Plants can communicate, learn, and remember, emit and hear sounds and recognise their kin. Our stereotypical ideas of what a plant is, are being challenged.
The field of plant sciences demonstrates that plants not only communicate, they also have senses, can learn, have memory, and can recognize their kin (Biedrzycki and Bais, 2010; Gagliano, 2018; Manusco and Viola, 2018). Previously, such attributes were ascribed only to species possessing a brain. Whether plants are intelligent and conscious is passionately debated. For example, by pointing out that plants lack a brain or nervous system, and challenging the act of anthropomorphising plants. Resistance is common when new ideas threaten an established paradigm (Kuhn, 1996). And as scientific evidence on plant senses, memory and learning are growing, we are asked to look at our understanding of plants anew.
Historically, humanity has not always acknowledged intelligence, or even life, in other species. In the history of the modern west, plants often appeared only in the background; in science, in religion, and in daily life. This phenomenon is called plant blindness, a term coined in 1999 to highlight how plants are consistently perceived as less important than animals (Knapp, 2019). We seem to forget life on earth would not be possible without photosynthesis. In ancient Greece, often perceived as the cradle of Western civilisation, philosopher and scientist Aristotle stated that plants had a kind of soul. Yet this soul was inferior to that of animals and humans. Aristotle viewed plants as “deficient animals” and he did not refer to any of the capacities that are reported by plant science in recent years (Marder, 2012).
A paradigm shift took place in Western civilisation when Darwin introduced the revolutionary concepts of evolution and natural selection in ‘On the Origin of Species’ in 1859. Humans could now see themselves as connected to other lifeforms, rather than separate from them. It also helped people see beyond the Cartesian understanding of animals, as machines without thought or self-consciousness (Harrison, 1992). Plants and their intelligence fascinated Darwin. In the 1880 book “The Power of Movement in Plants he introduced the root-brain hypothesis. He suggested that he ‘brain’ of the plant was positioned in the tip of the root. His son Francis continued his work and declared in 1908 that plants are intelligent beings, provoking a heated debate between two camps either supporting or rejecting his claim (Trewavas, 2016).
In more recent history, ‘plant intelligence’ was first mentioned in the popular 1973 book “The Secret Life of Plants”. Its authors claimed that plants are sentient, have musical preferences, and can respond to human thought. It became a bestseller, but much of the science was later discredited, damaging the credibility of the field of plant intelligence (Pollan, 2013). Still, research continued and in the 1980s important discoveries were done demonstrating that plants communicate with other plants, insects, and mammals (Pollan, 2013).
Plants communicate, often as defense, through volatile organic compounds. These compounds can sometimes be perceived by us as the aromas of plants. Plants often use these volatile organic compounds to attract pollinators or to defend themselves when eaten by animals, by making their leaves unpalatable, or even toxic (Manusco and Viola, 2018). For example, when antelopes eat the leaves of acacia trees, the leaves start to produce tannins that make them unappetising, difficult to digest, and even toxic (Yam, 1990). Plants can also attract an animal that can rid them of their attacker (Pollan, 2013).
More recent research shows how plants have capacities similar to the human senses of smell, sound, sight, and touch. Plants receive the chemical signals of other plants, which they use for communication. The cells on the surface of a plant can have receptors for these volatile organic compounds, which then send signals onwards to the entire plant organism (Manusco and Viola, 2018).
It was as recent as 2011 that plant scientist Monica Gagliano, through her research on plant bio-acoustics with corn, could confidently say that plants emit sounds and hear them, and modify their behaviour in response (Gagliano, 2018). Two years later Appel and Cocroft (2014) reported that mustard plants can ‘hear’ the sounds of caterpillars eating on their leaves, and respond with excretion of increased amounts of mustard oils for protection.
Plants can also absorb wavelengths of ultraviolet and infrared light (Manusco and Viola, 2018), which means they can perceive light in a much wider band of frequencies of the electromagnetic spectrum than humans can. Lastly, plants are sensitive to pressure, and heat and cold. This is most visible for the human eye in plants like the sensitive plant (Mimosa pudica) which responds to touch by closing its leaves, and in carnivorous plants such as the Venus flytrap (Dionaea muscipula).
Plants can also perceive competitors and grow away from them (Pollan, 2013) and can recognise kin and provide them with the nutrients needed for survival (Biedrzycki and Bais, 2010). We also know that trees communicate with each other through mycorrhizal networks (Gorzelak et al., 2015) leading us to see intelligence as an emergent property of a community of plants, such as a forest.
Italian plant scientist Stephano Manusco argues that plants have 15 more senses than humans. Such as a sense of gravity, electromagnetic fields and humidity, and the ability to calculate numerous chemical gradients. Of course, humans are also capable of sensing gravity, temperature, pain, balance, and several other internal stimuli that are part of the human nervous system (Daley, 2019). In addition, it has been proven that plants have memory, and are capable of associative learning. Monica Gagliano worked with the sensitive plant or Mimosa pudica (2018). This plant responds to touch or disturbance by closing its leaves. In a fairly straightforward experiment Gagliano dropped potted plants, however without leading to any harm. Soon enough, the plant registered this and no longer responded to the event by closing its leaves. This learned behaviour was retained at least up to 28 days.
Following these new insights, can it be claimed that plants are intelligent, even though they do not possess a brain or nervous system? This question has sparked a lot of debate, and various definitions of intelligence have been reviewed to answer this question. In a 2007 review, 70 definitions of intelligence were gathered and the recurring aspects were; a property that an individual displays when interacting with its environment, an ability to succeed or profit towards a goal, and last, an ability to adapt to different objectives or environments (Trewavas, 2016). Manusco himself chooses the broad definition that ‘intelligence is the ability to solve problems’ (Manusco and Viola, 2018).
Overall, terminology such as memory, learning, or even more radical, intentionality, and choice when it comes to plants, has been resisted and even ridiculed (Gagliano, 2018; Pollan, 2013). Critics of plant intelligence state that plants do not have a brain or nervous system, and we are simply anthropomorphising them.
When in 2006 a group of authors proposed a new field of research they labeled “plant neurobiology” (perhaps not suitable for a species without a brain or nervous system) this again sparked heated debates (Pollan, 2013). For example, critics like plant biotechnologist Devang Mehta, point out that ascribing intelligence or consciousness to plants is unnecessarily anthropomorphising them, and that intelligence does not equate consciousness (Livni, 2018).
Prominent plant scientist Monica Gagliano also experienced significant resistance from peers in her scientific career (Pollan, 2013, Gagliano, 2018). She demonstrated that the sensitive plant Mimosa pudica is capable of learning and remembering, but many peers did not accept this vocabulary. Memory and learning can only be applied to animals. Only after years of rejection and scornful remarks, her research paper was finally published (Gagliano, 2018).
It is clear that research into plant intelligence is challenging our stereotypical ideas on what plants are. Such resistance and ridicule of ideas that challenge a settled paradigm is a common pattern in the history of science. American physicist and philosopher Thomas S. Kuhn wrote his classical book ‘The Structure of Scientific Revolutions‘ in 1962. He highlighted that the history of science is not linear, but characterised by radical paradigm shifts. Investment in rules and facts that characterise a paradigm, make it resistant to change, and it can take several decades for a new paradigm to fully settle (Kuhn, 1996).
Copernicus anticipated that his heliocentric model of the universe, with the sun at the center of our solar system, would be very controversial. He waited more than 30 years to publish his book, just before his death. It was not until almost a century later that his ideas were accepted. The church placed Galileo Galilei under house arrest for the remainder of his life when he vocalised his support for the heliocentric worldview. Similarly, Newton’s work was not generally accepted for more than half a century after the ‘Principia’ appeared (Kuhn, 1996). And this pattern continues today; various Western school curricula have added quantum mechanics only in recent years. Concepts feel counterintuitive for students, and in conflict with the classical worldview they are familiar with (Krijtenburg-Lewerissa et al., 2017). Yet Max Planck, recognised as the father of quantum mechanics, was awarded the Nobel Prize in 1918, already over a century ago (“The Nobel Prize in Physics 1918,” n.d.).
Kuhn also remarked that a new paradigm is almost always the work of a young scientist, or someone new to the field, since they are not yet so invested in the current paradigm. The work of animal researcher Jane Goodall confirms this observation. She reported that the chimpanzees she researched in Gombo in 1960 were intelligent animals capable of emotion. The prevailing belief that only humans can feel emotions was not restricting this young researcher (“Leakey and Goodall,” 2019). In her own way, Goodall helped pave the path towards a less anthropocentric worldview. We can also see it in the groundbreaking work of plant scientist Monica Gagliano. This relatively young academic was initially trained as an animal ecologist. Therefore, she was able to bring novel ideas to the field of plant science.
Following the many new insights in plant science, we are starting to see plants in a new light. We have mostly been blind to the intelligence of plants, and their importance for life on earth to exist and evolve. The growing acknowledgment of plants challenges the dominant anthropocentric paradigm. Rights of Nature and ethical discussions on the dignity of plants are bringing these new perspectives in the national legislation of countries around the world. Perhaps this is paving the way for seeing all species as equal, possessing agency and consciousness. When we can see plants in their full right, as beings that deserve rights, then we can move beyond ideas of ownership or even stewardship. We will begin to learn what in Gagliano’s words a ‘non-hierarchical collaboration’ can look like.
This article is adapted from an essay I wrote for the Ethnobotany MSc at the School of Anthropology and Conservation, University of Kent, England.
Hero image: Palm fern. Photo credit: Image by Michael Gaida from Pixabay
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