Can plants think? The burgeoning field of plant neurobiology has a lot to say on the matter

Though they lack brains, scientists are figuring out new ways to probe whether plants "think"

By Matthew Rozsa

Staff Writer

Published September 30, 2022 4:00PM (EDT)

Brain Shaped Bonsai Tree (Getty Images/Viaframe)
Brain Shaped Bonsai Tree (Getty Images/Viaframe)

Americans like to mow their lawns, but blades of grass aren't supposed to all have the same length. Left un-sheared, an all-natural lawn contains grasses of wildly varying heights, more akin to an unruly, uncombed head of hair right after a long night's sleep. A lawn is not a single organism, but a large community of plants that have individual heights; being mowed is not the natural state for a blade of grass.

This raises a disturbing question: When a human mows a lawn, is that the equivalent of mass torture to the grass — assuming the grass can "feel" or "think" the way we can? The proposition is not as outlandish as it might seem. Recent research suggests that plants are far from the stationary automatons that most of us think of them as. And though they don't have brains in the same way most animals do, plants seem to possess a different set of evolutionary tools that suggest they may experience consciousness, albeit in a radically different way from us. 

"There are numerous definitions but the most simple and relevant is this: Consciousness is a feature of living systems allowing them awareness of their external and internal conditions."

One such theory of how this might work is known as the "Cellular Basis of Consciousness" theory. This posits that all life, from the smallest single-celled organism and on upwards to the largest animals on Earth, possesses something akin to consciousness.

"In our Cellular Basis of Consciousness (CBC) Theory, consciousness evolved with the very first cells and all cellular life is endowed with consciousness which is essential for their agency, survival and evolution," explained Dr. Frantisek Baluska, a scientist at the University of Bonn's Department of Plant Cell Biology, in an email interview with Salon. Baluska has published articles in scholarly journals from BioEssays to Philosophical Transactions B on the subject of plant consciousness. According to CBC Theory, every cell that exists possesses the innate qualities necessary to possess a level of self-awareness. It points out that individual cells are able to interact with their surroundings in a manner that clearly displays a sense of agency. Baluska and other scholars like psychologist Dr. Arthur S. Reber and neurobiologist Dr. Stefano Mancuso have argued that there are structures in all cells that endow each organism with a certain amount of consciousness.

"There are numerous definitions but the most simple and relevant is this: Consciousness is a feature of living systems allowing them awareness of their external and internal conditions," Baluska wrote to Salon when asked to define consciousness in the sense used to understand the inner lives of plants.

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Dr. Paco Calvo has an upcoming book, co-authored with Natalie Lawrence, called "Planta Sapiens: Unmasking Plant Intelligence." Calvo works at the MINT Lab (Minimal Intelligence Lab) at the University of Murcia in Spain, and provided Salon with his own definition of consciousness.

"There is no one single, agreed-upon definition of what 'consciousness' is," Calvo told Salon by email. To the extent that a coherent definition can be deduced based on what scientists know for sure about biology, however, Calvo speculated that "consciousness relates to the presence of 'feelings, subjective states, a primitive awareness of events, including awareness of internal states.'" Within that context, Calvo pointed out that scientists have already demonstrated that a number of non-human animals possess sentience, from cephalopods (like octopuses) to insects (like ants). As the list continues to grow, it is reasonable to at least wonder if plants as well as neurologically-wired organisms will be found to have self-awareness.

"Sentience, we may say, makes sense for life, as an essential underpinning to the business of living," Calvo explained. "And it is very unlikely that plants are not far more aware than we intuitively assume." To the "skeptics" who insist that consciousness must be tied to a central nervous system, and that plants would not need to evolve consciousness in the first place, "even if 'consciousness', as understood in vertebrates, is generated by complex neuronal systems, there is no objective way of knowing that subjective experience has not evolved with entirely different kinds of hardware in other organisms," Calvo argued. "We have no evidence to conclude that no brain means no awareness. It is certainly true that we cannot yet know if plants are conscious. But we also cannot assume that they are not."

Calvo added, "Plants, not unlike, say, locked-in patients, might well have significant conscious experience, although there is no way for us to intuit it nor for them to communicate it to us."

"Sentience, we may say, makes sense for life, as an essential underpinning to the business of living," Calvo explained. "And it is very unlikely that plants are not far more aware than we intuitively assume."

Not everyone is convinced by the various theories that exist for plant consciousness. Dr. David G. Robinson of the University of Heidelberg's Centre for Organismal Studies co-authored a 2021 article from the scholarly journal Biochemical and Biophysical Research Communications which addressed another theory that pro-plant consciousness proponents claims backs up their belief. On that occasion, Robinson discussed Integrated Information Theory, which attempts to identify the fundamental properties of consciousness and then ascertain the physical bodies that correspond with them. More broadly, Robinson was dismissive of those scholars who say that plants can be conscious beings.

"I can only refer you to the article of Mallatt et al. (2020), where the 'debunking of myths' was painstakingly carried out," Robinson wrote to Salon. "Since plants don't have a brain, Mancuso in his 2015 book talks about 'distributed intelligence' to explain the fact that many animal-like properties (hearing, seeing, chemical signaling, etc) are shown by epidermal cells. He equates this with consciousness, but in all these cases these are genetically programmed responses which are not centrally coordinated and there is no indication of feedback. This is not consciousness."

Robinson added, "There is a huge popular following for books (e.g. from [Dr. Monica] Gagliano) humanizing plants, telling us that plants can communicate with us. This is shamanism, pure humbug — it's fool's gold. We learn nothing about plants by reading this literature."

While it is likely an exaggeration to dismiss the ideas about plant consciousness as "pure humbug," it is fair to say that they remain unproved. Indeed, if they were validated, they would have remarkable implications in terms of the ethics of how humans interact with plants.

"We should acknowledge that plants are complex living systems which deserve dignity, as it is stated in the Swiss Constitution through amendment from 2008," Baluska argued. "As animals, humans and plants are in close co-evolution and have the same biological origins, we should treat them as living organisms deserving dignity."

Calvo noted that, even if humans only acknowledge that plants have a very primitive form of consciousness, that should still make us feel "uneasy" at the realization that "plants are agents, and not mere objects or resources to be exploited more or less wisely."

"Most people would dismiss the very possibility that plants are sentient at the outset, negating the need for an ethical standpoint, and arguing that it would lead to absurd implications," Calvo pointed out. "And I must confess that for many years, the ethical implications of the proposition that sentience might extend well beyond the animal world hadn't troubled me. But the parallels that are emerging between the ways that plants sense, understand and respond to their environments and the ways that animals do, are making it increasingly difficult to avoid these questions. In fact, our success at tackling the ecological crisis may depend upon facing these issues head-on."

By Matthew Rozsa

Matthew Rozsa is a staff writer at Salon. He received a Master's Degree in History from Rutgers-Newark in 2012 and was awarded a science journalism fellowship from the Metcalf Institute in 2022.

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