Hardwood cell structure

In hardwoods, morphological structural elements in longitudinal series comprise the segmented structure termed vessel . Vessels, which are exposed in transverse section, constitute about 10-46% of the stem volume in deciduous hardwoods and are cells of relatively large diameters (50-300 p.m). Vessels have in short the appearance of open vertical tubes within the wood structure because their end walls have partially dissolved. By comparison, the hardwood vessel diameter can be as much a 10 times the diameter of a softwood fiber. 

Wood has a well-defined cellular structure. In softwoods and ring-porous hardwoods the cells produced in the early part of the growing season, the earlywood, are larger than those produced later in the season, the latewood. In diffuse-porous hardwoods the cells are more uniform in size. These differences in cell structure are responsible for the variation in density observed among wood of different species and even in wood of the same species, depending on local growth conditions." ... 

In view of these findings it seems likely that structural and chemical differences in the very inhomogeneous lignin substrate should lead to a specialization in the respective degrading microorganisms, particularly the oxidative enzymes expressed by each organism. Bacteria also attack softwood and hardwood cell walls. They have been described as primary wood colonizers [92]. 

The two types of wood differ, however, in their nature and structure. The main structural characteristic of the hardwoods (which are botanically known as angiosperms, plants that flower to pollinate for seed reproduction) is that in their trunks or branches, the volume of wood taken up by dead cells, varies greatly, although it makes up an average of about 50% of the total volume. In softwoods (from the botanical group gymnosperms, which do not have flowers but use cones for seed reproduction) the dead cells are much more elongated and fibrous than in hardwoods, and the volume taken up by dead cells may represent over 90% of the total volume of the wood. 

The basic structure of all wood and woody biomass consists of cellnlose, hemicelluloses, lignin and extractives. Their relative composition is shown in Table 2.4. Softwoods and hardwoods differ greatly in wood stmctnie and composition. Hardwoods contain a greater fraction of vessels and parenchyma cells. Hardwoods have a higher proportion of cellulose, hemicelluloses and extractives than softwoods, but softwoods have a higher proportion of lignin. Hardwoods ate denser than softwoods. 

Lignin in the true middle lamella of wood is a random three-dimensional network polymer comprised of phenylpropane monomers linked together in different ways. Lignin in the secondary wall is a nonrandom two-dimensional network polymer. The chemical structure of the monomers and linkages which constitute these networks differ in different morphological regions (middle lamella vs. secondary wall), different types of cell (vessels vs. fibers), and different types of wood (softwoods vs. hardwoods). When wood is delignified, the properties of the macromolecules made soluble reflect the properties of the network from which they are derived. 

Trees are classified into two major groups termed softwoods (gymnosperms) and hardwoods (angiosperms). The botanical basis for classification is whether or not the tree seed is naked as in softwoods or covered as in hardwoods. A more familiar classification, which with some exceptions is valid, is based on the retention of leaves by softwoods or the shedding of leaves by hardwoods. Thus the softwoods are often referred to as evergreen trees and hardwood as deciduous trees. The major difference with regard to wood anatomy is the presence of vessels in hardwoods. Vessels are structures composed of cells created exclusively for the conduction of water. Softwoods lack vessels but have cells termed longitudinal tracheids which perform a dual role of conduction and support. 

Acetylation of cellulose to the triacetate has been carried out without breaking down of the structure with acetic anhydride containing pyridine to help open up the cell wall structure and to act as a catalyst (71). This led Stamm and Tarkow (72) to test the liquid phase reaction on wood. High dimensional stabilization without break down of the structure was obtained, but excessive amounts of chemical were used. They hence devised a vapor phase method at atmospheric pressure that proved suitable for treating veneer up to thicknesses of 1/8 inch. Acetic anhydride pyridine vapors generated by heating an 80-20% mixture of the liquids were circulated around sheets of veneer suspended in a box lined with sheet stainless steel. Hardwood veneer,... 

Cellulose never occurs in pure form in softwood and hardwood, it constitutes about 40 to 50% of the weight, in flax 70 to 85%, whereas, cottonseed hairs, which are the purest source, contain more than 90% (Table II). In these materials, cellulose macromolecules serve as a structural material within the complex architecture of the plant cell walls. Commercial production of cellulose is concentrated on the highly pure sources like cotton or easily harvested sources like wood. 

Hardwoods. Various cell types are found in hardwoods see box). Hardwood anatomy is more varied or complicated than that of the softwoods, but most structural concepts are analogous. 

Warts. Warts are conelike or droplike protuberances, sometimes found covered with an amorphous deposition, that are scattered in a random pattern on the inner fiber-wall surface in most softwoods and the fibers of some hardwood species (Figure 24). The wart structure, known collectively as the warty layer (IT), is manufactured by the living cell protoplast before cell autolysis 18, 19). The warty layer is ligninlike in nature but has no apparent physiological role it prob-... 

Extraneous Components. The extraneous components (extractives and ash) in wood are the substances other than cellulose, hemi-celluloses, and lignin. They do not contribute to the cell wall structure, and most are soluble in neutral solvents. The detailed chemistry of wood extractives can be found elsewhere (26). A review of extractives in eastern U.S. hardwoods is available (27). 

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. 

Figure 18. TEM photo showing the structure of fibers from the hardwood Acacia (3250-3420 years old) from the Tomb of Tjanefer (Thebes). Checks (CH) were frequently noted within the middle lamella (ML) and Sj cell wall regions and to a lesser extent within the Ss itself Bar 2.0 pm.

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