Xylem vessels appear as adjacent hollow rings in transverse section

Steps:

• Xylem vessels form continuous tubes from stacked dead cells with perforated end walls.
• In a stained transverse stem section, vessels show as circular or polygonal lumens surrounded by lignified walls.
• Adjacent vessel elements align in radial files, appearing side-by-side without visible end walls.
• Correct drawing depicts four uniform, hollow structures with thick walls, matching vessel anatomy.

Why D is correct:

• It shows four adjacent, hollow polygonal rings with thickened lignified walls, aligning with the definition of xylem vessels as empty conduits in cross-section.

Why the others are wrong:

• A: Depicts solid-filled centers, but vessels are hollow due to cell death and protoplast loss.
• B: Shows irregular, non-aligned shapes, violating the uniform radial arrangement of vessel files.
• C: Includes thin-walled cells with contents, resembling phloem sieve tubes rather than lignified xylem.

Final answer: D