Lignin, hemicelluloses, and cellulose are the three major chemical components of wood, in approximate proportions of 1:1:2, respectively (Sarkanen, 1971; Higuchi, 1997; Ralph, 1997; Hu, 1999). In the formation of secondary xylem (wood), lignin is deposited between the cells [compound middle lamella (CML), and cell corners (CC)] and in the layers of the secondary cell walls (of the three layers, the S2 layer is predominant; see Figure). Lignin surrounds the preformed hemicelluloses and cellulose to form a strong and hydrophobic secondary wall.
Figure. Chemical imaging of lignin in stem secondary xylem cells of Populus trichocarpa by confocal Raman microscopy (Schmidt et al., 2009). Lignin signal intensity is the strongest in CC, followed by compound middle lamella (CML). Lignin is also distributed in the S2 layer of the cell wall, with a higher concentration in the outer wall (towards CML).
References
Higuchi, T. 1997. Biochemistry and Molecular Biology of Wood. Springer-Verlag, Berlin-Heidelberg-New York.
Hu, W.-J, et al. 1999. Nature Biotechnol. 17:808-812.
Ralph, J., et al. 1997. Science. 277:235-239.
Sarkanen, K. V. 1971. Precursors and their polymerization. pp. 95-163. In Sarkanen, K. V. and Ludwig, C.H. (eds.), Lignins, Occurrence, Formation, Structure and Reactions. Wiley-Interscience, New York, NY.
Schmidt, M., et al. 2009. Planta. 230:589-597.