The ascent of sap in the xylem tissue of plants is the upward movement of water and minerals from the root to the crown. Xylem is a complex tissue consisting of living and non-living cells. The conducting cells in xylem are typically non-living and include, in various groups of plants, vessels members and tracheids. Both of these cell types have thick, lignifiedsecondary cell walls and are dead at maturity. Although several mechanisms have been proposed to explainthe phenomenon, the cohesion-tension mechanism has the most evidence and support. Although cohesion-tension has received criticism, for example due to the apparent existence of large negative pressures in some living plants, experimental and observational data favor this mechanism. The more recently proposed compensating pressure theory favors a version of vital theory proposed by Jagdish Chandra Bose. However, experimental evidence has not supported it
An alternative theory based on the behavior of thin films has been developed by Henri Gouin, a French professor of fluid dynamics. The theory is intended to explain how water can reach the uppermost parts of the tallest trees, where the applicability of the cohesion-tension theory is debatable. The theory assumes that in the uppermost parts of the tallest trees, the vessels of the xylem are coated with thin films of sap. The sap interacts physically with the walls of the vessels: as a result of van der Waals forces, the density of the film varies with distance from the wall of a vessel. This variation in density, in turn, produces a "disjoining pressure", whose value varies with distance from the wall. As a tree's leaves transpire, water is drawn from the xylem's vessels; hence, the thickness of the film of sap varies with height within a vessel. Since the disjoining pressure varies with the thickness of the film, a gradient in the disjoining pressure arises during transpiration: the disjoining pressure is greater at the bottom of the vessel and less at the top of the vessel. This spatial difference in pressure within the film results in a net force that pushes the sap upwards towards the leaves.
