Good Vibrations

Stachys byzantina - Lambs Ears @DBG LAH 004

Is that why they’re called Lamb’s Ears?

Can plants hear? At first glance, this seems like a silly question. Plants don’t have ears, so of course they can’t hear. But wait—do organisms need ears to hear? What is hearing, anyway?

Given that sound consists of a series of vibrations that are propagated through air, water, or another substance, then hearing must be the ability to sense those vibrations. And while our ears are very good at sensing vibrations, there are other options.

Think of the last time you were at a loud concert. Didn’t you feel the bass vibrating through your entire body? Or consider insects, who can sense vibrations with their antennae. It’s clear that when it comes to “hearing,” ears aren’t required. Maybe, asking if plants can hear isn’t such a silly question after all!

I recently ran across a fascinating study by H. M. Appel and R. B. Cocroft, published in Oecologia back in 2014, that asks that very question. The scientists wanted to know if plants could hear sound. While there have been plenty of studies involving the effect of music—everything from rock to classical—on plant germination and/or growth, music doesn’t occur in nature. These researchers wanted to know if a naturally occurring sound would elicit a response.

Given that plants are often eaten by other creatures, it occurred to the scientists that if plants would respond to anything, it would be the sound of chewing insects.

As their subjects, the researchers chose cress (Arabidopsis thaliana), a small weed native to Europe. A member of the cabbage family, it is easily grown in a lab and is often used in research. The Cabbage White Caterpillar (Pieris rapae) would make the perfect predator. As you can guess from its name, it loves to dine on cress.

To test their hypothesis, the scientists made a recording of the caterpillars munching on cress leaves. As a control, they also made a recording of wind.

For their actual experiment, leaves on one side of each plant were exposed to simulated munching noises, while those on the other side were exposed to the sound of the wind. As further insurance, they hooked up yet other leaves to the same sound equipment, but didn’t play back any noises. After the leaves were primed in this way, the hungry, hungry caterpillars were turned loose and allowed to chew off about a third of each leaf.

Finally, the leaves were removed from the plants and their chemical content was analyzed.

When a plant is attacked by insects, it often produces defensive chemicals. Some make the leaves taste bad, others may be toxic, but the goal is to make the bugs go away and chew somewhere else. Cress plants respond in this way—they produce compounds in response to predation by caterpillars.

Surprisingly (or did you guess already?), the plants that heard the recordings of the chewing caterpillars produced these defensive chemicals even though they weren’t being eaten yet. And when the final analysis was done, after all the leaves had been chomped on, these leaves had produced more of the anti-caterpillar chemicals than the leaves that just heard wind or silence.

Somehow, the leaves sensed that these sounds meant they were in mortal danger and took pre-emptive action. Think how this ability benefits wild plants. If they can put on their armor, so to speak, before the battle begins, they stand a much better chance of coming out intact.

I’ve previously written about how plants communicate a potential threat via chemicals exchanged by the roots. Chemical warnings are helpful, but the ability to sense vibrations occurs much faster. The question is, how do they hear these sounds?

I have a houseplant on my desk. Now I have a sudden urge to examine it carefully. Are those tiny ears under the leaves?

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