The Vegetable Kingdom

What Is a Plant?

Ask a child to draw a plant and you get a stem, two leaves and a flower. A botanist needs longer, because the word has to stretch from a moss the size of a fingernail to a giant sequoia and still exclude mushrooms, seaweed and coral. The modern definition rests on a single ancient event. Around 1.5 billion years ago (botanists debate the date) a single-celled organism engulfed a cyanobacterium and, instead of digesting it, kept it. That captive became the chloroplast, and every descendant of that cell belongs to the group now called Archaeplastida: the glaucophytes, the red algae and the green lineage that runs from pond scum to oak.

Within that green lineage sit the land plants, or Embryophyta, named because they shelter the embryo inside the parent's tissues. They share a toolkit: cells walled with cellulose, a stiff polymer of glucose; a large water-filled vacuole that keeps the cell firm; a life cycle that alternates between two generations; and a pigment set built around chlorophyll a and b. By that definition the kingdom holds mosses, liverworts, hornworts, ferns, conifers and flowering plants. Kew's State of the World's Plants report counted about 391,000 named vascular species in 2016, and the number climbs by roughly 2,000 a year.

How we know is written in the cell. Under the electron microscope a chloroplast shows two membranes, the inner one still built like a bacterial envelope, and inside sits a small circular genome whose genes read like those of living cyanobacteria. In 1905 the Russian botanist Konstantin Mereschkowski guessed that chloroplasts had once been free-living; DNA sequencing in the 1970s proved him right. The glaucophyte Cyanophora paradoxa, a freshwater alga, even keeps a thin bacterial cell wall around its chloroplasts, a relic of the original capture.

The borders of the kingdom are not where most people think. Fungi are not plants: their walls are chitin, the material of insect shells, they feed by absorption, and on the tree of life they sit beside animals. The brown kelp of a Californian forest, Macrocystis pyrifera, is a stramenopile that acquired its chloroplasts second-hand from a red alga, so most botanists leave it outside too. Coral is an animal with algal lodgers. Meanwhile a moss on a Scottish wall, a duckweed on a Thai pond and a baobab in Madagascar are all full citizens.

Did you know? When Linnaeus split nature into three kingdoms in 1735, he called one Regnum Vegetabile; "vegetable" meant any plant whatsoever, so a Georgian naturalist could call an oak a vegetable without a flicker of irony.

The definition matters because plants are the economy of the biosphere. Land plants hold around 450 billion tonnes of carbon, roughly 80 per cent of all living biomass on Earth, and almost every calorie an animal eats was first fixed by a green cell. Drawing the line correctly tells us which organisms share the inheritance of that first captured bacterium, and that inheritance is what this book sets out to explore. A plant, in the end, is a cell that caught the Sun and never let go.

The Plant Cell

In 1665 Robert Hooke put a sliver of cork under his microscope and saw, as he wrote in Micrographia, "a great many little boxes". They reminded him of monks' rooms, so he called them cells. He was looking at the dead, emptied walls of bark tissue, and he calculated that a cubic inch of cork held more than 1,200 million of them. The word stuck, and so did the thing he had noticed without understanding it: a plant cell is defined first by its wall.

The cell wall is built from cellulose, chains of glucose bundled into microfibrils that are stronger, weight for weight, than steel, glued together with pectin and hemicellulose. Inside the wall lies the living cell: a membrane, cytoplasm, a nucleus, mitochondria and the two organelles that make a plant cell unmistakable. The chloroplasts turn light into sugar. The central vacuole, a sac of watery sap, can fill 90 per cent of a mature cell and press the cytoplasm into a thin film against the wall. That pressure, called turgor, is what holds a lettuce leaf crisp; typical values of 0.5 to 1 megapascal are several times the pressure in a car tyre. Let the vacuole lose water and the cell goes slack, which is all that wilting is.

Plant cells are also joined in a way animal cells are not. Tiny channels called plasmodesmata, named by Eduard Strasburger in 1901, thread through the walls and connect the cytoplasm of neighbour to neighbour, so a leaf is in some ways a single continuous living space partitioned by cellulose. Sugars, hormones and even viral genomes pass along them.

How we know owes much to Matthias Schleiden, who in 1838 argued that every plant is built of cells and every part of a plant grows from them; his friend Theodor Schwann extended the claim to animals a year later, and cell theory was born. Modern evidence comes from the electron microscope and from giant cells kind enough to be experimented on: the internodal cells of the pond alga Chara are several centimetres long, so physiologists could push microelectrodes into them and measure turgor directly. A cotton fibre is a single cell two to three centimetres long; the fibres of ramie, Boehmeria nivea, are longer still.

Did you know? Cellulose is the most abundant organic polymer on Earth; plants make well over a hundred billion tonnes of it every year, enough to rebuild every building on the planet several times over.

The plant cell's design explains the plant's way of life. A wall means no crawling, no swallowing and no muscles, so plants grow instead of moving, taking in water, light and air across their surfaces. A vacuole means a cell can become large and cheap, inflated with water rather than filled with expensive protein, which is why a tree can be enormous and still mostly air and sap. Hooke's little boxes turned out to be the architecture of nearly all the green on Earth.

The Chloroplast

Every green cell carries a colony of former bacteria. A chloroplast is a lens-shaped body about five to ten micrometres long, bounded by two membranes, and a single mesophyll cell in a spinach leaf may hold up to a hundred of them. Inside, a third membrane system folds into flattened sacs called thylakoids, stacked like coins into grana, and it is on these stacks that chlorophyll sits and the light reactions of photosynthesis run. The fluid around them, the stroma, is where carbon dioxide is turned into sugar.

The odd thing about chloroplasts is that they are not quite part of the cell. They carry their own small circular genome of roughly 120 to 160 thousand base pairs and about 120 genes; they make some of their own proteins on bacterial-style ribosomes; and they reproduce by pinching in two, using an FtsZ ring like a dividing bacterium. A cell cannot build a chloroplast from scratch; it can only inherit one.

That is exactly what Konstantin Mereschkowski proposed in 1905: that the green bodies of plants were once free-living cyanobacteria taken in by a colourless host. The idea was mocked for sixty years. In 1967 Lynn Margulis, then publishing as Lynn Sagan, revived it in a paper that by her own account some fifteen journals had rejected, and in the 1970s the sequencing of chloroplast ribosomal RNA by Linda Bonen and W. Ford Doolittle, among others, showed it matched cyanobacteria far better than it matched the plant's own nucleus. In 1986 teams in Japan published the complete chloroplast genomes of tobacco and the liverwort Marchantia, and the case closed. The capture, a primary endosymbiosis, happened once, around 1.5 billion years ago, and gave rise to all red algae, green algae and land plants.

Chloroplasts are also surprisingly mobile. Under weak light they spread across the top of a cell to catch photons; under strong light they slide to the side walls to avoid damage, a response first described by Gustav Senn in 1908 and traced in 2001 to the blue-light receptor phototropin 2. Elsewhere the story has repeated: the freshwater amoeba Paulinella chromatophora captured its own cyanobacterium about a hundred million years ago and is still domesticating it, and the sea slug Elysia chlorotica steals chloroplasts from the alga it eats and keeps them photosynthesising for months.

Did you know? Most chloroplast genes have migrated to the plant's nucleus over the ages; a modern chloroplast imports more than 2,000 proteins that it can no longer make for itself.

What it means is that the plant kingdom is a partnership rather than a lineage. The chloroplast's cyanobacterial ancestry explains why herbicides that block bacterial-type ribosomes damage plants, why plastid DNA is the botanist's favourite barcode for identifying species, and why a leaf looks, under the microscope, like a city of captive bacteria doing the work of the Sun. Every plant is, at bottom, a successful kidnapping.

Why Plants Are Green

The green of a leaf is the light it refuses. Chlorophyll a, the pigment at the heart of every plant's photosynthesis, absorbs strongly in the blue around 430 nanometres and in the red around 660; chlorophyll b, its helper, fills in the gaps at 450 and 640. Between them lies the green and yellow part of the spectrum, from roughly 500 to 570 nanometres, which the pigments largely let through or bounce back. What reaches your eye is the leftover. A leaf is green for the same reason a red door is red: it is the colour it does not keep.

That was shown elegantly in 1883 by Theodor Engelmann, who laid a filament of the alga Spirogyra under a tiny spectrum cast by a prism and added oxygen-hungry bacteria. The bacteria crowded around the red and blue bands, where oxygen was being released, and ignored the green. Twenty years later, in 1903, the Russian botanist Mikhail Tswett ground up leaves, poured the extract through a column of chalk and watched it separate into bands of green and yellow. He called the method chromatography, the writing of colour, and it remains the chemist's basic tool.

Chlorophyll has company. Carotenoids such as beta-carotene and lutein absorb blue and blue-green light, pass energy on to chlorophyll and soak up excess light that would otherwise do damage; they are yellow and orange, and they are always present. In autumn, when a deciduous tree withdraws nitrogen from its leaves and dismantles chlorophyll, the carotenoids are simply unmasked, which is why a birch turns yellow. Reds are different. Anthocyanins are manufactured afresh in the dying leaf, at some cost to the tree, and botanists still debate why; the leading idea is that they shade the leaf's machinery while nutrients are salvaged. Red maples, Acer rubrum, in New England and the Japanese maples of Kyoto do it spectacularly; most European trees, with fewer red-leaved species, barely bother.

Not every photosynthesiser settled on green. Red seaweeds carry phycoerythrin, which absorbs the green light that chlorophyll wastes, and green light is exactly what survives deepest in clear sea water, since red is soaked up in the first ten metres and blue scatters. That is why the deepest-living photosynthetic organism ever found, a crustose red alga discovered off the Bahamas in 1984 at 268 metres, is red. Variegated leaves, the white-splashed hostas and figs of garden centres, are mostly chimeras in which some cells have lost the ability to make chlorophyll at all.

Did you know? No one is sure why plants pass up the green light at the peak of the Sun's output; some researchers suggest early photosynthesisers evolved beneath a layer of purple microbes that had already taken the green.

The colour of the kingdom is therefore a record of choices made billions of years ago and never revised. Satellites now read it from orbit, measuring the "red edge" where leaves stop absorbing and start reflecting, to map the planet's growing seasons. Green is not what plants are; it is what they leave for us.

Extremes of the Kingdom

The smallest flowering plant on Earth would fit through the eye of a needle. Wolffia, the rootless duckweed found on still water from Australia to Amazonia, is a green speck under a millimetre long that weighs around a tenth of a milligram; its flower, a single stamen and a stigma with no petals, sits in a pit on its upper surface. At the other end of the scale the parasite Rafflesia arnoldii of Sumatra and Borneo opens a single rust-red flower up to a metre across, weighing seven kilograms or more and smelling of carrion to attract the flies that pollinate it. Joseph Arnold and Stamford Raffles found the first in 1818. The plant has no leaves, stems or roots of its own; it lives as threads inside a jungle vine and appears only to flower.

Tallest is a separate contest. The titan arum, Amorphophallus titanum, also from Sumatra, pushes up a flower spike around three metres high, the largest unbranched inflorescence known, and warms it to spread a stench that botanical gardens from Kew to Bonn advertise with queue barriers. It is technically hundreds of small flowers on one stalk, so Rafflesia keeps the single-flower record.

The biggest plant of all is almost invisible. In Shark Bay, Western Australia, a seagrass meadow of Posidonia australis stretching about 180 kilometres was shown in 2022, by genetic sampling, to be one clone, probably around 4,500 years old, spreading by its runners a few centimetres a year since the bay first flooded. The deepest roots belong to the shepherd's tree, Boscia albitrunca of the Kalahari, reported in 1974 reaching 68 metres down a mine working in South Africa's Northern Cape. The fastest growth is bamboo: some species of Phyllostachys have been measured adding 91 centimetres in a single day, which is faster than a clock's minute hand is long.

Botanists know these numbers because someone measured them, usually with a tape, a balance and patience. The seagrass record needed 18,000 genetic markers from samples across the bay; the duckweed figures come from plants raised in laboratory flasks, where Wolffia is now studied as a possible protein crop. Some extremes are about disguise rather than size. The stone plants, Lithops, of Namibia and South Africa are a pair of swollen leaves that mimic the pebbles around them, with a translucent window on top that lets light down to photosynthetic tissue buried in the soil; William Burchell described the first in 1811 after picking up what he thought was a curiously shaped stone.

Did you know? The giant water lily Victoria amazonica unfurls leaves up to three metres across, ribbed underneath like a bridge truss, and Joseph Paxton, who first flowered it in Britain, said its leaf inspired the Crystal Palace of 1851.

Extremes are the kingdom's way of showing what a body plan can do. Freed from the need to move, a plant can be a speck, a metre-wide carrion lure, a clonal meadow older than the pyramids or a tree that drinks from the bottom of a mine. Each record is a limit that evolution found and then, somewhere, quietly tested.

Naming Plants

Every plant with a scientific name owes it to one book. On 1 May 1753 Carl Linnaeus published Species Plantarum, two volumes describing around 5,900 species, and in its margins he did something he regarded as a convenience: beside each long descriptive name he wrote a single word, the "trivial name". Combined with the genus, it gave a two-word label, Quercus robur, Zea mays, Rosa canina. The binomial proved so handy that botanists adopted it universally and set 1753 as the starting line: no plant name published before that date counts. His own scheme of classifying plants by counting stamens and pistils was abandoned within a century, but his names stayed.

The rules are now kept in the International Code of Nomenclature for algae, fungi and plants, revised every six years at an International Botanical Congress, most recently in Madrid in 2024. Its central principle is priority: the earliest validly published name wins. Its anchor is the type specimen, a single pressed sheet, chosen when a species is described, to which the name is permanently tied. Linnaeus's own herbarium, bought by the young Englishman James Edward Smith in 1784 and now kept in a vault at the Linnean Society in London, holds the types for thousands of the world's commonest plants.

Names tell you who a plant is related to, which is why they change. For two centuries families were defined by flower structure; since 1998 the Angiosperm Phylogeny Group, an international collective of botanists, has redrawn them from DNA. Its fourth system, APG IV in 2016, recognises 416 families and 64 orders of flowering plants and moved some old friends: the lily family lost most of its members, and the figwort family was broken up and scattered. The hardest naming fight of recent decades concerned Acacia. When DNA showed the genus had to be split, the 2005 congress in Vienna let the roughly 1,000 Australian wattles keep the name, so the African thorn trees became Vachellia and Senegalia.

Common names are cheerful, local and frequently wrong. Spanish moss, Tillandsia usneoides, hanging from live oaks in Louisiana, is neither Spanish nor a moss but a bromeliad, a cousin of the pineapple. Douglas fir, the great timber tree of the Pacific Northwest, is not a fir; its name Pseudotsuga menziesii means "false hemlock" and honours Archibald Menzies, while the common name honours David Douglas, who sent its seed to Britain in 1827. The Jerusalem artichoke is a sunflower from North America.

Did you know? A scientific name must be unique within the plant kingdom, but botany and zoology keep separate lists, so Pieris is both a shrub and a butterfly, and Morus is both a mulberry and a gannet.

Names are the index of everything else we know. A seed bank, a drug company and a customs officer all need to be sure that the plant in the packet is the one in the record, and the binomial, with its type sheet and its rules, is the thread that makes that certainty possible. Linnaeus's marginal convenience has become the one thing on which every botanist in the world agrees.

The Herbarium

A herbarium is a library in which the books are plants. Each specimen is a plant, or a piece of one, pressed flat, dried and glued or stitched to a stiff sheet of paper, with a label recording who collected it, where and when. The idea is usually credited to Luca Ghini, professor of botany at Bologna and then Pisa, who in the 1540s taught his students to press plants so they could be studied out of season. His own collection is lost, but those of his circle survive: the hortus siccus of Gherardo Cibo, begun around 1532, and the volumes of Ulisse Aldrovandi in Bologna are among the oldest in the world.

Dried, a plant lasts almost indefinitely. The Muséum national d'Histoire naturelle in Paris holds around 8 million sheets, the largest herbarium on Earth; the Royal Botanic Gardens, Kew, around 7 million; New York, St Petersburg, Geneva, Missouri and Leiden each count in the millions. Index Herbariorum, the world register, lists about 3,500 collections holding close to 400 million specimens between them. Among them lie treasures. Cambridge keeps the plants Charles Darwin collected on the Beagle, including his Galápagos specimens of 1835, which John Stevens Henslow had begged him to label by island; their island-by-island differences later fed his thinking.

The most valuable sheets are types, the single specimens to which scientific names are anchored. When a botanist suspects a new species, the first task is to check the types, which is why herbaria lend sheets to one another by the thousand and why the Natural History Museum in London still guards Hans Sloane's 265 bound volumes from the 1690s, the foundation of its collection.

Since the 2000s the sheets have been photographed. Paris digitised its entire collection between 2008 and 2012, and image portals now pool hundreds of millions of records, so a botanist in Nairobi can read a label written in Lima in 1802. That has turned herbaria into instruments for measuring change. In 1987 Ian Woodward compared leaves of eight temperate tree species collected over 200 years and found their stomatal density had fallen by 40 per cent as atmospheric carbon dioxide rose. Richard Primack's team in Massachusetts matched flowering dates on old sheets with Henry Thoreau's notebooks from Concord and showed that spring now arrives weeks earlier than in the 1850s. Sheets record where a plant once grew and no longer does, which seeds an expedition carried home and which beetles bored the leaves.

Did you know? Specimens collected by Linnaeus's students in the 1750s, by Joseph Banks on the Endeavour in 1770 and by Alexander von Humboldt in the Andes in 1802 can still be sampled for DNA.

A pressed plant is a fact that can be checked. Photographs fade and descriptions mislead, but a sheet dated and placed by the hand that collected it is evidence of a kind that history rarely leaves. The herbarium is where botany keeps its receipts.

How Many Plants Are There?

Nobody knows how many kinds of plant there are, but botanists have got better at admitting it. The Royal Botanic Gardens, Kew, which keeps the world's checklist, reports around 350,000 accepted species of vascular plant once synonyms have been pruned; the figure was nearer 391,000 in its 2016 estimate, and the gap shows how much of the count is bookkeeping. Add perhaps 20,000 mosses, liverworts and hornworts. To that total about 2,000 new species are added every year: in 2019 alone, 1,942 plants were named, from new orchids to a tree from the Cameroon highlands.

Most of them are not new to anyone but science. In 2010 a team led by Daniel Bebber examined the history of recently described species and found that the typical plant waits decades in a herbarium cupboard between being collected and being named, and that more than half of the species still undescribed may already be sitting pressed on a shelf. Estimates of the true total therefore rest on the rate at which the drawers are opened. Lucas Joppa and colleagues calculated in 2011 that around 10 to 20 per cent of flowering plants remain undescribed, which would put the final tally somewhere above 400,000.

The species are not spread evenly. In 2000 Norman Myers and colleagues defined biodiversity hotspots: regions with at least 1,500 endemic plants that had lost 70 per cent of their original habitat. The 25 they listed, since expanded to 36, hold about 44 per cent of all plant species on 1.4 per cent of the land. The Cape Floristic Region at the tip of Africa packs roughly 9,000 species into an area the size of Portugal, nearly 70 per cent of them found nowhere else. Madagascar's 11,000-odd plants are more than 80 per cent endemic. Colombia leads the world in orchids, with over 4,000. The Amazon tells the same story in trees: in 2013 Hans ter Steege and more than a hundred co-authors pooled data from 1,170 forest plots and estimated that the basin holds about 16,000 tree species, of which a mere 227 "hyperdominants" make up half of its 390 billion trees, while thousands of rare species may each number fewer than a thousand individuals.

That rarity is the point. A species confined to one mountain in Ecuador can be described and declared extinct in the same decade. The 2020 edition of Kew's assessment estimated that two in five plant species are threatened, and many will vanish between collection and description, known only from a sheet.

Did you know? Britain, one of the most intensively botanised places on Earth, has about 1,500 native plant species; Colombia, roughly five times the size, has more than 25,000.

Counting plants is not a hobby. The figure sets the scale of what is at stake in forest and field, tells conservationists where a hectare protects most, and reminds the rest of us that the green world is still mostly unread. The catalogue of the kingdom is open, and most of its pages are blank.

Plants, Fungi and Algae

For most of history the living world had two sides: things that moved and things that did not. Mushrooms, seaweeds and plants fell together on the still side. The separation took a century. Ernst Haeckel added a third kingdom, Protista, for microbes in 1866; in 1969 the American ecologist Robert Whittaker proposed five kingdoms, giving fungi a kingdom of their own on the grounds that they feed by absorbing, animals by swallowing and plants by photosynthesis. A generation learnt it; then DNA rearranged it.

The surprise came in the 1990s, when comparisons of ribosomal and protein genes showed that fungi are closer to animals than either is to plants. The two share an ancestor that swam with a single rear flagellum, which is why the group is called Opisthokonta, "hind-tailed". Fungi build their walls from chitin, the material of insect cuticle, store energy as glycogen as we do, and have none of the cellulose, chloroplasts or starch of plants. A mushroom, in short, is a relative of ours that gave up moving. There may be 2 to 4 million species of fungi, of which about 150,000 have names.

"Algae" is not a group at all but a habit. The word covers everything that photosynthesises in water without being a land plant, and that spans several branches of the tree. Cyanobacteria are bacteria. Red and green algae are true Archaeplastida, kin of plants. Brown algae are stramenopiles, cousins of the diatoms and of potato blight; the giant kelp Macrocystis pyrifera of Californian and Tasmanian coasts grows to 45 metres and up to half a metre a day, and it is no more a plant than a sea urchin is. Diatoms, dinoflagellates and euglenids each obtained chloroplasts second-hand from engulfed algae. Together these groups do roughly half the planet's photosynthesis.

Among the green algae lies the plant kingdom's ancestry. The charophytes, freshwater algae such as the stoneworts, Chara, and the delicate filaments of Spirogyra and its relatives, are the closest living relatives of land plants; genome comparisons published in 2014 and 2018 point to the Zygnematophyceae, the group that includes Spirogyra, as the sister lineage, a finding that surprised botanists who expected the more complex stoneworts. A pond scum, it turns out, is our best model of what crawled ashore around 470 million years ago.

Fungi and algae meet in the lichen. Simon Schwendener proposed in 1867 that a lichen was a fungus farming an alga, an idea his contemporaries found distasteful; it is now textbook, and in 2016 Toby Spribille showed that many lichens also harbour a second fungus, a yeast, in their outer layer. About 20,000 lichen species crust rocks from Antarctica to the Atacama.

Did you know? The largest known organism is a fungus: a single Armillaria ostoyae in Oregon's Malheur National Forest spreads under nearly 10 square kilometres of woodland.

The lesson is that greenness and stillness are not signs of kinship. A plant is defined by its lineage, and the greengrocer's shelf, with its mushrooms and its nori, is a reunion of three kingdoms that last shared a cell more than a billion years ago.

How a Plant Senses the World

A plant has no eyes, ears or nerves, yet it knows which way is up, where the light is, what time it is and when it is touched. In 1880 Charles Darwin and his son Francis published The Power of Movement in Plants, the record of years of patient experiments with seedlings and little paper caps. Shading the tip of a canary grass seedling stopped it bending toward a window; shading the lower stem did not. The tip, they concluded, saw the light and sent a message downward. In 1926 Frits Went caught the messenger in a block of agar and named it auxin. Phototropism turned out to be chemistry: light on one side shifts auxin to the shaded side, which grows faster, and the shoot curves.

Gravity is sensed by weight. In the root cap, special cells contain dense starch grains called statoliths that settle to the bottom like sand in a tilted jar; the cells read their position and redirect auxin, so a root laid flat turns down within hours. Gottlieb Haberlandt and Bohumil Němec proposed the idea independently in 1900, and experiments in orbit have confirmed it. Time is kept by an internal clock. In 1729 the French astronomer Jean-Jacques d'Ortous de Mairan shut a sensitive plant, Mimosa pudica, in a dark cupboard and found its leaves still folded at night and opened by day, the first evidence of a circadian rhythm in any organism.

Touch can be fast. Brush a Mimosa and its leaflets fold in a second as water rushes out of hinge cells at the leaf base. The Venus flytrap, Dionaea muscipula, from a few bogs in the Carolinas, shuts in around a tenth of a second, but only if two trigger hairs are touched within about twenty seconds; it counts, using electrical impulses that John Burdon-Sanderson recorded in 1873, at Darwin's urging, and found much like those of a nerve. Plants also smell: the parasitic dodder, Cuscuta, grows toward the scent of a tomato, and wounded sagebrush warns its neighbours with volatiles. The phytochrome pigment that measures red light doubles as a thermometer, a discovery of 2016. Mature sunflowers do not follow the Sun; only young plants swing east to west, and once in flower they face east for good.

Did you know? The ultrasonic clicks recorded from drought-stressed tomato plants in 2023, reported as plants "screaming", are most likely the pop of air bubbles breaking in water-starved stems; nobody is listening to them, and no plant is in pain.

Whether any of this amounts to intelligence is a live argument. In 2006 a group of researchers proposed a field of "plant neurobiology"; the next year 36 colleagues replied, under the title "no brain, no gain", that plants have signalling but no neurons, synapses or brain, and that the metaphor misleads. Most botanists side with the sceptics while conceding the plants' point: a rooted organism that integrates light, gravity, touch, scent, heat and time without a nervous system has solved the problem of knowing the world in its own way. The sensitive plant sensed the day long before anyone sensed it could.