Adaptations for Phloem and Xylem Transport

Steps:

• Identify companion cells' role in phloem: they support sieve tubes by providing energy for active loading of sugars.
• Note xylem vessel elements' role: they form dead, hollow tubes for passive water transport under tension.
• Evaluate options for accurate features: look for energy supply in companion cells and unobstructed flow in xylem.
• Select option matching these functions without errors.

Why D is correct:

• Companion cells have numerous mitochondria for ATP production to drive active transport (definition of active loading in phloem); xylem vessels lack cytoplasm and end walls, enabling mass flow of water (cohesion-tension theory).

Why the others are wrong:

• A: Companion cells transport bidirectionally with sieve tubes; xylem lignification provides support, not directional transport.
• B: Companion cells aid transport, not support; xylem vessels lack nuclei as they are dead cells.
• C: Companion cells have nuclei for protein synthesis, not nucleoli support; xylem has perforation plates, not random gaps.

Final answer: D