ABSTRACT. Cytoplasmic streaming in the internodal cell of Nitella ceased quickly following the application of the SH-group modifying reagent Nethylmaleimide (NEM). Centrifugation and NEM application were combined in a single cell mounted in a double-chambered vessel. This arrangement made possible the separate treatment of the streaming endoplasm and stationary cortex with NEM. Streaming occurred normally when the endoplasm was kept intact while the cortex was treated with the reagent. In the reciprocal case, in which endoplasm was treated while cortex was untreated, no streaming occurred. The results obtained suggest that myosin molecules are not intermingled with actin filaments localized on the cortex but reside in the endoplasm, since it is known that NEM inhibits actomyosin-type ATPase conspicuously while it has little effect on F-actin. It was tentatively concluded that the cytoplasmic streaming in Nitella results from the sliding of the endoplasmic myosin molecules alongside the stationary F-actin filaments lying on the cortex.

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