Wood: Elements, Nature and Quality

Secondary xylem is a type of wood which is produced in secondary growth of woody dicotyledons. However, all secondary xylem is not wood, e.g., secondary xylem formed after secondary growth in herbaceous dicots. On the other hand, in the Conifers group of Gymnosperm, the secondary xylem developed after secondary growth is wood.

Timber: Timber is a type of wood that has been processed for commercial purposes like beams, planks, etc. Any wood capable of yielding a minimum dimensional size can be termed timber. Timber is a high-quality wood. The term ‘wood’ is used to refer to the substance that makes up the tree. Those woods which are adapted for building purposes are timbers. The primary function of wood is to support the tree and to keep it growing straight and tall – so that it can be able to absorb sunlight for photosynthesis. The term ‘timber’ is used to refer to the wood at any stage after the tree has been felled. This can include the raw material, also known as rough timber or processed material.

Elements of wood:

Xylem fibre or wood fibre – most strong element

Tracheids – strong element

Vessel – weak element

Xylem parenchyma – weak element Xylem ray – extremely weak element Wood quality depends on the characters of the above elements of the secondary xylem. In stem, there are three portions. At the outside of the stem, there is the bark. Bark may be divided into two parts: outer corky dead bark and inner living bark. Next to the bark is wood. Lastly, there is a pith that is dark in colour. Pith is excluded. Wood obtained from the woody dicotyledons plant stem have the following elements:

• The wood of woody dicots is commercially known as hardwood.
• The vessel, tracheid and fibre elements are present.
• Multiseriate wood ray and broad ring present.

Wood obtained from the gymnosperm plant is commercially called softwood have the following characteristics:

• Vessel and fibre are absent.
• Tracheid present.
• Uniseriate wood ray.
• Narrow ring.

Sapwood: Sapwood, also called alburnum, is the outer, living layers of the secondary wood of trees, which engage in the transport of water and minerals to the crown of the tree. It is located next to the bark and is lighter in colour. Cells are living and non-lignified. Cells are loosely arranged. Conduction function is present and takes part in physiological performance. They contain sugar and other organic materials. Tannin, resin, gums etc are absent in these cells due to which cells are easily infected by fungus and insects. Sapwood is light and non-durable. It may vary in thickness.

• Function: Food storage, water and mineral conduction.

Heartwood: Heartwood, also called duramen, is the dead, central wood of trees. Its cells usually contain tannins or other substances that make it dark in colour and sometimes aromatic. In heartwood, cells are non-living and lignified. Cells are compactly arranged. The conductive function is absent. Tannin, resin, oil, gums, colouring matter, aromatic substances are deposited in the heartwood. So wood becomes compact. These woods are darker in colour and form the bulk of the timber. Function It gives strength to the plant.

Change from sapwood to hardwood

The protoplasm of the living cell will be loosed from the sapwood to the heartwood. Removal of food materials from the cell. Cell sap is withdrawn. The water content of the cell wall is gradually reduced. Deposition of many substances ( oil, gum, resin, tannin, colour material, aromatic substances are deposited in the cell wall and cell cavity of the xylem. Lignification of parenchyma cell. Tyloses closed the cell cavity. Ultimately xylem becomes physiologically functionless and become an inactive supporting column.

Softwood Hardwood Earlywood or springwood Latewood or autumn wood Porous wood (Ring porous wood and diffuse-porous wood) Non-porous wood

Annual ring: The secondary xylem in the stem of perennial plants commonly consists of concentric layers, each layer of which presents a seasonal increment. Transverse section of the axis, these layers appears as rings and are called annual rings or growth rings. These are called annual rings because temperate regions and tropical regions are in woody plants and there is an annual alternate of growing and dormant period. Each layer represents the growth of one year.

Tyloses: A ballon like enlargement of parts of the cell walls projecting into the cell lumen. It is the characteristic feature of certain families (identifying characters). The structures are found in both primary and secondary xylem but mainly found in secondary xylem. Tyloses have generally been considered to be absent from the wood of gymnosperm. However, sometimes in the tracheid of wood of gymnosperm (soft pine), tyloses are found. Tyloses are of common occurrence in angiosperms wood. The size and number of tyloses depend on cell lumen size and the number of other tyloses. It may become large or small. Wood quality becomes good due to the presence of tyloses because it blocks vessel lumina, prevents the rapid entrance of fungal elements, water, air, infection etc. Tyloses have economic importance in the use of wood. They are factors of durability.

Distribution of wood or xylem parenchyma: Wood or xylem parenchyma is a weak element. In some gymnosperm wood parenchyma is absent. The position of parenchyma with the vessel is discussed here. Wood parenchyma is distributed in three ways: Terminal wood parenchyma or apotracheal, Diffuse or metatracheal wood parenchyma and Vasicentric or paratracheal wood parenchyma.

Apotracheal parenchyma is subdivided into the diffuse, boundary and banded

Terminal wood parenchyma or boundary wood parenchyma (apotracheal) Apotracheal means parenchyma not associated with the vessel. In some gymnosperm plant wood, wood parenchyma is absent, in others (e.g., Lasia and Pseudosuga), and in some angiosperm wood (Magnolia and Salix), wood parenchyma cells occur only in the last formed tissue of the annual ring. Such wood has terminal wood parenchyma. 2. Diffuse or metatracheal wood parenchyma When parenchyma occurs not only in the region but also remains scattered throughout the annual ring, some of the cells lying the tracheids and fibre tracheids the plant has diffuse or metatracheal wood parenchyma (e.g., in Malus, Quercus, Diospyos). 3. Vasicentric or paratracheal wood parenchyma Paratracheal means parenchyma associated with the vessel. When parenchyma occurs at the edge of the annual ring and elsewhere only about vessels and does not occur isolated among tracheid and fibres, the plant possesses vasicentric or paratracheal wood parenchyma (e.g., in Acer, Fraxinus etc.).

Ray cell Individual ray cells may be homocellular and heterocellular. Homocellular: All the ray cells are elongated along the long axis that is composed of cells of one from only. Heterocellular: Some ray cells are elongated and some are vertical that means composed of two morphological types. The entire ray system pf a wood may consists of either homocellular or heterocellular rays or of combinations of the two types of rays. Small rays may be grouped so as to appear to be one large ray. Such groups are called aggregate rays.

Quality of timber on the basis of:

Weight: Weight depends on proportion of cell lumen and cell wall thickness (fibre, tracheid). When the lumen is small , i.e. where the wood is dense, it is heavy wood. Abundance of thick walled fibres makes a wood heavy. When all the walls are thin and lumen of cell is large and proportion of fibre is low, the wood is light. Strength: It is related with weight. Presence of large proportion of fibres or fibre-tracheid makes a wood strong. Dense and heavy woods are usually strong.

Resistance to decay by the action of fungi and bacteria is largely dependent upon the chemical nature of the wood. Both light and heavy wood are durable. Toughness: It is the resisting quality of the shocks. The interlocking of the grain may be due to the presence of broad ray around which the fibre bends in the course. Wood with interlocked fibre are not good for work. Steadiness: It is the quality that shape of article keep all right with distortion. Steadiness is the property of retaining the shape of manufactured articles such as furniture and doors etc. Texture: Proportion of wood elements. Coarse texture: When vessel is large and ray is broad, then it is called coarse texture. Fine texture: When vessel size is small and ray is narrow, then it is called soft or fine texture. 7. Grain: It means orientation and direction of fibre. Depending on the orientation and direction grains are of six types: straight, irregular, diagonal, spiral, interlocked and weavy. The term grain is concerned with the direction, alignment and grouping of wood fibre. Straight grained timber: Fibre and other elements are more or less parallel to the long axis, no ornamental fibre. Irregular: Not paralleled to vertical axis, ornamentation present, so gives rise to attractive figure.

Irregular: Not paralleled to vertical axis, ornamentation present, so gives rise to attractive figure. Diagonal: It is the milling defect and results from otherwise straight grained. Originally it is straight grain. Spiral grain: It is produced when the fibre follow spiral course in the living tree. The twist may be left or right handed. It reduces the strength of the timber and it therefore a serious defect. Interlocked: It is produced when the fibre may be arranged in spirals in alternate layers being inclined in opposite directions. One right handed and another left handed. Wavy: grain which grows in a wavy fashion up the trunk. When the direction of the fibre is constantly changing so that a line drawn paralleled with then appears as a wavy line or on a longitudinal surface. Besides those above quality, polishing quality, working quality, treatability, seasoning are also impotent for the quality of timber.

Chemical Composition of Wood

The four major components of wood are extractives, ash forming, minerals, lignin and cellulose. The extractives are not part of the wood structure but they contribute to such properties of wood as colour, odour, taste and resistance to decay. They include tannins, starch, colouring matters, resins, oils, fats and waxes. Some of the extractives like tannins, resins and oils etc. have industrial importance. Ash forming minerals, lignin and cellulose make up the wood structure, the most abundant constituent comprises 70% of the wood. It is of importance in industrials like paper, explosives, synthetic textiles and plastics. Lignin comprises about one-fourth of the woody structure. It cements the structural units of wood and thus imparts rigidity to the wood. The ash forming minerals which are left as an ash when the wood is burnt, make up from 0.2 to 1.0 per cent of the wood. The ash content is of importance in industries dealing with extractives and in the rayon industry where a low ash content is desirable.