Reaction wood cells

Studies of cell morphology using physical metliods not only gave information on the Ugnin distribution in normal and reaction wood cells, but also proved that lignin is a genuine natural product of aromatic nature and not an artifact as claimed even comparatively recently 134). Several detailed reviews of work on this topic are available (27, JO, 62, 126, 146, 148). 

Wood structure within a given tree species is not uniform but varies depending on the conditions under which the tree is growing. For example, trees compensate for exposure to wind or other types of bending pressure by the production of reaction wood. In softwood, the formation of reaction wood is induced on the compressed side of a bending trunk (compression wood), whereas in hardwood, reaction wood is formed on the elongated side of the trunk (tension wood). Reaction wood cells are morphologically similar to normal wood cells but differ in their cell wall structure and chemical composition. 

Other distinct classes of wood in a tree include the portion formed in the first 10—12 years of a tree s growth, ie, juvenile wood, and the reaction wood formed when a tree s growth is distorted by external forces. Juvenile fibers from softwoods are slightly shorter and the cell walls thinner than mature wood fibers. Reaction wood is of two types because the two classes of trees react differentiy to externally applied stresses. Tension wood forms in hardwoods and compression wood forms in softwoods. Compression wood forms on the side of the tree subjected to compression, eg, the underside of a leaning tmnk or branch. Tension wood forms on the upper or tension side. Whereas in compression wood, the tracheid cell wall is thickened until the lumen essentially disappears, in tension wood, tme fiber lumens are filled with a gel layer of hemiceUulose. 

Kojima Y, Yoon SY, Kayama T (1988b) A study of production of CTMP from hardwood Part 3 Characterization of pulps produced by CTMP O-i process J Jpn Tappi 42 953-962 Lange PW (1954) The distribution of lignin in the cell wall of normal and reaction wood from spruce and a few hardwoods Sven Papperstidn 57 525-532 Luft JH (1961) Improvements in epoxy resin embedding method J Biophys Biochem Cytol 9 409-414... 

Hemicelluloses in reaction woods are quite different from those in the normal woods, namely, galactan and P-(l-3)-gIucan in compression wood and galac-tan in tension wood. It is also well known that a remarkable amount of a water-soluble polysaccharide, arabinogalactan, is contained in the heartwood of larch. Since this polysaccharide occurs mainly in the lumen of tracheids and is not a cell wall component, it may not be included in hemicelluloses. Although structures and distributions of hemicelluloses have been comprehensively studied in the last 20 years, their physiologic meanings in a cell wall are not known yet. This must be the most important point for the future study of hemicelluloses. 

Chemical modification will be defined for this chapter as any chemical reaction between some reactive part of a wood cell wall component and a simple single chemical reagent, with or without catalyst, that forms a covalent bond between the two components. This excludes in situ polymerizations of monomers in the lumen structure of the wood and those reactions that result in cell wall-penetrating polymer systems that do not result in any cell wall attachment. It is well known that lumen-filling polymer treatment results in large improvements in mechanical properties, but these are mainly a result of the properties of the new polymer introduced [ 1 ]. 

Imai, T., Guto, H., Matsumura, H., and Yasuda, S., 1998, Determination of the distribution and reaction of polysaccharides in wood cell-walls by the isotope tracer techniques. VII. Double radiolabeling of xylan and pectin in magnolia Magnolia kobus DC) and comparison of their behaviors during kraft pulping by radiotracer technique. J. Wood Sci. 44 106-110. 

Because these types of degradation are chemical in nature, it should be possible to eliminate them or decrease their rate by modifying the basic chemistry of the wood cell wall polymers. Chemical modification of wood is any chemical reaction between some reactive... 

These techniques demonstrate that it is possible to change the basic chemistry and, therefore, the properties of wood cell wall polymers through chemical reactions. These chemical modifications can greatly enhance the properties of wood products. 

The macroscopic level of consideration takes into account fiber length and differences in cell growth such as earlywood, latewood, reaction wood, sapwood, heartwood, mineral content, resin content, etc. Difierences in growth chemistry can cause significant differences in the strength of wood. 

A hierarchical succession of steps marks out the wood quality chain, beginning with the intrinsic characteristics and features of the wood cell (Table 5.1). The spatial arrangement hints at the relationships between characteristics, properties and product specifications, but they are less obvious than is usually acknowledged. For furniture one would expect adequate density (revealed in properties such as stiffness, strength and hardness), straight grain, absence of reaction wood and tight knots... 

The occurrence of growth stresses, and the parallel deduction that particular cells in reaction wood tissue must generate stresses, is not in doubt. Analytical models have been developed that offer a plausible description of these phenomena. However, the fundamental biophysics and biochemistry remain controversial and unresolved. 

In wood, as in all of the above model compounds, the formaldehyde absorption and subsequent reaction depends on the presence of an aqueous phase. This phase may be a monomolecular layer of water on the cell surface, or water on the cured UF-resin film, but the largest reservoir of water is within the wood cell. As indicated, wood may contain two types of water (a) free or capillary water, and... 

Bonding of chemicals to wood cell wall components--cellulose, hemicellulose and lignin—can change the physical and chemical properties of the wood. For example, reaction of southern pine with simple epoxides results in a modified wood which is resistant to attack by subterranean termites in laboratory tests (1). Wood modified with acetic anhydride, dimethyl sulfate, 0-propiolactone and epoxides are highly resistant to attack by microorganisms in standard soil block laboratory tests (2,3). Southern pine modified by reaction with acetic anhydride and propylene and butylene oxides has a reduced tendency to swell in the presence of water (4). 

It is speculated that the biological resistance of chemically modified wood is due to chemical alteration of cellulosic substrate so that the very specific hydrolytic enzymatic reactions cannot take place. Resistance may also be due, in part, to reducing the available cell wall moisture to below a level required for biological attack. The decrease in swelling of wood in contact with moisture—that is, dimensional stability—which results from chemical modification of wood is due to the bulking action of the added chemical to the cell wall. The bulked wood cell walls are kept in a swollen state as long as the bonded chemical is retained. In this swollen condition, wood cannot expand or contract further in response to contact with water. 

Evidence that chemical reaction has taken place with wood cell wall hydroxyl groups is evident from the infrared (IR) spectra of methyl isocyanate-modified southern pine (Figure 3). All samples run in the IR were first milled to pass a 40-mesh screen and extracted first with benzene/ethanol (2/1, v/v) followed by water in a Soxhlet extractor. Any unreacted reagent and isocyanate homopolymer formed during the reaction with wood would be removed by this extraction procedure. The spectrum for unreacted... 

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