DEVELOPMENT AND ULTRASTRUCTURE OF THE PRIMARY PHLOEM IN THE SHOOT OF EPHEDRA-VIRIDIS (EPHEDRACEAE)

Sieve cell differentiation in the primary phloem of Ephedra viridis is first indicated by an increase in thickness of the wall, which begins in the corners of the cell, and next by the proliferation of smooth t ubular endoplasmic reticulum (ER). As differentiation proceeds, cister nae of rough ER form stacks along the wall, losing their ribosomes in the process. Concomitantly, all of the mitochondria, plastids, and ER become parietal in distribution, and the vacuoles collapse. Nuclear de generation is pycnotic and accompanied by the formation of tubular inv aginations of the nuclear envelope into the peripheral chromatin. At m aturity, an anastomosing network of smooth ER borders the plasmalemma, interconnecting aggregates of smooth tubular ER located primarily opp osite the sieve areas. In addition to ER, the mature sieve cell contai ns mitochondria, plastids, and remnants of the degenerate nucleus, all of which are parietal in distribution. P-protein is lacking at all st ages of sieve cell development. Sieve pore and compound median cavity development is similar to that reported for the sieve cells of conifer s. Albuminous cells are associated with the sieve cells of the metaphl oem throughout the shoot but with sieve cells of the protophloem only in the node. Among their cytoplasmic components are broad bundles of m icrofilaments spatially associated with a complex system of rough and smooth ER.