ULTRASTRUCTURAL FEATURES OF DEVELOPING SIEVE ELEMENTS IN LEMNA MINOR L.—SIEVE PLATE AND LATERAL SIEVE AREAS

Both intact and cut duckweed plants were prepared for electron microscopy. Plants which are prepared intact do not exhibit callose formation during development of sieve-plate pores. Future pore sites can be recognized by the presence of median cavities that are unassociated with callose platelets. These cavities are first seen in the region of the compound middle lamella and are lined by a plasmalemma. As end walls thicken, the cavities increase in size until open pores of uniform width are formed. Mature sieve plates of intact-prepared plants are also devoid of callose. Fully opened pores are lined by a plasmalemma and are only traversed by an occasional tubule of endoplasmic reticulum. Plants which have been cut prior to fixation possess mature sieve plates containing callose. The pores of developing sieve plates in cut plants exhibit small amounts of callose. Except for the lack of callose, lateral wall connections between sieve elements and contiguous cells are similar in development and mature state to those reported for other species.