(botany) A food-conducting cell in the phloem of vascular plants
The phloem is the vascular tissue that carries out the function of translocation (i.e. the process of transporting photosynthate materials from the photosynthetic plant organs to various parts of the plant). The phloem is comprised of the following major components: (1) sieve element, (2) companion cell, (3) phloem sclerenchyma, and (4) phloem parenchyma.
The sieve element refers to the main conductive cell in the phloem. It is a living cell with a protoplast although the nucleus is absent at maturity. There are also fewer organelles. It is a special type of sclerenchyma cell. However, the cell wall is not lignified.
The sieve element is closely associated with a specialized parenchyma cell that assists by providing energy during translocation. In angiosperms, the sieve element is referred to as sieve tube (element) and the cell associated with it is called a companion cell. In gymnosperms and other primitive vascular plants, the sieve element is called a sieve cell and the associated cell is an albuminous cell. Only the sieve tube and the companion cell have an ontogenic relationship, which means that they arise from a common mother cell. The sieve element is connected to the specialized cell (companion cell or albuminous cell) through plasmodesmata. The sieve elements have sieve areas, which are groups of pores. In sieve tubes, the sieve areas are found in the end walls (called sieve plates) whereas in sieve cells, they are mostly located in the lateral walls and rarely in the end walls.