Lignification is the process by which plants deposit lignin into cell walls, making them stronger, more rigid, and more resistant to compression and decay. Lignin is a complex polymer that is added primarily to secondary cell walls after cells have finished expanding.
Lignification is especially important in xylem vessels, fibres, and other structural tissues. The addition of lignin allows stems to remain upright, helps water-conducting tissues withstand the negative pressures generated during transpiration, and increases resistance to mechanical damage. Lignified tissues are typically harder and less flexible than non-lignified tissues.
Lignification also contributes to plant defense. Lignified cell walls are more difficult for many fungi, bacteria, and herbivores to penetrate or digest. Plants may increase lignification around wounds or infection sites as part of their defensive response.
The degree of lignification varies among species and tissues. Young, actively growing tissues generally contain less lignin than mature stems, woody branches, roots, seed coats, and vascular tissues.
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