Concept
Formal grammar as the answer to "that's just a metaphor"
Plant growth is modelled in practice by a small symbolic rewriting system, and spots and stripes by two substances spreading and reacting at different rates with no template anywhere. The structure is produced each time by a rule short enough to write on a card, so the fern is an instance rather than a comparison.
Where it appears
Treated in
- Part Four — The Ground UnderneathThe MathematicsThe chaos game, done properly: randomness and determinism converging on the identical structure, checkable with a pencil.