Growing Things
These are some examples of particle trails making plant-like patterns
Starting from one particle which moves along a path, at random intervals it spawns new particles going in slightly different directions, which follow modified versions of the rules followed by the original particle.
Pressing the 'go' button will produce a different random plant every time.
Articles
Oil spill
Parameters
Now there are some parameters to control the branching rate, and some symmetry by restricting the choice of branching angles.
Simple Cracks
Simple cracks - each particle travels in a straight line and randomly releases shorter-lived particles which travel off in deifferent directions.