(cytoplasmic streaming) The movement of *cytoplasm
within cells. The viscosity of cytoplasm can vary as a result of changes in the nature of protein molecules, particularly actin. In the fluid form it flows freely and can be observed carrying organelles and other inclusions in the cell. Cyclosis is very variable in plant cells. For example, in leaves of Elodea (waterweeds) it is more active in cells in the midrib region than in cells near the margin. The rate of flow increases with rise in temperature but is normally less than 0.1 mm per second. It is probable that contractile forces generated by F-actin microfilaments in the cytoplasm contribute to the movement. The streaming of cytoplasm probably assists the distribution of metabolites between cells via *plasmodesmata
, augmenting diffusion, which is a slower process. It is also believed to contribute to the passage of material through sieve tubes and is more pronounced in young phloem sieve tubes than in the older elements. Cyclosis is not seen in prokaryotic cells.