Popular Science Monthly/Volume 43/August 1893/How Plants and Animals Grow
| HOW PLANTS AND ANIMALS GROW. |
By Dr. MANLY MILES.
TOO little is known in regard to the chemistry of foods, or the specific use made of their proximate constituents in the processes of nutrition, to serve as a rational guide in formulating diets, or estimating the relative nutritive value of different articles of food. Our methods of chemical investigation are not as yet sufficiently delicate and refined to enable us to trace the unobtrusive transformations of matter and energy involved in the nutrition of living beings.
Liebig's chemical theories of nutrition are now discarded by physiologists as fallacious and misleading, but they are, nevertheless, confidently adopted by popular writers on the economy of foods and diets, who are not aware of the progress made in a more consistent knowledge of physiological processes. The history of biological science furnishes numerous instances of error arising from the undue prominence given to non-essential details which are readily observed, while the dominant factors in the phenomena under investigation, which are not so obvious, are overlooked or assigned a subordinate position.
The progressive development of the cell theory of organic structure, and the several steps which have led to a recognition of protoplasm as a factor in evolution and in the processes of vegetable and animal nutrition, may be profitably reviewed to illustrate the erroneous inferences made from superficial and defective observation. In 1755 Rosenhoff described the Proteus animalcule, now familiarly known as the Amæba, without being aware of the importance of the discovery in furnishing a type of the form or conditions of matter required for the manifestations of life.
Bichat laid the foundation for the study of the minute structure of animals in his work on anatomy, published in 1801, in which the different organs of the body were described as made up of tissues, to each of which was assigned a special function, and the attention of anatomists was then given to the distribution and arrangement of these structural elements, while their intimate relations, arising from a common origin, were not detected.
The next step of real progress was made in 1838 by Schleiden, who traced all vegetable tissues to the common form of nucleated cells from which they had their origin, and in 1839 Schwann perfected the cell theory of organization by extending the same conception to animal tissues. Cells were then recognized as the ultimate units of organic structure, which were variously modified to adapt them to diverse special purposes. This cell theory of organized structure was generally adopted, and cells were defined as closed membranes or sacs, containing a more or less fluid substance which served to nourish them. The cells were looked upon as independent units, which multiplied by a process of budding or by self-division, and a new factor was introduced in the discussion of the mooted question as to what constitutes the individual in plants. Schwann "regarded the plant as a cell community in which the separate elements were like the bees in a swarm," and this appeared to be a logical inference from the accepted cell theory of organization.
This view was, however, based on an erroneous assumption as to the essential constituents of the cell, and the progress of discovery gradually led to the demonstration of a material and physiological bond of union in the various tissues of plants. In 1835 Dujardin made the discovery that the bodies of Foraminifera, a group of animals of simple organization, including the Amæba, were composed of a glairy contractile substance, which he called sarcode (rudimentary flesh), and in 1840 Von Mohl called attention to the importance of the inner lining of the cell wall of plants, which he designated the primordial utricle, with its inclosed contents, to which he gave the name of protoplasm (primitive plastic or organizable matter), and these, he claimed, represented the essential constituents of the cell and active elements in plant nutrition.
The identity in essential features of Dujardin's sarcode and Von Mohl's protoplasm was pointed out by Cohn in 1850, and fully demonstrated by Max Schultze in 1858, who, adopting the term protoplasm, defined the cell as "a unit-mass of nucleated protoplasm, with or without a cell wall," and vegetable and animal physiology were thus placed on a common correlated basis. The original cell theory was materially modified and, in fact, superseded by the conception that the units of organized structure are masses of protoplasm, more or less intimately related, from and by which organic matters, including the cells and various tissues, were formed.
In 1868 Prof. Huxley translated protoplasm into the significant phrase, "the physical basis of life," and all vital activities were assumed to be the result of its inherent properties. While admitting the general pertinence of this assumption, we should not fail to notice that many of the inferences from the facts then known have not been verified in the progress of knowledge, and recent investigations have materially modified our views as to the real composition and constitution of protoplasm.
From what was known in regard to protoplasm twenty years ago, it appeared to be reasonable to assume with Huxley that there is "one kind of matter which is common to all living beings, and that their endless diversities are bound together by a physical as well as an ideal unity"; that vegetable and animal protoplasm are strictly identical; that "an animal can not make protoplasm, but must take it ready made from some other animal or some plant"; or, in other words, that the protoplasm made by plants from mineral matters is, in fact, the physical basis of animal life.
At the present time we may look upon protoplasm as the physical basis of life in the sense that some form of it is the essential and active constituent of every living cell or tissue, whether vegetable or animal, and that it is only formed through the physiological activities of living organisms. In the absence of life, protoplasm can not be formed, and, so far as we can perceive, there are no manifestations of life without it; but we can no longer assume that it is a substance of the same chemical composition and constitution in all the varied conditions under which it appears in the different groups of plants and animals, or even in the different organs of the same individual. Protoplasm is a convenient name for living substance, but we must bear in mind that it is the most complex and unstable of organic substances, and varies widely in structure, specific properties, and probably in chemical composition.The general properties of protoplasm may be readily observed in the simplest organic forms, like the Amæba, that are usually described as simple masses of protoplasm without structure or any distinction of parts. It should be remarked, however, that numerous species of Amæba have been described, differing in form and to some extent in habits, and there may also be differences in their protoplasm which we are unable to detect with our present means of investigation.
Under the low powers of the microscope an Amæba appears as a semi-transparent, jelly-like mass, which glides along with a flowing movement of its apparently homogeneous substance, sending out armlike projections from any part of its body to close around substances which it can feed upon, and rejecting other materials unsuitable for its nutrition. The processes of prehension, digestion, assimilation, respiration, excretion, and reproduction are carried on by the entire body, or by any part of it indifferently. The body of an Amæba, as we observe it, is not, however, a simple mass of protoplasm, as it evidently contains particles of undigested food, with particles representing the various stages the elements of the food pass through in being built up into protoplasm, together with the various waste products on the way to be excreted, so that what we call protoplasm, as represented in an Amæba, contains many extraneous substances; and substantially the same statement may also be made in regard to the differentiated protoplasm of the higher plants and animals.
From this it must be seen that it is practically impossible to obtain samples of pure protoplasm for analysis, and, even if this could be done, a chemical analysis of living protoplasm can not be made; but there is, however, evidence to show that there must be a wide difference in the chemical properties of living and of dead protoplasm. Carmine and other coloring matters, for example, do not color living protoplasm, but give a brilliant stain to dead protoplasm; and other observations show that living substance has properties that interfere with or limit the ordinary chemical and physical reactions of dead matter.
There are other considerations in regard to the composition of protoplasm which require a reference to the food of the higher animals, which is usually said to consist of the so-called proteids, fats, and carbohydrates, to which should be added certain mineral constituents or salts, with oxygen introduced by the lungs. These groups of food-stuffs have not the physiological significance that was formerly attached to them, and they do not represent definite chemical compounds which have a specific rôle in the processes of nutrition, as each group includes a great variety of complex compounds. The proteids or albuminoids, as they are sometimes called, are a group of organic substances containing carbon, hydrogen, and oxygen, with, from fifteen to eighteen per cent of nitrogen and a variable quantity of ash constituents, and they present marked differences in their general appearance and properties. The white of an egg, the casein of cheese, the glutin of wheat, and the legumin of peas and beans are often referred to as typical proteids, but they in fact represent several kinds of proteids which differ in many properties, and can not be assumed to have precisely the same physiological significance and value as nutrients.
The group of fats includes a great variety of compounds composed of carbon, hydrogen, and oxygen, without nitrogen, and their properties are various. The carbohydrates are likewise composed of carbon, hydrogen, and oxygen, without nitrogen, and they include starch, sugar, cellulose, woody fiber, and allied substances, differing in form and various properties, so that their physiological value can not be the same. Oxygen is the most abundant element of the animal body as a whole, and it stands next to carbon in the percentage composition of the proximate constituents of the tissues. Its significance as a food element is too often overlooked, but it is undoubtedly as important a factor in tissue-building as any other food constituent.
Protoplasm was formerly looked upon as a proteid, but it is now generally admitted that its composition and structure are very much more complex than any form of proteid. The chemical composition of living protoplasm, as already pointed out, can not be determined, but there is evidence that proteids, fats, and carbohydrates enter into the composition of its complex molecules, and it gives rise to all three of these groups of nutrients in the processes of destructive metabolism, and it should also be noted that several varieties of proteid matter have been detected in dead protoplasm.
Energy has been defined as the power of doing work, and it is expended in the work involved in building protoplasm out of the simpler proteids, fats, and carbohydrates from which it is formed. An essential constituent of the complex molecules of protoplasm which is neglected in chemical analysis is the potential energy stored up as a result of the constructive process, which is liberated in the form of heat in destructive metabolism. The properties of living protoplasm, and its role in the vital activities of plants and animals, have been more definitely determined than its chemical constitution, and although it is generally admitted to be the dominant factor in nutrition, there is yet much to learn in regard to its properties and specific action in its diverse forms.
Living protoplasm, or, in other words, living substance, must be looked upon as constantly undergoing changes that vary with the function required of it. These changes, without attempting to distinguish between them as chemical, physical, or more strictly biological, are conveniently expressed by the general term metabolism.
Dr. M. Foster says: "We may picture to ourselves this total change which we denote by the term 'metabolism' as consisting, on the one hand, of a downward series of changes (katabolic changes), a stair of many steps, in which more complex bodies are broken down into simpler and simpler waste bodies, and, on the other hand, of an upward series of changes (anabolic changes), also a stair of many steps, by which the dead food, of varying simplicity or complexity, is, with the further assumption of energy, built up into more and more complex bodies. The summit of this double stair we call 'protoplasm.' Whether we have the right to speak of it as a single body in the chemical sense of that word, or as a mixture in some way of several bodies; whether we should regard it as the very summit of the double stair, or as embracing as well the topmost steps on either side, we can not at present tell. Even if there be a single substance forming the summit, its existence is absolutely temporary: at one instant it is made, at the next it is unmade. Matter which is passing through the phase of life rolls up the ascending steps to the top, and forthwith rolls down on the other side."
The greater activity of the nutritive processes in young and growing animals, with a gradual decline to maturity and old age, are matters of common observation. Dr. Minot has, however, shown that "with the increasing development of the organism and its advance in age we find an increase in the amount of protoplasm.[1] This seems to indicate that katabolism is relatively more active in young organisms, and that they use protoplasm in tissue-building as fast as it is formed. In old age, on the other hand, the anabolic processes resulting in the formation of protoplasm are not diminished as rapidly as the katabolic transformations of protoplasm into new tissues, to replace the waste arising from the wear and tear of the system, and a general decline of the bodily powers follows.
As we pass from the simpler to the higher forms of life we find a gradual transition from the comparatively homogeneous protoplasm of the lowest, to the highly differentiated protoplasm of the highest forms which provide for a division of labor in the physiological activities of the different organs. In the highest organisms the functions of prehension, digestion, assimilation, respiration, etc., as in the Amœba, are still carried on through the agency of protoplasm, but it is distributed to various organs, each of which has a special function.
Plant cells are not independent units as assumed in the cell theory of organic structure, as recent investigations, with improved microscopes and more exact methods of research, have shown that the protoplasm of adjacent cells is connected by slender threads which pass through minute openings in the cell walls, and this has been observed in so many cases that the continuity of their protoplasm is believed to be the rule in the structure of plants. The various tissues and cells of the higher plants have, therefore, a common bond of union in the connecting threads of protoplasm which determine their harmonious action.
The higher powers of the microscope likewise show that the protoplasm of plants is not homogeneous, but contains numerous granules which repeat themselves indefinitely by a process of self-division, each granule having a genetic relation to pre-existing granules of the same kind. Besides the granules, each protoplasmic cell has a nucleus which in the same manner is formed by the self-division of a pre-existing nucleus. The granules, and especially the nucleus, may prove to be important factors in the perpetuation of ancestral characters, and consequently more intimately involved than other elements in the grand mystery of life.
The chlorophyll granules which constitute the green coloring matter of plants were supposed to be formed from the protoplasm in which they appear; but they are now known to arise from the pre-existing self-propagating granules of protoplasm.
The conception of ascending steps of constructive metabolism resulting in the formation of protoplasm and the storing of energy, with correlated descending steps, by which protoplasm is transformed into less complex compounds (destructive metabolism), with a liberation of energy, serves as a key to the complex processes of nutrition which enables us to trace their conformity to general laws that are readily recognized, and clears up the obscurity arising from the multiplicity of details which from other points of view could not be brought into consistent relations.
In the light of these principles the relations of protoplasm to the leading features of vegetable nutrition may be traced in brief outlines, as a prelude to the more highly differentiated processes of animal nutrition. The latest discoveries in physiology all tend to verify the conclusion that the simple chemical elements and binary compounds, which constitute the food of plants, are built up by successive steps of gradually increasing complexity into protoplasm, or living substance, the ultimate product of constructive metabolism, and that the energy expended in the work performed is stored in the form of potential energy as an essential element or condition of its constitution. From the instability of the exceedingly complex molecules of protoplasm, destructive metabolism immediately follows, and the proximate constituents of plants known as proteids, starch, cellulose, etc., are formed in the downward steps of its progress with a liberation of a portion of the stored energy in the form of heat. The heat liberated in these first steps of destructive metabolism is not, however, sufficient to maintain an independent temperature in plants, as it is used in vaporizing the water exhaled in the processes of growth, or lost by radiation from the extended surface of foliage. The energy expended in vaporizing water must be considerable, as experiments show that for each pound of dry organic substance formed by the plant about three hundred pounds of water are exhaled in the form of vapor.[2]
The products of the downward steps of metabolism are numerous, some of which, as waste matters, are either excreted, as in the case of carbonic acid and water, or deposited in the more stable tissues; while others called plastic products, including proteids, starch, fats, etc., are stored as reserve materials to be used in constructive metabolism when needed by the plant. These reserve materials are not as a general rule stored in the place where they are formed, and they can only be transported when changed to a soluble form, which is brought about by certain "soluble ferments," which are also products of the destructive metabolism of protoplasm. Starch formed in the leaves is changed to glucose and transported to other parts of the plants where it is reconverted into starch and stored for use, sooner or later, in constructive metabolism. Some of these reserve materials are apparently built up again into protoplasm before they are resolved into their ultimate products. This is seen in the starch deposited in oily seeds which is used in forming protoplasm, and the oil is then formed as a product of its metabolism.
The many forms of organic acids—as the malic, tartaric, oxalic, citric, etc.—and a variety of alkaloids and other bodies are also products of destructive metabolism, which may be deposited in the various tissues as waste materials, or some of them may be changed by soluble ferments into forms which may be again utilized in the economy of the plant. The organic acids and tannin of green fruits, for example, are converted into sugar in the process of ripening by ferments formed from the protoplasm of the fruit cells.
In general terms the processes of nutrition in plants may, then, be said to consist in the construction of protoplasm from the elements of their food, with a storing of energy, and the conversion of this protoplasm into the various organic substances entering into the composition of their tissues (starch, cellulose, etc.), with a liberation of energy, and all vital activities are included in these upward and downward transformations of matter and energy.
In the higher animals the various functions are more highly specialized, but we still find that protoplasm is the essential living substance of every tissue and the dominant factor in nutrition. It differs, however, from vegetable protoplasm in many of its properties, and it can not, as formerly assumed, be formed by plants, or built up from the simpler elements that plants feed on, and from which they construct vegetable protoplasm.
The proteids, fats, and carbohydrates which constitute the food of animals are, as we have seen, products of the destructive metabolism of the protoplasm of plants, and it was formerly supposed that they are transformed into animal proteids and fats, without any marked change in their chemical constitution. This assumption is, in fact, the basis of the fallacious theory of nutritive ratios, but it can not be reconciled with the known facts of animal nutrition. There appears to be conclusive evidence that the proteids, fats, and carbohydrates of the food can not be converted into the proteids and fats of the animal body without undergoing profound disruption and reconstruction through the agency of animal protoplasm; or, in other words, the products of vegetable protoplasm can not be made available in the construction of animal tissues without being resolved into simpler compounds and formed anew in the laboratory of animal life.
Without noticing the many details that would only tend to divert the attention of the general reader from the significance of the results produced, the essential or fundamental processes in the nutrition of the higher animals may be broadly stated as follows: In the first place, the proteids, fats, and carbohydrates of their food undergo a series of changes in the processes of digestion (using this word in its widest sense) that reduces them to simpler compounds and, in fact, almost to their elements, with a liberation of energy which is made available in the reconstruction of the disintegrated food constituents.
The activities of the various organs of nutrition are primarily directed to the elaboration of a nutritive fluid, the blood, which is distributed to all of the tissues through the circulatory apparatus provided for that purpose, funishing them the pabulum for their nutrition, and receiving the excretory and other products arising from their metabolism. "An average uniform composition of the blood" is maintained through the action of numerous glandular organs, and the drafts made upon it in the constant repair of the different tissues. The various ferments required in the disintegration or digestion of the food elements that are being transformed into blood are products of the destructive metabolism of the protoplasm of the special secreting organs and of the general tissues.
From the common nutritive fluid, the blood, protoplasm is formed in all the tissues of the body, and we must look upon the characteristic elements and products of these tissues as the result of its destructive metabolism. In each organ of the body the protoplasm appears to have special endowments adapted to their specific functions, but these diverse activities are correlated to serve a common purpose in the life of the individual. The contraction of muscles, the specific secretions of the glandular organs, including the salivary glands, the liver, the pancreas, the mammary glands, etc., and, in fact, the products of all the metabolic tissues, as well as their characteristic structural elements, must be considered as the resulting products of the downward steps of the metabolism of protoplasm.
As in plants the food elements are built up into protoplasm before they are converted into the proximate constituents of plant tissues (proteids, fats, starch, etc.), so in animal nutrition it appears that the proteids, fats, and carbohydrates, together with oxygen introduced by the lungs, which constitute their food, must pass through the intermediate phases of blood and protoplasm before they appear as animal proteids and fats, or enter into the composition of the different tissues of the animal body, so that a genetic or specific relation of particular tissues to special food constituents can not be traced.
For example, the muscles, from their comparative bulk, contain a large proportion of the nitrogen of the body, and they are spoken of as nitrogenous tissues, but they are not formed directly from the proteid or nitrogenous constituents of the food. Like all other tissues, they have their origin in protoplasm that is built up from the common nutritive fluid, the blood, which is elaborated, as we have seen, from the disintegrated elements of fats and carbohydrates as well as proteids. Moreover, nitrogen is no more essential to the formation of muscle than carbon or oxygen, or even water, which are more abundant constituents of all living tissues.
It must then be evident that we can not formulate the proportions of the proximate principles of foods that will serve the best purpose in animal nutrition. The extended and profound series of changes that intervene between the food constituents on the one hand and the resulting animal tissues on the other are too complex to enable us to trace any direct chemical relations between the initial elements and their final products. Aside from these physiological considerations there are insuperable obstacles in the way of prescribing diets that are even approximately suited to the requirements of any particular individual, or group of animals, arising from individual peculiarities and inherited ancestral habits. Experiments have not as yet been made with a sufficient number of individuals or with a sufficient variety of foods to warrant any generalizations as to what constitutes a normal diet for either man or beast under even average conditions.
The recent recognition of energy as one of the most important factors in physiology has led to the rejection of the purely chemical theories that were formerly quite generally accepted in regard to the rôle of particular food constituents in the processes of nutrition. An assumed combustion of food constituents is no longer required to explain the phenomena of animal heat, which is now known to be but a phase in the transformations of energy in the processes of nutrition.
Energy is expended in building organic substance, or, in other words, in converting food-stuffs of any kind into protoplasm, the summit of the double stair of life, and its potential energy is the transformed or stored energy of the constructive process. This combined energy, in accordance with the law of conservation, may be liberated in the form of heat to a greater or less extent in various ways by the more or less complete disintegration of the organic substance in which it is stored. If the process of disintegration is carried on until the organic substance is resolved into its original elements, the heat liberated is the exact equivalent of the energy expended in its construction.
In living organisms the descending steps of metabolism are but successive phases of normal vital activities, resulting in the formation of a definite series of organic substances which contain less potential energy than the protoplasm from which they are formed, and heat must therefore be liberated as they are elaborated. Dead organic matters may be torn apart by microbes and resolved by a widely different series of descending steps into their original elements, as in the processes of fermentation and putrefaction, with a complete transformation of their potential energy into heat. The same ultimate result may likewise be obtained by burning organic substances, but the intervening steps and products of the destructive process are less numerous and of a different character than those produced by vital activities, while the heat liberated is still the transformed energy of the constructive process.
Plants derive the energy required to convert simple chemical elements into the complex molecules of protoplasm from the heat and light of the sun, while on the other hand the energy expended in the constructive processes of animals is exclusively derived from the potential energy of their food (stored energy of plants), and a disintegration and apparent waste of the material, or chemical constituents of foods, become necessary to liberate their needed supplies of energy.
As all the food constituents contribute to the blood-making processes, they all in like manner through digestive disintegration contribute to the supplies of energy required in the animal economy. The energy provided in foods in the potential form is quite as important in building animal tissues as the chemical elements entering into their composition, as when liberated in the form of heat it is utilized in constructive metabolism and stored again as potential energy in the animal tissues formed. The destructive metabolism taking place in these tissues, as an essential concomitant of their vital activities, again liberates energy in the form of heat, which, with that derived from the digestion of foods, is used, so far as needed, in the reconstructive process, and the balance appears as animal heat.
We have noticed the recently discovered continuity of the protoplasm of plants, but we can not fairly infer that there is a similar continuity of the protoplasm of the higher animals that have a highly specialized nervous system which brings the different organs and functions into harmonious action more completely and efficiently than they could be by simple threads of protoplasm like those which unite the cells of plants. The widely different products of destructive metabolism in the various tissues of plants and animals, aside from other considerations, furnish conclusive evidence that while the general rôle of protoplasm is everywhere the same, it must differ materially in composition and constitution in the different conditions in which it is found. As stated by Dr. Foster, "It is obvious that the varieties of protoplasm are numerous, indeed almost innumerable. The muscular protoplasm which brings forth a contractile katastate must differ in nature, in composition—that is, in construction—from glandular protoplasm whose katastate is a mother of ferment. Further, the protoplasm of the swiftly contracting striped muscular fiber must differ from that of the torpid, smooth, unstriated fiber; the protoplasm of human muscle must differ from that of a sheep or frog; the protoplasm of one muscle must differ from that of another muscle in the same kind of animal, and the protoplasm of Smith's biceps must differ from that of Jones's."
What determines these differences and gives direction to such diverse metabolic activities? Chemical and physical considerations fail to clear up the mystery of life and its varied manifestations. We may look upon protoplasm as the physical basis of life, and consider vital activities as resulting from its inherent properties; but this does not aid us in gaining a better knowledge of the mysterious endowments of living matter. What gives rise to these diverse properties in different species and in different organs of the same individual? We can not attribute them to organization or structure, as so far as we know these are the result of vital activities, and can not, therefore, be the cause. As forcibly stated by Prof. Huxley in his paper on the cell theory (1852), cells "are no more the producers of the vital phenomena than the shells scattered in orderly lines along the sea-beach are the instruments by which the gravitative force of the moon acts upon the ocean. Like these, the cells mark only where the vital tides have been and how they have acted."
It has been suggested that the various factors in nutrition, including even "structure" and "composition," must be looked upon as modes of motion in accordance with the concepts of modern physics, and from this point of view the body of a man has been compared to a fountain. "As the figure of the fountain remains the same, though fresh water is continuously rising and falling, so the body seems the same, though fresh food is always replacing the old man, which in turn is always falling back to dust. And the conception which we are urging now is one which carries an analogous idea into the study of all molecular phenomena of the body."
The pertinence and significance of these physical considerations should not, however, lead us to assume that life is but a form of energy. We can not doubt that energy is the motive power in living beings, and that its transformations and activities which are evident in all organic processes are properly-considered as modes of motion, but we must discriminate between the motive power that does the work and the directing force which guides it in the lines along which it acts and determines the results produced. We are unable to detect any difference in the potential energy of living and of dead protoplasm, but we recognize an immense difference in their significant properties—a difference so wide that life can not be defined as a form of energy.
The manifestations of energy in organic processes are readily perceived, and there are definite standards with which to measure them, but our most delicate means of research throw no light on the purely vital endowments of protoplasm, which not only direct and control its activities, but are transmitted in well-defined characters from parent to offspring. There is no life without preexisting life from which it is derived, and the physical basis through which it acts, or is made manifest, furnishes no satisfactory explanation as to its real essence and constitution.
In discussing the economies of foods and diets, if we keep in mind the significant facts that vital activities direct and determine the transformations of energy and the collocations of matter in plants and animals, in accordance with the nutritive requirements of every organ and tissue, and that in the higher animals the food supplies of the various tissues that differ so widely in composition and function are derived from the same common pabulum, the blood, which under the varying conditions of supply and demand maintains a comparatively uniform composition—the futility of assigning to each or any element of the food a specific role in the processes of nutrition must be obvious.
