From the excellent Modern Farmer: an article about how new studies show that "plants can tell when they're being eaten, and they don't like it."
Turns out, the thale cress actually produces some mustard oils and sends them through the leaves to deter predators (the oils are mildly toxic when ingested). And the study showed that when the plants felt or heard the caterpillar-munching vibrations, they sent out extra mustard oils into the leaves. When they felt or heard other vibrations? Nothing. It's a far more dynamic defense than scientists had realized: the plant is more aware of its surroundings and able to respond than expected.
(Image via Flickr user Carolyn Conner)