Transpiration aids in pulling water up through the plant by causing

• A. Water absorbed by the roots from the soil.
• B. Carbon dioxide uptake for photosynthesis.
• C. Oxygen production in leaves.
• D. The loss of water vapor from leaves that helps pull water up through the plant.

Answer: (D)

Transpiration drives the ascent of water through the plant by creating a suction in the leaf’s water column. When water evaporates from the surface of the leaves (through the stomata), it leaves behind a low pressure, or negative tension, in the xylem. Water molecules are highly cohesive, so this tension pulls more water up from the roots, and adhesion helps the column cling to the xylem walls as it moves upward. This pull is what moves water from the roots all the way to the leaves to replace what was lost as vapor.

The other processes listed aren’t the mechanism for pulling water upward. Water uptake from the soil supplies the water, but transpiration itself is the force that generates the upward pull. Carbon dioxide intake for photosynthesis and oxygen production are separate metabolic processes, not the physical cause of the water column being drawn upward.