This critical review discusses how lignin can fulfill its biological functions with a focus on the potential importance of covalent bonds between lignin and polysaccharides. Biological function and definition of lignin are discussed. There are several direct and indirect indications that covalent bonds between lignin and polysaccharides are common in wood, and there are mechanistic explanations for the formation of these bonds, with the exception of phenyl glycosides. Grasses might have a unique pathway for formation of links between lignin and hemicellulose. The monolignol structures might be evolutionary "designed" for forming covalent bond to polysaccharides during polymerization, and hemicelluloses might have the ability to some extent control the lignin structure and frequency of covalent bonds between lignin and polysaccharides. These bonds represent both technical problems and possibilities.