Plants evolve like every other form of life, and they've come up with their own forms of aggression, defense, and so on. In the nineteenth century, most theories concerned animals--nature red in tooth and claw, all that. But now scientists are thinking about nature green in root and stem. We realize that plants, in their ceaseless struggle to survive, have evolved everything from complex symbiosis with other animals, to signaling mechanisms to warn other plants, to outright chemical warfare.