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In a review article published this week in the journal Trends in plant scienceLincoln Taiz, a professor at the University of California, Lincoln and his colleagues oppose the suggestion of "plant neurobiologists" that plants have a conscience.
According to Taiz et al, plants do not think, they grow. Image credit: Johannes Plenio.
The question of whether plants have many of the same mental characteristics as animals, such as consciousness, cognition, intentionality, emotions, and the ability to feel pain, has been debated since creation of plant neurobiology as a field in 2006.
In 2016, Mount Sinai neuroscientist Todd Feinberg and Washington State University evolutionary biologist Jon Mallatt conducted an extensive study 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 met these criteria were vertebrates, arthropods, and cephalopods (eg, octopus, squid).
"Feinberg and Mallatt have concluded that only vertebrates, arthropods and cephalopods have the threshold of brain structure for consciousness," said Professor Taiz.
"And if there are animals that are not conscious, then you can be pretty confident that plants, which do not even have neurons – let alone brains – do not have either."
Proponents of plant awareness draw parallels between electrical signaling in plants and the nervous system in animals.
But Professor Taiz and his co-authors argue that promoters draw this parallel by describing the brain as something as complex as the sponge.
The Feinberg-Mallatt model of consciousness, on the other hand, describes a specific level of organizational complexity of the brain required for a subjective experience.
Plants use electrical signals in two ways: to regulate the distribution of charged molecules across membranes and to send messages remotely in the body.
In the first case, the leaves of a plant could bend because the movement of the ions caused a movement of the water out of the cells, which modified their shape; and in the latter case, an insect bite on a leaf could initiate distant leaf defense responses.
Both actions may appear as if a plant chooses to respond to a stimulus, but Professor Taiz and his colleagues point out that these responses are genetically coded and have been refined over generations of natural selection.
A frequently cited study of plant learning is the habitual habituation of Mimosa Pudica.
In this experiment, a plant is released and its leaves curl up in defense. After being dropped several times, but without serious damage, the leaves stop weaving. When the plant is shaken, the leaves curl, ostensibly eliminating motor fatigue as a cause of failure to respond to a fall.
"The shaking was actually pretty violent. Because the agitated stimulus was stronger than the diminishing stimulus, it did not permanently exclude sensory adaptation, which did not involve learning, said Professor Taiz.
"Experiments associated with peas claiming to demonstrate classical Pavlovian conditioning are also problematic because of the lack of sufficient controls."
The researchers hope that further research will solve the problems left unanswered by current plant neurobiology experiments using stricter conditions and controls.
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Lincoln Taiz et al. Plants do not possess nor require consciousness. Trends in plant science, published online July 3, 2019; doi: 10.1016 / j.tplants.2019.05.008
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