Hardwood parenchyma

Triterpenoids occur in hardwood parenchyma resin, and closely related sterols are also present in softwoods (Fig. 5-6). Sterols typefied by the abundant /3-sitosterol, mostly have a hydroxyl group in the C-3 position. They also appear as the alcohol component in fatty acid esters (waxes). Triterpenoids and sterols are sparingly soluble substances contributing to pitch problems in pulping and paper making. Some trees contain polyterpenes and their derivatives known as polyprenols. Betulaprenols, present in birch wood, belong to this category of substances (Fig. 5-7). 

In hardwoods, parenchyma cells may be organized into numerous vertical strands and can occupy up to 25-50% of the wood volume (2). However, such strands usually occupy a relatively small percentage of the total wood volume in conifers (1-2%). In many softwoods and hardwoods the vertical strand parenchyma are essentially absent (2). 

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,... 

Wood also contains 3—10% of extraceUulat, low molecular weight constituents, many of which can be extracted from the wood using neutral solvents and therefore ate commonly caUed extractives. These include the food reserves, the fats and their esters in parenchyma ceUs, the terpenes and resin acids in epitheUal ceUs and resin ducts, and phenoUc materials in the heartwood. Resin materials occur in the vessels of some hardwood heartwood. 

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. 

The anatomy of softwoods will be described first because it is less complex than hardwoods. The two main cell types which constitute softwoods are tracheids, which conduct and support, and parenchyma which store food. These two cell types can be further classified as to their orientation, that is longitudinal or transverse. Cells oriented in the longitudinal direction have the long axis of the cell oriented parallel to the vertical axis of the tree trunk whereas transversely oriented cells have their long axis at right angles to the vertical axis of the tree stem. 

Obviously, softwood anatomy is relatively simple as only two types of cells, longitudinal tracheids and ray parenchyma, constitute the bulk of the wood. Hardwoods have a more complex anatomy as more kinds of cells are present. The roles of conduction and support are carried out by different cells and in addition to the transverse ray parenchyma, food-storage cells oriented in the longitudinal direction are present. Parenchyma oriented longitudinally are called longitudinal or axial parenchyma. Vessel segments perform the conduction role, and fibers the support role. 

Thus, most hardwood species contain four types of cells, vessel segments, fibers, transverse and axial parenchyma, whereas most softwood species possess two types longitudinal tracheids and transverse parenchyma. 

Considerable variation in the amount of transverse and longitudinal parenchyma exists among hardwood species. For example, basswood has approximately the same as softwoods, that is about 10%, while some oak species approach k0% parenchyma. As in softwoods, the parenchyma are usually brick-shaped cells although some variations of this shape occurs. The rays, composed of transverse parenchyma, range from one to thirty-plus cells wide. The ray illustrated in Figure 18 is seven cells wide. Thus the higher parenchyma volume is due to wider rays and the additional presence of axial parenchyma which is rather rare in softwood species. 

Based on the wood anatomical descriptions presented, it is obvious that hardwoods and softwoods differ considerably from each other. For example, vessels are present in hardwoods and absent in softwoods. In hardwoods more cell types, shorter cells, more parenchyma and a more variable arrangement of cell types occur. The relative uniformity of softwood anatomy is the result of the preponderance of a single type cell, the longitudinal tracheid. 

Water conduction in a tree is made possible by pits, which are recesses in the secondary wall between adjacent cells. Two complementary pits normally occur in neighboring cells thus forming a pit pair (Fig. 1-5). Water transport between adjacent cell lumina occurs through a pit membrane which consists of a primary wall and the middle lamella. Bordered pit pairs are typical of softwood tracheids and hardwood fibers and vessels. In softwoods the pit membrane might be pressed against the pit border thus preventing water transport, since the torus is impermeable. The pits connecting tracheids, fibers, and vessels with the ray parenchyma cells are half-bordered. Simple pits without any border connect the parenchyma cells with one another. 

Hardwoods contain several cell types, specialized for different functions (Fig. 1-9). The supporting tissue consists mainly of libriform cells, the conducting tissue of vessels with large cavities, and the storage tissue of ray parenchyma cells. In addition, hardwood contains hybrids of the above-mentioned cells which are classified as fiber tracheids. Although the term fiber is frequently used for any kind of wood cells, it more specifically denotes the supporting tissue, including both libriform cells and fiber tracheids. In birch these cells constitute 65 to 70% of the stem volume. 

Hardwood rays consist exclusively of parenchyma cells. The ray width varies in the tangential direction. In aspen wood the rays form one row, in birch wood and oak wood 1 -3 and 1 -30 rows, respectively. The height varies from one up to several hundred tiers. The rays account for 5-30% of the stem volume. 

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. 

The hardwood resin is located in the ray parenchyma cells which are connected with the vessels. It consists of fats, waxes, and sterols. The accessibility of the resin depends on the pore dimensions as well as on the mechanical stability of the ray parenchyma cells. Considerable variations occur among different wood species (Table 5-3). For instance, the accessibility of the resin in birch is much lower than in aspen. 

TABLE 5-3. Characteristics of Parenchyma Cells in Hardwoods and the Formation of Heart wood"... 

The fatty acids occur mostly as esters and are the major components of the parenchyma resin in both softwoods and hardwoods. The most important esters are fats (glycerol esters), usually present as triglycerides. Esters of other alcohols, which usually are aliphatic alcohols or of terpenoid nature, are known as waxes. 

Figure 12. SEM of calcium oxalate crystals in the ray parenchyma of the wood radial surface of a tropical hardwood. (Reproduced from Ref. 39. Copyri t 1982, American Chemical Society.)...

The parenchyma-cell content of hardwoods is, on the average, much greater than that of softwoods. This situation is a result of the wider rays (1-50 cells) and greater ray volume of hardwoods, and also the relatively high proportion of longitudinal parenchyma (2). Additionally, the rays are all parenchyma—no ray tracheids. 

Sculpturing. Pits, The regions on vessel elements where contact is made with adjacent vessels, fibers, and parenchyma are distinctly pitted (Figure 25). The arrangement, size, and shape of these pits are often species-dependent and are thus valuable in wood identification efforts (2). However, hardwood pits are not an open route for rapid intercellular transport (Figure 20) consequently, there is a need for vessel element perforations as a more effective route for fluid translocation. 

Figure 25. SEM of vessel pitting in hardwoods. (A) Vessel vessel pitting (VP) and vessel ray parenchyma pitting (RP) as seen from a transverse radial perspective in cottonwood (F = fibers). (B) Individual vessel element of cottonwood isolated by chemical pulping. (C) Isolated earlywood vessel element of white oak (FP = fiber vessel pitting).

Figure 1.2. The transverse and tangential-longitudinal faces of the hardwood poplar, Populus sp. The wood comprises vessels arranged in radial groups and fibres both in the axial system of cells, as well as uniseriate parenchyma in the ray system. Magnification x 210.

Axial parenchyma cells (also called longitudinal parenchyma) are generally very abundant in hardwoods. Like vessel elements and fibres, axial parenchyma cells are derived from the axially-elongated fusiform initials of the vascular cambium but, whereas vessel elements and fibres (except septate fibres) remain unsegmented, axial parenchyma cells are formed by the transverse segmentation of the derivatives of fusiform initials. Axial parenchyma cells, therefore, tend to lie in vertical files... 

The elassification of various ray types present in hardwoods is complex. Rays are broadly defined as homogeneous or homocellular if they have only procumbent parenchyma cells, or heterogenous or heterocellular if they have axially-elongated parenchyma cells associated with their margins. 

Figure 2.17. Some hardwood extractives, (a) In hardwoods the resin is almost entirely located in the ray parenchyma. The extract consists mainly of fatty acids and esters (not shown), triterpenoids being present in some species, (b) Polyphenolic extractives Ellagic acid 3,5,4 -trihydroxystilbene Robinetin, 3,7,3 ,4 ,5 -pentahydroxyflavone Okanin, 3,4,2 ,3 ,4 -pentahydroxy chalcone Melacacidin, 7,8,3 4 -tetrahydroxyflavan 3,4-diol.

The three principal portions of a tree are the wood or xylem, the inner bark or phloem, and the outer bark. During the growing season, xylem is laid down on the inside, and phloem on the outside, of the vascular cambium. In the wood of the Gymnospermae (softwoods), all of which are arborescent and which began to develop some 300 million years ago, the principal wood element is the tracheid, whereas the 100-million-years younger, arborescent Angiospermae (hardwoods) are characterized by the presence of fibers and vessels. Both woods also contain parenchyma cells, especially in the rays. 

Although soft rot cavities are commonly visualised in the axial tracheids of softwoods and the fibres of hardwoods, the walls of other cells such as vessels and parenchyma also show cavity attack. 

Not all cells within wood, however, are degraded equally. In general, wood fibers from hardwoods decay faster and to a greater extent than tracheids from conifers. Within deciduous wood, the fibers and ray parenchyma cells may be totally degraded while vessels remain relatively free from attack. In a recent study of wood decay under natural conditions, fibers and parenchyma cells in Acer and Tilia were completely degraded, but vessel elements were not (3i). Advanced stages of this type of white rot consisted entirely of vessels (Figure 5). 

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