Final answer:
Water sticks to the sides of the inside of a stem due to adhesion, which is the attraction between water molecules and other molecules. Adhesion, in combination with capillarity, facilitates the upward movement of water within the plant's xylem vessels against gravity.
Step-by-step explanation:
Water adheres to the sides of the inside of a stem by a process known as adhesion. In the context of plant biology, when water moves up from the roots of plants to the leaves as a result of capillary action, it is primarily because of adhesion, the attraction between water molecules and other types of molecules, such as those found in plant cell walls. This attraction allows the water to "climb" against gravity within the narrow spaces of the xylem vessels found in the stem.
Another property related to adhesion is capillarity, which is the ability of a liquid to flow in narrow spaces without the assistance of external forces like gravity. This occurs due to the adhesive forces between the liquid and surrounding solid surfaces. In plants, capillarity contributes to the overall movement of water through the xylem, enabling it to reach great heights against gravity.
While cohesion is the attraction between similar molecules (water molecules in this case), which contributes to water's surface tension and helps with the continuous columns of water within the plant, adhesion is key for the initial adhesion of water to cell walls that aids in the upward movement of water within plants.