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Vertical spread of decay is resisted by this wall, plugging xylem vessels:

  1. Wall 1

  2. Wall 3

  3. Wall 4

  4. Stomata

The correct answer is: Wall 1

The vertical spread of decay is resisted by Wall 1 because it contains the xylem vessels, which transport water and nutrients throughout the plant. Stomata, labeled as option D, are small openings in the surface of the leaf that allow for gas exchange. While Wall 3 and Wall 4 may play a role in supporting the plant, they do not directly plug the xylem vessels and are therefore not the correct options.