Protoplast Structure in Sieve Tube Elements

Summary The microscopic structure of sieve tube elements was studied in fresh hand sections from Cucurbita pepo, Glycine max and Solanum tuberosum. The sections were stained in aqueous solutions of either iodine (Lugol's) or eosin. Photomicrographs from these preparations show linear cytoplasmic structures attached to sieve plates, and linear files of particles associated with cytoplasmic strands. In addition, phase contrast observations of fresh Cucurbita pepo phloem exudate revealed cytoplasm constituents including membranes, fibrillar elements, 0. 1–0.2 μ in diameter, and mitochondria‐like particles floating free, or associated with the fibrillar elements. Plastids, 5 μ in diameter, also occurred in the fluid exudate, sometimes associated with the aggregates of structural material. Cytoplasmic structures in sieve tube elements and in fresh phloem exudate are interpreted as the remains of damaged transcellular strands. In the transcellular streaming concept of phloem transport, transcellular strands are an essential part of the sieve tube element, and present evidence leaves little doubt that these structures really exist.