text: According to cohesion-tension theory, water transport in xylem relies upon the cohesion of water molecules to each other and adhesion to the vessels wall via hydrogen bonding combined with the high water pressure of the plants substrate and low pressure of the extreme tissues (usually leaves).
  As in blood circulation in animals, (gas) embolisms may form within one or more xylem vessels of a plant. If an air bubble forms, the upward flow of xylem water will stop because the pressure difference in the vessel cannot be transmitted. Once these embolisms are nucleated , the remaining water in the capillaries begins to turn to water vapor. When these bubbles form rapidly by cavitation, the "snapping" sound can be used to measure the rate of cavitation within the plant . Plants do, however, have physiological mechanisms to reestablish the capillary action within their cells .