Which process primarily drives water movement in the xylem from roots to leaves?

• A. Root pressure
• B. Capillary action
• C. Cohesion-tension from transpiration pull
• D. Active pumping by root hairs

Answer: (C)

Water movement in the xylem from roots to leaves is driven primarily by cohesion-tension created by transpiration. When water evaporates from the leaf surfaces, it generates a pull, or negative pressure, in the leaf xylem. Because water molecules cling to each other (cohesion) and to the walls of the xylem (adhesion), this pull is transmitted down the entire water column, lifting water upward from the roots. This cohesive, tension-driven pull can move water through tall plants as long as the soil can supply water and the leaf transpiration rate is sustained.

Root pressure can contribute a small upward push, especially in some plants or conditions, but it isn’t the main force moving water up the plant. Capillary action and mere capillary rise are insufficient to explain ascent in tall vegetation. Water transport in the xylem is passive, powered by evaporation and the physical properties of water, not by active pumping by root hairs.