A sieve-tube is a characteristic element of phloem, which functions in the transport of food materials, such as sugars and proteins, synthesized within a plant. A sieve-tube consists of elongated, thin-walled, living cells arranged in a longitudinal row and forming a connected series by means of perforations in their walls through which pass strands of cytoplasm. These perforations occur in groups known as sieve-plates in the end or lateral walls.
Sieve-tubes are remarkable in that their cells lack nuclei when mature. They are closely associated with parenchymatous companion cells with prominent nuclei.