Tracheid walls are festooned with pits. The functional diameter of vessels is greater than that of tracheids. Water is pulled up the xylem by the force of transpiration , water loss from leaves. Mature corn plants can each transpire four gallons of water per week. Transpiration rates in arid-region plants can be even higher.
Water molecules are hydrogen bonded to each other.
Water lost from the leaves causes diffusion of additional water molecules out of the leaf vein xylem, creating a tug on water molecules along the water columns within the xylem. The loss of water from the root xylem allows additional water to pass throught the endodermis into the root xylem. Cohesion is the ability of molecules of the same kind to stick together. Water molecules are polar, having slight positive and negative sides, which causes their cohesion. Inside the xylem, water molecules are in a long chain extending from the roots to the leaves.