Wood anatomy softwoods

The cell structure of softwoods is relatively simple compared to that of the hardwoods (see Figure 2.4). Most of the cells found in conifer ous woods are tracheids, which comprise 90-95% of the volume of the wood. Tracheids are fibre-like cells with lengths of approximately 100 times their diameter. Average tra-cheid length ranges from 2 to 6 millimetres

Drawing Tracheid

among coniferous species, with a corresponding diameter range of approximately 20-60 pm (1 pm = 0.001 mm). The relative diameter of tracheids is a basis for classifying texture among conifers. Texture can be estimated by how clearly individual tracheids can be seen on a cross-sectional surface with a hand lens, and is therefore a valuable aid in wood identification. For example yew (Taxus spp.) is fine textured, redwood (Sequoia sempervirens) is coarse textured.

Figure 2.4 Diagrams of representative cell types (a) Softwood cell types: Tracheids (A, enlarged view, A') make up 90—95% of the volume of the wood. The rest is mainly ray tissue in the form of ray parenchyma cells (B) or ray tracheids (C). Some species (e.g. larch) also have a small percentage of epithelial cells (D) that line the resin canals or longitudinal parenchyma cells

(E). (b) Hardwood cell types: Vessel elements (A) may vary in shape and size. In hardwoods, tracheids (B) serve an intermediate function between vessel elements and fibres. Fibres (C) are long, slender straight cells that, in comparison to vessel elements, are smaller in size and have thick cell walls. They impart strength to the wood. Parenchyma cells, used for transport and storage, may be longitudinal (D) or radial. They are similar in cross-section to tracheids, though parenchyma are shorter. Ray cells may be upright (E) or procumbent

For a given species, average tangential diameter of tracheids is fairly consistent. Across a growth ring, late-wood is distinguished from early-wood by decreased radial diameter and increased cell wall thickness. The transition may be gradual in some woods, abrupt in others. The early-wood-late-wood contrast may be slight in some woods (even-grained woods) or may be pronounced (uneven-grained woods). In uneven-grained woods such as hard pines or larches, there may be as much as a threefold difference in density (relative density 0.3-0.9) from early-wood to late-wood.

Some coniferous species have resin canals, tubular passageways lined with epithelial cells, which exude resin or pitch into the canals. Resin canals are a constant feature of genera in the family Pinaceae (the pine family), including Pinus (pine), Picea (spruce), Larix (larch) and Pseudotsuga (Douglas fir), and are therefore an important feature for initial identification screening of unknown wood samples. Resin canals are largest and most numerous in the pines, usually distinct to the naked eye. In other species, magnification may be required to locate them. The resin from canals may bleed through paint films and result in yellowish speckling of finished surfaces.

Some softwoods also have a longitudinal cell type called longitudinal parenchyma (parenchyma is the collective name for cells specialized for the storage of food materials). If present, these cells represent a very small percentage of the wood's volume and are of importance mainly as an identification feature.

The rays in softwoods are narrow, usually one cell wide (except for occasional rays with horizontal resin canals in some species) and therefore cannot be seen without magnification. With a hand lens they are barely visible as light streaks across radial surfaces. Microscopic examination of softwood rays provides valuable information on how many cells high or wide a ray is, the type(s) of cell(s) present and the type of pitting and other features present on the cell walls. This information is very important for identification of softwoods.

Most narrow softwood rays contain two types of cells, ray tracheids and ray parenchyma. A softwood ray with both types of cells is termed heterocellular or heterogeneous. A ray composed of either ray tracheids or ray parenchyma alone is termed homocellular or homogeneous (it should be noted that these terms have different meanings in hardwoods).

The pitting that occurs between ray parenchyma cells and early-wood longitudinal tracheids is called cross-field pitting. Cross-field

Wood Longitudinal

Windowlike pits

Smooth ray tracheids

Dentate ray tracheids

Windowlike pits

Figure 2.5 Radial microscopic views of eastern white pine, Pinus strobus (a) and Scots pine Pinus sylvestris (b) showing cross-field pitting. The term 'cross field' refers to the area where a ray parenchyma cell (seen here where the cells run horizontally) crosses an earlywood longitudinal tracheid (seen here where the cells run vertically). The two cell types share a cell wall and the shape inside the cell wall (here the pitting is described as 'windowlike') can be used as a diagnostic feature in softwoods. An additional diagnostic feature can be seen in the ray tracheids above and below the cross fields. In the case of eastern white pine (a) the ray tracheids are smooth, whilst in Scots pine (b) the ray tracheids are jagged in appearance and are described as 'dentate'

Windowlike pits

Smooth ray tracheids

Dentate ray tracheids

Windowlike pits

Figure 2.5 Radial microscopic views of eastern white pine, Pinus strobus (a) and Scots pine Pinus sylvestris (b) showing cross-field pitting. The term 'cross field' refers to the area where a ray parenchyma cell (seen here where the cells run horizontally) crosses an earlywood longitudinal tracheid (seen here where the cells run vertically). The two cell types share a cell wall and the shape inside the cell wall (here the pitting is described as 'windowlike') can be used as a diagnostic feature in softwoods. An additional diagnostic feature can be seen in the ray tracheids above and below the cross fields. In the case of eastern white pine (a) the ray tracheids are smooth, whilst in Scots pine (b) the ray tracheids are jagged in appearance and are described as 'dentate'

pitting may be window-like (Figure 2.5), pinoid, piceoid, cupressoid, or taxodoid. For further information see Grosser (1977), Hoadley (1990), Phillips (1960), Schweingruber (1990) and the anatomical features list produced by the International Association of Wood Anatomists (IAWA).

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  • sm proudfoot
    What are the function of the resin canals in wood?
    7 years ago

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