Page:Proceedings of the Royal Society of London Vol 69.djvu/490

470 Alcohols and anaesthetics when dilute may accelerate streaming, but when more concentrated always retard it. Alkaloids, which are strong nerve or muscle poisons, have relatively little action upon the streaming protoplasm of plants.

Weak electrical currents may accelerate streaming, strong ones always retard it, while sudden shocks produce a temporary cessation. The latent period of recovery decreases as the temperature rises up to the optimum. Weak constant currents lower the optimal and maximal temperatures for streaming. Cells are more sensitive to electrical stimuli at moderately high temperatures than at very low or very high ones, and the nucleus is fatally affected before the cytoplasm.

The electrical conductivity of the protoplasm undergoes a slight temporary increase on death, and it differs in the living cell from that of the cell-sap and of the cell-wall. The effect produced by a weak constant current bears no relation to its direction with regard to the plane of streaming.

There is little or no analogy between a shock-stoppage of streaming and a muscular contraction, or between a nerve-muscle preparation and a streaming cell. No permanently differentiated nervous mechanism exists in plants, although temporary better-conducting channels may appear as the result of prolonged stimulation or at certain stages in the development of growing organs. Excitatory changes may be transmitted in the protoplasm of a cell of Chara or Nitella at from 1 to 8 or 20 mm. per second (18 C.), but the rate of propagation from cell to cell in tissues varies from Ol to 2 mm. per minute at 18 C. Neither the chloroplastids nor any of the visible floating particles in the proto- plasm have any power of independent movement, but are passively carried with the moving stream.

The only kind of energy which appears capable of producing streaming movements under the conditions existing in plant-cells is surface-tension energy, and this is probably brought into play by the action of electrical currents which traverse the moving layers, and are maintained by chemical action in the substance of the protoplasm. These currents may act upon more or less regularly arranged bipolar particles of emulsionised protoplasm in such a manner as to reduce their surface-tension on the anterior side, or increase it on the posterior one, hence producing streaming movement in a definite direction.