Sieve tube element

In plant anatomy, sieve tube elements, also called sieve tube members, are a specialised type of elongated cell in the phloem tissue of flowering plants. The ends of these cells are connected with other sieve tube members, and together they constitute the sieve tube. The main function of the sieve tube is transport of carbohydrates, primarily sucrose, in the plant (e.g., from the leaves to the fruits and roots). Unlike the water-conducting xylem vessel elements that are dead when mature, sieve elements are living cells. They are unique in lacking a nucleus at maturity.

At the interface between two sieve tube members in angiosperms are sieve plates, pores in the plant cell walls that facilitate transport of materials between them. Each sieve tube element is normally associated with one or more nucleated companion cells, to which they are connected by plasmodesmata (channels between the cells). Each companion cell is derived from the same mother cell as its associated sieve tube member. Sieve tube members have no cell nucleus, ribosomes, or vacuoles. Thus, they depend on companion cells to provide proteins, ATP, and signalling molecules. In leaves, companion cells help move the sugar that is produced by photosynthesis from the mesophyll tissue into the sieve tube elements.

Sieve cells are long, slender, conducting cells of the phloem that do not form a constituent element of a sieve tube, but which are provided with relatively unspecialized sieve areas, especially in the tapering ends of the cells that overlap those of other sieve cells. Sieve cells are typically associated with gymnosperms, because angiosperms have the more derived sieve tube members and companion cells in their phloem. They have a narrower diameter and are more elongated compared to sieve tube members. Sieve cells are associated with albuminous cells (also called Strasburger cells), which lack starch, thus making it possible to differentiate them from phloem parenchyma.

...
Wikipedia