Scientists have known for many years that plants emit sounds, and also that they respond to sounds—for instance, they make noises when fluids move rapidly up and down their stems. Plants also move towards or away from sounds as they also do when stimulated by light. From time to time, claims have appeared that plants grow better when spoken to encouragingly. The Daily Mail reported in 2009 the Royal Horticultural Society's conclusion that “talking to your plants can really help them grow faster.” Charles Darwin’s great granddaughter, Sarah, who took part in the Society's experiment, turned out to be the champion talker: reading from On the Origin of Species caused her plant to grow nearly two inches taller than the control. Elementary school science projects include experiments “to find out whether plants respond to human speech,” asking students to think about how plants communicate, what types of messages they send one another, and how vibrations affect plants.
As an organic gardener, I knew that legume roots were colonized by nitrogen-fixing bacteria, but it didn’t occur to me to ask how the bacteria in the soil found the rootlets to grab on to. If I’d thought about it, probably I’d have said these were random encounters, or that perhaps the rootlets emitted some kind of chemical that was attractive to the bacteria. It would never have occurred to me to think that bacteria might have been responding to vibrations in the soil made by disturbances when the rootlets expanded and grew. But after listening to Monica Gagliano at the conference, and then reading many of her articles published in the scientific literature, I know that this is very much a possibility. Gagliano is an Italian scientist who worked in the laboratory of Stefano Mancuso, one of the most important contemporary researchers in the field of plant intelligence. Currently she's a postdoctoral research fellow at the Centre for Evolutionary Biology at the University of Western Australia.
Gagliano claims that plants not only communicate with soil bacteria and other creatures, but that they communicate with each other, by means of vibrations which she is willing to call sound. “Vibrations are sound,” she replied when I asked her if she made any distinction between them. For a scientific realist sound is a feature of the external world and consists of waves inside a medium that are set in motion by vibrations. Plants, obviously, vibrate; their cells, for example, vibrate at the microscopic level continuously, just as human cells do. Perhaps the problem humans have in accepting plant communication via sound is that we commonly think of communication proceeding from vocal cords to ears, and of course plants have neither. But it doesn’t take long to realize that sound communication doesn’t need vocal cords—just think of musical instruments, or animals signaling by sounding parts of their bodies—and that sound vibrations can be felt in various parts of the body, not just the ears—think of the deep bass of an organ, for example. Assuming that communication is the transfer of information, then for it to take place there must be a transmitter, a receiver, and the information itself. On those grounds, plants qualify. Evolutionary biologists believe, further, that living creatures must have evolved through natural selection so that communication is a specific adaptation to their environment, not merely a by-product of some other activity.
Gagliano reports that plants not only emit sounds but also detect them and modify their behavior accordingly. ”Preliminary investigations of both emission and detection of sound by plants indicate that plants have the ability to detect acoustic vibrations and exhibit frequency-selective sensitivity (i.e., plants respond to the same range of frequencies that they emit themselves) that, in turn, generates behavioral modifications. Hence, the relevant question is not about whether plants have evolved to detect and respond to sound waves or vibrations in their environment, but how and why they do it (italics in original).1/ These are all good questions, yet it seems to me that numerous additional experiments are needed before one can be reasonably certain that sound communication is normal among plants. Scientists may more confidently proceed to the “how” and “why” after more instances of the “what” are confirmed. Incidentally, I’m making the distinction here between sound communication between plants, versus that between plants and non-plants (e.g., bacteria, insects); I believe we have a good deal more evidence for the latter, and much less for the former.
For readers who’d like an overview of plant communication within the larger field of plant intelligence, I recommend a New Yorker essay by Michael Pollan which appeared less than a year ago. (Thanks to Tyler Kinnear who called that essay to my attention.) Gagliano’s research is mentioned, but the essay focuses on the work of her mentor, Stefano Mancuso, and also gives a good deal of space to the skeptics who wonder whether it’s a stretch to call plants intelligent or communicative. Of course, skeptics also cast doubt on animal intelligence and communication, claiming that the human varieties are different in kind as well as degree. Still, it seems to me that considering sound communication among animals, and even among plants, offers perspective on sound communication among humans, its evolution and functions, which must be helpful in considering sound, music, and sustainability over the long term for all living creatures.
1/ Monica Gagliano, "Green Symphonies: a call for studies of acoustic communication in plants." Behavioral Ecology doi:10.1093/beheco/ars206, 25 November 2012, pp. 789-796.