the rays. Trachieds perform both the function of support and conduction, vessels are mainly conductive tissue, fibers are chiefly supportive tissues, ray cells are for horizontal transmission and xylem parenchyma is for storage and to a limited extent for conduction.
Many of the Indian forest trees are highly valuable as they provide wood for various purposes. Wood used for construction is called timber.
The timber yielding plants / trees are classified as soft woods and hard woods
Soft woods
Cochlospermum religiosum, Bombax ceiba, Cedrela toona, Thuja occidentalis, Abies balsamea , Tsuga canadensis, Pinus palustris, Pinus strobus, Sequoia sempervirens, Picea mariana
Hard woods
Hardwikia binata, Shorea robusta, Fagus grandifolia, Prunus serotina, Castanea dentata, Tilia americana, Tectona grandis, Dalbergia sissoo, Diospyros ebenum, Switenia mahogany
After a certain period of growth, the cambium cuts off secondary xylem towards its inner face and secondary phloem towards its outer face. In the secondary xylem, the peripheral zone is light in colour, rich in starch content and other nutrients. This is called the sapwood while the central portion, which is dark and oily, is called heartwood.
The wood consists of many types of cells. The following are the most important.
Vessels: These are tubular cells placed end to end, with open ends and have a number of pits. These are dead cells and have a lignified wall. The arrangement and the size of the vessels are used in the characterization of wood.
1. Trachieds: These are fusiform cells, much like vessels but have their end walls intact.
These are also non living cells with lignified, pitted walls, 2. Ray trachieds: Elongate cells with bordered pits.
3. Xylem fibres: These are elongated sclerenchyma cells found in the wood.
4. Medullary rays: These are sheets or ribbons of tissue extending radically in the xylem. The cells are thin walled and parenchymatous.
Based on the structural peculiarities, wood is characterized and identified as follows.
• Porous and non-porous wood: In a transverse section vessels appear as large pores when compared with trachieds. Hence wood with vessels is known as porous wood. While a wood having only trachieds is called non-porous wood. Usually angiosperm wood is porous, while gymnosperm wood is non porous. The presence or absence, and arrangement of pores serve as ideal criteria for identification of woods
The porous wood is again divided into two types viz. Ring porous wood and Diffuse porous wood. In ring porous wood, there is differentiation between autumn and springwood in an annual ring so that there is a clear distinction between rings as autumn wood and spring wood alternate.
The pores are arranged in concentric rings. Ring porous wood is seen in Ulmus spp, Quercus spp, Betula spp etc.
In diffuse porous wood, the pores are scattered in the ring ( Betula, Juglans etc.) and there is no clear demarcation between autumn wood and spring wood. The increase in the size of the pores from autumn wood to spring wood is gradual and it is not possible to draw a demarcating line between the two.
• Early wood and Late wood: The development of the secondary vascular tissues in a plant is seasonal. The extent of wood formation during any part of the year is intimately associated with the physiological activities of the plant. During spring, where maximum growth is seen and demand for nutrients is more, the extent of wood formation is more and also the vessels have a broad lumen, during autumn, the extent of wood formation is less and the vessels are narrow. The terms spring wood and autumn wood are however replaced by early wood and late wood respectively, because the two parts of the wood do not clearly represent the two seasons. The wood formed during the early part of the growing season is called Early wood and the one formed late in the growing season is called Late wood. The Early wood and late wood together constitute an annual ring. Annual rings are also called growth rings.
• Sapwood and Heartwood: Xylem elements formed first are most active physiologically and gradually their activity ceases. The peripheral part of the wood containing living cells and still performing conduction is said to be sapwood and the central party consisting of non-conducting elements is said to be the heartwood. The sapwood and heartwood are also called laburnum and duramen, respectively.
Sapwood helps in conduction and support while, heartwood provides only support. Sapwood is light in colour and has living cells. The heartwood is completely non living and black in colour due to the presence of tannins, oils, gums, resins etc. the heart wood is very hard, highly valuable and used for high quality furniture, cabinet making etc.
• Storied and Non-storied wood: In transverse section of stems, xylem elements are arranged serially. In tangential sections however the xylem elements may or may not show the stratification. Thus a stratified wood is called a storied wood. In a storied wood the ends of cells of
one tier appear at approximately same level and several clear-cut tiers (strata) are seen. In a non- storied wood the wood elements are not arranged in clear-cut tiers. The cells of one tier overlap the other. Storied wood is seen in Quercus, while non-storied wood is seen in Ficus. From the phylogenetical standpoint; storied wood is regarded as more advanced than non-storied wood.
• Grains: The size, shape and arrangement of cells in the wood contributing to the variation in appearance is referred to as the grains of wood. Grains may be coarse, fine or arranged cross wise.
In a spiral grain, the cell systems wind spirally around the tree. Wood structure, annual rings and wood ray together constitute the conspicuous grain of wood. In trees like Quercus, where the wood rays are large and dense, they take a high polish and are referred to as the silver grains. In certain trees, numerous adventitious buds remain dormant under the bark and represent areas of parenchyma. In a cross section, the fibres and other xylem elements swing around the buds or
‘eyes’. Such grains are called bird’s eye grains.
• Compression wood: In many conifers, the under side of the branches has a type of wood called compression wood. It is darker than the sap wood and reddish in color resembling the heartwood.
It is also called red wood because of its color. It is more brittle than the normal wood.