Tension wood

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. 

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.

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. 

Meier, H. (1962). Studies on a galactan from tension wood of beech (Fagus Silvatica L). Acta Chem. Scand. 16, 2275-2283. 

Timell TE (1969) The chemical composition of tension wood. Sven Papperstidn 72 173-181 Timell TE (1986) Compression wood in gymosperms, Vol. 1. Springer, Berlin Heidelberg New York Tokyo, 294, 295, 299, 311... 

Bentum ALK, Cote WA Jr, Day AC, Timell TE (1969) Distribution of lignin in normal and tension wood Wood Sci Technol 3 213-218... 

Poplar (Populus euramericana) MWLb - tension wood 497 513 1010 1 03... 

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. 

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. 

Figure 31. Cross-sectional sutFace of a young quaking aspen stem. Note the white arcs tension wood(TW) (NW = normal wood). (Reproduced from Ref. 39. Copyright 1982, American Chemical Society.)...

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

The salient features of tension wood are summarized in Table II. Most notable are the increased volume of fibers, the high cellulose content, low lignin content, and the special wall architecture of tension wood fibers. 

The extra cellulose content of tension wood tissue is most commonly due to the presence of fiber G-layers. However, the layers themselves are not really gelatinous. On the contrary, they are quite highly crystalline, and this fact, together with the axial orientation of their microfibrils, renders this layer easily distorted in the horizontal plane (i.e., normal to the fiber axis). 

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). 

The reduced vessel volume of tension wood, together with thickened fiber walls, can lead to a higher than normal basic density. This general situation, coupled with a difference in wood chemistry, could cause a variable response of such tissue to both chemical and physical treatments or to microbial degradation when compared to normal hardwood xylem. 

Tension wood fiber layering varies with species and tension wood severity, and may consist of Sj + S2 + S3 + G, 4-S2 + G, or Si + G, where G is the gelatinous layer. 

As a percentage of total wood, tension wood has more cellulose, less lignin, and fewer xylose residues than normal wood. 

Figure 33. Cross-sectional views of tension wood in a young quaking aspen stem. (Reproduced from Ref. 39. Copyright 1982, American Chemical Society.) (A) Light micrograph of a section that was selectively stained to differentiate the gelatinous layers in G-ftbers. (B) SEM of a surface of tension wood fiber zone. The Gravers, which are loosely attached to the rest of the fiber wall, were dislodged during specimen preparation and...

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