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Planet Earth

A String of Unusual Experiments Claim to Show Plants Can Think. Few Scientists Are Buying It

For years, an Australian researcher has drawn headlines with tantalizing evidence that plants can think. A growing chorus of scientists question her claims.

By Anna FunkAugust 28, 2019 1:00 PM
George Romney Emma Hamilton Sensibility - Wikimedia Commons
Mimosa plants have long inspired intrigue with their ability to move when touched. (Credit: stipple engraving, R. Earlom 1789, after painting by G. Romney, Wellcome Collection/Wikimedia Commons)

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When The Secret Life of Plants came out in 1973, Lincoln Taiz was a graduate student, just embarking on what would become a many-decades long career in plant biology. Plants, the book revealed, can make their own trace elements through fusion, just like the sun. More, they can recognize people. If someone committed a crime in front of them — plants’ fear could be measured with a simple lie detector test. And the book took it one step further, claiming that plants are conscious.

Taiz didn’t buy it.

“I could see the senior people in my field getting very exercised about this,” he recalls. “It’s embarrassing to plant biologists to have people believing stuff like that.”

The plant science community, he says, “teamed up with animal biologists and they did experiments to try and repeat some of these things, and of course it was all completely false.”

Yet the idea that plants may be sentient has not gone away; in fact, it has continued to gain interest — even in the scientific community. Monica Gagliano at the University of Sydney is now one of the most outspoken researchers on the subject. Her thrilling claims can be found in a laundry list of news outlets from the Economist, to Forbes, and yes, Discover. On Monday, The New York Times became the latest outlet to profile her.

But in science, extraordinary claims require extraordinary evidence — and an increasing number of plant scientists are pointing out: The evidence just isn’t there.

Plant Neurobiology

Decades after The Secret Life of Plants, in 2006, a subset of plant biologists reignited a second renaissance in thinking about whether plants can think. They christened a new field that they called “plant neurobiology,” a term that makes some plant experts cringe. But one of proponents’ main arguments for using the neuro language was that plant cells communicate with each other in a way similar to animal cells — using action potentials, a form of bio-electricity.

Mimosa pudica - shutterstock
The Mimosa pudica plant folds up its leaves when touched. (Credit: Tamara Kulikova/Shutterstock)

“[They were] really claiming that plants have neuron-like cells, and that they behave just like neurons in animals,” explains Taiz, now a professor emeritus at the University of California, Santa Cruz. “And then they were extrapolating from that to say that plants, like animals, have feelings, emotions and all kinds of anthropomorphic [human-like] qualities. And all of this was wrong.”

In the years since, the study of plant neurobiology has continued to grow. And in 2014, a team of researchers published one of the most famous studies in the developing field. Led by Gagliano, then at the University of Western Australia, the team experimented with sensitive plants, Mimosa pudica. The species folds up its leaves when it’s disturbed — a defensive strategy that presumably deters hungry herbivores.

Gagliano’s team found that if you drop a potted Mimosa repeatedly, it will eventually stop folding its leaves. But if you switch to a different disturbance — a vigorous shake — the plant will fold its leaves again. But drop the same plant again a month later, and still nothing happens. No folding. The team concluded that the plant is smart enough to not only know the difference between a drop and a shake, but it’s also capable of learning that being dropped isn’t a threat worthy of folding up. They also took their claim a step further. The team claimed all of this is evidence that the plant can remember well into the future.

The study sparked a media frenzy that continues today.

The latest era of interest in plant consciousness ramped up when writer Michael Pollan (The Omnivore’s Dilemma; The Botany of Desire) saw Gagliano present her Mimosa-dropping work at a scientific conference and wrote about it in a 2013 New Yorker article, The Intelligent Plant.

Taiz didn’t buy it.

Mimosa pudica - Discover/Shutterstock
(Credit: Discover, after Richard Duppa 1812 illustration/Courtesy Biodiversity Heritage Library and Jamesbin/Shutterstock)

Then in 2016, Gagliano and her team claimed they had trained pea plants to exhibit a Pavlovian-style response to a breeze from a fan. Again, they concluded, their results meant that plants could learn and remember, just like animals do.

Other researchers began trying to replicate Gagliano’s studies. Kasey Markel, a researcher at the University of California, Davis, tried replicating Gagliano’s 2016 Pavlovian pea experiment. And it didn’t work — he didn’t get the same results.

Armed with this new support from Markel, Taiz published an article in Trends in Plant Science last month — the same journal that published the announcement of “plant neurobiology” 13 years earlier. In it, he outlined his rationale for why Gagliano’s studies were flawed, and why none of the plant neurobiology studies point to plant sentience. He put it all in context of a definition of biological consciousness.

Gagliano wasn’t impressed. “This piece is simply someone’s opinion,” she said, in an email.

But Taiz isn’t alone. “It’s crazy the way these experiments were designed. And even crazier that it was accepted for publication,” says Mannie Liscum, a plant scientist at the University of Missouri who was not involved in these studies.

Pavlov’s Plants?

Markel, a graduate student, was particularly interested in Gagliano’s experimental work with pea plants (Pisum sativum). Instead of ringing a bell before serving dogs food like Pavlov did in the late 19th century, she blew on plants with a fan before turning on a grow light. Instead of salivating, the idea was that the plants could be conditioned to grow in the direction where they expected the light to appear.

To test the idea, she put week-old pea plants inside a Y-shaped pipe, so that they’d reach the junction as they grew and would have to “choose” whether to grow left or right. For three days, the plants were kept in the dark except for three 1-hour training sessions. During these sessions, half of the plants were given a light on one side of the Y, and a fan on the other. The other half were always exposed to a light and fan together. From one session to the next, the fan and light would switch sides.

Then, she tested the plants to see what they had learned by leaving them alone in the dark. The control plants, those growing with no fan, all continued to grow in whichever direction they had last seen the light, Gagliano reported. But the test plants, growing with a fan, grew opposite of where they had last seen the light. They had been conditioned by the fan.

Markel found this intriguing. “I was like, if it’s true, it’s the coolest thing ever,” he says. But he noticed the paper, published in Scientific Reports, had a few comments posted from other researchers, biologists Andrew Zink and Robyn Crook from San Francisco State University, who had significant concerns about the study design. He decided to investigate — to replicate the findings, with the eventual goal of building on them.

Gagliano's Pea Plant Experiment - Jay Smith
(Credit: Jay Smith/Discover after Gagliano et al. 2016 Scientific Reports)

So in 2017, Markel reached out to Gagliano and her team, hoping to get their input on how to confirm their findings. He says he emailed her six times: before, during and finally after running the experiment. Gagliano didn’t reply. Nor did senior author Martial Depczynski, a coral reef expert at the Australian Institute for Marine Science. Another researcher who worked on the original study, Mavra Grimonprez, had left academia to become a yoga instructor, Markel says.

“I wrote [Gagliano] a very excited and impressed email, like, ‘Hey I think your work is really cool. I’m interested in replicating it. I’m interested in expanding on it,’ ” says Markel.

For her part, Gagliano says she simply missed Markel’s emails when he reached out to her. “Yes, it is correct that he — like many other students — contacted me back in 2017. I always make my best effort to reply to all emails I receive, but invariably I never get to some. It is unfortunate that his original email(s) got lost in that category arriving at a time when I was away a lot and no longer working at the university,” she told Discover in an email. Gagliano declined to discuss her work over the phone.

Markel soldiered on without the team’s input, gleaning methods from the paper and making the improvements suggested by Zink and Crook. He beefed up Gagliano’s sample size, using 20 to 30 plants for each test instead of Gagliano’s 13.

But he couldn’t repeat their results. He didn’t find any conclusive evidence that the plants can be trained to grow in different directions in response to a fan. At the time, he just dropped it, moving on to other research pursuits.

Failure to Replicate

Then, last February, the hit science podcast Radiolab ran an episode called Smarty PlantsIn it, hosts Robert Krulwich and Jad Abumrad interviewed Gagliano about her work on how plants can learn. Some of Markel’s friends heard the podcast.

“A bunch of my friends were like, ‘Hey Kasey! Isn’t that the experiment that you’re talking about replicating?’ ” recalls Markel. “I’m like, close, same author, same broad vein.”

Markel picked up his replication study where he had left off. He wrote up a journal article showing that the Y-maze study wasn’t replicable. His paper is now in review at a scientific journal, but he says he’s struggled to publish it. Editors have told him it’s not innovative enough; that it’s not ground-breaking research to just repeat someone’s study, even if the outcome is different.

“No one’s replicated pretty much any of the novel [new and unusual] results from Gagliano’s lab. She has a lot of really really bold, really novel results,” says Markel. “But it’s like one paper and then nothing else. That’s kind of the trend.”

And Markel has a hunch about why his Y-maze results don’t match Gagliano’s. Her team reported that 100 percent of the control plants — the ones left alone in the dark with no fan during the test phase — grew toward the last place they had seen the light. So when eight out of 13 plants (60 percent) grew opposite the light, as if conditioned by the fan, it was a major difference.

Markel’s controls didn’t grow that way, though, they were more like a 50-50 split, with some growing toward the last light, some growing opposite. So, although his botanical test subjects performed similarly in the presence of the fan, with slightly over half growing away from the last light — the result is subtle, and could be explained by random chance.

That 50-50 split makes more sense than Gagliano’s result, says Mannie Liscum, a University of Missouri expert on plant phototropism — the way plants grow toward light. Liscum says that when plants are left in the dark, they recalibrate their position based on gravity, a process called gravitropism. In other words, they naturally grow straight up. If they then hit the fork in the Y-maze, they’d be forced to go one way or another, randomly. So, you’d expect half-ish to grow left, and half-ish to grow right. And that’s exactly what they did when Markel ran the experiment.

Smarty Plants?

Other scientists are also questioning Gagliano’s methods. Psychologist Robert Biegler, who studies memory and learning at the Norwegian University of Science and Technology, wrote a formal response to Gagliano’s Mimosa-dropping work. His reply, also published in Oecologia, outlined why her methods were flawed and her conclusions even worse.

He thinks the Mimosa experiment gives “insufficient evidence” for Gagliano’s claims, and that it would make more sense that the plants were fatigued, not learning. The reason they would respond to a vigorous shake after “getting used to” being dropped was just because the shake is so much more violent — it triggers a response at a higher threshold.

“Their conclusions go beyond their data,” he wrote.

Australian forest - shutterstock 1106802731-1
A road winds through the Boranup Karri Forest in Western Australia. (Credit: Victor Yong/Shutterstock)

Gagliano is not backing down from concluding that her work shows plants are cognitive, subjective, sentient organisms. In fact, she lays this all out on her personal website. “I have … extended the concept of cognition to plants, re-igniting the discourse on plant subjectivity, sentience and ethical standing,” her bio reads.

These ideas remain at the fringe of the plant sciences. “Their view is the minority — definitely the minority view when it comes to plant biology,” says Taiz, the plant physiologist. The traits Gagliano and others are attributing to sentience, he says, are just plant adaptations.

“The thing that has always bothered me about it,” says Taiz, “is that everything about plant behavior can easily be explained on the basis of natural selection — [of] adaptive traits.”

And a chief point of criticism from other plant scientists is that Gagliano hasn’t followed up on either of her major findings — and neither has anyone else. Her Mimosa­-dropping experiment was complete by at least 2013, when Pollan saw the results presented at a conference. Her peas-in-the-Y-maze experiment was published in 2016. And as a response to criticisms, like Biegler’s, she’s replied, formally, with a reiteration of her results.

“What’s kind of astounding is there isn’t a follow up paper. If I had [found] this phenomenon, I would have like three or four follow-up papers,” says Markel. “The fact that nobody’s reproduced it, and that the original authors haven’t reproduced it … does seem to be fairly strong evidence to me that they don’t really stand by their results,” says Markel. “But, I don’t know. The paper isn’t retracted.”

Liscum shares a similar sentiment, saying, “The fact that there’s been nothing else that’s come out of that is a little bothersome. Because if stuff is exciting, people usually start working on it.”

“I don’t know whether people aren’t working on it because they just don’t believe it, which is possible, or someone’s tried and they can’t reproduce it,” Liscum says.

“It’s being kept afloat by public interest,” says Taiz. “Everybody gets interested in sort of the science fiction-y stuff.”

What is Consciousness, Anyway?

Gagliano’s more recent efforts are focused on the philosophy and moral implications of the sentience of plants. For instance, in a Moral Systems and Behavioral Science paper published in August 2018, she and the other authors write: “We also argue that serious attention should be given to the increasing recognition of plant agency, sentience and even consciousness and feelings.”

But Taiz and other biologists see this entire philosophical discussion as irrelevant. Instead, he says, “It all has to do with the biology.”

“So the question,” Taiz says, “is do plants have the mechanisms that are needed for consciousness? By all of our understanding and definitions of consciousness, they don’t.”

Liscum thinks some amount of conscious, sentient, feeling, seeing language can be a helpful teaching tool when it comes to plants. But there’s a difference between taking a metaphor seriously versus taking it literally. “Are [plants] able to respond to their environment appropriately and actively, like animals can? Yes. Do they do it by the same mechanisms? No,” he says. “If we’re going to be very literal about it, plants do not have consciousness in a metazoan [animal] sense. That is absolutely clear.”

“The difference [is], are plants actively thinking? … There’s no evidence that they are,” Liscum says. “They’re there, and their environment’s there, and they just respond to it. That’s not sentience.”

Plants are responsive to their environments. They grow toward light. They emit chemicals when they’re damaged (think: the smell of fresh cut grass.) Researchers have even watched, in real time, how a plant getting munched on one leaf sends a signal to the rest of its extremities.

But do plants’ actions demonstrate that they are conscious? There’s not currently scientific evidence to support that.

“Plants are important,” Liscum says. “Without them, you won’t have anything to eat and you can’t breathe. And that’s enough. That’s good enough. You don’t need to give them neurons. You don’t need to give them consciousness.”

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