What actually makes a tree a tree?

A tree is a plant that has built a trunk and branches to hold its leaves up in the light, and the building material is wood. Nobody has a perfect rule: botanists admit there is no universally recognised precise definition of a tree. A popular one, from the tree guide author Alan Mitchell, says a woody perennial that can reach 6 metres or more on a single stem. That leaves out bananas, which are not woody, but lets in the larger palms.

Wood is the real secret. It is made mostly of secondary xylem, tissue that a woody plant adds to itself year after year, and it does two jobs at once: it holds the plant up, and in the living sapwood it carries water and minerals. Inside, wood is a composite, a bit like reinforced concrete. Fibres of cellulose are strong when pulled, and they sit in a matrix of lignin and hemicelluloses that resists squashing. Lignin lends rigidity and does not rot easily.

Here is the surprise: "tree" is a shape, not a family. Trees are not a single branch of the plant family tree. Many unrelated plants independently evolved a trunk and branches, because growing taller beats your neighbours to the sunlight. Flowering trees, conifers and tree ferns each took their own route.

The word lignin has a human story too. A Swiss botanist, A. P. de Candolle, first mentioned it in 1813, describing a fibrous, tasteless material that dissolved in weak alkaline solutions. He named it from the Latin word lignum, meaning wood. Two centuries later, an estimated three trillion mature trees stand on Earth, all of them leaning on that same stiff, rot-resistant chemistry.