- An opinion piece in the journal Trends in Plant Science emphatically argues that plants are not conscious.
- The article questions the soundness of widely covered studies that mimosa plants (Mimosa pudica) and peas (Pisum sativum) display learning behaviors that amount to having a consciousness.
- Plants do not have a brain or anything resembling it, the authors point out, and to possess consciousness a structurally complex brain is required.
- Monica Gagliano, who has reported learning behaviors in plants, rejects this view saying that the criteria used to determine animal consciousness cannot be uncritically extrapolated to plants, and that the opinion piece fails to cite sound evidence.
While the world wonders whether sentient robots are in the offing, scientists continue to grapple with an ancient mystery: Are plants conscious?
A new opinion piece in the journal Trends in Plant Science seeks to nip this debate in the bud. “We are arguing that the likelihood of consciousness in plants is effectively nil,” Lincoln Taiz, a co-author and professor emeritus of molecular, cell and developmental biology at the University of California, Santa Cruz, told Mongabay in an email.
It’s the latest salvo in a heated debate sparked by the publication of studies widely covered in the media that suggest that plants exhibit learning behavior. Monica Gagliano, an evolutionary ecologist at the University of Western Australia, is a leading proponent of the idea that plant might possess consciousness, based on her experiments with mimosa (Mimosa pudica) and pea plants (Pisum sativum), which have generated tremendous interest and controversy.
In 2014, her team published findings that mimosa plants “learn” not to fold their leaves in response to being dropped multiple times and not suffering any harm. In 2016, she and her colleagues published a paper in Nature’s Scientific Reports on “learning by association in plants,” based on experiments with peas that reportedly displayed “learned” behaviors.
Gagliano rejected Taiz’s position in an emailed response, noting that “crucially in science, opinions are cheap, but can be valuable (and perhaps correct) if they are well-informed, balanced and based on the best available experimental research. The piece fails in just about all these criteria.”
Taiz cites Gagliano’s work in the piece only to caution that the latter’s conclusions were hastily drawn. What we see in plants is actually an adaptive response, Taiz and his colleagues argue, adding that the studies were not rigorous enough and needed to be replicated.
The argument against intelligent plant life rests primarily on two legs: given that even animal brains need a level of structural complexity to support consciousness, what chance do brainless plants have? They even lack neurons, opponents point out. Second, that the behaviors observed in flora can be explained by other things, not consciousness.
The opinion piece draws heavily on the work of evolutionary biologist Jon M. Mallatt and neuroscientist Todd E. Feinberg, who studied the evolution of consciousness in animals and found that only vertebrates (including fish), arthropods (like insects and crabs), and cephalopods (like octopuses and squids), possess the hardware for consciousness.
Gagliano said she believes their work has been unjustifiably extrapolated to plants.
“It selectively piggybacks on one hypothesis — proposed by Feinberg and Mallat on the evolution of consciousness (in animals) — and appropriates it as the premise to affirm a pre-conceived and ontologically reductionist idea regarding (plant) consciousness,” she said.
But Mallat and Feinberg themselves disagree.
“We fully agree with their point that consciousness needs complex cellular networks for fast information-processing, which effectively means it requires neurons and brains, which plants lack,” they told Mongabay in a joint comment. They have expounded on their ideas in two books: The Ancient Origins of Consciousness (2016) and Consciousness Demystified (2018).
They also appear to agree with Taiz that actions such as moving toward a food source or avoiding harm are adaptive behaviors. Highly complex reflexive actions do not qualify as proof of consciousness. For the two authors, possessing a consciousness means “neural complexity, using many elaborate senses to build mapped representations (‘mental images’) of the world in which one’s behavior occurs, learning complex new actions from experience, or self-delivery of analgesics to avoid pain.”
Taiz and colleagues also contend that plants do not need consciousness because it bestows no evolutionary advantage. “They are perfectly able to carry out their physiological functions by means of genetic and epigenetic adaptations, without any need to invoke consciousness,” they write. In their view, having a brain or brain-like mechanisms would be costly for plants, depleting their energy unnecessarily with no real benefits.
The debate about consciousness in plants has implications for humans, one of which is whether plant neurobiology is a valid science. Can you belong to a discipline that studies plant consciousness if its very existence is in question? The Society for Plant Neurobiology was founded in 2005, and a year later a 2006 article heralded the birth of “plant neurobiology” as a new field of plant biology. In 2009, in the face of criticism, the organization changed its name to the Society of Plant Signaling and Behavior.
The debate is not merely a question of semantics. It reveals deeper fractures in the biological sciences about fundamental questions, such as: What is a brain? And what does it mean to have a consciousness. This is how Gagliano frames the question. “When we, as scientists, accept a particular idea without questions — for example, the idea that consciousness is a process that exclusively arises through the structure and dynamics of the brain — we are prone to bias of reasoning and at risk of resting our thinking on erroneous assumptions,” she said.
Taiz, L., Alkon, D., Draguhn, A., Murphy, A., Blatt, M., Hawes, C., … Robinson, D. G. (2019). Plants neither possess nor require consciousness. Trends in Plant Science. doi:10.1016/j.tplants.2019.05.008
Gagliano, M., Vyazovskiy, V. V., Borbély, A. A., Grimonprez, M., & Depczynski, M. (2016). Learning by association in plants. Scientific Reports,6(1). doi:10.1038/srep38427
Gagliano, M., Renton, M., Depczynski, M., & Mancuso, S. (2014). Experience teaches plants to learn faster and forget slower in environments where it matters. Oecologia,175(1), 63-72. doi:10.1007/s00442-013-2873-7
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Malavika Vyawahare is a staff writer for Mongabay. Find her on Twitter: @MalavikaVy
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