4. Plants have friends and enemies – Recent studies suggest that plants behave differently depending on who their neighbours are. When surrounded by ‘friendly’ plants, including genetic relatives or helpful species that limit pests and weeds, they grow slowly, perhaps to share resources. But when they detect a rival such as fennel, which secretes chemicals to inhibit other plants, they grow far more aggressively. Plants recognise these friends and foes thanks to chemical signals emitted from their leaves or roots, and some studies even imply that plants can detect sounds produced by their neighbours.
Plants can also alert their fellow flora to attacks from herbivores or parasites. When a tomato plant is attacked by aphids, for instance, it releases volatile chemicals into the air. The plants that pick up on these signals respond by producing their own chemicals to repel the parasites and even attract wasps that prey on aphids. Other species use symbiotic fungi living on their roots as messengers.
