Animals and plants depend for their existence upon a nutritive fluid, which permeates their structure; it is the element from which all their secretions are formed, and their organs are nourished.
The food of animals is composed of previously organized matters, and is conveyed into a reservoir called a stomach, where it undergoes a process of solution, previously to entering the circulation. At this period, the animal and the plant again present points of resemblance, the lymphatics or absorbent vessels take up the products of digestion, and convey them to the blood-vessels, where mingling with the current of the blood, they are conveyed to the lungs, there to undergo a process of oxygenation before they become fitted for the renovation of the tissues of the body. Such is the nature of the food of man, that it contains all the elements necessary and adapted for transformation into bone, muscle, brain, and parenchyma, as well as the other tissues of the body; besides other elementary matters, which, though they form a very insignificant portion of
animal textures, from their constant presence in the vital fluid, evidently perform some important offices in the general economy of life; they are partly, perhaps, occupied in forming constituents of secretions.
Plants do not require a stomach,—the humus or soil to which they are fixed is the laboratory, where the nutritive matter is prepared in a state fit for absorption by the spongioles of their roots, and these correspond to the lymphatics of animals; after being taken up by the spongioles, this new fluid mingles with the sap, and passes to the leaves or breathing apparatus of plants, where carbonic acid gas combines with the crude vital liquid, and converts it into a condition fit for all the offices to be performed by the plant: viz. the growth of tissues, and the elaboration of secretions.
The tissues, however, of plants, though more simple in their nature, present a much more varied character than those of animals, when the different species are compared.
The bones of animals which give them their form, are invariably constituted of phosphate and carbonate of lime, deposited in a matrix of gluten; muscle, nerve, brain, tendons, and ligaments, have nearly, if not completely, an identical composition throughout the whole range of the animal kingdom: their secretions, however, vary much more considerably, as also do the secretions of vegetables. But vegetable tissue may contain, as in the stems of
grasses, a considerable amount of silex, and some notable quantity of sulphur, and so essential to their existence is the former element, that they cannot live without its presence in the soil, and also with it an alkali, to render it soluble. A large amount of soda, is an invariable attendant upon the structure of marine plants, as potash is of those growing on the land.
Thus, whether we regard the health of animals, or vegetables, we discover, that besides the matters which are absolutely indispensable for the nutriment of the tissues which undergo rapid transformation, those of a more permanent and durable nature require in an almost insensible degree, a restitution of elements; and though not apparently absolutely necessary to preserve vitality in the being, yet have so marked an influence over it, as to indicate an extensive bearing of each individual part, on the whole associated entity.
The elementary tissues of both kingdoms have been traced, in whatever form they may be found, to a cellular origin. The minutest vegetable germ, is a cell containing a granular matter within it, and even man himself, in his embryonic state, may be represented as an insignificant point in the realms of space; and might be placed side by side with the smallest particle of living matter, without suffering by the comparison.
The laws by which the development of these elementary cells is regulated, so that each advances