The remarkable ability of plants to respond to their environment has led some scientists to believe it’s a sign of conscious awareness. A new opinion paper argues against this position, saying plants “neither possess nor require consciousness.”
Many of us take it for granted that plants, which lack a brain or central nervous system, wouldn’t have the capacity for conscious awareness. That’s not to suggest, however, that plants don’t exhibit intelligence. Plants seem to demonstrate a startling array of abilities, such as computation, communication, recognising overcrowding, and mobilizing defences, among other clever vegetative tricks.
To explain these apparent behaviours, a subset of scientists known as plant neurobiologists has argued that plants possess a form of consciousness. Most notably, evolutionary ecologist Monica Gagliano has performed experiments that allegedly hint at capacities such as habituation (learning from experience) and classical conditioning (like Pavlov’s salivating dogs). In these experiments, plants apparently “learned” to stop curling their leaves after being dropped repeatedly or to spread their leaves in anticipation of a light source. Armed with this experimental evidence, Gagliano and others have claimed, quite controversially, that because plants can learn and exhibit other forms of intelligence, they must be conscious.
Nonsense, argues a new paper published today in Trends in Plant Science. The lead author of the new paper, biologist Lincoln Taiz from the University of California at Santa Cruz, isn’t denying plant intelligence, but makes a strong case against their being conscious.
“Since the ‘plant neurobiology’ group emerged back in 2006, claiming that plants have their own nervous systems and many of the same features of consciousness and volition as animals, it has been the subject of a veritable feeding frenzy in the media not seen since the publication of Secret Life of Plants in the early ‘70s,” Taiz told Gizmodo.
To some, it may seem obvious that plants aren’t conscious, but just because something seems obvious doesn’t mean it’s true.
“The great thing about the scientific community is that it doesn’t automatically dismiss such claims, as long as they appear to be backed up by some sort of evidence,” said Taiz. “We know from history that a lot of things that seemed obvious at first glance are not necessarily true, so it’s necessary to evaluate the evidence and, if necessary, repeat the experiments or observations that were used to obtain it.”
In particular, Taiz and his colleagues took issue with the claim made by some plant neurobiologists that a brain and central nervous system are not requirements for consciousness, and that electrical signalling within plants effectively performs the same function. In an effort to debunk this claim, the authors turned to previous work by neuroscientist Todd Feinberg and evolutionary biologist Jon Mallatt, who recently described the specific neurological requirements for subjective awareness.
Feinberg and Mallatt concluded that vertebrates (e.g. fish and mammals), arthropods (crustaceans and insects), and cephalopods (e.g. squids and octopuses), have the basic neurology required for consciousness. Taiz and his colleagues went a step further, claiming that plants lack the requirements for consciousness. What’s more, the authors showed that plants, from an evolutionary perspective, don’t require 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 specialisation and complexity,” Taiz told Gizmodo. “Such threshold brains, according to their criteria, are found only in the vertebrates, arthropods, and cephalopods. Of course, there are still many unanswered questions, but the point we make in the article is that Feinberg and Mallat’s analysis make consciousness in plants highly unlikely.”
To which he added: “It would take ‘extraordinary evidence,’ to use [Carl] Sagan’s phrase, to salvage their hypothesis.”
As Taiz and his colleagues pointed out, plants appear to be intelligent on account of their genetically programmed behaviours. For example, plants use electrical signals to regulate the distribution of charged molecules across their membranes, which causes leaves to curl. When an insect munches on a leaf, these same signals can trigger a defence response. To an outside observer, this may seem as if the plant is consciously responding to the environment and making choices, but these responses are simply pre-programmed actions, the authors argue.
“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 write in the study. “Plant development and behaviour can be regarded as a series of [unintentional] consequences emerging from internal and external signalling networks that have evolved through natural selection.”
Alex Jordan from the Max Planck Institute of Animal Behaviour said that, if we’re to understand the evolution of cognition, consciousness, and intelligence, we have to be open to the idea that these traits may exist in nonhuman life.
“As such, I am very happy to see the discussion and debate on non-human consciousness extending beyond mammals to fish, invertebrates, and plants,” Jordan, who wasn’t involved with the new paper, told Gizmodo. “I think this is an essential step if we are to ultimately understand our place in the diversity of life. 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.”
Nature has value, whether it’s a mammal, fish, bug, plant, or even a river. Just because something is not conscious doesn’t mean we’re justified in treating it poorly. At the same time, however, we need to recognise actual consciousness because it enables us to recognise the presence of pain, discomfort, and anguish. As 18th-century English philosopher Jeremy Bentham once said:
The question is not, Can they reason? nor Can they talk? but, Can they suffer?
It may seem paradoxical, but if we mistakenly believe that plants can suffer, it could diminish our appreciation and concern for creatures that actually experience suffering. It’s therefore critically important for us to recognise when consciousness is resident in an entity, and when it’s not.