Our knowledge of these matters is not yet far enough advanced to permit a definite statement as to whether there is any difference between the protoplasm of plant tissues and that of animal origin in their modes of action, or in the physical-chemical changes which constitute the vital phenomena in the two groups of living organisms. Thus far, no such differences have been discovered. Hence, in the following discussions, no attempt is made to differentiate between animal and plant protoplasm. Most of the facts and principles which are here presented have been developed as the result of the study of the physiological chemistry of animal life. No similar careful study of plant chemistry has yet been carried out; but preliminary studies seem to indicate that the same general principles apply to all protoplasm, regardless of whether it is of plant or of animal origin. It is possible, of course, that further studies of plant protoplasm will render necessary some modifications of some of these views as applied to the growth of plants; but they are believed to represent the best which is now known of the physical chemistry of the plant-cell activities. HETEROGENEOUS STRUCTURE OF THE CELL Examination of cell protoplasm under the microscope reveals that it is not a simple homogeneous mass. In the first place, it has a definite structure, composed of (_a_) a nucleus; (_b_) numerous granular bodies of different sizes and kinds; and (_c_) a clear mass of colloidal material, which (if observed under the ultramicroscope, or photographed by ultra-violet light) is apparently made up of very minute particles of many different types of materials; the whole mass, in the case of plant protoplasm, being generally surrounded by (_d_) a differentiated layer known as the cell-wall. The actual internal structural arrangement of the clear colloidal mass is uncertain; but its properties indicate that it may be considered to be like a mass of foam (resembling a compact mass of soap-bubbles) the compartments of the foam being, of course, very minute and the films themselves almost infinitely thin, the contents of each compartment being probably liquid, and the whole composing a typical colloidal gel of complex composition. This conception may not be accurate in every detail, but it seems to fit very closely the conditions and reactions of cell protoplasm. Furthermore, it is obvious that the definite structure, or form, of the cell is essential to its life; since, if the structure be destroyed by any kind of mechanical injury (freezing of the cell contents, resulting in the puncturing of the membranes by ice crystals; rupturing of the films, or cell-walls, by grinding with sharp sand, etc.) so as to bring about an intermingling of the parts which are segregated from each other in the organized structure, there results an immediate exhibition of abnormal chemical actions, accompanied by the liberation of carbon dioxide, and the death of the cell.