How water reaches the upper branches of a tree
Water moves upward through a tree's xylem largely because evaporation from leaves creates tension in connected water columns. Water molecules hold together through cohesion, allowing this pull to extend down the transport pathway. The trunk does not contain a heart that pumps water from the roots to the crown.
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A pull transmitted through water
Xylem contains conduits that transport water and dissolved minerals. As water evaporates from leaves, curved water surfaces within the leaf generate tension that draws replacement water upward.
Cohesion between water molecules helps transmit this pull through continuous columns. Water enters from the soil through the roots to replace what is lost.
The main upward driver in a transpiring tree is a pull associated with leaf evaporation. Root pressure can contribute in some plants and conditions, but it cannot explain water transport in all tall trees.
- Leaves lose water to the atmosphere.
- Tension develops in the transport system.
- Connected water columns transmit the pull.
This system has vulnerabilities. Air bubbles can interrupt a conduit, a problem called embolism, and plants differ in how their xylem structures limit or tolerate that risk.