Cohesion-tension theory drives xylem transport

Steps:

• Recall that water transport in xylem relies on transpiration pull from leaves.
• Identify key molecular properties: cohesion (water molecules stick together), adhesion (water sticks to xylem walls), and polarity (enables hydrogen bonding).
• Link these to upward flow: cohesion maintains water column continuity, adhesion prevents collapse, polarity supports bonding.
• Confirm all three are essential for the process.

Why A is correct:

• Cohesion-tension theory states that transpiration creates tension, pulling cohesive water columns upward via adhesion to xylem vessels, with polarity enabling these interactions.

Why the others are wrong:

• B omits polarity, which underlies hydrogen bonding for cohesion and adhesion.
• C omits adhesion, needed to counter gravity and vessel collapse.
• D omits cohesion, essential for continuous water column under tension.

Final answer: A