Water moves upward through xylem in plants by which action?

• A. Capillary action
• B. Osmotic pressure
• C. Active transport
• D. Bulk flow

Answer: (A)

Capillary action is the key idea here. Water’s cohesion to itself and adhesion to the walls of the narrow xylem vessels create a continuous column that can rise against gravity. In plants, this capillary rise works in concert with the pull generated by transpiration at the leaves, which helps draw the water upward through the xylem. Osmotic pressure involves water crossing membranes due to solute differences, not lifting water up through xylem tubes. Active transport uses cellular energy to move substances, not to raise water. Bulk flow describes movement driven by pressure differences in general, but the specific action that allows water to ascend in the xylem’s narrow tubes is capillary action.