On the Ascent of Sap

The article "On the Ascent of Sap" by Dixon, along with related research, explores how fluids move upward in plants. While root pressure contributes, it's insufficient to overcome gravity, especially in tall trees. The primary driving force is the cohesion hypothesis, which explains that water molecules stick together (cohesion) and to the walls of the xylem vessels (adhesion). This creates a strong, continuous column of water. Evaporation from leaves, known as transpiration pull, creates a tension that lifts this water column against gravity. Dixon, with John Joly, developed this tension theory, demonstrating that water transport occurs through passive vessels.123