Hydrogen bonding drives cohesion in xylem water transport

Steps:

• Identify key processes in xylem: adhesion (water to vessel walls), cohesion (water molecules to each other), evaporation (from leaves), hydrostatic pressure (root push).
• Recall hydrogen bonding: strong attraction between water molecules via partial charges on oxygen and hydrogen.
• Link to cohesion: it maintains continuous water column in xylem via these bonds, enabling transpiration pull.
• Eliminate others: adhesion involves bonds to walls, not water-water; evaporation is phase change; pressure is mechanical force.

Why B is correct:

• Cohesion depends on hydrogen bonding, which creates tensile strength in water columns, as per the cohesion-tension theory of xylem transport.

Why the others are wrong:

• A: Adhesion relies on bonds between water and hydrophilic xylem walls, not water molecules.
• C: Evaporation is a physical process driven by heat and vapor pressure gradient, independent of hydrogen bonding.
• D: Hydrostatic pressure is generated by root osmosis, a mechanical force without direct hydrogen bonding involvement.

Final answer: B