Lignin

Definition
noun
An organic substance that acts as a binder for the cellulose fibers in certain plants adding strength and stiffness to the cell walls
Supplement
Lignin is an organic substance belonging to a group of aromatic alcohols. It is naturally produced by certain plants (especially woody plants and certain algal species). It is found in the secondary cell walls of plants where it serves as a binder for cellulose fibers and provides stiffness to the cell walls. It was first described by the Swiss botanist Augustin Pyramus de Candolle in 1813. He first called it lignine from the Latin word lignum meaning wood.¹ That is because it is present in wood primarily to provide structural support and protection against degradation by microorganisms. It fills the spaces in the cell wall particularly in xylem tracheids, vessel elements and sclereid cells. Since lignin is hydrophobic it assists in the efficient water transport across the vascular tissue. Ecologically, the importance of lignin is associated with the carbon cycle. It takes carbon from the atmosphere into the woody plant tissues and therefore acts as an important reserve of carbon in the form of humus as the plant tissue slowly decomposes.
Word origin: Latin lignum (wood)
See also:

• cell wall

Reference(s):

¹ E. Sjöström (1993). Wood Chemistry: Fundamentals and Applications. Academic Press.