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JB: This is Earth and Sky. If an insect is damaging a plant, the plant may send out an airborne signal -- like an odor -- attracting just the right predators -- perhaps flies or wasps to attack the offending insect.

DB: Penn State entomologist Jack Schultz told us that not only can plants send chemical signals to insects -- they can send them to other plants.

Jack Schultz: If you put an unwounded plant in the presence of a wounded plant something will pass through the air -- or at least the evidence is -- that a signal will pass through the air from the wounded plant to the unwounded plant, and turn on defenses in the unwounded plant.

JB: So far, scientists can only see the effects of these chemical signals that fly through the air.

Jack Schultz: We just don't have the instrumentation or the gadgets necessary to collect those molecules outside in open air and demonstrate that Molecule A went from Plant 1 to Plant 2 or from the plant to an insect, and is the cause of the change in the behavior of the receiver organism.

DB: Schultz is working with engineers to develop devices that might one day detect the complex signals sent out and received by plants. Tomorrow -- more on plant senses and defenses. Our thanks to the National Science Foundation -- where discoveries begin. We're Block and Byrd for Earth and Sky.

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