ULTRASTRUCTURAL OBSERVATIONS ON PINACEAE LEAF PHLOEM I. THE SPRING CONDITION

Abstract

Pine and spruce leaves were fixed for electron microscopy during their period of rapid differentiation (April‐May 1970). The sieve cells are in radial rows and mature from the outside towards the inside of the leaf. The sieve cell initials have large nuclei and many organelles. As the sieve cells mature, the walls become thick and lamellar, the tonoplast breaks down, and most of the organelles disappear. The plastid stroma becomes fibrillar. The plastids then rupture, releasing fine fibrils into the mictoplasm. The plasmodesmata between phloem parenchyma cells and albuminous cells occur in expanded regions of the cell wall. The sieve areas have little callose in the spring. Copyright © 1972, Wiley Blackwell. All rights reserved