Finally these forces are communicated to water molecules within the xylem because each water molecule is bound to the next by hydrogen bonds . Water Potential and the leaf Evaporation from the leaf sets up a water potential gradient between the outside air and the leaf's air spaces. The gradient is transmitted into the photosynthetic cells and on to the water-filled xylem in the leaf vein. Water Movement in Xylem through TACT Mechanism Four important forces combine to transport water solutions from the roots, through the xylem elements, and into the leaves. The previous section describes transpiration more fully.
Adhesion is the attractive force between water molecules and other substances.
Because both water and cellulose are polar molecules there is a strong attraction for water within the hollow capillaries of the xylem. Cohesion is the attractive force between molecules of the same substance. Water has an unusually high cohesive force again due to the 4 hydrogen bonds each water molecule potentially has with any other water molecule. It is estimated that water's cohesive force within xylem give it a tensile strength equivalent to that of a steel wire of similar diameter. This is called capillary action.