122 THEORETICAL AND GENERAL 



Food-materials exercise both a direct and an indirect effect upon 

 streaming. Acids, alkalies, and metallic poisons all retard streaming, and 

 may cause a temporary shock-stoppage when suddenly applied. 



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 cells of plants. 



Weak electrical currents may accelerate streaming, strong ones always 

 retard it, sudden shocks produce temporary cessation. The latent period 

 of recovery decreases as the temperature rises up to a certain limit. 

 Beyond this it increases. Weak constant currents lower the optimal and 

 maximal temperatures for streaming. 



A shock-stoppage is more readily produced by electrical stimuli at 

 moderately high temperatures than at very low or very high ones, and 

 in general the cells are more sensitive in the former case. 



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 cell-wall. The nucleus is fatally affected by electrical 

 currents before the cytoplasm. 



The effect produced by a weak constant current is not influenced 

 by 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 develop- 

 ment of growing organs. 



Stimuli may be transmitted in the protoplasm of a cell of Chara or 

 Nitella, at from i to 8 or even 20 mm. a second (i8C.), but the rate 

 of propagation from cell to cell in tissues varies from o-i to 2 mm. per 

 minute at i8C. 



The chloroplastids have no active power of movement of their own, 

 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 electric 

 currents traversing the moving layers, and maintained by chemical action 

 in the substance of the protoplasm. These currents may be supposed 

 to act upon regularly-arranged bipolar particles of protoplasm in such 

 a manner as always to lower the surface-tension on the anterior faces, 

 and raise it on the posterior ones. 



