noun, plural: sieve tubes
A sieve element is the main conductive component of the phloem. It is closely closely associated with a special type of parenchyma cell. In angiosperms, the sieve element is referred to as sieve tube (element) and the cell associated with it is referred to as a companion cell. In gymnosperms and other primitive vascular plants, the sieve element is a sieve cell and the associated cell is called an albuminous cell.
A sieve tube is a tube made up of sieve-tube elements joined end-to-end. Therefore, it is a series of cells forming a tube through which the photosynthate materials flow through. Each cell is referred to as sieve-tube element (or sieve-tube member). It is considered the more advanced type of sieve element, especially in terms of sieve areas. A sieve area is a group of pores that allow the passage of materials between sieve-tube elements. The sieve area in sieve tube is found in the end walls where it is referred to as a sieve plate. There are two types of sieve plates: simple and compound. A simple sieve plate has only one sieve area whereas a compound sieve plate has more than one. Through these sieve areas, the protoplasts between sieve tube elements are uninterrupted. Translocation is carried out through cytoplasmic streaming.