Identifying vascular cells by structural features in plant stems

Steps:

• Examine cell walls: Thick, lignified walls indicate xylem vessel elements (P).
• Check for sieve plates and enucleate cytoplasm: These define phloem sieve tube elements (Q).
• Look for small, dense, nucleated cells adjacent to sieve tubes: These are companion cells (R).
• Confirm positions: Xylem typically central, phloem peripheral in stems.

Why D is correct:

• Xylem vessel elements have perforated ends and lignified walls for water transport; phloem sieve tube elements form tubes with companion cells for sugar conduction.

Why the others are wrong:

• A: Mislabels Q as phloem tube (should be sieve tube) and R as xylem (companion cells are phloem).
• B: Swaps Q and R; xylem vessels aren't companion cells, and sieve tubes aren't xylem.
• C: Reverses P and Q; companion cells aren't sieve tubes.

Final answer: D