Intracellular Transport Apparatus of Phloem Fibers.

Rotational streaming of cytoplasm occurs in the form of longicellular currents in immature and relatively mature fibers of bean stems. Plastids carried by these currents move continuously on their rotational course from one end of the cell to the other in relatively straight lines. A distance of approximately 2 millimeters, the average length of the fibers, is covered in about 3 minutes (3.6 cm per hour). The same type of streaming occurs in immature phloem fibers that develop within 24 hours after bean seeds are planted and before the plants appear above the soil surface. Rotational streaming also occurs in xylem fibers of bean stems and in cells which appear to be fibers in stems of young cucumber, tomato, sunflower, and flax.