Closely associated with its power of holding water is its tendency to assume an architectural form. In large vegetable cells, such as those of the hairs within the flowers of Tradescantia, the protoplasm may be seen, under the microscope, arranged in threads containing granules which are incessantly streaming up and down them. The spaces between the threads are filled with water. Such mobile protoplasm cannot be said to have a structural form. But in the greater number of cells, and especially in animal cells, the protoplasm is disposed in a network, with usually a tendency for the strands of the network to set in lines. In attempting to define these very variable networks, the microscopist is obliged to speak with caution. He finds it very difficult to distinguish between appearances which he is justified in regarding as inherent in the cell-substance, whether alive or dead, and appearances which he may have induced by the action of reagents whilst preparing the tissue for examination. Rarely can he assert that he sees a network in a living cell. When examining a dead cell, he is bound to recognize that the preservatives and hardening reagents which he used may have caused the proteins to coagulate in a particular pattern. If he obtains the same pattern with several different methods, he infers that the appearance which he sees is that of a structure existing in the living cell; but he is never quite sure that it is not an arrangement produced by reagents after death. The tendency of protoplasm to dispose itself in the form of a network or sponge-work is of the greatest interest in its bearing upon the theory of its activity in effecting chemical change. The body itself, as we shall find later, is a network of tissues enclosing lymph. The lymph in the tissue-spaces contains foods and waste products in solution. The tissues are constantly taking from it the former, and discharging into it the latter. Every cell is, microscopically, a tissue. The strands of its protoplasm are perpetually sorting foods from its cell-juice, adding to its cell-juice waste products. By diffusion, foods, including oxygen, pass from lymph to cell; waste products, including carbonic acid, pass from cell to lymph. If water be added to gum, the gum swells. The mixture is homogeneous. Diffusion takes place slowly through the mucilage. When water is taken up by protoplasm, the protoplasm swells; but the mixture is not homogeneous. The protoplasm expands as a wet sponge expands, although the relation of the enclosing reticulum to the water which it encloses is far more complicated. It is, as it were, a sponge made of gum. Some water is combined with the protoplasm; the remainder fills its spaces. There is an active surface relation between the free water and the protoplasmic threads. As water rises in a capillary tube, as it passes from the inside to the outside of a flannel shirt, so it circulates within the cell.