The Glasshouse Engine

1. An empty cell grows a plant if at least two of its four edge neighbours are plants.
2. A plant becomes a grazer if an edge neighbour is a grazer. Otherwise it stays a plant.
3. A grazer survives if an edge neighbour is a plant. Without a plant, it becomes fungus when an edge neighbour is fungus; otherwise it leaves empty ground.
4. Seeded fungus stays fungus. An empty cell beside both a fungus and a plant becomes fungus, unless two plants already claim it for plant growth.
5. A rock stays rock when the world advances. Rocks are not food or neighbours that trigger growth. You can repaint them with any brush.

Fungus never appears without a seeded fungus. The starter keeps its original three-state layout; choose the Fungus brush to introduce it. Rock is terrain that stays fixed during steps, not a locked tile. Reset starts without rocks; Clear removes them.

All cells update together from the previous step. Outside edges are empty; they do not wrap. These are fictional rules for a coding toy, not a scientific ecology model.