Resins canals

These are narrow short cells (usually less than 0.2 mm in length) which are often lost in screening of chemical pulps, but are usually found in mechanical pulps. In addition, there are also epithelial cells which, in the original wood, are found surrounding the spaces which make up the resin canals. These canals may be up to 0.3 mm in diameter, and contain large quantities of resin, the chemistry of which is discussed more fully later in this chapter. 

The oleoresinous exudate or "pitch of many conifers, but mainly pines, is the raw material for the major products of the naval stores industry. The oleoresin is produced in the epithelial cells which surround the resin canals. When the tree is wounded the resin canals are cut. The pressure of the epithelial cells forces die oleoresin to the surface of die wound where it is collected. The oleoresin is separated into two fractions by steam distillation. The volatile fraction is called gum turpentine and contains chiefly a mixture of monoterpenes but a smaller amount of sesquiterpenes is present also. The nonvolatile gum rosin 5 consists mainly of llie dilerpenuid resin acids and smaller amounts of esters, alcohols and steroids. Wood turpentine, wood rosin and a fraction of intermediate volatility, pine oil are obtained together by gasoline extrachon of the chipped wood of old pine stumps. Pine oil is largely a mixture of the monoterpenoids terpineol. borneol and fenchyl alcohol. Sulfate turpentine and its nonvolatile counterpart, tall oil, 5 are isolated as by-products of the kraft pulping process. Tall oil consists of nearly equal amounts of saponified fatty acid esters and resin acids. 

Fig. 1-2. Transverse section of xylem and phloem of red spruce (P/cea rubens). CZ, cambial zone DP, differentiating phloem MP, mature phloem with sieve cells (sc) and tannin cells (tc) DX, differentiating xylem with ray cells and tracheids (tr) MX, mature xylem, earlywood (EW) with resin canals (rc), lined with epithelial cells (ec) LW, latewood. Note that each ray continuous from the xylem, through the cambial zone, and into the phloem. Light micrograph by L. W. Rees. Courtesy of Dr. T. E. Timell.

Resin canals are intercellular spaces building up a uniform channel network in the tree. Horizontal canals are always located inside the rays which... 

Sometimes terms pathological and physiological resin are used. Pathological resin, located in resin canals, is mainly composed of resin acids and monoterpenes and protects the wood against biological damage. Physiological resin, located in the ray parenchyma cells, is rich in fats and constitutes a supply of reserve food. Hardwoods contain only this type of resin. 

Many softwoods contain resin canals. Both vertical and horizontal (radial) canals can exist in the same wood (Fig. 5-2). The resin generated by the epithelial cells surrounding the canal is called oleoresin. In sapwood oleore-... 

Fig. 5-2. Resin canals in Norway spruce (Picea abies) (Back, 1969). (A) Horizontal resin canal in a ray (tangential section) originating from the inner annual rings. The canal is surrounded by epithelial cells which secrete resin into the canal cavities. (B) Horizontal resin canal in a ray (tangential section) originating from the outer annual rings. The canal is filled with epithelial cells because of their swelling during sample preparation. (C) Vertical resin canal (cross section).

The oleoresin present in the resin canals of certain conifers, especially pine, is secreted as a viscous fluid when the tree is wounded. Pine oleoresin contains about 25% volatile components known as "volatile oil" (or turpentine) the nonvolatile residue consists mainly of resin acids. 

The lipophilic fraction, extractable with nonpolar solvents (ethyl ether, dichloromethane, etc.) consists mainly of fats, waxes, terpenes and terpenoids, and higher aliphatic alcohols (cf. Sections 5.3.1 and 5.3.2). Terpenes, resin acids, and sterols are located in the resin canals present in the bark and also occur in the cork cells and in the pathological exudate (oleore-sin) of wounded bark. Triterpenoids are abundant in bark /3-sitosterol occurs in waxes, as an alcohol component, and the cork cells in the outer bark (periderm) of birch contain large amounts of betulinol (cf. Fig. 5-6). 

Resin ducts or resin canals are tubelike voids that are both longitudinally and radially oriented throughout the xylem of some softwoods see Figure 9). These ducts are lined with specialized parenchyma, called epithelial cells, that secrete into the duct a substance called oleoresin (Figure 14A) (3). 

Figure. 1.13. An axial resin canal in Pinus Figure 1.14. A radial resin canal in Larbc radiata with associated epithelial decidua, x 200. parenchyma cells on either side, x 145...

Studies on Pinus species have shown that the nature and amount of extractives depend upon the percentage of heartwood present and thus on tree age. In Piims radiata, heartwood starts forming once the trees are about 12 to 15 years old. Heartwood extractives occur in greatest amount in inner growth rings near the pith (Uprichard, 1971 Lloyd, 1978) especially in the butt log of mature trees (Table 2.5). The high level of resin in the inner zone appears due to a process of enrichment with sapwood extractives via the transverse resin canals (Harris, 1965). Resin acids predominate in heartwood and comprise from 70-80% of total extractives, however in sapwood there are approximately equal amounts of resin acids and fatty acids (Table 2.6). An important feature of the resin constituents of pines is that a mixture of resin acids in turpentine occur in the resin canals, and the fatty acid esters and unsaponifiable materials occur in the ray parenchyma resin. In some processes, for example refiner mechanical pulping some separation of these chemical components can occur. 

Figure 2.16. Some important extractives of softwoods, (a) A mixture of resin acids in turpentine is present in resin canals of Pinus radiata. (b) Ray parenchyma resin of Pinus radiata consists mainly of fatty acid esters and unsaponifiable materials (a major component of which is 3-sitosterol). (c) Polyphenolics found in Pinus radiata heartwood. (d) Some polyphenols that are significant in certain other softwoods.

Understanding adhesive-wood cell interactions is more difficult because of the tremendous variability in wood cell types. With tracheid, parenchyma, and fiber cells, vessels, resin canals, and ray cells that vary in composition and structure in the earlywood, latewood, sapwood, and heartwood domains, there is a tremendous variety of bonding surfaces, each of which may interact differently with the adhesives. The most dramatic difference is often between wood species because of the large difference in cellular architecture. Bonding of different species often requires changes in adhesive formulation to control penetration into the wood. Although some work has been done on determining penetration into cell lumens and walls [6], this information is usually not related to the performance of the bonded assembly. 

In softwoods, woods formed by cone-bearing trees (e.g., fir, pine, and spruce) with naked seeds, the xylan contains mainly tracheids (90%). Tracheids are considerably elongated cells (around 40 pm in diameter and between 2 and 8 nun in length), which ensure both sap flow, by means of numerous bordered pits situated on the radial cell walls, and mechanical strength. In softwoods, the earlywood is characterized by cells with large radial diameters and thin walls, and hence relatively large cavities. Latewood cells have a much smaller radial diameter and thicker walls, which result in much smaller cavities (Figure 40.4). In addition, some softwoods have resin canals. Parenchyma cells surround these canals and actively secret resin into the canals, and ultimately into the heartwood. 

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