Plants Neither Possess Nor Require Consciousness, Biologists Say

In a review paper published this week in the journal Trends in Plant Science, University of California, Santa Cruz’s Professor Lincoln Taiz and colleagues argue against the suggestion from ‘plant neurobiologists’ that plants possess consciousness.

According to Taiz et al, plants don’t think, they grow. Image credit: Johannes Plenio.

The topic of whether plants possess many of the same mental features as animals, such as consciousness, cognition, intentionality, emotions, and the ability to feel pain, has been under debate since the establishment of plant neurobiology as a field in 2006.

In 2016, Mount Sinai neuroscientist Todd Feinberg and Washington State University evolutionary biologist Jon Mallatt conducted a broad survey of the anatomical, neurophysiological, behavioral, and evolutionary literature from which they were able to derive a set of criteria for the emergence of consciousness.

The only animals that satisfied those criteria were the vertebrates, arthropods and cephalopods (e.g., octopuses, squids).

“Feinberg and Mallatt concluded that only vertebrates, arthropods, and cephalopods possess the threshold brain structure for consciousness,” Professor Taiz said.

“And if there are animals that don’t have consciousness, then you can be pretty confident that plants, which don’t even have neurons — let alone brains — don’t have it either.”

Proponents of plant consciousness draw parallels between electrical signaling in plants and nervous systems in animals.

But Professor Taiz and co-authors argue that the proponents draw this parallel by describing the brain as something no more complex than a sponge.

The Feinberg-Mallatt model of consciousness, by contrast, describes a specific level of organizational complexity of the brain that is required for subjective experience.

Plants use electrical signals in two ways: to regulate the distribution of charged molecules across membranes and to send messages long-distance across the organism.

In the former, a plant’s leaves might curl up because the movement of ions resulted in movement of water out of the cells, which changes their shape; and in the latter, an insect bite on one leaf might initiate defense responses of distant leaves.

Both actions can appear like a plant is choosing to react to a stimulus, but Professor Taiz and colleagues emphasize that these responses are genetically encoded and have been fine-tuned through generations of natural selection.

One frequently referenced study on plant learning is the apparent habituation of Mimosa pudica.

In this experiment, a plant is dropped, and its leaves curl up in defense. After being dropped many times, but sustaining no serious damage, the leaves stop curling. When the plant is shaken, the leaves do curl, ostensibly ruling out motor fatigue as a cause of the lack of response when dropped.

“The shaking was actually quite violent. Because the shaking stimulus was stronger than the dropping stimulus, it doesn’t definitively rule out sensory adaptation, which doesn’t involve learning,” Professor Taiz said.

“Related experiments with peas purporting to show Pavlovian classical conditioning are also problematical because of the lack of sufficient controls.”

The researchers hope that further research will address the questions left unanswered by current plant neurobiology experiments by using more stringent conditions and controls.

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Lincoln Taiz et al. Plants Neither Possess nor Require Consciousness. Trends in Plant Science, published online July 3, 2019; doi: 10.1016/j.tplants.2019.05.008