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Wood: Properties, Structure, Function

 

Wood: Properties, Structure, Function

Wood is a complex biological structure, a composite of many chemistries and cell types acting together to serve the needs of a living plant. Attempting to understand wood in the context of wood technology, we have often overlooked the key and basic fact that wood evolved over the course of millions of years to serve three main functions in plants― conduction of water from the roots to the leaves, mechanical support of the plant body, and storage of biochemicals.

 

There is no property of wood—physical, mechanical, chemical, biological, or technological—that is not fundamentally derived from the fact that wood is formed to meet the needs of the living tree. To accomplish any of these functions, wood must have cells that are designed and interconnected in ways sufficient to perform these functions.

 

These three functions have influenced the evolution of approximately 20,000 different species of woody plants, each with unique properties, uses, and capabilities, in both plant and human contexts. Understanding the basic requirements dictated by these three functions and identifying the structures in wood that perform them allow insight to the realm of wood as an engineering material.

 

A scientist who understands the interrelationships between form and function can predict the utility of a specific wood in a new context. The objective of this chapter is to review the basic biologi- cal structure of wood and provide a basis for interpreting its properties in an engineering context. By understanding the function of wood in the living tree, we can better understand the strengths and limitations it presents as a material.

 

The component parts of wood must be defined and delimited at a variety of scales. The wood anatomical expertise necessary for a researcher who is using a solid wood beam is different from that necessary for an engineer designing a glued-laminated beam, which in turn is different from that required for making a wood–resin composite with wood flour.

 

Differences in the kinds of knowledge required in these three cases are related to the scale at which one intends to interact with wood, and in all three cases the properties of these materials are derived from the biological needs of the living tree. For this reason, this chapter explains the structure of wood at decreasing scales and in ways that demonstrate the biological rationale for a plant to produce wood with such features.

 

Although shrubs and many vines form wood, the remainder of this chapter will focus on wood from trees, which are the predominant source of wood for commercial and engineering applications and provide examples of virtually all features that merit discussion.

 

 

Biological Structure of Wood at Decreasing Scales

The Tree

A living, growing tree has two main domains, the shoot and the roots. Roots are the subterranean structures responsible for water and mineral nutrient uptake, mechanical anchor- ing of the shoot, and storage of biochemicals. The shoot is made up of the trunk or bole, branches, and leaves (Raven and others 1999). The remainder of the chapter will be concerned with the trunk of the tree.

 

If one cuts down a tree and looks at the stump, several gross observations can be made. The trunk is composed of various materials present in concentric bands. From the outside of the tree to the inside are outer bark, inner bark, vascular cambium, sapwood, heartwood, and the pith (Fig. 3–1). Outer bark provides mechanical protection to the softer inner bark and also helps to limit evaporative water loss. Inner bark is the tissue through which sugars (food) produced by photosynthesis are translocated from the leaves to the roots or growing portions of the tree. The vascular cambium is the layer between the bark and the wood that produces both these tissues each year.

 

The sapwood is the active, “living” wood that conducts the water (or sap) from the roots to the leaves. It has not yet accumulated the often- colored chemicals that set apart the non-conductive heart- wood found as a core of darker-colored wood in the middle of most trees. The pith at the very center of the trunk is the remnant of the early growth of the trunk, before wood was formed.

 

 

Softwoods and Hardwoods

Despite what one might think based on the names, not all softwoods have soft, lightweight wood, nor do all hardwoods have hard, heavy wood. To define them botanically, softwoods are those woods that come from gymnosperms (mostly conifers), and hardwoods are woods that come from angiosperms (flowering plants).

 

In the temperate portion of the northern hemisphere, softwoods are generally needle-leaved evergreen trees such as pine (Pinus) and spruce (Picea), whereas hardwoods are typically broadleaf, deciduous trees such as maple (Acer), birch (Betula), and oak (Quercus). Softwoods and hardwoods not only differ in terms of the types of trees from which they are derived, but they also differ in terms of their component cells.

 

Softwoods have a simpler basic structure than do hardwoods because they have only two cell types and relatively little variation in structure within these cell types. Hardwoods have greater structural complexity because they have both a greater number of basic cell types and a far greater degree of variability within the cell types. The single most important distinction between the two general kinds of wood is that hardwoods

 

Figure 3–1. Macroscopic view of a transverse section of a Quercus alba trunk. Beginning at the outside of the tree is the outer bark (ob). Next is the inner bark (ib) and then the vascular cambium (vc), which is too narrow to see at this magnification. Interior toward the vascular cambium is the sapwood, which is easily dif- ferentiated from the heartwood that lies toward the in- terior. At the center of the trunk is the pith (p), which is barely discernible in the center of the heartwood.

Figure 3–1. Macroscopic view of a transverse section of a Quercus alba trunk. Beginning at the outside of the tree is the outer bark (ob). Next is the inner bark (ib) and then the vascular cambium (vc), which is too narrow to see at this magnification. Interior toward the vascular cambium is the sapwood, which is easily dif- ferentiated from the heartwood that lies toward the in- terior. At the center of the trunk is the pith (p), which is barely discernible in the center of the heartwood.

 

have a characteristic type of cell called a vessel element (or pore) whereas softwoods lack these (Fig. 3–2). An important cellular similarity between softwoods and hardwoods is that in both kinds of wood, most of the cells are dead at maturity, even in the sapwood. The cells that are alive at maturity are known as parenchyma cells and can be found in both soft- woods and hardwoods.

 

 

Sapwood and Heartwood

In both softwoods and hardwoods, the wood in the trunk of the tree is typically divided into two zones, each of which serves an important function distinct from the other. The actively conducting portion of the stem in which parenchyma cells are still alive and metabolically active is referred to as sapwood. A looser, more broadly applied definition is that sapwood is the band of lighter colored wood adjacent to the bark. Heartwood is the darker colored wood found to the interior of the sapwood (Fig. 3–1).

 

In the living tree, sapwood is responsible not only for conduction of sap but also for storage and synthesis of bio- chemicals. An important storage function is the long-term storage of photosynthate. Carbon that must be expended to form a new flush of leaves or needles must be stored some- where in the tree, and parenchyma cells of the sapwood are often where this material is stored.

 

The primary storage forms of photosynthate are starch and lipids. Starch grains are stored in the parenchyma cells and can be easily seen with a microscope. The starch content of sapwood can have important ramifications in the wood industry. For example, in the tropical tree ceiba (Ceiba pentandra), an abundance of starch can lead to growth of anaerobic bacteria that pro- duce ill-smelling compounds that can make the wood commercially unusable (Chudnoff 1984). In southern yellow pines of the United States, a high starch content encourages growth of sap-stain fungi that, though they do not affect the strength of the wood, can nonetheless decrease the lumber value for aesthetic reasons (Simpson 1991).

 

Figure 3–2. A, the general form of a generic softwood tree. B, the general form of a generic hardwood tree. C, trans- verse section of Pseudotsuga mensiezii, a typical soft- wood; the thirteen round white spaces are resin canals. D, transverse section of Betula allegheniensis, a typical hardwood; the many large, round white structures are vessels or pores, the characteristic feature of a hardwood. Scale bars = 780 µm.

Figure 3–2. A, the general form of a generic softwood tree. B, the general form of a generic hardwood tree. C, trans- verse section of Pseudotsuga mensiezii, a typical soft- wood; the thirteen round white spaces are resin canals. D, transverse section of Betula allegheniensis, a typical hardwood; the many large, round white structures are vessels or pores, the characteristic feature of a hardwood. Scale bars = 780 µm.

 

Living cells of the sapwood are also the agents of heartwood formation. Biochemicals must be actively synthesized and translocated by living cells. For this reason, living cells at the border between heartwood and sapwood are responsible for the formation and deposition of heartwood chemicals, one important step leading to heartwood formation.

 

Heartwood functions in long-term storage of biochemicals of many varieties depending on the species in question. These chemicals are known collectively as extractives. In the past, heartwood was thought to be a disposal site for harmful byproducts of cellular metabolism, the so called secondary metabolites. This led to the concept of the heartwood as a dumping ground for chemicals that, to a greater or lesser degree, would harm living cells if not sequestered in a safe place. We now know that extractives are a normal part of the plant’s system of protecting its wood.

 

Extractives are formed by parenchyma cells at the heart- wood–sapwood boundary and are then exuded through pits into adjacent cells. In this way, dead cells can become occluded or infiltrated with extractives despite the fact that these cells lack the ability to synthesize or accumu- late these compounds on their own.

 

Extractives are responsible for imparting several larger-scale characteristics to wood. For example, extractives provide natural durability to timbers that have a resistance to decay fungi. In the case of a wood like teak (Tectona grandis), known for its stability and water resistance, these properties are conferred in large part by the waxes and oils formed and deposited in the heartwood.

 

Many woods valued for their colors, such as mahogany (Swietenia mahagoni), African blackwood (Diospyros melanoxylon), Brazilian rosewood (Dalbergia nigra), and others, owe their value to the type and quantity of extractives in the heartwood. For these species, the sapwood has little or no value, because the de- sirable properties are imparted by heartwood extractives.

 

Gharu wood, or eagle wood (Aquilaria malaccensis), has been driven to endangered status due to human harvest of the wood to extract valuable resins used in perfume making (Lagenheim 2003). Sandalwood (Santalum spicatum), a wood famed for its use in incenses and perfumes, is valuable only if the heartwood is rich with the desired scented extrac- tives. The utility of a wood for a technological application can be directly affected by extractives. For example, if a wood like western redcedar, high in hydrophilic extractives, is finished with a water-based paint without a stain blocker, extractives may bleed through the paint, ruining the product (Chap. 16).

 

 

Axial and Radial Systems

The distinction between sapwood and heartwood, though important, is a gross feature that is often fairly easily ob- served. More detailed inquiry into the structure of wood shows that wood is composed of discrete cells connected and interconnected in an intricate and predictable fashion to form an integrated system that is continuous from root to twig. The cells of wood are typically many times longer than wide and are specifically oriented in two separate systems of cells: the axial system and the radial system.

 

Cells of the axial system have their long axes running parallel to the long axis of the organ (up and down the trunk). Cells of the radial system are elongated perpendicularly to the long axis of the organ and are oriented like radii in a circle or spokes in a bicycle wheel, from the pith to the bark. In the trunk of a tree, the axial system runs up and down, functions in long-distance water movement, and provides the bulk of the mechanical strength of the tree.

 

The radial system runs in a pith to bark direction, provides lateral transport for biochemicals, and in many cases performs a large frac- tion of the storage function in wood. These two systems are interpenetrating and interconnected, and their presence is a defining characteristic of wood as a tissue.

 

 

Planes of Section

Although wood can be cut in any direction for examination, the organization and interrelationship between the axial and radial systems give rise to three main perspectives from which they can be viewed to glean the most information.

 

These three perspectives are the transverse plane of section (the cross section), the radial plane of section, and the tangential plane of section. Radial and tangential sections are referred to as longitudinal sections because they extend parallel to the axial system (along the grain).

 

The transverse plane of section is the face that is exposed when a tree is cut down. Looking down at the stump one sees the transverse section (as in Fig. 3–3H); cutting a board across the grain exposes the transverse section. The trans- verse plane of section provides information about features that vary both in the pith to bark direction (called the radial direction) and also those that vary in the circumferential direction (called the tangential direction). It does not provide information about variations up and down the trunk

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The radial plane of section runs in a pith-to-bark direction (Fig. 3–3A, top), and it is parallel to the axial system, so it provides information about longitudinal changes in the stem and from pith to bark along the radial system. To describe it geometrically, it is parallel to the radius of a cylinder, and extending up and down the length of the cylinder. In a practical sense, it is the face or plane that is exposed when a log is split exactly from pith to bark. It does not provide any in- formation about features that vary in a tangential direction.

 

The tangential plane is at a right angle to the radial plane (Fig. 3–3A, top). Geometrically, it is parallel to any tangent line that would touch the cylinder, and it extends along the length of the cylinder. One way in which the tangential plane would be exposed is if the bark were peeled from a log; the exposed face is the tangential plane. The tangential plane of section does not provide any information about features that vary in the radial direction, but it does provide information about the tangential dimensions of features.

 

All three planes of section are important to the proper observation of wood, and only by looking at each can a holistic and accurate understanding of the three-dimensional struc- ture of wood be gleaned. The three planes of section are determined by the structure of wood and the way in which the cells in wood are arrayed. The topology of wood and the distribution of the cells are accomplished by a specific part of the tree stem.

 

 

Vascular Cambium

The axial and radial systems and their component cells are derived from a part of the tree called the vascular cambium. The vascular cambium is a thin layer of cells that exists between the inner bark and the wood (Figs. 3–1, 3–4) that produces, by means of many cell divisions, wood (or secondary xylem) to the inside and bark (or secondary phloem) to the outside, both of which are vascular conducting tis- sues (Larson 1994). As the vascular cambium adds cells to the layers of wood and bark around a tree, the girth of the tree increases, and thus the total surface area of the vascular cambium itself must increase, and this is accomplished by cell division as well.

 

The axial and radial systems are generated in the vascular cambium by two component cells: fusiform initials and ray initials. Fusiform initials, named to describe their long, slender shape, give rise to cells of the axial system, and ray initials give rise to the radial system. For this reason, there is a direct and continuous link between the most recently formed wood, the vascular cambium, and the inner bark. In most cases, the radial system in the wood is continuous into the inner bark, through the vascular cambium.

 

In this way wood, the water-conducting tissue, stays connected to the inner bark, the photosynthate-conducting tissue. They are interdependent tissues because the living cells in wood require photosynthate for respiration and cell growth and the inner bark requires water in which to dissolve and transport the photosynthate. The vascular cambium is an integral feature that not only gives rise to these tissue systems but also links them so that they may function in the living tree.

 

 

Growth Rings

Wood is produced by the vascular cambium one layer of cell divisions at a time, but we know from general experience that in many woods large groups of cells are produced more or less together in time, and these groups act together to serve the tree. These collections of cells produced together over a discrete time interval are known as growth incre- ments or growth rings.

 

Cells formed at the beginning of the growth increment are called earlywood cells, and cells formed in the latter portion of the growth increment are called latewood cells (Fig. 3–3D,E). Springwood and summerwood were terms formerly used to refer to earlywood and latewood, respectively, but their use is no longer recommended .

 

In temperate portions of the world and anywhere else with distinct, regular seasonality, trees form their wood in annual growth increments; that is, all the wood produced in one growing season is organized together into a recognizable, functional entity that many sources refer to as annual rings. Such terminology reflects this temperate bias, so a preferred term is growth increment, or growth ring. In many woods in the tropics, growth rings are not evident. However, continuing research in this area has uncovered several characteristics whereby growth rings can be correlated with seasonality changes in some tropical species.

 

Woods that form distinct growth rings, and this includes most woods that are likely to be used as engineering materials in North America, show three fundamental pat- terns within a growth ring: no change in cell pattern across the ring; a gradual reduction of the inner diameter of con- ducting elements from the earlywood to the latewood; and a sudden and distinct change in the inner diameter of the conducting elements across the ring (Fig. 3–5). These pat- terns appear in both softwoods and hardwoods but differ in each because of the distinct anatomical differences between the two.

 

Figure 3–3. A, illustration of a cut-away tree at various magnifications, correspond- ing roughly with the images to its right; at the top, at an approximate magnification of 100×, a softwood cell and several hardwood cells are illustrated, to give a sense of scale between the two; one tier lower, at an approximate magnification of 50×, is a single growth ring of a softwood (left) and a hardwood (right), and an indication of the radial and tangential planes; the next tier, at approximately 5× magnification, il- lustrates many growth rings together and how one might produce a straight-grained rather than a diagonal-grained board; the lowest tier includes an illustration of the relative position of juvenile and mature wood in the tree, at 1× magnification. B,C, light microscopic views of the lumina (L) and cell walls (arrowheads) of a softwood (B) and a hardwood (C). D,E, hand-lens views of growth rings, each composed of earlywood (ew) and latewood (lw), in a softwood (D) and a hardwood (E). F, a straight-grained board; note that the line along the edge of the board is parallel to the line along the grain of the board. G, a diagonal-grained board; note that the two lines are markedly not parallel; this board has a slope of grain of about 1 in 7. H, the gross anatomy of a tree trunk, showing bark, sapwood, and heartwood.

Figure 3–3. A, illustration of a cut-away tree at various magnifications, correspond- ing roughly with the images to its right; at the top, at an approximate magnification of 100×, a softwood cell and several hardwood cells are illustrated, to give a sense of scale between the two; one tier lower, at an approximate magnification of 50×, is a single growth ring of a softwood (left) and a hardwood (right), and an indication of the radial and tangential planes; the next tier, at approximately 5× magnification, il- lustrates many growth rings together and how one might produce a straight-grained rather than a diagonal-grained board; the lowest tier includes an illustration of the relative position of juvenile and mature wood in the tree, at 1× magnification. B,C, light microscopic views of the lumina (L) and cell walls (arrowheads) of a softwood (B) and a hardwood (C). D,E, hand-lens views of growth rings, each composed of earlywood (ew) and latewood (lw), in a softwood (D) and a hardwood (E). F, a straight-grained board; note that the line along the edge of the board is parallel to the line along the grain of the board. G, a diagonal-grained board; note that the two lines are markedly not parallel; this board has a slope of grain of about 1 in 7. H, the gross anatomy of a tree trunk, showing bark, sapwood, and heartwood.

 


Figure 3–4. Light microscopic view of the vascular cambium. Transverse section showing vascular cam- bium (vc) and bark (b) in Croton macrobothrys. The tissue above the vascular cambium is wood. Scale bar = 390 µm.
Figure 3–4. Light microscopic view of the vascular cambium. Transverse section showing vascular cam- bium (vc) and bark (b) in Croton macrobothrys. The tissue above the vascular cambium is wood. Scale bar = 390 µm.

 

Figure 3–4. Light microscopic view of the vascular cambium. Transverse section showing vascular cam- bium (vc) and bark (b) in Croton macrobothrys. The tissue above the vascular cambium is wood. Scale bar = 390 µm.

Figure 3–4. Light microscopic view of the vascular cambium. Transverse section showing vascular cam- bium (vc) and bark (b) in Croton macrobothrys. The tissue above the vascular cambium is wood. Scale bar = 390 µm.

B, gradual transition from earlywood to latewood in Picea glauca. C, abrupt transition from earlywood to latewood in Pseudotsuga mensiezii. D–F, hardwoods. D, diffuse- porous wood (no transition) in Acer saccharum. E, semi- ring-porous wood (gradual transition) in Diospyros virgin- iana. F, ring-porous wood (abrupt transition) in Fraxinus americana. Scale bars = 300 µm.

 

 

Non-porous woods (or softwoods, woods without vessels) can exhibit any of these three general patterns. Some softwoods such as Western redcedar (Thuja plicata), northern white-cedar (Thuja occidentalis), and species of spruce (Picea) and true fir (Abies) have growth increments that undergo a gradual transition from the thin-walled wide- lumined earlywood cells to the thicker-walled, narrower-lumined latewood cells (Fig. 3–5B).

 

Other woods undergo an abrupt transition from earlywood to latewood, such as southern yellow pine (Pinus), larch (Larix), Douglas-fir (Pseudotsuga menziesii), baldcypress (Taxodium disticum), and redwood (Sequoia sempervirens) (Fig. 3–5C). Because most softwoods are native to the north temperate regions, growth rings are clearly evident. Only in species such as ar- aucaria (Araucaria) and some podocarps (Podocarpus) does one find no transition within the growth ring (Fig. 3–5A).

 

Some authors report this state as growth rings being absent or only barely evident. Porous woods (or hardwoods, woods with vessels) have two main types of growth rings and one intermediate form. In diffuse-porous woods, vessels either do not markedly differ in size and distribution from the earlywood to the latewood, or the change in size and distribution is gradual and no clear distinction between earlywood and latewood can be found (Fig. 3–5D). Maple (Acer), birch (Betula), aspen/cot- tonwood (Populus), and yellow-poplar (Liriodendron tulip- ifera) are examples of diffuse porous species.

 

This pattern is in contrast to ring-porous woods wherein the transition from earlywood to latewood is abrupt, with ves- sel diameters decreasing substantially (often by an order or magnitude or more); this change in vessel size is often ac- companied by a change in the pattern of vessel distribution as well. This creates a ring pattern of large earlywood ves- sels around the inner portion of the growth increment, and then denser, more fibrous tissue in the latewood, as is found in hackberry (Celtis occidentalis), white ash (Fraxinus americana), shagbark hickory (Carya ovata), and northern red oak (Quercus rubra) (Fig. 3–5F).

 

Sometimes the vessel size and distribution pattern falls more or less between these two definitions, and this condition is referred to as semi-ring-porous (Fig. 3–5E). Black walnut (Juglans nigra) is a temperate-zone semi-ring-porous wood. Most tropical hardwoods are diffuse-porous; the best- known commercial exceptions to this are the Spanish-cedars (Cedrela spp.) and teak (Tectona grandis), which are generally semi-ring-porous and ring-porous, respectively.

 

Few distinctly ring-porous species grow in the tropics and comparatively few grow in the southern hemisphere. In gen- era that span temperate and tropical zones, it is common to have ring-porous species in the temperate zone and diffuse- porous species in the tropics. The oaks (Quercus), ashes (Fraxinus), and hackberries (Celtis) native to the tropics are diffuse-porous, whereas their temperate congeners are ring-porous. Numerous detailed texts provide more informa- tion on growth increments in wood, a few of which are of particular note (Panshin and deZeeuw 1980, Dickison 2000, Carlquist 2001).

 

 

Cells in Wood

Understanding a growth ring in greater detail requires some familiarity with the structure, function, and variability of cells that make up the ring. A living plant cell consists of two primary domains: the protoplast and the cell wall. The protoplast is the sum of the living contents that are bounded by the cell membrane. The cell wall is a non-living, largely carbohydrate matrix extruded by the protoplast to the exterior of the cell membrane. The plant cell wall protects the protoplast from osmotic lysis and often provides mechanical support to the plant at large.

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For cells in wood, the situation is somewhat more complicated than this highly generalized case. In many cases in wood, the ultimate function of the cell is borne solely by the cell wall. This means that many mature wood cells not only do not require their protoplasts, but indeed must completely remove their protoplasts prior to achieving functional maturity. For this reason, a common convention in wood literature is to refer to a cell wall without a protoplast as a cell.

 

In the case of a mature cell in wood in which there is no protoplast, the open portion of the cell where the protoplast would have existed is known as the lumen (plural: lumina). Thus, in most cells in wood there are two domains; the cell wall and the lumen (Fig. 3–3B,C). The lumen is a critical component of many cells, whether in the context of the amount of space available for water conduction or in the context of a ratio between the width of the lumen and the thickness of the cell wall. The lumen has no structure per se, as it is the void space in the interior of the cell. Thus, wood is a substance that has two basic domains; air space (mostly in the lumina of the cells) and the cell walls of the component cells.

 

 

Cell Walls

Cell walls in wood give wood the majority of its properties discussed in later chapters. Unlike the lumen, which is a void space, the cell wall itself is a highly regular structure, from one cell type to another, between species, and even when comparing softwoods and hardwoods. The cell wall consists of three main regions: the middle lamella, the primary wall, and the secondary wall (Fig. 3–6). In each region, the cell wall has three major components: cellulose microfibrils (with characteristic distributions and organization), hemicelluloses, and a matrix or encrusting material, typically pectin in primary walls and lignin in secondary walls.

 

In a general sense, cellulose can be understood as a long string-like molecule with high tensile strength; microfibrils are collections of cellulose molecules into even longer, stronger thread-like macromolecules. Lignin is a brittle matrix material. The hemicelluloses are smaller, branched molecules thought to help link the lignin and cellulose into a unified whole in each layer of the cell wall.

 

Figure 3–6. Cut-away drawing of the cell wall, including the structural details of a bordered pit. The various lay- ers of the cell wall are detailed at the top of the drawing, beginning with the middle lamella (ML). The next layer is the primary wall (P), and on the surface of this layer the random orientation of the cellulose microfibrils is de- tailed. Interior to the primary wall is the secondary wall in its three layers: S1, S2, and S3. The microfibril angle of each layer is illustrated, as well as the relative thick- ness of the layers. The lower portion of the illustration shows bordered pits in both sectional and face view.

Figure 3–6. Cut-away drawing of the cell wall, including the structural details of a bordered pit. The various lay- ers of the cell wall are detailed at the top of the drawing, beginning with the middle lamella (ML). The next layer is the primary wall (P), and on the surface of this layer the random orientation of the cellulose microfibrils is de- tailed. Interior to the primary wall is the secondary wall in its three layers: S1, S2, and S3. The microfibril angle of each layer is illustrated, as well as the relative thick- ness of the layers. The lower portion of the illustration shows bordered pits in both sectional and face view.

 

 

To understand these wall layers and their interrelationships, it is necessary to remember that plant cells generally do not exist singly in nature; instead they are adjacent to many other cells, and this association of thousands of cells, taken together, forms an organ, such as a leaf. Each of the individual cells must adhere to one another in a coherent way to ensure that the cells can act as a unified whole. This means they must be interconnected to permit the movement of biochemicals (such as photosynthate, hormones, cell- signaling agents) and water. This adhesion is provided by the middle lamella, the layer of cell wall material between two or more cells, a part of which is contributed by each of the individual cells (Fig. 3–6). This layer is the outermost layer of the cell wall continuum and in a non-woody organ is pectin rich. In the case of wood, the middle lamella is lignified.

 

The next layer formed by the protoplast just interior to the middle lamella is the primary wall (Fig. 3–6). The primary wall is characterized by a largely random orientation of cellulose microfibrils; like thin threads wound round and round a balloon in no particular order, where any microfibril angle from 0° to 90° relative to the long axis of the cell may be present. In cells in wood, the primary wall is thin and is generally indistinguishable from the middle lamella.

 

For this reason, the term compound middle lamella is used to denote the primary cell wall of a cell, the middle lamella, and the primary cell wall of the adjacent cell. Even when viewed with transmission electron microscopy, the compound middle lamella often cannot be separated unequivocally into its component layers.

 

The remaining cell wall domain, found in virtually all cells in wood (and in many cells in non-woody plants or plant parts), is the secondary cell wall. The secondary cell wall is composed of three layers (Fig. 3–6). As the protoplast lays down the cell wall layers, it progressively reduces the lumen volume. The first-formed secondary cell wall layer is the S1 (Fig. 3–6), which is adjacent to compound middle lamella (or technically, the primary wall). This layer is a thin layer and is characterized by a large microfibril angle. That is to say, the cellulose microfibrils are laid down in a helical fashion, and the angle between the mean microfibril direction and the long axis of the cell is large (50° to 70°).

 

The next wall layer is arguably the most important cell wall layer in determining the properties of the cell and, thus, the wood properties at a macroscopic level (Panshin and deZeeuw 1980). This layer, formed interior to the S1 layer, is the S2 layer (Fig. 3–6). This is the thickest secondary cell wall layer and it makes the greatest contribution to the over- all properties of the cell wall. It is characterized by a lower lignin percentage and a low microfibril angle (5° to 30°).

 

The microfibril angle of the S2 layer of the wall has a strong but not fully understood relationship with wood properties at a macroscopic level (Kretschmann and others 1998), and this is an area of active research.

 

Interior to the S2 layer is the S3 layer, a relatively thin wall layer (Fig. 3–6). The microfibril angle of this layer is rela- tively high and similar to the S1 (>70°). This layer has the lowest percentage of lignin of any of the secondary wall layers. The explanation of this phenomenon is related directly to the physiology of the living tree. In brief, for water to move up the plant (transpiration), there must be adhesion between the water molecules and the cell walls of the water conduits. Lignin is a hydrophobic macromolecule, so it must be in low concentration in the S3 to permit adhesion of water to the cell wall and thus facilitate transpiration. For more detail on these wall components and information on transpiration and the role of the cell wall, see any college- level plant physiology textbook.

 

 

Pits

Any discussion of cell walls in wood must be accompanied by a discussion of the ways in which cell walls are modified to allow communication and transport between the cells in the living plant. These wall modifications, called pit-pairs (or more commonly just pits), are thin areas in the cell walls between two cells and are a critical aspect of wood structure too often overlooked in wood technological treatments. Pits have three domains: the pit membrane, the pit aperture, and the pit chamber.

 

The pit membrane (Fig. 3–6) is the thin semi-porous remnant of the primary wall; it is a carbohydrate and not a phospholipid membrane. The pit aperture is the opening or hole leading into the open area of the pit, which is called the pit chamber (Fig. 3–6). The type, number, size, and relative proportion of pits can be characteristic of certain types of wood and furthermore can directly affect how wood behaves in a variety of situations, such as how wood interacts with surface coatings.

 

Pits of predictable types occur between different types of cells. In the cell walls of two adjacent cells, pits will form in the wall of each cell separately but in a coordinated location so that the pitting of one cell will match up with the pitting of the adjacent cell (thus a pit-pair). When this coordination is lacking and a pit is formed only in one of the two cells, it is called a blind pit. Blind pits are fairly rare in wood. Understanding the type of pit can permit one to determine what type of cell is being examined in the absence of other in- formation. It can also allow one to make a prediction about how the cell might behave, particularly in contexts that involve fluid flow. Pits occur in three varieties: bordered, simple, and half-bordered.

 

Bordered pits are thus named because the secondary wall overarches the pit chamber and the aperture is generally smaller or differently shaped than the pit chamber, or both. The portion of the cell wall that is overarching the pit chamber is called the border (Figs. 3–6, 3–7A,D). When seen in face view, bordered pits often are round in appearance and look somewhat like a doughnut (Fig. 3–6). When seen in sectional view, the pit often looks like a pair of V’s with the open ends of the V’s facing each other (Fig. 3–7A,D).

 

In this case, the long stems of the V represent the borders, the secondary walls that are overarching the pit chamber. Bordered pits always occur between two conducting cells, and sometimes between other cells, typically those with thick cell walls. The structure and function of bordered pits, particularly those in softwoods (see following section), are much-studied and considered to be well-suited to the safe and efficient conduction of sap. The status of the bordered pit (whether it is open or closed) has great importance in the field of wood preservation and can affect wood finishing and adhesive bonding.

 

Simple pits lack any sort of border (Fig. 3–7C,F). The pit chamber is straight-walled, and the pits are uniform in size and shape in each of the partner cells. Simple pits are typical between parenchyma cells and in face view merely look like clear areas in the walls.

 

Figure 3–7. Light micrographs and sketches of the three types of pits. A,D, longitudinal section of bordered pits in Xanthocyparis vietnamensis; the pits look like a vertical stack of thick-walled letter Vs. B,E, half-bordered pits in Pseudotsuga mensiezii; the arrow shows one half- bordered pit. C,F, simple pits on an end-wall in Pseu- dotsuga mensiezii; the arrow indicates one of five simple pits on the end wall. Scale bars = 20 µm.

Figure 3–7. Light micrographs and sketches of the three types of pits. A,D, longitudinal section of bordered pits in Xanthocyparis vietnamensis; the pits look like a vertical stack of thick-walled letter Vs. B,E, half-bordered pits in Pseudotsuga mensiezii; the arrow shows one half- bordered pit. C,F, simple pits on an end-wall in Pseu- dotsuga mensiezii; the arrow indicates one of five simple pits on the end wall. Scale bars = 20 µm.

 

Half-bordered pits occur between a conducting cell and a parenchyma cell. In this case, each cell forms the kind of pit that would be typical of its type (bordered in the case of a conducting cell and simple in the case of a parenchyma cell) and thus half of the pit pair is simple and half is bordered (Fig. 3–7B,E). In the living tree, these pits are of great im- portance because they represent the communication between conducting cells and biochemically active parenchyma cells.

 


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