Xylem

The xylem is a complex vascular tissue consisting of fibrous and woody elongated cells that carry water and dissolved minerals from the roots through the stem and leaves. It helps support the plant and is the main constituent of wood. Wood, or secondary xylem, is derived from vascular cambium, and its formation encompasses a series of developmental processes. The xylem comprises dead cells with thickened cell walls rich in cellulose, hemicelluloses, and lignin. It is responsible for providing mechanical support and conducting water and minerals for the plant.

Xylem parenchyma cells are characterised by simple pits, where numerous plasmodesmata are localised. Thus, they are well connected and form a 3D network essential for water and solute transport and storage. The living parenchyma cells in the xylem are responsible for storing and transporting nutrients and water throughout the plant.

As the tree ages and the vascular cambium layer produces new sapwood, the inner xylem cells compact and harden, so they are no longer able to transport water. They form the central support column known as heartwood, composed chiefly of cellulose fibers bound together by lignin. Softwood trees, like willows, poplars, and usually conifers, do not have heartwood.

In summary, the xylem is a complex vascular tissue that carries water and dissolved minerals from the roots through the stem and leaves. It is the main constituent of wood, which is derived from vascular cambium. Xylem parenchyma cells are important for water and solute transport and storage. In contrast, the living parenchyma cells in the xylem are responsible for storing and transporting nutrients and water throughout the plant. As the tree ages, the inner xylem cells compact and harden to form heartwood, composed mostly of cellulose fibers bound together by lignin.