Tracheid compression

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. 1-18. Transverse section of compression wood tracheids in tamarack (Larix laricina), showing intercellular spaces (IS), middle lamella (M), the outer (S,), and the inner (S2) layer of the secondary wall, and the lumen (L). The So layer contains narrow, branched helical cavities (HC) as well as two wide drying checks (C), an artifact. Transmission electron micrograph. Courtesy of Dr. T. E. Timell.

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... 

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. 

Figure 1.18. Same stem normal tracheids to the upper side (upper micrograph) and compression wood tracheids to the lower side (lower micrograph), x 250.

Figure 1.19. Compression wood tracheids in Pinus radiata showing helical splits in the S2 wall layer and intercellular spaces, x 1100.

Figure 1.20. The S2 wall layer of a compression wood tracheid in Pinus radiata. Note the large microfibril angle, x 3750.

Compared with normal wood, this tissue is characterized by shorter tracheids, higher lignin and hemicellulose content, and lower cellulose content. This reaction wood is easily identified on smooth surfaces, in particular in a transverse view. When compression wood is formed, the growth rings appear darker, reddish brown, and often wider than on the opposite side. Therefore, when compression wood develops in the same side for several years, the cross section of the stem tends to be oval with an eccentric pith in the core this is typical of branches or stem of bent trees. 

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