<P> The more recently proposed compensating pressure (CP) theory favors a version of vital theory proposed by Jagdish Chandra Bose . However, experimental evidence has not supported it </P> <P> 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 . </P> <P> 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 . (Disjoining pressure is a difference in pressure from that which prevails in the bulk of a liquid; it is due to the liquid's interaction with a surface . The interaction may result in a pressure at the surface that is greater or less than that which prevails in the rest of the liquid .) 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 (where the film is thickest) and less at the top of the vessel (where the film is thinner). This spatial difference in pressure within the film results in a net force that pushes the sap upwards towards the leaves . </P>

Adhesion and cohesion in the ascent of xylem sap