By further observations one may determine that the substance of the amoeba is receptive and assimilative, that it is respiratory, taking in oxygen and giving off carbonic dioxide, and that it is also secretory. If the amoeba be watched long enough, it may be seen to undergo division, thus producing another individual of its kind. We say, therefore, that it exhibits the power of reproduction. All these properties manifested in close association in the amoeba are exhibited in the bodies of higher organisms in a greater degree of perfection, and also in separation, particular organs often being set apart for the performance of one of these particular functions. We should, however, bear in mind that in the simple protoplasm of the amoeba is found the germ of all the activities of the higher animals. It will be convenient now to turn our attention to the microscopic examination of a plant that is sufficiently transparent to enable us to look within its living parts and observe the behavior of protoplasm. The first thing that strikes one is the continual activity of the living substance within the boundaries of a particular cell. This movement sometimes takes the form of rotation around the walls of the cell (Fig. 81 _A_). In other instances the protoplasm marks out for itself new paths, giving a more complicated motion, called circulation (Fig. 81 _B_). These movements are the result of chemical changes taking place within the protoplasm, and they are usually to be observed in any plant or animal organism. [Illustration: Fig. 81.--(_A_) Rotation of Protoplasm in the Cells of Nitella. (_B_) Highly Magnified Cell of a Tradescantia Plant, Showing Circulation of Protoplasm. (After Sedgwick and Wilson.)]