Trakeids

Tracheids are elongated, tapering cells that serve as the primary water-conducting and supportive elements in the xylem of most vascular plants. They are dead at maturity, with lignified secondary cell walls and no cytoplasm, forming long, hollow tubes that run through stems and roots.

Water moves between tracheids through bordered pits in their walls, enabling lateral transfer and maintaining continuity of the water column even when end walls block direct flow. In many gymnosperms, pits include a torus that can seal against the pit aperture to limit air entry during cavitation.

The patterns of secondary wall thickenings in tracheids vary among species and include annular (ring-like), spiral, scalariform (ladder-like), and reticulate forms. These thickenings provide mechanical strength and some flexibility, helping tracheids withstand tension from transpiration and environmental stress while preserving a continuous conduit for water.

Tracheids are present in all major lineages of vascular plants, including lycophytes, ferns, gymnosperms, and angiosperms. In gymnosperms they are often the dominant water-conducting cells; in many angiosperms, vessel elements provide a more efficient conduit for rapid transport alongside tracheids. Evolutionarily, tracheids represent an early form of xylem, with vessels evolving later in several lineages to increase conduction efficiency.

In addition to transport, tracheids contribute to structural support for stems and roots, and their durability under drought and freezing conditions helps maintain water transport under challenging environmental conditions.