Tracheids

Tracheids are elongated cells that form part of the xylem in all vascular plants. They function as water-conducting and supporting elements and are dead at maturity, leaving behind a lignified cell wall with a hollow lumen.

Structurally, tracheids are tapered at the ends and align end to end to create continuous conduits. The secondary walls often display thickening patterns such as annular, spiral, scalariform, or reticulate. Water moves between tracheids mainly through pits in the cell walls; these bordered pits connect adjacent cells and allow lateral water flow. In many gymnosperms, bordered pits can include a torus that can occlude the pit aperture to limit cavitation.

Distribution and development vary among plant groups. Tracheids are present in the xylem of all vascular plants, including ferns, conifers, and many angiosperms. In gymnosperms and many primitive plants, tracheids are the principal or sole conductive cells, whereas many angiosperms also possess vessel elements that form larger, perforated conduits; nevertheless, tracheids remain important in many species for conduction and structural support. Tracheids arise from procambial cells during xylem development; after the protoplast dies, the cell becomes hollow and the thickened wall matures.

Evolution and significance: tracheids are among the most ancient xylem components and provide both hydraulic conduction and mechanical support. Their narrow diameter and interconnected pit system help limit rapid spread of embolisms, contributing to drought tolerance in some species. The combination of tracheids and, in many plants, vessel elements reflects an evolutionary balance between safety and efficiency in water transport.