<P> The lignols that crosslink are of three main types, all derived from phenylpropane: 4 - hydroxy - 3 - methoxyphenylpropane, 3, 5 - dimethoxy - 4 - hydroxyphenylpropane, and 4 - hydroxyphenylpropane . The former tends to be more prevalent in conifers and the latter in hardwoods . </P> <P> Lignin is a cross-linked polymer with molecular masses in excess of 10,000 u . It is relatively hydrophobic and rich in aromatic subunits . The degree of polymerisation is difficult to measure, since the material is heterogeneous . Different types of lignin have been described depending on the means of isolation . </P> <P> Three monolignol monomers are precursors, all of which are methoxylated to various degrees: p - coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Figure 3). These lignols are incorporated into lignin in the form of the phenylpropanoids p - hydroxyphenyl (H), guaiacyl (G), and syringyl (S), respectively . Gymnosperms have a lignin that consists almost entirely of G with small quantities of H. That of dicotyledonous angiosperms is more often than not a mixture of G and S (with very little H), and monocotyledonous lignin is a mixture of all three . Many grasses have mostly G, while some palms have mainly S. All lignins contain small amounts of incomplete or modified monolignols, and other monomers are prominent in non-woody plants . </P> <P> Lignin fills the spaces in the cell wall between cellulose, hemicellulose, and pectin components, especially in vascular and support tissues: xylem tracheids, vessel elements and sclereid cells . It is covalently linked to hemicellulose and therefore cross-links different plant polysaccharides, conferring mechanical strength to the cell wall and by extension the plant as a whole . It is particularly abundant in compression wood but scarce in tension wood, which are types of reaction wood . </P>

Lignin is the most abundant macromolecule on earth