The term "L-system" is coined after biologist Aristid Lindenmayer, who proposed a formalism to model the growth of plants. Such a system develops a string of symbols from a simple starting point (the axiom) to an increasingly complex structure.

Interpreting this string as a list of commands for drawing in turtle graphics style, one can generate a rich variety of natural-looking fractal geometry. A host of variations and extensions to the originally simple idea exist, aiming at ever more natural and complex plant shapes.

The applet on this page demonstrates how these systems evolve over the course of several iterations. So far, you can only choose from a fixed set of rules, but maybe I'll build in some on-line rule creation when I get around to it. This L-system variant and the examples are largely based on the chapter on fractals in Gary W. Flake's great book "The Computational Beauty of Nature". If you're really interested in the topic, the one book to read is of course "The Algorithmic Beauty of Plants" by Przemyslaw Prusinkiewicz and Aristid Lindenmayer.