xylem

Xylem is one of the two transport tissues in vascular plants, the other being phloem. It functions primarily to transport water and dissolved minerals from roots to aerial parts and provides mechanical support through its lignified cell walls. In most plants, xylem also stores some nutrients in parenchyma cells.

Xylem consists of several cell types. The conducting elements are tracheids and vessel elements; both are typically dead at maturity and form hollow tubes that allow water flow. Tracheids are long, spindle-shaped cells with tapered ends and pits; vessel elements are shorter, wider cells that align end-to-end to form vessels with perforation plates. In gymnosperms, tracheids predominate; many angiosperms possess vessels in addition to tracheids. Xylem fibers provide structural support, and xylem parenchyma stores and transports nutrients within the tissue. The cell walls are heavily lignified.

Water movement through xylem is driven largely by transpiration from leaves and the cohesion-tension mechanism. Evaporation creates negative pressure, pulling water upward; cohesion among water molecules helps sustain continuous columns. Root pressure and capillary action can contribute locally, but are insufficient to explain long-distance ascent.

Xylem is produced by the vascular cambium in woody plants and by primary meristems during primary growth. In mature stems and roots, it forms the bulk of wood and contributes to the plant’s support system. Its arrangement and composition vary among plant groups and developmental stages.