In the state of life, during the interchange of action which thus incessantly goes on between physical agents and vital organs, the laws to which inorganic matter is subject are resisted, controlled, and modified. Physical and chemical attractions are brought under the influence of a new and superior agency, with the laws of which we are imperfectly acquainted, but the operation of which we see, and which we call the agency of life. Air, water, heat, electricity, are physical agents, which subvert the most intimate combinations of inorganic bodies, resolving them into their simple elements, and recombining these elements in various modes, and thus forming new bodies, endowed with totally different properties; but the physical and chemical agencies by which these changes are wrought in the inorganic, are resisted, controlled, and modified by the living body: resisted, for these physical agents do not decompose the living body; controlled and modified, for the living body converts these very agents into the material for sustaining its own existence Of all the phenomena included in that circle of actions which we designate by the general term life, this power of resisting the effects universally produced by physical agents on inorganic matter, and of bringing these very agents under subjection to a new order of laws, is one of the most essential and distinctive. All vital processes are processes of supply, or processes of waste. By every vital action performed by the organized body, some portion of its constituent matter is expended. Numerous vital actions are constantly carried on for the sole purpose of compensating this expenditure. Every moment old particles are carried out of the system; every moment new particles are introduced into it. The matter of which the organized, and more especially the animal, body is composed, is thus in a state of perpetual flux; and in a certain space of time it is completely changed, so that of all the matter that constitutes the animal body at a given point of time, not a single particle remains at another point of time at a given distance. All the wants of the economy of the plant are satisfied by a due supply of air, water, heat, cold, electricity, and light. Some of these physical agents constitute the crude aliment of the plant; others produce in this aliment a series of changes, by which it is converted from crude aliment into proper nutriment, while others act as stimulants, by which movements are excited, the ultimate object of which is the distribution of the nutriment to the various parts of the economy of the plant.