[The nature of the trophic tract in vascular plants]

Modern data about the origin, structure and functions of plant plasmodesmata and "symplast" are reviewed. Etymology of "symplast" means the continuality of cell protoplasts in a plant body. Originally this term signified the continuality of sieve tubes only as a system of photosynthate transport and distribution. Content of their cavities seemed to be identical to cytoplasm. Later, the idea was shown to be a mistake. Sieve tube content is identical to vacuolar exudates. It has been shown that the plasmodesmata, vacuome of parenchyma and sieve tubes of phloem arised during prokaryote/eukaryote endosymbiogenesis as a transport network for photosynthate distribution. They continued to be the same during all the time of evolution of vascular plants. Functional role of the apoplast is similar when symplastic transport is blocked and apoplast becomes the compensatory channel. Both, the symplast and the apoplast, are continual in plant cell systems in contrast to cytoplasm whose continua-lity is secondary and has no serious significance as transport communication. It is not accorded with the phenomena of cell and tissue differentiation and cannot be accepted without additional evidences. The concept of plant symplast as a cytoplasm continuum could be reviewed under the influence of new facts.