Filamentous Component of Secondary Phloem Sieve Elements in Pinus strobus L.

Filamentous material in differentiating and mature sieve elementsof white pine is found to originate from plastids that undergoa gradual breakdown. In addition to the usual loose arrangement,filamentous material may display different degrees of condensationand form lattice- and honeycomb-like figures or discrete circularbodies. Endoplasmic reticulum undergoes a vesiculation processand forms a vesicular, membranous meshwork; the fine filamentousmaterial is often found enclosed in the loops of endoplasmicreticulum membranes. Ribosomes temporarily form noticeable aggregates,but are absent from mature sieve elements.