Putting the Breaks On: Regulating Organelle Movements in Plant Cells Motile Fibrils and Cytoplasmic Streaming in Germinated Pollen The Presence of Contractile Proteins in Pollens ami Their Role in Cytoplasmic Streaming Explaining the “Pulse of Protoplasm”: The search for molecular mechanisms of protoplasmic streaming Organelle trafficking, the cytoskeleton, and pollen tube growth