Let's talk about PVZ, that is to say, Plants Vs. Zombies. It starts very simply: an eccentric genius breeds new strains of plant life (and fungus) to beat back the zombie hordes. It starts with basically turrets firing in straight lines, but it quickly evolves into something more. MUCH more. The plants become capable of movement of their own volition, begin sprouting without being planted there, and even acting on their own authority, as we see how Crazy Dave no longer even instructs us in PVZ: Battle for Neighborville. The plants have begun evolving into monstrous states without any experimentation from Dave, as shown by the colossal cyclops tree, Dreadwood. Forget the zombie horde, the real problem of PVZ is going to be the revolt of the plants.
