Lignin is a complex organic polymer deposited within plant cell walls that provides rigidity, strength, and resistance to compression. It is especially abundant in woody tissues, stems, roots, bark, and the water-conducting cells of xylem, where it helps plants remain upright and transport water under tension without the vessels collapsing.
Lignin is formed from phenolic compounds produced by the plant and becomes integrated into cell walls after the cells have largely finished expanding. As lignin accumulates, tissues become tougher, more durable, and less flexible. This is one reason young shoots are soft while older stems become woody.
Because lignin is highly resistant to decomposition, it breaks down much more slowly than cellulose and other plant carbohydrates. Lignin-rich materials such as wood chips, bark, and mature stems persist longer in soils and contribute to long-term organic matter formation.
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