One morning in May 1755, in Uppsala, Linnaeus saw a Lotus corniculatus come into flower, sent to him by François Boissier de Sauvages, a botanist from Montpellier. The plant came from the Mediterranean shores, and it had taken several months to acclimatise to the Swedish cold. Linnaeus returned to the greenhouse in the late afternoon, and the yellow flowers had vanished. The next morning they were back in place, perfectly fresh. What Linnaeus had just observed is what botanists call nyctinasty, from the Greek nux, night, and nastós, compact. The leaves rise and gather around each cluster of flowers, the peduncles fold, the branches bend towards the ground. From this Linnaeus would draw Somnus plantarum, then the idea of his clock garden.
It is with scenes of this kind that Stefano Mancuso and Alessandra Viola proceed. The first directs the LINV, the International Laboratory of Plant Neurobiology at the University of Florence, and has published more than 250 scientific papers. The second is a science journalist, and it is she who holds the pen for the general reader. Brilliant Green appeared in Italian in 2013 under the title Verde brillante, the French Albin Michel edition (L’Intelligence des plantes) dates from 4 April 2018, translated by Renaud Temperini with a preface by Michael Pollan.
A book that pleads more than it demonstrates
Six parts: an introduction, five chapters, a conclusion. The first chapter argues that the refusal to grant intelligence to plants is not scientific but cultural. The second describes what a plant physically is. The third takes stock of its senses, the fourth of its ways of communicating, the fifth tackles intelligence itself. The conclusion shifts towards plant rights and technology.
Let me say it straight away, this is not a book of experiments. Mancuso and Viola proceed by cataloguing botanical examples and rereading old observations, and quantified protocols are rare here. Anyone looking for detailed set-ups will be disappointed. Anyone looking for a critique of scientific anthropocentrism will be served to the point of surfeit.
The opening argument is an image. In 1509, Charles de Bovelles published his Liber de sapiente, with a ladder of beings that has become famous. The rock: Est, it is, and that is all. The plant: Est et vivit, it is and it lives. The animal: Sentit, it feels. Man: Intelligit, he alone understands. This Renaissance pyramid, says Mancuso, has survived the Enlightenment, the scientific revolution and the hundred and fifty years that separate us from On the Origin of Species. It still holds, right into the laboratories. Further back stands Aristotle, for whom the plant is deprived of soul understood as a moving principle, and so left stuck on the border between the living and the non-living.
The fate reserved for those who work on plants
Chapter 1 lines up the cases, and they are damning. Mendel and his peas founded genetics, his conclusions were ignored for forty years, until the animal genetics boom. In the 1940s, Barbara McClintock demonstrated on maize that the dogma of genome constancy is false. Her Nobel came in 1983, after analogous research carried out on animals in the early 1980s had confirmed the phenomenon. Mancuso is explicit, it was that animal rediscovery that won her the prize. Same mechanics for interfering RNA, Nobel 2006 to Andrew Fire and Craig Mello, who rediscovered on the worm Caenorhabditis elegans what Richard Jorgensen had found twenty years earlier on the petunia. The cell itself was discovered on plants. Nobody remembers.
Hence the hardest question in the book, put more or less in these terms: what brilliant researcher would devote themselves to plants rather than animals, knowing that most forms of recognition will be closed to them?
A plant is not an individual
Chapter 2 rests on an etymology and a bravura passage. The etymology first. “Individual” comes from the Latin in, not, and dividuus, divisible. Our body cut in two dies. A plant cut in two gives two parts that live separately. So a plant is not an individual, it is a colony, and a tree is closer to a beehive than to a horse. This modularity is a defence strategy for a being that cannot flee: never concentrate its functions in a few vital organs, the way someone afraid of theft spreads their money across several hiding places. Some species can be grazed down by ninety or ninety-five per cent and start again from the small surviving core.
The bravura passage is the duel between the paramecium and the euglena. Round one, food: the paramecium detects it and moves, so does the euglena, which swims with its flagella. Even. Round two, electricity: the paramecium’s body is run through by electrical signals that cross its single cell, which earned it the nickname swimming neuron. Now the euglena is crossed by the same kind of impulses. Even. Round three, the euglena photosynthesises, and to do it better it has developed a rudimentary form of sight that lets it intercept light frequencies and position itself in the best spot. So why has nobody ever called the euglena a swimming neuron? Mancuso answers that there is no rational explanation. It is green, that is all.
The senses, and an attack on a standard protocol
Chapter 3 is the densest in botany. Three families of photoreceptors are named, phytochromes, cryptochromes and phototropins, which absorb in the red, the far red, the blue and the ultraviolet. They are distributed across the leaves, the young stems, the tendrils, the buds, the apices, right into the green wood. The roots, for their part, flee the light as though they had a phobia of it.
From this comes a head-on criticism, and it is the passage I found the most courageous. Arabidopsis thaliana, the model plant of all plant molecular biology, is grown in the laboratory on transparent gel, roots kept in full light. A completely abnormal situation, and therefore a stressful one. The rapid growth observed there is usually read as a sign of well-being. Mancuso argues the opposite: the root grows fast because it is trying to escape.
For smell, it is a matter of volatile organic compounds, with receptors spread over the surface of the cells, as if we carried millions of small noses distributed all over our body. One molecule is named, methyl jasmonate, produced under stress, whose message amounts to saying “I am not well”. The fact that stopped me: very distant species use the same words to say the same thing, which suggests not a single plant language but a common root shared by different languages. A tomato under attack from herbivores alerts other plants hundreds of metres away. And on the state of our knowledge, Mancuso offers an apt comparison: we are in the position of Champollion before 1822. We know that certain signs carry messages, their number is tiny compared to the whole set of molecules emitted, and the message is never carried by a single molecule but by a combination in determined proportions.
Taste brings the long history of the Venus flytrap. On 24 January 1760, Arthur Dobbs, landowner and governor of North Carolina, described the plant in a letter to the English botanist Peter Collinson and named it Fly Trap Sensitive. Collinson passed the specimens on to John Ellis, who named the species Dionaea muscipula and wrote to Linnaeus in 1769 to assert its carnivorous nature, sweet bait, red glands, closure of the two lobes, three spines standing upright in the centre of each. Linnaeus refused. He filed the flytrap among the sensitive plants, alongside Mimosa pudica, involuntary movement in response to a stimulus. Ellis, hardly famous, described what he saw. Linnaeus, at the height of his glory, bent the observation to fit his theory of the order of nature.
On hearing, the mechanism proposed goes through the mechanosensitive channels present in every cell, with the comparison of the nightclub, where the bass makes the body vibrate to the point that deaf people perceive it. Two facts held me there. Recent work shows that roots perceive a very wide range of vibrations and that their directional growth can be steered by them, a phenomenon Mancuso calls phonotropism. And in 2012, an Italian study established that roots produce sounds, without anyone knowing how. It has been named clicking, because it sounds like clicks, and the accepted hypothesis attributes it to the rupture of cell walls during growth. Mancuso specifies that these sounds are not produced deliberately, and he adds a sentence I envy him: what purpose it serves for roots to perceive vibrations, we do not yet know.
The pollen market, with its honest traders and its swindlers
Chapter 4 is the most entertaining to read. The lima bean attacked by the plant-eating mite Tetranychus urticae emits a blend of volatiles that attracts Phytoseiulus persimilis, a specialised carnivorous mite, which exterminates the assailants. The plant recognises its aggressor and summons that aggressor’s precise enemy.
The case of maize is more political. In the United States, the crop was decimated for years by Diabrotica virgifera, whose larvae develop near the roots. The old European varieties and wild maize produced beta-caryophyllene, whose sole function was to call for help from nematodes with a taste for those larvae. Modern selection, obsessed with yield and cob size, removed this capacity without meaning to. Losses estimated at around one billion dollars a year worldwide. The solution came from genetic engineering, by reintroducing into modern varieties the caryophyllene production gene borrowed from oregano. To give maize back an innate characteristic, a transgenic plant had to be manufactured.
The lupin plays fair: it tints blue the petals of flowers already visited, to signal to bees that neither pollen nor nectar is left there. Orchids, for their part, are on the side of the swindle, about a third of species according to the book. Ophrys apifera pushes mimicry through three senses at once: the shape and colours of the female of certain non-social hymenopterans, the consistency and hairiness of her surface, and the scent, by secreting pheromones identical to those of females ready to mate. The male copulates with the flower, and a spring mechanism sticks the pollinia onto his head. The detail that settles the matter: during flowering, these hymenopterans prefer the flower even when females are available. Arum palestinum attracts the fruit fly by producing the aroma of fermenting fruit, closes its inflorescence over her for a whole night, covers her in pollen and releases her without giving her anything. Amorphophallus titanum, the largest inflorescence in the world, reproduces the smell of a putrefying corpse for the blowfly.
Darwin, the root, and the charge of dilettantism
Chapter 5 contains the only real experimental story in the book, and it is superb. In 1880, Darwin published The Power of Movement in Plants with his son Francis, the result of hundreds upon hundreds of experiments on plant movement, mostly concerning the root rather than the aerial part. In the final paragraph, where he always places his decisive conclusions, he states that the root contains something comparable to the brain of a lower animal. Mancuso insists on the correct reading, nobody ever claimed there is a real brain in a root, but a plant analogue that fulfils several of its functions.
Julius von Sachs, hostile, tasked his assistant Emil Detlefsen with redoing the experiments, notably those concerning root behaviour after removal of the root cap. Detlefsen obtained something else. The work was badly done, the book notes, the assistant being held in low regard in that laboratory. Sachs then accused the Darwins, father and son, of having worked as dilettantes. Wilhelm Pfeffer, a former student of Sachs, in turn redid the manipulations and found exactly the Darwins’ results. Sachs would say of his Lehrbuch der Pflanzenphysiologie that it is nothing but a heap of undigested facts. And on 2 September 1908, at the opening of the annual congress of the British Association for the Advancement of Science, Francis Darwin publicly declared that plants are intelligent beings, which he would confirm that same year in an article of more than thirty pages published by Science.
From this comes the thesis. The brain is not a magical organ, and the brain of our greatest genius, separated from the body, would be no more intelligent than his stomach. In plants, cerebral functions are not isolated, they are co-present in every cell. Mancuso here uses the vocabulary of artificial intelligence and speaks of an embodied agent. Then he argues by symmetry: a plant has no lungs, no liver, no stomach, no kidneys, and yet it fulfils all these functions, so why would the absence of a brain prevent it from being intelligent?
The root apex becomes the computing centre. It measures from a few tenths of a millimetre in Arabidopsis to about two millimetres in maize, it is white, it is the living part that lengthens, and it deploys a very intense electrical activity based on action potentials, very similar to those of animal neurons. Even a very small plant can have more than fifteen million apices, rye or oats several tens of millions, a tree reasonably hundreds of millions, a figure given with the precision that no study has established it. Each apex continuously measures gravity, temperature, humidity, the electrical field, light, pressure, chemical gradients, heavy metals, sound vibrations, the presence of oxygen and carbon dioxide. The list, Mancuso specifies, is not exhaustive and grows longer year after year.
Two well-held analogies follow. The network first, with Arpanet designed modularly by DARPA to survive a nuclear attack, the destruction of most nodes not compromising the whole, against isolated mega-computers of the IBM Sequoia type. Then the swarm, with this difference that he underlines: in animals the swarm is made of many individuals, in plants the dynamic plays out inside a single plant, between its roots. Each apex would simply maintain a given distance from its neighbours, which would be enough to produce a coordinated exploration of the soil, with no higher will and no director. He recalls that before the 1970s, the flight of starlings passed for such a mystery that serious journals were considering telepathy.
And he pushes further than I expected. If we were to encounter an extraterrestrial intelligence, would we know how to recognise it? Probably not. Man, incapable of conceiving of intelligences other than his own, is looking less for an extraterrestrial intelligence than for his own intelligence mislaid somewhere in space. Why suppose that other beings would use our means of communication, all based on wave phenomena, when part of life on Earth communicates by producing chemical molecules?
The two experiments, and what they are worth
Kin recognition, dated 2007, is the only experiment in the book described with a protocol and a sample size. Two identical pots. In the first, thirty seeds from the same mother plant. In the second, thirty seeds from different mothers. The latter develop a very large number of roots, occupy the ground, secure water and nutrients at the expense of their neighbours. The former, in a space just as confined, produce far fewer roots and favour aerial growth. So a plant does not follow an automatism of the neighbour equals rival type, it recognises first, and if it detects a genetic affinity it chooses to cooperate. Mancuso draws two equally revolutionary hypotheses from this: either plants are far more evolved than we believe, or altruism also exists in so-called primitive life forms. Neither the species nor the publication is given in the text, and the experiment in all likelihood corresponds to the work of Susan Dudley and Amanda File on Cakile edentula, published in 2007, but that is a reader’s connection, the book does not say so.
The other is viticultural, at Montalcino, in Tuscany. A producer plays music in his vineyards, in collaboration with the LINV and with the Bose company, which funded the research. More than five years of observation. The exposed vines grow better, ripen earlier, give grapes richer in flavour, colour and polyphenols, and the disoriented insects keep their distance, which makes it possible to cut insecticides sharply. In 2011, EUBRA selected the experiment among the hundred projects called on to change the green economy. The point of explanation matters more than the result: it is not the musical genre that acts, the plant tells no styles apart, it is the frequencies. The low ones, between 100 and 500 hertz, promote germination, growth and root elongation. The higher ones inhibit.
It must be said honestly what is missing. No paper is cited for Montalcino, the funder is a loudspeaker manufacturer, and the authors say so themselves. The statistic on the advance of lianas at the expense of trunk-bearing species is not sourced, nor is the estimate of a third of orchids being deceitful. Conversely, all the historical material is dated and precise. This is a book whose strongest claims are often the least supported, and whose scholarship is impeccable as soon as it speaks of the past.
The language, and what it costs
Two hands, and you can hear it. Constant address to the reader, rhetorical questions in cascade, numerous exclamation marks. Above all, a single metaphorical register, economic and social, run through from beginning to end. The plant in the shade is poor, the one in the sun is rich, the one that stretches to outgrow its rival behaves like an entrepreneur investing in its future, the root proliferating along a gradient becomes a mining company opening galleries, pollen and nectar form a market with its honest traders and its swindlers, insects are pony express riders and fruits are gift parcels for the postmen. Lianas are called shirkers, the orchid the lady of the con. The anthropomorphism is deliberate, barely held in check by safeguards that always arrive after the fact.
This stance has a price, and it explains the reception. In 2007, Amedeo Alpi and thirty-six co-signatories, among them Michael Blatt, Lincoln Taiz and Chris Somerville, published in Trends in Plant Science a letter titled “Plant neurobiology: no brain, no gain?”, concluding that the discipline added nothing to the understanding of plant physiology. Mancuso and his colleagues replied in the same journal that all metaphors have value. Round two in 2019 with “Plants Neither Possess nor Require Consciousness”. The Society for Plant Neurobiology, founded in 2005, had in the meantime become the Society of Plant Signaling and Behavior. What we learn from this battle is useful, and it is not what one might think: the disagreement almost never bears on the observations, it bears on the right to use the words brain, neuron, learning, memory. A quarrel over vocabulary and territory, not over facts. Francis Hallé, who belongs to neither camp, long refused the word intelligence before accepting it by redefining it as the capacity to solve the problems of one’s life through learning and memorisation.
The book ends on Switzerland, the first nation in the world to have recognised rights for plants. Its ethics committee, which brought together philosophers, molecular biologists, naturalists and ecologists, concluded unanimously that the indiscriminate destruction of plants is morally unjustifiable. Mancuso takes care to specify that dignity does not forbid use, just as recognising the dignity of animals has not taken them out of the food chain. Then come the plantoids, robots of plant inspiration, the Greenternet, a network that would make available in real time the parameters measured continuously by roots and leaves, and the phytocomputers. This is the announcement of the next book.
What I took away from it fits in one sentence, the one on the Italian back cover. Plants could very well live without us, whereas we, without them, would die out in short order. And yet our languages have manufactured “to vegetate”.
Read in May 2018. Entry written in 2026.
