A number of years ago, some wacky botanists suggested that trees might communicate with each other by releasing chemicals into the air, presumably for the purpose of signaling danger. Most other botanists dismissed this out of hand, saying that these substances would be far too diluted by the time they wafted to other trees that there would be no signal left, not to mention that other trees couldn't possibly have receptors for these signals.
Well, evidence for just this has been growing over the past decade, with work showing that plants do indeed release many substances that do things like facilitate communication between the organs of a single plant and between plants, as well as repel destructive herbivores. A recent paper in Current Opinion in Plant Biology (Protective perfumes: the role of vegetative volatiles in plant defense against herbivores, Sybille B Unsicker, Grit Kunert and Jonathan Gershenzon, 2009, 12:1-7, discussed this week on the BBC World Service radio program, Science in Action with Jon Stewart) adds to the understanding of the role of such chemical release in plant defenses.
Details of the plant immune system have been known for a long time, but these have generally concerned how the plant launches a cascade of internal defensive responses to attack. When a bug bites, the plant responds by killing the area around the attacker, and launching a systemic chemical defense. Now it seems that volatiles released by plants can actually repel bugs that come to feed, attracted to this huge meal that can't get away. At least one volatile mimics a compound produced by aphids when they've been attacked, warning other aphids to stay away. Other volatiles inhibit pests from depositing their eggs on or in leaves. Indeed, more mechanisms are being documented all the time.
The system is more complicated than just repelling herbivores, however. Some of these substances actually attract herbivore predators and parasitoids. For example, researchers attached artificial caterpillars to the leaves of trees infested with autumnal moth, and birds attacked these caterpillars much more frequently than those on uninfested trees, presumably because of the volatiles being released that attracted them. Other attackers are more susceptible to parasitoids when they are on an infested tree, again presumably attracted by the substances being released by the tree.
Among other things this teaches us, it is yet another beautiful example of the importance of signaling in evolution. We find this a fascinating area of research, and yet more support for the idea that if you are open to looking for cooperation, you'll find it everywhere.