Xylem conduits in plants transport water from soil to leaves through a pulling force generated when water evaporates at the surface of leaves creating a negative pressure gradient.


"The transport system that drives sap ascent from soil to leaves is extraordinary and controversial. More than a century ago, H. H. Dixon (1896) proposed that a pulling force was generated at the evaporative surface of leaves and that this force was transmitted downward through water columns under tension to lift water much like a rope under tension can lift a weight. The cohesion–tension theory (C–T theory), as it is known, supposes both adhesion of water to conduit walls and cohesion of water molecules to each other." (Tyree 2003: 923)

Journal article
Plant hydraulics: The ascent of waterNatureFebruary 23, 2017
Melvin T. Tyree