Wood, rays

Fig. 1. Schematic section, where AR = annual ring, BP = bordered pits, F = wood fiber, FWR = fusiform wood ray, HRD = horizontal resin ducts,...

Most of the cells of wood are long, narrow hollow fibers and tubular-shaped cells arranged with their long axes parallel to the axis of the tree trunk. Certain food storage cells lie in radial bands, termed wood rays, which are perpendicular to the tree axis. The walls of this complex system of plant cells form the basic framework and material of all wood substance, All wood substance is composed of two basic chemical materials, lignin, and a polysaccharidic system, which is termed holoceliulose. The latter embraces cellulose and the hemicelluloses, a mixture of pentosans, hexosans and polyuronides, and in some instances small amounts of pectic materials. Wood cell wall tissue also always retains small amounts of mineral matter (ash). 

These facts preclude the surface treatment with monomers to form a wood-polymer shell around the wood. Ray cells in the radial direction are often connected to the longitudinal cell structure making shell loading improbable. 

More than 95% of the parenchyma cells in softwoods are associated with the wood rays (ray parenchyma). In sapwood, they continue their living... 

Softwood species Volume % of wood Ray cells Parenchyma cells (%) Mean pit size of parenchyma cells (Atm)... 

Figure 4. The location of wood rays. (A) Portion of an oak tree cross section showing that rays are oriented normal to tree growth rings and along the tree radius. (Photo by W. J. McCleary.) (B) SEM of wood rays (R), vessels or pores (V), and wood fibers (F) as seen in a two-plane view...

Arranged horizontally or radially in the tree are the wood rays, which, as mentioned earlier, are composed predominantly of small, bricklike, and often living cells called parenchyma see Figures 4, 7, and 11). These cells function in radial translocation but have a major role as a storage receptacle, and frequently contain extraneous materials such as starch, fats, oils, various sugars, and inorganic depositions such as calcium oxalate crystals or silica (Figure 12). 

Clearly shrinkage anisotropy is a eomplex issue. A number of faetors ean contribute and the relative importance of each will vary between timbers. In some cases a large microfibril angle might be significant, as in corewood and in compression wood. Ray tissue will be important in species such as beech and oak. Contrasting earlywood and latewood densities is a likely cause in Douglas fir, but would be irrelevant for a tropical hardwood. The effects of elastic anisotropy would be more apparent in low density softwoods. 

The extent of lumen penetration into earlywood, latewood and wood rays is preferably determined by examination of the cross section of the bondline, and many techniques have been successfully used for this purpose light microscopy [29, 30], transmitted and reflected microscopy [31-33], (epi-)fluorescence microscopy [2, 10, 26, 31, 34-40], fluorescence confocal laser scanning microscopy (CLSM) [34, 41 5], scanning electron microscopy (SEM) [32, 46 8], transmission electron microscopy (TEM) [49-52], SEM in combination with an energy-dispersive analyzer for X-rays (SEM/EDAX) [43, 53], X-ray microscopy [54], autoradiography [11] or combination of different microscopy techniques [55]. Kamke... 

Figure 16.1. Detailed wood structure Cross section of white oak tree trunk a. outer bark (dry dead tissue) b. inner bark (living tissue) c. cambium d. sapwood e. heartwood f. pith g. wood rays. From U.S. Department of Agriculture (1999) Wood Handbook Wood as an Engineering Material. USDA Forest Products Society, Madison, WI and reproduced with permission.

Wood rays Rays Band tissue that radiate from pith to phloem and ensure nutrient storage. 

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