Cambridge University Science Magazine Skip To Content
Articles
Biomedicine
Biology
Physics
Technology
Society


Whenever we talk about intelligence, we often refer to the seemingly miraculous feats of the human mind such as communication, learning and problem-solving. However, scientists also know that animals such as ants (e.g. swarming), dolphins (e.g. language comprehension) and cephalopods (e.g. tool use) display complex behaviours that mirror and sometimes even outperform human beings. And that’s just the tip of the iceberg! Recent research suggests that intelligence may exist in lifeforms that are not animals at all: plants. This piece explores the latest discoveries in plant cognition and considers how they challenge our assumptions regarding the nature of intelligence.

First: what do scientists mean by intelligence? Unfortunately, there is not a simple answer to this question. There is no universally agreed upon definition of intelligence. Researchers agree that it involves the ability to learn new information and use that knowledge to adapt to novel situations and accomplish goals. As a result, most early intelligence research focused on behaviour, or what it means to act intelligently. It’s believed that a wide range of cognitive processes are involved in producing intelligent behaviour such as memory, learning and abstract reasoning. In humans, intelligence is measured using standardised cognitive assessments such as IQ tests, which attempt to capture an individual’s ability to reason and solve problems relative to others in the population. Scientists utilise similar evaluations in non-human animals such as monkeys and mice, although conceptual and methodological difficulties remain when trying to translate these findings between different species.

In addition to behaviour, recent approaches have attempted to uncover the ‘biological basis’ of intelligence, for which the presence of a nervous system is thought to be an essential component. Many studies in both humans and other animals have consistently linked intelligent behaviour to large assemblies of nerve cells (or neurons). These networks interact with each other over time (e.g. milliseconds, hours, days, etc.) to decode patterns of information and execute appropriate behavioural responses. This is possible due to the elaborate structure and function of neurons that emerged over billions of years of evolution. This biological backdrop enables these cells to communicate with thousands of others via electrochemical signals to form complex systems that carry out specialised tasks.

Therefore, it seems likely that the reason certain species are capable of intelligent behaviour has a lot to do with the connectedness and efficiency of these neural systems. In other words, to be intelligent requires one to have a highly integrated and proficient nervous system. But what about other organisms such as plants? Should they also be considered intelligent even if they don’t exhibit behaviours we typically think of as intelligent nor have a nervous system?

Biologist Dr. Monica Gagliano (University of Sydney in Australia) is convinced that plants also have cognition. During a recent panel discussion at the World Science Festival 2019 in New York City titled “Intelligence Without Brains”[1], Dr. Gagliano asserted that plants are far more sophisticated than we thought and has the data to support this claim.

First, Gagliano sought to understand more about how plants communicate. Prior to her work, scientists had shown that plants communicate in three ways: light, chemicals and touch. Plants use these media to transmit signals to and from other neighbouring plants to minimise ecological conflicts. For example, if two plants are growing in proximity, they might bump into one another. When this happens, one plant may need to change its behaviour depending on what species (e.g. hostile competitor) is growing next to it. But what happens when you seal off these means of communication? Could they still communicate?

Gagliano grew chillies (plant of interest), basils (good guy) and fennels (bad guy), and isolated them in boxes so they couldn’t smell, touch or ‘see’ each other. To her surprise, the baby chillies still knew which plant (basil or fennel) was growing next to them. But how? The answer: sound. The chillies somehow ‘heard’ and ‘distinguished’ which plant was growing next to them. As a follow-up experiment, Gagliano used laser technology to check whether plants could also emit sounds rather than just receive them. Again, she was amazed to find that their roots not only produce sounds, but ones that can also be heard by humans! She insists that if you place your ear to the ground in a quiet forest and wait long enough, you’ll hear them too.

But how does this demonstrate intelligence in plants? To investigate further, Gagliano performed another experiment, the results of which defy our assumptions of a nervous system-based model of intelligence. She revealed that plants not only communicate through sound (‘language’) but also display two other hallmarks of intelligence mentioned above: learning and memory.

In Mimosa pudica, also known as the sensitive plant, Gagliano examined habituation, a type of learning whereby an organism’s response to a stimulus decreases after recurrent presentations of that same stimulus over time. She routinely dropped the plant, which responds to stress by rapidly closing its leaves, from a set height over 60 consecutive trials. During the first few trials the plant closed its leaves as it hit the ground. After these initial drops, however, something incredible happened: the plants stopped closing their leaves. They seemed to learn that the drops weren’t harmful. Gagliano inferred that this behaviour arose since the plants lose about 40% of their photosynthetic capacity whenever they close their leaves. Therefore, if the stimulus isn’t dangerous, there’s no need for them to sacrifice valuable nutrients. But it didn’t stop there. She repeated her experiment and found that they remembered this response for up to 28 days! Finally, she took plants grown in either low or high light environments and switched them into opposite conditions. She found that the plants grown in a high light environment and transferred to a low light one quickly adapted by closing their leaves. Most interesting was the other group, which maintained its learned response to disturbance even after being moved to a more comfortable environment. In other words, this group learned context and circumstances can change.

So, where does this leave humans? I argue these findings suggest we need to move beyond the stereotypical nervous system-cognitive assessment model of intelligence and be open to the possibility that intelligence permeates nature. Although plants lack brains like us and other animals, they use some of the same chemicals (e.g. calcium signalling, dopamine, serotonin, etc.) that neurons use to transmit information. Our inability to agree on what’s required for an organism to act or be intelligent could be due not only to the limitations of our theoretical and experimental tools, but also our assumptions about how intelligence evolved in the first place. Plants already outnumber us in terms of biomass. Who knows, maybe they’re smarter than us as well. Hopefully we’ll figure it out before they decide to take over…




Ivan L. Simpson-Kent is a 3rd year PhD student at Wolfson College in Medical Science (Developmental Cognitive Neuroscience), studying how the development of brain networks produces intelligence in childhood and adolescence.


References:

  1. Dr. Monica Gagliano: "Intelligence Without Brains" at the World Science Festival 2019,