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Abstract
Electron microscopical observations of the phloem of Brassica chinensis L. leaves, healthy or infected by turnip yellow mosaic virus, have shown that the «slime», characteristic proteic component of the sieve cell protoplast, is present as tubules or fibrils, free in the cellular cytoplasm or localized in the cisternae of the endoplasmic reticulum. In this paper it was thought to follow the ontogensis and evolution of the «slime» in parallel with the differentiation of the sieve tube and at the same time to contribute to the solution of the problem concerning the origin and probable function of this proteic material.