Reaction wood

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. 

Cellulose is the most abundant polysaccharide in nature with approximately 180 billion tons produced and broken down every year (Engelhard 1995). Cellulose, which occurs as microfibrils, is the component responsible for the excellent load bearing properties of plant cell walls (for a summary for the cellulose content of the different cell wall layers, see Table 6-1). The cellulose microfibrils in wood fibers are important raw material for the pulp and paper industries, and those in cotton and hemp for the textile industries. Moreover, the renewable plant fibers have substantial potential to replace man-made fibers in fiber-reinforced thermosets and thermoplastics to produce environmentally friendly materials (Mohanty et al. 

2001 Klemm et al. 2005). However, despite the potential for broad commercial use, the detailed structure of plant fibers and the underlying mechanisms of cellulose biosynthesis remain poorly understood. 

Compression wood is heavier, harder, and denser than the normal wood. Its tracheids are short and thick-walled (even in earlywood) and in cross section rounded so that empty spaces remain between the cells. The S, layer is thicker than in a normal wood while the S J layer is absent. The layer contains helical cavities that parallel the microfibrils and reach from the 

Tension wood differs less from normal wood than compression wood. It contains thick-walled fibers, terminated towards the lumen by a gelatinous layer (Fig. 1-19). This so-called G layer consists of pure and highly crystalline cellulose oriented in the same direction as the fiber axis. For this reason the cellulose content of tension wood is higher and the lignin content lower than in normal wood. 

and Day, A. C. (1979). Wood Structure and Identification, 2nd ed. Syracuse Univ. Press, Syracuse, New York. 

(1967). Wood Ultrastructure. Univ. of Washington Press, Seattle. 

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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. 

State of subdivision. The smaller the pieces of a solid reactant—the smaller the state of subdivision—the faster the reaction. Wood shavings burn faster than solid wood, for example, because they have more surface area in contact with the oxygen with which they are combining (for a given mass of wood). In a sense, this is also a corollary of factor 4. 

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). 

In view of the enormous impact which symmetry-based rules concerning the stereochemistry of concerted addition and cyclization reactions (Wood-ward-Hoffmann rules) have had in recent years a detailed introduction to this subject has been added. 

In all reactions, wood of white fir Abies concolor) has been used. For the alkaline nitrobenzene reactions, extractive-free —20+40 mesh heart-wood sawdust containing 28.0% Klason lignin was used. Sequential extraction of the original sawdust with alcohol-benzene, 95% ethanol, and hot water gave extractives amounting to 4.9, 0.5, and 1.2%, respectively. In the other oxidation reactions, nominal 5 -in. wood chips, commonly used in pulping procedures, were employed. The mixed sap wood-heart-wood chips contained 26.5% Klason lignin and sequential extractives of 3.3, 0.5, and 3.2%, respectively. 

In the selection of starting material, one should be aware that there may be a variation in lignin composition depending on the part of a tree from which the wood is taken. Therefore, sapwood free of reaction wood should be used for the preparation of a reference lignin from a particular species. Size reduction and removal of extractives from the wood can be accomplished according to the directions given by Bjorkman (1956). 

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... 

Morohoshi N, Sakakibara A (1971) The chemical composition of reaction wood Mokuzai Gakkaishi 17 393-399... 

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. 

By reacting wood with AN, cyanoethylation occurs [Reaction (4)]. The early work was conducted for improving dimensional stability and decay resistance [18,19]. In this case, before reaction, wood was pretreated with NaOH aqueous solution and the degree of reaction was generally low. 

Reaction wood in softwoods is normally concentrated on the underside or lower side of the affected tree or branch. Because the wood in such regions appears to be subjected to compressional forces, this wood is given the nontechnical designation of compression wood (Figure 30). In hardwoods the location and concentration of reaction wood tissue is normally opposite that in softwoods, i.e., on the upper side of branches and leaning stems, and the tissue is called tension wood (Figure 31). Reaction wood in softwoods and hardwoods is also located in the vicinity of points on any tree bole where branches originate. 

Although concentrated in and around branches and in obviously leaning tree boles, reaction wood zones can be frequently scattered... 

Tension Wood. Reaction wood is not found as consistently in... 

Although much is known about reaction wood, the specific effects of its characteristics on such processes as wood preservation. 

Table II. Major Structural and Chemical Characteristics of Reaction Wood...

Chemistry. For both softwoods and hardwoods, the interpretation of general trends in changing wood chemistry along the tree radius is complicated by the potential occurrence and distribution of reaction wood. Nevertheless, hardwoods show relatively little change in cellulose or lignin contents from pith to bark and from tree base to top (2). However, softwoods have tree cores that are about 3-20% lower in cellulose and up to several percent higher in lignin content (probably due at least in part to the presenc e of cx)mpression wood) (2). 

Growth Characteristics. As a fibrous product from living trees, wood is subjected to many environmental influences as it is formed and during its lifetime. These environmental influences can increase the variability of the wood material and, thus, increase the variability of the mechanical properties. To reduce the effect of this inherent variability, standardized testing procedures using small, clear specimens of wood are often used. Small, clear specimens do not have knots, checks, splits, or reaction wood. However, the wood products used and of economic importance in the real world have these defects. Strength estimates derived from small clear specimens are reported because most chemical treatment data have been generated from small clear specimens. 

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. 

Carbon dioxide is also fixed in the dark by photosynthetic organisms by the so-called Wood-Werkman reaction (Wood and Stjemholm, 1962). The CO2 assimilated, however, rarely exceeds that formed by dark respiration i.e. there is no net CO2 uptake. On the other hand, the amount of organic carbon derived from CO2 may be as high as 30% in heterotrophic bacteria and 90% in mixotrophic organisms. In the natural environment, non-photo-synthetic CO2 fixation by these organisms, together with the above-mentioned dark fixation by photosynthetic organisms, may under some condi-... 

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