HEARING AN ENEMY. “Chili seedlings quicken their growth when a nasty sweet fennel plant is nearby, sealed off from the chilies in a box that only transmits sound, not scent. The fennel releases chemicals that show the plants’ growth, so the researchers think the chili plants grow faster in anticipation of the chemicals — but only because they hear the plant, not because they smell it.”
http://www.livescience.com/27802-plants-trees-talk-with-sound.html