Epilogue: The Aware Plant

“Intelligence” is a loaded term. Everyone from Alfred Binet, the inventor of the much-debated IQ test, to the renowned psychologist Howard Gardner has had a different understanding of just what it means to classify someone as “intelligent.” While some researchers consider intelligence a unique propensity of human beings, we have seen reports that animals—from orangutans to octopuses—possess qualities that fall under some definitions of “intelligence.” Applying definitions of intelligence to plants, however, is much more contentious, though the question of intelligent plants is hardly a new one. Dr. William Lauder Lindsay, who doubled as a physician and a botanist, wrote in 1876: “It appears to me that certain attributes of mind, as it occurs in Man, are common to Plants.”

Anthony Trewavas, an esteemed plant physiologist based at the University of Edinburgh in Scotland and one of the early modern purveyors of plant intelligence, points out that while humans are clearly more intelligent than other animals, it is unlikely that intelligence as a biological property originated only in Homo sapiens. In this vein he sees intelligence as a biological characteristic no different from, say, body shape and respiration—all of which evolved through the natural selection of characteristics present in earlier organisms. We saw this quite clearly in Chapter Four in the “deaf” genes shared by plants and humans. These genes were present in a common ancient ancestor of plants and animals, and Trewavas has proposed that rudimentary intelligence was present there as well.

Controversy arose among plant biologists when a group of scientists who study various aspects of plant function defined a new field in 2005 that they called “plant neurobiology,” which aims to study the information networks present in plants. These scientists saw many similarities between plant anatomy and physiology and the neural networks in animals. Some of these similarities are obvious, such as the electrical signaling we encountered in the Venus flytrap and Mimosa plants, and some more divisive, such as the architecture of plant roots being similar to the architecture of neural networks found in various animals.

This latter hypothesis was originally put forward in the nineteenth century by Charles Darwin and has been picked up again over the past few years, particularly by Stefano Mancuso from the University of Florence and Frantiöek Baluöka from the University of Bonn, two of the pioneers in the field of plant neurobiology. Many other biologists who study plants, including a number of very prominent scientists, criticized the ideas behind plant neurobiology, claiming that its theoretical basis is flawed and that it has not added to our understanding of plant physiology or plant cell biology. They strongly felt that the plant neurobiologists had gone too far in drawing parallels between plant and animal biology.

Many proponents of plant neurobiology would be the first to explain that the term itself is provocative and therefore useful for encouraging more debate and discussion about the parallels between the ways plants and animals process information. Metaphors, as pointed out by Trewavas and others, help us make connections that we might not normally make. If by using the term “plant neurobiology” we challenge people to reevaluate their understanding of biology in general, and plant biology specifically, then the term is valid. But we must be clear: no matter what similarities we may find at the genetic level between plants and animals (and, as we have seen, they are significant), they are two very unique evolutionary adaptations for multicellular life, each of which depends on unique kingdom-specific sets of cells, tissues, and organs. For example, vertebrate animals developed a bony skeleton to support weight, while plants developed a woody trunk. Both fill similar functions, yet each is biologically unique.*

(* Following the initial controversy, and after much discussion, the plant neurobiologists changed the name of their professional organization in 2009 from the Society for Plant Neurobiology to the more accepted Society of Plant Signaling and Behavior (though “behavior” is also an interesting term not intuitively connected to plants).)

While we could subjectively define “vegetal intelligence” as another facet of multiple intelligences, such a definition does not further our understanding of either intelligence or plant biology. The question, I posit, should not be whether or not plants are intelligent—it will be ages before we all agree on what that term means; the question should be, “Are plants aware?” and, in fact, they are. Plants are acutely aware of the world around them. They are aware of their visual environment; they differentiate between red, blue, far-red, and UV lights and respond accordingly. They are aware of aromas surrounding them and respond to minute quantities of volatile compounds wafting in the air. Plants know when they are being touched and can distinguish different touches. They are aware of gravity: they can change their shapes to ensure that shoots grow up and roots grow down. And plants are aware of their past: they remember past infections and the conditions they’ve weathered and then modify their current physiology based on these memories.

If a plant is aware, what does this mean for us regarding our own interactions with the green world? On one hand, an “aware plant” is not aware of us as individuals. We are simply one of many external pressures that increase or decrease a plant’s chances for survival and reproductive success. To borrow terms from Freudian psychology: the plant psyche is devoid of an ego and a superego, though it may contain an id, the unconscious part of the psyche that gets sensory input and works according to instinct. A plant is aware of its environment, and people are part of this environment. But it’s not aware of the myriad gardeners and plant biologists who develop what they consider to be personal relationships with their plants. While these relationships may be meaningful to the caretaker, they are not dissimilar to the relationship between a child and her imaginary friend; the flow of meaning is unidirectional. I’ve heard world-famous scientists and undergraduate research students alike use anthropomorphic language with abandon as they describe their plants as “not looking too happy” when mildew has taken over their leaves or as “satisfied” after they’ve been watered.

These terms represent our own subjective assessment of a plant’s decidedly unemotional physiological status. For all the rich sensory input that plants and people perceive, only humans render this input as an emotional landscape. We project on plants our emotional load and assume that a flower in full bloom is happier than a wilting one. If “happy” can be defined as an “optimal physiological state,” then perhaps the term fits. But I think that for all of us, “happy” depends on much more than being in perfect physical health. In fact, we’ve all known people afflicted with various ailments who considered themselves happy, and otherwise healthy individuals who are generally miserable in mood. Happiness, we can agree, is a state of mind.

A plant’s awareness also does not imply that a plant can suffer. A seeing, smelling, feeling plant can no more suffer pain than can a computer with a faulty hard drive. Indeed, “pain” and “suffering,” like “happy,” are very subjective terms and are out of place when describing plants. The International Association for the Study of Pain defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” Perhaps “pain” for a plant could be defined in terms of “actual or potential tissue damage,” as when a plant senses physical distress that can lead to cell damage or death. A plant senses when a leaf has been punctured by an insect’s jaws; a plant knows when it’s been burned in a forest fire. Plants know when they lack water during a drought. But plants do not suffer. They don’t have, to our current knowledge, the capacity for an “unpleasant . . . emotional experience.” Indeed, even in humans, pain and suffering are considered separate phenomena, interpreted by different parts of the brain. Brain-imaging studies have identified pain centers deep within the human brain that radiate out from the brain stem, while the capacity for suffering, scientists believe, is located in the prefrontal cortex. So if suffering from pain necessitates highly complex neural structures and connections of the frontal cortex, which are present only in higher vertebrates, then plants obviously don’t suffer: they have no brain.

The construct of a brainless plant is important for me to accentuate. If we keep in mind that a plant doesn’t have a brain, it follows then that any anthropomorphic description is at its base severely limited. It allows us to continue to anthropomorphize plant behavior for the sake of literary clarity while remembering that all such descriptions must be tempered by the idea of a brainless plant. While we use the same terms—“see,” “smell,” “feel”—we also know that the overall sensual experience is qualitatively different for plants and people.

Without this caveat, anthropomorphism of plant behavior left unchecked can lead to unfortunate, if not humorous, consequences. For example, in 2008 the Swiss government established an ethics committee to protect the “dignity” of plants.* A brainless plant likely does not worry about its dignity. And yet if a plant is aware, it means everything for us regarding our own interactions with the vegetal world. Maybe the Swiss attempt at bestowing dignity on plants mirrors our own attempt at defining our relationship with the plant world. As individuals, we often seek our place in society by comparing ourselves with other people. As a species, we seek our place in nature by comparing ourselves with other animals. It’s easy for us to see ourselves in the eyes of a chimpanzee; we can identify with a baby gorilla clinging to her mother. John Grogan’s dog, Marley, like Lassie and Rin Tin Tin before him, evokes very deep feelings of empathy, and even people who are not necessarily dog lovers can see human characteristics in our canine friends. I’ve known bird keepers who claim their parrots understand them, and fish lovers who see human behavior in their marine life. These examples clearly show that “human” may be only a flavor, albeit an interesting one, of intelligence.

* This committee was formed to further define dignity in terms of plants, as the Swiss Federal Constitution requires “account to be taken of the dignity of living beings when handling animals, plants and other organisms.” See www.ekah.admin.ch/en/topics/dignity-of-living-beings/index.html.)

So if humans and plants are similar in that both are aware of complex light environments, intricate aromas, different physical stimulations, if humans and plants both have preferences, and if both remember, then do we see ourselves when looking at the plant?

What we must see is that on a broad level we share biology not only with chimps and dogs but also with begonias and sequoias. We should see a very long-lost cousin when we gaze at our rosebush in full bloom, knowing that we can discern complex environments just as it can, knowing that we share common genes. When we look at ivy clinging to a wall, we are looking at what, save for some ancient stochastic event, could have been our fate. We are seeing another possible outcome of our own evolution, one that branched off some two billion years ago.

A shared genetic past does not negate eons of separate evolution. While plants and humans maintain parallel abilities to sense and be aware of the physical world, the independent paths of evolution have led to a uniquely human capacity, beyond intelligence, that plants don’t have: the ability to care.

So the next time you find yourself on a stroll through a park, take a second to ask yourself: What does the dandelion in the lawn see? What does the grass smell? Touch the leaves of an oak, knowing that the tree will remember it was touched. But it won’t remember you. You, on the other hand, can remember this particular tree and carry the memory of it with you forever.