Wood as a Material: Structure, Species and Properties
Gross structure
A cross-section of a log shows bark, cambium, sapwood, heartwood and pith. Sapwood is the living, sap-conducting outer wood - pale, wet and much less durable. Heartwood is dead, darker and rich in extractives, so it resists decay and insects far better. Annual rings mark each season's growth; medullary rays run radially and give quarter-sawn faces their figure.
Hardwood, softwood and bamboo
Conifers give softwoods (tracheids, no vessels); broadleaved trees give hardwoods (vessels or pores). The words describe botany, not hardness: simul (Bombax ceiba) is a hardwood yet very soft and light. Assam species worth memorising: sal (Shorea robusta) - heavy, termite-resistant, posts and beams; gamari - light, easy to work, furniture and plywood; segun/teak - stable, durable; nahor - very hard, beams; agar - resinous, aromatic, high value; simul - light, masks and packing. Bamboo is a grass: no secondary thickening, no heartwood, vascular bundles instead of rings.
Properties that decide selection
Density and strength, durability, dimensional stability, workability, grain and figure, colour. Wood is hygroscopic: moisture content drifts towards the equilibrium moisture content of the surrounding air, and shrinkage is tangential > radial > longitudinal, which is why a flat-sawn board cups.
Worked example
A chair leg needs stiffness and shock resistance → sal or nahor. A drawer bottom needs stability and low weight → plywood or gamari. A carved mask must be light and cut cleanly → simul.
Traps
Bamboo is not a hardwood; "soft" is not the opposite of hardwood; thick sapwood means poor durability; grain direction governs planing tear-out.