Scientists have proposed various models to explain the ascent of xylem sap from roots to leaves. One model was based on the idea that root pressure pushed the liquid up the stem. Although root pressure does exist, it cannot account for the ascent of sap in trees. The current model of xylem transport relies on an alternative to pushing: pulling. The evaporative loss of water from the leaves indirectly generates a pulling force—tension—on the water in the apoplast of the leaves, which pulls the xylem sap upward.
Consider the magnitude of what xylem accomplishes in transporting a large amount of water over a great distance within the plant. A single maple tree 15 meters tall has been estimated to have some 177,000 leaves, with a total leaf surface area of 675 square meters—half again the area of a basketball court. During a summer day, that tree loses 220 liters of water per hour to the atmosphere by evaporation from the leaves. So to prevent wilting, the xylem needs to transport 220 liters of water from the roots to the leaves every hour. Water can be pulled upward through the tiny tubes in the xylem of plants because of the remarkable cohesion of water.
Textbook Reference: Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension