The walls of xylem vessels are strengthened by a substance that also helps hold the whole plant upright. Which substance is it?

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Lignin is laid down in these walls and is stiff enough to stop the tubes collapsing under the pull of the water column, and the same stiffening adds rigidity to the stem as a bonus. Cellulose tempts students because every plant cell wall contains it, but ordinary cell walls stay flexible, so cellulose alone cannot account for the extra stiffness of this tissue.