This is a attemps for a Diffusion-Limited Aggregation (DLA):

It is a physical model invented in 1981 (Witten & Sander)
to simulate how particles that scatter (by random walk) aggregate around a nucleus when they stick to it upon contact.//

Principle: two types of particles:
1. stabilized particles, the first of which is the “seed”
2. moving particles, following a Brownian path
When type 2 encounters type 1 (touches it), it becomes type 1 (stops moving)
... and one (or more) new type 2 particule is generated to replace it.

The result:
A fractal, tree-like, very “organic” structure—found in:
• the growth of ice or salt crystals,
• electrical discharges (Lichtenberg figures),
• corrosion, the formation of bacterial colonies,
• or even patterns such as corals, snow, and roots.