An extracellular matrix part that consists of lignin in the form of a three-dimensional polymeric network. Lignins are complex racemic aromatic heteropolymers derived from a variety of phenylpropane monomers coupled together by an assortment of carbon-carbon and ether linkages. Lignin is crucial for structural integrity of the cell wall and stiffness and strength of the stem. In addition, lignin waterproofs the cell wall, enabling transport of water and solutes through the vascular system, and plays a role in protecting plants against pathogens.