586 TEE POPULAR SCIENCE MONTHLY 



come to an end. We have seen that such a cell has also lost the character- 

 istic vital potential difference between exterior and interior. Response 

 to stimulation thus depends on semipermeability, which implies polar- 

 izability 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 control- 

 ling 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 in- 

 volves 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. Evi- 

 dence 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. 3 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 stimu- 

 lation as seen in the larva? of the marine annelid Arenicola ; these larvae 

 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 



s 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. 



