Wood fiber secondary wall

However, due to the very high strength of the wood fiber secondary wall, the material resulting from mechanical homogenization of wood fibers also contains a substantial part of larger fibril bundles and even residual fibers and fiber fragments (Figure 8.4). 

In normal wood tissue, the fiber secondary wall consists of three fairly distinct layers. The outermost layer or Si is very thin (0.1-0.2 jjim) and exhibits an average microfibril angle (for the layer as a whole) of about 50-70° (2). The bulk of the secondary wall is made up of the S2 layer, which is typically several micrometers thick (Figure 18). Here the microfibrils are usually oriented to the fiber axis at a relatively small angle (5-20°). The thickness and small microfibril angle... 

Fiber secondary wall exterior to the G-layer is just as lignified, if not more so, than in normal wood fibers. 

Figure 1.3e Microstructure of wood fiber cell wall P primary cell wall layer, SI, S2, and S3 are the inner, middle, and outer layers of the secondary wall, respectively. Reprinted with permission from Abdul Khalil etal. (2012]. Copyright 2012 Elsevier.

Lignin in the true middle lamella of wood is a random three-dimensional network polymer comprised of phenylpropane monomers linked together in different ways. Lignin in the secondary wall is a nonrandom two-dimensional network polymer. The chemical structure of the monomers and linkages which constitute these networks differ in different morphological regions (middle lamella vs. secondary wall), different types of cell (vessels vs. fibers), and different types of wood (softwoods vs. hardwoods). When wood is delignified, the properties of the macromolecules made soluble reflect the properties of the network from which they are derived. 

Figure 10. Transverse section of coalified Cyrilla wood through wound tissue. Note differential response of the thick fiber wall layers to coalification. The outermost wall layers have produced an isotropic, orange-brown material whereas the bulk of the secondary wall has yielded a pale yellowy strongly anisotropic coal substance. A ring of dark red material has been produced from the inner secondary wall in the fiber-tracheid shown in the left center of the photograph and a semiopaque, granular product is seen in an adjacent iracheary element. 323X...

Fig. 1-19. Transverse section of a tension wood fiber in American beech (Fagus grandifolia), showing the middle lamella (M), primary wall (P), the outer (S,) and middle (S. ) layers of the secondary wall, the thick gelatinous layer (G), and the lumen (L). Transmission electron micrograph. Courtesy of Dr. T. E. Timell.

Fig. 7-3. UV absorbance (222 nm, 0.5 p,m section thickness) by various morphological regions of spruce fibers delignified to various lignin contents by the kraft and acid sulfite method (Wood and Goring, 1973). S, secondary wall P, primary wall CCP, primary wall at the cell corner.

During the division and enlargement phases of wood cell development, the cell wall is a thin, deformable, and extensible envelope of material referred to as the primary wall. Near the cessation of cell enlargement, however, a secondary wall may begin to be manufactured to the lumen side of the primary wall. Wood fibers, vessel elements, and certain other xylem or phloem elements that function in passive conduction and/or support normally develop a secondary wall (Figure 5). 

Perhaps as early as the manufacture of the secondary wall, differentiating wood fibers (and most other wood cells) and the regions... 

Figure 5. Cross-sectional view of fully differentiated wood fibers. (A) SE M of a Douglas-fir. Note the thick secondary walls (S) and the fiher lumen (L). (Reproduced from Ref 39. Copyright 1982, American Chemical Society.) (B) TEM of two adjacent fibers in white spruce. Key P, primary wau, S, secondary wall L, lumen and ML, middle lamella. (Reproduced with permission from Ref. 38. Copyright 1974, Forest Products...

Figure 16, Schematic of what is widely considered to be (at least in principle) the general wall architecture of normal wood fibers. Key ML, middle lamella F, primary wall and Sj, Sg, and So, layers of the secondary wall, (Adapted from Ref, i5.)...

The other significant structural feature of tension wood fibers is the nature of the rest of the secondary wall, which may lack an S3 or S3 and S2 (36) (Figure 34). 

An important characteristic of these wood cells is the heterogeneous distribution of cellulose, hemicelluloses and lignins over the cell walls. For example, lignins are located mainly in the middle lamellae (ML) and glue the individual fibers together. A smaller quantity is also distributed throughout the secondary wall with a higher concentration in the S3 layer. 

Both cotton and wood fibers have a thin primary wall that consists of a loose, random fibrillar network and surrounds the relatively thick secondary wall. The primary wall and adjacent intercellular substance between contiguous cells in wood is referred to as the compound middle lamella. In both wood and cotton the secondary wall usually consists of three layers designated SI, S2, and S3. The SI and S3 layers usually are... 

The distribution of constituents is quite simple in cotton. The secondary walls of cotton fibers consist almost entirely of highly crystalline cellulose. Almost all the hemicelluloses and extraneous materials (waxes, pectins, and certain nitrogenous substances) are contained in the cuticle and primary wall layers. In wood, on the other hand, the non-cellulosic materials are deposited in all regions of the cell walls from the lumen through the compound middle lamella. 

Figure 6. Cylindrical cavities formed in the secondary walls of wood fibers by soft-rot fungi. Note hyphae of these organisms developing within the secondary walls (A) and causing dissolution of cell-wall substance in very close proximity to the hyphae (B) (13)...

The secondary wall found in wood cells is composed of two or three layers, known as SI, S2, and S3, respectively. In each of these layers, the cellulose microfibrils are "spirally-wound" at a different angle to the major axis of the tracheid. This variation in microfibril angle imparts strength to the fiber structure in a variety of directions. Within the bast or schlerenchyma cells found in flax, hemp, jute, and kenaf, the secondary wall is less thick than that of wood, but contains layers of similarly spirally-wound microfibrils embedded in a hemicellulose and pectin-rich matrix. This "composite structure" imparts potentially high strength to regions of the cell wall. Figures 9.1 and 9.2 show a schematic representation of flax fiber and a section of an elementary fiber with its fibrillar structure in its secondary cell wall [31]. 

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