Plant Scientists on Plant Sentience

Sitting amongst tall sunflower plants, this person looks as if they are communing with the plants as fellow sentient beings.

Image by Anthony Tran, courtesy of Unsplash

What role do plants play in your life?

Plants are everywhere.  In our landscapes, our food, our clothes, our buildings, even our plastics.  Some of us grow and care for plants.  Others study and work with plants.  But for most of us, plants fade into the backdrop of our lives. 

Plants are an integral, living part of our ecosystem. Yet they are often treated like objects mechanically going about their functions.  What if we thought about plants as subjects, as fellow sentient beings capable of doing something interesting, something worth considering more closely? What if plants are trying to tell us something and we don’t know how to listen?

Imagine looking at plants with a new kind of curiosity. 

Plant scientists are discovering that plants can learn, remember, communicate, and even anticipate the future.  Scientists and philosophers are using a variety of terms to describe this behavior: plant sentience, plant consciousness, plant intelligence, plant cognition, plant thinking, even plant neurobiology. 

While this might sound fascinating, it’s also incredibly controversial.  It’s not the science itself that’s at issue.  It’s the idea that plants are sentient beings, that they process information, make decisions, and even think.  This crosses an important line:  the line that sets humans apart from the rest of nature. 

Why we might be resistant to the idea of plants as sentient beings.

If humans are the only intelligent beings capable of making decisions, it’s natural to assume that we’re in charge. And that we’re entitled to treat nature as a resource.  But if plants have their own kind of intelligence and world-making abilities, we have to reevaluate these assumptions. 

Our industrial society is built upon the domination and control of nature.  But the resulting ecological crises are requiring us to revise this approach.  One way to begin reevaluating the relationships between humans and the rest of nature is by taking a closer look at the behaviors and abilities of plants.

Plant Movement

Plants are rooted in the ground.  They don’t move around like humans and animals do.  This makes some people think that plants must be stupid, that they don’t have the intelligence required for movement.  But it’s only from our human time-scales that plants appear not to move. 

Patient observant individuals like Charles Darwin recognized the movement of plants long ago.  Now, with time-lapse photography, we can see this movement on our time-scale.  Check out this video of a morning glory plant whipping its vines around in big circular movements in search of a support to twist around.  Plant scientists have shown that this isn’t a mere mechanical motion but involves anticipation, planning, strategy, and precise adjustments.  

If the support is moved, the plant changes its course, similar to how a predator hunts its prey.

If we think about movement only in human terms, plants seem quite limited.  But if we recognize how active plants are even while rooted in the ground, their abilities seem all the more amazing.  Seeing plants as active and intentional makes it easier to consider that they could be sentient beings.

Plant Communication

We tend to think of communication in terms of information passing back and forth between individuals.  I speak out loud, the sound waves impinge on your ear, and your brain processes the sounds into meaning. 

Plants don’t have eyes, ears, or mouth, so they obviously don’t communicate in the ways we’re used to.  But plant scientists have observed that plants absolutely do communicate:  they take in information from their environment and coordinate this information into a response. 

Plant scientists have shown that plants perceive temperature and electricity, they detect and distinguish between different types of light, they perceive chemicals in the atmosphere and in the soil, they detect and respond to touch, they emit and respond to sound waves, they pass information through underground networks of micorrhizal fungi, and they may be able to perceive visual information.

Plants communicate with themselves, between their roots and their shoots (the tips of the plants).  They communicate with other plants.  And they communicate with other organisms like animals and pathogens. 

Check out this multifaceted example of plant communication.

When part of a plant is attacked by a pathogen, the attack is communicated to the rest of the plant (through action potentials or chemical signals) so that the plant can mount a coordinated response.  The plant can tell the pathogen to go away by converting nutrients into chemicals that the pathogen doesn’t like.  The plant can also emit chemicals into the air to ask beneficial organisms to come and help remove the pathogen.  Most interestingly, the plant can emit chemicals to warn their plant neighbors about the pathogen, so the other plants can prepare for a possible attack.

If we think of communication only in human terms, we automatically exclude plants.  But if we recognize the many different ways in which beings can take in information and respond, it’s apparent that plants communicate even without eyes, ears, or mouth.

We’ve been thinking about communication in terms of individual organisms exchanging meaning via a language of sounds, chemicals, etc.  Plant scientists are also investigating how we can go beyond language and think more holistically. They are studying how meaning emerges dynamically through interactions and relationships between plants, animals, and humans. After all, we are interdependent organisms sharing an ecosystem, sharing meaning.

Plant Intelligence

Plants move.  Plants communicate.  But are plants intelligent?  First off, there’s no agreed-upon definition of intelligence – or whether the same definition should apply to both humans and plants.

And then there’s the issue of the brain.  We’re conditioned to believe that intelligence requires a brain.  Plants don’t have brains like we do.  For some plant scientists, this is enough to label plant intelligence as crazy. 

But others are investigating how the plant’s anatomy can support brain-like behavior even without a brain.  Some plant scientists theorize that the plant’s vascular system, which has networks of electro-sensitive cells, allows plants to store memories, make predictions, and engage in abstract processing.  

Plants process information even without a brain, neurons, or a nervous system.

Other scientists and philosophers are looking at intelligence in a new way.  Instead of talking about individual plants as intelligent (or not), they are looking at the relationships between plants and their environments. 

Scientist Anthony Trewavas defines plant intelligence as the ability to engage with the environment in an embedded and interactive way through meaningful physical relationships that are dynamically changing. 

Philosopher Paco Calvo studies the plant-environment system as a dynamic whole.  The ability to think and behave intelligently arises from the system as a whole rather than from an individual part.

Intelligence or adaptation?

Researchers who don’t believe in plant intelligence will tell you that this scientific data does not prove plant intelligence, it merely shows how plants adapt to their environment.  This is a key question, to understand the difference between intelligence and adaptation.

Trewavas says that for intelligence, individual plants must exhibit behaviors such as learning and memory that make them more likely to survive in a changing environment.

Calvo explains that intelligence requires goal-oriented behavior in which the plant can anticipate the future and adapt flexibly within their own life span.

Two types of behavior that exhibit this kind of intelligence are habituation and conditioning.

Plant Learning - Habituation and Conditioning

Scientist Monica Gagliano has provided evidence for both habituation and conditioning in plants.

Habituation is the ability to form habits, to memorize something about your environment and then react to situations based on those memories. 

To study habituation, Gagliano used a sensitive plant called Mimosa that folds its leaves when touched or disturbed.  To create the disturbance, Gagliano dropped the plants repeatedly, causing them to fold their leaves when they hit the ground.  After repeated droppings, the plants habituated themselves to the dropping.  They recognized that they weren’t actually in danger, and they stopped folding their leaves when they hit the ground.  They still folded their leaves in response to other disturbances, because it was only the dropping they were habituated to.  The plants retained this memory for as long as 28 days.

Forming habits is a rather primitive type of learning, but conditioning is more sophisticated.  It requires abstract processing, or forming an association between two different stimuli “in our heads”.

You’ve probably heard of rats being trained to go through a maze to get the reward of food at the end.  Gagliano trained plants to grow through a maze.  For plants, the reward is light (plants grow towards the light).  The plants were able to learn which fork of the maze to grow towards in order to end up at the light.  Here’s the catch.  Gagliano trained them to associate light with wind (a blowing fan), even though plants generally avoid wind.  When the plants felt the wind, they had to associate the wind with the light at the end of the maze, and make the decision to grow towards the wind so they could eventually reach the light.

Plant sentience

We could debate forever about whether or not plants can communicate, whether or not they are intelligent and can learn, and whether or not they are sentient beings.  The answers depend on how we define these words and what we are willing to believe about the world. 

For some of us, the world feels more ordered if we have sharp distinctions, if we know which organisms belong in which categories.  Humans are intelligent, plants aren’t, and so humans are in control.  But where has this control led us?  To ecological disaster.

Suppose instead that there are no sharp lines between categories like intelligence and non-intelligence.  Suppose that humans aren’t set apart from other beings because we have something that they don’t have.  Suppose that it’s not about who has what, but about what emerges from our relationships, about the meanings that are created when our differences come together in interesting and provocative ways.

Plant sentience is not the stuff of science fiction. 

Plant scientists have uncovered interesting and compelling evidence about plant sentience. In the midst of intensifying ecological crises, how can we afford to ignore the possibility that plants could be fellow sentient beings capable of working with us to restore harmony and balance? Check out the Findhorn story for an example of this. Also my own story about how the plant world helped me make a necessary life change.

It’s time to re-imagine our relationships with plants, to co-create new kinds of meaning, to work together to create a mutually flourishing world.