Understanding Plant Abilities without Animal Metaphors

"At the end of the day, whether or not plants are intelligent is a social question, not a scientific one."¹

—Zoë Schlanger

The way we represent reality—how we name objects, beings, or phenomena—is a product of our cognitive structures, knowledge paradigms, and cultural heritage. For instance, early European voyagers, faced for the first time with unfamiliar New World animals, likened them to known fauna from the Old World. Their late-fifteenth-century letters described capuchin monkeys as “cats with the face of human beings,” iguanas as “serpent[s] with thick clawed feet,” and the opossum as a Frankenstein-like creature with “its anterior half like a fox and its posterior half like an ape, with feet like those of a human being and ears like those of an owl.”² While resorting to analogies helped European explorers convey novelty in familiar terms, it also limited their ability to see the novel organisms on their own terms, implying instead that these animals were simply “downgraded” versions of their known Old World counterparts. The explorers were prisoners of their own cosmovision: They could not help seeing New World fauna in such a way. 

While these historical examples may seem dramatic, contemporary scientists are no different. They describe the unknown through recourse to the known, and the known is shaped by culture, history, and social values. So rather than mocking the gratuitous use of analogy by European naturalists, we might ask where such tendencies persist today, and how they impact our attempts to understand novel organisms or phenomena. 

Here, I explore this question in the case of contemporary plant scientists who attempt to make sense of plants through recourse to animals. While such a conceptual move may be understood as an attempt to “upgrade” plants, granting them greater complexity or perhaps even moral consideration in our eyes, I argue that the pervasive use of animal metaphors to describe plants constrains, rather than expands, frontiers of knowledge. I contend that this move, which hinders our capacity to understand plants, is grounded in conscious or unconscious views of animal superiority. ³ To illustrate my point, I’ll explore how two remarkable capacities of plants—kin recognition in Arabidopsis thaliana⁴ and leaf mimicry in the vine Boquila trifoliolata⁵—have been interpreted as cases of “plant vision.”

Perhaps it goes without saying that plants and animals are distinct outcomes of biological evolution. Together they account for most macroscopic organisms on Earth, and their divergence occurred around 1.6 billion years ago.⁶ Although they share basic genetic and metabolic features, plants and animals differ in essential aspects, such as their sources of energy, capacities for movement, and presence of neuronal cells. These differences have shaped their modes of interaction with the environment. We must keep evolutionary history in mind when we apply categories once reserved for humans and some nonhuman animals to plants. 

Despite this evolutionary history, botanical studies have a long tradition of using animal metaphors and analogies to describe plant features,⁷ dating as far back as the 3rd century BCE naturalist Theophrastus and continuing through modern botany.⁸ The tradition has intensified in the past decade, as plant scientists increasingly apply categories once reserved for animals—such as intelligence, agency, and consciousness—to plant life, often drawing parallels between plant and animal capabilities.⁹

Animal metaphors are especially prevalent in descriptions of “advanced” plant abilities, i.e., complex features that challenge mainstream conceptions of plant behavior. See, for example, excerpts from the titles of some recent academic articles: “insight into plant vision,” “plants talk, but are they deaf?” “‘brain-like’ status of the root apex,” “plant synapses,” “plants that cry for help,” and “plants have two minds as we do.”¹⁰  In fact, metaphor lies at the heart of the field known as “plant neurobiology.”¹¹ In 2007, a group of plant scientists critiqued the nascent field, pointing out that plants lack brains, neurons, and synapses.¹² In a rebuttal, the University of Edinburgh’s Anthony Trewavas argued that “plant neurobiology is a metaphor,” and that “metaphors can have substantial value” because they stimulate scientists’ imagination and promote experimental approaches.¹³

Of course, metaphors help convey messages in a simple way, particularly to audiences outside of the scientific community. But they may also trap us in a conceptual framework that curtails explanatory possibilities. Indeed, Trewavas’s rebuttal illustrates this risk. He draws on Darwin’s comparison of the root apex to the brain of lower animals, lists four essential features of brains—control of behavior, sensory integration, memory, and decision-making—and then maps each one onto root behavior. He interprets changes in root growth in response to moisture, gravity, or light as control of behavior;¹⁴ the simultaneous influence of these factors on roots is sensory integration;¹⁵ and the roots’ responses to them¹⁶ are evidence of memory and judgment, or decision-making. Thus, for Trewavas, roots that proliferate in patches of resource-rich soil would be evidence of processes involving memory, evaluation, judgment, and decision-making. Accordingly, he concludes that Darwin’s brain metaphor is correct.

This kind of reasoning reveals the risks of letting metaphor guide interpretation. Use of metaphors can become axiomatic, creating ideal conditions for confirmation bias; in this case, starting with a conclusion (roots behave as brains), and then accommodating facts to “prove” it.¹⁷ The metaphor also oversimplifies root foraging behavior, potentially misleading readers into thinking we fully understand it, when in reality, as a paper published in Science in 2020 stated, we still “lack a comprehensive theory for root system responses to their environment.”¹⁸ More importantly, by forcing plant behavior into a framework based on animals, the fundamental question—how an organism without a brain, neurons, or nervous system can be capable of intelligent behavior—is bypassed.¹⁹ 

Critics have long warned against using such metaphors (and analogies) in plant neurobiology.²⁰ Metaphorical concepts like plant vision and plant neurons may obscure rather than clarify the unique mechanisms plants use to perceive and respond to their environments, especially when such terms import unsupported assumptions from animal neurobiology. In short, such metaphors can constrain mechanistic explanations of plant behavior. Yet despite awareness of this zoocentrism in plant neurobiology, animal-based metaphors continue to appear in the work of the field’s leading proponents.²¹

A notable case of a zoocentric framework being applied to plants is the claim that the existence of eyelike organs in plants confers them vision.²² This purported “plant vision” is said to account for two unique phenomena: kin recognition in Arabidopsis thaliana and leaf mimicry in Boquila trifoliolata.²³

In the case of Arabidopsis, research has shown that plants growing alongside genetically related (“kin”) neighbors reorient their leaves, resulting in decreased mutual shading. This response is not observed when Arabidopsis grows alongside non-kin neighbors. The study’s authors concluded that kin recognition was mediated by photoreceptors’ perception of specific light cues, particularly red:far-red ratios and blue light profiles. Their evidence included that genetic mutants deficient in these photoreceptors failed to show the same leaf reorientation, or showed a weakened response. 

Plant biologists František Baluška and Stefano Mancuso interpreted these findings as evidence of plant-specific vision. Yet the mechanism underlying shade-avoidance responses—phytochrome-mediated detection of red:far-red light ratios—has been well-established for decades.²⁴ Because plants absorb radiation in the red region and not the far-red region of the electromagnetic spectrum, the phytochrome senses the presence of neighbors because the red:far red ratio decreases, activating an internode elongation response that involves the hormone gibberellin.²⁵ Plant physiologist Carlos L. Ballaré and his colleagues demonstrated that when the red:far-red signal was manipulated using transparent, far-red–absorbing filters, plants no longer responded.²⁶ In other words, the plants were “fooled”; they did not show elongated internodes despite being surrounded by neighbors. If we characterize light-sensing as “vision,” then by analogy, we might say these plants were “blindfolded.” But as I argue below, interpreting such photoreceptive responses as vision risks conflating well-characterized physiological processes with more speculative claims.

The case of Boquila trifoliolata presents a different and arguably more puzzling example—one that has also been framed through the metaphor of vision. While reported cases of plant mimicry are relatively rare compared to animal mimicry, Boquila is a remarkable case. This climbing plant, endemic to the temperate rainforest of southern South America, can mimic the leaves of more than 20 model species, including recently introduced ones.²⁷ Leaf traits mimicked include shape, color, vein conspicuousness, leaf/leaflet orientation, toughness, spiny tips, and size. Intriguingly, mimicry can occur without physical contact with the model species. Furthermore, an individual plant can mimic multiple hosts as it climbs across them. 

Several hypotheses have been proposed to explain Boquila mimicry. One suggests horizontal transfer of (epi)genetic factors mediated by a vector, likely airborne microorganisms.²⁸ Preliminary evidence supports a microbial role: Endophytic bacterial communities found in Boquila leaves appear to correlate with the pattern of leaf mimicry,²⁹ and recent studies in other systems suggest this hypothesis is plausible.³⁰ Nevertheless, some scientists argue that “the only realistic explanation of Boquila trifoliolata mimicry is the existence of plant eyes (plant ocelli).”³¹ They base this speculation on cases of eye-like structures that allow cyanobacteria and dinoflagellates to sense light direction and consequently orient their movement.³² Other scientists have also considered that plants can “see” due to their perception of light orientation, intensity, and/or quality mediated by photoreceptors.³³

Light is the most essential resource for plants. They have evolved numerous adaptations to quantitative and qualitative light variation; several photoreceptors allow them to perceive and respond to different light wavelengths, regulating development, growth, and responses to the environment.³⁴ However, if we interpret any plant response to any light feature as “plant vision,” then phototropism,³⁵ photosynthesis, chlorophyll fluorescence,³⁶ and many other light-related processes could be considered plant vision too. Following this rationale, the production of the pigment melanin by melanocytes in human skin in response to UV radiation—a.k.a. tanning—would constitute a sort of “skin vision.”

Researchers investigated the hypothesis of “plant vision” by testing whether Boquila could mimic plastic plants.³⁷ They report to “demonstrate that plant vision possibly via plant-specific ocelli is a plausible hypothesis.”³⁸ However, their experimental design had several shortcomings that undermine this conclusion. Most notably, the study conflated three variables. The Boquila plants in the two experimental groups differed not only in their distance to the plastic plants, but also in their ontogenetic stage and light microenvironment. The latter two factors are well known to affect leaf shape,³⁹ meaning any observed differences in leaf traits cannot be attributed solely to proximity to the model, plastic plants.⁴⁰

This example underscores my point: When the metaphor that plants have eyes—as animals do–is taken too literally, it can solidify into dogma, creating conditions for confirmation bias or misinterpretation of evidence. Even if we momentarily accept the idea that Boquila had “eyes,” that alone would not account for its capacity for leaf mimicry. After all, vision is not equivalent to mimicry. All mollusks can see, but only a few, like certain octopus species, are able to mimic their environment and other animals. Without a brain or nervous system, Boquila would still require a mechanism that links vision to leaf phenotype. Abandoning the vision metaphor allows us to explore alternative hypotheses and explanations for this unique behavior, such as horizontal gene transfer and volatile-mediated plant-to-plant signaling.⁴¹

"Truth is a mobile army of metaphors, metonyms, anthropomorphisms, in short, a sum of human relations which were poetically and rhetorically heightened, transferred, and adorned, and after long use seem solid, canonical, and binding to a nation."⁴²

—Friedrich Nietzsche

The concept of a scala naturae, a linear hierarchy in which animals are superior to plants and fungi, has long shaped Western thought.⁴³ While often attributed to Aristotle, the term and its rigid formulation were actually developed later, during the Scholastic period, when medieval thinkers synthesized Aristotelian philosophy with Christian theology to construct an ascending typology of beings.⁴⁴

Today, the idea that animals are superior to plants is scientifically untenable. While plants and animals share several features—as do any pair of taxa with a common ancestor—they also differ in fundamental ways. The claim that neither is superior might sound like a truism, but given the recalcitrant use of animal metaphors to describe advanced plant features, it bears repeating. Paradoxically, those plant neurobiology scholars who attempt to “upgrade” the status of plants through animal metaphors do so by implicitly reinforcing a hierarchy in which animals remain the gold standard.

More importantly, when those analogies reify into a framework—and even a dogma—they can constrain our quest to understand plants. Important plant features may go unrecognized simply because they are not compatible with the metaphor or analogy.⁴⁵ (This holds even in cases where superficial structural similarities arise from convergent evolution, such as between plant stomata and insect spiracles; such analogies may be biologically informative but can mislead if overextended into nomenclature.)

That said, metaphors are not inherently problematic. Some argue that they have a heuristic value,⁴⁶ even going so far to say that anthropomorphism itself can be a useful tool for understanding plants or animals.⁴⁷ The issue arises when metaphors are mistaken for mechanisms—as evidenced by the advocates of “plant vision” who have gone so far as to “identify” cornea-like, lens-like, and retina-like structures in plants.⁴⁸ The slippage between metaphor and mechanism has long plagued scientific interpretation. Writing in the mid-20th century, botanist Agnes Arber reflected on how the tree-of-life metaphor had distorted botanical observations, as researchers strained to fit plant morphology into a preconceived phylogenetic structure. She suggested that the metaphor of a bush, or even a sheaf, might better capture polyphyletic relationships.⁴⁹ More recently, evolutionary biologists have shown that the traditional tree-of-life metaphor can obscure the complexity of evolutionary processes.⁵⁰ Such examples highlight how dominant metaphors not only shape, but can also constrain, the imagination and inquiry of scientists.

In short, metaphors are only as constraining or productive as the ways we deploy them. I think that metaphors are best suited to describing the known—not to guiding pursuits into the unknown. When addressing plants’ unique capacities, we require creative ideas and thinking outside the box. Likening plants to animals might seem at first glance an example of such creative thinking, but it often reflects the opposite—a clinging to traditional frames. Above all, scientific inquiry must remain rigorous, avoiding both overinterpretation of evidence and retrofitting data to suit pre-existing beliefs. 

My aim here is not to ban animal metaphors from plant research, but rather to invite awareness of the risks associated with such an approach. Before invoking such metaphors, researchers might ask:

• Is it possible to describe the phenomenon without the recourse to the metaphor? 
• Is the metaphor constraining the range of questions that can be posed to further understand the phenomenon? 
• Does the metaphor capture the essential, defining aspects of the phenomenon?  

Plants are capable of astonishing feats, many of which remain poorly understood—and many more amazing capacities are yet to be discovered. Rather than fueling polarized debates over whether plants are “intelligent” or “conscious,” researchers would be better served by deciphering—conceptually or experimentally, but always with scientific rigor—the mechanisms by which plants perform intelligent acts.⁵¹ We must open our minds to find radically new phenomena in plants and name them accordingly. 

This is simpler than it may seem. For example, the newly discovered organelle that, through symbiosis, allows a marine alga to fix nitrogen has been appropriately named a nitroplast in consonance with the chloroplast, another organelle that originated through bacterial symbiosis.⁵² A zoocentric lens might have named this organelle the “small intestine of plants” based on its functional role. In the same vein, when describing plant functions and traits, we might prefer terms like “perception of light” over “vision,” “photoreceptors” over “eyes,” and “internal signaling system” over “nervous system.” 

We must focus on plants as plants, rather than filtering their behavior through zoocentric (or anthropocentric) lenses; we must be vigilant not to make the same mistake as the European explorers who described New World fauna using Old World analogies. In our quest to achieve a deeper understanding of plants and their interactions with the biotic and abiotic environment, we should strive to consider the plant’s perspective—however unattainable it may be. 

Acknowledgments: I thank Rachael Petersen and Zoë Schlanger for the invitation to present my work at the conference Thinking with Plants and Fungi (Harvard Divinity School). My attendance was funded by Tarleton State University. I am very grateful to Rachael Petersen, Christine Webb, Natalia Schwien Scott, and an anonymous reviewer for thoughtful comments and suggestions that significantly improved an earlier version of the manuscript.