<P> Transpirational pull results from the evaporation of water from the surfaces of cells in the leaves . This evaporation causes the surface of the water to recess into the pores of the cell wall . By capillary action, the water forms concave menisci inside the pores . The high surface tension of water pulls the concavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches . </P> <P> Transpirational pull requires that the vessels transporting the water be very small in diameter; otherwise, cavitation would break the water column . And as water evaporates from leaves, more is drawn up through the plant to replace it . When the water pressure within the xylem reaches extreme levels due to low water input from the roots (if, for example, the soil is dry), then the gases come out of solution and form a bubble--an embolism forms, which will spread quickly to other adjacent cells, unless bordered pits are present (these have a plug - like structure called a torus, that seals off the opening between adjacent cells and stops the embolism from spreading). </P> <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), Askenasy (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>

The only living tissue in plant xylem is