tracheid

A tracheid is a type of water-conducting cell in the xylem of vascular plants. Tracheids are elongated, tapered cells with thick lignified secondary walls and are dead at maturity. They form long, continuous tubes by stacking end-to-end, allowing water movement through the xylem via openings in the side walls known as bordered pits.

Structure and development: Tracheids develop from cambial or procambial tissue and acquire a robust secondary cell wall enriched in lignin. They lack perforation plates; water moves between adjacent tracheids through bordered pits in the end walls. At maturity, the cytoplasm disintegrates, leaving a hollow lumen that conducts water and minerals.

Function: The primary roles of tracheids are conduction of water and dissolved minerals from roots to shoots and providing structural support to the plant. Their narrow diameter and bordered-pit connections can reduce the risk of cavitation compared with larger conduits, contributing to safe transport under stress conditions.

Distribution and evolution: Tracheids are found in all vascular plants, including lycophytes, ferns, gymnosperms, and many angiosperms. In many angiosperms, vessel elements provide a more rapid, efficient water-conducting pathway, but tracheids remain common, especially in gymnosperms and primitive angiosperms. Evolutionarily, tracheids are considered the ancestral xylem element, with vessel elements representing a later specialization in some lineages.