[Investigations on the fine structure of the phloem : III. The "trumpet cells" of Laminaria species].

The trumpet cells in the medulla of the Laminaria-cauloid are living cells with normal organelles and a persistent nucleus. The cross wall consists of a single primary pith field with 20,000-30,000 plasmodesmata of normal structure and dimension. They have a density of 50-60/μm(2). There are indications that the rim of the pores and the surface of the cross wall (beside the pores) are covered with a callose layer.