Season 5: Episode 6

The Primrose Path

If a being doesn’t have ears, can it hear? And if it doesn’t have a mouth, can it talk?  In this episode, we spend a golden afternoon conversing with the flowers, plants, and trees. 

Guests

Extras

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Credits

Transcript

[00:00] INTRODUCTION

BEE BUZZING

AMY: I was just sitting in front of my computer trying to figure out how to start this episode, which is all about plants and pollinators and sound. And I heard this noise across the room. And it made me wonder, wait, did I leave my phone on vibrate?

BEES BUZZING

AMY: And then I walk over to track down the sound and then I see it's an enormous bee, just outside my window, moving from flower to flower on my snapdragons, which are in full bloom right now. (laughing) Nice!

BEES BUZZING

MUSIC

AMY: I grabbed my sound gear and walked out into my yard, and everywhere I looked—and listened—the bees were on a bender. They were drinking nectar with so much gusto that they made me want to try some. They disappeared into the blossoms and lost themselves there, then stumbled out later with pollen all over their legs and wings, and flew off into the morning sun.

BEE BUZZING

AMY: This is pollination in action. Plants can't pick themselves up and move toward potential mates—they need to attract helpers to move the sperm from the male plants to the ovaries of the females. So the flowers have essentially thrown a nectar-drinking party because they want to have sex. And it works. Animals like these insects pollinate around three-quarters of all flowering plants. Without them, a lot of plants would die, including many of the foods we depend on daily.

BUZZ

AMY: The amorous intentions of plants are the hidden force behind a lot of activity on our planet. Take the sound I heard earlier—on the surface, this seems like an interaction between two animals: a human and a buzzing bee. But the flowers are actually driving all the action. They produced the nectar which attracted the bees, that then attracted me. All of which is to say: flowers can do things. They are doing things. Making things happen around them. Like all living things, plants using whatever resources are available to get their needs met. And one of the things that's available in abundance is sound.

BUZZ

AMY: So what, if anything, are they doing with all of that acoustic information?

AMY: Welcome to Threshold, I’m Amy Martin, and we are living through a botanical paradigm shift. A growing body of research is disrupting and reorganizing what scientists thought they knew about trees, flowers, fungi, and all things vegetal. In our last episode, we met Heidi Appel and Rex Cocroft, the two researchers who proved that plants can tune into the vibrations made by insects doing unfriendly things to them, like caterpillars biting into their leaves. 

CATERPILLAR CHOMP

AMY: But what about all of these sounds made by friendly visitors?

BUZZ BUZZ

AMY: Is it possible that the plants can hear pollinators happily buzzing among the blossoms? While I’m wandering around in my yard, listening to bees, are the flowers listening too?

LILACH: I wondered about this question.

AMY: Dr. Lilach Hadany didn't only wonder about this question, she decided to try to answer it. In this episode, we’re going to follow her down that primrose path into a whole new world of planty possibilities.

INTRO MUSIC

[04:17] SEGMENT A

AMY: Flowers are a time-honored part of human courtship ritual. I mean, just try to imagine a wedding with no flowers. And Dr. Lilach Hadany says this is no accident. Attracting suitors is exactly what flowers were designed to do. It's just that plants use them to attract creatures with six legs. 

LILACH: It is actually well known that the plant signals to the pollinator. It is large, and colorful, and emits smells.  

AMY: Lilach is an evolutionary theoretician at Tel-Aviv University. And she says scientists have studied how plants send pollinators these kinds of visual and chemical signals through their flowers for a long time. But the assumption has been that sound wasn’t really a factor; that there was no acoustic communication going on between the plants and the pollinators. And this seemed odd to her. 

LILACH: Plants are communicating all the time. Why wouldn't they use sound?

AMY: Almost all flowering plants need to attract pollinators, and a lot of those pollinators make a fair amount of noise. Was all of that sound really just wasted on the plants?

LILACH: Pollination seemed to me like one of the cases where responding to sounds would be immediately beneficial for the plant. So there was an evolutionary puzzle here.

MUSIC

AMY: It was a puzzle she wanted to solve, so she designed an experiment focused on nectar—the sweet liquid plants make as a reward for animals that visit their flowers. Producing nectar is costly for plants, so it’s to their advantage to time the production of it really precisely—right when they're most likely to be visited by the birds and bees they most want to attract. And breezing by isn’t enough; ideally, the pollinators will get very interested and hang out a while, and then spread the love around.

LILACH: If a flower is pollinated by an animal that makes sounds, then it is possible to prepare pollination at exactly the time that the pollinators are likely to be around. So it is not necessarily responding to a single pollinator. But a single pollinator is a good indication that other pollinators may be around.

AMY: So the plant might be kind of…in very metaphorical terms, the plant might be “thinking”—in quotes—“There's the sound of one pollinator. So there's probably going to be others around. So maybe it would help me to produce more nectar.”

LILACH: So I would put it that natural selection could act on the plants to produce improved reward in times where pollinators are likely to be around.

AMY: So this was what Lilach decided to test.

MUSIC

AMY: When plants are exposed to sounds made by pollinators, do they change something about their nectar production? 

LILACH: If you hear a pollinator nearby, would the plant say, “me, me, me!” Sort of.

AMY: Because if they did, that would be a sign that they were “hearing” the pollinators somehow.

LILACH: And it might be preparing for fertilization.

AMY: Lilach and her team drained the nectar from a bunch of beach evening primroses—tough little plants with big yellow blossoms. Then they played the kinds of buzzes that pollinators like bees might make.

BUZZ

AMY: Did you have, like, a little portable speaker? I'm trying to picture what it looked like.

LILACH: So the speaker is portable. We were trying to mimic the bee going around the bush rather than standing in front of the flower and buzzing.

AMY: I didn't ask Lilach if costumes were involved, but I really hope so. They also played other sounds to the plants—sounds that had nothing to do with pollination. And sometimes they played them no sounds at all.

LILACH: And after three minutes we test the newly produced nectar. And we discovered that indeed the flowers that are exposed to the sound of a bee had a higher concentration of sugar then the ones that were not exposed to these sounds or that were exposed to irrelevant sounds.

AMY: Lilach was shocked. There was a measurable change. When the plants were exposed to the sounds of bees, they appeared to sweeten their nectar.

LILACH: It was like, no, this cannot be true. And then we redid the whole experiment.

AMY: They actually redid it multiple times, inside and outside.

LILACH: Eventually it was done several times by different people in different contexts. So we were convinced.

AMY: Heidi Appel and Rex Cocroft had shown that some plants respond to an acoustic stimulus by sending out a chemical weapon. Lilach and her team demonstrated that they can also respond by making a love potion. Sweetening themselves up, maybe so pollinators will linger longer on their flowers, and come back again later. And while Heidi and Rex were focused on the vibroscape—acoustic waves moving through the body of the plant—Lilach's work was about airborne sound. Bee buzzes just like the ones I heard through my window.

AMY: One of the sentences I loved from the paper was this one. “We found that the flowers vibrated mechanically in response to these sounds, suggesting a plausible mechanism where the flower serves as an auditory sensory organ.” So in non-science speak, it's almost like you're hinting that flowers are kind of ears? Is that right?

LILACH: So I think it is something like the external ear. It's an amplifier of the sound.

AMY: Ah-ha.

LILACH: And then we can think—when we think of flowers—are they selected to be visible to the pollinator or to hear the pollinators better?

AMY: Wow. I love that! That's so cool!

LILACH: It really converted my view of walking in fields with flowers. Suddenly I find myself...little small ears and who would be a good ear?

AMY: Everyone agrees we need more studies on more species to figure out what’s really going on here, but when this research was published in 2019, it catapulted Lilach into the global spotlight. You may have read about it in National Geographic, or The Atlantic, or any number of other places. There’s something thrilling about western science validating the very ancient idea that plants are not passive set pieces—they are main characters. They’re doing things. And they’re highly relational. They’re in constant communication with their surroundings and with other living things, especially insects. And that led Lilach to ask another daring question: if plants can detect sound, could they also be producing it? 

AMY: We’ll have more after this short break.

Break

[11:47] SEGMENT B

AMY: Welcome back to Threshold, I’m Amy Martin, and I’m going to ask that question from before the break again, using terms that plant scientists would definitely not use: now that we know that plants can hear, can they also talk? Or at least make sounds of some sort?

TOMATO PLANT

AMY: The answer is yes. And this is what it sounds like.

TOMATO PLANT

AMY: This is the sound of a stressed out tomato plant. It’s making these sounds as it’s drying out—most likely as little air bubbles pop inside its planty veins. These noises were recorded by Lilach Hadany and her team at Tel-Aviv University.

LILACH: We used most of the time tomato and tobacco, but we recorded several other plants also, and we directed two sensitive ultrasonic microphones at the plant 

AMY: OK, so I'm picturing a plant almost like it's being interviewed. It has microphones set up around it.

TOMATO DRY

LILACH: And then we heard these clicks.

AMY: They’re happening in the ultrasonic range, too high for us to hear.  They’ve been converted into our audible range, and condensed in time. When Lilach and her team first heard them, they were skeptical. They thought…

LILACH: Perhaps it's not the plant. Perhaps it's an insect or other sounds.

AMY: So they repeated the experiment over and over and over, including in a soundproofed box in the basement of a building, where no other sounds could intrude.

LILACH: Then we recorded them also in the greenhouse and in the botanical garden. And then we tried like wheat and corn and grapevine. So basically we had to be ultra, ultra careful. And we were sure that the sounds are emitted by the plants themselves.

MUSIC

AMY: Lilach was surprised to find herself in the spotlight again after this research was published in March of 2023. News outlets around the world covered the story. 

NEWS PERSON 1: Researchers have found that plants can make noise.

NEWS PERSON 2: There is an actual sound that tomato plants make when they’re thirsty. A whimper, or scream if you will.

NEWS PERSON 3: Well this discovery here is probably one of the biggest in the last few years.

NEWS PEOPLE: I wonder if our dogs can hear them? Our dogs often bark at something random. Probably they’re like, hey, the plants are hungry! Feed them!

AMY: We’ve actually known that plants make ultrasonic clicks and pops since at least 1966, when they were documented by a botanist named John Milburn. But those sounds were detected by sticking wires inside plants, so it wasn’t clear if the vibrations ​​stayed trapped inside the stems and leaves or if they made it out into the surrounding ecosystem. Lilach’s research showed that the noises are indeed audible outside of the plant, and that plants make more of them when they're stressed. Like, when they need water or after they've been cut. And together, those two facts point to something important, and pretty exciting: the sounds plants make have the potential to shape the behavior of other living things. For example, picture a field full of tomato plants drying up on a hot summer afternoon, each of them popping away. 

TOMATO POPS

AMY: It might make quite a racket in the ultrasonic range. And those sounds might be meaningful to the right kinds of listeners…

LILACH: For an organism with the relevant hearing ability, like a moth flying through the field in early summer may be hearing plenty of sounds and also getting a lot of information.

AMY: Maybe all those little popping sounds cause the moth to avoid those plants, or to feed more heavily on them. Or to respond in some other way. And maybe that affects other animals, like bats. AMY: Lilach’s study didn’t test all that—she had to answer the most basic questions first. But now we know that the sounds plants produce at least have the potential to influence all sorts of other living things. In short, plants are talking. So now we can who’s listening? 

AMY: I'm curious how you see the future of this field of plant bioacoustics. What are you hoping for? 

LILACH: So I really think we are just seeing the edge of the iceberg at the moment. And expect that we would record plenty of sound, plenty of sounds from different plants under different circumstances. And we might be able to understand the acoustic interactions of plants with their environment, which is the most exciting part for me. I'm hoping that within a few years we will be somewhere completely different, because now we know that there is this layer of acoustic information. That we can investigate.

AMBI: walking through Vindeln experimental forest

AMY: OK, I'm wandering around in a Swedish forest looking for people who might be attaching microphones to trees.

AMY: I’m in northern Sweden now, walking through a dense and fairly dark woods. It’s kind of storybookish, with lots of lichen hanging off of the branches. But instead of forest gnomes, there are mysterious pieces of scientific equipment tucked among the mossy rocks. 

AMY: Signs of research projects around but no signs of researchers.

AMY: This is an experimental forest, where scientists from area universities come to do research on one of Sweden’s most valuable and plentiful natural resources: trees.

AMY: Hej!

JONATAN: Hej Amy!

AMY: Hi!

AMY: I’ve spotted the scientists I’m looking for. Dr. Jonatan Klaminder and his three-person crew, who are here to set up an experiment that will run throughout the summer.

JONATAN: My name is Jonatan Klaminder. I'm a professor at Swedish University of Agriculture Sciences in Umeå. I'm totally driven by curiosity, so you can’t say a field that I'm really interested in. I'm interested in so many things.

AMY: One of those things is soil bioacoustics. Jonatan has done some really interesting studies on the sounds that ants and earthworms make underground. But lately, he’s turned his attention to trees.

JONATAN: Well trees is a soil organism, if you think about it. I mean, the root system is underground. And it's difficult to not think about the trees if you're in Sweden. I mean forestry is an important part of our economy. And then, of course, I like being out among trees. So trees, there's many reasons why we should keep track of the trees. Trees are important.

AMY: The person that I talked to that maybe had the closest connection to your work was Lilach Hadany in Tel Aviv. Is what you're doing a continuation of her work?

JONATAN: Yeah, absolutely. I mean, her work is really important because they show that it work for tomato. But they don't have bark. Trees in Sweden have bark. 

MUSIC

AMY: All trees have some kind of bark, actually—or at least a protective bark-like layer. Jonatan wanted to know if these ultrasonic pops were loud enough to make it through that layer of soundproofing where they could be heard in the open air. And he also wanted to know if they could be heard in a real-world setting.

JONATAN: When you have rain, when you have wind, when you have cars. That's what we're testing. So if we can hear them, well, that is the million dollar question. We’re the first to try this, so we'll see. We’ll see.

AMY: I’m joining this team on their very first day in the field, as they figure out where to set up their equipment and how to keep it running, outdoors, all summer long. The first order of business is to set up a tent. It’s big and green and wedged somewhat awkwardly into a bumpy little spot. But it’s not meant for sleeping in.

JONATAN: So this is our lab. We're actually building our outdoor lab. We have some electronics that can't handle rain. But the field workers, they can handle the rain, right? It’s just the electronics.

AMY: The field workers—Lena, Mathias and Sebastian—look vaguely skeptical but choose not to comment. Maybe because what they’re really wondering is if they’re going to be able to handle the clouds of bloodthirsty mosquitoes tracking our every move.

JONATAN: I mean, the mosquitoes love you the most, Mathias. But, Sebastian, you have been drinking bear blood, so they don’t, they won’t touch you.

AMY: I don't know, I've been seeing a lot of them on him.

JONATAN: Yeah, put on a jacket, maybe. It’s in the car.

AMY: Jonatan is pursuing this question of whether trees make sounds that can be heard in the open air, in part, for a very simple reason. Because we don’t know the answer. 

JONATAN: So I mean we’re exploring the unknown, and I think that's the driver.

AMY: But he’s also interested in potentially applying what he learns. After decades of steady increases, forests in Sweden have started growing more slowly, possibly because of drought. That’s a problem for the forestry industry, but also for all of us. Trees are one of nature’s best carbon capture and sequestration systems; every ring on a tree represents more planet-warming gasses sucked out of the atmosphere and locked away, potentially for hundreds of years. But when trees slow their growth, they draw less carbon out of the air.

JONATAN: Our forests have lost productivity. And we don't really know why. There's different theories. And one of the theories is that if you have a really, really dry year, the trees suffer from cavitation, which is bubbles that forms inside the xylem. 

AMY: Sai-lem, or zai-lem, is spelled X-Y-L-E-M. Remember that, Scrabble players. It’s the part of the tree responsible for water transport. Trees take in water through their roots, and pump it up through the xylem to their needles or leaves high above.

JONATAN: The basic is that a tree pumps up water through the xylem, and downwards it pumps sugar through the phloem. 

AMY: It’s quite remarkable that they can do this when you think about it—every tree is a carbon-eating, gravity-defying water pump. But an essential part of that process is keeping the pump primed. Water molecules want to hang together; they kind of pull each other along in a little train if there’s enough of them present. But if the roots can’t find water, and that train of molecules is broken…

JONATAN: The water transport’s going to cease. And not all trees can just get rid of these bubbles.

AMY: Once that link in the water train is broken, it’s really hard for the tree to reconnect it. So these bubbles, or dry patches in the xylem, can stay dry, even after the water returns.

JONATAN: You might knock out the water transport for several years. So that bubble could be fatal for that part of the xylem. And if you have many bubbles of course, then you shut off the water transport, and the worst case scenario is that the trees die.

AMY: The process of these bubbles forming, and the water transport shutting down—that’s what makes these pops and clicks. That’s cavitation. 

JONATAN: But so far, no one has managed to hear those cavitation explosions in an outdoor setting through bark. So it's a challenge. 

AMY: If Jonatan is able to detect these sounds in the open air, through the bark, the next logical question would be the same one Lilach Hadany is asking: who or what might be listening?

AMY: Do you suspect that the trees are producing sound that is ecologically relevant?

JONATAN: So now we're definitely speculating…

AMY: But maybe, just as a thought experiment, a bark beetle might be able to hear the pops of a tree under stress. That's an insect that kills a lot of trees in Sweden, the U.S., and other countries.

JONATAN: So if that's information is around, why should the beetle not use it? That's not far-fetched I think. I mean, it can be something mind-blowing there. But it could also be the dead end. I mean that's kind of exciting.

AMY: Yeah it is!

AMY: Maybe, in the future, foresters might be able to listen to trees the way doctors listen to our hearts and lungs to keep tabs on our health. Maybe the sounds trees make can serve as a kind of early warning system, letting us know when they’re suffering from drought before it becomes acute. And maybe we can figure out which species of trees, or varieties within species, can handle dry conditions better, so we can grow more climate-resilient forests. Not just in Sweden, but everywhere.

JONATAN: Maybe you can put the drone with a microphone and fly over to the forest and sort of get an overview, how many trees in your forest suffers from this. 

AMY: Ah ha.

JONATAN: We aim for something applied that we need to do some basic research for before we can actually do something.

AMY: Yeah, to see if it’s even detectable.

JONATAN: It could be a total failure.

AMY: (laughs) That’s science, right?

JONATAN: That's science. Been there, done that….

AMY: All of the things that Jonatan hopes to learn start with that embrace of uncertainty. That willingness to do bold science.

AMY: You seem like you enjoy this.

JONATAN: Yes.

AMY: Why?

JONATAN: I mean, because it's unknown. And, I mean, when I was younger, I was a junkie for adrenaline. I can't do that now because I should be old and smart and wise. So I do, this is like academic bungee jumping. I mean, it's high risk. I like that. It’s high risk, high reward, but it could be high risk, low reward also, but it's I mean. Our community are steered towards… the research community is steered towards getting a number. What's goes up and what goes down. But the processes behind them are, I think, more interesting than just getting the numbers. I want to understand how soils and forests functions.

AMY: A couple of months later I checked in with Jonatan to see how things went. He assured me that the fieldworkers had not been carried off by mosquitoes. He also sent this:

TREE CAVITATION SOUND

AMY: These are the sounds of cavitation happening inside a birch tree. Jonatan’s team captured them in the open air.

TREE CAVITATION SOUND

AMY: As with the plants that Lilach Hadany recorded, these are ultrasonic sounds that have been converted into our hearing range. So Jonatan’s gamble paid off. He now has documentation that when trees dry out—or at least, when some trees dry out—the sounds they make are loud enough to push through the bark, and could at least theoretically be detected by other organisms. Including humans, if we have the right equipment. And if we decide we want to listen.

AMBI: mountain meadow

AMY: I'm on a hiking trail in western Montana, looking up at beautiful, rugged mountain peaks. And there are wildflowers all around me...purple, yellow, red, violet blue. And there are also tons and tons of bees right here, moving from flower to flower to flower.

MUSIC

AMY: There's a bee with its entire head stuck into the trumpety flower of a larkspur. (laughter) Just going for it. And I wonder if it's possible that that flower heard that bee and maybe sweetened its nectar just a little bit. Obviously the bees are doing things, but maybe the flowers are doing things too.

AMY: I really like plants. I like to look at them, smell them, eat them, and just be around them. But even as I appreciate plants, I've always positioned myself as the perceiver, and them as the perceived. I’ve assumed I was the subject and they were the object. But now I realize that’s just scientifically inaccurate. Plants are perceivers as well. They are listening—possibly even tuning into some of the same sounds I am—and they’re also talking in their way. They’re in a sort of conversation with the life around them; there is acoustic information getting passed between the pests and pollinators and plants that has the potential to affect all the participants. And if we start tuning in, that conversation could shape our behavior, too. And it’s hard to imagine how that could be anything but good for us.

CREDITS

AMY: This episode of Threshold was written, reported, and produced by me, Amy Martin, with help from Erika Janik and Sam Moore. Music by Todd Sickafoose. Post-production by Alan Douches. Fact checking by Sam Moore. Special thanks to Lilach Hadany, her collaborator Yossi Yovel, and Jonatan Klaminder for sharing their plant recordings. Threshold is made by Auricle Productions, a non-profit organization powered by listener donations. Deneen Wiske is our executive director. You can find more about our show at thresholdpodcast.org.