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How does water travel from tree roots to leaves through xylem?

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temperate softwoods and hardwoods selected to show variations — wood Temperate softwoods (left column) and hardwoods (right column), selected to highlight natural variations in colour and figure: (A) Douglas fir, (B) sugar pine, (C) redwood, (D) white oak, (E) American sycamore, and (F) black cherry. Each image shows (from left to right) transverse, radial, and tangential surfaces. Click on an individual image for an enlarged view.

Water travels from a tree's roots to its leaves primarily through the xylem via a process known as the cohesion hypothesis (or the transpiration-cohesion hypothesis).12

This movement is driven by transpiration, the evaporation of water from the leaves through stomata. As water evaporates, it creates a negative pressure, or "transpiration pull," that draws water upward through the plant's vascular system. This process relies on two critical physical properties:132
* Cohesion: Water molecules stick together, allowing them to form a continuous, unbroken column within the xylem vessels.12
* Adhesion: Water molecules stick to the walls of the xylem tracheids and vessels, which helps support the column against gravity.12

While root pressure—caused by the active uptake of ions and osmotic water entry—can push water upward, it is generally insufficient to move sap to the tops of tall trees. Consequently, the passive pull generated by transpiration remains the primary mechanism for water transport in trees.14