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sieve tube[′siv ‚tüb]
a conducting element of flowering plants. A sieve tube is a monostichous strand stretched the length of the cells; the end walls of the cells bear sieve plates containing sieve areas, which have numerous perforations lined with callose. Simple, usually horizontal plates have one sieve area (pumpkin, ash), whereas complex, slanted plates have several sieve areas (linden, grape, passionflower, rice). Each segment of the sieve tubes is adjoined by a strand of narrow companion cells. As the tubes develop, the tonoplasts in the cells are destroyed, the cytoplasm mixes with the cell fluid, and the organelles and nucleus deteriorate.
Most plants have sieve tubes that function for one year. However, the sieve tubes of grapes function for two years, those of lindens for several years, and those of some palms for decades. At the end of the vegetative period the sieve perforations are completely covered with callose, as are both sides of the sieve plates. The mass of callose on the sieve plates is called definitive callus. After the sieve tubes and their companion cells cease functioning, they disorganize and are obliterated.
L. I. LOTOVA