Plants and the Memories We Never Knew They Could Have

On a basic level, plants are demonstrating the ability to think and remember. For them, it might not be about survival of the fittest, but the smartest.
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Source: Sten Porse / Wikimedia

Mimosa pudica. It’s a name that sounds like it belongs attached to a mixed gelatinous alcoholic beverage, not a plant that has researchers discussing whether or not one of our leafy counterparts in the vegetable kingdom might have the ability to remember what has happened to it in the past.

So, this isn’t about a bubbly champagne and orange juice party favourite, but another casual (albeit firmly rooted in a pot of dirt with no ability to speak) observer judging how lousy a tipper some people can be after being served their boozy mimosa?

Before Monica Gagliano, an evolutionary ecologist at the University of Western Australia, even began her study into M. pudica, this creeping perennial herb had established a reputation for shying away and responding to physical contact. It’s why gardeners refer to it as the ‘sensitive’ or ‘touch-me-not’ plant—its leaves fold inwards as a defensive reaction to the lightest caress.

Source: Wiki Commons

Obviously, the Mimosa pudica does not have a brain, so where does all this talk of having memories come into play? Gagliano decided to study why this particular plant’s leaves always seemed to assume a defensive position when touched. She selected 56 specimens and with the help of a specially designed, measured vertical rail, began gently dropping them six inches to a padded landing zone.

As she expected, the plants followed their usual playbook and folded their leaves up.
These handful of drops gave Gagliano a base from which to work from. She then began submitting her specimens to repeated drops at five-second intervals, 60 drops in total. She duplicated the process another six times over the course of one day, and as the number of drops increased, the physical reactions by her test subjects decreased. Their leaves remained outward, almost like a person stretching their arms skyward while enjoying a rollercoaster.

Was this proof of the plants adaption to the scenario, or were they just exhausted from having the proverbial rug pulled out from under them hundreds of times? Gagliano took a sample of tested specimens and gave them all a good shake to check their reaction to this new form of startling stimuli. Leaves curled up, showing that they responded as expected to the new stimuli and weren’t experiencing exhaustion.

Source: University of Western Australia

The basic concept of plants potentially possessing traits that might make an individual wonder if they could be sentient beings is not a new one. 1973’s “The Secret Life of Plants” detailed experiments based around an eclectic mix of grounded plant science with some oddball approaches involving bananas and onions being hooked up to a polygraph machine. Going even further back, a Bengali polymath named Sir Jagadish Chandra Bose theorized that plants had both feelings and memories… and this was a hundred years ago! He might have had the right idea; he just didn’t find a way to prove his theory the way Gagliano has.

Gagliano bypassed subjecting her plants to lie detector tests, but instead repeated her experiment every week for a month. It was a “been there, done that” scenario for the plants – drop after drop, their leaves remained open.

Gagliano’s findings have raised the ire of some members of the scientific community who simply don’t believe plants have the necessary genetic make-up to form memories. For centuries scientists have said that plants don’t think, they only do what they’ve been genetically programmed to do. After all, it’s that inherited infrastructure that makes up for plants not having a brain. Gagliano had this to say this in her published paper on her study:

“…the process of remembering may not require the conventional neural networks and pathways of animals; brains and neurons are just one possible, undeniably sophisticated, solution, but they may not be a necessary requirement for learning.”

While they do lack brains and nervous systems as we understand them, plants (such as the talented M. pudica) have a complex, calcium-based signalling network within their cells. This system functions a bit like the memory making process of animals, and some scientists suspect that this network is at the heart of the memory formation process that seems to occur within some plants – or perhaps within all plants and we just haven’t figured out how to test for it yet, who knows?

Another theory is that the key to memory formation in plants lies in the mysterious realms of the RNA that makes up their genetic code. In plants, a portion of the RNA string know as the COOLAIR plays an important role in the flowering of plants in springtime, by sensing how long the plants have been exposed to cold—blooming only after a certain number of cold days have passed. If the COOLAIR section of the RNA is deactivated, the plants will not flower. This complex mechanism could also hold the key to memory in plants.

A hairpin loop from a pre-mRNA. Highlighted are the nucleobases (green) and the ribose-phosphate backbone (blue). By Vossman – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=7115139

But it gets even better. Plants might not just remember things, they may act upon these memories. Don’t worry, we’re not talking about the Day of the Triffids… yet, but the results are still very impressive.

Photo Source: Wiki Commons

In more recent experiments, Gagliano found memory and learning demonstrated in the behavior and reactions of seedlings from pea plants (Pisum Sativum) as well.

Image Source: https://www.nature.com/scientificreports

The seedlings could be trained to learn by association—meaning that they learned to associate a certain stimulus with a reward, and then reacted to the stimulus alone.

In the experiment seedlings were provided with an environmental cue in the form of a fan blowing air on their leaves. That cue was accompanied by light, and the plants grew in the direction of the light and the fan. When the light cue was removed, the pea seedlings still grew most in the direction of a fan that was blowing air on them.

Photo Source: Rasbak – Own work, CC BY-SA 3.0,

The results of this experiment were fascinating for a number of reasons. Firstly, Gagliano used classic conditioning techniques that were most famously applied by Ivan Pavlov in his creation of Pavlov’s Dogs. In this experiment, the dogs were trained by ringing a bell before each feeding. Eventually, the sound of the bell alone was enough to provoke salivation—even without food present—as the dogs had associated the bell with the food. In a similar way, the plant’s reaction continued even after the “food” (light) had been removed.

One of Pavlov’s dogs with a surgically implanted cannula to measure saliva, preserved in the Pavlov Museum in Ryazan, Russia – Wikipedia

Secondly, this shows that plants display aspects of associative learning in how they grow and shape themselves. This means that they don’t just respond to stimulus that occurs around them, and not only do they “remember”, they also choose and predict a course of action and act upon those memories. This is similar to the foraging or hunting behaviour we might see from animals (such as with a lion stalking prey in an area where they have found prey before), but entirely unexpected from a plant without a nervous system or higher brain functions.

Photo Source: Wiki Commons

What does this mean? In a broad sense, it is challenging some of the most basic ideas of biology and the relationship human beings have with the world around them. The previous well-defined line between the plant and animal kingdoms is becoming a bit more… fuzzy. There is a grey area now, where long-held concepts such as “only animals have nervous systems and can learn” may have been proven to be not quite as true as we believed.

The ramifications of these findings are wider than you might think—millions of people worldwide have adopted vegetarian or vegan diets. Some specifically believe that a plant-based diet is a more ethical choice than eating animals because animals have feelings and memories.

In a specific sense, it is not absolutely clear where we will end up, how this will specifically change our understanding of the world (and our relationship with it), or what this new knowledge about plants might mean for vegans and their diet; but scientific discoveries like this help remind us that our world is often more interesting than we could imagine. Even aspects of science which we believe we completely understood might still have something to offer us if we look closely enough and challenge our previously held assumptions.

As it turns out, plants might not care if you are, in fact, a cheapskate when it comes to tipping. But they have presented a possibility that has scientists and researchers talking to one another about approaching our way of assessing plant behaviour differently—maybe over a champagne mimosa or two.


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