Adhesion occurs when one type of molecule is attracted to a different kind of molecule. Water exhibits adhesive properties in the presence of polar surfaces—like glass or cellulose in plants. Regarding glass, the positively charged hydrogen molecules in 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 water, the water molecules bind to the tube surface, and the water level inside the tube rises. The smaller the tube diameter, the farther the water rises, because more water molecules are exposed to the glass surface. Capillary action continues as long as the adhesive force is greater than the pull of gravity.
Plants use the adhesion of capillary action and cohesion between water molecules to move water up from the roots to the leaves. In plants, xylem vessels consist of long, narrow cells called tracheary elements, which transport water. Because water molecules have an attraction to cellulose, they cling to the xylem cell wall. Cohesive forces between water molecules also attract the water molecules to each other. Together, these forces of adhesion and cohesion create a column of water molecules that gradually moves up the xylem vessels.
