Cognitive Science: An Introduction/Non-Human Communication

Communication in Plants
Plants have primitive ways to communicate with each other. When one tree gets attacked by caterpillars, it releases a chemical into the air that nearby trees smell through holes in the leaf (called stomata). They respond by increasing the amount of certain chemicals in their leaves to make them unpalatable. Trees can warn each other. Trees release different chemicals, depending on whether they are under a bacterial or a bug attack, and nearby trees respond accordingly.

Many insect-eating arthropods also respond to this warning signal so that they can come eat the insects devouring the leaves. We have no reason to believe the trees are intending to communicate with each other, or with the arthropods. A system of communication simply evolved.

Fruit ripens in the presence of ethylene in the air—even very minute amounts of it. This can be interpreted as a primitive sense of smell. Ripening fruits also emit ethylene, resulting in many fruits on the same tree ripening at once. What's happening here is that fruits are communicating with each other to make ripening simultaneous.