A biologist, Lindenmayer, asked: what if a plant is a SENTENCE that rewrites itself? Start with a seed symbol, replace every symbol by a rule, repeat — then read the string as turtle commands (draw, turn, branch). A handful of characters unfolds into ferns, trees, and snowflakes. Growth as grammar. Slide the iterations and watch the plant unfold.
An L-system (Lindenmayer, 1968) is a parallel string-rewriting grammar: an axiom plus production rules applied to EVERY symbol each step. Reading the result as turtle graphics (F draw, +/− turn, [ ] push/pop state for branching) produces self-similar plants and fractals; the string length grows by the rule’s expansion factor per iteration (e.g. F→F+F-F triples the F-count each pass). It models real phyllotaxis and is a staple of procedural botany. A fail-loud self-check throws unless the rewrite grows the string by the exact expected factor. ◆ real generative grammar, node-verified.
The turtle angles/step are art choices; the GRAMMAR (parallel rewriting, exact growth factor, branching via a stack) is the exact content. Deeper iterations are truncated in the render for legibility.