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The Silence of the Yams

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What a Plant Knows: A Field Guide to the Senses

By Daniel Chamovitz
Scientific American/FSG, 2012

“Think about this: plants see you,” Daniel Chamovitz writes at the beginning of his debut book on plant perception. This provocative opener seems to promise a much-needed reassessment of the popular understanding of plant life, which has remained more or less static for centuries. Meanwhile, the past few decades have seen a booming interest in revisiting the ethics of human-animal relationships, leading to the rise of animal studies programs and the mainstreaming of vegetarianism. Isn’t it time for plants to get a piece of the action?

Unfortunately, What a Plant Knows never follows through on its initial syntactic salvo. Instead, Chamovitz spends the remaining six chapters quietly backpedaling away from his opening. Thus when Chamovitz, the director of the Manna Center for Plant Biosciences at Tel Aviv University, says that plants “see,” he doesn’t quite mean it: He means that they respond to environmental stimuli that we experience visually, like light. When he says that plants see “you,” Chamovitz means they respond to a few attributes of your appearance that we consider visual, like the color of your shirt. He might just as easily have said that plants don’t see you at all: They simply register the different wavelengths of light in their immediate environments, including light reflected from your shirt as you walk by.

Such corrections and qualifications may seem a mere matter of semantics, but they make a dramatic difference to the importance and impact of What a Plant Knows. They also reveal a great deal about our current culture. Chamovitz’s book appears at a crucial time in the evolution of Western understandings of nonhuman life—we are in the midst of a shift in both popular and scholarly attitudes towards other living beings that a number of theorists have dubbed “the nonhuman turn.” As organic and sustainable food options grow fashionable among the smart set, that other smart set, academia, has taken such interests a step further. Political theorists like Bruno Latour and Jane Bennett now argue for a reassessment of ancient divisions between nature and society, or even between subjects and objects. Academics with a more scientific bent in fields like economics and psychology have also begun to question the supposed transcendence of the human mind, investigating experiments that suggest that human decision-making may be much more predictable than we would like to admit—and that much of it may spring from the physical details of our environments rather than from our moods, philosophies, or personalities.

What we are seeing is a widespread reassessment of the commonplace wisdom about the mind of man and the mindlessness of everything else. All the packaging of What a Plant Knows aligns it with this cultural moment. From the promise of its title and opening sentence to the individual chapter headings (“What a Plant Sees, What a Plant Smells, What a Plant Feels,” and so on), it plays on the frisson of anthropomorphic phrasings to suggest that the humble and oft-overlooked members of the plant kingdom may harbor minds of their own.

Each chapter begins with the suggestion of such a possibility, only to lead the reader through a series of experiments that quietly undermine it. The book ends with the comfortable return of an enlightened enthusiasm for the plant world, one that admits that plants may not see or smell in the senses we mean, but that nevertheless suggests that they still respond to more elements of their surroundings than we ever thought they did.

The most dramatic example of this general pattern occurs in the book’s fourth chapter, “What A Plant Hears,” which begins with an evocation of the noisiness of the natural world:

Forests reverberate with sounds. Birds sing, frogs croak, crickets chirp, leaves rustle in the wind. This never-ending orchestra includes sounds that signal danger, sounds related to mating rituals, sounds that threaten, sounds that appease. A squirrel jumps on a tree at the crunch of a breaking branch; a bird answers the call of another. Animals constantly move in response to sound . . . But even as the forest chatters and crackles, plants remain ever stoic, unresponsive to the din around them. Are plants deaf to the clamor of a forest? Or are we just blind to their response?

Like so many of Chamovitz’s intriguing questions, this one has a disappointing answer: not even the corn has ears. Humans are not blind to plant perception; instead, plants are deaf to the world around them.

Coming to that conclusion, however, is an absorbing process in itself, one that involves a detour through the annals of pseudoscience. We meet the enterprising mid-century housewife Dorothy Retallack, who fought off empty nest syndrome by enrolling in coursework at a local college. There, her introduction to biology class required her to design an original experiment. Retallack, “a unique mixture of a social conservative who believed that loud rock music correlated with antisocial behavior . . . and a New Age religious spiritualist,” decided to subject plants to classical music and rock music to observe the effects of such sounds on growth. Despite the many flaws in her experimental methods and the inability of scientists to replicate her findings, Retallack’s conclusions—that rock music damages plants, while classical music causes them to flourish—have found widespread acceptance in the alternative healing community.

Chamovitz narrates such strange byways of scientific progress with clarity and interest. Still, the body of the book often feels like one long bait-and-switch, with initial excitement at the idea of plant cognition gradually bartered away for a safer, more scientific notion of plants as unconscious responders to their environments. Nor is this process really hidden—even as he uses the language of cognition, Chamovitz takes great pains to explain to his audience that his book is not The Secret Life of Plants, the 1973 New Age bestseller that argued for plant sentience. The very fact that he must make such a distinction demonstrates the internal inconsistencies that plague What a Plant Knows: the contradiction between the formal and linguistic suggestions of sentience used to entice the reader on the one hand, and the book’s argument that such sentience does not really exist on the other.

Chamovitz tries to own and disown such anthropomorphism at the same time, concluding in his epilogue that the language of his chapters remains useful. Such language enables us “to anthropomorphize plant behavior for the sake of literary clarity,” though it’s necessary to remember “that all such descriptions must be tempered by the idea of a brainless plant.” It’s hard to imagine how using misleading anthropomorphic language constitutes “literary clarity.” Chamovitz’s stance on plants is transparent by the end of the book, but equally transparent is his reason for describing plant activity in human terms: his language has everything to do with marketing, and nothing to do with his actual position on plant life.

Despite the misleading language draped over the science of What a Plant Knows, Chamovitz demonstrates a masterful ability to explain one fascinating experiment after another. He describes a great number of them in this brief book, and each turns out to be equally crucial to the determination of exactly how plants react to their environments. The reader learns that the snapping shut of a Venus flytrap, for example, entails an electrical process surprisingly similar to the firings of human neurons. (The reader also learns along the way that the flytrap got its association with Venus from the adolescent imaginations of eighteenth century scientists, who saw a resemblance between the plant’s two delicate, pink, hair-tipped lobes and the lovelier parts of the goddess of love.)

The flytrap’s action requires what Chamovitz describes as rudimentary memory: the plant must have two of its hair-like sensors triggered within the space of twenty seconds to snap shut, so it technically “remembers” any single stimulation for twenty seconds and “waits” to see if another hair will be triggered to “decide” whether it should snap shut or not. While a twenty second memory may seem unimpressive, Chamovitz describes another sequence of experiments demonstrating that seedlings can remember an early trauma for weeks, and that such memory is used by damaged plants to determine the direction of future growth.

To learn more, scientists have been subjecting young seedlings to forms of torture and amputation worthy of a horror film, pricking and cutting away at their growth buds and early leaves (called cotyledons) to see how the plants respond. Experimenters have discovered that, with the top growth bud “decapitated,” plants will focus their growth away from any damage their leaves experience. The French botanist Michel Thellier found that even slight damage to a cotyledon will cause a plant to focus its growth away from its damaged side: “Thellier didn’t have to mangle the cotyledon to get the response; he would just prick the leaf four times with a needle at the same time as the decapitation, and this minor wound was enough to get asymmetrical growth of the lateral buds.” Even more surprising, plants appear to be able to remember and differentiate the kinds of harm done to their leaves:

Thellier stabbed one of the cotyledons as he had before, but then he removed both cotyledons several minutes later. He found that the plant retained the memory of the stabbing: once the central bud was removed, the lateral bud opposite the original wounded cotyledon grew more than the one on the side of the [other] wounded cotyledon.

In other words, plants grow away from trauma, and can remember that trauma—here, the stabbing of a leaf on one side—even if leaves on all sides of the plant are amputated only minutes later. How they can store such memories when the damaged parts have been removed remains an open question.

These experiments in plant memory stick with you because Chamovitz excels at outlining the design of scientific experiments. With considerable skill, he manages to highlight the deductive thinking behind each new experiment to nail down the meaning of previously intriguing but inconclusive observations. But these examples also remain memorable as markers of what What a Plant Knows does not know. Chamovitz is a scientist, and despite his generous attention to nonhuman life, he’s eager to avoid appearing non-scientific. (At one point he relates the cautionary tale of an organization of scientists who described themselves as specialists in “plant neurobiology,” until they had to concede to concerns from the scientific community and renamed themselves the Society of Plant Signaling and Behavior.) While his wary professionalism leaves him unwilling to credit plants with consciousness, emotional states, or the capacity for suffering, he does posit that plants are “aware.” “Plants are acutely aware of the world around them,” he concludes. “They are aware of their visual environment; they differentiate between red, blue, far-red, and UV lights and respond accordingly.”

Chamovitz’s terminology here is perhaps more controversial than he realizes. To note that a plant registers and responds to something does not seem adequate to justify the use of a term like “awareness,” which implies a capacity for attention that his book would suggest plants do not possess. In Chamovitz’s terms, your building’s thermostat is aware: it manages to respond to the temperature of its environment, changing itself and its relation to the world in the process. Ironically, by avoiding questions of philosophy, Chamovitz has allowed his scientific rationalism to lead him straight into a kind of modern-day animism. It’s not just thermostats that suddenly become “aware” by Chamovitz’s terms—it’s everything from computers to the human immune system.

These issues are the most fascinating aspect of What a Plant Knows, and there’s not nearly enough discussion of them. What a plant knows depends entirely on how you define knowing. The same goes for seeing, hearing, feeling, and the rest of the book’s chosen terms. Chamovitz takes too little care of this, and relies rather heavily on that college-student staple, Merriam-Webster, for definitions. His project could have benefited from increased awareness of philosophy and cognitive science—or at least of the Oxford English Dictionary.

Even when Chamovitz approaches issues that lie outside the bounds of scientific enquiry, his determination to avoid speculation ultimately hinders him. Describing the chemical signals emitted by plants subjected to physical trauma, for example, he notes that neighboring plants pick up on such signals and change their own physical and chemical composition accordingly. He admits that this observation could be interpreted a number of different, and philosophically troubling, ways:

While the phenomenon of plants being influenced by their neighbors through airborne chemical signals is now an accepted scientific paradigm, the question remains: Are plants truly communicating with each other . . . or are the healthy ones just eavesdropping on a soliloquy by the infested plants, which do not intend to be heard? When a plant releases a smell in the air, is it a form of talking, or is it, so to say, just passing gas? While the idea of a plant calling out for help and warning its neighbors has allegorical and anthropomorphic beauty, does it really reflect the original intent of the signal?

Chamovitz dutifully details the experiments that demonstrate that these interplant communications are probably not intentional in the strict sense: while plants are responding to each other, the chemicals emitted by damaged leaves seem primarily devoted to ensuring the damaged plant’s individual survival, as the chemicals are the sole means it has to warn its own leaves of oncoming danger.

What Chamovitz doesn’t tell us is that such a process is analogous to phenomena we observe across kingdoms and species. Plants, when attacked, emit a chemical that appears to communicate their distress between their own leaves, but also to creatures around them, provoking an appropriate response. Fish do the same when grasped by a predator, releasing what is called an alarm odor, a kind of chemical scream that—evolutionary biologists have speculated—actually draws more predators, increasing the possibility that the prey may be released in the subsequent squabble. Rabbits scream, too, as anyone who has ever lived in the country and heard that haunting, baby-like shriek can attest. Scientists speculate that fish and rabbits, whether they scream chemically or sonically, do so for the same reasons: to attract interest and competition, thereby increasing their chances of survival and reproduction.

Humans scream as well, of course—in moments of terror, and even in moments of passion. We scream in terror to attract attention, just as animals do, although that attention is (ideally) more altruistic. Perhaps our howls of passion are more interesting. Primatologists have suggested that female apes cry out during sexual encounters to draw more males to the scene of the action, thereby increasing their chances of producing children with the best partner around. It is not a far cry, literally, from the human world. Apes respond to the sound of orgasm just like the astonished diner in When Harry Met Sally: “I’ll have what she’s having.”

What such interspecies analogies should show is how little we know about the “intent,” causes, and mindsets that drive behavior, including our own. In order to know what a plant knows, such questions—of what humans know, and what animals know, and how we know what knowing is—must be asked. But for now many of these questions appear untestable; they may remain so forever. The empirically unproveable nature of the questions suggested by What a Plant Knows makes strict empiricists uneasy, because it opens a hole in human knowledge big enough for all the charlatans and mountebanks of pseudoscience to pass through.

But this is precisely why bright minds must turn to these issues, and make the best sense of them that they can. Philosophers are trying. Cultural critics are trying. A few scientists are trying, too. One wishes that Chamovitz, who has produced such an accessible and valuable catalogue of experiments on the subject, would get on board.

It is of course possible to draw your own conclusions on plant cognition from Chamovitz’s work. It seems, provisionally, that much of what humans accomplish with their brains is paralleled by plants at the genetic level. Whether brains or genes constitute the cleverer way to solve problems, or what the comparison between the two reveals about human presumptions to define cleverness and problem solving in general, remains open to debate. Clocking in at a measly 140 pages of large print, What a Plant Knows certainly has room for such debate—what it lacks is the confidence to step outside the strictures of the laboratory and engage in it.

The impossibility of writing such a book without addressing these questions, combined with Chamovitz’s hesitation about discussing anything that isn’t strictly scientific, leads him to treat these issues only hastily. Thus at the end of the work, after settling on the questionable terminology of awareness, he lyrically lingers over a strand of ivy: “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.” The idea is profound; the ideology is not. When Chamovitz considers plants, he is all the more thankful for the unique nature of man: There but for the grace of God go I, he thinks. “While plants and humans maintain parallel abilities to sense and be aware of the physical world,” he triumphantly concludes, “the independent paths of evolution have led to a uniquely human capacity, beyond intelligence, that plants don’t have: the ability to care.”

When it comes to such sweeping statements of human uniqueness, Chamovitz’s normally reliable endnotes are, tellingly, nowhere to be found. Here is one moment where our exceedingly cautious author may be right—it’s possible that humans, or at least the so-called higher animals, are unique in their capacity to care—but he certainly isn’t scientific. If these final lines have no scientific source, what are they doing in such a strictly scientific book? They imply a strange, superfluous desire to restore man to his proper place at the top of the chain of being, despite the lack of any solid evidence for the propriety of such a place. They suggest, in other words, that even strict empiricists shelter some beliefs they would prefer to hide from the light of serious scientific scrutiny.

Iain Mulder lives in New York.