QUOTE : Lignin is a linkage of three aromatic alcohols—coumaryl, coniferyl, and sinapyl—which fill the spaces in cell walls that are not already occupied by other substances, even ousting water molecules to do so. It thus forms a very strong hydrophobic net, cementing all the cell-wall elements in place and providing strength and rigidity to the xylem. It also provides an important barrier to fungal and bacterial infections. When a tree is invaded by disease, it seals off the infected section with a wall of lignin so that the disease cannot spread. Lignin is so tough that getting rid of it is a costly process in pulp-and-paper plants. The acids needed to break down lignin in pulpwood are the chief pollutants such mills contribute to the environment.