Which mechanism primarily drives water movement up the xylem from roots to leaves?

• A. Capillary action
• B. Transpiration pull
• C. Active pumping by roots
• D. Osmotic flow

Answer: (B)

Water moves up the xylem mainly because transpiration from the leaves creates a pull that draws water upward. When water evaporates from stomata, it leaves behind a negative pressure (tension) at the top of the xylem. That tension is transmitted down the continuous chain of water molecules all the way from the leaves to the roots, effectively pulling the entire column upward.

Water molecules stick to each other through cohesion, and they also cling to the walls of the xylem through adhesion, helping to maintain an uninterrupted column that can be pulled up against gravity. This cohesion–tension (transpiration) mechanism is the dominant driver of bulk water transport in most plants, especially taller ones. Other processes, like capillary action, provide some help but are not sufficient on their own to move water long distances. Roots’ uptake is driven by osmosis into root cells, and root pressure can contribute in some cases, but the main ascent of water through the plant is caused by the tension created by transpiration at the leaves.