In order that the substance of living bodies should be able to undergo these constant changes while preserving the same form and structure in minute details—that they should be, as it were, in a constant state of flux while remaining sensibly unchanged, it is necessary that the molecules of which they are built up should be so combined as to be easily separated and as easily united—be, as it is termed, labile or flowing; and this is brought about by their chemical composition, which, while consisting of few elements, is yet highly complex in structure, a large number of chemical atoms being combined in an endless variety of ways.
The physical basis of life, as Huxley termed it, is protoplasm, a substance which consists essentially of only four common elements, the three gases, nitrogen, hydrogen, and oxygen, with the non-metallic solid, carbon; hence all the special products of plants and animals are termed carbon-compounds, and their study constitutes one of the most extensive and intricate branches of modern chemistry. Their complexity is indicated by the fact that the molecule of sugar contains 45, and that of stearine no less than 173, constituent atoms. The chemical compounds of carbon are far more numerous than those of all the other chemical elements combined; and it is this wonderful variety and the complexity of its possible combinations which explain the fact, that all the various animal tissues—skin, horn, hair, nails, teeth, muscle, nerve, etc., consist of the same four elements (with occasionally minute quantities of sulphur, phosphorus, lime, or silica, in some of them), as proved by the marvellous fact that these tissues are all produced as well by the grass-eating sheep or ox as by the fish or flesh-eating seal or tiger. And the marvel is still further increased when we consider that the innumerable diverse substances produced by plants and animals are all formed out of the same three or four elements. Such are the endless variety of organic acids, from prussic acid to those of the various fruits; the many kinds of sugars, gums, and starches; the number of different kinds of oil, wax, etc.; the variety of essential oils which are mostly forms of turpentines, with such substances as camphor, resins, caoutchouc, and gutta-percha; and the extensive series of vegetable alkaloids, such as nicotine from tobacco, morphine from opium, strychnine, curarine, and other poisons; quinine, belladonna, and similar medicinal alkaloids; together with the essential principles of our refreshing drinks, tea, coffee, and cocoa, and others too numerous to be named here—all alike consisting solely of the four common elements from which almost our whole organism is built up. If this were not indisputably proved, it would scarcely be credited.
Professor F.J. Allen considers that the most important element in protoplasm, and that which confers upon it its most essential properties in the living organism—its extreme mobility and transposibility—is nitrogen. This element, though inert in itself, readily enters into compounds when energy is supplied to it, the most striking illustration of which is the formation of ammonia, a compound of nitrogen and hydrogen, produced by electric discharges through the atmosphere. Ammonia, and certain oxides of nitrogen produced in the atmosphere in the same way, are the chief sources of the nitrogen assimilated by plants, and through them by animals; for although plants are continually in contact with the free nitrogen of the atmosphere, they are unable to absorb it. By their leaves they absorb oxygen and carbon-dioxide to build up their woody tissues, while by their roots they absorb water in which ammonia and oxides of nitrogen are dissolved, and from these they produce the protoplasm which builds up the whole substance of the animal world. The energy required to produce these nitrogen-compounds is given up by them when undergoing further changes, and thus the production of ammonia by electricity in the atmosphere, and its being carried by rain into the soil, constitute the first steps in that long series of operations which culminates in the production of the higher forms of life.
But the remarkable transformations and combinations continually going on in every living body, which are, in fact, the essential conditions of its life, are themselves dependent on certain physical conditions which must be always present. Professor Allen remarks: 'The sensitiveness of nitrogen, its proneness to change its state of combination and energy, appear to depend on certain conditions of temperature, pressure, etc., which exist at the surface of this earth. Most vital phenomena occur between the temperature of freezing water and 104° F. If the general temperature of the earth's surface rose or fell 72° F. (a small amount relatively), the whole course of life would be changed, even perchance to extinction.'
Another important, and even more essential fact, in connection with life, is the existence in the atmosphere of a small but nearly constant proportion of carbonic acid gas, this being the source from which the whole of the carbon in the vegetable and animal kingdoms is primarily derived. The leaves of plants absorb carbonic acid gas from the atmosphere, and the peculiar substance, chlorophyll, from which they derive their green colour, has the power, under the influence of sunlight, to decompose it, using the carbon to build up its own structure and giving out the oxygen. In the laboratory the carbon can only be separated from the oxygen by the application of heat, under which certain metals burn by combining with the oxygen, thus setting free the carbon. Chlorophyll has a highly complex chemical structure very imperfectly known, but it is said to be only produced when there is iron in the soil.
The leaves of plants, so often looked upon as mere ornamental appendages, are among the most marvellous structures in living organisms, since in decomposing carbonic acid at ordinary temperatures they do what no other agency in nature can perform. In doing this they utilise a special group of ether-waves which alone appear to have this power. The complexity of the processes going on in leaves is well indicated in the following quotation:—
'We have seen how green leaves are supplied with gases, water, and dissolved salts, and how they can trap special ether-waves. The active energy of these waves is used to transmute the simple inorganic compounds into complex organic ones, which in the process of respiration are reduced to simpler substances again, and the potential energy transformed into kinetic. These metabolic changes take place in living cells full of intense activities. Currents course through the protoplasm and cell-sap in every direction, and between the cells which are also united by strands of protoplasm. The gases used and given off in respiration and assimilation are floated in and out, and each protoplasm particle burned or unburned is the centre of an area of disturbance. Pure protoplasm is influenced equally by all rays: that associated with chlorophyll is affected by certain red and violet rays in particular. These, especially the red ones, bring about the dissociation of the elements of the carbonic acid, the assimilation of the carbon, and the excretion of the oxygen.'[12]
It is this vigorous life-activity ever at work in the leaves, the roots, and the sap-cells, that builds up the plant, in all its wondrous beauty of bud and foliage, flower and fruit; and at the same time produces, either as useful or waste-products, all that wealth of odours and flavours, of colours and textures, of fibres and varied woods, of roots and tubers, of gums and oils and resins innumerable, that, taken altogether, render the world of vegetable life perhaps more varied, more beautiful, more enjoyable, more indispensable to our higher nature than even that of animals. But there is really no comparison between them. We could have plants without animals; we could not have animals without plants. And all this marvel and mystery of vegetable life, a mystery which we rarely ponder over because its effects are so familiar, is usually held to be sufficiently explained by the statement that it is all due to the special properties of protoplasm. Well might Huxley say, that protoplasm is not only a substance but a structure or mechanism, a mechanism kept at work by solar heat and light, and capable of producing a thousand times more varied and marvellous results than all the human mechanism ever invented.
But besides absorbing carbonic acid from the atmosphere, separating and utilising the carbon and giving out the oxygen, plants as well as animals continually absorb oxygen from the atmosphere, and this is so universally the case that oxygen is said to be the food of protoplasm, without which it cannot continue to live; and it is the peculiar but quite invisible structure of the protoplasm which enables it to do this, and also in plants to absorb an enormous amount of water as well.
But although protoplasm is so complex chemically as to defy exact analysis, being an elaborate structure of atoms built up into a molecule in which each atom must occupy its true place (like every carved stone in a Gothic cathedral), yet it is, as it were, only the starting-point or material out of which the infinitely varied structures of living bodies are formed. The extreme mobility and changeability of the structure of these molecules enables the protoplasm to be continually modified both in constitution and form, and, by the substitution or addition of other elements, to serve special purposes. Thus when sulphur in small quantities is absorbed and built into the molecular structure, proteids are formed. These are most abundant in animal structures, and give the nourishing properties to meat, cheese, eggs, and other animal foods; but they are also found in the vegetable kingdom, especially in nuts and seeds such as grain, peas, etc. These are generally known as nitrogenous foods, and are very nutritious, but not so easily digestible as meat. Proteids exist in very varied forms and often contain phosphorus as well as sulphur, but their main characteristic is the large proportion of nitrogen they contain, while many other animal and vegetable products, as most roots, tubers, and grains, and even fats and oils, are mainly composed of starch and sugar. In its chemical and physiological aspects protein is thus described by Professor W.D. Haliburton:—'Proteids are produced only in the living laboratory of animals and plants; proteid matter is the all-important material present in protoplasm. This molecule is the most complex that is known; it always contains five and often six or even seven elements. The task of thoroughly understanding its composition is necessarily vast, and advance slow. But, little by little, the puzzle is being solved, and this final conquest of organic chemistry, when it does arrive, will furnish physiologists with new light on many of the dark places of physiological science.'[13]