Trees are either softwoods (gymnosperms or conifers) or hardwoods (angiosperms or flowering plants). Softwoods are not always softer than hardwoods. The Australian native cypress pine (Callitris) is a softwood of greater density and harness than many hardwoods, while balsa (Ochroma) is a hardwood.
The microscopic structure of wood resembles a bundle of straws glued together; each straw represents a cell with a cellulose wall and a hollow centre (lumen), and lots of fine perforations through the walls. The cell walls are impregnated with lignin, a natural polymer that glues the cells together and gives wood its strength. Most cells are oriented longitudinally (parallel to the trunk of the tree), with some radially aligned.
