come to an end. We have seen that such a cell has also lost the characteristic vital potential difference between exterior and interior. Response to stimulation thus depends on semipermeability, which implies polarizability of the membranes bounding the irritable cells or elements. This conclusion is one of far-reaching importance, because it localizes the primary change in electrical and hence in other forms of stimulation at the plasma membranes. Some membrane-process forms the first stage of the response to stimulation. The membrane is thus not to be regarded as a mere passive diffusion-preventing barrier between living substance and surroundings, but as the essentially sensitive and controlling portion of the cell.
Although there is much evidence that the initial event in stimulation is a surface-process and involves a change in the chemical and physical properties of the plasma membrane, the precise nature of this change is imperfectly understood at present. It seems, however, clear that it involves a temporary loss or lowering of semipermeability: i. e., the osmotic properties of the membrane are altered, and along with these its state of electrical polarization. This change forms the condition of the other and more complex changes in the interior of the stimulated cell. Evidence of a temporary loss of semipermeability comes from a number of sides, and is seen in the irritable tissues of both animals and plants. Many motor mechanisms in plants depend on this change; e. g., the movements of the sensitive plant, of the Venus' fly-trap, the tentacles of the sundew, etc. Turgid cells arranged in special ways lose their turgor on stimulation and collapse; the resulting movements may be so rapid—e. g., in the Venus' fly-trap—as to simulate muscular contraction. Yet the effect is undoubtedly due to a loss of water caused by a change in the osmotic properties of the plasma membranes.[1] Phenomena of just this kind are not seen in animal cells, where osmotic distension or turgor plays a less important part than in plants; but in gland-cells, many of which are under nervous control, closely similar changes follow upon stimulation. Water and dissolved substances are rapidly lost from the cells, which in many cases shrink at the same time. Electrical variations accompany these processes in both the plant and the animal, and are probably directly due to the change in the membranes. An especially clear parallelism between increase of membrane-permeability and stimulation as seen in the larva? of the marine annelid Arenicola; these larvæ are minute worm-like organisms a third of a millimeter long, actively muscular, and swimming freely by their cilia. When brought into pure sodium chloride or other appropriate salt solution the muscles instantly contract strongly and the contraction is invariably accompanied by a
- ↑ The term plasma membrane is applied by some botanists to the entire layer of protoplasm between cell-surface and vacuole-surface. The most external surface-layer, to which ordinarily the term is applied, can not in fact be sharply separated from the inner protoplasm.
