A variant of the diffusion-limited growth model was proposed
by Kaandorp [Kaa1992, Kaa1994] to simulate the growth of marine
sessile organisms; namely corals and sponges.
Both types of organisms develop by adding successive layers
to the structure formed so far (growth by accretion). The layer
size is determined locally by the availability of nutrients that diffuse
in the water surrounding the organism.
In proximity to regions of the organism that have high
curvature, the gradient of nutrient concentration is high, and the structure
grows fast. Near flattened areas the gradient of the concentration is lower,
and the structure develops more slowly. Consequently, a dip may occur near
the center of a flattened area, causing the emergence of a pair of new branches.
The development of a hypothetical sponge simulated using this technique
is shown in Animation 2. | 
