SURFACE ASSOCIATIONS BETWEEN PARTICLES AND THE ENDOPLASMIC RETICULUM IN PROTOPLASMIC STREAMING

S ummary Protoplasmic streaming was observed in Tradescantia virginiana staminal hair cells using a bright field oil immersion obective and particle movements were followed in the cytoplasm. By recording and plotting particle positions in every fourth frame of a cine film, different velocities were measured for particles which appeared to follow a common pathway in a transvacuolar strand. From the same kind of analysis, various patterns of particle movement were revealed on the surface of the nucleus. Phase contrast observations of leaf hair cells of Primula obconica and Perilla frutescens showed that the endoplasmic reticulum was particularly well developed in regions of active protoplasmic streaming. Tubules of the endoplasmic reticulum were mainly in parallel orientation, with occasional lateral connections and anastomoses. Groups of parallel, endoplasmic reticulum tubules also appeared to be in continuity with the nucleus and, although these elements approached the nucleus at different angles from the parietal cytoplasm, they were more usually orientated with the long axis of the cell. Particles moved along the endoplasmic reticulum surfaces, sometimes in opposite directions on adjacent tubules, and a dynamic association was observed between the endoplasmic reticulum and mobile particles. This association was well illustrated when endoplasmic reticulum developed in a microscopically undifferentiated region of parietal cytoplasm. As the endoplasmic reticulum developed, the undifferentiated region was apparently transformed and became part of the endoplasmic reticulum. Occasionally, a centre of endoplasmic reticulum development was observed as a compact, complex pattern of tubules which gradually became orientated in a more linear manner as it expanded. This differentiation occurred in association with unusual, concentric, particle movement at the development centre. Finally the endoplasmic reticulum elements formed parallel pathways as commonly seen in actively streaming cytoplasm. These observations are incompatible with current theories of protoplasmic streaming which propose that particles are carried along by the fluid endoplasm or the endoplasmic reticulum. An extension of earlier theories has been made to explain the movement of particles in relation to the activities of the endoplasmic reticulum. Associations of particles and membranes were also observed in cytoplasm of cells involved in the synthesis and transport of organic nutrients. This structural association may be part of a common transport mechanism within parenchyma cells and over long distances in the vascular system.