( Originally Published 1909 )
THE facts dealing with the physiology' of organisms, the activities associated with that which we call life are often designated Organic Functions. The terms animal physiology, plant physiology and human physiology are in common use and often suggest to the lay reader that the functions or workings of the organs of plants, animals or man are quite distinct, so much so as to require discussion in different treatises. This is true only as a matter of detail, for in the past fifty years it has been made evident that in general principles all living things are fundamentally similar. One of the most important summaries of this similarity is Huxley's famous essay, 'The Border Territory Between the Animal and Vegetable Kingdoms,' written in 1876, extracts from which follow.
In the second edition of the 'Règne Animal,' published in 1828, Cuvier devotes a special section to the 'Division of Organised Beings into Animals and Vegetables,' in which the question is treated with that comprehensiveness of knowledge and clear critical judgment which characterize his writings and justify biologists in regarding them as representative expressions of the most extensive, if not the profoundest, knowledge of his time. He affirms that living beings have been subdivided from the earliest times into animated beings, which possess sense and motion, and inanimated beings, which are devoid of these functions and simply vegetable.
Altho the roots of plants direct themselves toward moisture and their leaves toward air and light, altho the parts of some plants exhibit oscillating movements without any perceptible cause and the leaves of others retract when touched, yet none of these movements justify the ascription to plants of perception of will. From the mobility of animals Cuvier, with his characteristic partiality for teleological reasoning, reduces the necessity of the existence in them of an alimentary cavity, or reservoir of food, whence their nutrition may be drawn by vessels, which are a sort of internal roots; and, in the presence of this alientary cavity he naturally sees the primary and the most important distinction between animals and plants. logical reasoning, reduces the necessity of the existence in them of an alimentary cavity, or reservoir of food, whence their nutrition may be drawn by the vessels, which are a sort of internal roots; and in the presence of this alimentary cavity he naturally sees the primary and the most important distinction between animals and plants.
Following out his teleological argument, Cuvier remarks that the organization of this cavity and its appurtenances must needs vary according to the nature of the aliment and the operations which it has to undergo before it can be converted into substances fitted for absorption, while the atmosphere and the earth supply plants with juices ready prepared and which can be absorbed immediately. As the animal body required to be independent of heat and of the atmosphere, there were no means by which the motion of its fluids could be produced by internal causes. Hence arose the second great distinctive character of animals, or the circulatory system, which is less important than the digestive, since it was unnecessary, and therefore is absent, in the more simple animals.
Animals further needed muscles for locomotion and nerves for sensibility. Hence, says Cuvier, it was necessary that the chemical composition of the animal body should be more complicated than that of the plant; and it is so, inasmuch as an additional substance nitrogen enters into it as an essential element; while in plants nitrogen is only accidentally joined with the three other fundamental constituents of organic beings carbon, hydrogen and oxygen. Indeed, he afterward affirms that nitrogen is peculiar to animals, and herein he places the third distinction between the animal and the plant. The soil and the atmosphere supply plants with water composed of hydrogen and oxygen and carbonic acid containing carbon and oxygen. They retain the hydrogen and the carbon, exhale the superfluous oxygen and absorb little or no nitrogen. The essential character of vegetable life is the exhalation of oxygen, which is effected through the agency of light. Animals, on the contrary, derive their nourishment either directly or indirectly from plants. They get rid of the superfluous hydrogen and carbon and accumulate nitrogen. The relations of plants and animals to the atmosphere are therefore inverse. The plant withdraws water and carbonic acid from the atmosphere, the animal contributes both to it. Respiration that is, the absorption of oxygen and the exhalation of carbonic acid is the specially animal function of animals and constitutes their fourth distinctive character.
Thus wrote Cuvier in 1828. But in the fourth and fifth decades of this century the greatest and most rapid revolution which biological science has ever undergone was effected by the application of the modern microscope to the investigation of organic structure, by the introduction of exact and easily manageable methods of conducting the chemical analysis of organic compounds and finally by the employment of instruments of precision for the measurement of the physical forces which are at work in the living economy.
That the semifluid contents (which we now term protoplasm) of the cells of certain plants, such as the Charae, are in constant and regular motion was made out by Bona ventura Corti a century ago; but the fact, important as it was, fell into oblivion and had to be rediscovered by Treviranus in 1807. Robert Brown noted the more complex motions of the protoplasm in the cells of Tradescantia in 1831, and now such movements of the living substance of plants are well known to be some of the most widely prevalent phenomena of vegetable life.
Agardh and other of the botanists of Cuvier's generation who occupied themselves with the lower plants had observed that, under particular circumstances, the contents of the cells of certain water-weeds were set free and moved about with considerable velocity and with all the appearances of spontaneity as locomotive bodies, which, from their similarity to animals of simple organization, were called 'zoospores.' Even as late as 1845, however, a botanist of Schleiden's eminence dealt very skeptically with these statements, and his skepticism was the more justified since Ehrenberg in his elaborate and comprehensive work on the infusoria, had declared the greater number of what are now recognised as locomotive plants to be animals.
"At the present day," writes Huxley, "innumerable plants and free plant cells are known to pass the whole or part of their lives in an actively locomotive condition, in nowise distinguishable from that of one of the simpler animals, and while in this condition their movements are, to all appearances, as spontaneous as much the product of volition as those of such animals.
"Hence the teleological argument for Cuvier's first diagnostic character the presence in animals of an alimentary cavity, or internal pocket, in which they can carry about their nutriment has broken down, so far, at least, as his mode of stating it goes. And, with the advance of microscopic anatomy, the universality of the fact itself among animals has ceased to be predicable. Many animals of even complex structure which live parasitically within others are wholly devoid of an alimentary cavity. Their food is provided for them, not only ready cooked but ready digested, and the alimentary canal, become superfluous, has disappeared, and, again, the males of most Rotifers have no digestive apparatus. Finally amid the lowest forms of animal life the speck of gelatinous protoplasm,. which constitutes the whole body, has no permanent digestive cavity or mouth, but takes in its food anywhere and digests, so to speak, all over its body.
"But altho Cuvier's leading diagnosis of the animal from the plant will not stand a strict test, it remains one of the most constant of the distinctive characters of animals. And if we substitute for the possession of an alimentary cavity the power of taking solid nutriment into the body and there digesting it, the. definition so changed will cover all animals, except certain parasites, and the few and exceptional cases of non-parasitic animals which do not feed at all. On the other hand, the definition thus amended will exclude all ordinary vegetable organisms. Cuvier himself practically gives up his second distinctive mark when he admits that it is wanting in the simpler animals.
"The third distinction is based on a completely erroneous conception of the chemical differences and resemblances between the constituents of animal and vegetable organ-isms, for which Cuvier is not responsible, as it was current among contemporary chemists. It is now established that nitrogen is as essential a constituent of vegetable as of animal living matter and that the latter is, chemically speaking, just as complicated as the former. Starchy sub-stances, cellulose and sugar, once supposed to be exclusively 'confined to plants, are now known to be regular and normal products of animals. Amylaceous and saccharine substances are largely manufactured, even by the highest animals. Cellulose is widespread as a constituent of the skeletons of the lower animals and it is probable that amyloid substances are universally present in the animal organism, tho not in the precise form of starch.
"Moreover, altho it remains true that there is an inverse relation between the green plant in sunshine and the animal, in so far as under these circumstances the green plant decomposes carbonic acid and exhales oxygen while the animal absorbs oxygen and exhales carbonic acid, yet the exact researches of the modern chemical investigators of the physiological processes of plants have clearly demonstrated the fallacy of attempting to draw any general distinction between animals and vegetables on this ground. In fact, the difference vanishes with the sunshine, even in the case of the green plant, which in the dark absorbs oxygen and gives out carbonic acid like any animal. On the other hand, those plants, such as the fungi, which contain no chlorophyll and are not green, are always, so far as respiration is concerned, in the exact position of animals. They absorb oxygen and give out carbonic acid. Thus, by the progress of knowledge, Cuvier's fourth distinction between the animal and the plant has been as completely in-validated as the third and second, and even the first can be retained only in a modified form and subject to exceptions."
But has the advance of biology simply tended to break down old distinctions without establishing new ones? With a qualification, to be considered presently, the answer to this question is undoubtedly in the affirmative. The famous researches of Schwann and Schleiden in 1837 and the following years founded the modern science of histology or that branch of anatomy which deals with the ultimate visible structure of organisms as revealed by the microscope, and from that day to this the rapid improvement of methods of investigation and the energy of a host of accurate observers have given greater and greater breadth and firmness to Schwann's great generalization that a fundamental unity of structure obtains in animals and plants, and that, however diverse may be the fabrics or tissues of which their bodies are composed, all these varied structures result from the metamorphosis of morphological units (termed cells in a more general sense than that in which the word `cells' was at first employed), which are not only similar in animals and in plants respectively; but present a close resemblance when those of animals and those of plants are compared together.
"The contractility which is the fundamental condition of locomotion," continues Huxley, "has not only been discovered to exist far more widely among plants than was formerly imagined, but in the plants the act of contraction has been found to be accompanied, as Dr. Burdon Sanderson's interesting investigations have shown, by a disturbance of the electrical state of the contractile substance comparable to that which was found by Du Bois Reymond to be a concomitant of the activity of ordinary muscle in animals. Again, I know of no test by which the reaction of the leaves of the Sundew and of other plants to stimuli, so fully and carefully studied by Mr. Darwin, can be distinguished from those acts of contraction following upon stimuli, which are called 'reflex' in animals.
"On each lobe of the bilobed leaf of Venus fly trap are three delicate filaments which stand out at right angles from the surface of the leaf. Touch one of them with the end of a fine human hair and the lobes of the leaf instantly close together in virtue of an act of contraction of part of their substance, just as the body of a snail contracts into its shell when one of its 'horns' is irritated.
"The reflex action of the snail is the result of the presence of a nervous system in the animal. A molecular change takes place in the nerve of the tentacle, is propagated to the muscles by which the body is retracted, and causing them to contract, the act of retraction is brought about. Of course the similarity of the acts does not necessarily involve the conclusion that the mechanism by which they are effected is the same, but it suggests a suspicion of their identity which needs careful testing."
The results of inquiries into the structure of the nervous system of animals converge toward the conclusion that the nerve fibers, which have been regarded as ultimate elements of nervous tissue, are not such, but are simply the visible aggregations of vastly more attenuated filaments, the diameter of which dwindles down to the limits of our present microscopic vision, greatly as these have been extended by modern improvements of the microscope, and that a nerve is, in its essence, nothing but a linear tract of specially modified protoplasm between two points of an organism one of which is able to affect the other by means of the communication so established. Hence it is conceivable that even the simplest living being may possess a nervous system. And the question whether plants are provided with a nervous system or not thus acquires a new aspect and presents the histologist and physiologist with a problem of extreme difficulty, which must be attacked from a new point of view and by the aid of methods which have yet to be invented.
"Thus it must be admitted," he says again, "that plants may be contractile and locomotive ; that, while locomotive, their movements may have as much appearance of spontaneity as those of the lowest animals, and that many exhibit actions comparable to those which are brought about by the agency of a nervous system in animals. And it must be allowed to be possible that further research may reveal the existence of something comparable to a nervous system in plants. So that I know not where we can hope to find any absolute distinction between animals and plants, unless we return to their mode of nutrition and inquire whether certain differences of a more occult character than those imagined to exist by Cuvier, and which certainly hold good for the vast majority of animals and plants, are of universal application.
"A bean may be supplied with water in which salts of ammonia and certain other mineral salts are dissolved in due proportion, with atmospheric air containing its ordinary minute dose of carbonic acid and with nothing else but sunlight and heat. Under these circumstances, unnatural as they are, with proper management, the bean will thrust forth its radicle and its plumule ; the former will grow down into roots, the latter grow up into the stem and leaves of a vigorous bean plant, and this plant will, in due time, flower and produce its crop of beans just as if it were grown in the garden or in the field.
"The weight of the nitrogenous protein compounds, of the oily, starchy, saccharine and woody substances contained in the full-grown plant and its seeds will be vastly greater than the weight of the same substances contained in the bean from which it sprang. But nothing has been supplied to the bean save water, carbonic acid, ammonia, potash, lime, iron and the like in combination with phosphoric, sulphuric and other acids. Neither protein, nor fat, nor starch, nor sugar, nor any substance in the slightest degree resembling them has formed part of the food of the bean. But the weights of the carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur and other elementary bodies contained in the bean-plant and in the seeds which it produces are exactly equivalent to the weights of the same elements which have disappeared from the materials supplied to the bean during its growth. Whence it follows that the bean has taken in only the raw materials of its fabric and has manufactured them into bean-stuffs.
"The bean has been able to perform this great chemical feat by the help of its green coloring matter, or chlorophyll, for it is only the green parts of the plant which, under the influence of sunlight, have the marvelous power of decomposing carbonic acid, setting free the oxygen and laying hold of the carbon which it contains. In fact, the bean obtains two of the absolutely indispensable elements of its substance from two distinct sources. The watery solution, in which its roots are plunged, contains nitrogen but no carbon; the air, to which the leaves are exposed, contains carbon, but its nitrogen is in the state of a free gas, in which condition the bean can make no use of it, and the chlorophyll is the apparatus by which the carbon is extracted from the atmospheric carbonic acid, the leaves being the chief laboratories in which this operation is effected.
"The great majority of conspicuous plants are, as everybody knows, green, and this arises from the abundance of their chlorophyll. The few which contain no chlorophyll and are colorless are unable to extract the carbon which they require from atmospheric carbonic acid and lead a parasitic existence upon other plants, but it by no means follows, often as the statement has been repeated, that the manufacturing power of plants depends on their chlorophyll and its interaction with the rays of the sun. On the contrary, it is easily demonstrated, as Pasteur first proved, that the lowest fungi, devoid of chlorophyll or of any substitute for it as they are, nevertheless possess the characteristic manufacturing power of plants in a very high degree. Only it is necessary that they should be supplied with a different kind of raw material; as they cannot ex-tract carbon from carbonic acid, they must be furnished with something else that contains carbon. Tartaric acid is such a substance, and if a single spore of the commonest and most troublesome of molds 'Penicillium' be sown in a saucerful of water in which tartrate of ammonia, with a small percentage of phosphates and sulphates is contained, and kept warm, whether in the dark or exposed to light, it will in a short time give rise to a thick crust of mold, which contains many million times the weight of the original spore in protein compounds and cellulose. Thus we have a very wide basis of fact for the generalization that plants are essentially characterized by their manufacturing capacity by their power of working up mere mineral matters into complex organic compounds.
"Contrariwise, there is a no less wide foundation for the generalization that animals, as Cuvier put it, depend directly or indirectly upon plants for the material of their bodies; that is, either they are herbivorous or they eat other animals which are herbivorous. But for what constituents of their bodies are animals thus dependent upon plants? Certainly not for their horny matter; nor for chondrin, the proximate chemical element of cartilage; nor for gelatin ; nor for syntonin, the constituent of muscle; nor for their nervous or biliary substances; nor for their amyloid matters; nor, necessarily, for their fats.
"It can be experimentally demonstrated that animals can make these for themselves. Bus that which they cannot make, but must in all known cases obtain directly or indirectly from plants, is the peculiar nitrogenous matter, protein. Thus the plant is the ideal proletariat of the living world the worker who produces the animal, the ideal aristocrat, who mostly occupies himself in consuming, after the manner of that noble representative of the line of Zähdarm, whose epitaph is written in 'Sartor Resartus.'
"Here is our last hope of finding a sharp line of demarcation between plants and animals, for, as I have already hinted, there is a border territory between the two kingdoms, a sort of no man's land, the inhabitants of which certainly cannot be discriminated and brought to their proper allegiance in any other way.
"Some months ago Professor Tyndall asked me to examine a drop of infusion of hay, placed under an excellent and powerful microscope, and to tell him what I thought some organisms visible in it were. I looked and observed, in the first place multitudes of 'Bacteria' moving about with their ordinary intermittent spasmodic wriggles. As to the vegetable nature of these; there is now no doubt. Not only does the close resemblance of the 'Bacteria' to unquestionable plants, such as the 'Oscillartoria' and the lower forms of 'Fungi,' justify this conclusion, but the manufacturing test settles the question at once. It is only needful to add a minute drop of fluid containing 'Bacteria' to water in which tartrate, phosphate and sulphate of ammonia are dissolved, and in a very short space of time the clear fluid becomes milky by reason of their prodigious multiplication which, of course, implies the manufacture of living bacterium-stuff out of these merely saline matters.
"But other active organisms, very much larger than the bacteria, attaining, in fact, the comparatively gigantic dimensions of one-three-thousandth of an inch or more, incessantly crossed the field of view. Each of these had a body shaped like a pear, the small end being slightly in-curved and produced into a long curved filament, or cilium, of extreme tenuity. Behind this, from the concave side of the incurvation, proceeded another long cilium, so delicate as to be discernible only by the use of the highest powers and careful management of the light. In the center of the pear-shaped body a clear, round space could occasionally be discerned, but not always, and careful watching showed that this clear vacuity appeared gradually and then shut up and disappeared suddenly at regular intervals. Such a structure is of common occurrence among the lowest plants and animals and is known as a contractile vacuole.
"The little creature thus described sometimes propelled itself with great activity, with a curious rolling motion, by the lashing of the front cilium, while the second cilium trailed behind; sometimes it anchored itself by the hinder cilium and was spun around by the working of the other, its motions resembling those of an anchor buoy in a heavy sea. Sometimes, when two were in full career toward one another, each would appear dexterously to get out of the other's way; sometimes a crowd would assemble and jostle one another with as much semblance of individual effort as a spectator on the Grands Mulets might observe with a telescope among the specks representing men in the valley of Chamounix.
"The spectacle, tho always surprising, was not new to me. So my reply to the question put to me was that these organisms were what biologists call 'Monads,' and tho they might be animals, it was also possible that they might, like the 'Bacteria,' be plants. My friend received my verdict with an expression which showed a sad want of respect for authority. He would as soon believe that a sheep was a plant. Naturally piqued by this want of faith, I have thought a good deal over the matter, and as I still rest in the lame conclusion I originally expressed must even now confess that I cannot certainly say whether this creature is an animal or a plant."
Thus in the question of making proteid there are all gradations from plants able to make proteid from inorganic matter to plants which are as much animal as vegetable in structure, but are animal in the dependence on other organisms for their food. The singular circumstance observed by Meyer that the torula of yeast, tho an indubitable plant, still flourishes most vigorously when supplied with the complex nitrogenous substance pepsin, the probability that the potato blight is nourished directly by the protoplasm of the potato-plant, and the wonderful facts which have recently been brought to light respecting insectivorous plants all favor this view and tend to the conclusion that the difference between animal and plant is one of degree rather than of kind and that the problem whether, in a given case, an organism is an animal or a plant may be essentially insoluble.
This conception that animals and plants differ not in kind has been of great importance in the development of biology as a unified science of life. But, altho the differences in the organic functions are of degree rather than of kind, it is true that the differences of degree are clear enough, except in certain unicellular organisms which are on the border line, to enable us to distinguish between animals and plants and to make it convenient to divide physiology, the science of function, into plant and animal physiology. These differences will be clearer by adding to Huxley's general comparison of plants and animals a concrete comparison of an animal and a plant. For this purpose one of Huxley's most famous students, T. Jeffery Parker, chose the unicellular animal 'Amoeba' and the unicellular plant 'Haematococcus,' also known as 'Sphaerella.' Both of these live in water. In their nutrition there are some differences, for Amoeba can take in solid food (other small animals and plants), formed of protoplasm as complex as its own. These it breaks up by means of digestive juices into soluble proteids and then recombines them to form its own protoplasm. Haematococcus has not this ability to take in solid food; it never feeds in the ordinary sense of the word. Nevertheless it must take in food in some way or other or the decomposition of its protoplasm would, soon bring it to an end. The water in which it lives is never pure, but always contains certain mineral salts in solution, especially nitrates, ammonia salts and often sodium chloride or common table salt. These salts, being crystalloids, can and do diffuse into the water of the organism so that we may consider its protoplasm to be constantly permeated by a very weak saline solution, the most important elements contained in which are oxygen, hydrogen, nitrogen, potassium sodium, calcium, sulphur and phosphorus. It must be remarked, however, that the diffusion of these salts does not take place in the same uniform manner as it would through parchment or other dead membrane. The living protoplasm has the power of determining the extent to which each constituent of the solution shall be absorbed.
"If water containing a large quantity of Haematococcus is exposed to sunlight minute bubbles are found to appear in it, and these bubbles, if collected and properly tested, are found to consist largely of oxygen. Accurate chemical analysis has shown that this oxygen is produced by the decomposition of the carbon dioxide contained in solution in rain-water, and indeed in all water exposed to the air, the gas, which is always present in small quantities in the atmosphere, being very soluble in water.
"As the carbon dioxide is decomposed in this way, its oxygen being given off, it is evident that its carbon must be retained. As a matter of fact, it is retained by the organism, but not in the form of carbon. In all probability a double decomposition takes place between the carbon dioxide absorbed and the water of organization, the result being the liberation of oxygen in the form of gas and the simultaneous production of some extremely simple form of carbohydrate i.e., some compound of carbon, hydrogen and oxygen, with a comparatively small number of atoms to the molecule.
"The next step seems to be that the carbohydrate thus formed unites with the ammonia salts or the nitrates absorbed from the surrounding water, the result being the formation of some comparatively simple nitrogenous compound. Then further combinations take place, substances of greater and greater complexity are produced, sulphur from the absorbed sulphates enters into combination and proteids are formed. From these finally fresh living protoplasm arises.
"From the foregoing account, which only aims at giving the very briefest outline of a subject as yet imperfectly understood, it will be seen that, as in Amoeba, the final result of the nutritive process is the manufacture of proto-. plasm, and that this result is attained by the formation of various substances of increasing complexity. But it must be noted that the steps in the process of constructive metabolism are widely different in the two cases. In Amoeba we start with living protoplasm that of the prey which is killed and broken up into diffusible proteids, these being afterward recombined to form new molecules of the living protoplasm of Amoeba. So that the food of Amoeba is, to begin with, as complex as itself, and is broken down by digestion into simpler compounds, these being afterward recombined into more complex ones. In Haematococcus, on the other hand, we start with extremely simple compounds, such as carbon dioxide, water, nitrates, sulphates, etc. Nothing which can be properly called digestion i.e., a breaking up and dissolving of the food, takes place, but its various constituents are combined into substances of gradually increasing complexity, protoplasm, as before, being the final result.
"To express the matter in another way: Amoeba can only make protoplasm out of proteids already formed by some other organism; Haematococcus can form it out of simple liquid and gaseous inorganic materials.
"Speaking generally, it may be said that these two methods of nutrition are respectively characteristic of the two great groups of living things. Animals require solid food containing ready made proteids and cannot build up their protoplasm out of simpler compounds. Green plants i.e., all the ordinary trees, shrubs, weeds, etc. take only liquid and gaseous food and build up their protoplasm out of car-bon dioxide, water and mineral salts. The first of these methods of nutrition is conveniently distinguished as holozoic, or wholly animal, the second as holophytic, or wholly-vegetal.
"It is important to note that only those plants or parts of plants in which chlorophyll is present are capable of holophytic nutrition. Whatever may be the precise way in which the process is effected, it is certain that the de-composition of carbon dioxide which characterizes this form of nutrition is a function of chlorophyll, or to speak more accurately, of chromatophores, since there is reason for thinking that it is the protoplasm of these bodies, and not the actual green pigment, which is the active agent in the process.
"Moreover, it must not be forgotten that the decomposition of carbon dioxide is carried on only during daylight, so that organisms in which holophytic nutrition obtains are dependent upon the sun for their very existence. While Amoeba derives its energy from the breaking down of the proteids in its food, the food of Haematococcus is too simple to serve as a source of energy, and it is only by the help of sunlight that the work of constructive metabolism can be carried on. This may be expressed by saying that Haematococcus, in common with other organisms containing chlorophyll, is supplied with kinetic energy (in the form of light or radiant energy) directly by the sun.
"As in Amoeba, destructive metabolism is constantly going on, side by side with constructive. The protoplasm becomes oxidized, water, carbon dioxide and nitrogenous waste matters being formed and finally got rid of. Obviously then absorption of oxygen must take place, or in other words, respiration must be one of the functions of the protoplasm of Haematococcus as of that of Amoeba. In many green i.e., chlorophyll containing plants this has been proved to be the case; respiration i.e., the taking in of oxygen and giving out of carbon dioxide is constantly going on, but during daylight is obscured by the converse process, the taking in of carbon dioxide for nutritive purposes and the giving out of the oxygen liberated by its decomposition. In darkness, when this latter process is in abeyance, the occurrence of respiration is more readily ascertained."