Heat disrupts xylem via cavitation, spares phloem temporarily

Steps:

• Heat stem to 60°C for 10 min at 1m height, targeting vascular tissues.
• Phloem transport uses living sieve tubes with pressure-flow; short heat exposure does not fully denature proteins, so sap continues.
• Xylem transport is passive through dead vessels; heat vaporizes water, causing air embolisms that block flow.
• Result: Phloem flow persists between roots/leaves, xylem halts.

Why C is correct:

• Xylem sap stops due to thermal cavitation (gas bubble formation in vessels per cohesion-tension theory), while phloem continues via resilient osmotic gradient.

Why the others are wrong:

• A: Fails to account for xylem blockage by embolisms.
• B: Incorrectly assumes phloem stops, ignoring sieve tube heat tolerance.
• D: Wrongly predicts phloem halt despite its brief resilience.

Final answer: C