An ontogenetic study of secondary phloem sieve elements of Pisum sativum L., fixed on the intact plant for electron microscopy, indicates that the connecting strands across the sieve plates are continuities of the endoplasmic reticulum. Each connecting strand is surrounded by a callose cylinder. The peripheral cytoplasm of the nucleate "young" sieve elements contains longitudinally oriented tubules of endoplasmic reticulum. As the sieve elements develop, nuclear material is extruded into the cytoplasm by way of a fibrotubular body which is structurally distinct from the slime body. When the cells are fully expanded the slime bodies disperse. This process is followed by breakdown of a number of organelles including the nucleus and tonoplast. This apparently leaves the endoplasmic reticulum free in the cell lumen.