Xylem

Xylem is the water-conducting vascular tissue of trees, responsible for transporting water and dissolved mineral nutrients from the roots up through the trunk and branches to the leaves. This upward flow is driven primarily by transpiration, the evaporation of water from leaf surfaces, which creates tension that pulls water up through the xylem vessels like liquid through a straw.

In trees, xylem accumulates year after year as wood. Each growing season, the cambium produces a new layer of xylem cells on its inner side, creating the annual growth rings visible in cross-section. Spring wood (early wood) features larger cells for rapid water transport during the growing season, while summer wood (late wood) has smaller, denser cells. Only the most recent annual rings (the sapwood) are actively conducting water; older inner xylem becomes heartwood.

Several devastating tree diseases attack the xylem vascular system. Dutch elm disease, oak wilt, and laurel wilt are all caused by fungal pathogens that colonize xylem vessels, blocking water transport and causing wilting. The tree's own defensive response of forming tyloses (balloon-like plugs in vessels) can actually worsen the blockage. Systemic insecticides and fungicides delivered by trunk injection rely on the xylem transport system to distribute the product throughout the canopy.