The wood represents the central harder part of all shrubs and trees inner to the pharmacognostic bark, that is the part of a plant body which lies inner to the cambium ring. Wood consists of mainly the xylem tissues (vessels, tracheids and fibres) and the associated parenchymatous cells, the wood parenchyma. Both the primary and secondary xylems constitute the wood of a plant. With the continuous formation of secondary xylem tissues by the cambium, the primary or original xylem tissues are gradually pushed towards the centre and become more compact. In course of time, as the plant increases in girth, these primary xylem tissues die due to over congestion as well as accumulation of tannins, oils, gums, resins, etc. and formation of plugging tyloses (balloon-like ingrowths) in their cavities. These dead xylem tissues give rise to the hard, durable and densely coloured central wood, called the heartwood. Thus the function of the heartwood is no longer conduction of water, but simply to give mechanical support to the plant. The outer region consisting of the newly formed secondary xylem tissues, which are living, lighter in colour and comparatively softer in texture, constitute the sapwood. This part of the wood is used by the plant for conduction of water and salt solutions from the root to the leaf.
External characters of a wood, as seen by cutting the wood transversely, include the annual rings, which are evident as bands crossing the piece of wood from one radial surface to the other. Each ring represents ts the total growth of the wood in a year, i.e., the spring wood and the summer wood, the latter being much darker forms a dark line on the outer edge of each annual ring. Medullary rays, which cross the annual rings at right angles as parallel lines, are also visible on the surface. Between the rays small holes or pores, representing the vessels, are evident. Associated with pores there are small patches or bands of xylem parenchyma, which are lighter in colour then the remainder of the xylem. The remainder of the wood consists of fibres, which give the hardness and strength of the wood. The type and arrangement of the fibres in the wood determine the splitting properties or grain type of the wood, which may be straight grained, if the fibres are straight and arranged parallel to each other. This type of wood splits very easily. The wood is said to possess an interlocked grain when it contains wavy fibres, which are arranged crossing each other at an angle of about 30 degrees. This type of wood splits with great difficulty.
Histologically, woods are composed of five types of cell elements, which are tracheids, vessels, fibres, wood parenchyma and medullary ray parenchyma. Tracheids form the great bulk of the woods of the Conifers, e.g., Pinus, Ephedra and Aznus, Vessels are characteristic of the Angiospermous woods and are absent from the Coniferous woods. They occur in various sizes and with different types of secondary thickenings on their walls in various groups of wood (see Fig. 9). Wood parenchyma occurs in association with the vessels in most woods, whereas the medullary ray parenchyma transverses the wood tissue forming the medullary rays in a radiating pattern. Fibres constitute the major bulk of the wood and determine the texture of the wood.
Woods used pharmaceutically are obtained both from the heart wood and sapwood, but the great bulk comes from the heartwood. Examples of such woods include: Deal wood from Anus sylvestris Linn and Picea excelsa, Quassia wood from Picrasma excelsa Guaiacum wood from Guaiacum officinale, Red Saunders from Pterocurpus santalinus and Sandalwood from Santalum album. They are hriefly discussed in the following pages.
