3. You say that plants have a sense of smell?
Sure. But to answer this we have to define for ourselves what “smell” is. When we smell something, we sense a volatile chemical that’s dissolved in the air, and then react in someway to this smell. The clearest example in plants is what happens during fruit ripening. You may have heard that if you put a ripe and an unripe fruit together in the same bag, the unripe one will ripen faster. This happens because the ripe one releases a ripening pheromone into the air, and the green fruit smells it and then starts ripening itself. This happens not only in our kitchens, but also, or even primarily, in nature. When one fruit starts to ripen, it releases this hormone which is called ethylene, which is sensed by neighboring fruits, until entire trees and groves ripen more or less in synchrony.
Another example of a plant using smell is how a parasitic plant called dodder finds its food. Dodder can’t do photosynthesis, and so has to live off of other plants. The way it finds its host plant is by smelling. A dodder can detect minute amounts of chemicals released in the air by neighboring plants, and will actually pick the one that it finds tastiest! In one classic experiment scientists showed that dodder prefers tomato to wheat because it prefers the smell.
3B. How about hearing?
This is a bit trickier because while loads of research support the idea that plants see, smell, taste and feel, support for plant auditory prowess is indirectly proportional to the amount of anecdotal information we have about the ways in which music may influence how a plant grows. Many of us have heard stories about plants flourishing in rooms with classical music. Typically, though, much of the research on music and plants was, to put it mildly, not carried out by investigators grounded in the scientific method. Not surprisingly, in most of these studies, the plants thrived in music that the experimenter also preferred.
From an evolutionary perspective, it also could be that plants haven’t really needed to hear. The evolutionary advantage created from hearing in humans and other animals serves as one way our bodies warn us of potentially dangerous situations. Our early human ancestors could hear a dangerous predator stalking them through the forest, while today we hear the motor of an approaching car. Hearing also enables rapid communication between individuals and between animals. Elephants can find each other across vast distances by vocalizing subsonic waves that rumble around objects and travel for miles. A dolphin pod can find a dolphin pup lost in the ocean through its distress chirps. What’s common in all of these situations is that sound enables a rapid communication of information and a response, which is often movement—fleeing from a fire, escaping from attack, finding family.
But plants are rooted, sessile organisms. While they can grow toward the sun, and bend with gravity, they can’t flee. They can’t escape. They don’t migrate with the seasons. As such, perhaps the audible signals we’re used to in our world are irrelevant for a plant.
All that being said, I have to cover myself hear by pointing out that some very recent research hints that plants may respond to sounds. Not to music mind you, which is irrelevant for a plant, but to certain vibrations. It will be very interesting to see how this pans out.
4. Do plants communicate with each other?
At a basic level, yes. But I guess it centers around how you define communication. There is no doubt that plants respond to cues from other plants. For example, if a maple tree is attacked by bugs, it releases a pheromone into the air that is picked up by the neighboring trees. This induces the receiving trees to start making chemicals that will help it fight off the impending bug attack. So on the face of it, this is definitely communication.