Dynamics of cytoplasmic streaming in a plant cell 1 2

Abstract

A part of the large cylindrical cell of Nitella expansa, which showed vigorous cytoplasmic streaming, was compressed and flattened between a pair of parallel glass walls and the velocity distributions of the cytoplasm and the cell sap were measured. The velocity and the velocity gradient of the endoplasm were changed as the width of the stream was modified. From these measurements we calculated both the motive force responsible for the cytoplasmic streaming and the force resisting the shifting at the endoplasm-ectoplasm boundary. In the calculation it was assumed that (1) the motive force was independent of the load applied and (2) the sliding resistance was proportional to the velocity of the outermost layer of the stream. The shear stress/shear rate relation was obtained from the endoplasmic drop isolated in vitro according to the agar capillary method developed earlier by the authors. Thus the motive force was found to be 1.7 dyn cm⁻² in good agreement with the data previously reported. It was further known that the major part of the motive force was used in overcoming the resistance to the sliding at the endoplasm-ectoplasm boundary. Less than 1 per cent of the motive force was necessary to bring about the shear of the cell sap in the normal cell. The resistance per unit sliding velocity was of the order of 230 dyn sec cm⁻³.