Lignin is highly resistant towards chemical and biological
degradation, and confers mechanical resistance to wood.
The highest concentration of this recalcitrant polymer is
found in the middle lamella, where it acts as a cement
between wood fibers, but it is also present in the layers of the
cell wall (especially the secondary cell-wall), forming,
together with hemicelluloses, an amorphous matrix in which
the cellulose fibrils are embedded and protected against
biodegradation.
Lignin composition in terms of the H:G:S ratio varies between different vascular plant groups.
Woody gymnosperms (softwoods) have the highest lignin
content, and their lignin is made up mostly of G units. By
contrast, lignin of woody angiosperms (hardwoods) consists
of S and G units, and that from non-woody angiosperms contains
also H units. Lignin composition between the different
wood tissues and cell-wall layers also varies. For example,
middle-lamella lignin typically has a lower S/G ratio than
lignin from the secondary wall.