Question

What is xylem?

Answer 1

Xylem is the plant vascular tissue that conducts water and dissolved mineral nutrients from the roots up through the stem to the leaves and other parts. It also provides structural support.

Key points
- Function: long-distance transport of water and minerals (upward) and mechanical support; storage in some cells.
- Main cell types:
- Tracheids: elongated, lignified cells that conduct water (found in all vascular plants).
- Vessel elements: shorter, wider, stacked cells with open end walls forming continuous tubes (common in flowering plants); these conduct water more efficiently than tracheids.
- Xylem fibers: provide strength.
- Xylem parenchyma: living cells that store substances and help lateral transport.
- Most conducting xylem cells (tracheids and vessel elements) are dead at maturity and have thick, lignified cell walls.
- Transport mechanism: primarily the cohesion–tension mechanism driven by transpiration from leaves, which pulls a continuous water column upward; root pressure and capillarity can also contribute.
- Types: primary xylem (formed during primary growth) and secondary xylem (wood, formed by vascular cambium during secondary growth; produces growth rings).

Contrast with phloem: xylem moves water/minerals (mostly dead cells, generally upward); phloem moves sugars and signaling molecules (living cells, can move in multiple directions).