process in which cell walls become thicker and stronger by impregnation with lignin.

lignification (Wikipedia)
An example of a possible lignin structure.

Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically, lignins are polymers made by cross-linking phenolic precursors.

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