Cell structure hardwoods

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In comparing the anatomy of the hardwoods with that of the softwoods several general differences are apparent. There are many more cell types present in hardwoods, and there is more variation in their arrangement. Rays in hardwoods vary widely in size, from invisibly small to conspicuous to the eye. Hardwoods do not have resin canals as such but may have gum canals in rays.

Hardwood trees have evolved specialized conductive cells called vessel elements, which are distinct in having relatively large diameters and thin cell walls (see Figure 2.4). They form in the tree in end-to-end series in which the end walls become perforated, thus forming continuous vessels ideal for sap conduction. Vessel elements stand out as the largest diameter cells in a given hardwood species. When vessels are cut transversely, the exposed open ends are referred to as pores. Pores vary in size among and within species. In certain woods such as chestnut and oak the largest pores up to 300 pm in diameter can be easily seen without magnification, whereas in some species such as holly the pores are no larger than 40 pm in diameter and are barely perceptible with a hand lens. Among hardwoods, pore size serves as a measure of texture. Oaks having large pores are coarse textured; maple has small-diameter pores and is fine textured.

In some (temperate) species such as oak, ash, elm and sweet chestnut the largest pores are concentrated in the early wood. Such woods are said to be ring-porous; they are inherently uneven-grained and therefore have a distinct growth-ring related figure. Ring porous structure results in uneven density and expectedly affects woodworking behaviour such as uneven resistance to abrasive paper and uneven retention of pigmented stains. Woods (maple, birch, lime, poplar etc.) whose pores are more uniform in size and evenly distributed across the growth ring are said to be diffuse-porous. Such woods may show inconspicuous figure, or figure may be associated with uneven pigmentation or density of fibre mass in the outer late wood. Most diffuse-porous woods of the temperate regions have relatively small diameter pores, but among tropical woods, some diffuse porous species (e.g. mahogany) have rather large pores. A third classification, semi-ring porous (also called semi-diffuse porous), refers to woods in which the first-formed pores in a growth ring are large, but decrease in size gradually to small pores in the late wood, without apparent p-J',. . o p-J',. . o

Chestnut Cell Structure

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Figure 2.6 Hardwoods are classified as (a) ring-porous (e.g. ash, chestnut, oak), (b) semi-ring-porous (e.g. walnut), or (c) diffuse-porous (e.g. lime, birch) on the basis of the pore size and distribution within a growth ring when the end-grain (transverse section) is viewed with a hand lens (10X)

distinction of early-wood and late-wood layering. Walnut is an important example of this type. Ring porosity is illustrated in Figure 2.6.

Not all vessels have contents, but where present, contents can be a valuable aid in identification. Tyloses are bubble-like structures that form in the cell cavities of some species (see, for example, Figure 2.8 B). Other significant vessel contents include whitish or chalky deposits, and reddish or brown gummy deposits.

Hardwoods have three other types of longitudinal cells: fibres, tracheids and parenchyma cells. All are uniformly small in diameter

PARENCHYMA ARRANGEMENTS

Arrangement of a longitudinal parenchyma (P) is classified on the basis of relationship to pores (V) as seen in cross-section

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P

L

P

V

V

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V

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P

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Apotracheal

Apotracheal

Paratracheal

diffuse-in

diffuse

scanty

aggregates

  • Apotracheal diffuse and paratracheal scanty parenchyma are not visible with hand-lens magnification.)
  • Growth ring boundary)

(Growth ring boundary)

Paratracheal confluent

Marginal

Butternut

(Apotracheal diffuse and paratracheal scanty parenchyma are not visible with hand-lens magnification.)

Paratracheal confluent

Marginal

Banded

White ash

Honeylocust

Yellow-poplar

Hickory

Ramin

White ash

Honeylocust

Yellow-poplar

Hickory

Figure 2.7 The arrangement of longitudinal parenchyma (P) is classified on the basis of the relationship to vessels (V) when viewed in cross-section

(mostly in the range of 15-30 pm), and therefore can be seen individually only under a microscope. Fibres are present in all woods and are characteristically long and needle-like with tapering, pointed ends and relatively thick walls. On transverse surfaces, masses of fibres appear as the darkest areas of the tissue. Among hardwood species, tracheids and parenchyma cells range from absent or sparse to fairly abundant. They are thinner walled cells than fibres and when present in sufficient numbers, the resulting areas of tissue usually appear lighter in colour than adjacent fibre masses. Parenchyma and tracheids can be distinguished with certainty only with microscopic examination. Tracheids are elongated cells roughly similar to tracheids in softwoods. Parenchyma cells occur as vertical series of rather short cells arranged end to end. On transverse surfaces viewed with a hand lens, characteristic patterns of lighter tissue (mostly parenchyma, but sometimes including tra-cheids) are valuable in identifying hardwoods.

The various arrangements of parenchyma are referred to in Table 2.1 and illustrated in Figure 2.7. Longitudinal parenchyma is termed paratracheal if associated with the pores and apotracheal if independent of the pores. A complete halo or sheath of parenchyma more or less concentric with a pore is termed vasi-

centric. Cells of parenchyma along the growth ring boundary, referred to as marginal parenchyma, are termed initial if they occur at the inner edge of the growth ring or terminal if they occur at the outer edge. Single isolated cells of apotracheal parenchyma are called diffuse parenchyma. Diffuse parenchyma in short tangential lines is described as diffuse-in-aggre-gates parenchyma. More or less continuous tangential or wavy lines or bands of parenchyma within the growth ring are called banded parenchyma. Paratracheal parenchyma with wing-like lateral extensions is termed aliform. When parenchyma forms a continuous tangential or diagonal zone connecting two or more pores, it is termed confluent.

Rays are quite variable among hardwood species. The size of rays is expressed by cell count as viewed microscopically on tangential sections, in particular, ray width or seriation of the largest rays present. In woods such as chestnut and willow, the rays are uniseriate, that is, only one cell wide, and therefore visible only with a microscope. At the other extreme, such as the white oaks, the largest rays are up to 40-seriate and up to several inches in height. Rays in oak are conspicuous to the unaided eye.

Although hardwood rays consist entirely of ray parenchyma cells, there are two types of p

Banded

Butternut

Ramin these distinguished on the basis of overall shape. As seen in radial view, procumbent ray cells are elongated horizontally; upright ray cells are either square or they are vertically oriented. If only one type of cell is present in the ray, it is termed homocellular or homogeneous. If both types are present, it is called heterocel-lular or heterogeneous.

In some species, the rays are of fairly even height and are spaced at even horizontal levels throughout the tree. These storied rays appear as even horizontal rows when the wood is sectioned tangentially. Ripple marks, horizontal striations of lighter and darker wood 0.4 mm to 0.8 mm apart, frequently seen in tropical woods are due to storied rays.

The distinct appearance of rays on radial surfaces is termed ray fleck, and in many woods, such as oak, plane, beech and lace wood, their prominent rays are a characteristic feature of the figure. Rays also profoundly influence physical and mechanical behaviour. Rays, especially larger ones, represent planes of weakness in the wood. Shrinkage stresses associated with the seasoning of wood may develop checks through the ray tissue. Also, the restraining effect of the rays contributes to differential radial and tangential shrinkage, a common cause of cupping of flat sawn boards and radial cracking of timbers.

Detailed illustrations of microscopic wood anatomy beyond the scope of the present work are provided by Brazier and Franklin (1961), Détienne and Jacquet (1983), Fahn et al. (1986), Grosser (1977), Hart and Jay (1971), Hoadley (1990), Panshin and deZeeuw (1980), Phillips (1960) and Schweingruber (1990).

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