Softwood, cells

Figure 1. Chemical composition of a softwood cell wall.

Figure 3. Chemical components in a typical softwood cell wall. Values are given as percentages.

Wood, a lignocellulosic polymer composite, contains a fibrous structure of cellulose, hemicellulose, and a three-dimensional network of lignin and several extractives (Table 4.7). Figure 4.18 shows the chemical components of a typical softwood cell wall [650]. 

Fig. 4.18. Chemical components in a typical softwood cell wall (values are given as percentages) [650]. (Reprinted with permission from [650]. Copyright (1990) American Chemical Society.)...

Coils are characteristic of softwood cells. They are highly elongated cells (1-5 mm) with beveled ends. In the walls of the pits, there is a plurality of coils, for the transmission of water between adjacent cells. The shape of certain pits is sufficiently characteristic to allow identification of the species of wood. Coils pine can be identified under a microscope by the characteristic large rectangular pits. The wood fibers are a major component of hardwood, while they are not present in conifers. These are elongated (0.7-1.6 mm), but shorter than the coils of relatively thick cell walls and sharp ends [19]. 

A cross-sectional view of kenaf is shown in Figure 3d. The ultimate cells are nearly cylindrical with thick cell walls. Kenaf fibers are shorter and coarser than those of jute. Both chemical (kraft) and mechanical pulps have been produced from kenaf, and successful demonstration mns of newsprint have been made for the Dallas Morning Nem, the St. Petersburg Times, and the Bakersfield Californian with a furnish of 82% kenaf chemithermomechanical pulp and 18% softwood kraft pulp. Kenaf fiber is also considered a substitute for jute and used in sacking, rope, twine, bags, and as papermaking pulp in India, Thailand, and the former Yugoslavia. RoseUe bleached pulp is marketed in Thailand. 

This portion is called springwood (Sp) or eadywood. During the summer or later in the growing season, the cell wall thickness increases to 8—12 pm and the outside diameter decreases from 29—47 pm in short-leaf pine. These cells form summerwood (Sm) orlatewood. The sequential combination of seasonal cell types leads to the characteristic aimual ring (AR) of trees, which is more or less distinct in softwoods, depending on the species. 

Upon maturation of both softwoods and hardwoods, the parenchyma cells at the core die. This portion of the wood is called heartwood and often contains polyphenols, davones, and other colored compounds that do not occur in the contrasting sapwood. A clear, visual distinction usually exists between heartwood and sapwood, depending on the species. Heartwood compounds, eg, dihydro quercetin (taxifofin,... 

Other distinct classes of wood in a tree include the portion formed in the first 10—12 years of a tree s growth, ie, juvenile wood, and the reaction wood formed when a tree s growth is distorted by external forces. Juvenile fibers from softwoods are slightly shorter and the cell walls thinner than mature wood fibers. Reaction wood is of two types because the two classes of trees react differentiy to externally applied stresses. Tension wood forms in hardwoods and compression wood forms in softwoods. Compression wood forms on the side of the tree subjected to compression, eg, the underside of a leaning tmnk or branch. Tension wood forms on the upper or tension side. Whereas in compression wood, the tracheid cell wall is thickened until the lumen essentially disappears, in tension wood, tme fiber lumens are filled with a gel layer of hemiceUulose. 

The cells that make up the stmctural elements of wood are of various si2es and shapes and are firmly bonded together. Dry wood cells may be empty or pardy filled with deposits such as gums, resias, or other extraneous substances. Long and poiated cells, known as fibers or tracheids, vary gready ia length within a tree and from species to species. Hardwood fibers are - 1 mm long, and softwood fibers are - 3 to 8 mm. 

Carbohydrates. Carbohydrates are the principal components of the cell wall, comprising 65—75% by weight of the dry wood. Total hydrolysis yields simple sugars, primarily glucose and xylose in hardwoods and glucose and mannose in softwoods. Minor amounts of galactose, arabinose, and rhamnose are present. 

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. 

Glucomannans (GM) and galactoglucomannans (GGM), common constituents of plant cell walls, are the major hemicellulosic components of the secondary cell walls of softwoods, whereas in the secondary cell walls of hardwoods they occur in minor amounts. They are suggested to be present together with xylan and fucogalactoxyloglucan in the primary cell walls of higher plants [192]. These polysaccharides were extensively studied in the 1960s [6,193]. 

AG type II is most abundant in the heartwood of the genus Larix and occurs as minor, water-soluble components in softwoods. Certain tree parts of western larch (I. occidentalis) were reported to contain up to 35% AG [378]. The polysaccharide is located in the lumen of the tracheids and ray cells. Consequently, it is not a cell-wall component and, by definition, not a true hemicellulose. However, it is commonly classified as such in the field of wood and pulping research. This motivated us to include the larch AG in the review. 

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. 

In softwoods, the main cell type is the tracheid, which is often mistakenly referred to as a fibre. Tracheids constitute over 90% of the volume of most softwoods, and are the principal paper-making cells of softwoods. Their average length is usually between 2 and 4 mm, with a lengthrwidth ratio (aspect ratio) often in excess of 100 to 1, but there is a wide distribution of tracheid lengths, and it is possible for some to be as short as 1 mm and for others to be as long as 5 mm (Table 2.1). The lumen, or central cavity, is several times wider than the cell wall thickness. There is also a difference between spring wood (i.e. cells synthesised in the early part of the annual... 

Table 2.1 The cell dimensions of typical hardwood and softwood.

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).

Deka and Saikia (2000) treated softwood Anthocephalus cadamba) with methanolic solutions of PF, MF or UF resins. At aronnd 34 % resin loading, the wood samples exhibited volnme increases of 14 % (PF), 12 % (MF) and 9 % (UF), which was nearly equal to the calculated volume of polymer added, showing that most of the resin was located in the cell wall. Resin loadings higher than 34 % resulted in no further volume increase of the wood. Increases in ASF also exhibited the same behaviour, with the highest values (around 70 % for PF and MF, and 50 % for UF) found at about 34 % resin loading. Both the MOE and the MOR of treated samples increased with resin loading. 

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. 

Hemicelluloses are quite difficult to extract from cell walls of softwoods (9,10) and are usually destroyed or depolymerized during the chemical pulping of these raw materials. However, other hemicelluloses, primarily xylans, can be extracted by cold, dilute sodium hydroxide from grasses and many hardwoods in very high yields (9,77). These xylans are deacetylated in an alkaline medium and are for the most part insoluble (hemicellulose A). A partially water soluble fraction (hemicellulose B) has also been... 

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