Cell wall tracheid structure

Figure 2.3 A schematic representation of the structure of the primary (P) and secondary (SI, S2 and S3) cell walls of a softwood tracheid (ML = middle lamella).

It should be obvious that the structure of the longitudinal tracheid is well suited to perform the dual roles of conduction and support. Since water is translocated up the tree via the tracheids, the orientation of the long axis of the tracheid parallel to the vertical stem permits a longer passageway prior to interruption by a cell wall. The rigid cell walls, of varying thickness, provide adequate support. 

Figure 8. View of internal cell walls of springwood longitudinal tracheids. The circular dome-like structures are bordered pits which permit liquid flow between contiguous longitudinal tracheids. The smaller egg-shaped pits in clusters lead to adjacent transversely oriented ray cells. 400X (Courtesy of N. C. Brown Center for Ultrastructural Studies, S.U.N.Y. College of Environmental Science and Forestry)...

The other major cell wall structure found on longitudinal tracheids is termed a ray crossing and is illustrated in Figures 7 and 8. Ray crossings consist of pits which interconnect longitudinal tracheids to ray parenchyma. Due to the diverse structure of ray crossing pits they are extremely useful in the identification of wood and wood fibers. However, since identification is beyond the scope of this review, a description of the different types of pits found in ray crossings is not included. 

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. 

Besides vessel members and tracheids, parenchyma cells and fibers also occur in the xylem (see Fig. 1-3). Xylem fibers, which contribute to the structural support of a plant, are long thin cells with lignified cell walls they are generally devoid of protoplasts at maturity but are nonconducting. The living parenchyma cells in the xylem are important for the storage of carbohydrates and for the lateral movement of water and solutes into and out of the conducting cells. 

Bailey and his co-workers worked out the basic structure of the softwood tracheid (Figure 2.12) in the 1930s, e.g. Bailey and Kerr (1935) the orientation of the microfibrils within the cell wall was determined by polarizing microscopy and by... 

Fig. 1.—Simplified Structure of the Cell Wall of a Fiber or Tracheid (Wardrop ).

The secondary wall found in wood cells is composed of two or three layers, known as SI, S2, and S3, respectively. In each of these layers, the cellulose microfibrils are "spirally-wound" at a different angle to the major axis of the tracheid. This variation in microfibril angle imparts strength to the fiber structure in a variety of directions. Within the bast or schlerenchyma cells found in flax, hemp, jute, and kenaf, the secondary wall is less thick than that of wood, but contains layers of similarly spirally-wound microfibrils embedded in a hemicellulose and pectin-rich matrix. This "composite structure" imparts potentially high strength to regions of the cell wall. Figures 9.1 and 9.2 show a schematic representation of flax fiber and a section of an elementary fiber with its fibrillar structure in its secondary cell wall [31]. 

An interesting structure can be found in wood, for example in the different layers of conifer tracheids, where helically arranged cellulose micro-fibril bundles are found (Fig. 9.7). The spiral-Uke lay-up and the angle of the strengthening fibers in the stem walls and of cellulose micro-fibril bundles in the cell walls is optimized according to the types and combinations of... 

The organization of a typical softwood tracheid or hardwood fiber is presented in Fig. 1. In the primary wall (P) the cellulose microfibrils form a disordered network, whereas the outer layer of the secondary wall (Si) has a crossed, fibrillar structure. The middle layer of the secondary wall (S2) represents the bulk of the cell. Here, the cellulose microfibrils run almost parallel to the fiber axis. The microfibrils in the inner layer of the secondary wall (S3), sometimes referred to as the tertiary wall, are oriented at a steep angle to the fiber axis. Toward the central lumen, the S3 layer is often terminated by a warty membrane. 

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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