Answer: Strong cohesive forces between water molecules from hydrogen bonding.
- A Pressure generated by guard cells at the leaf surface under most conditions studied
- B Continuous secretion of mucilage inside xylem vessels in most observed cases
- C Active pumping of water by living xylem vessel cells in standard reference material
- D Strong cohesive forces between water molecules from hydrogen bonding
Correct answer: D. Strong cohesive forces between water molecules from hydrogen bonding
Explanation: Water molecules are held together by hydrogen bonds (cohesion) and adhere to the hydrophilic xylem walls (adhesion), allowing the column to withstand the tension generated by transpirational pull.
Water can travel through cell walls (apoplast) or cell-to-cell through the cytoplasm (symplast) - but the Casparian strip at the endodermis blocks the apoplast route entirely, forcing everything through a selectively permeable membrane (symplast) before it can enter the xylem.
Concept context
Water and mineral absorption, the ascent of sap, transpiration, and phloem transport via the pressure flow hypothesis.