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.
The current model of xylem transport—called the transpiration–cohesion–tension mechanism—relies on the evaporative loss of water from leaves to pull water up the plant.
The process begins at the leaves. The concentration of water vapor in the atmosphere is lower than in the intercellular spaces of the leaf. Because of this difference, water vapor diffuses from the intercellular spaces, through the stomata to the outside air, in a process called transpiration.
Within the leaf blade, water evaporates from the moist walls of the mesophyll cells and enters the intercellular spaces. As water evaporates from the aqueous film coating each cell, the film shrinks back into tiny spaces in the cell walls, increasing the curvature of the water surface and thus increasing its surface tension.
The increased tension in the surface film draws more water into the cell walls, replacing that which was lost. The resulting tension in the mesophyll draws water from the xylem of the nearest vein into the apoplast surrounding the mesophyll cells.
The removal of water from the veins, in turn, establishes tension on the entire column of water contained within the xylem, so that water is drawn upward all the way from the roots.
Water can be pulled upward through tiny tubes because of the remarkable cohesion of water—the tendency of water molecules to stick to one another by hydrogen bonding. The integrity of the column is also maintained by the adhesion of water to the xylem walls. Overall, the narrower the tube, the greater the tension the water column can withstand without breaking.
In summary, the key elements of water transport in the xylem are: the transpiration of water molecules from the leaves by evaporation; the tension in the xylem sap resulting from transpiration from the leaves; and cohesion of water molecules in the xylem sap, from the leaves to the roots. At each step between soil and atmosphere, water moves passively, requiring no expenditure of energy on the part of the plant.
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.