Using some cunning molecular trickery, they were able to prove that the plants determine whether neighboring plants are friend or foe based on the other plant's root secretions.
Plants don't have a nervous system. As a matter of fact, you can cut about any piece you like off pretty much any plant and get that piece to grow into a whole new plant. Because of this most people write off plants as stupid, self replicating machines, good only for oxygen and a shelter for wildlife.
Hopefully this little bit of information will show that there is much more to our photosynthesizing brethren than meets the eye.