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How does xylem transport water through a tree?

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Tall giant sequoia trees with reddish bark and thick trunks rise closely together in a dense forest at Sequoia National Park, California. — tree A stand of giant sequoia trees (*Sequoiadendron giganteum*) in Sequoia National Park, California.

In trees, water is transported from the roots to the leaves through xylem, a specialized vascular tissue composed of hollow, elongated cells known as tracheids and vessel members.1

The primary mechanism for this movement is the cohesion theory, also known as the transpiration-cohesion hypothesis. This process relies on two key physical principles:23

Transpiration Pull: As water evaporates from the leaves into the atmosphere, it creates a gradient of decreasing pressure that pulls water upward through the tree.23
Cohesion and Adhesion: Water molecules are held together by strong cohesive forces, forming a continuous column within the xylem. Simultaneously, adhesive forces between the water molecules and the walls of the xylem vessels prevent the column from breaking, allowing it to be lifted to great heights.23

While root pressure can occasionally push water up the stem—particularly in early spring or under specific environmental conditions—the vast majority of water transport in trees is driven passively by the tension created by transpiration.2