ABOVE: Mimosa species are a model system studied in plant neurobiology for their movements in response to physical stimuli. Their leaves close in response to touch.

Plant biologists argue that plants are definitively not conscious, in a paper published in Trends in Plant Science on July 3. They are pushing back against researchers who study plant neurobiology and have argued that plants have the ability to learn, respond to their environment, and have a form of consciousness.

Plant neurobiology debuted in the same journal in 2006 with a review paper. The initial resistance to the field was based on the name, as neurobiology implies having a nervous system, which plants don’t have, according to the latest report. “There is no evidence that plants require, and thus have evolved, energy-expensive mental faculties, such as consciousness, feelings, and intentionality, to survive or to reproduce,” the authors...

See “Can Plants Learn to Associate Stimuli with Reward?

The authors argue that proponents of plant consciousness are missing the complexity needed to evolve consciousness. “Our criticism of the plant neurobiologists is they have failed to consider the importance of brain organisation, complexity and specialisation for the phenomenon of consciousness,” coauthor Lincoln Taiz of the University of California, Santa Cruz, tells The Guardian.

Botanists who do think plants have cognitive abilities such as perception, learning, and consciousness have performed experiments suggesting plants are able to learn from past experiences and can be classically conditioned. Because of this they argue plants are conscious.

The paper cites the work of neuroscientist Todd Feinberg of the Icahn School of Medicine at Mount Sinai and evolutionary biologist Jon Mallatt of the University of Washington that compares simple and complex brains, such as those of insects and mammals, and finds that plants do not pass the tests for consciousness. “Feinberg and Mallat have advanced our understanding of the biological basis of consciousness with studies that suggest that in animals, at least, consciousness did not evolve with the first appearance of a nervous system, but with the evolution of a brain with a threshold level of functional specialization and complexity,” says Taiz to Gizmodo.

See “Consilience, Episode 1: Smarty Plants

Taiz says he thinks that the motivation for plant neurobiologists stems from the environmental crisis. “They want to raise people’s consciousness about plants as living organisms and reach them on an emotional level. I’m very sympathetic to the motivations, but it is clouding their objectivity,” he says to The Guardian.

Plant neurobiologist Monica Gagliano of the University of Sydney who was not involved with the paper criticizes the authors for not working towards a better scientific understanding of what consciousness is, according to The Guardian. “If we think we already know how things are and fail to continuously question our own assumptions, but construct our claims on a system of beliefs we are dearly attached to, then we are in deep trouble and miss the opportunity for true scientific discovery to occur,” says Gagliano to The Guardian. Gagliano studies plant communication and consciousness and is one the leading researchers in plant neurobiology.

The main question comes down to how consciousness is defined, whether by having a brain and nervous system or by the learning and perception abilities afforded by that brain. To understand the evolution of cognition, consciousness, and intelligence, we need to be open to the idea that these can exist in nonhuman life, says Alex Jordan from the Max Planck Institute of Animal Behaviour who was not involved with the paper, to Gizmodo. “I am very happy to see the discussion and debate on non-human consciousness extending beyond mammals to fish, invertebrates, and plants,” Jordan tells Gizmodo. “However, I agree with Taiz [and his co-authors] that the evidence for plants as conscious entities is currently lacking, and while parallels between plant physiology and neurobiology can be drawn, these are not equivalent systems.”

Chia-Yi Hou is an intern at The Scientist. Email her at chou@the-scientist.com.

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