Intelligent Life

Can we acknowledge agency in plants?

PICTURE A TALL jamun tree (Java plum, Syzygium cumini) growing in a busy urban neighborhood in an Indian city. It is summer and time for the jamun to bear fruit — a predictable, long-term behavior. The blazing summer has unfortunately brought a drought to this semi-arid urbanscape, an unpredictable disruption that could either pass quickly or affect the tree for a prolonged period. Underground concrete structures and tarmac roads add layers of restriction to the tree’s root expansion decisions. Between energy expended to bear fruit and environmental pressures like drought, the tree becomes vulnerable to invasion by herbivores and parasites. The jamun tree must actively contend with immediate threats, manage resources amid short- and medium-term pressures, and balance the evolutionary pull to reproduce. How does the jamun do it?

We are only beginning to uncover the fascinating behaviors [plants] exhibit, the lives they lead, and the stories they can tell.

The jamun may decide to invest in growing its roots to power through concrete and soil in search of water, or it may forgo fruiting in response to the drought and reproduce when conditions improve. Each of these decisions has consequences that determine resource allocation, fruiting, and growth. But they are decisions. Indications of agency.

OUR PERCEIVABLE UNIVERSE is a vast stage filled with action: Stars burn, subatomic particles spin, sunflowers turn dutifully to face the sun, and you open articles to read them. Take a moment to imagine the events leading up to you reading this. You opened a magazine, scanned this page, then perhaps poured yourself a warm cup of tea and began to read. Acting on the intent to read suggests you possess what we would call agency. Could we imagine a similar flow of events leading to the jamun tree sending its roots farther underground during an especially hot summer? Or for a sunflower plant turning to face the sun? Could we say the plant acts with agency, or do we reduce its behavior to a mere response to a stimulus?

Since it was contemplated by early Western philosophers like Aristotle, running through Hume, agency — the capacity to act with intent — has been considered an exclusively human trait. Many philosophical and scientific definitions of agency are still restricted to humans, or a few select species, because having a mind capable of thought is considered a prerequisite to action. Bound by the confines of our minds, we only perceive constructs such as thought and consciousness in our species. By and large, this has led to the all-pervading idea of human superiority among life on Earth.

Plant behaviors are thus not mere reactions to changes but proactive and predictive actions.

Our exclusionary approach to human constructs like agency promotes a strong sense of human exceptionalism at a time when our species would do well to understand that we are one of many actors in nature, woven together in a mutually dependent tapestry of life. Plants are critical actors in this intricate fabric. Yet, despite being living entities much like us, plants are often cast off as inanimate objects. Even discussions around their importance often portray them as resources and commodities, rather than fellow inhabitants of the planet. Plants are complex beings, and we are only beginning to uncover the fascinating behaviors they exhibit, the lives they lead, and the stories they can tell.

plant and roots artwork

Is the fear of anthropomorphizing holding us back from a truly inclusive and expansive view of the abilities of plants? We argue that it is, and perhaps it is time to revisit some of our long-held assumptions about the abilities of plants.

SCIENTIFIC RESEARCH SHOWS that plants detect and integrate a plethora of information they receive from their environment before mounting appropriate responses. Plant behaviors are thus not mere reactions to changes but proactive and predictive actions. Their responses are complex, nuanced, and dependent on the context they find themselves in. Some of these are shaped by evolution, while others may be plastic responses executed rapidly. (Plastic responses are changes in an organism’s behavior, morphology, and physiology in response to the environment that may or may not be long-lasting.) Scientific inquiry into the lives of plants presents us with incredible insights into their behavior and brings to light similarities plants and humans share in the way we interact with the world around us.

Take, for instance, the fact that plants can recognize self and kin. Biologists Richard “Rick” Karban, of the University of California, Davis and his colleagues recently discovered that, like most plants, when damaged by herbivores, sagebrush (Artemisia tridentata) in the Sierra Nevada of California gives off chemical signals (volatile organic compounds) that facilitate rapid communication across various parts of the plant. These chemical signals trigger the plant to release proteinase inhibitors that give the attacking herbivores digestive troubles.

Their study also found that when a sagebrush plant is exposed to volatile cues released by a genetically identical plant (that is, a plant grown from its own cuttings), it suffered less damage than a sagebrush that was exposed to volatile cues from a plant it is not directly related to — a form of kin recognition. “Our results clearly demonstrate chemotype-dependent communication,” study co-author Finnish biologist Patrick Grof-Tisza told CORDIS, the European Union research website. “This may be analogous to language dialects in human speech.”

Plants’ recognition of genetically related individuals can shape how they respond to their neighbors as well. Plants like the American searocket (Cakile edentula) have been seen to act differently around genetic relatives: They limit their root growth, perhaps to allow their kin to share resources in the soil. Other plants, like the annual ragweed (Ambrosia artemisiifolia), form more robust fungi-based mycorrhizal networks — which improve their water and nutrient absorption capabilities — with their kin compared to strangers. All these fascinating behaviors suggest the plants recognize self and kin, and alter their behavior and decisions accordingly.

Plants communicate. Sitting at the heart of most ecosystems, they are crucial nodes for exchanging energy and information. Innovative research has provided fascinating insights into how plants communicate with other plants, animals, and microbes (some of which are symbiotic and housed inside plant tissues).

Visual cues are one of the most apparent ways that plants communicate. Many flowering plants use dramatically colored flowers to visually communicate the readiness of pollen and nectar to attract insect pollinators. Some of these visual signals can even trick pollinators into visiting them.

Take the bee orchids, for instance. Most orchids are pollinated by bees and wasps (Hymenoptera), and the bee orchid (Ophrys apifera) is no different. The flowers of this orchid, which is found in many European countries, mimic the solitary bee species that pollinate them. The orchid’s velvety lip looks so much like a female bee that male bees often visit the flower mistaking it for a female. The flowers also emit a distinct fragrance to convince the male bee that the flower is a female suitor. The male then attempts to mate with the flower and leaves with a generous helping of pollen. Not a successful visit for the bee, but definitely so for the plant!

Apart from visual appeal, many plants also include enticing smells in their flowers to attract pollinators to visit them. Think of the wonderful smells of mangoes and figs that attract a host of animals to eat the fruit, and in the process, act as seed dispersers. Or the stinky corpse flower (Amorphophallus titanum), whose putrid odor attracts pollinators like carrion beetles and flies from near and far.

sunflower artwork

The incredible layers of interspecies communication between plants and pollinators also extend to sound. Plants are great listeners. Some can detect sound vibrations made by hungry caterpillars or slugs chewing on their leaves and mount a defense by secreting chemicals that taste bad or even kill the predator.

Even more surprising is that plants can tell the difference between vibrations caused by predators chewing on them and those caused by wind, or even pollinators’ wings, and respond accordingly. Israeli researcher Marine Veits and colleagues, for instance, found that beach evening primroses (Oenothera drummondii) produce a sweeter nectar when they “hear” the sound of a bee buzzing nearby.

Heliotropic plants… have been shown to predict the sun’s position and orient their leaves in that direction even before sunrise.

The researchers presented the primroses with recordings of various sounds — including the wing-beats of its pollinators, hawk moths by night and early morning, and bees at dusk and in the morning — as well as “synthetic sound-signals at similar frequencies.” Their experiments showed that when the primrose hear the bees and moths they can increase the sugar content in their nectar within three minutes. The flowers, they said, served as the plant’s ears, picking up the sound of the pollinators’ wings while tuning out irrelevant sounds like wind.

Plants also are able to anticipate and prepare for some environmental cues. Heliotropic plants (those that track sunlight), such as the Cornish mallow (Malva multiflora), have been shown to predict the sun’s position and orient their leaves in that direction even before sunrise.

Plants can switch strategies in the face of unpredictable environmental changes. For instance, researchers found that oak and pine trees can switch to surface layer water as their primary water source after rainfall instead of deeper water reserves. Such predictive ability is evident even in the early stages of their lives, wherein a seed has to obtain and integrate various information even before it germinates. Soil is an incredibly unpredictable and dynamic natural system. Seeds germinate based on the environmental conditions they face in the soil, such as temperature, moisture, age, and nutrient quality, and the presence of specific volatile organic compounds. And they can delay germination until favorable conditions arise. Studies have shown that a seed’s interpretation of its environment is likely a result of epigenetic memory transferred from the maternal parent; a mother’s learnings are genetically passed on to help her young navigate the world.

plant and grasshopper artwork

Similarly, plants are sensitive to light of various wavelengths and can detect changes in the quantity and quality of light they receive. Noticing and evaluating a change in light conditions is an unceasing task for plants because they grow in heterogeneous environments with many neighbors. While they may not be able to control a competing tree or a large building from springing up next to them, plants can perceive their neighbors blocking light and take corrective action. Depending on the nature of the threat to their sustenance, they can grow away from their neighbors, and show shade avoidance behaviors such as changing the angle of the leaf or shade tolerance behaviors such as increasing the efficiency of light capture by leaves. They can assess the size of their neighbors and adjust investments in vertical growth accordingly. Plants may also choose not to grow vertically and shade their neighbors if they are recognized as kin.

For most of us, plants move slowly if they move at all. This is partly true. Most movements in plants, like the dramatic emergence of a shoot or even the adventurous explorations of tendrils, happen at a time scale that is not very evident to the human eye. We often see the outcome of the movement, not the movement itself. There are some plants, however, that exhibit rapid movement. The white mulberry tree (Morus alba) catapults pollen by moving its stamen at mind-boggling speeds (over 600 km/hr). To the dismay of many flying insects, the carnivorous Venus flytrap (Dionaea muscipula) can snap shut in roughly 100 milliseconds! These rapid movements are, ironically, too fast for the human eye.

The touch-me-not plant, (Mimosa pudica), is the poster child of prompt action in the plant world. The plant moves at a pace that perfectly suits human chronoperception. Most of us have likely spent generous amounts of time as children (and possibly adults) touching mimosa plants to trigger their leaves to shut — a defensive strategy used by the plant. When the plant senses vibrations or touch, it will close its leaves and droop to discourage potential predators from feeding on it. It may also shut its leaves to prevent dehydration when the environment gets too warm.

plants artwork

The touch-me-not plant, (Mimosa pudica), is the poster child of prompt action in the plant world.

In the early 1900s, the captivating responsiveness of the touch-me-not plant caught the eyes of the legendary Indian scientist Jagadish Chandra Bose.

Bose — who invented the crescograph, an instrument to measure growth in plants — believed that plants were complex and intelligent life forms who “feel pain and understand affection” just as much as humans do. Often denied access to laboratories run by British colonizers, he created machinery and experimental setups in his small Kolkata lodgings that blurred the lines between physics and biology. By studying the bioelectric potential within a mimosa plant, Bose demonstrated that its rapid movements were triggered by a complex network of pathways carrying electrical signals analogous to an animal nervous system.

“Since the nervous reactions in animals and plants are so essentially similar,” Bose wrote more than a century ago, “delay in full recognition of this fact will undoubtedly retard the advance of science.”

Several studies investigating plant behavior since Bose’s groundbreaking work have gathered more evidence to suggest that the action potential in plants is similar in design to the action potential in humans (nerve impulses).

Current knowledge demonstrates that plants also use molecules like GABA (gamma-aminobutyric acid) and glutamate, which are common animal neurotransmitters, for internal communication. Curiously, plants are also affected by anesthesia, just like humans. All these present readily apparent commonalities in the way plants and animals communicate information between different parts of their bodies.

Scientific investigations have clearly demonstrated that plants deal with unpredictable changes in their environment, including attacks by herbivores, damage, and disease, among other things. Their lives are intricately linked to microbes, animals, and other plants. In an ever-changing world, it is important for plants to rapidly detect and respond to changes, many of which happen simultaneously. Without the luxury of moving away, plants have developed sophisticated ways to detect, integrate, and respond to information. They can choose to respond differently based on the specific conditions they face. They can also predict and prepare in advance for changes; they show evidence of memory and learning. If this isn’t agency, then what is?

LANGUAGE INFLUENCES THOUGHT and actions. Language is how nearly all human knowledge is acquired and shared. Thoughts, emotions, feelings, interpretations, perspectives, and so much more flow between people through language. There are over 7,100 languages spoken around the world. Each language reflects the unique worldview of the place, society, and culture it originated from, and that has consequences for both the individuals and societies that use languages to communicate.

bird and plant artwork

The Siona tribe from the Ecuadorian Amazon region, for example, have a deep-rooted ethnobotanical and spiritual relationship with plants like the capi vine (Banisteriopsis caapi) that is used to make Ayahuasca, a hallucinogenic drink. The depth of their connection with this plant can be glimpsed in the fascinating ways that they distinguish and name varieties that modern science sees as a single plant. For instance, sese-yajé or sise-yajé is a variety of Ayahuasca distinguished by the visions of hunting it induces in the drinker. In Shadows in the Sun: Travels to Landscapes of Spirit and Desire, Wade Davis, an ethnobotanist, writes of the incredible relationships between Indigenous Amazonian communities and the plants that lived alongside them:

Modern science… is simply too rigid to capture the complexities with which our species can perceive an environment and communicate it to others.

The Ingano of the upper Putumayo in Colombia, for example, recognize seven kinds of ayahuasca. The Siona have eighteen varieties, which they distinguish on the basis of the strength and colour of the visions, the trading history of the plant, the authority and lineage of the shaman, even the tone and key of the incantations that the plants sing when taken on the night of a full moon. None of these criteria makes sense botanically, and, as far as modern science can distinguish, the plants are referable to one species, Banisteriopsis caapi. Yet the Indians can readily differentiate their varieties on sight even from a considerable distance in the forest. What’s more, individuals from different tribes, separated by large expanses of forest, identify these same varieties with amazing consistency …

Fourteen categories in all, not one of which can be determined based on the rules of our own science.

For most of us reading this, it would seem virtually impossible to imagine a connection so deep with plants or any other actors from the natural world. Modern science, as it is typically practiced, is simply too rigid to capture the complexities with which our species can perceive an environment and communicate it to others. It would benefit society to acknowledge the worldviews, perspectives, and languages of Indigenous communities that share rich and non-exploitative relationships with the natural world.

One reason that natural habitats are often well-preserved around Indigenous communities could be their nature-inclusive language, which helps both articulate and shape thought and action. Fostering inclusive language and vivid expression in modern science and society can nurture empathy and spark meaningful insight at every level of the collective. Embracing commonalities and seeing that we are a unique part of the design (not above it) is a fulfilling first step.

It is heartening to see that some researchers are pushing the boundaries of our scientific understanding of plant behavior and using unrestricted language to describe it, while challenging the human exceptionalism that has plagued Western thought for centuries. Scientists and philosophers like veteran molecular biologist Anthony Trewavas, who has been writing about plant intelligence since the early 2000s, ecologist Monica Gagliano — a trailblazer in the relatively new field of plant bioacoustics; cognitive scientist and philosopher of biology Paco Calvo, whose interdisciplinary work combines insights from biology, philosophy; and cognitive science; philosopher Michael Marder, whose work has focused on building philosophies that take into account plants as beings with their own form of subjectivity; and many others are expanding the perspectives of science and examining plant lives unconstrained by fears of anthropomorphism. By rejecting anthropocentrism, they are radically transforming our understanding of plants and broadening the prevailing view of what intelligence is.

While many of the findings in the fields of plant behavior and intelligence may be at the incipient stage, they have opened the door for rigorous scientific investigations and conversations around the idea of agency in other organisms. Our understanding of plant biology and behavior will progress when modern science embraces the commonalities between plants and humans, the common processes and behaviors that are recognized by traditional knowledge systems.

New scientific approaches arising from open-minded inquiry have allowed us to gain a richer and deeper understanding of plant intelligence. Many groundbreaking discoveries about plant behavior were made by those who challenged existing ideas and engaged with questions at the margins of conventional scientific thought. Science progresses one step at a time when it finds an insight into the natural world and investigates it rigorously. We ask only that science does not mistake rigidity for rigor and that it accommodates the full spectrum of human inquiry.

The next time you see a tree like the jamun, or oak, or beech, growing in your neighborhood, take a moment to think about its life. The tree observes the world, contends with climate change, and does all that it can to survive. It is kind to family and other well-intentioned visitors. It does what it can to protect itself from threats. It possesses the capacity to act and exercises that capacity in ways we are only now learning to perceive.

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