Brilliant Green

The Surprising History and Science of Plant Intelligence

Stefano Mancuso,Alessandra Viola, Joan Benham

• Publisher: Island Press
• Year: 2015
• ISBN:
  9781610916042
• Topic: Scienze biologiche
• Subtopic: Scienza generale

Are plants intelligent? Can they solve problems, communicate, and navigate their surroundings? Or are they passive, incapable of independent action or social behavior? Philosophers and scientists have pondered these questions since ancient Greece, most often concluding that plants are unthinking and inert: they are too silent, too sedentary -- just too different from us. Yet discoveries over the past fifty years have challenged these ideas, shedding new light on the extraordinary capabilities and complex interior lives of plants.In Brilliant Green, Stefano Mancuso, a leading scientist and founder of the field of plant neurobiology, presents a new paradigm in our understanding of the vegetal world. Combining a historical perspective with the latest in plant science, Mancuso argues that, due to cultural prejudices and human arrogance, we continue to underestimate plants. In fact, they process information, sleep, remember, and signal to one another -- showing that, far from passive machines, plants are intelligent and aware. Through a survey of plant capabilities from sight and touch to communication, Mancuso challenges our notion of intelligence, presenting a vision of plant life that is more sophisticated than most imagine.Plants have much to teach us, from network building to innovations in robotics and man-made materials -- but only if we understand more about how they live. Part botany lesson, part manifesto, Brilliant Green is an engaging and passionate examination of the inner workings of the plant kingdom.Financial support for the translation of this book has been provided by SEPS: Segretariato Europeo Per Le Pubblicazioni Scientifiche.

CHAPTER 1

The Root of the Problem

In the beginning, there was green: a chaos of plant cells. Then God created the animals, ending with the noblest of them all: man. In the Bible, as in many other cosmogonies, man is the supreme fruit of the divine work, the chosen one. He appears near the end of Creation, when everything awaits him: ready to be subjugated and ruled by the “master of Creation.”

In the Biblical account, the divine work is completed in a time frame of seven days. Plants are created on the third day, while the most presumptuous of all living creatures comes into the world—at last—on the sixth. This sequence approximates present-day scientific findings, according to which living cells capable of performing photosynthesis first appeared on the planet more than three and a half billion years ago, while the first Homo sapiens, so-called modern man, only appeared 200,000 years ago (a few seconds ago, in the evolutionary time frame). But arriving last hasn’t kept human beings from feeling privileged, even though current knowledge on the subject of evolution has drastically reduced our role of “master of the universe,” downgrading our status to that of “newcomer”—a relative position that brings no a priori guarantee of supremacy over other species, despite what our cultural conditioning would have us believe.

The idea that plants possess a “brain” or a “soul,” and that even the simplest plant organisms can feel and react to external stresses, has been proposed over the centuries by numerous philosophers and scientists. From Democritus to Plato, from Fechner to Darwin (to cite only a few examples), some of the most brilliant minds of all time have been exponents of the intelligence of plants, some attributing to them the capacity to feel, others imagining them as humans with their heads in the ground: sensitive living beings, intelligent and endowed with all human faculties, except those precluded by their . . . odd position.

Dozens of great thinkers have theorized and documented the intelligence of plants. Yet the belief that plants are less intelligent and evolved beings than invertebrates, and that on an “evolutionary scale” (a concept without basis in fact but still fixed in our mentality) they’re barely above inanimate objects, persists in human cultures everywhere and manifests itself in our everyday behavior. No matter how many voices are raised in support of recognizing plant intelligence on the basis of experiments and scientific discoveries, infinitely more oppose this hypothesis. It’s as if by tacit agreement religions, literature, philosophy, and even modern science promulgate in Western culture the idea that plants are beings endowed with a level of life (not to speak of “intelligence,” for the moment) lower than that of other species.

Plants and the Great Monotheistic Religions

“And of every living thing of all flesh, two of every sort shalt thou bring into the ark, to keep them alive with thee; they shall be male and female. Of fowls after their kind, and of cattle after their kind, of every creeping thing of the earth after his kind; two of every sort shall come unto thee, to keep them alive.” With these words, according to the Old Testament, God told Noah what to save from the universal Flood so that life would continue on our planet. Obeying God’s instructions, before the Flood Noah loaded onto the ark birds, animals, and every creature that moved: “clean” and “unclean” creatures, in pairs, to assure the reproduction of every species.

And plants? Not a word about them. In Holy Scripture the plant world not only isn’t considered equal to the animal world, it isn’t considered at all. It is left to its fate, probably to either be destroyed by the Flood or to survive it along with other inanimate things. Plants were so unimportant that there was no reason to care about them.

And yet the contradictions this passage contains are soon evident. The first becomes obvious as the narrative continues. After the ark’s slow coming aground, when the rain has stopped for several days, Noah sends a dove to bring back news of the world. Is there dry land anywhere? Are there places above water nearby? Are they inhabitable? The dove returns with an olive branch in its beak: a sign that some lands have reemerged and that on them life is possible again. Noah therefore knows (even if he doesn’t say it) that without plants there can be no life on Earth.

The dove’s news is soon confirmed, and in a short while the ark has come to rest on Mount Ararat. The great patriarch debarks, lets the animals off, and then gives thanks to God. His duties are fulfilled. And what does Noah do next? He plants a vineyard. But where does the original vine come from, if it isn’t mentioned elsewhere in the story? Noah brought it with him before the Flood, aware of its usefulness, though not that it was a living being.

In this way, almost without the reader’s realizing it, the idea that plants are not living creatures comes through the story in Holy Scripture. In Genesis, two plants, the olive and the grape vine, are associated with the value of rebirth and of life, though the vital quality of the plant world in general goes unrecognized.

All three of the Abrahamic religions have implicitly failed to recognize that plants are living beings, in effect grouping them with inanimate objects. Islamic art, for example, respecting the prohibition against representing Allah or any other living creature, is passionately devoted to the representation of plants and flowers, so much so that the floral style is emblematic. Without stating it outright, this shows the belief that plants are not living beings—otherwise representing them would be forbidden! In the Koran, there is actually no explicit ban on representing animals; the prohibition is transmitted through the hadith, the sayings of the prophet Mohammad that form the basis for the interpretation of Islamic law, by virtue of the fact that in Islam there is no God but Allah and everything comes from him, and everything is him—which evidently doesn’t mean plants.

The relationship between humans and plants is totally ambivalent. For example, the same Judaism which is based on the Old Testament forbids the gratuitous destruction of trees and celebrates the new year of trees (Tu Bishvat). The ambivalence comes from the fact that on the one hand we humans are intimately aware that we can’t exist without plants, and on the other hand we’re unwilling to recognize the role they play on the planet.

It’s true that not all religions have the same relationship to the plant world. Native Americans and other indigenous peoples recognize its undeniable sacredness. If some religions have sacralized plants (or rather, parts of them), others have gone so far as to hate or even demonize them. For example, during the Inquisition, plants believed to be used in potions by women accused of witchcraft—garlic, parsley, and fennel—were put on trial along with the witches! Even today, plants with psychotropic effects receive special treatment: some are banned altogether (How do you ban a plant? Could you ban an animal?), others are regulated, still others are considered sacred and used by shamans in tribal ceremonies.

The Plant World According to Writers and Philosophers

Hated, loved, ignored, or sacralized, plants are part of our lives and so of our art, folklore, and literature. In the works they create, the imagination of artists and writers helps construct a vision of the world. What does art tell us about the relationship between human beings and the plant world? Though there certainly are important exceptions, in general, writers depict the plant world as a static, inorganic part of the countryside, passive as a hill or a mountain chain. Consider, for example, Robinson Crusoe (1719) by Daniel Defoe, where plants are depicted as part of the landscape but never as living organisms. For the first hundred pages, the whole plot of the novel is based on Robinson’s search for other living organisms on the island . . . while he is literally surrounded by them in the form of plants. More recently, in Suddenly in the Depths of the Forest (2005) by Amos Oz, a small village is under a curse that prevents any form of life except humans . . . while the village is completely encircled by the plants of the forest.

In philosophy, as we have noted, inquiries into plants’ nature have animated the discussions of great minds for centuries. Whether plants had life (or a “soul,” as they called it then) was an endlessly debated question centuries before Christ. In Greece, birthplace of Western philosophy, opposing positions on this matter long coexisted: on one side Aristotle of Stagira (384/383–322 BCE) thought that the plant world was closer to the inorganic world than to the world of living things; on the other, Democritus of Abdera (460–360 BCE) and his followers showed a high estimation of plants, even comparing them to human beings.

In classifying living things, Aristotle divided them according to the presence or absence of soul, a concept which for him had nothing to do with spirituality. To understand it, we need to consider the root of the word animate, which even today means “having the ability to move.” In one of his works, he wrote: “Two characteristic marks have above all others been recognized as distinguishing that which has soul in it from that which has not—movement and sensation” (On the Soul). On the basis of this definition, and with the support of such observations as were possible in those times, Aristotle initially considered plants to be “inanimate.” But then he had to reconsider. After all, plants could reproduce! How could one argue that they were inanimate? The philosopher then opted for a different solution and gave them a low-level soul, a plant soul created expressly for them, which in practical terms only permitted them reproduction. If plants couldn’t be thought of as equal to inanimate things, because they could reproduce, still—Aristotle decided—they shouldn’t be considered all that different from them, either.

Aristotelian thinking influenced Western culture for many centuries, especially in certain disciplines such as botany, where it held sway almost until the beginning of the Enlightenment. So it’s little wonder that philosophers long considered plants to be “immobile” and not worth further consideration.

However, from antiquity to the present day, some philosophers have paid the highest honors to the plant world. For example, almost a century before Aristotle, Democritus described plants in a completely different way. His philosophy was based on atomistic mechanics: every object, even if it appeared to be immobile, was composed of atoms in continuous motion, separated in a vacuum. According to this vision of reality, everything moved, and thus at an atomic level even plants were mobile. Democritus even compared trees to upside-down humans, with their head set in the ground and their feet in the air—an image that would often recur through the centuries.

The Aristotelian and Democritean conceptions in ancient Greece thus often gave rise to a kind of unconscious ambivalence, which held that plants were simultaneously inanimate beings and intelligent organisms.

The Fathers of Botany: Linnaeus and Darwin

Carl Nilsson Linnaeus (1707–1778), usually known as Carl Linnaeus, was a physician, explorer, and naturalist whose many interests included the classification of all plants. For this reason, he is often known as “the great classifier,” which only partly does him justice, since in addition to his work of classification he conducted intensive research throughout his lifetime.

Linnaeus’s ideas concerning the plant world were idiosyncratic almost from the start. First, he identified “reproductive organs” in plants, and he made the “sexual system” the principal taxonomic criterion upon which he based his work of classification. In a bizarre contradiction, this decision earned him both the first university chair and also condemnation for “immorality.” (It was known that plants had a sex. But studying this in order to classify plants? . . . how scandalous.) Then the scientist proposed another innovative theory, which only by accident drew less criticism than the first: Linnaeus maintained, with surprising determination and simplicity, that plants . . . sleep.

Even the title of Somnus Plantarum (The Sleep of Plants), his treatise of 1755, didn’t observe the caution used by scientists in those days to protect their theories from possible attacks. In fact, based on scientific knowledge of that time and on his own observations of the different positions assumed by the leaves and branches during the night, it was relatively easy for Linnaeus to assert that plants sleep. But it would be several centuries before sleep was recognized as a fundamental biological function related to the brain’s most evolved activities, and so his idea was not even contested.

Today the same theory has plenty of opponents, and even Linnaeus, if he had known the many functions of sleep, would probably interpret his own observations differently and would deny the existence of an activity in plants that could be compared to an activity of animals. In fact, he did deny it in another instance: that of insectivorous plants. Linnaeus was quite familiar with plants that ate insects, such as Dionaea muscipula (the Venus flytrap), for example. And he certainly had the experience of observing one as it enclosed, trapped, and digested an insect. Yet that reality (a plant eating an animal) was so incompatible with the rigid pyramidal organization of nature, in which plants were relegated to the lowest level of life, that Linnaeus, like his contemporaries, sought a myriad of other possible explanations rather than acknowledge plain evidence. Without any regard to scientific confirmation of his assertions, from time to time he therefore hypothesized that the insects didn’t die at all, and that they chose to remain inside the plant of their own volition and for their own convenience, or that they landed on the plant by chance and not because they were attracted to it. Or even that the plant trap closed by chance, and so couldn’t possibly lure an animal. Ambivalence toward the plant world still had its hold on the mind of the great Swedish botanist!

Not until Charles Darwin published his treatise on insectivorous plants in 1875 did a scientist finally assert the existence of plant organisms that feed on animals. But even Darwin, with his characteristic caution, didn’t go so far as to call them “carnivores” (as we do today), though he was perfectly aware of plants that prey on rats and other small mammals, such as several supercarnivores belonging to the genus Nepenthes. Some “insectivores!”

We shouldn’t be dismayed by Darwin’s caution, any more than we are by Galileo’s, or the caution of other scientists in centuries past. It’s because of their “diplomacy,” in fact, that certain revolutionary ideas could slowly filter through the collective consciousness—and into a scientific community that was very conservative. But let’s return to Linnaeus for a moment, and ask ourselves: how was it possible for him to assert so boldly that plants sleep, without being shunned or persecuted by his peers? This isn’t hard to answer: for a long time it was thought that his theory had no basis in fact, so it wasn’t even worth refuting. And furthermore, who cared whether plants slept, when sleep wasn’t believed to have any particular function?

Today, we know how many important vital and cerebral functions are linked to this physiological process. But until the turn of this century, even modern science maintained that only the most evolved animals sleep. In 2000, this was disproved by the Italian neuroscientist Giulio Tononi, who showed that even the fruit fly, one of the “simplest” insects in existence, takes its well-deserved rest. Then why shouldn’t plants? Maybe the only possible explanation is that this idea doesn’t fit with how we think about the vegetal world.

Humans Are the Most Evolved Beings on the Planet. Or Are They?

With few or no exceptions, unfortunately, the idea of the plant world and the so-called Pyramid of Living Things that we’ve taken with us down through the centuries is the one contained in the Liber de sapiente (Book of Wisdom), published in 1509 by Charles de Bovelles (c. 1479–1567). An illuminated illustration from the book is worth more than a thousand words: it shows the living and nonliving species in ascending order. It starts with rocks (which are given the following lapidary comment: Est, meaning they exist and that’s all; they have no further attributes), continues to plants (Est et vivit: thus a plant exists and is alive, but nothing more) and animals (Sentit: an animal is endowed with senses), and finally comes to man (Intelligit: only man has the faculty of understanding).

The Renaissance idea that among living creatures, some species are more or less evolved and endowed with greater or lesser vital capacities, is still in vogue. It is part of our cultural humus, and nearly impossible for us to give up, despite the passage of more than 150 years since the publication in 1859 of The Origin of Species, the foundational work given us by Charles Darwin to understand life on our planet—a book so important that the great biologist Theodosius Dobzhansky wrote: “Nothing in biology makes sense except in the light of evolution.” The theories of the great British scientist, who w...