ULTRASTRUCTURAL FEATURES OF DEVELOPING SIEVE ELEMENTS IN LEMNA MINOR L.—THE PROTOPLAST

The occurrence and precise arrangement of phloem tissue is reported for the first time in the root of the duckweed, Lemna minor L. Since these plants are small enough to be prepared intact for electron microscopy, the induction of artifacts as a result of mechanical manipulation during specimen preparation is virtually eliminated. Ultrastructural features of the complete sequence of sieve-element development are presented and are in some respects similar to what has been described in other monocotyledons (e.g., nuclear degeneration, aggregation of ER, elaboration of electron-dense crystalline plastid inclusions). Mature sieve elements in Lemna are enucleate, lack P-protein, contain a plasmalemma, ER-aggregations, mitochondria, plastids and, in many instances, intact vacuoles. It is suggested that structural modification required for the sieve element to assume a transport function may not be very drastic, apart from the degeneration of the nucleus.