LIGNIN

n. complex polymer which combines with cellulose to form the woody cell walls of plants (Biology)

Lignin (sometimes "lignen") is a complex chemical compound most commonly derived from wood and an integral part of the cell walls of plants. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood. It is the most abundant organic polymer on Earth after cellulose, employing 30% of non-fossil organic carbon and constituting from a quarter to a third of the dry mass of wood. The compound has several unusual properties as a biopolymer, not least its heterogeneity in lacking a defined primary structure.
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Lignin, n the heteropolysaccharides contained in the cell walls of plants that provides dietary fiber for digestion. [more]Lignin - Community and Resources

Noun
1. a complex polymer; the chief non-carbohydrate constituent of wood; binds to cellulose fibers to harden and strengthen cell walls of plants
(hypernym) polymer
(substance-holonym) wood

(n.)
A substance characterizing wood cells and differing from cellulose in its conduct with certain chemical reagents.