Sensing sound by touch instead?
Critics of Gagliano's research point out that no one has found structures resembling a mouth or ears on corn or any other plant. Nor do the group's studies prove that plants "talk" among themselves.
"This is pretty provocative and worth following, but it doesn't really provide a lot of evidence that these are acoustic communications," said Richard Karban, a University of California, Davis, expert in how plants communicate via chemical signals.
But simpler life forms manage just fine without complex sound receptors and producers. Walnut sphinx caterpillars whistle by forcing air out of holes in their sides. Flying insects perform death drops when they sense a bat's sonar clicks. Earthworms flee the vibrations of oncoming moles. (Listen to caterpillars communicate with their butts)
Of course, there may be another explanation for the apparent response to sound reported by Gagliano. One that could also account for the century of researchers and home gardeners (including Charles Darwin) who manipulated plant growth with music.
Could a sense of touch be why plants seem to respond to sound?
Even humans can perceive sound without hearing it, said Frank Telewski, a botanist at Michigan State University and an expert on how trees respond to wind.
"How many times have you sat next to someone who has their car stereo at full blast? You can really feel it pounding in your chest," he said.
Trees perceive and respond to touch, like wind or an animal passing on a trail. And like the wind, sound is a wave that travels through air.
In fact, a tree needs wind to grow, Telewski said. "If you stake down a seedling, you do it a little bit of disservice, because a tree needs to perceive motion. It's like physical therapy for the tree. If you stake it too tight, it does not allow the plant to produce stronger tissues."
But Telewski is open to the idea of plant communication by sound. He said in the last few years, researchers in China have shown they can increase plant yields by broadcasting sound waves of certain frequencies. Other groups have investigated how different frequencies and intensities of sounds change gene expression. Their studies find that acoustic vibrations modify metabolic processes in plants. Some of the beneficial vibrations also drive away pesky insects that munch on crops.
"We're not there yet," Telewski said of the effort to prove plants communicate. "Sometimes a fantastic hypothesis can turn out to be true, but there has to be fantastic evidence to support it."
Karban, from UC Davis, notes that the plant field is not very receptive to new ideas. The idea that plants could talk via scent, or volatile chemicals, was roundly pooh-poohed in the 1980s, but Karban and others went on to prove that plants including sagebrush warn their neighbors of impending danger by wafting chemical signals into the air. "At times in my career I've tried to push new ideas and it's been very difficult," Karban said.
Gagliano remains undeterred by the skepticism.
"I was guided to sound by the long tradition in folklore of people talking to plants and listening to plants and plants making sounds," Gagliano said. "I wanted to see if there was any scientific basis for something that stays so stubbornly in our culture."
But the corn root clicks are at the lower end of the human hearing range. "In theory, we could hear it, but realistically, these were emitted from roots in the ground, so the truth is we probably wouldn't hear it," she said. And the fizzy bubble bursts in xylem are ultrasonic, about 300 kiloHertz, detectable only by insects and some other animals.
This spring, Gagliano and her collaborators will screen more plants for communication skills. "We will see whether some groups of plants might be more chatty than others, and if some plants have specific requirements for sound," she said. They also plan to record sounds emitted from plants and play them back and see what kind of response, if any, they produce in other plants.
"Shamans say they learn from the plant's sounds. Maybe they are attuned to things we don't pay attention to," Gagliano said. "It's really fascinating. We might have lost that connection and science is ready to rediscover it."
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Image Gallery: Carnivorous Plants Stinky Seduction: Flowers Use Shocking Scents to Attract Bugs Nature's Giants: Tallest Trees on Earth This article originally appeared on LiveScience. Copyright 2013 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.