Charles Darwin believed wholeheartedly that plants can hear. There was a logical reason for this thinking: Evolution, in its exemplary fashion, surely gave plants with the ability to hear an advantage over others. He decided to test his scientific hypothesis himself by playing the bassoon to his mimosa plant, hoping to provoke a response. None was forthcoming, and Darwin never succeeded in proving his theory, concluding from his failure that he had conducted a “fool’s experiment.”
Even though many people since then have thought that Darwin was correct – indeed, music has even been composed to gladden potted plants – no one ever succeeded in proving it. Indeed, the assumption was always that plants are not capable of hearing.
Among those agreeing with that perception was Daniel Chamovitz, a biologist and author of a 2012 book of popular science called “What a Plant Knows: A Field Guide to the Senses,” which was a bestseller in Israel and published in the United States to much acclaim. Surveying studies focusing on the sensory mechanisms of plants, Prof. Chamovitz, today is the president of Ben-Gurion University, wrote that “in lieu of any hard data to the contrary, we must conclude for now that plants are deaf and that they did not acquire this sense [of hearing] during evolution.”
That sentence does not appear in the book’s updated edition. In fact, 120 years after Darwin played his bassoon, it turns out that the intuition of the naturalist who wrote “On the Origin of Species” was right. In a breakthrough experiment published in 2019, Israeli scientist Lilach Hadany proved that plants – or at least the evening primrose – can hear. And that isn’t even her most amazing discovery.
“We proved that when we play a recording of a honeybee to evening primroses, within three minutes, they produce nectar that is significantly sweeter,” says Prof. Hadany, who teaches at the School of Plant Sciences and Food Security at Tel Aviv University. “But they don’t do that when we play them a recording of a bat.” In other words, not only do the plants hear – they can also distinguish between sounds of different animals and react accordingly. “When I look at flowers today, I see ears everywhere,” she declares.
Chamovitz, who took part in that study, was not surprised that an exciting new detail emerged from it about plants’ sensory capabilities. “It’s hard to surprise me about plants, because I’m aware of their marvelous abilities,” he says. “Precisely because plants can’t escape in times of danger, they were compelled to develop fascinating mechanisms for coping with the environment, and we’re only starting to scratch the surface in terms of understanding them.”
And that was just one study, about one species. An array of experiments conducted in recent years in Israel and internationally have highlighted the intriguing capabilities of plants. Not only do they have acutely honed sensory mechanisms capable of processing information about dozens of environmental variables, such as heat, dryness, salinity, light and minerals – they also possess genuine cognition. Scientists now say that plants can remember, think strategically, analyze situations, learn, cooperate with one another, do mathematical calculations and engage in trade of a sort. An Israeli study even found that garden sweet pea plants gossip about one another. No less.
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Those involved in this research hope to have an impact on what they see as people’s patronizing attitudes toward the plant world. They are finding that although plants may lack consciousness per se, they are definitely far more intelligent than previously thought. Their discoveries are undermining the human tendency to regard plants at best as a backdrop for our own activities. Some scientists are convinced that recognition of plants’ capabilities should be taken a step farther: that they should no longer be treated as objects but as subjective entities possessing inherent rights.
Interacting with fauna
It’s not surprising that it was Lilach Hadany who succeeded in proving Darwin’s hypothesis. An evolutionary biologist, she says she became involved in her experiment out of her “preoccupation with an evolutionary riddle.” Her thinking was that, “if plants are completely deaf, that would not be useful to them in evolutionary terms, because they are involved in a great deal of interaction with a great many animals that emit a great many sounds.”
Together with her Tel Aviv University colleagues, Prof. Yossi Yovel and Dr. Yuval Sapir, Hadany wondered which mechanism in plants would benefit most from reacting to sound.
“The pollen, of course!” she says, explaining that pollinators are what help flora produce offspring, and reproduction is what it’s all about. “In the interaction with animals that emit sounds, a rapid reaction is important because the pollinating insects can always move to the neighboring plant.” It’s worthwhile, therefore, for a plant to invest energy in producing sweeter nectar, so it will be more attractive to pollinators, as that will help it produce offspring more efficiently.
Because plants can’t escape in times of danger, they had to develop fascinating mechanisms for coping with the environment, and we’re only starting to scratch the surface in terms of understanding them.Daniel Chamovitz
In the first stage of the experiment, the scientists discovered that the evening primrose is especially sensitive to the buzzing of the honeybee. But how does the flower “hear” the bee? To identify the organ the plant uses to pick up sounds, the researchers repeated the experiment, with one difference: This time they covered the flower with a bell jar. The result was unequivocal: the plants did not produce the sweeter nectar. The scientists deduced that the hearing mechanism is in the flower itself.
“The flower serves as an external earpiece, the place where vibrations from the air become vibrations among the plant cells,” Hadany explains. “We generally think about colorful petals as the organ intended to attract the pollinators’ visual attention, but our experiment indicates that there may be another selection factor that led to the flower taking the form it did.”
The evening primrose was the first plant Hadany and her colleagues examined, but they hypothesize that other plant species also react to sound. Moreover, she adds, the research also raises a question about the pollinating insects themselves: “We know that the buzzing they emit is a by-product of flight, but perhaps they buzz more than is necessary so that plants will ready themselves for them and prepare them tasty nectar with a high energy value?”
Not far away, in a different laboratory, also in Tel Aviv’s Faculty of Life Sciences, Yasmine Meroz reached a particularly surprising conclusion: Plants can calculate an average. For real.
Dr. Meroz, who comes from the realm of physics, discovered this when, in the course of her research, she examined the movement of sunflowers vis-a-vis light (phototropism). “We know that plants grow in the direction of light, and it’s easy to provide controlled stimuli at different times and intensity of light” to test that, she explains. By illuminating the plant at a certain intensity for a specified time, and then examining the plant’s angle of inclination toward the source of light, the intensity of its reaction can be determined. If light at a higher level of intensity or for a longer time is added afterward, the plant reacts accordingly: Its angle of inclination toward the light will be greater.
Meroz: “The interesting thing is when you project light on the plant for a minute, and then darken the space for a few minutes and then supply light again for a minute. The plant responds as though it were illuminated for two consecutive minutes – in other words, it somehow comes up with a total length of time. When we change the duration and the intensity of the light – so that we’re projecting it half of the time, twice the time and so forth – we discover that it bends toward the source of the light according to the average duration and intensity of the light provided. Not only does the plant ‘know’ how to tally or accumulate stimuli, it also ‘knows’ how to calculate an average. That is definitely a complex process of information processing. It’s exciting, because it leads us to the hypothesis that plants carry out decision-making processes.”
Besides being able to calculate an average, plants can also learn. That was proved by an Italian scientist, Monica Gagliano, in the course of her work at the University of Western Australia. Like Darwin, Dr. Gagliano and her colleagues examined the learning abilities of the mimosa plant, which closes its leaves in response to a threat.
In a 2014 experiment, Gagliano and her colleagues dropped a mimosa plant from a certain height without causing it any damage, and examined its reaction. This was not a random choice: Falling is not an experience the mimosa (or any other plant) would have undergone through its evolution, so it’s ideal for examining the learning process.
The researchers expected that the first time the plant fell, it would “sense” a threat and close its leaves – and that indeed happened. However, it quickly learned: When it found that no harm came to it from being dropped, the leaves began to remain open when that happened.
Two years later, Gagliano and her colleagues proved the existence of even more complex learning, involving a connection between different environmental factors. The scientists planted green peas at the base of a Y-shaped labyrinth. Air blew in from only one of the channels of the maze, and the light also emanated from there. The plants naturally chose to grow in the direction of the light and the air. When the experiment was repeated, this time without light, only with air, the scientists discovered that the plants grew in the direction of the air source alone. They had internalized the conditioning: Where there is air there is also light. “Our results show that associative learning is an essential component of plant behavior,” the researchers wrote in an article in Nature in 2016. “We conclude that associative learning represents a universal adaptive mechanism shared by both animals and plants.”
Three years earlier, Canadian researchers had discovered that plants of the Impatiens pallida species, known as the pale jewelweed, identify whether the plants adjacent to them are relatives or not, and form a strategy accordingly. When the flower is planted next to its relatives, it grows normally; but when it’s planted next to strangers, it devotes a larger amount of its resources to growing leaves, which comes at the expense of its roots. This apparently gives the jewelweed an advantage in competing for sunlight, and reduces sharing of minerals through the roots with non-relatives.
There are plants that can estimate in advance the height of neighboring flowers that will compete for light. Michal Gruntman, an expert in plant sciences who’s also from Tel Aviv University, found that a lovely yellow flower called the creeping cinquefoil reacts differently when planted next to far taller plants, as compared to being planted in a flowerbox next to plants slightly shorter than it is.
Dr. Gruntman: “We saw that the plants are capable of estimating the height their neighbors will reach in the future and thus assess the degree of competition they will have over sunlight; in the wake of that information they can select an appropriate strategy. When the flowers were densely surrounded by relatively low plants, they stretched upward – but when they were densely surrounded by tall plants, they ‘gave up’ growing high and compensated for this by increasing the size of their leaves. That allows them to cope better with the lack of light they will experience later due to the shade their tall neighbors will cast on them. We discovered that the flowers ‘know’ how to make a decision about when to use which strategy on the basis of estimating the future height of their competitors – even before the latter have reached their final height.”
Efforts are now underway to harness the various capabilities of plants for technological purposes. The European Union recently invested 7 million euros in an ambitious international research project called GrowBot, which hopes to utilize the movement of plants to create a new generation of robots.
Dr. Meroz’s laboratory is one of nine labs taking part in the project. “The idea is to develop a robot inspired by climbing plants,” she notes. “Most of the robots that exist today were designed to imitate the animal world. They have legs or wheels and move like animals do. Their bodies don’t change shape, and they can move from place to place. But if you place a robot like that in the ruins of a city, for example, in a place that is unstructured and doesn’t have a regular pattern, it will have difficulty moving around – just like most animals or humans.”
What sort of flora can overcome obstacles easily? Climbing plants. “Those kinds of plants are able to proceed from tree to tree, to climb on walls and stones and even to crack rocks if needed,” Meroz says. “There is a fundamental difference between the movement of a plant and the movement of an animal. Instead of self-propulsion, as in the case of animals, the plants add matter to themselves. In other words, they grow and in that way they move. So a plant-based robot will constantly change its shape and add matter to itself.”
Meroz’s role in the project is to analyze plant behavior and convert it into mathematical models, on the basis of which the climbing robot will be able to “decide” where to grow to.
“Let’s say the plant gets light from the left side, but on the right side there’s a hook it can latch onto,” she says. “So we check which is more important for it and how it makes its calculations. Using time-lapse photography that monitors climbing plants, one can absolutely see thought. You see that they don’t latch onto everything they encounter; they check and then decide whether it’s stable and whether it goes upward or sideways. There’s a strategy. The answers to these questions become mathematical models which in the future will be entered into the robots’ control system.”
Meroz notes that “we tend to think of plants as being closer to a floor tile than to something that’s alive, and I imagine that’s because plants move too slowly for us to perceive their movement, so we find it difficult to detect their ‘wisdom’ with the naked eye.” However, she continues, “If we imagine a parallel universe in which plants move at our pace, then we would probably not treat them like tiles, but as being close to animals.”
To illustrate how sophisticated their movement is, Meroz created an art installation together with the artist Liat Segal. The work, called “Tropism,” currently on display as part of the exhibition “Plan(e)t,” atthe art gallery of Tel Aviv University, includes 20 giant robotic stalks – about twice as high as average human height – that move independently by means of sensors, in accordance with the changing light in the venue. The result is mysterious and hypnotic, enabling visitors to view phototropism on an immense scale.
Plants can remember, think strategically, analyze situations, learn, cooperate with one another, do mathematical calculations and engage in trade of a sort. An Israeli study even found that garden sweet pea plants gossip about one another. No less.
Tamir Klein always admired trees, he says, so it’s not surprising that he chose to devote his life to studying their world – and to undertaking research about the communication they engage in with their surroundings. Yes, trees communicate. They communicate with one another, with fungi and also with bacteria.
A study that Dr. Klein, a plant ecophysiologist, conducted at the University of Basel, showed that trees don’t just communicate with each other. They also “trade” nutrients – carbon, above all – by means of a subterranean network of fungi. The hypothesis that trees share nutrients was already commonly held, but Klein was the first to succeed in showing that the process in fact occurs in the forest.
The result astounded the world of science: It turns out that 40 percent of the carbon at the ends of tree roots reaches them not by means of photosynthesis (that is, via the plants’ leaves), but from networks that develop between the trees. In other words, trees conduct a thriving cooperative economy.
I meet Klein in a greenhouse at the Weizmann Institute of Science in Rehovot, together with research assistant Yaara Oppenheimer-Shaanan, and doctoral student Sophie Obersteiner. Dr. Oppenheimer-Shaanan removes a sealed plastic cover from a transparent flowerbox of Pistacia trees, revealing a splendid system of roots. The group is examining the communication between the tree and the bacteria in the soil. It’s known that plants and certain bacteria act symbiotically – the vegetation affords the bacteria a source of carbon, and the bacteria supply the plants with various minerals – but little is known about how they interact.
Thanks to the research in the Weizmann Tree Lab, we now know a little more. The scientists noticed that the roots of cypress trees in the forest secrete different substances in summer and winter, and decided to examine those substances in the lab. They found that the cypress secretes 13 different compounds in a particular way in the summer, during the dry period; with their aid, the trees are effectively attracting bacteria to their roots to help them in their hour of need.
And the bacteria do help. An examination of the minerals and other substances found in the leaves during the dry time showed an increase in the amount of iron, zinc and phosphates. In other words, the trees “called” upon specific bacteria that were able to help them cope with the effects of the dryness. To understand how sophisticated this is, it’s important to know that soil usually contains some 10 million different types of bacteria. From among this vast diversity, the trees attracted particular bacteria via the “smart” communication systems in their roots, for the purpose of helping them respond to the dryness. How do they do that?
“If one thinks of the matter a tree secretes as taking the form of a letter [of an alphabet], then it’s possible to effectively combine letters into words and through them to ‘summon’ the desired bacteria,” Klein explains. “Let’s say the Pistacia tree has a problem that only the bacillus bacterium can alleviate, so it secretes specific letters that call only it to come.” According to Klein, when the tree gets water again, it secretes different substances, thus broadcasting other needs to the surroundings.
“We found the language that the plant uses to speak with the specific bacterium it needs,” adds Oppenheimer-Shaanan.
In another Weizmann Institute greenhouse, Klein and his team are examining the communication between plants and fungi.
“It was thought that between trees of different species there was competition, but today we know that below the surface, deals are made for mutual aid by means of fungi,” Klein says. “We discovered that each type of tree has a network of connections with specific fungi. It’s like a club in which a fungus of a particular type gathers several types of trees around it, and within that club each participant knows what it needs to give and what it can receive.”
Pea plants are also able to communicate with their surroundings, especially when they’re in distress. “When a plant is suffering from something, it secretes a wide range of substances and thus communicates with the plants around it,” says Ariel Novoplansky, director of the Swiss Institute for Dryland Environmental and Energy Research at Ben-Gurion University. “It’s a language they understand.”
Prof. Novoplansky discovered that green pea plants situated next to a plant of the same species that is experiencing heat or dryness, will themselves respond to the stress of that situation, even if not directly affected by it.
Moreover, a plant that responds to distress signals will itself broadcast such signals to neighboring plants, setting in motion a sort of chain reaction. “It’s a kind of gossip about approaching danger that passes between plants,” Novoplansky says.
He and his colleagues also found that a plant’s response to distress signals from its neighbors can have significance for the future: A plant that picks up a message from a neighbor suffering from dryness will remember it. When that plant is exposed to such a situation later on, it has a better chance of survival than plants that did not receive prior signs of distress from their neighbors.
These discoveries give rise to some fundamental insights about plants: They are capable of sensing, learning and remembering. What’s sublime about this is that plants do all this without a brain or a nervous system. Which leads to another thought: Is our focus on the brain – as the organ in which consciousness, intelligence and decision-making processes reside – exaggerated? Most scientists who study plants would say it is.
“Let’s not forget that we humans also have memories that are not in the brain, but in the muscles and in the immune system,” Prof. Chamovitz says. “If I play the piano, my muscles remember the movements. If I am sick, my immune system remembers the bacteria. We ostensibly need a brain to conduct the conversation that you and I are having now, but plants prove that it’s possible to survive for millions of years without having this conversation.”
Some scientists are convinced that recognition of plants’ capabilities should be taken a step farther: that they should no longer be treated as objects but as subjective entities possessing inherent rights.
“I think neurons are overrated,” Lilach Hadany adds. “A computer doesn’t have neurons – there are other ways to process information. Plants are living beings that respond to their environment, remember information, process it, react accordingly and also communicate. For humans, acoustic communication is important, and people who don’t have that capacity may be suspected of not being able to think. So in this sense I believe that our discovery – that plants, too, have acoustic communication – will help promote plants and raise awareness of their capabilities. We know plants react to light, which means they have a type of sense of sight. They also react to vaporous substances, meaning they have a type of sense of smell. It has also been proved that they respond to contact, so there’s a sense of touch. And now it’s been discovered that they react to sounds, too, so a certain sense of hearing has been added. They definitely possess higher capabilities than has been customarily thought.”
Ethics of eating
Novoplansky’s research about gossip and memory in plants served as inspiration for a controversial piece the Basque philosopher Michael Marder published in The New York Times in 2012. “Imagine a being capable of processing, remembering and sharing information – a being with potentialities proper to it and inhabiting a world of its own,” he wrote. “Given this brief description, most of us will think of a human person, some will associate it with an animal, and virtually no one’s imagination will conjure up a plant. Since [last] Nov. 2, however, one possible answer to the riddle is Pisum sativum, a species colloquially known as the common pea.”
Marder describes Novplansky’s experiment and then wonders: Given what we know about the consciousness of peas, is it ethical to grow and eat them? Many did, and will, guffaw at that. But Marder insists that at the very least we must ask that moral-philosophical question. A year later, he did just that in his book, “Plant-Thinking: A Philosophy of Vegetal Life.”
“The research findings… at the very least, ought to prompt us to rethink our relation to plants,” he writes in the article. “Is it morally permissible to submit to total instrumentalization living beings that, though they do not have a central nervous system, are capable of basic learning and communication? Should their swift response to stress leave us coldly indifferent, while animal suffering provokes intense feelings of pity and compassion?
“Evidently, empathy might not be the most appropriate ground for an ethics of vegetal life,” he continues. “But the novel indications concerning the responsiveness of plants, their interactions with the environment and with one another, are sufficient to undermine all simple, axiomatic solutions to eating in good conscience. When it comes to a plant, it turns out to be not only a what but also a who — an agent in its milieu, with its own intrinsic value or version of the good.”
Gagliano raises similar issues. In her book “Thus Spoke the Plant” (2018), she refers to the high cognitive abilities of flora as evidenced in various studies. She then asks whether the time hasn’t come to grant them basic legal rights, as animals have. Research findings cast doubt on the classic dichotomous division between flora and fauna, she writes, adding that such scientific developments emphasize even more urgently the need to grant legal rights to the environment – that will also encompass plants.
Gabliano describes how the discourse on animal rights initially drew responses of mockery and ridicule, but in the end succeeded in narrowing somewhat the traditional gap between laws for humans and the absence of laws for all other denizens of the planet.
No few scientists will likely claim that Marder and Gagliano have gone a step too far. For his part, Chamovitz says there is no moral issue about eating plants, simply because they feel no pain.
“We, the animals, feel pain because we have specific types of receptors that are programmed to respond to pain,” he explains. “Plants have no such receptors. The point of departure for the moral question about consuming plants is that when we cut a plant it suffers – but that is anthropomorphic thinking. It’s completely ethical to eat plants. In fact, many plants are built so that their fruit will attract animals to eat them, and in that way they scatter their seeds and create generations of offspring.”
Chamovitz also cautions against attributing consciousness to plants. Their various mechanisms, he says, amazing as they may be, prove nothing about the existence of consciousness, only about a more advanced sort of life than we thought previously. Other scientists second this cautionary note. One of them is Marcelo Sternberg, from the School of Plant Sciences and Food Security at Tel Aviv University. “In all these studies that attribute intelligence to plants, one has the feeling of hyper-anthropomorphism. We are not obligated to understand systems of nature only by way of anthropomorphic attitudes, Prof. Sternberg says.
“Plants are plants, and we are us. Plants react to the environment due to evolutionary processes that took place over millions of years and were intended to produce healthy, fertile offspring. It’s a fascinating world, which contains more than meets the eye. But it’s important to give plants enough respect in order not to turn them into lower-level beings. We need to look at them for what they are. After all, they were here long before us and will probably also be here long after us.”