The history of biological science shows that the conceptions which men have held concerning the nature of plant and animal growth have undergone a series of revolutionary changes as the technique of, and facilities for, scientific study have developed and improved. For a long time, it was thought that life processes were essentially different in character than those which take place in inanimate matter, and that the physical sciences had nothing to do with living changes. Then, too, earlier students had only vague notions of the actual structure of a living organism. Beginning with the earliest idea that a plant or an animal exists as a unit organism, to be studied as such, biological science progressed, first to the recognition and study of the individual organs which are contained within the organism; then to the tissues which make up these organs; then (with the coming into use of the microscope as an aid to these investigations) to the cells of which the tissues are composed; then to the protoplasm which constitutes the cell contents; and finally to the doctrine of organic evolution as the explanation of the genealogy of plants and animals, and the study of the relation of the principles of the physical sciences to the evolutionary process. The ultimate material into which organisms are resolved by this process of biological analysis is the cell protoplasm. But protoplasm is itself made up of a complex system of definite chemical compounds, which react and interact according to the laws of physical science. Hence, any study of the chemistry of plant growth is essentially a study of the chemical and physical changes which take place in the cell protoplasm.