Tracheids hardwood

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. 

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. 

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. 

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. 

Wood can be considered as a biological composite of hollow tubes of cellulose fibers held together by a lignin matrix gluing material. Liquids are transported up and down the trees through the tubular plumbing system. About 90 percent of the wood tissue of softwood trees consists of fiber tracheid cells for liquid conduction and support. The hardwood trees evolved after the softwoods and have specialized water conduction cells called vessels. 

Virtually all cavities of wood cells are interconnected, as mentioned above for the vessel members of hardwoods. Tiny openings in the cell walls of overlapping ends of tracheids provide vertical passageways for water rising from cell to cell in conifers. Thin walls of ray cells are perforated for the radial transport of sugars, and tangential connections provide for... 

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. 

Vasicentric tracheids are found close to the vessels in some hardwoods, particularly in the earlywood of ring porous species. They are short tracheid-like cells with profuse sidewall pitting. They are often longitudinally bent and flattened transversely on account of the lateral expansion of the adjacent vessels. 

These trends are valid for many species both hardwoods and softwoods. Bao et al. (2001) found for seven softwoods that tracheid length in corewood was 21-52% shorter than in outerwood, and for the three hardwoods studied the fibres were about 24% shorter in corewood. 

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. 

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. 

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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