Every organ is excited to action, or its function is called into operation by means of some external body. The external bodies capable of exciting and maintaining the functions of living organs, consist of a definite class. Because these bodies belong to that department of science which is called physical, they are termed physical agents. They are air, water, heat, cold, electricity, and light. Without the living organ, the physical agent can excite no vital action: without the physical agent, the living organ can carry on no vital process. The plant cannot perform the vital process of respiration without the leaf, nor, with the leaf, without air. The physical agent acts upon the living organ; the living organ reacts upon the physical agent, and the action between both is definite. In the lung of the animal a certain principle of the air unites, in definite proportions, with a certain principle of the blood; the oxygen of the air combines with the carbon of the blood; the air is changed by the abstraction of its oxygen; the blood is changed by the abstraction of its carbon. Atmospheric air goes to the lung, but atmospheric air does not return from the lung; it is converted into a new substance by the action of the organ: it is changed into carbonic acid by the union of a given quantity of oxygen, which it transmits to the organ, with a given quantity of carbon which the organ conveys to it. Venous blood goes to the lung, but venous blood does not return from the lung; it is converted, by the instrumentality of the organ, into a new substance, into arterial blood, by giving to the air carbon, and by receiving from the air oxygen. In this manner the change in the physical agent is definite and uniform; and the change in the living substance is equally definite and uniform. It is this determinate interchange of action between the living organ and the physical agent that constitutes what is termed a vital process. All vital processes are carried on by living organs; the materials employed in all vital processes are physical agents; the processes themselves are vital functions. All the changes produced by all the organs of the plant upon physical agents, and all the changes produced by all physical agents upon the organs of the plant, constitute all the vital processes of the plant—comprehend the whole sum of its vital phenomena. The root, the trunk, the woody substance, the bark, the ascending vessels bearing sap, the descending vessels bearing secreted fluids, the leaves, the flowers, these are the living organs of the plant. Air, water, heat, cold, electricity, light, these are the physical agents which produce in these organs definite changes, and which are themselves changed by them in definite modes; and the whole of these changes, taken together, comprehend the circle of actions, or the range of functions performed by this living being.