Author: Kate Giffin
Editors: Henry Ertl, Sarah Bassiouni, Sophie Hill and Jennifer Baker
The year was 1633, and Galileo Galilei was being placed under house arrest for a heretical idea: that the Earth revolves around the sun. While he is now considered one of the fathers of modern science, at the time the Roman Inquisition of the Catholic Church declared him “vehemently suspect of heresy.”
Galileo’s story is just one example of how controversial science can get. Throughout history, some ideas that initially seemed outlandish were shown to be facts, while others were proven incorrect and forgotten. The scientific process is purposely slow and careful. Experiments must be repeated many times and facts must be proven with evidence.
While most scientists today are not placed under house arrest by the Church for their hypotheses, there are still hotly debated ideas. At the moment, scientific debate is raging over a humble group of organisms integral to our everyday life: plants.
The traditional view of plants is that they are passive organisms. While they may respond to some changes in the environment like light and water, most people do not consider them to be capable of making choices or behaving with intention.
This mainstream view is being challenged by scientists and philosophers who believe that plants have more abilities than we give them credit for. They hypothesize that plants can learn, form long-term memories, and communicate with other organisms. In fact, this group of scientists theorize that plants are intelligent beings. If proven true, their claims have the potential to rewrite everything we know about the evolution and biology of intelligence.
Opposing this radical view are the traditionalists. These scientists point to the differences between the bodily structures of plants and animals. They argue that plants do not have any structures resembling the brain that produces all behavior and thoughts in animals. Any plant behaviors are merely reflexive responses to changes in their environment, not intentional actions on the part of the plant. This group argues that there is a lack of robust evidence for anything resembling intelligence in plants.
The only way to resolve the debate over plant intelligence is to conduct scientific experiments testing whether plants exhibit what researchers consider to be intelligent behavior, such as learning.
In the midst of the 1960s boom in psychedelic subcultures, some scientists attempted to test the learning abilities of plants as a way of understanding the nature of consciousnesses. However, discussing plant “behavior” was not only unorthodox, but essentially taboo. Publishing scientific papers on this topic was nearly impossible. The papers that were published were often plagued by methodological issues such as not controlling for outside variables that could affect the results.
After 1970, the field went quiet and no papers have been published about learning in plants– until recently. In 2016, the debate was sparked anew by a splashy paper that presents data showing associative learning in plants. While the paper was praised in the media, it raised controversy among scientists. Its ideas were fought out in the court of public scientific opinion with a series of back-and-forth papers.
Can plants learn? The supporting evidence
In 2016, the paper “Learning by Association in Plants”6 by Monica Gagliano and a team of researchers presented startling data that plants are capable of associative learning.
So, what is associative learning? This type of learning is also known as classical or Pavlovian conditioning and was famously described in the experiments involving Ivan Pavlov’s dogs. During associative learning, a signal (also called a cue or stimulus) that previously did not have meaning becomes associated with a meaningful event. Take the sound of a bell– by itself, it has no meaning to a dog. However, if you ring the bell every time you give the dog a juicy piece of meat, like Pavlov did, the dogs will learn that the bell is associated with the delicious meat. That is, the learner will begin to associate the previously neutral stimulus with a meaningful experience or object. Eventually, the dogs started to have the same reaction to the bell as they had to the meat. They started salivating in anticipation of a delicious meal. After associative learning has occurred, giving the learner the unconditioned stimulus (bell) will cause the same reaction that they originally had to the meaningful stimulus (meat).
Dr. Monica Gagliano and her team set out to find out if plants could undergo a similar learning process. They used a fan as the neutral, unconditioned stimulus, like Pavlov’s bell. For the biologically relevant stimulus, they chose to use a blue light. Light is as vital to plants as meat is to dogs–without it, they quickly wither and die. In fact, plants will move towards light, a reflex called phototropism. If you’ve ever noticed a houseplant bending towards a sunny window, you’ve witnessed phototropism in action. Dr. Gagliano and her team reasoned that if they could get the plants to learn an association between the fan and the light, the plants would move in the direction the fan taught them the light would be in.
To test this idea, the researchers created a maze for plants. They kept it simple– no twisty turns or dead ends for these seedlings, just two choices. The researchers fitted a Y-shaped PVC tube over each pot. The plants could grow towards the left or the right arm of the pipe.
The scientists chose to test the humble garden pea, Pisum sativum. Pea plants not only produce delicious food, but they also grow rapidly in the early stages, allowing the experiment to progress quickly. Perhaps most importantly, the young plants only have one tendril, so they could only grow into the left or right arm of the maze.
Then, just like Pavlov pairing the bell and meat for his dogs, the scientists started pairing the light and the fan for the plants. To train the plants, they shone the blue light into one arm of the maze at the same time that the fan was blowing air onto the plants. This was meant to teach the plants that fan = light. Half of the plants were taught that the fan was always on the side where the light would be and half had the fan on the opposite arm of the light. Each training session had one hour of light exposure, and the training was repeated three times a day for three days to give the plants plenty of time to learn. The researchers also included a control group to make sure that the light was an attractive stimulus. For this group there was no fan, just the light.
After training, the researchers tested whether the plants had learned the connection between the fan and the light. This time, they only turned on the fan and recorded which direction the seedlings grew in. If the plants had learned the association between the fan and the light, they would grow towards the side of the maze where the fan had signaled the light would be. For example, if the plants had learned that the light is always on the side of the fan, then when the fan was blown into the right side of the maze the plants would grow into the right side of the maze in search of light.
The majority of pea plants grew towards the side of the maze where the fan had told them the light would be: 62% when the fan was on the same side as the light and 69% when the fan was on the opposite of the light. This was evidence that the plants had learned the association between the light and the fan.
Meanwhile, all of the plants in the control group, which did not get any exposure to the fan, grew towards the arm where the light was last seen. This was an example of phototropism, the reflex where plants grow towards light. This result showed that light was a good reward for these plants because they will grow towards it.
Can plants learn? Evidence against.
Gagliano’s paper on associative learning in plants created quite the splash. Numerous popular publications covered the story (such as The Conversation, Aeon, Atlas Obscura, and a Radiolab podcast episode), sparking a new conversation on plant intelligence.
But in science, a single study does not a fact prove. It is important to do experiments more than once, a process called replication. Otherwise, the findings could have been a fluke–a random happening due to any number of variables and conditions. If the same results are seen with each replication, the experiment is said to be reproducible and the findings are considered to be valid. Through this process, the results of a single experiment can be validated and accepted as fact.
Crucially, experiments should be replicated and reproduced by multiple independent groups of scientists so that we can trust that the results are not due to one very particular setup in one specific lab. Kasey Markel was the scientist who chose to replicate the buzzy paper on plant learning. He published the paper “Lack of evidence for associative learning in pea plants”7 in 2020 detailing the results of his replication attempt.
Markel’s work featured the same basic setup as the previous study. Markel used pea plants with the same Y-shaped maze. They exposed the plants to the fan and the light. Then, they tested the plants by providing only the fan and marking which direction the plants grew in.
In the replication experiments, there was no evidence that the plants were able to learn the association between the fan and the light. There was no pattern to the direction the plants were growing in.
Perhaps the most damning finding of the study was in the control group. This group of plants didn’t get exposed to the fan, but they did have the blue light shone on them from different sides of the maze. In the original study by Gagliano’s team, 100% of the control plants grew towards the side where they last saw the light. The researchers explained this as expected due to the plants’ innate phototropism.
However, in Markel’s replication study only slightly more than half of the control plants grew towards the last presentation of light. While this was a consistent tendency, it was not statistically significant and nowhere near the 100% response from the original study. This finding was concerning, as the experiment relies on light being an effective motivator for the plants to change their behavior or growth pattern. If the plants do not want to grow towards where the light was last seen, then there is no reason for them to learn the association between the light and the fan. The entire design of the original experiment was suddenly called into question.
The authors’ debate
The 2020 replication study directly compared Markel’s data to Gagliano’s, presenting the data side by side in graphs. This direct criticism of their original study prompted Gagliano’s team to publish a short essay responding to the claims of Markel’s replication study.
Their main critique of the replication study was that the plants did not have a reliable phototropic response because they didn’t grow towards the light each time. This meant that the light was not an effective unconditioned stimulus–it simply wasn’t as attractive to the plants as meat was to Pavlov’s dogs. This may have disrupted the plants’ ability to form associations.
In their response, Gagliano’s team suggested that perhaps light from other sources was leaking into the mazes, interfering with the plant’s phototropism. In a rebuttal to Gagliano’s essay, Markel argued that differences between experimental setups were inconsequential. He conceded that his setup had more background light, but dismissed it as a reason why his plants did not reliably grow towards the last presentation of the light.
Instead, Markel claims that a mere hour of light exposure is not enough to convince a plant to grow in one direction. Phototropism is a well-documented phenomenon that even the ancient Greeks were familiar with, but it usually occurs over the timescale of many days of consistent light in one direction. Basically, one hour of light exposure is extremely short in plant time. Markel cited previous research finding that one hour of light stimulation was not very effective in producing a phototropic response.
So where does this leave us? Until another replication study (or, preferably, many) comes along to settle the score, it is impossible to make a conclusive decision about associative learning in plants.
Studying the learning abilities of plants not only has the potential to transform the way we think about plants, but also the nature, evolution, and biology of intelligence. If plants can learn the way animals learn, the whole history of intelligence could be rewritten. This would be a truly incredible finding–but only if it can be replicated.
Kate Giffin is a Neuroscience PhD candidate in the lab of Dr. Ben Singer. She studies the interactions between the brain and the immune system, specifically to understand how inflammation can cause mental health changes such as memory loss and anxiety. When Kate is not marveling at the brain, she is probably outside marveling at some strange plant.