light no plant-life is possible, and since without plant-life no animal-life can exist, it may be said that without light no life whatever would exist. Hence, although light plays an essential role as a direct source of energy only in the plant, it is as indispensable for the maintenance of life upon the earth's surface as the chemical energy of food.

The places in the plant where light effects the cleavage of carbonic acid are the green parts of the plant-body, and hence especially the leaves. This can best be demonstrated by the experiment on assimilation already described.1 This experiment shows that in the part played by the rays of light in the cleavage of carbonic acid in the green plant-cell, two factors are present, the intensity and the wave-length of the rays. The efficiency of the light increases with the intensity, so that in a brighter light, more carbonic acid is split up than in a feebler one. Moreover, with the same intensity the rays of red light (not those of yellow, as botanists formerly supposed) are the most effective. Engelmann ('81, 1 ; '94) in a series of researches placed this beyond all doubt by a microscopic method that depends upon the action on bacteria of the oxygen set free in the cleavage of carbonic acid. At the same time these researches confirmed the view that the cleavage of carbonic acid in the green plant-cell takes place in the chlorophyll-bodies only, and established the fact that the cleavage begins at once upon the admission of light and ceases immediately upon darkening. Hence the dependence of this property of the chlorophyll-body upon light is extremely close.

The heat that comes into the living organism from the outside, partly by radiation and partly by conduction, plays, like light, a role in the chemical transformations within living substance. Since with increasing temperature the power of decomposition increases in all chemical compounds, the heat that is introduced takes part especially in the processes of decomposition in the living substance. The rdle of heat as a source of energy may be recognised especially clearly in the so-called cold-blooded animals. These are better termed animals possessing a changeable temperature (poikilothermal), since in contrast to the so-called warmblooded animals, or animals possessing a uniform temperature (homothermal), the temperature of their bodies changes continually with that of their environment : with a high external temperature they may have a body-temperature equal to that of the warmblooded animals. When the temperature of the medium in which

they live is high, these animals, such as insects and reptiles, are extremely lively, move about much, and show in general an intense activity. With decreasing temperature the liveliness of their movements decreases, and at 0° in many cases vital activity is hardly to be observed in them, the transformation of energy has almost ceased. " Wherever we look into the realm of living organisms," says Pfliiger (75, 1), "we see how the intensity of vital processes, and hence decomposition, varies proportionately with the temperature. When I observe the lively, moving, nimble lizard in summer, and then see how the same animal, exposed to a temperature below 0°, becomes gradually quiet and sinks into a death-like torpor, and inquire what is the reason why the animal becomes again so active in warmth, appearance tells me that it is because heat has been introduced into the organs ; heat puts the atoms into vibration and promotes dissociation." The heat that is introduced serves in this way directly as a source of energy for the work of the organism.

This completes the enumeration of the sources from which the organism receives energy. The other forms of energy have almost no importance in this respect. At present it is wholly impossible to follow the tortuous paths taken by the energy that is introduced in its changes through the living body. Scarcely a beginning has been made in investigating the transformations that this energy undergoes under the various conditions found by it in living substance. There is here needed a long series of exhaustive special researches and especially a detailed knowledge of metabolic processes, before an intelligible conception can be formed of the mechanism of these transformations. The field of physiological energetics offers rich problems full of reward for the future, which thus far have been scarcely noticed. Only the final links of the chain of metamorphoses, the outward achievements of the living organism, are now known with certainty.

The evolution of energy outward, especially that of mechanical energy, which expresses itself in the movements of the living body, is undoubtedly the most evident of all vital phenomena ; it is more or less the first criterion of life for the untrained observer, and perhaps this is the reason why physiology from early times has made the phenomena of movement a favourite object of research. Less evident, because either uncommon or difficult to observe, is the production, on the part of living substance, of other forms of energy, such as light, heat and electricity.

All living substance moves, i.e., the single points of its material system change their positions in space. There results, according to the special conditions, a shifting of the single particles, the external form remaining the same, a change in the external form, a change of place of the whole (locomotion), or several of these changes at the same time. But although motion in itself is a general phenomenon of life, all forms of living substance do not show the same kind of motion. The variety of modes of motion that may be observed in different organisms is very great. Nevertheless, all may be classified in accordance with the manner of their occurrence into a few large groups, of which only certain ones, on account of their wide distribution, possess any considerable importance. Since the • motion of living substance is the most evident vital phenomenon, and special interest is therefore lent to it, we are justified in considering it somewhat in detail.

It is useful first to classify the various modes of motion into : (d) Movements by change of the specific gravity of the cell. (*) Movements by secretion on the part of the cell. In passive movements the cause lies outside the part that is moved. Passive movements in living substance are, therefore, not a vital phenomenon of the elements that are moved, but the expression of vital phenomena in the environment. The movement of the red blood-corpuscles, the streaming of the blood-plasma in the blood-vessels of the human body, are passive movements ; for the blood-corpuscles and the plasma possess no intrinsic power of movement; they are only passively driven by the activity of the heart, which works like a suction- and force-pump in the system of branching tubes filled with blood. This streaming of the blood in the fine capillary vessels can be observed very beautifully under the microscope, if a frog, paralysed by the South American arrow poison, curare, be placed upon a cork plate and the web between the toes of the hind leg be stretched out by needles over an opening

in the plate. A picture full of interest for every observer will then be presented (Fig. 86). The much-branched network of the capillary system is seen, and in it the blood flows with its apparently yellow corpuscles so slowly that one can easily follow every individual corpuscle as it winds its way in the clear plasma through the fine canals and sinuosities. Even in the single cell such passive movements are found. The fine granules that lie embedded in the protoplasm of the naked cells of rhizopods show a streaming movement, especially in the long, thread-like pseudopodia of marine species ; this so-called granular streaming presents a spectacle as fascinating as the streaming of the blood in the capillaries, although going on much more slowly. Like pedestrians in the street, or like ants, the

granules take their selfestablished paths, now in a centrifugal, now in a centripetal direction, now standing still, now turning about, and now again proceeding. This granular streaming does not come about by the active progressive movement of the granules themselves ; but by their being passively dragged along by the liquid protoplasmic ground - sub - stance in which they lie embedded, and which has constantly an active flowing motion. of passive movements that occur in the living cell is the so-called Brownian molecular movement. There lives in fresh water a small, unicellular, green alga, Closterium, of a delicate crescent-shape (Fig. 87, 7). In its protoplasm at each end of its body is a vacuole of liquid, in which as a rule lie fine granules which show Brownian motion By strong magnification it may be seen that the granules are continually dancing about each other with a delicate trembling motion, but without moving to any considerable distances. The dancing continues tirelessly and unceasingly. Here the object in which this peculiar motion is seen is living. More frequently, however, it can be observed in dead cells, and it has long been known in the so-called salivary corpuscles in the saliva, which are dead leucocytes (white blood-corpuscles). These leucocytes are swollen into a spherical form by the absorption of water, and possess a nucleus surrounded by granular protoplasm (Fig. 87, 77).

FIG. 86. — Capillary circulation in the web of the frog's foot. (After Ranke.) Upon strong magnification the granules of this swollen protoplasm show clearly molecular motion. That the strange Brownian molecular movement does not occur in living organisms exclusively, follows from the fact that all light, microscopic granules of whatever kind, when suspended in water or any other easily moving liquid, show it. Among the most beautiful lifeless objects adapted for this purpose and occurring in the organism are the fine crystals (Fig. 87, ///) in the calcareous sacs that lie in the body-cavity of the frog on each side of the spinal column between the transverse processes of adjacent vertebrae. If some of the white substance be placed in a drop of water and examined under a cover-glass with a high power of the microscope, the wonderful picture of this restless, trembling dance of lifeless crystals is pre-

FIG. 87. — Browniau molecular movement. /. Closterium (after Strasburger). In the vacuoles, K> at the two ends of the crescent-shaped body there are numerous granules in active molecular motion. //. A so-called salivary corpuscle, a dead and spherically contracted leucocyte from the human saliva, in the swollen contents of which the granules are in dancing motion. ///. Crystals from the calcareous sacs of the frog ; when put into water they show a restless, dancing motion.

sented in its most graceful form, especially in the smaller crystals.1 When the English botanist Brown in the year 1827 discovered such peculiar motions in plant-cells, it was believed that the motion of the fine granules was an active one, resulting from the vibrations of their molecules, and it was accordingly termed " molecular motion." In accordance with more modern ideas this view became untenable, and for a long time the significance of the puzzling phenomenon was not understood. But in the year 1863 Wiener, and soon afterwards Exner, studied very carefully the physical conditions of the motion, and found an explanation that is in entire accord with our present ideas of the molecular condition of liquids. In fact, the behaviour of the molecules of a liquid even requires such phenomena of motion of small light par-

ticles suspended in it. As is well known, the molecules in a liquid are conceived to be in constant motion, crowding together, bounding against one another, pushing away, moving off and again colliding. This motion of the molecules cannot be seen even with the strongest magnifying powers, for liquids appear homogeneous because their molecules are too small to be perceived even microscopically. But the result of the motion can be observed in small, light granules suspended in the liquid; if the molecules possess the given kind of motion, they must strike the particles continually, so that with their delicate mobility the latter are put into a trembling, dancing motion. Hence, the so-called Brownian molecular movement of small granules is a purely passive movement caused by the constant slight impulses given to the granules by the dancing molecules of the liquid. An excellent proof of the correctness of this view is afforded by the fact that the Brownian movement gains in intensity with increasing temperature of the liquid. This might have been predicted from the fact that the motion of the molecules of the liquid is greater the higher the temperature ; it finally becomes so great that the individual molecules are driven violently apart, that is, the liquid evaporates.

Movements that are caused by swelling of the cell-walls constitute a variety intermediate between passive movements and all those mentioned below, which latter depend upon the activity of living substance. The phenomenon of swelling, as is well known, is due to the fact that between the molecules of a dry, expansible body, brought into a moist environment, molecules of water become stored, being attracted so strongly by the molecules of the body that they force the latter powerfully apart ; during the process the volume of the body becomes markedly increased. If the swollen body comes again into an environment free from water, e.g., dry warm air, it gradually gives off its water, diminishes its volume proportionately and shrinks; upon being again moistened, it swells again. The organic products of the metabolism of plants, especially the cellulose walls of plant-cells, are peculiarly prone to swell. This is not associated in any way with the life of the plant-cell, but goes on for an indefinite time in the cellulose of dead cells, in the same manner as in that of living cells. In order that a movement in one direction may be brought about by the increase in volume caused by the swelling or by the decrease in volume caused by the drying of an expansible object, such as the stem of a leaf or a membrane, the different sides of the object must be capable of swelling differently, one side strongly, the other feebly or not at all. Were all parts equally capable, there would result a uniform enlargement toward all sides. If, however,

one side of an elongated structure swells more than the opposite, the former becomes lengthened more than the latter, and the result is a bending of the whole structure, which takes place suddenly or gradually as the swelling is rapid or slow. The well-known resurrection-plants (Sdaginella lepidopJiylla), which of late have frequently come to Europe from the American deserts, are characteristic objects for the observation of swellingmovements. During a drought their leaf-stalks are brought together like the fingers in a closed fist, but when moistened they bend out as in the open hand, the leaf-stalks strongly swelling

FIG. 88.— Seed of the crane's bill (Erodium cicutarium), a, in the dry, 6, in the swollen state. upon their inner side. The well-known rose of Jericho, which is simply the dry, dead branch of a crucifer (Anastatica) growing in the Arabian deserts, behaves similarly. Its spreading when placed in water has led to the common belief that the rose of Jericho is resurrected to a new life, while in reality the phenomenon depends merely upon the swelling-movements of the dead branch. Selaginella, however, is a real resurrection-plant in so far as it can remain for years completely dry without losing its capacity of life. The seeds of many species of crane's bill likewise show very clearly the phenomena of swelling-movements. Erodium cicutarmm has seeds that are provided with a long stalk

beset with hairs ; in a drought this stalk is rolled up like a corkscrew into a beautiful spiral (Fig. 88, a), but when moistened it becomes straightened, one turn after another unrolling itself by swelling and extension of the inner side (Fig. 88, &). The movements of the so-called elaters on the spores of the horse-tail are very interesting and striking because of their rapidity. The ripe spores of the horse-tail are spherical cells surrounded by a cellulose-wall. This wall is split into two bands the elaters (Fig. 89), which run in a spiral from above downward around the whole ball, being fastened to each other and to the spore itself at a certain spot in the equator. If the spores, slightly moistened, be brought under the microscope, the two bands are seen to lie in two parallel spirals and form a closed capsule about the spore (Fig. 89, a). If they be allowed to dry, the two spirals become extended into straight bands (Fig. 89, &}

FIG. 89.— Spore of a horse-tail, a. The elaters in the moist state are curled around the cell 6. The elaters in the dry state are rapidly spread apart, through the drying and shortening of their outer sides. If, while observing with the microscope, one breathes upon them in this extended state, they are seen to coil themselves in spirals about the spore with excessive rapidity, their outer surfaces extending by swelling. At the moment when the moisture of the breath disappears, the bands extend again with equal rapidity ; and the experiment can be repeated, like all experiments on swelling, as often as one wishes.

Swelling-movements are very common among plants, and some of them play an important role in plant life. The great power that can be developed by swelling can be realised from the fact that huge rocks can be split with wedges of swelling wood. With movements caused by a change of the cell-turgor, we begin the consideration of those phenomena of motion that presuppose normal life in the object in which they appear. With the death of their substratum they are extinguished. Turgescence-movements are chiefly found among plants ; and it is necessary, therefore, that certain peculiarities of the plant-cell be recalled.

The plant-cell, as is well known, is a cylindrical capsule, the walls of which are formed by an elastic membrane of cellulose. FIG. 90. — Scheme of cell-turgor of a plant-cell ; Ji, cell-membrane ; p, primordial utricle ; k, nucleus ; c, chlorophyll-bodies ; s, cell-sap ; e, infiltrating salt solution. In A, the cell is in complete turgescence, the primordial utricle lies close to the cell-membrane. In B the turgor has decreased as a result of the action of a salt solution, the cell has become smaller, but the primordial utricle still lies in contact with the cell-membrane. In C the turgor has become still less, the primordial utricle is beginning to be pulled away from the cell-membrane, which latter has reached its minimum. In D the primordial utricle has contracted completely, because the osmotic effect of the salt solution acting from the outside has reached a very high degree. (After de Vries.)

The inner surface of the capsule fs covered by a thin but continuous protoplasmic layer, the so-called primordial utricle, which encloses like a sac or bladder a liquid, the cell-sap, and as a rule sends through the large vacuole strands of protoplasm which branch lengthwise and crosswise (Fig. 90; in this figure the strands are wanting). Various chemical substances, which have been produced by the vital activity of the cell, are dissolved in the sap. In its usual uninjured condition the protoplasm is impermeable to these substances, hence they cannot diffuse from the interior to the outside through the primordial utricle. But the protoplasm is likewise impermeable to many substances that are dissolved in the water outside the cell, and which, therefore,

cannot diffuse into the cell. Now it is known that the molecules of such soluble substances as salts, sugar, etc., attract water, every molecule taking to itself a number of molecules of water. The molecules of the former are said to act " osmotically." As Van t'Hoff has recently shown by his important researches, the osmotic pressure is proportional to the number of molecules dissolved in the unit of volume. If, therefore, there are stored within the cell-sap strongly osmotic substances, and outside the cell in the water substances that are less osmotic, and if the wall of the primordial utricle is impermeable to these dissolved substances, an equalisation by diffusion cannot take place ; but, since the primordial utricle allows pure water to pass through it unhindered, water must be drawn by the osmotic substances of the sap into the interior and held there permanently. The result of this process is that the pressure in the primordial utricle becomes constantly greater the more osmotic substances are dissolved in the sap, i.e.. the more the concentration of the sap increases. The primordial utricle of the cell, therefore, must be extended from within outward ; and this tension, stretching the elastic cellulose wall, is the turgor of the cell. It is evident that the turgor will become greater, that the cell must be put more upon the stretch, the more osmotic substances accumulate in the sap and the less in the surrounding medium.

Prom this brief consideration it is clear that the turgor of the cell can be changed in different ways. First, the quantitative relations of the osmotic substances within and without the cell can change, by the concentration outside or inside becoming increased or decreased. If, e.g., substances in solution be added gradually to the surrounding medium, water will be drawn out constantly from the interior, and the turgor will decrease. This phenomenon has been termed, with little appropriateness, plasmolysis. Further, the turgor can likewise be changed by the wall of the primordial utricle from some cause becoming permeable to the substances in solution in the cell-sap. Then an equalisation by diffusion must take place, and the tension of the cell-wall must disappear. Finally, a change in turgor will take place when the tension of the primordial utricle increases or decreases because of active changes in its protoplasm. If, e.g., the protoplasm contracts, the contraction will partially or wholly overcome the osmotic pressure opposing it, and the result will be that a corresponding quantity of water minus the osmotic substances will be pressed out from the sap through the primordial utricle. When the contraction of the primordial utricle ceases, the osmotic substances of the sap will attract more molecules of water, and the turgor will again increase.