Tracheid(redirected from tracheidal)
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a dead lignified plant cell that functions in water conduction. Tracheids are found in the xylem of all higher plants except certain angiosperms, such as cereals and sedges, in which the water-conducting function is performed by vessels, or tracheae. Tracheids are usually polygonal in cross section; their walls have annular, spiral, or scalene thickenings or rimmed pores. The cells range in length from fractions of a millimeter to 3–5 mm (pine, larch) and even 10 mm (agave). In the process of evolution, tracheids developed into fibrous tracheids with limited water-conducting ability and into specialized mechanical elements known as libriform fibers.