Identifying companion cells in phloem tissue

Steps:

• Observe cell size and location: X appears as a small, nucleated cell adjacent to a larger elongated cell in vascular tissue.
• Recall phloem structure: Phloem transports sugars and consists of sieve tube elements supported by companion cells.
• Distinguish from xylem: X lacks lignified walls and perforation plates typical of water-conducting vessels.
• Rule out unrelated cells: X shows dense cytoplasm and connections via plasmodesmata, matching companion cell function.

Why D is correct:

• Companion cells are specialized parenchyma cells with nuclei and dense cytoplasm that load/unload sugars into adjacent sieve tube elements, as defined in plant vascular anatomy.

Why the others are wrong:

• A: Xylem vessel elements are dead, hollow tubes with thick, lignified walls for water transport, not nucleated.
• B: Stem cells are undifferentiated meristematic cells in growth regions, lacking phloem-specific features.
• C: Phloem sieve tube elements are enucleate, elongated cells with sieve plates, unlike the nucleated X.

Final answer: D