“The fact that we could go to international symposia in biology and have to sit in animal neuro sections because there was no such thing as a plant neuro section was just getting a little annoying.”

Van Volkenburgh is a plant biologist at the University of Washington, and in that paper, she and her colleagues coined a new field of research: plant neurobiology.

A little more than a decade ago, Elizabeth Van Volkenburgh and five other scientists published a paper that set off a heated debate in the scientific community, a debate that never really got resolved.

This is Consilience, a podcast from The Scientist magazine. I’m Ben Henry. Today, we’re talking with scientists about how to compare plants to animals—and whether or not we can use words we associate with animals, like learning or sex, in reference to plants.

A lot of other biologists thought their colleagues were going overboard, and an argument ensued over how to talk about the science at hand. Which is actually a very old problem for the scientific community.

“My interpretation of this whole business with plant consciousness is it’s sort of a replay of what went on during the discovery of sex in plants.”

Lincoln Taiz is a retired professor of biology at the University of California, Santa Cruz, and he’s been a vocal skeptic of plant neurobiologists. He’s also an expert on an older semantic dispute, one that arose in the late 17th century when botanists had just discovered that plants reproduce sexually. To call plant reproduction sex at the time though, or to make any comparison between the way plants reproduce and the way humans do, was extremely controversial.

“It was fought over, and there was huge controversies for another hundred and fifty years. . . people were so biased against the idea that there was sex going on in a flower.”

For anybody not familiar with plant sex, flowers produce pollen and eggs, and the two have to come together for plants to reproduce. Nowadays no one disputes that this falls under the biological category of sexual reproduction, and we even use words like male and female when talking about flowers. It took us a long time to get here, though.

“At first the research on this question was very objective, applying the scientific method,. . . Well, the field was divided between the sexualists and the asexualists. Some of the proponents of the sexual theory went way overboard. And they said, well, if plants have sex, they must experience passion and lust.

“One of the biggest proponents of this idea was Erasmus Darwin, Charles Darwin’s grandfather. He wrote a book called The Loves of the Plants, which was an epic poem. . . personifying stamens and pistils as if they were husbands and wives. Just the use of the term husbands and wives to describe flowers created part of the backlash because they were personifying sex in plants . . . This was not helpful really, to treat the flower in an anthropomorphic way.”

Botanists of the day kept using words we typically associate with people to talk about plants. And that muddied the water around the actual biology. It made it hard to communicate.

A few hundred years later, we’ve settled that dispute, but scientists still argue about how to appropriately compare plants to animals.

If you ask the skeptics, plant neurobiology is a case of overzealous personification. In defense of the neurobiology crowd, plants do generate electrical charge using ion channels as part of their elaborate system of internal signaling. But still, the comparison between all of that and the nervous systems of animals just really doesn’t sit right with some people.

And recently, there’s been a new source of contention.

Monica Gagliano, an evolutionary biologist at the University of Western Australia, generated evidence that plants can learn.

Gagliano and some colleagues recreated Pavlov’s famous experiment in associative learning—that’s the one where a dog learns to associate a treat with a bell—but they did it using plants. If you want to get into the details of this experiment, I wrote about it for The Scientist, and we’ll put a link to that story in the episode description.

But according to Gagliano, the results of this experiment are clear.

“This is learning. This is exactly what Pavlov did. If this was a bee, or an animal of any kind, it would be no doubt that this was considered learning.”

Not everyone is so convinced, of course, but Gagliano says the evidence is all there, scientists just don’t want to use the word learning in relation to plants.

There’s good reason to be skeptical of the evidence though. For one thing, nobody has an explanation for how, on a molecular level, plants could store, recall, and act upon memories the way they would need to, in order to learn. For Lincoln Taiz, the whole concept doesn’t yet pass the smell test.

“Since plants lack neurons, this seems quite a stretch, so right off the bat, I’m very skeptical.”

There are some potential problems with the methods of the study itself, but Taiz also thinks this is just another case of anthropomorphizing plants.

“We discovered decades ago that plants have signal transduction mechanisms that allow them to sense the environment. But now, once that’s established, there’s this new romantic school of thought, that not only do they respond to their environment but they do it consciously. So I see this as, these are basically romantics.”

Gagliano, for her part, is fine with swimming against the current. After all, scientists live in the world of unconfirmed hypotheses—that’s their job.

“Humans are the ones we know the best—it’s ourselves, right? And we are still struggling to talk about intelligence, consciousness, memories, learning with us. Let alone when we move to animals, and let alone when we move even further away from our own kingdom.

“But we, you know, we are here to explore, so why not?”

If you want to read more about Gagliano’s experiment, go to the-scientist.com. And if you’re interested in the weird and rich history of plant sex, Lincoln Taiz and his wife Lee wrote a book about it, called Flora Unveiled, which will be published later this month.

I’m Ben Henry, and this is Consilience, a podcast from The Scientist magazine.