<P> The cohesion - tension theory is a theory of intermolecular attraction that explains the process of water flow upwards (against the force of gravity) through the xylem of plants . It was proposed in 1894 by John Joly and Henry Horatio Dixon . Despite numerous objections, this is the most widely accepted theory for the transport of water through a plant's vascular system based on the classical research of Dixon - Joly (1894), Eugen Askenasy (1845--1903) (1895), and Dixon (1914, 1924). </P> <P> Water is a polar molecule . When two water molecules approach one another, the slightly negatively charged oxygen atom of one forms a hydrogen bond with a slightly positively charged hydrogen atom in the other . This attractive force, along with other intermolecular forces, is one of the principal factors responsible for the occurrence of surface tension in liquid water . It also allows plants to draw water from the root through the xylem to the leaf . </P> <P> Water is constantly lost through transpiration from the leaf . When one water molecule is lost another is pulled along by the processes of cohesion and tension . Transpiration pull, utilizing capillary action and the inherent surface tension of water, is the primary mechanism of water movement in plants . However, it is not the only mechanism involved . Any use of water in leaves forces water to move into them . </P> <P> Transpiration in leaves creates tension (differential pressure) in the cell walls of mesophyll cells . Because of this tension, water is being pulled up from the roots into the leaves, helped by cohesion (the pull between individual water molecules, due to hydrogen bonds) and adhesion (the stickiness between water molecules and the hydrophilic cell walls of plants). This mechanism of water flow works because of water potential (water flows from high to low potential), and the rules of simple diffusion . </P>

Where is the xylem found in a plant