Axial tracheids

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. 

Fig. 7.4 A-C. A model of control of morphogenesis by vectors of auxin wave propagation in the final phase (autolysis) of axial tracheid maturation in secondary xylem of the stem. A Trajectories of vectors of auxin wave propagation (a) in the cambial region and in differentiating secondary tissues as seen in radial (b) and transverse (c) sections. The breakdown of the cytoplasm is initiated when some critical angle (a) between the vector of auxin wave propagation and the cell axis is attained. B and C Vector trajectories associated with a model of regulation of earlywood and latewood differentiation. Ph conducting phloem Ph dividing phloem Cj cambial initial dividing xylem ...

Plate 1 Photomicrograph of pine showing axial tracheids (T) and ray tissues (R) Bar= 100 pm Reproduced with permission, Mike Hale. 

The outer secondary cell wall (SI) is comparable in thickness to the primary wall and consists of four to six lamellae which spiral in opposite directions around the longitudinal axis of the tracheid. The main bulk of the secondary wall is contained in the middle secondary cell wall (S2), and may be as little as 1 fim thick in early woods and up to 5 fim in summer wood. The microfibrils of this part of the wall spiral steeply about the axial direction at an angle of around 10 to 20°. The inner secondary wall (S3), sometimes also known as the tertiary wall, is not always well developed, and is of no great technological importance. 

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. 

A. Fibers 1. Libriform fibers 2. Fiber tracheids 3. Vasicentric tracheids B. Axial parenchyma C. Vessel elements Ray parenchyma 1. Procumbent cells 2. Upright cells Homocellular rays 1 or 2 Heterocellular rays 1 and 2... 

Transverse Section. The transverse section of southern yellow pine is normally quite simple and homogeneous. Its axial system is essentially composed of wood tracheids with only a relatively small number of parenchyma cells. An SEM micrograph of a transverse southern pine surface before exposure is shown in Figure 9. 

Figure 1.1. The transverse and tangential-longitudinal faces of the softwood European larch, Larix decidua. The wood comprises longitudinal tracheids forming the axial system of cells, and radial parenchyma mostly in uniseriate rays. Axial and ray canals are also present. Magnification X 125.

Softwoods are built up primarily of axially-elongated cells termed tracheids. Tracheids have no living cell contents at functional maturity and comprise thick-walled conduits with their tips densely interlaced (Figure 1.1). Their length varies... 

Some softwoods also have axially-elongated cells termed axial parenchyma cells present (Figure 1.12), sometimes referred to as longitudinal or wood parenchyma. These cells differ from tracheids in having thinner walls and a protoplast that may live for several years. The cell protoplasts die when the surrounding wood cells undergo the transition from sapwood to heartwood. Axial parenchyma cells often... 

Cells that look superfieially like axial parenchyma but in fact have bordered pits and assist with eonduetion are termed strand tracheids. Strand traeheids arise by the subdivision of axially-elongated eells that might otherwise have developed into normal undivided traeheids. Strand tracheids have no living eontents at funetional maturity. Some evolutionists believe that they represent an intermediate stage between the traeheid and the parenchyma cell. In some woods, such as larch and Douglas fir, they replaee the parenchyma in the latewood. 

Figure 1.11. Helical thickenings overlying Figure 1.12. A strand of axial parenchyma the S3 layer in Pseudotsuga menziesii. A cells containing starch grains in the wood of growth ring boundary divides this photo Dacrydium cupressinum. x 630. with latewood tracheids to the left, x 400.

Figure 1.16. Ray axial parenchya cells (upper) and ray tracheids (lower) in Pinus radiata. x550.

Fibres are usually classified into fibre tracheids, libriform fibres, and septate fibres. Libriform fibres (Figures 1.27 and 1.28) are longer then fibres tracheids and have moderate to very thick walls and simple pits. Their function is one of support. The shorter fibre tracheids have moderately thick walls and bordered pits. They function in both conduction and support although their occurrence in vesselled woods suggests that their function is primarily one of support. It is likely that they represent an intermediate evolutionary form between the softwood tracheid and the true libriform fibre. The fibres in some woods have their fibre lumens divided into chambers by septa. Such fibres are known as septate fibres. The septa only cross the fibre lumen and do not connect to the primary wall. They are produced by a late sequence of division in the fibre prior to death of the cytoplasm. Septate fibres resemble axial parenchyma in some woods and are most abundant in woods where the latter are poorly represented. This has led to the general belief that septate fibres have evolved as an alternative site for the storage of starches, oils and resins. 

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