Chapter IV: Section I: Animal and Vegetable Life

_Functions of Plants._--It is the primary function of plants to convert the inorganic matter of the soil and air into organised structures of a highly complex nature. The food of plants is purely mineral, and consists chiefly of water, carbonic acid, and ammonia. Water is composed of the elements oxygen and hydrogen; carbonic acid is a compound of oxygen and carbon; and ammonia is formed of hydrogen and nitrogen. These four substances are termed the _organic elements_, because they form by far the larger portion--sometimes the whole--of organic bodies. The combustible portion of plants and animals is composed of the organic elements; the incombustible part is made up of potassium, sodium, and the various other elements enumerated in another page. The organic elements are furnished chiefly by the atmosphere, and the incombustible matters are supplied by the soil.

Water in the state of vapor forms, according to the temperature and other conditions of the atmosphere, from a half per cent. to four and a half per cent. of the weight of that fluid--about 1.25 per cent. being the average; carbonic acid exists in it to the extent of 1/2000th; and ammonia forms a minute portion of it--according to Dr. Angus Smith, one grain weight in 412.42 cubic feet of air (of a town), or 0.000453 per cent. It is remarkable that the most abundant constituents of atmospheric air--oxygen and nitrogen--are not assimilable by plants, although these elements enter largely into the composition of vegetable substances. In the soil, also, the part which ministers to the wants of vegetables is relatively quite insignificant in amount.

Plants are unendowed with organs of locomotion, their food must therefore be within easy reach. Every breeze wafts gaseous nutriment to their expanded leaves, and their rootlets ramify throughout the soil in search of appropriate mineral aliment. But no matter how abundant, or however easy of reach may be the food of plants, the vegetable organism is incapable of partaking of it unless under the influence of light. Exposed to this potent stimulus, the plant collects the gaseous carbonic acid and the vaporous water, solidifies them, decomposes them, and combines their elements into new and organised forms. In effecting these changes--in conferring vitality upon the atoms of lifeless matter--the plant acts merely as the _mechanism_, the light is the _force_. As the work performed by the steam-engine is proportionate to the amount of force developed by the combustion of the fuel beneath its boiler, so is the rapidity of the elaboration of organic substances by plants proportionate to the amount of sunlight to which they are exposed. It is an axiom that matter is indestructible; we may alter its form as often as we please, but we cannot destroy a particle of it. It is the same with _force_: we may convert one kind of it into another--heat into light, or magnetism into electricity--but our power ends there; we can only cause force, or _motion_, to pass from one of its conditions to another, but its _quantity_ can never be diminished by the power of man.

The principle of the Conservation of the Forces gives us a clear explanation of the fact that animals can obtain their food only through the medium of the vegetable kingdom. Plants are stationary mechanisms; they have no need to develop motive power, as animals have, in moving themselves from place to place. Their temperature is, we may say, the same as that of the medium in which they exist. Such beings as plants do not, therefore, require the expenditure of force to maintain their vitality; on the contrary, their mechanisms are, for a beneficent purpose, constructed for the _accumulation_ of force. The growing plant absorbs, together with carbonic acid, water, and ammonia, a proportionate amount of light, heat, and the various other subtile forces which have their abiding place in the sun-beam--

"That golden chain,
Whose strong embrace holds heaven and earth and main."

Co-incidentally with the conversion of the mineral constituents of the food of plants into organised structures--albumen, fibre, and such like substances--the light, and the heat, and the various other forces likewise suffer a change. Although the precise nature of the new force into which they are converted is still a mystery--one, too, which may never be revealed to us--still we know sufficient of it to satisfy us that it can only exist in connection with organic or organised structures. It is owing to its presence that the elements of these structures (the natural state of which is mineral) are bound together in what may be aptly designated a constrained state; or, as Liebig aptly expresses it, like the matter in a bent spring. So long as the organic structure retains its form, it will be a reservoir of latent force--which will manifest itself in some form during the recoil of the atoms of the matter forming the structure to their original mineral, or statical condition: so the bent spring, when the pressure is removed, returns to its original straight form.

_Animal Life._--The chief manifestation of the life of a plant is the accumulation of force; very different are the functions of animal life. It is only by the continuous _expenditure_ of force that the vitality of animals is preserved; the heat of a man's body, his power of locomotion, the performance of his daily toil, even his very faculty of thought, are all dependent upon, and to a great extent proportionate to, the amount of organised matter disorganised in his body. It is by the conversion of this organised matter into its original mineral state of water, carbonic acid, and ammonia, that the force originally expended in arranging, through the agency of plants, its atoms, is again restored, chiefly in the form of heat and animal motive power.

Animals, as a class, are completely dependent upon vegetables for their existence. There is every reason to believe that the most lowly organised beings in the scale of animal life, even those of so simple a structure as to have been long regarded as vegetables or as plant-animals, are incapable of organising mineral matter. The so-called vegetative life of animals--for I believe the term to be exceedingly inexact--is applied to their growth, that is, to the increase in their weight. This increase takes place by their power of reorganising, or of assimilating to the nature of their own organisms, certain of the substances elaborated by plants, and destined to become food for animals.