Wood : A secondary xylem is called as wood. It secondary growth by cambium and constitutes the bulk of the plant body in dicot stems and dicot roots.
Wood can be classified into the following categories:
(a) Hard wood : It is the wood produced by angiosperms. It consists mainly of xylem vessels (95%) and hence called porous wood.
(b) Soft wood : It is the wood produced by gymnosperms. It consists mainly of xylem tracheids and hence called non-porous wood.
(c) Heart wood : It is the central core of wood formed during secondary growth. It consists of dead cells. The cells are dark in colour due to the presence of extractives like gums, resins, tannins etc. It is more durable and resistant to attack by microbes and insect pests.
(e) Sap wood : It is the peripheral part to the wood formed during secondary growth. It consists of living cells. The cells are lighter in colour (compared to heart wood) as extractives are absent. It is less durable and resistant to attack by microbes and insects.
(f) Early wood : It is the wood formed during favourable seasons (spring and summer). The vessels and tracheids are larger in dimension.
(g) Late wood : It is the wood formed during unfavourable seasons (autumn and winter). The vessels and tracheids are smaller in dimension.
(h) Ring porous wood: Ring porous wood is the wood produced by plants growing in temperate regions, where seasons are pronounced and it shows distinct early wood (with prominent large vessels and tracheids) and late wood (with smaller/narrow vessels and tracheids).
(i) Diffuse porous wood : Diffuse porous wood is the wood produced by plants in tropical regions, where seasons are not much pronounced and it does not show distinct early wood and late wood.