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.
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.
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
.
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 3–4. 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.
.
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.
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.
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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