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How smart is a plant? It sounds like a trick question, but it isn’t.
We use the word “vegetable” to describe a human being who’s incapable of thought or reaction. But the evidence suggests that this is unfair. Far from being passive lifeforms that grow toward light and take up water and do nothing else, plants can actively perceive and respond to their surroundings.
Plants communicate with each other, signal danger, share nutrients, distinguish kin from non-kin, and do other things commonly associated with intelligence. Plants can hear: their roots grow toward the sound of flowing water, they ramp up the production of toxins when exposed to the noise of a caterpillar munching leaves, and they make their nectar sweeter in response to the frequency of a bee’s buzz.
The study of plant intelligence has a poor reputation, mostly because of the infamous 1960s lie-detector experiments which allegedly showed that houseplants perceive thoughts and emotions. This was pure crankery. However, newer and more rigorous science suggests that plants do possess some surprising talents.
Can plants learn?
One of the stars of plant intelligence studies is Mimosa pudica, also called the touch-me-not or the sensitive plant, a roadside weed native to the American tropics. It has an unusual adaptation to protect itself from damage: if its leaves are touched or disturbed, they curl up and close inward.
However, it’s energetically costly to do this. It would be beneficial for the plant if it could learn not to react to harmless stimuli. In a 2014 study, biologist Monica Gagliano and colleagues tested if they’re capable of that.
In the experiment, potted Mimosa plants were attached to a vertical rail which they could slide up and down freely. The researchers proceeded to drop the plants 15 cm onto foam rubber padding. This was enough of a shock to trigger the leaf-folding reflex, but not enough to harm the plant.
At first, their leaves folded up every time. But after several dozen repetitions, the plants learned that the small drops weren’t harmful. When the drop was repeated, they ignored it. It wasn’t because the plants were exhausted, either—because when they were exposed to a different stimulus, a vibrating plate, their leaves still curled up. It was only that one sensation where they learned to react differently.
Not only did the plants learn, they remember what they learned. As long as a month afterward, when the experiment was repeated, they continued not to react to the drop. They somehow retained the information and used it to guide their behavior. This represents learning in the strictest scientific sense, no different than Skinner’s operant conditioning experiments on pigeons or rats.
Pea plants can learn, too (maybe)
Another study from 2016, also by Gagliano and colleagues, argues that Pisum sativum, the common pea, is also capable of learning through Pavlovian conditioning.
In the experiment, pea seedlings were grown in Y-shaped pipes. In the training sessions, a fan was turned on to blow a current of air (a neutral stimulus) down one arm of the Y, then a blue light (attractive for photosynthesis) was shined down either the same arm as the fan, or the opposite arm. This was repeated for three days, randomly varying which arm of the pipe was used.
For the experimental phase, the light was turned off and the fan was turned on. The majority (between 60% and 70%) of the pea seedlings grew toward the arm of the Y where their prior experience predicted the light would appear. If they had been trained with fan and light together, they grew in the direction the fan was blowing. If they had been trained with fan and light in opposing arms, they grew away from the direction the fan was blowing.
Notably, the plants followed the fan cue even when it was different from the direction of their last exposure to light. As the authors write:
Thus, our results show that pea seedlings develop an association that facilitates growth towards the light based on the occurrence of a neutral cue. This learned behaviour prevails over innate positive tropism to light, which is thought to be the major determinant of growth direction in plants.
To be fair, other scientists have questioned this finding. Another biologist, Kasey Markel, replicated the study in 2020 and couldn’t reproduce the result. Gagliano and colleagues wrote a reply in which they argued that Markel’s experimental setup doesn’t appear to have been completely dark, creating a possibility that the seedlings were confused by background light.
Clearly, this is an area where more evidence is needed. Since this experiment is easy to replicate, we can hope that more scientists try it out and settle the question conclusively.
The chameleon vine
I saved the best for last. The most amazing case of plant intelligence is Boquila trifoliolata, sometimes called the chameleon vine. It’s a climbing vine native to South America that wraps around trees and woody plants. But Boquila has a startling talent: it’s a shapeshifter. It alters its leaves to imitate the leaves of whatever it’s growing on.
Native to Chile and Argentina, B. trifoliolata is the first plant shown to imitate several hosts. It is a rare quality—known as a mimetic polymorphism—that was previously observed only in butterflies, according to this study, published today in Current Biology. When the vine climbs onto a tree’s branches, its versatile leaves can change their size, shape, color, orientation, and even the vein patterns to match the surrounding foliage… If the vine crosses over to a second tree, it changes, even if the new host leaves are 10 times bigger with a contrasting shape.
“‘Chameleon’ Vine Discovered in Chile.” Science.org, Nsikan Akpan, 24 April 2014.
This is thought to be an adaptation to defend against leaf-eating animals. By imitating the leaves of unpalatable species, Boquila makes itself look less like a meal. But the why isn’t as important as the how. How is a plant capable of this?
Mimicry isn’t unknown in the plant kingdom. For example, parasitic mistletoes have leaves that resemble the leaves of their host plants. However, this can be explained as an ordinary case of host-parasite coevolution. Each species of mistletoe grows only on one type of tree, and natural selection has gradually reshaped them to mimic their hosts. Boquila is more flexible, not limited to one host or one form.
Ernesto Gianoli and his colleagues, who study Boquila, first guessed that it was capable of horizontal gene transfer: stealing genes that control leaf growth from other plants, possibly with help from microbes, and integrating them into its genome. That in itself would be an amazing talent, but further research disproved it. It turns out Boquila can also imitate the leaves of plastic plants, which have no DNA to copy.
The only other hypothesis, wild as it sounds, is that Boquila can see.
And why not? Plants, after all, are sensitive to light. As far back as 1905, the botanists Gottlieb Haberlandt and Francis Darwin (Charles’ son) proposed that some plant epidermal cells have a convex, lens-like shape. It’s possible that they could focus light rays into light-sensitive cells deeper down, the same way our eyes form an image.
This, of course, is an extremely speculative hypothesis. Even if plants have eyespot-like structures, it’s a long way from there to being able to recognize what they’re seeing, let alone using that information to guide their growth. We’re completely in the dark as to how a plant might be able to process this information without a visual cortex. However, there’s currently no other explanation for Boquila‘s talents.
A different kind of intelligence
These talents are all the more remarkable because plants don’t have brains or nervous systems. How they accomplish these things is still unknown—although research suggests they have electrochemical signaling based on molecules such as glutamate, which is also found in human neurons.
Besides, we don’t even have a full understanding of how humans process sensory information or store memories. The interior of the brain is a terra incognita to science. Until we understand the physical basis of intelligence much better, it would be premature to declare what talents other species do or don’t possess.
Now, obviously, plant intelligence isn’t like ours. A pea plant isn’t going to compose a sonnet, nor will a chameleon vine pass the mirror test. Their intelligence is slower, subtler, more specialized. But when it comes to the specific talents they need to survive, they can be surprisingly adept.
This realization ought to deepen our sense of belonging and kinship with the natural world. We humans don’t stand on a mountain, gazing down on nature like Olympian gods. We’re down here with the rest of the planet, among the leaves and the roots, with our feet in the same soil. Who knows what other kinds of heretofore unsuspected intelligence may be hiding all around us?