Adhesion occurs when one type of molecule is attracted to a different molecule. Water exhibits adhesive properties in the presence of polar surfaces, such as glass or cellulose in plants. For instance, when water is poured into a glass, the positively charged hydrogen molecules of water are more attracted to the negatively charged oxygen molecules in the silica than to the oxygen in neighboring water molecules.
Capillary action is a result of water’s adhesive tendencies. When a narrow glass tube is inserted into a beaker filled with water, the water molecules attach to the surface of the tube, and the water level inside the tube rises. The smaller the tube diameter, the farther the water rises as more water molecules are exposed to the glass surface. Capillary action continues as long as the adhesive force is greater than the cohesive forces between the water molecules.
In plants, adhesion and cohesion allow water molecules to move up from the roots to the leaves via transpiration. Plants contain xylem vessels that consist of long, narrow cells called tracheary elements, which transport water. As water molecules exhibit attraction to cellulose, they cling to the xylem cell wall and resist the downward pull of gravity. Cohesive forces between water molecules allow attraction between the water molecules within the tracheal cells. Together, these dynamic forces of adhesion and cohesion create a column of water molecules. The adhesion of water to cellulose is greater than the cohesive forces, enabling the water to move upward from the roots to the leaves.