The fact here is the responsive power of living protoplasm. It will be well to examine it first in its operation in an individual organism before we consider it in relation to the species. Reaction or response of a chemical and mechanical type takes place alike in dead matter and in living organisms, but certain stimuli will induce action in an organism which they could not possibly induce in a mineral. For in every cell, as Reinke well says, there are a chemist and an architect who guide its energies, and who have something quite different from chemistry and physics in view. Consider the following case. Every tuber of a potato plant is covered with a light skin composed of a corky substance intended to protect the internal structure from injury. This skin is produced by the action of the surface cells of the tuber. Chemically and physically these cells are just the same as the cells in the interior of the tuber. But the interior cells do not produce this corky substance, because it would be injurious to the plant if they did. The cells below the surface of the tuber, though they are by no means secluded from the chemical influences of the earth around them, behave quite differently from those actually in contact with the earth. Now let us take our tuber, slice it in half, and replace it in the earth again. If we look at it again in a few days we shall find that the interior cells, now exposed by the cutting, have done what they could not or did not do before—they have produced a layer of skin to cover the exposed surface of the tuber just as if they had been surface cells from the outset. This kind of response seems to take us quite out of the region of chemical and physical action as understood in the case of inorganic matter. It is a response directed to maintaining as far as possible the life and form of the organism, a thing which mere chemical action in mineral substances never does. It may perhaps, however, be argued that the actual contact with the earth has a possible chemical stimulus which is not communicable to cells even a hair’s-breadth below the surface, and that the cells laid bare by slicing react as they do simply because they are exposed to this stimulus. Let us take, then, another common and typical case of response to altered conditions in plant life.