Chloroplasts and other cytoplasmic granules in the freshwater coenocytic green algaDichotomosiphon tuberosusstreamed bidirectionally along the longitudinal axis of its tubular body. In response to light stimuli, the organelles migrated toward the apical regions and accumulated there. In the dark, they migrated toward the basal regions and stayed there. The cytoplasmic streaming and the light-regulated movement were inhibited by the presence of 5×10−4M colchicine, but not by 100μg/ml cytochalasin B. Local illumination with blue light caused reversible accumulation of the organelles in the illuminated zone. Single arrays of microtubules were found in the ectoplasmic layer of the alga, and both single and bundle arrays in the endoplasm. The endoplasmic microtubules disintegrated when the alga was treated for 24 h with colchicine. The involvement of the microtubules in the motive force generation for the bidirectional streaming and translocational movement of the organelles is discus
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