Plants: us, them, and intelligence

Editorial by Paola Bonfante – University of Turin, Accademia dei Lincei e Accademia delle Scienze di Torino , RSF Scientific Committee

Plants appeared on Earth 450 million years ago, and dominate our planet with their impressive biomass: 450 gigatons of carbon, compared to humans’ measly 0.061. We depend on them for the oxygen they produce and the food they provide. Plants play a crucial role in regulating atmospheric carbon dioxide levels, closely linked to climate change2Plants connect human health with environmental health in a hypothetical triangle. Many Sustainable Development Goals directly or indirectly identify plants as specific action targets. Significant European projects aim to maintain high crop productivity while drastically reducing pesticide and fertilizer use.

For these reasons, in recent years, the phenomenon of plant blindness, where many humans tend to overlook plants in a given environment3, has been diminishing. However, the media’s portrayal of plants and the surrounding world reveals two dominant perspectives4. There is a utilitarian view, where plants are seen not only as sources of “natural” food reminiscent of a bygone era but also as sources of beauty through scents and colors that enhance life. An example of this view can be found in the contemporary architectural trend of developing vertical forests, rooftops, and urban gardens. The other perspective looks at plants through an anthropomorphic lens, allowing the construction of feelings and relationships. In this narrative, plants take on “new age” nuances, becoming carriers of miraculous qualities and psychophysical benefits. It is the power of green.

Within this context, a debate arises, juxtaposing communicators and researchers with diverse expertise: Are plants intelligent? A Google search yields numerous book titles on the subject, ranging from the secret life of plants to their sensitivity, intentionality, and memory, even identifying a new discipline, plant neurobiology5. Some of these books introduce extreme elements, suggesting that plants not only learn but also “speak” to humans, reviving aspects of ancient vitalism/animism6.

It all starts with the definition of intelligence. If intelligence is the capacity of an individual to confront and resolve problems, promoting their own personal fitness, then plants are, without a doubt, intelligent (like all living beings). Plants, specifically, are able to respond to external stimuli and change in response to an evolving environment – showing an extraordinary capacity for adaptation. Nonetheless, this characteristic is inherent to biology’s definition of living: an individual that does not adapt to change perishes. Bacteria represent the category of living beings that might have the highest success rate on our planet as of today – mainly due to metabolic plasticity and their presence in almost every niche. Defining intelligence purely through biology, and thus equating it to capacity for adaptation, could put bacteria in the lead with regards to the race for the smartest organisms.

The capacity for adaptations that plants have also occurs through a series of movements: although plants are, by nature, sessile, they can still move certain parts of their body. This is mostly thanks to plant cell’s features, and their capacity to uptake or disperse water. Plants responde, thus, to light, bending their body towards to light source (known as phototropism); they also push their roots deeper (gravitropism) and their stem upwards (negative gravitropism), as well as responding to touch (thigmonasty).

The various responses plants can have to their environment can be summarized – in simple terms – stating that plants have senses, just like animals. However – and this isn’t a contradiction, per se – these senses can be explained by observing a cascade of biological events. For example, the process that regulates phototropism involves a group of blue light receptors, the synthesis of specific proteins, the irregular distribution of auxin towards the darker side of the stem, all resulting in a final curvature of the stem. All plant movement is, in fact, traceable to stimulus perception processes that activate specific signals within the plant. A reductionist approach allows researchers to explain plant reactions to environmental stimuli like this, without the need to involve variables like cognitive mechanisms, intentionality, or learning.

Intelligence is, in fact, associated with a system of psico-cognitive attributes, allowing whoever has these attributes to create abstract models of reality, make predictions, express judgment, have a certain degree of self awareness, and learn. Many of these cognitive behaviors are associated with the functions of the nervous system, more specifically: networks of neurons with specific and distinct functions. Since plants don’t have these kinds of networks, plant neurobiology might seem like an inaccurate and maybe even misleading metaphor. Actually, intelligence (in humans and in animals) has an important emotional aspect, based on the organism’s propensity for developing relationships, resulting in cooperation and altruistic behavior.

These emotional aspects are explored by Susanne Simard in her book Finding the Mother Tree10Starting with extensive studies where Simard found the passage of marked carbon from a donator plant to a receiving plant, through the mycelium of a mycorrhizal fungus11, she then described the forest as an interconnected community (WWW) in which cooperation thrives, and where the Mother Trees feed their young ones (saplings, born from their seeds) through a subterraneous network that symbiotic fungi build in the ground. This perspective has become extremely popular, and speaks to our need to have a ‘’positive’’ view of nature. Nonetheless, it is also heavily criticized due to the fact that the studies that comprise Simard’s theoretical base have many experimental weaknesses, often weren’t thoroughly peer-reviewed12,13, and have an excess of anthropomorphization.

Undoubtedly: today, plants attract attention from many different sectors. Cognitive science researchers ask themselves philosophically14 rich questions, while plant biologists decipher the increasingly complex mechanisms that determine how plants operate. Nonetheless, if asked about intelligence within the plant world, most agree with Karl Popper and his principle of falsifiability, and that as of right now the cognitive activity and learning processes of plants have not been experimentally proven.

If the concept of regeneration (at the core of discussions regarding reforestation and agriculture) is evaluated in terms of its importance within European15 (and not only) politics, the debate surrounding plant intelligence doesn’t seem to be about itself, rather it seems to be simply a projection for political relevance. Will governance think of the WWW of plants that communicate with man thanks to their intelligence, or will they adopt researcher’s reductionist view8?

Photo by Nagara Oyodo


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