Earlywood tracheids, cell walls

The middle layer (S2) forms the main portion of the cell wall. Its thickness in softwood tracheids varies between 1 (earlywood) and 5 (latewood) jiim and it may thus contain 30-40 lamellae or more than 150 lamellae. The thickness naturally varies with the cell types. The microfibrillar angle (Fig. 1 -16) varies between 10° (earlywood) and 20-30° (latewood). It decreases in a regular fashion with increasing fiber length. The characteristics of the S2 layer (thickness, microfibrillar angle, etc.) have a decisive influence on the fiber stiffness as well as on other papermaking properties. 

Figure 13. Scanning electron micrograph of transverse section of air-dried Picea sp. from a 1200 A.D. Thule site Herschel Island. The fractured surface shows oil-swollen secondary cell walls of latewood tracheids separated from the primary wall complex. This is shown in detail in Figure 14. The earlywood tracheids appear normal.

The types and relative abundances of cellulose, hemicellulose, and lignin vary among wood cell types, between earlywood and latewood, and within individual cell walls. These distributional differences affect the relative resistance of the various components to degradation and are best worked out for spruce wood tracheids and birch wood fibers, which constitute roughly 95 and 80%, respectively, of the volumes of the corresponding woods (2, 5). 

The percentages of total biopolymer that reside within the different cell wall layers are illustrated in Figure 3. These percentages depend both on the compositions described and on the relative volumes of the layers themselves. The relative volumes vary between earlywood and latewood, and will be discussed for earlywood. On the basis of microscopic observation (4), the CML region accounts for about 12% of the total tissue volume of spmce earlywood. This wall component contains approximately 4, 21, and 27%, respectively, of the total cellulose, hemicellulose, and lignin in spruce tracheids. The adjacent Si layer has a comparable volume and contains 9, 23, and 10% of the total cellulose, hemicellulose, and lignin, respectively. 

The S2 plus tertiary (T) cell wall layers account for about 75% of the tissue volume in earlywood and 80% in latewood. Because of its great thickness, the S2 + T layer contains the majority of the cellulose (87%), hemicellulose (56%), and lignin (63%) in earlywood tracheids. These fractions are even greater for latewood tracheids, which have thicker secondary walls. At the molecular level, arabinans and galactans are concentrated in the CML... 

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. 

FIGURE 36.4 Wood is formed by cell division in the cambium zone, hence the radial cell lines appear clearly in this figure, in spite of the large variation of radial diameter between earlywood and latewood. Some bordered pits allowing sap flow from one tracheid to the other can also be observed on radial cell walls. (ESEM Photograph Norway spruce (Picea abies), LERMAB-ENGREF.)... 

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... 

Some ehemieal pulp fibres respond very quickly. Thin-walled fibres rapidly become flexible on beating and may collapse while being beaten. Thieker walled cells require heavier beating to achieve the same degree of flexibility. Latewood tracheids of Douglas fir respond only slowly while their earlywood fibres are beaten very easily indeed. 

Dutch Elm. Like oak, elm is characterized by discrete areas with thin-walled vessels, tracheids, and parenchyma cells, and areas of thick-walled fibers. Large earlywood vessels line the border of annual rings. 

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. 

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