Comparing structural features of phloem sieve tube and xylem vessel elements

Steps:

• Xylem vessel elements are dead at maturity, lacking cytoplasm and nucleus, with perforated or absent end walls for water flow.
• Phloem sieve tube elements are alive at maturity, containing cytoplasm but lacking a nucleus, with sieve plates as modified end walls for sugar transport.
• Feature 1 (no cytoplasm) applies only to xylem, not phloem.
• Feature 2 (no end walls) applies only to xylem, as phloem has sieve plates; feature 3 (no nucleus) applies to both for efficient conduction.

Why D is correct:

• Mature sieve tube elements and xylem vessel elements both lack nuclei, as enucleation in phloem allows companion cell control while xylem cells die completely, per vascular tissue specialization for transport.

Why the others are wrong:

• A includes 1 and 2, but phloem retains cytoplasm and has end walls.
• B includes 1, but phloem sieve tubes contain cytoplasm.
• C specifies only 2, but phloem has modified end walls as sieve plates.

Final answer: D