Abstract: | The effect of external divalent cation salt solutions upon the association of an action potential and cessation of cytoplasmic streaming in Nitella was studied. Nitella cells remained excitable when immersed in solutions of CaCl2, MgCl2, BaCl2, and SrCl2. Cessation of streaming coincident with excitation occurred in solutions of CaCl2 or SrCl2 but not in solutions of MgCl2 or BaCl2. In cells exposed to solutions containing mixtures of MgCl2 and CaCl2, or MgCl2 and SrCl2, it was the Ca]/Mg] or Sr]/Mg] which determined the effect of an action potential upon the rate of streaming, rather than the absolute concentrations Ca++ or Sr++. The implications of these data are discussed with respect to the structure involved in the generation of cytoplasmic streaming and the relation of streaming to other types of biological motion. |