The Plant Which Dials 911
The picture below shows a drama at a corn seedling. When the caterpillar attacked the corn plant, the plant emitted a chemical signal which was found to be specific to the type of insect which was attacking the plant. Wafting through the air, this particular chemical signal was picked up by the type of wasp which lays its eggs in this particular type of caterpillar. To the rescue! The wasp lands on the damaged part of the corn seedling, following the chemical beacon, and attacks the caterpillar.
The article also reported studies of tobacco plants which also called parasitic wasps to attack their attackers. Those tobacco plants apparently emitted the chemical signal from the whole plant. Even if the attacking caterpillar was removed, the wasps would buzz around the plant for some time in response to the emitted chemical distress signal.
"Fine-tuned plant response to insect attack", Science News, December 20, 1997, Vol 152, p391.