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