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.