Sieve tube | definition of sieve tube by Medical dictionary
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sieve tube a series of long cells (sieve cells) lying end to end forming a tube, found in the PHLOEM of ANGIOSPERMS. Each sieve cell loses its nucleus when mature, but maintains the cytoplasm, which is continuous from one cell to another and crosses the cell boundaries via perforations in the wall called sieve plates. Associated with the sieve tube are modified PARENCHYMA cells each with a cytoplasm, nucleus and many MITOCHONDRIA, called companion cells. It is considered that the companion cells may regulate TRANSLOCATION of materials within sieve tubes since there are cytoplasmic connections between the sieve cells, and the companion cells and the sieve cells are anucleate.
References in periodicals archive
How a cell could function without a nucleus has long puzzled scientists, and we now believe that the close association of a sieve tube
member with a companion cell allows for its regulation.
Vessels and sieve tubes
are the vascular elements responsible for the transport of water, inorganic salts, and organic nutrients in the xylem and phloem, respectively, in plants.
It appears that branching and dropping of vascular bundles along the rachis was accompanied by a reduction in total vascular size as well as cross-sectional surface area of xylem elements and sieve tubes
in the bundle at successive internodes.