whatA bacterial colony on agar. Rich agar grows a smooth disc; starve it and the edge breaks into fingers, then fingers into fingers, a branching coral. One slider takes it from one to the other, and the colony records the slider's history as rings.

whyBen-Jacob's colonies showed that the same organism draws utterly different shapes depending only on how hungry it is. The piece shows the Mullins-Sekerka instability, the reason a starved edge cannot stay smooth, as a living thing.

howThe agar holds a nutrient that diffuses. The colony eats every scrap it sits on, so it is a perfect sink, and an empty cell touching the colony becomes colony with a chance proportional to the nutrient reaching it. When food arrives faster than the edge can eat, the disc stays round. When the edge fights over a thin supply, any bump reaches richer ground and shadows its neighbours, and the disc branches. The slider sets how rich the far agar is.