When I began as a stewardship volunteer over a decade ago, I was pretty “ho-hum” about plants. Like a lot of people, plants seemed to simply provide lovely decorations in the form of flowers or a green backdrop for the more interesting flying, crawling, singing, running, leaping world of animals. I always loved trees, and rolling landscapes, but just never really paid attention to what I saw as the static plants. I understand some frustrated botanists refer to this lack of attention as “plant blindness.” That was me in 2015.

To we mobile humans who take movement as a given, plants may seem very static and passive – but, as it turns out, they’re not at all! Like all living beings, they need to avoid and cope with danger and find partners for reproduction and they must do all of it while firmly rooted in one place for their whole post-seed lives! What I’m learning is that plants – one of the earliest living beings on land – managed to survive and reproduce in myriad and astonishing ways that I’d been unaware of all my life! I guess I never seriously considered the challenges of being a plant. Imagine trying to survive predators when you can’t run, fly, hide or fight! Imagine trying to produce and care for young when you’re forced to rely on insects, wind, rain, or a nearby stream to even mate – and then having to depend on other species and the elements to carry your seeds out into the world. What challenges plants have overcome in the millions of years they have lived on this earth!

Plants Interact with the World Around Them…but Not Quite Like We Do.

I thought, just as you may have, that plants sit passively in the earth and just cope as best they can. But no! In a book evocatively entitled, The Light Eaters by Zoë Schlanger, I learned that science is exploring the ways that plants perceive and react to sound in the form of vibration, like we do, but they don’t exactly “hear.” They sense and react to touch as electric pulses transported by chemical signals, like we do, but they don’t exactly “feel.” They can transmit and receive information from their surroundings like we do, but they can’t “speak.” As you’ll see, sound, touch and communication are crucial to how plants defend against predators and foster their offspring but it’s all done without the ears, mouths, nerves, etc. that we have. So for me, reading Schlanger’s book about recent plant research feels like I’m studying alien beings that have surrounded me all my life! And what’s especially fascinating is that though we seem so very different from plants, we may have more in common with these beings than I ever expected. So please join me in exploring the mysterious, unseen ways plants “hear, ” “feel” and “communicate”!

So, Can Plants “Hear?’ Well, Yes, But…

In the chapter on how plants “hear,” Schlanger describes collaborative research by two botanists, Dr. Heidi Appel and Dr. Rex Cocroft at the University of Wisconsin Madison. Botanists have known for some time that when plants are chewed or damaged in some way, they react by increasing or changing the natural toxins in their leaves to make them distasteful or even toxic to their predators. That’s pretty amazing! They began by recording the sound produced by the caterpillars of a Cabbage White butterfly (Pieris rapae) chewing on a leaf of a Mouse-eared Cress (Arabidopsis thaliana.) Evidently, those caterpillars love Mouse-eared Cress and are a common predator for that plant. They make quite a racket! If you’d like to hear it, scroll down on Appel’s faculty profile page at the University of Houston.

Then they removed the caterpillar, returned with their recording and simply played the sound of the caterpillar chewing to the plant. That grating “ratatat” noise alone set off minuscule vibrations in the plant and – ta-da! – the plant increased its leaf toxins even without the caterpillars present! The plant reacted as if it had “heard” a sound that it somehow associated with danger. I’m impressed!

Schlanger also reports that “researchers in China and the United States found that the tiny hairs on Mouse-eared Cress leaves function like acoustic antennae, picking up and vibrating at the frequency of incoming sounds.” Hairs on plants, called trichomes, have many forms and ways of helping plants. They help them retain moisture, fight off the effects of frost , drought or UV exposure and deter small predators by simply making plants fuzzy. But the study that Schlanger cites could add to the possibility that they also conduct sound for plants. It occurred to me that if it turns out to be true for one of our local hairy plants, Penstemon/Hairy Beardtongue (Penstemon hirsutus), that would be quite a noise! Only more research will tell.

Clearly, plants can sense sound, but it appears that it may only be sounds relevant to their survival. Appel and Cocroft repeated the experiment with the sound of a small fan to simulate a soft breeze or the sound of a plant-eating Leafhopper’s mating song which is as loud as the caterpillar chewing but rhythmically different. No chemical response. A soft wind isn’t threatening and leafhoppers don’t eat Mouse-eared Cress, so the plant didn’t react.

Bottle Gentian (Gentiana andrewsii), a local, native wildflower, depends on husky bumblebees to thrust their way into their closed blossoms and then buzz loudly to create enough vibration to cover the bee with pollen which it then carries off to other blossoms. Schlanger reports that research on some buzz-pollinated plants “can be induced to release their pollen when played a recording of bees buzzing.” So sound may only be relevant to plants if it means “man the barricades!” or “Here come the pollinators!” I guess playing Beethoven, singing or even a rhythmic rap is unlikely to impress or delight your average plant.

Plants React to Touch in Varied Ways

Plants That React to Touch Quickly

Along with reacting to sounds, plants react strongly to touch of many kinds. Reading the chapter in The Light Eaters entitled “Alive to Feeling,” I remembered my queasy fascination as a young girl when I learned about the Venus Fly Trap (Dionaea muscipula), the carnivorous plant that snaps shut almost instantly when an insect lands on it. So while reading Schlanger’s book, I went looking for info on this predatory plant. According to many sources, a few “trigger hairs” within the Fly Trap’s leaves sense the vibration of struggling live prey . The toothy- looking trap only closes completely, however, when a second trigger hair fires at about 20 seconds. It then waits for 5 more triggers to react before it starts digesting its food. In this way, the plant saves energy by not closing entirely or digesting until it’s sure that it has trapped a nutritious fly or spider, not a dead leaf or a pebble. Pretty complex behavior that I’d never have expected in a plant!

The leaves of the Sensitive Plant (Mimosa pudica) also react the moment it is touched. This touch response is believed to successfully dislodge predators. Here’s a link to see the plant reacting. (Photo by Hrushikesh in the public domain at Wikimedia Commons.)

Some Plants React to Touch By Changing Their Shape

Schlanger notes ” Scientists have long observed that virtually all plants are highly sensitive to touch of any kind and can change their growth accordingly.” Back in the 1970’s, a plant physiologist from Ohio named Mordecai Jaffe found that repeatedly touching common plants like cucumber, barley, or common beans with a fine brush could make these plants slow their upward or outward growth, making them shorter and sturdier. That shape might give the plant a survival advantage if frequently exposed to high winds or moving animals. When Jaffe stopped brushing, the plants quickly returned to growing upward. Catching up for lost time, maybe?

Schlanger explains, “Later, the genomics revolution made it possible to see just how impactful touch is on a deeper level.” Researchers touched the most common plant used in botany experiments, Mouse-eared Cress (Arabidopsis thaliana), by stroking it with a brush and then analyzed the plant’s genetic response. “Within thirty minutes of being touched, 10 percent of the plant’s genome had altered…Touched multiple times, the Arabidopsis cut its upward growth rate by as much as 30 percent, just as Jaffe had found years before.” Becoming shorter and sturdier could help a plant cope with heavy winds and movement of animals.

Many Plants React to Damage by Setting Up Chemical Defenses

Early on, Schlanger says, plant scientists determined that “plants will produce an immediate spike of electricity at the place where they’ve been cut, chewed or otherwise damaged. ” Schlanger visited the lab of Dr. Simon Gilroy, a biology professor at the University of Wisconsin Madison who studies, among other things, plant electricity and how it relates to plant wounding. In 2013, he and his research assistant at the time, Dr. Matsatsugo Toyota, developed a way to demonstrate how a plant reacts to damage. Like Jaffe, Gilroy’s chose to use Mouse-eared Cress , the plant Schlanger refers to as botany’s “lab rat.” Dr. Toyota altered the plant’s DNA so that its calcium would glow green under a florescent light microscope. Why calcium? It seems calcium increases in a plant when electricity passes through it. So they made a small cut at the end of one leaf, expecting to see a green florescent glow near the cut, showing that the plant had reacted to the electrical pulse caused by the cut. But they got a bigger reaction than they’d bargained for!

A green glow did appear near the cut as expected but then the green glow flowed out into the whole plant. In about two minutes, the whole plant reacted to the electrical pulse caused by a cut on one leaf. In human terms, the whole plant “knew” about the cut! What a sight! Here’s a link to a short, speeded-up video posted by Gilroy’s lab showing how the green glow near a tiny cut in a leaf on the right side of the plant created a flood of florescent green.

How Touch Works in Plants

The plant’s reaction to damage is much like ours, really. Schlanger reminds us that for humans, “Electricity is entangled in every aspect of our living. It is behind our ability to move, think, breathe.” Burning your finger, for example, produces an electrical signal that is instantly transported to your brain with the help of nerves and chemical neurotransmitters. You “feel” that in a split second. Plants are also “extremely electrically conductive” and the glutamate that plants produce and the calcium in them function as transmitters of electricity throughout a plant much like the neurotransmitters in our nervous system. But of course, plants have no nerves and no brain to initiate a reaction. According to Schlanger, Gilroy believes that there likely is a store of glutamate in plant cells that spills out from the cells that are cut; it then conducts the electrical impulse from cell to cell much like nerves do in humans. That combination of electricity and the glutamate released by the cut triggers the plant’s immune-related genes and they initiate the toxic flow throughout the plant that make its leaves less palatable to predators.

Schlanger reports that researchers have determined that trichomes trigger some plants to respond to the tiny footsteps of caterpillars and respond chemically.

It seems science can show us that plants sense touch, but, I remind myself, it’s not necessarily “pleasure” or “pain.” Those are animal kingdom words. What we seem to know is that when damage occurs, the plant registers the touch, has a chemical stress reaction and does something to defend itself. It appears to me there’s much more to be learned about that process in plants. But the simple fact that plants respond to touch is fascinating enough for me! I’ll watch for more research.

Plants Communicate If and When It’s Important

Plants Send and Receive Defensive Chemical Messages to Other Plants

In earlier blogs, I’ve described the discovery that trees connect through vast networks of parasitic mycorrhizal fungi that live in and are fed by the sugars in their roots. The fungi transport nutrients and water from distant locations back to the plant. Trees in the sun pass nutrition through the fungi to saplings in the shade. But much earlier, science began to learn how plants also “communicate” through the air.

Plants are amazing chemists. After all, they were the first to master turning sunlight, water and carbon dioxide into food in the form of sugars They can also concoct chemical scents called volatiles or pheromones that vaporize and float through the air. Once afloat, these signals can be “breathed in” by trees through the tiny holes on the underside of their leaves called stomata. The stomata actually open and close like mouths to take in air and release oxygen!

In 1983, David Rhoades, a University of Washington chemist and insect zoologist watched trees near the university being decimated by tent-producing caterpillars for several years – but then suddenly, the caterpillars began to die. He wanted to know why. Schlanger described his answer published in a journal called Plant Resistance to Insects. He found that the trees that the the caterpillars hadn’t yet reached were prepared for the caterpillar assault due to “airborne pheromonal substances” emitted by the damaged trees. Those scents had triggered healthy trees to flood their leaves with tannins, making them distasteful or even poisonous to the caterpillars infecting the neighborhood.

Many botanists and foresters at the time were understandingly skeptical of Rhoades’ findings at first. Trees communicating?! But six months after Rhoades’ article, Ian Baldwin and Jack Schultz of Dartmouth College put two sugar maple seedlings in the sterile environment of a growth chamber at their laboratory. The seedlings didn’t touch each other but they shared the same air within the chamber. The researchers tore a piece off the leaf of one seedling to simulate the action of a predator and as predicted, the torn seedling increased the tannin in its remaining leaves. And then, thirty-six hours later, the leaves of the untouched seedling inside the sterile chamber also filled with tannin. Signal received. Response initiated. The saplings had “communicated” chemically. What a step forward in understanding plants!

Some Plants “Recruit” Animals to Fight Off a Predator

In 1998, Consuelo De Moraes, a grad student at the University of Georgia (now an ecologist at ETH Zurich, a prestigious research university in Switzerland), studied a strange complicated interaction between a caterpillar, a tobacco plant and a parasitic wasp. She found that when a caterpillar chews a tomato or tobacco plant, the plant’s genes analyze the caterpillar saliva. Then plant then combines 10 different organic compounds available in the plant to create a specific volatile or scent. The scent vaporizes when released through the tiny holes on the underside of plant leaves (stomata), drifts in the air and attracts a certain tiny parasitic wasp that preys on that specific species of caterpillar, the one chewing the plant! It lays its eggs on the caterpillar and when the larvae hatch, they consume the host! Yecch! Evidently the message was received by the plant in the photo below! The defense may not save this plant in time, but it might save other tobacco plants around it.

Evidently, the same ability was found in tomato plants. Essentially, it seems, plants are mixing chemicals that attract a particular insect that will attack an insect larvae that is eating the plant. Could it be that someday science will learn how to use natural plant chemistry to protect plants from predators instead of insecticides? I have no idea if that would ever work, but the idea is out there among agricultural botanists.

Some Lure in Assistants to Nourish and Disperse Seeds

Many plants have evolved to use ants to “plant” their seeds. It’s called Myrmecochory. These plants evolved to produce a nutrient- rich, fatty substance at the tip of their seeds called elaiosomes. As an article on the the Xerces website explains, “The elaiosome is a delicious snack to an ant but just an expendable accessory for the seed!”

Ants drag whole seeds with their elaiosomes attached back to the ants’ underground nests and feed the fatty, nutritious substance to their young. They then discard the seed in their subterranean trash piles, where the seeds are protected from birds and weather. The seeds also nourished by the “compost.” Ant young benefit from being fed; the plants benefits from seed dispersal, protection and fertilization. Definitely a win-win situation, eh? Here’s a short slideshow of a bevy of native spring ephemerals in our parks that “recruit” ants for seed planting.

Some Plants “Trick” Bumblebees into Pollinating

Dr. De Moraes , the researcher i n Switzerland, found that Yellow Monkey Flowers (Erythranthe guttata) release a special scent when they have an exceptionally large amount of pollen. Bumblebees can detect that scent at quite a distance. They naturally buzz over to collect some and leave carrying Monkey Flower pollen to deposit on other blossoms. But producing big loads of pollen uses a lot of the Monkey Flower’s resources . So it will occasionally release its “come hither” scent when it actually doesn’t have much pollen, because, of course, it wants the smaller amount of available pollen to stick to the fuzzy bumblebee . A case of false advertising! We have lavender Monkey Flowers in Michigan but they are somewhat uncommon. And I haven’t yet found any research that shows that it’s as “deceptive” as its yellow relative out West.

Throughout The Light Eaters, Schlanger provides an amazing variety of research on these and other sensing capability of plants. Some are more speculative but show where the science might take us. Here I’ve limited myself to sharing a few research findings in three areas that intrigued me. The book and its extensive notes at the back can provide you with lots more to explore, if you’re so inclined.

Learning to Describe Plants, an “Alien” Species

As brilliant and intriguing as the research in The Light Eaters is, I approached the book and writing this piece with a certain uneasiness. I finished the book accepting Schlanger’s approach as sincerely reporting what she learned, but like me, she found herself forced into a mild case of that scientists’ anathema, anthropomorphism! (Merriam Webster defines it as “an interpretation of what is not human or personal in terms of human or personal characteristics.”) Do we really want to say that plants “hear,” “feel” or “recruit” for example? Those words draw useful analogies, but in using them, it’s easy to oversimplify the differences between the plant and animal kingdoms. That’s why I put quotes around them to remind you and I to take care when applying human analogies to other living things.

Reading Schlanger’s book and writing this blog, I occasionally felt like a respectful space traveler using earth vocabulary to understand and report on an alien species. It’s challenging to find language for living beings who live with us in our world, but perceive it so differently. Our language was created by humans to explain our human reality. The lives of plants are radically different ours, even with their similarities. Rooted in one place, plants evolved to use the tools of electricity and chemistry to solve the challenges common to all living beings on this planet. Evolution provided us with the same basic tools for the same ends, but we use them in radically different ways with radically different results. We humans use the portals of our eyes, ears, mouths and such translated through our nerves and brains. And our vocabulary is wedded to understanding the world that way.

Despite the language challenges, though, Schlanger’s book was another dazzling revelation. It allowed me to see beyond the seeming passivity of plants and appreciate them as fully living beings. They sense our shared world, react to it, even change it, but in ways not readily apparent, except to science. Hooray for science! Hooray for The Light Eaters! I hope you can catch at least a whiff of my excitement about seeing plants in a whole new light. Thanks for being here to share my surprise and delight!

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