Xyleem

Xyleem is sometimes encountered as a spelling variant of xylem, the vascular tissue in plants that conducts water and dissolved minerals from roots to shoots and provides structural support through lignified cell walls. It is a fundamental component of the vascular system in higher plants.

Xylem is composed mainly of dead, hollow cells at maturity that form conduits: tracheids and vessel elements. Tracheids are long, tapered cells with bordered pits; vessel elements are shorter and wider, aligning end-to-end to form vessels, a feature common in many angiosperms. Xylem also contains living parenchyma and ray cells, which store nutrients and facilitate lateral transport within the tissue.

Water transport occurs through the xylem by the cohesion-tension mechanism. Transpiration from leaves generates a negative pressure that pulls the water column upward; cohesion between water molecules and adhesion to hydrophilic xylem walls help maintain a continuous water column as it moves through pits in the cell walls. The secondary cell walls are lignified, providing mechanical strength to resist collapse under tension.

Developmentally, xylem includes primary xylem formed during embryogenesis and primary growth, and secondary xylem produced by the vascular cambium during secondary growth. The latter accumulates as sapwood, which conducts water, while older tissue becomes heartwood and loses conductive function. Growth rings in temperate species reflect yearly deposition of xylem tissue.

Xylem occurs in all vascular plants and shows variation in conduit type and arrangement among lineages, with vessels more prominent in many angiosperms and tracheids predominating in gymnosperms.