Development and Dispersal of P-Protein in the Phloem of Coleus Blumei Benth

The development of P-protein (slime) in the phloem of Coleus stem apices has been studied electron microscopically using material fixed in glutaraldehyde followed by osmium tetroxide. In phloem parenchyma cells the earliest-appearing groups of tubular P-protein commonly are seen as close association with clusters of ‘spiny’ vesicles similar to those reported in Phaseolus phloem (Newcomb, 1967). The vesicles break down as the P-protein masses enlarge, and are assumed to contribute to P-protein formation. Subsequently the groups of tubules are consolidated into a single spindle-shaped body aligned longitudinally in each phloem parenchyma cell or sieve element. The microtubules observed frequently in the vicinity of the young P-protein body may play a role in its consolidation or in the longitudinal alignment of its constituent tubules. Some P-protein bodies acquire a highly organized structure in which the tubules are arranged hexagonally around lightly staining centres. Disaggregation of the P-protein body occurs during disintegration of the cytoplasm and nucleus, and results initially in the presence of swirls of packed fibrils. During disaggregation, the tubules of the mature P-protein body, which are about 200 Å in diameter, are converted to fibrils about 70 Å in diameter in a process apparently with several intermediate stages. In longitudinal view the fibrils exhibit alternate electron transparent and dense bands that impart a striated appearance to the mass. During maturation of the sieve element the swirls of fibrillar masses separate into individual fibrils which become dispersed through the cell lumen.

Ontogeny of cambium and Phloem in the Epicotyl of Pisum sativum

The pattern of distribution and differentiation of the primary phloem, the cambium, and the secondary phloem, and the exact pattern of division of the initial cell and its derivatives have been studied in the epicotyl of pea plants by using electron microscopy. Three divisional patterns of the initial cell, in giving rise to the phloem cells, are recognized. The initial cell first divides periclinally to give rise to a transitional cell. This transitional cell then divides further (periclinally and/or anticlinally) to give rise to three sequences of phloem derivatives: (1) phloem parenchyma cells, (2) a companion cell and a sieve cell, and (3) a companion cell, a sieve cell, and a phloem parenchyma cell. The derived cells can all easily be distinguished from each other either by their position in the vascular bundle at low magnification or by the different types of plastids present in them. The general pattern of differentiation of the cytoplasm and the formation of the sieve plate and the sieve pores of the sieve element are essentially similar in the primary and the secondary phloem. However, the sieve element of the secondary phloem, unlike that of the primary phloem, possesses in its cytoplasm three kinds of inclusion bodies - an amorphous form, a "crystalline" form, and a tubular form; these are described and their nature discussed.