These scientists don’t think plants think

A surprisingly old idea, the notion that plants have consciousness, is facing renewed scepticism and scrutiny.

In the pages of Trends in Plant Science, a group of scientists put the controversial ideas of “plant neurobiology” under the microscope, in particular singling out plant consciousness for some wilting criticism.

A central precept in the Enlightenment of the seventeenth and eighteenth centuries was that the natural world and everything in it was akin to a machine; life was simply clockwork, governed by laws and mathematically predictable. 

The Romantic movement of the early nineteenth century saw a shift away from this world of automatons to a new vista of vital living things. In particular, German Naturphilosophie, or philosophy of nature, understood the world holistically, bringing together mind and spirit and nature into a tight weave.

During this period, modern biology as we know it began to form, presenting something new from the unending taxonomy of previous centuries. Amongst these new biologists were those who saw in the realm of plants, creatures with consciousness and feelings.

The idea stuck, and like so many notions in biology, plant consciousness finds a place in the writings of Charles Darwin too.

His 1880 work, The Power of Movement in Plants, has at its end a passage in which Darwin suggests that the tips of a plant’s roots are analogous in function to the brains of lower animals. The shadow of this analogy has cast long indeed.

The idea that plants might be conscious has found renewed vigour since a 2006 paper heralded the arrival of a new subfield of botany known as plant neurobiology (PN). 

PN researchers have argued that there are parallels between electrical signalling in plants and the nervous systems of animals, and even for a botanical equivalent of the nervous system based around hormones belonging to the auxin class acting like neurotransmitters. They hold that plants have intelligence, intention and can even learn. Some have revived Darwin’s idea that a root tip is a “brain-like command centre”.

But these ideas have not been received enormously well. Indeed, one of the authors of the current paper was among the many scientists to sign a letter published in 2007 arguing that plant neurobiology was a field without a subject of study: that is, plants simply don’t have neurobiology.

That author, Lincoln Taiz of the University of California, Santa Cruz, US, along with seven other colleagues from various international institutions, has now published a critical review of the state of play of the field of plant neurobiology. And the title says it all: “Plants neither possess nor require consciousness.” 

Taiz and colleagues survey several problems with PN, from the philosophical to the experimental. They argue that plant behaviour, initiated by internal electrical signalling, which is used, in part, for messaging across the large distances of the organism, are genetically preprogramed.

These behaviours have been mistakenly anthropomorphised, understood by projecting human traits on to non-human organisms, by PN researchers. In seeing something human in a plant’s reactions, advocates of PN have erroneously concluded that plants must have intention, intelligence and consciousness. The danger of this, says Taiz, “is that it undermines the objectivity of the researcher”.

Similarly, they dismiss, or more carefully parse, the significance of a number of key experimental findings in the field, concluding that much of PN’s empirical backing is far more equivocal that advocates admit.

One person the authors single out for special attention is Monica Gagliano, an evolutionary ecologist from the University of Sydney. Gagliano has published attention-grabbing papers detailing the results of experiments purporting to show learning in plants.

One of her experiments involved habituation, “a decrease in a behavioural response with repeated stimulation that does not involve either sensory adaptation or motor fatigue” that is considered the most basic form of learning in animals.

Specimens of the plant Mimosa pudica, which rapidly folds its leaves in response to mechanical stimulus, was dropped repeatedly and was shown to eventually stop rolling its leaves after a time. It had seemingly become habituated. The plants were then shaken, only to display the rolling response once more, a result that supposedly rules out motor fatigue. The experiment subsequently was used to support the claim of animal-like-learning in plants.

But Taiz sees problems with the experimental design. “The shaking was actually quite violent,” he says, “Because the shaking stimulus was stronger than the dropping stimulus, it doesn’t definitively rule out sensory adaptation, which doesn’t involve learning.” He sees similar problems with Gagliano’s work purportedly showing Pavlovian classical conditioning in pea plants, another form of learning only seen in higher animals.

But Gagliano sees in these experimental results the necessary hints that plant consciousness exists, writing about it in papers with provocative titles such as “Inside the Vegetal Mind: On the Cognitive Abilities of Plants”. Her passion is perhaps aided and abetted by her “her ideological connection with South American shamanic traditions”.

Even if Gagliano is right about plant learning, that is no guarantee at all that this implies consciousness.

Consciousness is the most baffling phenomenon on earth and philosophers refer to it as “the hard problem”. While we know that the physical brain underpins the conscious mind, there is an explanatory gap – an empty space where an explanation of how the latter arises from the former should fit.

Nonetheless, we can get some idea of the necessary equipment an organism would need in order to evolve consciousness. 

“Recently,” write Taiz and his co-authors, “Todd E. Feinberg and Jon M. Mallatt conducted a broad survey of the anatomical, neurophysiological, behavioural, and evolutionary literature from which they were able to derive a consensus set of principles that allowed them to hypothesise how and when primary consciousness, the most basic type of sensory experience, evolved.”

Based on their research “Feinberg and Mallatt concluded that the only animals that satisfied their criteria for consciousness were the vertebrates (including fish), arthropods (e.g., insects, crabs), and cephalopods (e.g., octopuses, squids).” 

Plants, notably, do not feature on this list.

This leads Taiz to conclude that “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.”

Although plants may not be conscious, the authors still hold that they “are nonetheless remarkable organisms, worthy of our admiration, respect, study, and efforts to conserve”.