Tension wood fibers

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.

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 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). 

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. 

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...

Figure 6-3. A gelatinous layer (G-layer) is formed on the inside of the S2 layer in tension wood fibers from Populus tremuloides. Courtesy of Geoff Daniel, Swedish University of Agriculture Sciences. With permission...

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. 

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. 

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 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. 

At the microscopic level, tension wood is much easier to identify when it is fully developed. Fiber cell walls are much thicker than normal, enclosing very small lumens. Secondary walls are loosely attached to the primary wall and thus are responsible for some of the differing mechanical properties. 

It is important to note material such as those plastics or wood that are weak in either tension or compression will also be basically weak in shear. For example, concrete is weak in shear because of its lack of strength in tension. Reinforced bars in the concrete are incorporated to prevent diagonal tension cracking and strengthen concrete beams. Similar action occurs with RPs using fiber filament structures. 

In the standing, living tree the bordered-pit membranes between softwood fibers act as valves to prevent the spread of air or bubbles into sap-filled cells in the event of tree injury and potential rupture to vertical water columns. Unfortunately, they perform a similar function in the processing of wood into commercial products. For example, during wood drying, substantial capillary and surface tension forces are developed upon water retreat from the fiber lumens through the pits, and the membranes move effectively (particularly in earlywood) to seal the apertures in the direction of water... 

Like all composite materials reinforced with fibers, wood has anisotropic mechanical properties of resistance to both compression and tension, but with values in the transverse directions considerably lower than those in the axial direction along the fibers (Figure 12.7). 

Once the fiber strands were placed into the bottom wood board, four other pieces of wood boards were attached to the bottom board by nails to form a wooden mold. The mold had a length of 177.8 mm, width of 38.1 mm, and height of 5.0 mm. Therefore, the panel from this mold will yield three beam specimens with a length of 177.8 mm, width of 12.7 mm, and thickness (height) of 5.0 mm. Each beam specimen has four parallel SMPF strands in one layer. Because each strand has 200 fibers and the diameter of each filament is 0.05 mm, it is estimated that a total of 3200 fibers are needed to have a fiber volume fraction of 9.9%. From this estimation, there are four layers of fiber strands through the thickness for the strands without pre-tension. Figure 7.10 shows a schematic of the top view and side view of the beam specimen. 

Aicher S, Hofflin L, Dill-Langer G (2001) Damage Evolution and Acoustic Emis-sin of Wood at Tension Perpendicular to Fiber. Holz als Roh- und Weikstoff 59 104-116... 

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