Xylem blockage halts water transport in plants

Steps:

• Water moves up plant stems through xylem via transpiration pull and cohesion.
• Identify factors affecting this pathway: air humidity influences transpiration rate, while physical barriers directly block flow.
• Evaluate options: humidity changes alter evaporation speed but not the conduit itself.
• Blockage in xylem physically obstructs the water-conducting vessels.

Why C is correct:

• Xylem vessels form continuous tubes for water ascent; blockage severs this path, stopping capillary action and cohesion-tension mechanism.

Why the others are wrong:

• A: Higher humidity reduces transpiration, slowing but not stopping water movement.
• B: Lower humidity increases transpiration, potentially accelerating water uptake.
• D: Phloem transports sugars, not water; insect damage there does not affect xylem flow.

Final answer: C