Protoplasmic streaming as a process of jet propulsion.

Abstract Protoplasmic streaming is proposed to be generally a countercurrent process in which tubular and vesicular membranous elements of the endoplasm jet-propel themselves in the direction of streaming by a pumping action in which the cytoplasmic matrix is ejected in the opposite direction. Movement of inclusions with the jet-propelled membranous elements apparently depends upon the fact that they are membrane encapsulated and enmeshed in a dense network of these elements, for which they have a high affinity. Circulation of matrix between massive lamellar elements of the endoplasmic reticulum also seems to be accomplished by a pumping action of the constituent double-membrane lamellae. The recoil of these massive elements and of membrane-encapsulated organelles may be the basis for their translational and rotational movements. The pumping of matrix by membranous elements is suggested to be an accompaniment of metabolic activity at their surfaces and the primitive basis for cell circulations. In this view, the very elements responsible for syntheses and other metabolic reactions, also provide the channels and motive force for partitioning, exposing, circulating, and transporting both the substrates upon which they act and the products of their actions. When protoplasmic streaming in various forms is analyzed as a countercurrent process involving jet propulsion, its many manifestations can be formulated as mere variations on a common theme, as postulated in the following: 1. 1. Cyclosis of plant-cell protoplasm reflects the continuous countercurrent circulation of matrix through the lumina of counter-moving membranous elements. 2. 2. Asters form by jet propulsion of membranous elements inward along the rays to the clear hyaline central region where they accumulate as a gelled mass. 3. 3. “Peristaltic” flow of axoplasm and “damming up” of constricted neurons occur as a result of the temporary “pile up” of jet-propelled elements of the axonal endoplasmic reticulum (and accompanying inclusions) at the nodes of Ranvier and other constricted regions. 4. 4. Myxomycete shuttle streaming primarily reflects the back-and-forth countercurrent circulation of matrix through the lumina of elements of the endoplasmic reticulum. The matrix and jet-propelled elements alternately accumulate in greater amounts at opposite ends of the stream. 5. 5. Protoplasmic streaming in fungal hyphae, viewed as a process of mass exchange rather than of bulk transfer, resolves the long-standing enigma of where the streaming protoplasm goes to. Membranous elements and inclusions make headway to certain regions at the expense of matrix which, being retrojected in tandem, accumulates at other regions. The cycles of waxing and waning of vacuolar size at opposite extremities of hyphae in which reversals of streaming occur, reflect the accumulation and evacuation of matrix retrojected from one end to the other by the jet-propelled elements. 6. 6. The countercurrent basis of protoplasmic streaming is revealed in organisms which feed by filament streaming. When only an interfacial film separates the protoplasm from the medium, as in a foraminiferan reticulopod, the movement of retrojected matrix is manifested because it carries the film and adhering particulate matter back to the cell body with it by adhesive drag. A gradient of dynamic interfacial tension probably promotes the flow of the film.