Wood Ultrastructure

The wood cell consists mainly of cellulose, hemicelluloses, and lignin (see Appendix). A simplified picture is that cellulose forms a skeleton which is surrounded by other substances functioning as matrix (hemicelluloses) and encrusting (lignin) materials. 

The microfibrils, which are 10-20 nm wide, are visible in the electron microscope without pretreatment. Microfibrils are combined to greater fibrils and lamellae, which can be separated from the fibers mechanically, although their dimensions greatly depend on the method used. 

Disordered cellulose molecules as well as hemicelluloses and lignin are located in the spaces between the microfibrils. The hemicelluloses are considered to be amorphous although they apparently are oriented in the same direction as the cellulose microfibrils. Lignin is both amorphous and isotropic. 

The cell wall is built up by several layers, namely (Figs. 1-12 and 1-13), middle lamella (M), primary wall (P), outer layer of the secondary wall (SJ, middle layer of the secondary wall (S2), inner layer of the secondary wall (S3), and warty layer (W). These layers differ from one another with respect to their structure as well as their chemical composition. The microfibrils wind around the cell axis in different directions either to the right (Z helix) or to the left (S helix). Deviations in the angular directions cause physical differences and the layers can be observed in a microscope under polarized light. 

The middle lamella is located between the cells and serves the function of binding the cells together. At an early stage of the growth it is mainly composed of pectic substances, but it eventually becomes highly lignified. Its thickness, except at the cell corners, is 0.2-1.0 /tm. The primary wall is a thin layer, 0.1 -0.2 jam thick, consisting of cellulose, hemicelluloses, pectin, 

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D. Eengel and G. Wegener, in Wood, Ultrastructure, Reactions, DeGmyter, Berlin, 1983, Chapt. 5. 

Source Reproduced from Wood Ultrastructure , W.A. Cote Jr., University of Washington Press, Syracuse, NY, 1967). 

Stevens, M. and Parameswaran, N. (1981a). Effect of formaldehyde-acid catalysed reactions on wood ultrastructure. Wood Science and Technology, 15(4), 287-300. 

Cote, W. A., Jr. (1967). "Wood Ultrastructure." Univ. of Washington Press, Seattle. 

Cote WA (1967) Wood ultrastructure an atlas of electron micrographs. University of Washing Press, Seattle, Washington... 

Green F, Larsen M, Highley TL. Ultrastructural morphology of the hyphal sheath of wood-decay fungi modified by preparation for scanning electron microscopy, in Biodeterioration Research, Vol. Ill, Plenum Press, New York, 1990. 

For a detailed description of the ultrastructure of wood and the cell wall, the reader is referred to the comprehensive texts listed above. Briefly, the cell wall of wood is composed of a number of discernable layers (Figure 2.2). These are divided into the primary (P) and secondary (S) layers the secondary layer is further subdivided into the Sj, S2 and S3 layers. The primary layer is the first to be laid down when the cell is formed and is composed of microfibrils, which have an essentially random orientation that allows for expansion of the cell to occur as cell growth takes place. The secondary layer is subsequently formed, with each of the sub-layers exhibiting different patterns in the way the microfibrils are oriented, as illustrated in Figure 2.2. Of these, the 83 layer occupies the... 

Figure 2.2 A schematic of the ultrastructure of the wood cell wall, showing the middle lamella, the main cell wall layers and the associated microfibrillar orientation.

Fengel, D. and Wegener, G. (1989). Wood Chemistry, Ultrastructure, Reactions. Walter De Gruyter, Berlin, Germany. 

Neam, W.T. (1974). Application of the ultrastructure concept in industrial wood products research. Wood Science, 6(3), 285-293. 

Peterson, M.D. and Thomas, R.J. (1978). Protection of wood from decay fungi by acetylation—an ultrastructural and chemical study. Wood and Fiber, 10(3), 149-163. 

Zolhfank, C. and Wegener, G. (2002). FTIR microscopy and ultrastructural investigation of sUylated solid wood. Hol orschung, 56(1), 39-42. 

Evidence supporting the original paradigm of lignin in wood as a random, three-dimensional network polymer is reviewed. More recent results which do not fit this simple picture are discussed. A modified paradigm is proposed in which lignin in wood is comprised of several types of network which differ from each other both ultrastructurally and chemically. When wood is deligni-fied, the properties of the macromolecules made soluble reflect the properties of the network from which they are derived. 

Wooding, F. B. P. and Kemp. P. 1975A. Ultrastructure of the milk fat globule membrane with and without triglyceride. Cell Tiss. Res. 165, 113-127. 

D. Fengel, G. Wegener, Wood— Chemistry Ultrastructure and Reactions, De Gruyter, Berlin, 1983. 

Figure 3. Softwood block showing three complete and part of two other growth rings in the cross-sectional plane lX). Individual springwood cells can be detected, whereas the smaller summer-wood cells cannot be seen as individual cells. Also note the absence of vessels and the uniformity of the wood. Two longitudinal surfaces (R—radial T— tangential) are illustrated. Food-storing cells can be easily detected on the radial surface (arrow). 47 X (Courtesy of N. C. Brown Center for Ultrastructural Studies, S.U.N.Y. College of Environmental Science and Forestry)...

Fengel D, Wegener G. Wood, chemistry, ultrastructure, reaction. Berlin, Germany Walter de Gruyter 1984. 

Timell, T. E. (1973). Ultrastructure of the dormant and active cambial zones and the dormant phloem associated with formation of normal and compression woods in P/ cea abies (Karst.). Tech. Publ.—State Univ. N.Y., Coll. Environ. Sci. For., Syracuse No. 96, pp. 1 -23. 

Timell, T. E. (1965). Wood and bark polysaccharides. In "Cellular Ultrastructure of Woody Plants" (W. A. C6te, Jr., ed.), pp. 127-156. Syracuse Univ. Press, Syracuse, New York. Timell, T. E. (1967). Recent progress in the chemistry of wood hemicelluloses. Wood Sci. Technol. 1,45-70. 

Erickson O, Goring DAI, Lindgren BO (1980) Structural studies on the chemical bonds between lignins and carbohydrates in spruce wood Wood Sci Technol 14 267-279 Fengel D, Wegener G (1984a) Wood chemistry, ultrastructure, reactions Walter de Gruyter, New York, 49-53... 

Fengel D, Wegener G (1984b) Wood chemistry, ultrastructure reactions Walter de Gruyter, New York, 157 pp... 

Eom TJ, Meshitsuka G, Ishizu A Nakano J (1987) Chemical characteristics of lignin in the differentiating xylem of a hardwood III Mokuzai Gakkaishi 33 716-723 Fengel D, Wegener G (1984) Wood chemistry, ultrastructure and reactions de Gruyter, Berlin,... 

Brown W, Cowling EB, Falkehag SI (1968) Molecular size distributions of lignins liberated enzymatically from wood Sven Papperstidn 71 811-821 Browning BL (1967) Methods of wood chemistry, Vol 1 Interscience, New York, 179-180 Fengel D, Wegener G (1984) Wood, chemistry, ultrastructure, reactions Walter de Gruyter, Berlin, 157-160... 

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