Structure of Timber
Structure of timber at macroscopic level
The trunk of a tree has three physical functions to perform: firstly, it must support the crown, a region responsible for the production not only of food, but also of seed; secondly, it must conduct the mineral solutions absorbed by the roots upwards to the crown; and thirdly it must store manufactured food (carbohydrates) until required. As will be described in detail later, these tasks are performed by different types of cell.
Whereas the entire cross-section of the trunk fulfils the function of support, and increasing crown diameter is matched with increasing diameter of the trunk, conduction and storage are restricted to the outer region of the trunk. This zone is known as sapwood, while the region in which the cells no longer fulfil these tasks is termed the heartwood. The width of sapwood varies widely with species, rate of growth and age of the tree.
Thus, with the exception of very young trees, the sapwood can represent from 10 to 60% of the total radius, though values from 20 to 50% are more common (Figs 1 and 2); in very young trees, the sapwood will extend across the whole radius. The advancement of the heartwood to include former sapwood cells results in a number of changes, primarily chemical in nature. The acidity of the wood increases slightly, though certain timbers have heartwood of very high acidity.
Substances collectively called extractives are formed in small quantities and these impart not only colouration to the heartwood, but also resistance to both fungal and insect attack. Different substances are found in different species of wood and some timbers are devoid of them altogether: this explains the very wide range in the natural durability of wood.
Many timbers develop gums and resins in the heartwood, while the moisture content of the heartwood of most timbers is appreciably lower than that of the sapwood in the freshly felled state. However, in exceptional cases high moisture contents can occur in certain parts of the heartwood. Known as wetwood these zones are frequently of a darker colour than the remainder of the heartwood and are thought to be due to the presence of micro-organisms, which produce aliphatic acids and gases (Ward and Zeikus, 1980; Hillis, 1987).
With increasing radial growth of the trunk by division of the cambial cells (see later), commensurate increases in crown size occur, resulting in the enlargement of existing branches and the production of new ones; crown development is not only outwards but upwards. Radial growth of the trunk must accommodate the existing branches and this is achieved by the structure that we know as the knot.
If the cambium of the branch is still alive at the point where it fuses with the cambium of the trunk, continuity in growth will arise even though there will be a change in orientation of the cells. The structure so formed is termed a green or live knot (Fig. 3). If, however, the cambium of the branch is dead – and this frequently happens to the lower branches – there will be an absence of continuity, and the trunk will grow round the dead branch, often complete with its bark. Such a knot is termed a black or dead knot (Fig. 4), and will frequently drop out of planks on sawing. The direction of the grain in the vicinity of knots is frequently distorted, and in a later section the loss of strength due to different types of knot will be discussed.
Structure at microscopic level
The cellular structure of wood is illustrated in Figs 52.5 and 52.6. These three-dimensional blocks are produced from micrographs of samples of wood 8 × 5 × 5 mm in size removed from a coniferous tree, known technically as a softwood (Fig. 52.5), and a broadleaved tree, known as a hardwood (Fig. 52.6).
In the softwoods about 90% of the cells are aligned in the vertical axis, while in the hardwoods there is a much wider range in the percentage of cells that are vertical (80–95%); the remaining percentage is present in bands, known as rays, aligned in one of the two horizontal planes known as the radial plane or quartersawn plane or more loosely, as the radial section (Fig. 1).
This means that there is a different distribution of cells on the three principal axes, which is one of the two main reasons for the high degree of anisotropy present in timber. It is popularly believed that the cells of wood are living cells, but this is certainly not the case. Wood cells are produced by division of the cambium, a zone of living cells that lies between the bark and the woody part of the trunk and branches (Fig. 1).
In winter the cambial cells are dormant and generally consist of a single circumferential layer. With the onset of growth in the spring, the cells in this single layer subdivide radially to form a cambial zone some ten cells in width. This is achieved by the formation within each dividing cell of a new vertical wall called the primary wall. During the growing season these cells undergo further radial subdivision to produce what are known as daughter cells.
Some of these will remain as cambial cells while others will either develop into bark if on the outside of the zone or change into wood if on the inside. There is thus a constant state of flux within the cambial zone with the production of new cells and the relegation of existing cambial cells to bark or wood. Towards the end of the growing season the emphasis is on relegation, and a single layer of cambial cells is left for the period when growth does not occur. To accommodate the increasing diameter of the tree the cambial zone must increase circumferentially, which is achieved by the periodic tangential division of the cambial cells.
In this case, the new wall is sloping and subsequent elongation of each half of the cell results in cell overlap, often frequently at shallow angles to the vertical axis, giving rise to the formation of spiral grain in the timber. The rate at which the cambium divides tangentially has a significant effect on the average cell length of the timber produced. The daughter cells produced radially from the cambium undergo a series of changes extending over a period of about three weeks; this process is known as differentiation.
Changes in cell shape are paralleled with the formation of the secondary wall, the final stages of which are associated with the death of the cell; the degenerated cell contents are frequently to be found lining the cell cavity. It is during the process of differentiation that the standard daughter cell is transformed into one of four basic cell types (Table 1).
Chemical dissolution of the lignin–pectin complex cementing the cells together will result in their separation, and this is a useful technique for separating and examining individual cells. In softwoods two types of cell can be observed (Fig. 7). Those present in greater number are known as tracheids, which are some 2–4 mm in length with an aspect ratio (length:diameter) of about 100:1. These cells, which lie vertically in the tree trunk, are responsible for both supporting and conducting roles.
The small block-like cells some 200 × 30 µm in size, known as parenchyma, are mostly located in the rays and are responsible for the storage of food material. In contrast, in hardwoods four types of cell are present albeit that one, the tracheid, is present in small amounts (Fig. 8). The role of storage is again primarily taken on by the parenchyma, which can be present horizontally in the form of a ray, or vertically, either scattered or in distinct zones.
Support is effected by long thin cells with very tapered ends, known as fibres; these are usually about 1–2 mm in length with an aspect ratio of about 100:1. Conduction is carried out in cells whose end walls have been dissolved away either completely or in part. These cells, known as vessels or pores, are usually short (0.2–1.2 mm) and relatively wide (up to 0.5 mm) and when situated above one another form an efficient conducting tube.
Tracheids can also be present in some hardwoods but represent a very small percentage of the total cell count. It can be seen, therefore, that while in softwoods the three functions are performed by two types of cell, in hardwoods each function is performed primarily by a single type of cell (Table 1). Although all cell types develop a secondary wall this varies in thickness, being related to the function that the cell performs. Thus, the wall thickness of fibres is several times that of the vessel (Table 1).
Consequently, the density of the wood, and hence its quality (Butterfield, 2003) and many of its strength properties will be related to the relative proportions of the various types of cell (as will be discussed later). Density, of course, will also be related to the absolute wall thickness of any one type of cell, for it is possible to obtain fibres of one species of wood with a cell wall several times thicker than those of another.
The range in density of timber is from about 120 to 1200 kg/m3 , corresponding to pore volumes of from 92 to 18%. Growth may be continuous throughout the year in certain parts of the world and the wood formed tends to be uniform in structure. In the temperate and subarctic regions and in parts of the tropics growth is seasonal, resulting in the formation of growth rings; where there is a single growth period each year these rings are referred to as annual rings (Fig. 1).
When seasonal growth commences, the dominant function appears to be conduction, while in the latter part of the year the dominant factor is support. This change in emphasis manifests itself in the softwoods with the presence of thin-walled tracheids (about 2 µm) in the early part of the season (the wood being known as earlywood) and thick-walled (up to 10 µm) and slightly longer (10%) in the latter part of the season (the latewood) (Fig. 5).
In some of the hardwoods, but certainly not all of them, the earlywood is characterised by the presence of large-diameter vessels surrounded primarily by parenchyma and tracheids; only a few fibres are present. In the latewood, the vessel diameter is considerably smaller (about 20%) and the bulk of the tissue comprises fibres. It is not surprising to find, therefore, that the technical properties of the earlywood and latewood are quite different from one another. Timbers with this characteristic two-phase system are referred to as having a ring‑porous structure (Fig. 6).
The majority of hardwoods, whether of temperate or tropical origin, show little differentiation into earlywood and latewood. Uniformity across the growth ring occurs not only in cell size, but also in the distribution of the different types of cell (Fig. 9): these timbers are said to be diffuse‑porous.
In addition to determining many of the technical properties of wood, the distribution of cell types and their sizes is used as a means of timber identification. Interconnection by means of pits occurs between cells to permit the passage of mineral solutions and food in both longitudinal and horizontal planes. Three basic types of pit occur.
Simple pits, generally small in diameter and taking the form of straight-sided holes with a transverse membrane, occur between parenchyma and parenchyma, and also between fibre and fibre. Between tracheids a complex structure known as the bordered pit occurs (Fig. 10).
The entrance to the pit is domed and the internal chamber is characterised by the presence of a diaphragm (the torus), which is suspended by thin strands (the margo strands). Differential pressure between adjacent tracheids will cause the torus to move against the pit aperture, effectively stopping flow. These pits have a profound influence on the degree of artificial preservation of the timber. Similar structures are to be found interconnecting vessels in a horizontal plane.
Between parenchyma cells and tracheids or vessels, semi‑bordered pits occur and are often referred to as ray pits. These are characterised by the presence of a dome on the tracheid or vessel wall and the absence of such on the parenchyma wall: a pit membrane is present, but the torus is absent. Differences in the shape and size of these pits provide an important diagnostic feature in the softwoods.
The general arrangement of the vertically aligned cells is referred to as grain. While it is often convenient when describing timber at a general level to regard these cells as lying truly vertically, this is not really true in the majority of cases; these cells generally deviate from the vertical axis in a number of different patterns. In many timbers, and certainly in most softwoods, the direction of the deviation from the vertical axis is consistent and the cells assume a distinct spiral mode, which may be either left- or right-handed.
In young trees the helix is usually left-handed and the maximum angle, which is near to the core, is frequently of the order of 4°, though considerable variability occurs both within a species and also between different species. As the tree grows, so the helix angle in the outer rings decreases to zero and quite frequently in very large trees the angle in the outer rings subsequently increases, but the spiral has changed direction. Spiral grain has very significant technical implications; strength is lowered, while the degree of twisting on drying and the amount of pick-up on machining increase as the degree of spirality of the grain increases (Brazier, 1965).
In other timbers the grain can deviate from the vertical axis in a number of more complex ways, of which interlocked and wavy are perhaps the two most common and best known. Since each of these types of grain deviation gives rise to a characteristic decorative figure.