Calculating jewelweeds? Conniving tomatoes? Sentimental willow trees? The concepts seem strange. But before we go further, let’s remember John Milton’s declaration: “Truth never comes into the world but like a bastard, to the ignominy of him that brought her birth.” So let us return to the source of these odd ideas.
Charles and Emma Darwin gave birth to 10 children. Charles Darwin thought his firstborn would become “the future Lord Chancellor of all England,” but William settled for the life of a banker in Southampton. George studied the evolution and origins of the solar system, becoming the Plumian Professor of Astronomy and Experimental Philosophy at Cambridge University. Leonard was a major in the Royal Engineers, a Liberal Unionist MP, president of the Royal Geological Society, and a scientific patron. Horace became mayor of Cambridge and a Fellow of the Royal Society, like his brother George. Henrietta lived to be 86 and edited her mother’s papers; like her sister Elizabeth, she had no children, and neither did William or Leonard.
There were also tragedies in the Darwin family: Mary died a few weeks after her birth in the fall of 1842, and Charles Waring, the tenth and last child, before the age of two. It was little Annie, however, who really broke her father’s heart, withering at the tender age of 10, in 1851, probably from tuberculosis. After that Darwin never again set foot in a church.
But of all the children, it was the seventh-born, Francis, who was his father’s spitting image: a born naturalist. From his boyhood, Franky, as he was called, could be counted on to skin animals and boil pigeons, mount worms on microscope slides, catch butterflies and plant seeds. At the family home, Down House, he was Charles’ little assistant, the mainstay, always tinkering in the greenhouse or the makeshift lab, never far from his father’s side. Together, father and son sparked each other’s passions. And their biggest passion was plants.
The Bible may have shortchanged vegetation with God's pact with Noah that included “the birds, the livestock, and all the wild animals, all those that came out of the ark with you” (Genesis 9:10). But the great Aristotle was no help either. For the Greek philosopher, plants were just a cut above stones living, just barely, but not sentient. This tradition survived through the Middle Ages, when plants were placed quite low on the Great Chain of Being. When the mechanists of the 17th and 18th centuries put their minds to explaining how plants sucked up water or opened their leaves, they saw them as little more than green machines. Animals may not have a soul, Descrates and his followers thought, but they move and have intentions. In the great moral economy of nature, plants existed way below them.
Darwin and Franky took personal offense at such callous anti-verdant verbiage. Staying up nights together to follow plants’ sleeping movements, planting seeds lovingly in biscuit tins on the chimneypiece, “cabbages and runner beans in floor pots, and nasturtiums, cyclamens, cacti, and telegraph plants” about the house, both felt a special affinity for their green companions, sensing, as Darwin wrote in his journal, their “aliveness.” They spoke and whispered to them unselfconsciously, often petting them and praising their cleverness. Franky would even serenade them with his bassoon, and father and son would get down on their knees, straining to detect a reaction.
So when Franky grew up, becoming a Fellow of the Royal Society (and later Sir Francis), he and his father decided to write a book, summarizing what they had learned together over the years. It was called “The Power of Movement in Plants,” and was published in November 1880, a year and a half before Darwin would be buried in Westminster Abbey. Here their experimental ingenuity was put to the test, demonstrating just how active plant life really was: leaves, roots, tendrils, flowers all were in constant, albeit slow, motion, sensing their environment and rising to its challenges. Far from inert or lacking in sentience, plants were living, breathing, operating creatures, painstakingly attuned to the world. It seemed almost could it be? as though they were thinking. In the very last sentence of the book, in fact, the Darwins dropped a bomb: “It is hardly an exaggeration to say,” they wrote, against all convention, “that the tip of the radicle thus endowed [with sensitivity] and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements.”
“The Power of Movement in Plants” revolutionized botany. Far from being stationary vegetables, just a rung above rocks, plants proved positively kinetic: They move away from and toward light (phototropism), oxygen (oxytropism), water (hydrotropism), current (electrotropism), chemicals (chemotropism), forces (magnetropism), and injury (traumatropism), to name a few. They were known to perform amazing acts, such as the Javan cucumber’s ability to create wings on its seeds so they could fly to greener pastures. (The Austrian flight pioneer Igo Etrich based his early glider designs on the shape of this family of seeds.) The Cuban vine Marcgriva Evenia has a leafy satellite dish-like attachment above its fruit clusters, which tricks the sonar systems of bats, its chief pollinators, into believing it’s the sexiest food in town. The echo from the leaves sounds constant from every angle, rendering the Marcgriva the “brightest” flower to the bats, who “see” with their ears. Then there’s the Venus Flytrap, with its carnivorous leaves to capture unsuspecting prey. Or consider the speedy pollen launchers of the bunchberry dogwood tree, which accelerate in less than a millisecond at 2,400 times the force of gravity. That’s 800 times what an astronaut experiences at liftoff.
But a plant brain? That was a different story. The very notion was ridiculed by the leading physiologists of the day, who accused Darwin and his son of being amateur romantics. Dismissively and rather rudely, the ‘root-brain’ hypothesis was cast aside.
Cries of distress
Fast forward to 2004. In that year, near Florence, the first laboratory in plant neurobiology was born. Sure, its founder Stefano Mancuso admitted, plants don’t have neurons like animals do. But they have signaling and sensing mechanisms that might as well be thought of as analogous to an animal brain, since in effect they carry out the same functions. Finally, 125 years after it was hatched, castigated, buried, and forgotten, Charles and Franky’s hypothesis was being revived. And there were good reasons to be excited.
Take, for example, the common tomato. When a caterpillar is chewing on it, experiments show, it can release a chemical signal that attracts parasitic wasps to come fight the caterpillar off. Tobacco does the same thing. Amazingly, the signals are not general distress calls, but are specific to certain predators, depending on the pest the plant is dealing with. A tomato that smart? It looks like plants can sense the digestive substances secreted by invading insects when they bite into them: saliva from different bugs sets off different chemical alarms, which invite specific species of wasps, nematodes or mites whatever the job requires.
Or consider the yellow jewelweed, which, under conditions of deprivation of light, and when surrounded by members of its own species, does not invest in leaf creation as you might expect, to soak up as much light for photosynthesis as possible, but rather pulls its punches, investing in stems instead as if showing consideration. And yet when a jewelweed is placed in a pot with an unrelated species, it is a fierce, all-out competitor. Clearly, it is able to sense who is kin and who is not, and direct its behavior accordingly. Plants apparently even share a camaraderie outside the family: When sagebush plants are put under distress by scientists, neighboring tobacco begins to produce its own defenses.
How are the plants communicating? Apparently, in different ways. Recently, the Israeli researchers Ariel Novoplansky and Omer Falik and their team from the Mitrani Department of Desert Ecology at Ben-Gurion University, showed in a series of elegant experiments how stressed pea and other plants share information through their roots, allowing neighbors to prepare themselves for an upcoming drought by closing their pores to retain water. The weeping willow, when under attack, produces chemicals that invading bugs have trouble digesting, but which are picked up through the air by neighboring willows, which then prepare for the worst before they’ve been attacked themselves. A recent study out of Bristol University using loudspeakers emitting humanly inaudible clicks, published in the journal Trends in Plant Science, showed that plants seem to communicate with each other via vibrations, and may in fact have their own language. How precisely all this happens we still do not know.
Plants have memories, too, believe it or not. When a Polish team led by Stanislaw Karpinski shone light on the leaves of a member of the cabbage family, chemical reactions ensued in leaves that were not exposed to the light. In fact, when the light was turned off, the leaves of the entire plant retained a specific memory of what they had been exposed to for several days. Red, blue, and white light, as well as different time durations, produced distinct memories. What are these memories good for? Karpinski showed that when light was focused on the plant for an hour before he infected it with a virus, the plant resisted infection, whereas if the virus was introduced before the light, the plant succumbed to the disease. Since light is different in different seasons and hours of the day, plants may be using this information to immunize themselves against characteristic pathogens. When Karpinski discovered that light information is relayed throughout the plant via a type of cell called a “bundle sheath” cell, which emits electrical signals, he announced dramatically that he had found in plants the equivalent of the nervous system in animals.
Unity of life
Which brings us back to Darwin and his follower, Mancuso, near Florence. If plants can see, hear, smell, taste and remember, might they have a “brain”? Darwin and Francis thought they could and did at the tip of the root, the meristem. It was there, the Darwins believed, where sensory inputs were integrated and from which movement of the entire plant was directed. It wasn’t all that surprising to them: After all, the anterior pole of all non-plant multicellular organisms specializes in taking in food, sensing, and information processing, whereas the other end, the posterior pole, houses the sexual organs, the excretory apparatus, and motility. Plants are simply anchored in the soil by their “heads,” so to speak, exposing their genitals to their pollinators. Having shown experimentally that this 1-1.5 mm “transition zone” in the meristem is in fact a kind of “command center,” directing cell division, growth, and movement in response to the environment via secretion of particular molecules transported by particular proteins. Mancuso concurred: “Plant neurobiology is a necessary proposition in the argument for the unity of life,” he writes. The differences between plants, animals, and humans are real, but we are all more similar than we have dared to imagine.
What does this mean for our moral world? The philosopher Michael Marder, in an op-ed in The New York Times this past April titled “If Peas Can Talk, Should We Eat Them?”, argues that since they are sentient, the same kinds of considerations demanded by animal rights advocates might equally apply to plants. Green creatures may not experience pain, nor “know” that they are acting for their own good, but that doesn’t mean we shouldn’t take the “vegetal good” into account. Short of calling for an immediate ban on eating greens, he suggests in his upcoming book, “Plant-Thinking: A Philosophy of Vegetal Life,” that “ethical eating demands that we respect plant communities, paying attention to both the methods of their cultivation and their reproductive possibilities.”
The critics scoff, if they do not laugh outright. Marder, they say, is confusing reaction with response, physics with communication, mechanics with sentience. When an electrical current through a wire attached to a bell rings it, do we consider that the bell is trying to tell us something? Do we say thermostats are intelligent? Or that overheated rocks that explode are responding to information “it’s getting bloody hot!” from their environment? Evolution may have helped plants develop sophisticated sensing devices, but just like ringing bells, thermostats and rocks, they have no intentions.
“The term plant neurobiology is as ridiculous as, say, human floral biology,” Daniel Chamovitz, director of the Manna Center for Plant Biosciences at Tel Aviv University, recently told Scientific American. “Plants do not have neurons, just as humans do not have flowers!”
But Chamovitz, who in the 1990s began to discover that the same genes that regulate light responses in plants figure in circadian responses in animals, appreciates that thinking and information processing are not the same thing. Clearly, you don’t always need a brain to get things done. In fact, while plants lack neurons, they do have receptors for neuroactive chemicals, like glutamate, which plays a role in human memory and learning. The very same drugs that inhibit glutamate receptors in humans affect plants, and studying them in that context has helped scientists understand cell-cell communication more clearly. In a beautiful new book, “What a Plant Knows,” Chamovitz discusses this and other facets of “plant consciousness” evenhandedly. Interestingly, the greatest opponents of a “vegetal philosophy” turn out to be vegans, who worry that when all the rest of us get wind of the talk of plant ethics, we’ll knee-jerkingly crawl back to an exclusively human-centric worldview, throwing the baby out with the bathwater. (In reply to the argument that “nonconscious intentionality” basically applies to all creatures capable of some degree of stimulus, Marder has replied: “We should certainly not reject the possibility of respecting communities of bacteria without considering the issue seriously.”) As one critic has argued: “In a world in which we kill 56 billion sentient beings a year for food (not counting fish), the idea that we need to think about plants or risk being accused of ‘self-righteous moralizing,’ is, on many levels, disturbing.”
In the end, there are those who believe that this is a debate about language. What do we really mean, after all, when we talk about “communication,” and “feelings,” “sensing” and “a brain”? With recent interest in what seem to be incredible feats of mushroom memory and learning, the language we use to describe certain kinds of behavior is increasingly being challenged. This is a good thing. At the very least, even if we end up rejecting a vegetal ethics, it seems sensible to open our minds to the existence of other, nonhuman, forms of knowing the world, whatever names we end up calling them. We stand to be enriched, rather than compromised, by such an understanding.
Yes, but will this craze take hold, you ask, eyeing your salad with concern. True, poor old Darwin and Franky were ridiculed for suggesting the existence of a plant “brain” (that’s okay they won their share of fame). And true, the animal rights movement has come a long way (50 years ago, did any one seriously imagine the extension of legal rights to chickens?). Still, the jury remains out over whether Milton’s warning will stand in this case. Time will tell though I have my doubts. In the meantime, enjoy contemplating the splendid varieties of experience in this most wonderful of worlds. And don’t forget to savor your asparagus.