The Vegetable Kingdom

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 .

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.

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.

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.

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.

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.

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…

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.

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.

"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.

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.

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.

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.

Every green thing alive today, from a pond's film of algae to a giant sequoia, runs on a chemical trick invented by bacteria. Cyanobacteria were the first organisms to split water with sunlight, using the energy to fix carbon dioxide into sugar and releasing oxygen as waste. Earlier photosynthesisers had used hydrogen sulphide or dissolved iron and gave off no oxygen at all. Water was everywhere, so the new chemistry had an inexhaustible feedstock, and its waste product would eventually remake the planet.

The next step was a merger. Around 1.5 to 2 billion years ago a single-celled eukaryote engulfed a cyanobacterium and, instead of digesting it, kept it. The captive became the chloroplast , and its descendants sit in every leaf. Konstantin Mereschkowski proposed this in 1905 and Lynn Margulis revived it in 1967; the chloroplast's own small circular genome settled the argument. From that one event descend the red algae, the green algae and all land plants.

Botanists now read this history in three overlapping archives: the rocks and their isotopes, the microfossils, and the genomes that carry the merger's signature in every cell. Oxygen in the air, iron ore in the ground and the chloroplast in a blade of grass are three outcomes of the same invention. Plants did not just colonise the Earth; their ancestors first had to make it breathable.

For nine-tenths of life's history the land was bare rock and crust. Water was the problem: a cell out of the sea dries, overheats and has nothing to float its sperm in. The plants that finally crept ashore were descended from freshwater green algae, and they solved the problem one invention at a time.

The first plant with a body we can draw is Cooksonia , from around 430 million years ago in the Silurian. Named by William Lang in 1937 after the Australian palaeobotanist Isabel Cookson, it was a few centimetres tall, a forking stem with no leaves and no roots, each branch tipped by a capsule of spores. Specimens from Devilsbit Mountain in County Tipperary, Ireland, are among the oldest known. Some already had stomata , the adjustable pores that let a plant take in carbon dioxide while rationing water, and a waxy cuticle sealing the surface.

By the end of the Devonian, some fifty million years later, plants had leaves, roots, wood and seeds, and the first forests were drawing carbon dioxide out of the air. The rock weathering they drove helped cool the planet, and botanists suspect the nutrients they washed into the sea contributed to a mass extinction there. Every soil on Earth, and the oxygen in every breath, dates from the moment a scrap of pond weed learned to keep its water in.

Walk along any damp wall and you are looking at the oldest design for a land plant still in business. Bryophytes , the mosses, liverworts and hornworts, number around 20,000 species: roughly 12,000 mosses, 7,000 liverworts and a few hundred hornworts. They have no true roots, no wood and no internal plumbing worth the name, so they stay small, a few centimetres tall, and drink through their whole surface.

Their trick is endurance. Many mosses can dry out completely, stop all metabolism and revive within minutes of wetting. Syntrichia caninervis survives on the crusts of the Mojave and Gobi deserts. In 2014 a team led by Peter Convey regrew Chorisodontium moss from a frozen core on Signy Island in Antarctica that had lain in permafrost for more than 1,500 years. Hornworts such as Anthoceros keep a single large chloroplast in each cell and house nitrogen-fixing cyanobacteria in cavities, and Takakia , found on the Tibetan plateau, belongs to a lineage that split from other mosses around 390 million years ago.

Mosses were the first plants to green the land, and they never left. They cushion the rainforest canopy, cap the tundra and insulate the taiga, and their bogs are the planet's most efficient long-term carbon store. The smallest plants are doing some of the biggest work.

Trees were invented more than once, and the first attempts look odd to modern eyes. At Gilboa in New York State, quarrying in the 1850s and again in 2010 exposed the floor of a forest around 385 million years old: hundreds of root mounds of Eospermatopteris , a plant eight metres tall with a trunk like a palm's and a crown of fronds, but no true leaves at all. At nearby Cairo a slightly older site preserves the roots of Archaeopteris , the first plant to combine proper wood, a branching crown of fern-like fronds and deep roots. By the end of the Devonian it had spread across the world and built the first real forests.

Why so much coal? One popular idea is that fungi had not yet evolved the enzymes to digest lignin , so dead wood piled up unrotted. Palaeobotanists debate this; a 2016 study found that the distribution of coal matches wet tropical basins sinking fast enough to bury peat, not a gap in the fungal toolkit. Either way, the carbon was taken out of the air for 300 million years, and we have put much of it back in two centuries.

The coal forests are gone, but their legacy is in every power station chimney and every fern unfurling in a shaded gully. We are still living on the carbon they buried.

Before the seed, every land plant had to mate in the wet. Mosses and ferns release sperm that swim to an egg through a film of water, and their spores leave with no food and no protection. The seed changed the rules. It wraps an embryo in a food store and a tough coat, lets it wait, and sends it off with a packed lunch.

Inside a modern seed sit three generations. The seed coat and the nucellus belong to the mother plant. The embryo, with its root tip, shoot tip and one or two seed leaves, is the child. The food store is the odd one out: in a pine it is the tissue of the female gametophyte, while in flowering plants it is endosperm , a tissue formed by a second fertilisation that belongs to neither parent exactly. That is what you eat in a grain of rice. Fertilisation itself no longer needs water. Pollen carries the male cells through the air and grows a tube to the egg, which is why conifers and grasses can breed in the dry.

The seed is the reason plants could cross deserts and mountains, the reason there are forests on dry continents, and the reason humans have stores, granaries and agriculture at all. A field of wheat is a field of embryos, each one carrying the Devonian's best idea.

When the coal swamps dried out at the end of the Carboniferous, seed plants inherited the Earth. The gymnosperms , plants with naked seeds borne on cones or stalks rather than inside a fruit, dominated the Permian and the whole Mesozoic. Glossopteris , a seed fern whose tongue-shaped leaves litter rocks in India, Australia, Africa, South America and Antarctica, was among the clues that persuaded Alfred Wegener the continents had once been joined. Around 1,000 gymnosperm species survive, in four lines.

Cycads , around 350 species, look like palms but are nothing of the kind. Their cones are pollinated by beetles and thrips that the plant lures with warmth and scent, and their roots host cyanobacteria. They are often called living fossils from the age of dinosaurs, yet DNA shows that today's species mostly arose in the last 12 million years; the lineage is old, the species are not. The loneliest plant in the world is a cycad: every Encephalartos woodii in cultivation is a cutting of a single male found in South Africa in 1895, and no female has ever been seen.

Flowering plants took most of the world from the gymnosperms, but not the high latitudes, the mountains or the long game. The cone, the resin and the fan-shaped leaf were here before the flower, and on present evidence they will be here after us.

On 22 July 1879 Charles Darwin wrote to his friend Joseph Hooker at Kew that the rapid appearance of the higher plants in the fossil record was 'an abominable mystery'. Flowering plants, or angiosperms , seemed to turn up in the mid-Cretaceous already diverse, with no obvious ancestors. Nearly 150 years later the mystery is smaller but not gone.

The living world offers a different clue. Amborella trichopoda is a sprawling shrub found only in the understorey of New Caledonia's rainforests. DNA places it as the sister of every other flowering plant, the first branch off the trunk, and its genome, sequenced in 2013, is a reference for what the ancestral flower might have carried. It has small creamy flowers, separate male and female plants, and wood without the water-conducting vessels that most angiosperms possess.

Molecular clocks suggest the angiosperm line split from other seed plants in the Triassic or even the Permian, long before the first fossil flower, so either the early flowers were rare and left no trace or the clocks run fast. Botanists still argue. Today around 300,000 of the roughly 390,000 known plant species are angiosperms, and almost everything on your plate, in your wardrobe and in your garden is one of them. Darwin's mystery was really the question of how the modern world began.

Split a bean and you find two fleshy halves; sprout a grain of maize and a single leaf spears up. The English naturalist John Ray noticed in 1682 that seedlings come in these two designs and used the number of seed leaves, or cotyledons , to divide flowering plants into two great groups. For three centuries botanists called them monocotyledons and dicotyledons, and the rule still holds up remarkably well.

The clearest signature is in the pollen . Monocots, like the gymnosperms before them, make grains with a single furrow. Eudicots make grains with three, and the first three-furrowed, or tricolpate, pollen appears in rocks around 125 million years old from Africa and Europe, giving the group a birthday the fossils can date. Two cotyledons turned out to be the ancestral state, shared with plants that are not eudicots at all.

The split still shapes the world. Monocots gave us the staple grains and the sugar cane; eudicots gave us timber, most fruit, pulses, cotton, coffee and nearly every vegetable in the greengrocer. A lawn and the hedge beside it belong to lineages that parted company more than 130 million years ago, and a gardener weeding one out of the other is sorting plants the way John Ray did, by counting leaves.

Grass is so ordinary that it is easy to forget how new it is. The grasses , family Poaceae, number around 11,500 species and cover roughly a third of the land, yet for most of the age of flowering plants they barely existed. Their rise reshaped landscapes, teeth and, in the end, human history.

The open grassland is a product of the Miocene. From around 20 million years ago, as the climate dried and cooled, grass-dominated plains spread across North America, Central Asia and Africa. The animals changed with them. Horses of the time grew hypsodont teeth, with high crowns that keep erupting to replace what wears away, because grass is full of silica and grit. Antelopes, rhinos and elephants did the same. Grasses, for their part, evolved to be eaten: they grow from meristems at the base of the leaf and the stem, so a grazing animal, a fire or a lawnmower removes the top and the plant simply carries on. Most other plants grow from the tip and stop when it is cut.

The savannas that spread across East Africa were the stage on which the first upright hominins walked, and when people began to farm, around 11,000 years ago, nearly everything they chose was a grass. Wheat, rice, maize, barley, sorghum and millet supply more than half of all the calories humans eat. The youngest great plant family is the one we depend on most.

For two centuries the plant family tree was drawn from the outside. Linnaeus sorted plants by counting stamens; later systems, from Bentham and Hooker in the 1860s to Cronquist in 1981, used flowers, fruit and leaves and the judgement of experts. The trees looked different depending on who drew them. In 1993 the view changed from the inside.

To keep the result usable, botanists formed the Angiosperm Phylogeny Group , an open committee that publishes a consensus classification: APG in 1998, revised in 2003, 2009 and 2016. It is the system behind the order of beds at Kew and the names in the floras. Amborella sits at the base, then the water lilies, then the star anise group, then the great mass of magnoliids, monocots and eudicots. The One Thousand Plant Transcriptomes project, published in 2019, extended the picture to algae and bryophytes using 1,124 species and found that land plants had doubled their genomes several times. Kew's Tree of Life, published in 2024, sampled 353 genes from around 9,500 species, covering nearly 60 per cent of all flowering plant genera.

The family tree now reaches from a charophyte alga to an orchid, and it is still being redrawn, partly by sequencing herbarium sheets collected two centuries ago. Names change because knowledge does. The tree is not a filing cabinet; it is a map of how the green kingdom actually happened.

Pull up a dandelion and you hold a taproot : one thick axis driving downward, with side roots branching from it. Pull up a tuft of grass and you hold the alternative, a fibrous root system of hundreds of thin strands of similar size spreading through the top of the soil. Both do the same jobs. They anchor the plant, they take up water and dissolved minerals, and they store food, which is why a carrot, a beetroot and a cassava are all swollen roots. Roots also hold the plant's reserve of patience: cut a dock to the ground and it returns from the crown of its taproot.

How much root does a plant have? In 1937 Howard Dittmer of the University of Iowa grew a single winter rye plant in a box of soil for four months and then counted. He found about 13.8 million roots with a combined length of around 620 km, and some 14 billion root hairs adding a further 10,600 km, more than the distance from London to Cape Town, all beneath one plant you could hold in your hand. Depth is the other surprise. Roots of honey mesquite, Prosopis glandulosa , were found 53 m down in an Arizona open-pit mine, and roots of the shepherd's tree, Boscia albitrunca , were traced to 68 m in the Kalahari.

All of it is hidden work. Roots are half the plant, sometimes more, and the half we never see decides whether the half we do see survives a drought.

Dig up almost any wild plant and the roots you find are not working alone. Around 90 per cent of land plant species pair their roots with soil fungi in a partnership called a mycorrhiza , from the Greek for fungus-root. The term was coined in 1885 by the German botanist Albert Bernhard Frank, who had been asked by the Prussian government to find a way of farming truffles and instead noticed that fungal threads wrapped every rootlet of the trees the truffles grew under. The fungus gathers phosphorus, nitrogen and water from a volume of soil far beyond the reach of roots, and the plant pays in sugar: typically 10 to 20 per cent of everything it makes by photosynthesis.

Bacteria are the other great partners. Peas, beans, clover and acacias form nodules on their roots that house rhizobia, bacteria able to turn nitrogen gas from the air into ammonia the plant can use. Hermann Hellriegel and Hermann Wilfarth demonstrated this in 1886: peas grown in sterilised sand starved unless a little unsterilised soil, with its bacteria, was added. Martinus Beijerinck isolated the organism in 1888. Inside a working nodule the plant makes leghaemoglobin, a relative of our own blood pigment, which keeps oxygen low and turns the nodule pink. Alder, sea buckthorn and the Australian she-oak do the same with a filamentous bacterium called Frankia .

What is not in doubt is dependence. A tree is a plant plus its fungi plus its bacteria, and most of the world's farmland runs on nitrogen fixed in nodules or on fertiliser that imitates them.

A stem holds leaves up to the light and flowers up to pollinators, and it is the plant's plumbing. Running its length are vascular bundles , each a cable of two tissues: xylem , dead hollow cells that carry water and minerals upward from the roots, and phloem , living cells that carry sugar from the leaves to wherever it is needed. Slice a sunflower or a bean stem across and the bundles form a ring near the outside, leaving a pith in the centre; this is the eudicot plan, and the ring is where a cambium can later make wood. Slice a maize stalk or a palm trunk and the bundles are scattered through the ground tissue like currants in a bun, the monocot plan, which is why a palm has no bark, no rings and cannot thicken once its trunk is formed.

The first person to measure what a stem does was an English clergyman, Stephen Hales, in Vegetable Staticks (1727). He sealed a sunflower in a pot, weighed it morning and evening and found it lost around 0.9 kg of water through its leaves on a hot day. He tied glass tubes to a cut grapevine and watched sap pressure from the roots push mercury up the tube, and he showed that water moved up the stem in the wood, not the bark. Marcello Malpighi had already found, in the 1670s, that stripping a ring of bark from a tree stopped sugar moving down but not water moving up, which is why ring-barking kills a tree slowly from the roots.

Whatever its shape, a stem is a bet on where the light will be, and the plant's plumbing is laid to serve it.

Wood is the stem's answer to time. In eudicot trees and conifers a thin sleeve of dividing cells, the vascular cambium , lies between the bark and the wood. Each season it lays down new xylem on its inner face and new phloem on its outer face, so a trunk thickens from a ring just under the bark while its centre is already dead. The xylem it makes is wood; the phloem, with the cork outside it, is bark. The cambium of a mature oak may add a layer only a few millimetres thick each year, yet over centuries that layer becomes a trunk two metres across.

Reading rings as a calendar was the idea of an astronomer. Andrew Ellicott Douglass, at Lowell Observatory in Arizona, began around 1904 matching ring patterns between pines to look for the sunspot cycle, and founded dendrochronology . By cross-matching the sequence of wide and narrow rings from living trees back through older and older beams, he could assign a calendar year to any ring. In 1929 a charred beam from Show Low, Arizona, closed the last gap in his sequence and gave dates to Pueblo Bonito and other ruins of the American Southwest. The method now reaches far further: bristlecone pines, Pinus longaeva , in the White Mountains of California, including living trees nearly 5,000 years old, give a continuous chronology of more than 9,000 years, and oak and pine preserved in the gravels of German rivers run back some 12,500 years. These chronologies calibrate radiocarbon dating…

Every plank is a diary. Cut a tree and you can read the weather of years nobody alive remembers.

A leaf is a solar panel built from the cheapest materials a plant can afford, and thrown away when it stops paying. The flat part is the blade , held out on a stalk, the petiole , with a network of veins that bring water in and carry sugar out. Cut the blade across and it is a sandwich. On top is a waxy cuticle over a single layer of transparent epidermis; beneath that stand the tall palisade cells, packed with chloroplasts and arranged like a crowd of upright bottles to catch the light; beneath those lies the spongy mesophyll , a loose honeycomb of cells with air spaces between them where carbon dioxide diffuses to the chloroplasts; and the lower epidermis closes the sandwich, punctured by stomata. A typical leaf is only a few tenths of a millimetre thick, because light does not usefully penetrate much further.

Leaf shape runs from needle to parasol. The raffia palm Raphia regalis of central Africa holds the record for the largest leaf, a compound frond up to about 25 m long; the giant water lily Victoria amazonica floats pads up to 3 m across on ribs strong enough to carry a child. At the other end, the tiny duckweed Wolffia is a green speck less than a millimetre long with no distinct leaf at all. Compound leaves, such as the pinnate frond of an ash or the palmate hand of a horse chestnut, divide the blade into leaflets that shed wind and heat better than a single sheet. Cactus spines are leaves reduced to defence; the tendrils of a pea are leaflets turned into hooks.

The leaf is where the plant meets the sky, and every detail of its design is about catching light while keeping water.

Every leaf is riddled with tiny mouths. A stoma (plural stomata, from the Greek for mouth) is a pore between two guard cells , kidney-shaped in most plants and dumbbell-shaped in grasses, which swell or sag to open or close the gap. There are commonly 100 to 1,000 of them in a square millimetre of leaf surface, mostly on the shaded underside, where the air is cooler and stiller. Across a single oak they number in the hundreds of millions. Through these pores every molecule of carbon dioxide a plant uses must enter, and through them nearly all the water a plant draws from the soil escapes as vapour: around 97 per cent of what the roots take up is lost this way.

The pores were first seen in the 1670s, when the microscope reached leaves. Marcello Malpighi in Bologna and Nehemiah Grew in London, working separately and publishing in rival volumes, both described them; Grew, in The Anatomy of Plants (1682), guessed that they let a leaf breathe out. Their role in water loss was settled only in the nineteenth century, and the potassium-pump mechanism in the 1960s and 1970s.

Today stomata sit at the centre of a global accounting. Together they return a large share of the rain that falls on land to the atmosphere, perhaps 40 per cent, which is why a forest cools its own region and why a drought that shuts them shows up in the carbon budget of the planet within weeks. The smallest valves on Earth help set the terms of its climate.

A coast redwood, Sequoia sempervirens , lifts water more than 100 m from its roots to its crown every day, with no pump, no moving parts and no energy spent on the lift. A column of water that tall presses down with about ten atmospheres, so no suction from the top could ever draw it up: a vacuum pump raises water barely 10 m. The answer, proposed in 1894 by Henry Dixon and John Joly in Dublin, is the cohesion-tension theory . Water molecules cling to one another by hydrogen bonds strongly enough that a thin thread of water in a narrow tube can be pulled, like a wire, without breaking. In a tree the pull comes from the leaves.

The pressures were made measurable by Per Scholander in 1965. His pressure chamber clamps a cut shoot with its stump protruding through a seal, and gas is pumped in until sap just appears at the cut; the gas pressure needed equals the tension the water was under. It is still the standard instrument of plant water research, carried into orchards and canopies in a steel bomb the size of a kettle. Measurements of stretched water columns and of xylem sap have confirmed the theory, and botanists no longer seriously debate it.

Every tree is a tightrope of water, and its height is the length of rope it can hold without the rope breaking.

Sugar made in the leaves must reach every root tip, bud, fruit and seed, and the leaf is often at the top. The tissue that carries it is the phloem , a living pipeline of sieve tubes : files of elongated cells whose end walls are perforated like a colander and whose contents, having lost nucleus and most organelles, are kept alive by a companion cell alongside. The sap they carry is syrup, commonly 10 to 30 per cent sucrose, moving at about half a metre to a metre an hour, which is slow beside xylem water but thousands of times faster than diffusion.

The proof came from an insect. An aphid feeds by threading its stylet into a single sieve tube and letting the pressure push sap into its gut; it taps a pressurised line. In 1953 John Kennedy and Thomas Mittler in Cambridge anaesthetised feeding aphids, cut them from their stylets and left the hollow mouthparts in place, where pure phloem sap went on oozing out for days to be collected and analysed. The stylet technique measured sap sugar, speed and pressure directly and confirmed that the tubes are under pressure, as Münch required.

Everything we harvest, from a potato to a date, was once sap in a sieve tube, delivered by pressure to the place the plant chose to grow.

At the tip of every shoot sits a dome of cells smaller than a pinhead, the shoot apical meristem , which never stops dividing. It builds the plant upward and, in bumps on its flanks, lays down the beginnings of every leaf. A tiny bud forms in the angle of each leaf, and most of those buds do nothing while the tip is alive. This is apical dominance : the growing tip exports the hormone auxin down the stem, and the flow holds the lateral buds in check. Pinch the tip out and the buds below burst into side shoots, which is the whole art of hedge-clipping and the reason a tomato grower nips out the top. Kenneth Thimann and Folke Skoog showed in 1934 that a dab of auxin on a decapitated stump keeps the buds asleep just as the tip did.

The pattern is older than the explanation. Karl Friedrich Schimper and Alexander Braun measured the angles in the 1830s. In 1868 Wilhelm Hofmeister gave a rule that still holds: each new primordium arises in the largest gap left by the previous ones, as far as possible from the youngest. In 1992 the physicists Stéphane Douady and Yves Couder dropped magnetised oil droplets at a steady rate onto a dish of oil, where each repelled the last; the drops arranged themselves in golden-angle spirals of their own accord. Botanists now trace the repulsion to auxin itself: each primordium drains the hormone from its surroundings, and the next can only form where enough remains.

A plant has no plan of itself; it has a tip that keeps going and a habit of filling the biggest gap.

A plant has no nerves and no brain, yet it bends to light, ripens a fruit on cue, drops its leaves on schedule and knows how tall to grow. The coordination is chemical. Five classical plant hormones , each made in minute amounts and carried in the sap or from cell to cell, run most of it, and each was found by someone puzzling over a plant that misbehaved.

Gibberellin came from a disease. Rice farmers in Japan knew the bakanae or foolish seedling disease, in which plants shoot up pale and spindly and fall over. In 1926 Eiichi Kurosawa showed that a filtered extract of the fungus responsible, Gibberella fujikuroi , made healthy seedlings do the same; Teijiro Yabuta's team crystallised the substance in the late 1930s. Plants turned out to make their own. Gibberellin lengthens stems, breaks seed dormancy and, sprayed on grapes, makes the big seedless berries of the supermarket.

Those genes, Reduced height-1 , came from a Japanese variety, Norin 10, by way of Norman Borlaug's breeding in Mexico in the 1950s, and were identified in 1999 as mutations in a protein called DELLA that normally restrains growth until gibberellin removes it; the mutant DELLA cannot be removed. The sd1 gene behind dwarf rice does the same job by cutting gibberellin synthesis. Newer hormones keep arriving: brassinosteroids in the 1970s and strigolactones, which control branching, in 2008.

Every meal, every forest and every lungful of oxygen comes back to one line of chemistry: six molecules of carbon dioxide and six of water, driven by light, become one molecule of sugar and six of oxygen. Written out, 6 CO2 + 6 H2O + light gives C6H12O6 + 6 O2. The equation looks like a schoolroom certainty, yet it took almost two centuries of stubborn, often eccentric experiments to assemble, one term at a time.

The last term was settled in a Californian laboratory. In 1941 Samuel Ruben and Martin Kamen gave the green alga Chlorella water labelled with the heavy isotope oxygen-18 and found the label in the oxygen bubbling out. The oxygen we breathe comes from split water, not from carbon dioxide. The sugar keeps the carbon, and the water donates the gas.

That reaction is the only large-scale route by which sunlight becomes food. Fossil fuel is ancient photosynthesis; bread, beef and cotton are recent photosynthesis. The willow in van Helmont's pot was quietly building itself from air, and so, by way of plants, are we.

Photosynthesis begins with an act of violence. Water is one of the most stable molecules on Earth, and the first thing a leaf does with sunlight is tear it apart. The machinery sits in the thylakoid membranes of the chloroplast, stacked like green coins, and it runs in two stages. The light reactions capture photons and turn them into chemical currency, ATP and NADPH ; the dark reactions, covered in the next chapter, spend that currency on carbon.

Here is how it runs. Photosystem II absorbs a photon at 680 nanometres and flings an electron away, leaving a hole so hungry that it pulls electrons from water. A cluster of four manganese atoms and one calcium, whose structure the Japanese team of Jian-Ren Shen resolved in 2011, cycles through five states and releases one oxygen molecule for every two waters consumed. The electron passes down a chain through plastoquinone and the cytochrome b6f complex to photosystem I , which boosts it with a second photon at 700 nanometres, high enough to reduce ferredoxin and make NADPH. Along the way protons pile up inside the thylakoid, and the gradient drives the turbine of ATP synthase , a rotary motor that Peter Mitchell's chemiosmotic theory of 1961 predicted and André Jagendorf confirmed in 1966 by making chloroplasts synthesise ATP in the dark with nothing but an acid bath.

Once the light reactions have charged the chloroplast with ATP and NADPH, the carbon work begins, and it is done by a cycle rather than a straight line. Three molecules of carbon dioxide enter; one three-carbon sugar leaves; everything else is regenerated to go round again. The route was mapped in a wooden hut at Berkeley by Melvin Calvin , Andrew Benson and James Bassham between 1946 and 1954, and it earned Calvin alone the Nobel Prize in Chemistry in 1961, a fact Benson resented for the rest of his long life.

The enzyme that does the bolting is RuBisCO , ribulose bisphosphate carboxylase-oxygenase, discovered as "Fraction I protein" in spinach by Sam Wildman in 1947. It is probably the most abundant protein on Earth: about half the soluble protein in a leaf, and an estimated 0.7 billion tonnes worldwide according to a 2019 census by Yinon Bar-On and Ron Milo. It is also astonishingly slow, fixing roughly three carbon dioxide molecules a second per active site, where typical enzymes manage thousands. Worse, it cannot fully tell carbon dioxide from oxygen, and perhaps one reaction in four at 25 degrees grabs oxygen by mistake, launching a wasteful salvage path called photorespiration . Plants cope by making more of it.

RuBisCO's habit of grabbing oxygen is a nuisance in a cool, damp English meadow and a disaster in the tropics, where heat raises the error rate and a plant that opens its stomata to breathe loses water by the litre. Twice over, evolution has answered with the same trick: concentrate carbon dioxide around the enzyme so that oxygen never gets a look in. One answer spreads the job across space; the other spreads it across time.

The cost is extra ATP; the reward is speed and thrift in hot, bright places. Maize, sorghum, sugar cane, millet and most tropical grasses are C4 plants, as are many amaranths and saltbushes. They make up only about three per cent of flowering plant species, around 8,000 in all, yet by estimates from Rowan Sage and colleagues they account for roughly a quarter of the carbon fixed on land each year. The trick has evolved independently more than sixty times, which suggests it is not hard to find once the climate demands it.

Both strategies tell the same story: RuBisCO is an ancient compromise, and the hottest, driest places on Earth have been quietly redesigning the leaf around its faults.

It is easy to imagine plants as the opposite of animals, taking in carbon dioxide while we give it out. The truth is that a plant breathes exactly as we do, in every cell, day and night, and the leaf's famous intake of carbon dioxide is only a daytime surplus laid over a constant background of exhalation. Nicolas-Théodore de Saussure measured plant respiration in the dark as early as 1804; what has changed since is our understanding of how much it costs.

Temperature sets the pace. The rate of respiration roughly doubles for every 10-degree rise, a ratio biologists call Q10 , and it climbs faster than photosynthesis, which saturates in heat. This is why a tropical night is costly, why a potato in a warm cellar shrinks and sweetens, and why warming forests are being watched so anxiously: a leaf's carbon balance, net gain by day minus loss by night, can tip towards zero when nights grow hot. In the light compensation point , the dim level at which a leaf's gain just matches its loss, lies the limit of every shaded forest floor.

A few plants turn respiration into heat on purpose. Roger Knutson measured the skunk cabbage in 1974; the Brazilian Philodendron selloum pushes its spadix to over 40 degrees; the sacred lotus of Asia holds its flowers at 30 to 36 degrees for days; and the titan arum of Sumatra warms its spike as it releases its stench of rotting flesh to lure carrion beetles. All use an alternative oxidase that dumps the energy of sugar straight into warmth. Every green thing is also, quietly, burning.

Air and water supply three of a plant's elements: carbon, hydrogen and oxygen, which together make up about 95 per cent of its dry weight. The remaining 5 per cent must be mined from the soil, and without it the big equation stalls. Botanists now recognise seventeen essential elements , defined by the test Daniel Arnon and Perry Stout set out in 1939: a plant must be unable to complete its life cycle without it, and nothing else can substitute. Nickel, the latest, joined the list only in 1987 when barley grown without it failed to make viable grain.

How we know comes from glass jars. In the 1860s Julius von Sachs in Germany and Wilhelm Knop raised plants to maturity with their roots in nothing but water and dissolved salts, then dropped one salt at a time and watched what went wrong. Hydroponics was born as a diagnostic tool, and the deficiency symptoms they catalogued still guide farmers: nitrogen shortage yellows the old leaves first, because the plant salvages it for new growth; iron shortage yellows the young leaves between green veins, the chlorosis familiar on rhododendrons planted in chalky soil, where iron is present but locked up at high pH.

Some plants have turned poison into property. Serpentine soils, rich in nickel, chromium and magnesium and poor in calcium, carry their own stunted floras from California to Cuba to the Balkans, and a few hundred hyperaccumulator species pack metals into their leaves at levels that would kill a cabbage, possibly as a defence against insects. Every bite of food, in the end, is seventeen elements dug from the ground.

Four fifths of every breath is nitrogen, and almost none of it is any use to a plant. The two atoms of the N2 molecule are held by a triple bond so strong that breaking it takes nearly a thousand kilojoules per mole, and no plant, animal or fungus has ever evolved the tool to do it. Only certain bacteria and archaea carry nitrogenase , an enzyme built around an iron-molybdenum cluster that is ruined by oxygen and must be shielded. Every protein and every strand of DNA in the living world depends on this narrow bottleneck.

The second way was built by chemists. In 1909 Fritz Haber in Karlsruhe squeezed nitrogen and hydrogen over an iron catalyst at high pressure and heat, and Carl Bosch at BASF scaled his table-top apparatus into a factory at Oppau that opened in 1913. The Haber-Bosch process now makes well over 100 million tonnes of nitrogen fertiliser a year, consuming around one to two per cent of the world's energy. Jan Willem Erisman and colleagues calculated in 2008 that roughly half the people alive are fed by its output; without it, the nitrogen in their bodies would never have left the air.

The Calvin cycle delivers a trickle of three-carbon sugar, and from that trickle a plant must build everything. Glucose is the common currency, and it is spent in three main ways: linked in pairs for transport, coiled in chains for storage, or bonded in straight ribbons for structure. The chemistry differs by the twist of a single bond, and the difference separates bread from timber.

For storage, glucose is strung into starch , in two forms: amylose , a mostly straight chain that coils into a helix, and amylopectin , a branched tree of chains that makes up around three quarters of the grain. Starch is laid down in granules with concentric growth rings; potato granules reach a tenth of a millimetre and were among the first objects Antonie van Leeuwenhoek drew. The classic test is a drop of iodine, which slides into the amylose helix and turns blue-black. Julius von Sachs used it in the 1860s to show that leaves make starch only in the light: he covered half a leaf with foil, left the plant in the sun, bleached the leaf in alcohol and painted it with iodine, and only the exposed half turned dark. Rice endosperm, wheat flour, potato, cassava, yam and banana are all starch stores raided by humans, and starch supplies well over half the calories eaten on Earth.

Sugar, starch and wall all begin as the same molecule, and the fate of a glucose is decided by one enzyme choosing one bond. Civilisation has run on that choice ever since the first loaf and the first plank.

A plant cannot run from a caterpillar, so it poisons it. Beyond the primary chemistry of sugar, protein and fat, plants make a vast catalogue of secondary metabolites , compounds with no role in growth whose purpose is to deter, sicken or kill. More than 200,000 are known, and the arms race that produced them, described by Paul Ehrlich and Peter Raven in 1964 as coevolution between plants and insects, has shaped a good part of the diversity of both.

Other families work differently. Tannins , phenolic compounds that make oak galls, tea and unripe persimmons astringent, bind proteins and make leaves indigestible. Glucosinolates in the cabbage family sit harmlessly in the cell until a bite mixes them with the enzyme myrosinase, releasing the hot mustard oils that flavour wasabi and horseradish; the cabbage white butterfly has evolved a protein that disarms the "mustard oil bomb", which is why its caterpillars eat brassicas untroubled. Cyanogenic glycosides , found in more than 2,500 species, release hydrogen cyanide when tissue is damaged. Bitter cassava, Manihot esculenta , the staple of 800 million people in Africa, Asia and South America, is one of them, and must be grated, soaked or fermented before it is safe; in drought years, when processing is rushed, outbreaks of the paralysis konzo follow in the Democratic Republic of Congo…

The smell of cut grass is a distress signal. Within seconds of a blade being torn, enzymes in the damaged cells turn membrane fats into a burst of six-carbon green leaf volatiles , chiefly the aldehyde known as cis-3-hexenal, and the scent that pleases a lawn-mower is, to an insect, news. Plants speak in chemistry, and their vocabulary is mostly terpenes , compounds built from five-carbon isoprene units that give pine its resin, mint its menthol, eucalyptus its cineole and lemon its limonene.

The idea that plants also signal to one another was first raised in 1983, when Ian Baldwin and Jack Schultz reported that poplar and sugar maple saplings in sealed chambers made more defensive tannins when neighbouring plants were damaged, and David Rhoades found something similar in willows near Seattle. "Talking trees" made headlines and then met hard criticism; the experiments were small and the chambers leaky, and for a decade the claim was treated as an embarrassment. The rescue came from the field. In the Great Basin desert of Utah, Richard Karban clipped sagebrush, Artemisia tridentata , and found that wild tobacco plants growing downwind suffered less damage from grasshoppers and cutworms over the following season, a result repeated many times since.

The modern verdict is careful. Plants do not "scream" and have no intent, but they emit, detect and respond to airborne chemistry, and neighbours, predators and parasites all listen in. The air over a meadow is full of messages, and we have only begun to read them.

A flower is a short shoot that has stopped growing and turned its leaves into something else. Botanists describe it as four whorls , rings of organs set one inside the next on a compressed stem, the receptacle. Outermost are the sepals, usually small and tough, which wrap the bud. Inside them sit the petals, the advertising. Then come the stamens , each a slender filament topped by an anther that makes pollen. At the centre stand one or more carpels , each with a sticky stigma, a stalk-like style and a swollen ovary holding the ovules that will become seeds. A buttercup has all four whorls in plain view; a grass flower has reduced them to scales; a tulip has sepals so petal-like that both whorls are simply called tepals.

Flowers rarely come singly. An inflorescence is the branching system that carries them, and its architecture is as diagnostic as the flowers: the spike of a plantain, the umbel of a carrot, the catkin of a hazel. The most deceptive is the capitulum of the daisy family. A sunflower head is not one flower but a platter of up to two thousand tiny florets. The outer ring of showy "petals" are sterile ray florets, each a single flower with its petals fused into one strap; the dark centre is thousands of disc florets, opening in spirals from the rim inwards over about a week.

Pollen is the plant kingdom's answer to a problem the first land plants never solved: how to get sperm to an egg without a film of water to swim through. A pollen grain is a tiny package, usually 20 to 50 micrometres across, holding a vegetative cell and, within it, a generative cell that divides to give two sperm cells. It is wrapped in two coats. The inner intine is cellulose; the outer exine is sporopollenin, one of the toughest organic materials known, resistant to acids, enzymes and heat, and sculpted into spines, ridges, pores and furrows that are specific to a family, a genus and often a species. A pine grain carries two air bladders for flight; a grass grain is a smooth sphere with one pore; a dandelion grain is a studded ball with three.

Because sporopollenin survives in mud for millions of years, pollen is also a record. In 1916 the Swedish geologist Lennart von Post showed a meeting in Kristiania, now Oslo, how counting grains layer by layer in a peat core revealed the forests that had grown above it since the ice retreated. Palynology was born. Cores from lakes and bogs on every continent now chart the return of birch and pine to Europe, the drying of the Sahara around 5,000 years ago, and the clearance of forest for farming. The ratio of tree pollen to grass and weed pollen is one of the most reliable ways to date when people first put a plough into a landscape.

Not every flower needs a visitor. Roughly one flowering plant in ten, and almost every conifer, throws its pollen to the wind, and the design shows it. Anemophilous flowers are small, dull and scentless, with no petals to speak of, long dangling anthers and feathery stigmas held out to comb the air. Hazel, Corylus avellana , hangs its male catkins in February before the leaves can get in the way; the female flowers are a tuft of red styles barely two millimetres long. Grasses, which cover a quarter of the land surface, are wind pollinated almost without exception, which is why a June hay meadow makes people sneeze. The method is wasteful. A single hazel catkin releases around four million grains; the chance of any one finding a stigma is minute, and the plants compensate with sheer numbers.

The third option is to pollinate yourself. Many plants do: thale cress, peas, barley and most wheat set seed from their own pollen, and violets even produce closed cleistogamous flowers in late summer that never open at all. Selfing is reliable and cheap, and a single seed can found a population, which is why so many weeds do it. But it comes at a cost, and the man who measured it was Charles Darwin. In 1876 he published The Effects of Cross and Self Fertilisation in the Vegetable Kingdom , reporting eleven years of greenhouse experiments. Crossed morning glories, Ipomoea purpurea , grew to an average of 86 inches against 65 for selfed ones, a ratio he gave as 100 to 76, and in most of his 57 species the crossed offspring were taller, heavier and more fertile. Inbreeding depression, as it is now called, is the price of a limited gene pool.

In 1793 a Berlin schoolmaster named Christian Konrad Sprengel published a book with a title that promised a great deal: Das entdeckte Geheimnis der Natur im Bau und in der Befruchtung der Blumen , the discovered secret of nature in the structure and fertilisation of flowers. Sprengel had spent years on his knees in meadows and had seen what no one had quite said: the colours, scents, nectar and spotted markings of flowers were not for our pleasure but for insects, and the insects, in exchange, carried pollen. He named the spots and streaks that point towards the nectar Saftmale , nectar guides. The book sold badly and he died poor, but Darwin read it, called it wonderful, and built on it.

Then there is sound. About eight per cent of flowering plants, including tomato, potato, aubergine, blueberry and cranberry, keep their pollen locked in anthers that open only by a pore at the tip. The way in is buzz pollination : a bumblebee grips the anther, disengages its wings and vibrates its flight muscles at a few hundred hertz, shaking the pollen out like salt from a cellar. Honeybees cannot do it, which is why every commercial tomato glasshouse from the Netherlands to Mexico now hires boxes of bumblebees, Bombus terrestris in Europe and Bombus impatiens in North America, a trade that began in Belgium in 1987.

In January 1862 Charles Darwin received a box of orchids from Madagascar. One of them, Angraecum sesquipedale , carried a nectar spur nearly 30 centimetres long with a few drops of nectar at the very bottom. He wrote that in Madagascar there must be moths with tongues able to reach it, and was mocked for it. In 1903 Walter Rothschild and Karl Jordan described a hawkmoth from the island with a proboscis of that length and named it Xanthopan morganii praedicta , the predicted one. The moth was finally filmed feeding at the orchid in 1992. The episode made the point that pollinators and flowers shape each other, and that a flower's design is a kind of portrait of its visitor.

Bats work the night shift in the tropics. Their flowers are large, pale, open at night, smell of sour milk or cabbage and hang clear of the foliage. The lesser long-nosed bat, Leptonycteris yerbabuenae , follows the flowering of columnar cacti and agaves up a "nectar corridor" from southern Mexico to Arizona each spring, and tequila's Agave tequilana is its crop. In Southeast Asia the cave nectar bat Eonycteris spelaea pollinates the durian; in Africa the baobab, Adansonia digitata , opens its white pendulous flowers for fruit bats at dusk.

Most flowers pay their pollinators. A sizeable minority do not. Of some 28,000 orchid species, about a third offer no nectar, no edible pollen and no reward at all, and get themselves pollinated by lying. The bee orchids of the genus Ophrys are the most notorious. Their lip is shaped, furred and coloured like a female solitary bee and, more importantly, scented like one: the flower makes the same blend of hydrocarbons that the female uses as a sex pheromone. A male attempts to mate with the flower, picks up two pollen packets on his head, and tries again on the next one. The French amateur Maurice-Alexandre Pouyanne worked this out watching the mirror orchid, Ophrys speculum , in Algeria, and published it in 1916; few believed him for decades. Sexual deception has since been found in hundreds of orchids from Europe to Australia, where some lure wasps and even male fungus gnats.

The carrion flowers lie about death. Rafflesia arnoldii of Sumatra and Borneo, a parasite with no leaves or stem, opens a flower nearly a metre across and up to 11 kilograms that smells of rotting meat and is pollinated by blowflies. The stapeliads of southern Africa, starfish-shaped succulents, pull the same trick at a hand's width. The European cuckoo-pint, Arum maculatum , goes further. Its spadix heats up by as much as 15 degrees on an April evening, volatilising a smell of dung that draws tiny owl midges down past a ring of hairs into a chamber. They are kept overnight, dusted with pollen as the male flowers open, and freed at dawn when the hairs wilt.

Flowering plants fertilise twice. When the pollen tube reaches the embryo sac it bursts and releases its two sperm cells. One fuses with the egg to make the zygote, the start of the embryo. The other fuses with the large central cell, which already carries two nuclei, to form a triploid tissue called endosperm . The embryo will become the plant; the endosperm is its packed lunch, and ours: the white of a rice grain, the flour of wheat and the flesh of a coconut are all endosperm. This double fertilisation was first seen in 1898 by the Russian botanist Sergei Nawaschin in lilies and fritillaries, and confirmed the next year in France by Léon Guignard. It is unique to flowering plants, and it means a plant invests in food for a seed only once that seed has been fertilised.

Some fruits manage without seeds. Parthenocarpy , fruit set without fertilisation, is the normal state of the cultivated banana, whose wild ancestors are full of hard black seeds. Seedless grapes such as Thompson Seedless are fertilised, but the embryo aborts early, leaving only a trace of seed. Seedless watermelons are sterile triploids; seedless citrus uses a mix of all three.

A seed that falls at its parent's feet grows up in its parent's shade, competing for the same light and water and sharing the same pests. Dispersal is the escape, and plants have more ways of leaving home than any animal has of travelling.

Water takes the coconut. Its husk is a buoyant fibre jacket and the seed can survive about three months afloat, which is how Cocos nucifera came to fringe almost every tropical beach before anyone planted it. Animals do the rest, by accident or design. A Eurasian jay, Garrulus glandarius , buries several thousand acorns each autumn and forgets enough of them to plant an oak wood; oaks spread north across Europe after the ice far faster than any acorn could roll. Fleshy fruit is a bribe: the seed passes through the gut and is dropped elsewhere with manure. In Africa and Asia elephants carry the large seeds of Balanites and other trees for tens of kilometres, and some Amazonian fruits, the avocado among them, seem to have been built for giant ground sloths and gomphotheres that vanished 10,000 years ago. Ants carry the seeds of more than 11,000 species, from bluebells to acacias, for the…

Dispersal decides where the next generation can be, and so it decides how fast a forest can move when the climate shifts. Oaks and jays managed about a kilometre a year after the ice. The climate is now moving faster than that.

A seed is a plant that has pressed pause. Inside its coat an embryo waits beside a store of food, dried to perhaps five per cent water, its metabolism almost at a standstill. Dormancy is the refusal to grow even when water and warmth are available, and it is one of the plant kingdom's most useful inventions, because the moment a seed falls is rarely the moment it should germinate. An acorn that sprouted in a warm October would face its first winter as a seedling; a desert annual that germinated after a single shower would die before the next one.

Some wait for smoke. Botanists in South Africa's fynbos knew that many of its shrubs germinated only after a bush fire, and in 1990 Johannes de Lange and Chris Boucher showed that smoke alone would do it. In 2004 a team in Perth led by Gavin Flematti identified the molecule: a small butenolide from burnt cellulose, now called a karrikin , from a Noongar word for smoke. Karrikins wake seeds across the kwongan of Western Australia and the Californian chaparral, and their receptor turns out to be ancient, present in plants that have never seen fire. Banksias go one step further and keep their seeds in woody follicles on the plant for years, opening only in the heat of a fire that has just cleared the ground of competitors.

Not every seed can wait. The acorn, the chestnut, cacao and mango are recalcitrant : they die if dried, and must grow within weeks. Which seeds can sleep, and for how long, now decides what seed banks can hold against extinction, and what the soil beneath a burned hillside can give back.

Sex is expensive. It needs a partner, a pollinator and luck, and it shuffles a set of genes that may already be perfectly suited to the spot. So many plants hedge, and some skip it entirely. A strawberry sends out runners , horizontal stems that root at the nodes and raise a daughter plant a hand's width from the parent. A tulip or an onion packs next year's shoot into a bulb of fleshy leaf bases; a crocus does the same with a corm of swollen stem; the potato stores it in a tuber, which is why every Maris Piper in Britain is genetically the same plant. Bamboo, ginger and bracken run underground rhizomes that can carry a single individual across a hillside. The Mexican hat plant, Kalanchoe daigremontiana , simply grows complete plantlets, roots and all, along the edges of its leaves and drops them.

Some plants fake sex. The common dandelion, Taraxacum officinale , goes through the motions of flowering and sets seed, but most European dandelions are triploid and produce seed by apomixis , without fertilisation, so every seedling is a copy of its mother. The result is hundreds of near-identical "microspecies" that have kept taxonomists arguing for a century. Hawkweeds and brambles do the same, which is why Gregor Mendel, who turned to hawkweeds after his peas, got results he could not explain.

A tree is not a kind of plant but a way of being one. Botany has no order or family called Trees; the habit, a tall woody stem lifting a crown above its competitors, has evolved independently dozens of times, in conifers, in oaks and eucalypts, in legumes, roses and mallows. The first to manage it, Archaeopteris , was a relative of the ferns that cast shade across the Devonian around 385 million years ago. What unites them all is secondary growth : a thin cylinder of dividing cells, the vascular cambium, lays down wood on the inside and bark on the outside year after year, so the stem thickens as it lengthens. Stiffen that wood with lignin and a plant can stand a hundred metres tall and live for millennia.

The exceptions prove the rule. A coconut palm, Cocos nucifera , is a monocot with no cambium at all; it reaches its full girth in a thickening zone just below the crown and never widens afterwards, which is why palms show no annual rings and why a wound in a palm trunk never heals over. Bamboos are grasses, hollow and jointed; the giant moso bamboo Phyllostachys edulis reaches its full height of 20 metres or more in a single spring, and culms of some species have been measured growing close to a metre in a day. The tree ferns of Australasia and the Andes hold themselves up with a sheath of fibrous roots. Dragon trees and the Joshua tree of the Mojave, both monocots, cheat with a second kind of cambium that botanists call anomalous.

Trees matter because the habit, once invented, reshaped the planet. They built the soils, slowed the rivers, drew down enough carbon to cool the Devonian and gave the rest of life a three-dimensional world to climb into. A tree is simply a plant that decided to stay, and to stay tall.

The oldest known single tree is a Great Basin bristlecone pine, Pinus longaeva , growing in the White Mountains of California at around 3,000 metres. Named Methuselah, it was cored in 1957 by Edmund Schulman, who counted its rings back to a germination date of about 2833 BC, which makes it close to 4,850 years old today. Its exact location is kept secret. Nearby a sibling called Prometheus was felled in 1964 by a graduate student with a permit and turned out to be older still, nearly 4,900 years. Bristlecones live so long by growing slowly in cold, dry, nutrient-poor dolomite where nothing can rot them, and by letting most of the trunk die back to a strip of living bark that feeds a few branches.

Size has its champions too. General Sherman, a giant sequoia, Sequoiadendron giganteum , in California's Sequoia National Park, holds about 1,487 cubic metres of trunk, the largest tree by volume, at 83.8 metres tall and over 11 metres across at the base. Hyperion, a coast redwood, Sequoia sempervirens , found in 2006 by Chris Atkins and Michael Taylor in Redwood National Park, is the tallest living thing at 115.9 metres; in 2022 the park closed its surroundings to visitors because trampling was damaging it. The tallest flowering plant is a mountain ash, Eucalyptus regnans , in Tasmania named Centurion, measured at 100.5 metres in 2018, and in Sabah a yellow meranti, Shorea faguetiana , climbed and taped in 2019 at 100.8 metres is the tallest tropical tree known.

These records matter because they are archives. Bristlecone rings underpin the calibration of radiocarbon dating; sequoia stumps record two millennia of fires; a dying baobab is a thermometer of a warming continent. The oldest trees have outlived the pyramids, and our job now is to let them outlive us.

A bare surface does not stay bare. On 27 August 1883 the volcano Krakatau blew apart and buried what was left of the island under hot ash and pumice tens of metres deep. Nine months later a visiting naturalist found a single spider spinning a web in the sterile rock. By 1886 there were ferns, grasses and blue-green algae; by 1897 a savanna of wild sugar cane; by the 1920s a closed forest of figs and casuarinas; and today a mixed forest of over 400 plant species in which not one was planted. Succession , the orderly replacement of one community by another, had run its course within a human lifetime.

Who decides the pattern was the great argument of twentieth-century ecology. Frederic Clements, writing from the Nebraska prairie in 1916, saw a community as a 'complex organism' developing towards a single climax fixed by climate. Henry Gleason replied in 1926 that a community was merely the species that happened to arrive and tolerate the site, an 'individualistic concept'. Modern ecology leans to Gleason, with a nod to Clements, and adds gap dynamics : Alex Watt showed in 1947 that a mature wood is a mosaic of patches of different ages, each started by a fallen tree.

The lesson is that forests are not finished things but processes. Every wood is at some point on a journey that fire, wind, flood or a single falling trunk can restart, and a 'natural' forest is simply one that has been allowed to keep walking.

Stand on the floor of a tropical rainforest and you are in a dim basement, with 1 or 2 per cent of the sunlight reaching you. The life is overhead. A mature rainforest sorts itself into strata : a shrub and sapling layer, an understorey of palms and slender trees, the main canopy at 25 to 45 metres where crowns interlock into a continuous roof, and above it the emergents , giants that break through into the wind, a Brazil nut in Amazonia, a kapok, or in Borneo a dipterocarp such as the yellow meranti measured at 100.8 metres in 2019. Most of the leaves, flowers, fruit and animals are in the top third.

Getting there became a discipline. Single-rope climbing techniques borrowed from cavers arrived in the 1970s; Francis Hallé's Radeau des Cimes , an inflatable raft lowered by airship onto the treetops of French Guiana in 1986, let scientists walk on the roof; and in 1990 the Smithsonian put a 42-metre construction crane in a forest park in Panama City, the first of a network that now spans Sarawak, Queensland, Switzerland and Germany, letting a gondola park above any crown. Walkways at Kakum in Ghana and Taman Negara in Malaysia opened the view to everyone.

The canopy is where a forest meets the sky: where photosynthesis happens, where rain is caught and where carbon is fixed. We built the cranes because the roof turned out to be the house.

Tropical rainforests cover about 6 per cent of the land and hold around half of its species, and most of their bulk is trees. The largest, the Amazon, spreads across nine countries and about 5.5 million square kilometres, and a census of 1,170 plots published by Hans ter Steege and colleagues in 2013 estimated that it holds some 390 billion trees of around 16,000 species. Yet half of those trees belong to just 227 species, the hyperdominants , led by the palm Euterpe precatoria and the rubber tree Hevea brasiliensis , while thousands of rare species are known from a single plot. A hectare near Yasuní in Ecuador can hold 650 tree species, more than the whole of North America.

The Congo basin, second in size at around 2 million square kilometres, keeps a different secret. In 2017 Simon Lewis and Greta Dargie mapped a peat swamp forest the size of England in the Cuvette Centrale, holding about 30 billion tonnes of carbon in its mud. Borneo's lowland forests are ruled by one family, the Dipterocarpaceae, whose winged seeds spin down from emergents 80 metres high and whose trees fruit almost all together once every two to ten years in mast events cued by El Niño droughts, flooding the forest with more seed than the bearded pigs can eat. In Australia's Daintree the oldest rainforest lineage on Earth has stood for perhaps 180 million years.

The rainforest is the planet's greatest living engine, and it runs on its own rain.

North of the deciduous woods, around the top of the world, runs the largest forest on Earth. The taiga , or boreal forest, covers about 17 million square kilometres of Canada, Alaska, Scandinavia and Russia, roughly a third of all forest and a tenth of the land, and it manages with a handful of species: spruce, pine, fir, larch, birch and aspen. The conifers are shaped for winter. A spire crown and down-sloping branches shed snow before it breaks them; needles with a waxy skin and sunken stomata lose little water when the ground is frozen and water is unobtainable; a dark colour and an evergreen habit catch every hour of a short spring.

Farther south, where winters are milder and wetter, broadleaves take over: oak, beech, maple, hornbeam and lime in Europe, oak and hickory, maple and beech in eastern North America, beech and oak again in Japan and Chile. Their trick is to throw away the leaf. As days shorten, a layer of cells at the leaf base, the abscission zone, weakens; chlorophyll is dismantled and its nitrogen withdrawn into the twig, exposing yellow carotenoids that were there all summer, while sugar maples and some oaks make fresh red anthocyanins on cold bright days. New England's fall colour is the sight of a tree salvaging its assets. In the Southern Hemisphere the Nothofagus beeches of Patagonia do the same.

Temperate and boreal forests are the lungs of the seasons: each northern summer the global carbon dioxide curve dips as they leaf out, and rises again as the leaves fall. Watch the Keeling curve and you are watching the north breathe.

Fire is a weather the forest expects. A lodgepole pine, Pinus contorta , of the Rocky Mountains holds many of its cones shut for years under a seal of resin that melts at around 45 to 60 degrees Celsius; only the heat of a crown fire opens them, and the seed falls onto freshly cleared ash. The strategy is called serotiny , and it was invented separately by the banksias of Australia, whose woody follicles open in the flames, and by jack pines, cypresses and the giant sequoia, whose seedlings barely survive except on burnt mineral soil. Other trees cheat death instead. Eucalypts keep dormant epicormic buds beneath thick bark and a swollen lignotuber at the base, and a blackened gum can be green again within weeks, while cork oaks and longleaf pines simply insulate themselves and wait.

People have been part of fire regimes for tens of thousands of years. Aboriginal Australians practised what the archaeologist Rhys Jones in 1969 called 'fire-stick farming': frequent cool burns in the right season that cleared travel routes, flushed game, encouraged yams and grass and kept fuel from building up. Bill Gammage's The Biggest Estate on Earth argued in 2011 that the first Europeans saw a continent that was, in effect, a managed park. In California the Karuk and Yurok burned oak woodlands for acorns and basketry plants until the practice was outlawed in 1850, and only in recent years has the state begun to welcome it back. A century of suppression in both countries let fuel pile up, and the fires of the twenty-first century are the bill. Australia's Black Summer of 2019 to 2020 burned around 24 million hectares and killed or displaced an estimated three billion vertebrates;…

A tree is a pump with no moving parts. Sunlight evaporates water from the leaves through their stomata; the loss pulls more water up the trunk; the roots draw it from the soil. A mature oak in full leaf can move several hundred litres a day this way, a big eucalypt or a Sitka spruce more, and almost all of it is not used but transpired , breathed into the air. Multiply by a forest and the numbers become geographic. Antonio Nobre's estimate for the Amazon is around 20 billion tonnes of water a day lifted into the sky by its trees, more than the river itself delivers to the Atlantic.

Some forests drink from the sky directly. In the cloud forests that wrap tropical mountains between about 1,500 and 3,000 metres, at Monteverde in Costa Rica or on Kinabalu in Borneo, trees comb fog from the air with mossy branches and epiphytes, and this 'horizontal precipitation' can add a third again to the measured rainfall. California's coast redwoods do the same: Todd Dawson showed in 1998 that in a dry summer around a third of a redwood's water, and two thirds of the water of the ferns beneath it, arrives as fog drip . When Monteverde's cloud base lifted in the warming of the 1980s and 1990s, the golden toad vanished and the trees began to dry.

Trees are the land's way of keeping its rain. Cut the forest and the river runs brown in the wet season and dry in the dry; keep it and the rain keeps coming back. Rain, it turns out, is partly grown.

Half of a dry log is carbon. Take the water out of wood and what remains is cellulose, hemicellulose and lignin, about 50 per cent carbon by mass, every atom of it pulled from the air as carbon dioxide. Add it up across the world's 4 billion hectares of forest and the figure, published by Yude Pan and colleagues in 2011 from inventories on every continent, comes to around 861 billion tonnes of carbon: 363 in living trees, 73 in dead wood, 43 in litter and 383 in the top metre of soil. The atmosphere at the time held about 830 billion tonnes. Forests, in other words, store as much carbon as the sky.

The surprise was the old-growth. Textbooks assumed that an ancient forest was carbon-neutral, growth balanced by decay. In 2008 Sebastiaan Luyssaert and colleagues gathered data from 519 plots of forest between 15 and 800 years old and found that stands continued to accumulate carbon well past their first few centuries, much of it in soil and dead wood. The argument that felling old forest and replanting is carbon-neutral does not survive the figures. Where the carbon sits also matters: in the Amazon most of it is in the trees, but in the boreal zone and in peat swamps the soil holds several times more than the wood above it, and a thawed or drained peatland becomes a source .

What this means is simple and hard. Forests are doing a third of our carbon clean-up for free, but they are a sink with a limit and a sponge that can be wrung out. Planting is useful; not felling is more so; and the only reliable way to keep carbon in wood is to keep the wood standing.

For most of history people did not clear woods so much as farm them. Cut a hazel, ash or oak to the ground and it does not die; the stump, or stool, throws up a thicket of straight poles that can be harvested every seven to twenty years, indefinitely. Coppicing supplied firewood, charcoal, hurdles, tool handles and the wattle of wattle-and-daub for thousands of years, and some English stools, cut every generation since the Middle Ages, are among the oldest living things in the country. Bradfield Woods in Suffolk has records of coppicing from 1252 and is still cut on rotation. Where deer or cattle grazed, the cut was made above head height instead, and the result, a pollard , is why the beeches of Burnham Beeches and the oaks of Spain's dehesa have their stout trunks and knuckled crowns.

Hedgerows are forests drawn as lines. England has around 400,000 kilometres of them, roughly half of what stood in 1945, and the ecologist Max Hooper noticed in the 1970s that a 30-yard stretch gains about one woody species per century, so a hedge with ten species is probably a thousand years old. Oliver Rackham taught a generation to read woods the same way, by ancient woodland indicators : bluebell, wood anemone, dog's mercury and yellow archangel spread so slowly that their presence marks ground wooded since at least 1600.

Living with trees was never a matter of leaving them alone. It was craft: a bargain of cutting and regrowing, planting and waiting, that kept both parties alive. The woods we love most are the ones someone looked after.

Stand on a mountain in the tropics and you can walk through most of the world's vegetation in a day. In June 1802 Alexander von Humboldt and Aimé Bonpland climbed Chimborazo in Ecuador to about 5,900 metres, higher than anyone had recorded, noting with barometer and thermometer which plants grew at which height. The picture Humboldt published in 1807, the Naturgemälde , was a cross-section of the volcano with the plants written onto its slopes: palms and cacao at the foot, tree ferns and cinchona higher up, grasses, then lichens, then snow. It was the first serious attempt to show that climate, not chance, decides where a plant can live. Vegetation, he argued, was a readable map of the physical world.

Satellites turned the map into a moving picture. The Normalised Difference Vegetation Index , devised in 1973 from the first Landsat images, compares red light, which chlorophyll absorbs, with near-infrared, which leaves reflect, so that living foliage glows on the image. Forty years of these records show the Earth greening: a 2016 study led by Zaichun Zhu found leaf area rising over a quarter to a half of the vegetated land since 1982, driven by extra carbon dioxide, warming in the north and, as a 2019 analysis showed, tree planting and intensive cropping in China and India.

The green planet, in other words, is not a fixed mosaic but a set of boundaries that move with the weather. Humboldt drew the first map; the satellites are redrawing it every sixteen days.

A grassland is a forest that keeps being interrupted. Across the North American prairie, the Eurasian steppe, the South American pampas and the African savanna, there is usually enough rain for trees to grow, and yet the trees do not win. What stops them is a trio of pressures: a dry season, fire, and the teeth of grazing animals. Grasses shrug all three off because they grow from the base. The dividing tissue of a grass leaf sits at the bottom of the blade, protected at or below the soil surface, so a bison, a wildebeest or a prairie fire removes only the top and the plant simply resumes. A tree sapling, growing from its tips, is killed outright. Grasslands and savannas cover somewhere between a quarter and two-fifths of the land surface, depending on how much scattered tree cover one allows.

Fire and grazing have been tested, not assumed. At the Konza Prairie in Kansas, ecologists have burned fixed watersheds on schedules of one, two, four and twenty years since the early 1970s; where fire is withheld, shrubs and then trees creep in within decades. In the Serengeti the migration of some 1.3 million wildebeest mows the short grass plains, and their dung feeds it. The savanna's umbrella thorn trees, Vachellia tortilis , once Acacia , survive by growing above the browse line and trading their foliage with giraffes, which can reach five metres up and tolerate the thorns.

Grasslands are the biome we eat. Wheat, rice, maize and the pasture of every cow are grasses, and the soils they built feed the world. Keep the fire, keep the grazers, and the grass keeps the trees at bay.

A desert is defined by what it lacks, under 250 millimetres of rain a year, and its plants are defined by how they cope. There are three strategies. Succulents store water and ration it; drought-dodgers sleep through the dry years as seed; and a few extraordinary plants simply endure, decade after decade, on fog and patience. Deserts and dry shrublands cover around a fifth of the land, from the Sahara's nine million square kilometres to the cold Gobi and the fog deserts of Namibia and Chile.

The endurers are stranger. Welwitschia mirabilis of the Namib, described in 1859 by the Austrian botanist Friedrich Welwitsch, grows only two leaves in its life, which lengthen from the base, fray in the wind and sprawl across the gravel for a thousand years or more, drinking fog. In the Mojave Desert of California the creosote bush, Larrea tridentata , spreads outward as a ring of clones; in 1980 Frank Vasek measured the growth of one ring, King Clone, and put its age at about 11,700 years, older than agriculture.

Deserts are growing as the climate dries, and the plants that have mastered them are among the slowest to replace. A saguaro arm takes a human lifetime; a creosote ring took the whole of the Holocene.

Climb a thousand metres and the temperature drops by roughly six degrees Celsius, so a mountain compresses the world's climates into a single afternoon. Humboldt saw this on Chimborazo and Spanish settlers had already named the bands: tierra caliente at the foot, tierra templada , tierra fría and the tierra helada of frost and snow. The sharpest line is the treeline . Christian Körner of Basel has measured it on every continent and found it falls where the mean temperature of the growing season is around six to seven degrees; below that, a tree's roots and growing tips cannot build tissue fast enough to be worth the trunk. Above the treeline the forest ends abruptly or frays into krummholz , 'crooked wood', spruces and pines pruned by ice-laden wind into mats a few centimetres tall that may be centuries old.

On the equator's mountains the problem is not seasons but days. Olov Hedberg, who studied the Afro-alpine flora in the 1950s and 1960s, called it 'summer every day, winter every night'. Kilimanjaro's giant groundsels, Dendrosenecio kilimanjari , stand up to five metres on trunks lagged with their own dead leaves, and the giant lobelias, Lobelia deckenii , close their rosettes at dusk around a reservoir of water that freezes at the surface and shields the bud. The Andes have the most extravagant of all: Puya raimondii , a bromeliad of Peru and Bolivia that grows for around 80 years, then raises a flower spike up to 10 metres carrying thousands of flowers, sets millions of seeds, and dies.

The summit flora has nowhere higher to go. When the climate climbs past the peak, the plants that grew only there are gone.

North of the treeline the ground itself is frozen. Permafrost , soil that has stayed below zero for at least two years, underlies roughly a sixth of the exposed land of the Northern Hemisphere, and in summer only a thin skin of it, the active layer, thaws: a few tens of centimetres in Siberia, perhaps a metre in warmer sites. Roots cannot pass that floor, the water above it cannot drain, and the growing season lasts six to ten weeks. Yet the sun does not set for months, and plants use every hour of it.

Antarctica is harder still. Only two flowering plants grow there naturally, the Antarctic hair grass, Deschampsia antarctica , and the pearlwort Colobanthus quitensis , both confined to the Peninsula and nearby islands; the rest of the continent's plant life is moss, lichen and alga. Their spread since the 1960s is itself a thermometer, and surveys on Signy Island found both expanding faster in the 2010s than in the previous fifty years.

The tundra is the planet's cold store, and its plants are both the lid and the thermometer. As the shrubs march north, they tell us the lid is lifting.

Land that is wet for most of the year poses a plant one problem above all: roots drown. Waterlogged soil runs out of oxygen within hours, and every wetland plant is an answer to that. Some pipe air down through spongy tissue; some lift their roots above the water; some, in the acid bogs, give up on soil nutrients altogether and eat insects. Wetlands cover a few per cent of the land, yet they hold a wildly disproportionate share of its birds, its fish and its carbon.

Swamps are wetlands with trees. The bald cypress, Taxodium distichum , of the American South stands in water on buttressed trunks and sends up woody knees , conical growths from its roots whose purpose botanists still debate, since experiments have failed to show that they carry air. Papyrus, Cyperus papyrus , the sedge of the Nile from which Egypt made its paper, grows five metres tall in the Sudd of South Sudan, one of the world's largest swamps. Common reed, Phragmites australis , found on every continent except Antarctica, pumps oxygen to its roots through hollow stems and is the raw material of thatch from Norfolk to the marshes of Iraq.

Humans have drained wetlands for centuries, from the Dutch engineers who emptied the English Fens after 1630 to the sugar fields of the Everglades. Rewetting them is now one of the cheapest ways to keep carbon in the ground, and the reeds come back on their own.

Plants came out of the water 470 million years ago, and some have gone back. Freshwater offers everything a leaf wants, light and water together, but it withholds oxygen and carbon dioxide, which diffuse through water ten thousand times more slowly than through air. The answers are written in the anatomy. Submerged leaves are thin, often a few cells thick, with no waxy cuticle and no stomata. Floating leaves put their stomata on the upper surface, the only one that sees air. And nearly every water plant is riddled with aerenchyma , spongy tissue full of air channels that runs from leaf to root and keeps the buried parts breathing.

The sacred lotus, Nelumbo nucifera , taught a different lesson. Its leaves emerge spotless from the mud because water cannot wet them: in 1997 Wilhelm Barthlott and Christoph Neinhuis showed that the surface is tiled with microscopic wax-coated bumps, so a droplet sits on the peaks like a ball on nails and rolls off carrying dirt with it. The lotus effect now coats paints, glass and fabrics. Lotus seeds are as remarkable: in 1995 Jane Shen-Miller germinated one from a dry lakebed in north-east China that radiocarbon dating put at around 1,300 years old.

Freshwater is less than one per cent of the planet's surface and home to a startling share of its plants, and of the threats to them. Drain a lake and the floating world simply dries.

Five small regions of the world share a climate of mild wet winters and hot dry summers, and between them hold around a fifth of the world's plant species on about two per cent of its land. They are the Mediterranean basin itself; California and its chaparral ; central Chile's matorral ; the Cape of South Africa's fynbos ; and the kwongan and mallee of south-western and southern Australia. All sit on the western edges of continents between about 30 and 40 degrees of latitude, where cold ocean currents and winter storms meet summer high pressure.

Fire is the second architect. The summer scrub is dry, resinous and dense, and in every one of the five it burns on a cycle of decades. The plants expect it. Many banksias hold their seeds in woody cones that open only in the heat of a fire, a habit called serotiny ; many Cape and Australian species germinate only after smoke, and in 2004 a team in Perth led by Gavin Flematti isolated the chemical responsible, a family of compounds they named karrikins after the Noongar word for smoke. Cork oak survives by growing bark thick enough to be harvested; Californian chamise resprouts from a swollen root crown within weeks of a burn.

These five regions are also where people most like to live, and vineyards, suburbs and more frequent fire are squeezing them. Nearly all of the Mediterranean basin's original vegetation has been altered since antiquity, and the Cape's lowland fynbos is among the most threatened habitat on Earth. Winter rain, summer fire and a hard little leaf made the richest scrub in the world; keeping it means keeping the fire on its old schedule.

An island is an experiment with the lid on. Few plants arrive, so the ones that do find empty niches and run riot, splitting into forms that would never survive among mainland competitors. Nowhere shows this better than Hawaii, the most isolated archipelago on Earth. Its native flowering plants descend from perhaps 290 natural arrivals in five million years or more, roughly one successful seed every 20,000 years, carried by wind, sea or a bird's gut; about 90 per cent of the species that resulted grow nowhere else.

Islands make plants woody. Darwin noticed it in On the Origin of Species and Carlquist catalogued it: lineages that are small herbs on the mainland turn into shrubs and trees on islands, where there are no real trees to compete with. The Galápagos have Scalesia , fifteen species of daisy that grow into forests of trees up to 20 metres, the botanical counterpart of Darwin's finches. Saint Helena had its cabbage trees and the Saint Helena ebony, Trochetiopsis ebenus , a mallow turned shrub, given up for extinct until George Benjamin found two plants on a cliff in 1980 and propagated them. Socotra, off Yemen, has the dragon's blood tree, Dracaena cinnabari , an umbrella of leaves on a trunk that bleeds red resin, one of about 300 endemic species on an island of 825.

Islands hold a disproportionate share of the world's plant species and of its extinctions: goats, rats, pigs and introduced weeds have done to the island floras what competitors never could. The experiment is not over, but we have taken the lid off.

A city is a new biome, barely a few thousand years old, and plants have already moved in. It is warmer than the country around it by one to three degrees on average and far more at night, a heat island of brick and tarmac. It is drier, more trampled, richer in nitrogen, split into thousands of tiny patches, and planted with species from every continent. For some plants it is a disaster; for a surprising number it is paradise.

Disturbance is the city's gift. When the Blitz opened thousands of bomb sites across London in 1940 and 1941, rosebay willowherb, Chamerion angustifolium , once an uncommon woodland plant, carpeted the rubble in magenta so completely that Londoners called it bombweed; Edward Salisbury, director of Kew, catalogued well over a hundred species on the bombed ground, including buddleja, Buddleja davidii , a Chinese shrub introduced in the 1890s that now seeds itself along every railway cutting and into every neglected wall in Britain.

Cities now cover some three per cent of the land and hold more than half of humanity, and the plants in them matter out of proportion: street trees cool pavements by several degrees, green roofs slow stormwater, and a buddleja on a wall feeds the butterflies of an entire street. The city is the biome we are building; we might as well build it green.

Farming was not invented once. It was invented at least seven times, on different continents, by people who had never heard of one another, within a few thousand years of the last ice age ending. The earliest well-dated centre is the Fertile Crescent , the arc of hills from the Levant through south-east Turkey to the Zagros, where einkorn, emmer wheat, barley, lentil, pea, chickpea, bitter vetch and flax, the eight founder crops, were being sown and harvested by around 11,500 years ago. Rice followed on the Yangtze and millet on the Yellow River, maize in Mexico, potato and quinoa in the Andes, taro and banana in the highlands of New Guinea, sorghum and pearl millet in the African Sahel, sunflower and squash in eastern North America.

The idea of multiple centres belongs to Nikolai Vavilov, who between 1916 and 1940 led expeditions to 64 countries and noticed that each crop was most varied in one region, its probable homeland. He named eight centres in 1935. Modern genetics has moved some lines and added others but kept his map: Zea mays in the Balsas valley of Mexico around 9,000 years ago, Oryza sativa on the lower Yangtze about the same time, and at Kuk Swamp in Papua New Guinea drainage ditches for taro and banana dated to at least 7,000 years ago, a wet-tropics agriculture owing nothing to grain.

Wheat is the most widely grown crop on Earth, sown on around 220 million hectares, more land than any other plant, and harvested at roughly 790 million tonnes a year. It is also a genetic oddity. Bread wheat , Triticum aestivum , carries three whole genomes stacked inside one cell, from three different wild grasses, joined by two accidents thousands of years apart.

The rising is chemistry. Wheat grain stores proteins called glutenins and gliadins; wetted and kneaded they knit into gluten , an elastic net that traps the carbon dioxide from yeast. The D genome brought the particular glutenins that make a loaf stand up. Durum, Triticum durum , a hard tetraploid descended from emmer, lacks them, which is why it makes pasta and couscous but poor bread. The bread wheat genome was finally read in full in 2018 by an international consortium: around 16 billion base pairs, five times the human genome, with more than 100,000 genes.

More than half of humanity eats rice every day. Oryza sativa is harvested at around 780 million tonnes of paddy a year, nearly nine-tenths of it in Asia, and it is the only major cereal grown standing in water. The water is not a need but a trick. Rice tolerates flooding because its roots carry aerenchyma , air-filled channels that pipe oxygen down from the leaves, while most weeds drown. A paddy is a weed-control system that happens to grow grain.

Terraces are the paddy's answer to slopes. The Ifugao terraces around Banaue in the Philippines, carved into the Cordillera and walled with stone and mud, climb to 1,500 metres and are watered by a canal system that gives every pool its turn. Their age is debated: tradition says 2,000 years, radiocarbon suggests several centuries. Either way they are engineering at the scale of a mountain range, maintained by hand.

No crop has changed more in our hands than maize. Its wild ancestor, teosinte , Zea mays subspecies parviglumis , is a bushy grass of the Balsas river valley in south-west Mexico whose "ear" is a spike of five to twelve kernels, each sealed in a stony case that passes unharmed through a deer. A modern maize ear carries 500 or more naked kernels on a cob that cannot shed them. For a century botanists doubted the two were even related.

Maize fed the Olmec, the Maya and the Aztec, and the Maya creation story in the Popol Vuh says the gods made people from maize dough. The dough had a secret. Maize is poor in available niacin , and a people living on it alone get pellagra, a disease of skin, gut and mind. Mesoamerican cooks soaked the grain in water with lime or wood ash, a process called nixtamalisation , which loosens the hull and frees the vitamin. When maize crossed to Europe and the American South without the ash, pellagra followed: in the United States it caused around 100,000 deaths in the early twentieth century before Joseph Goldberger traced it to diet in 1914.

In 1972 the historian Alfred Crosby gave a name to the greatest reshuffling of living things since the continents drifted apart. He called it the Columbian Exchange : the two-way traffic of plants, animals and diseases between the Old World and the New that began with the voyages of 1492. Westward went wheat, rice, sugar cane, coffee, bananas, citrus and grapes, with horses, cattle and smallpox. Eastward went maize, cassava, sweet potato, tomato, chilli, cacao, groundnut, pineapple, vanilla, tobacco and the potato. Within two centuries there were chillies in Sichuan and Goa, tomatoes in Naples, maize in the Balkans and West Africa, and cassava across the Congo basin.

The potato yields more calories per hectare than any grain and grows in cold wet places where wheat fails. Economists Nathan Nunn and Nancy Qian calculated in 2011 that it accounts for roughly a quarter of the Old World's population growth between 1700 and 1900. It also showed the danger of a single crop. Ireland by 1840 was growing one variety, the Lumper, on which three million people depended; when the water mould Phytophthora infestans arrived in 1845, the Great Famine killed around a million and drove more than a million to emigrate.

Grain is only a third of a meal. The rest of what farming grows falls into three families: the pulses , seeds of the pea family that carry protein; the roots and tubers , swollen storage organs that carry starch; and the oil crops , whose seeds and fruits carry fat. Each has its own history, and the first family carries a trick that keeps the others fed.

The roots feed the tropics. Cassava , Manihot esculenta , from the southern Amazon, tolerates drought and poor soil and keeps in the ground for two years, but its tissues hold cyanide-releasing compounds that must be grated, soaked or fermented away. Around 800 million people depend on it. West Africa's yam belt, led by Nigeria, grows most of the world's Dioscorea yams. And the sweet potato, Ipomoea batatas , poses a puzzle: an American plant, it was growing in Polynesia centuries before Europeans arrived, and its Polynesian name, kumara , resembles words in Quechua. Whether seafarers carried it or a seed floated, botanists still debate.

Plant a pip from a Bramley apple and you will not get a Bramley. Apples are self-incompatible and extravagantly heterozygous, so every seed is a new and usually mediocre individual. The orchard's answer is three thousand years old. Grafting joins a cutting of the tree you want, the scion , to the roots of another, the rootstock , so that their cambium layers knit and the scion grows on as a clone. Grafting was described by Theophrastus around 300 BC, and some scholars read it in Mesopotamian texts from around 2000 BC. Every Bramley descends from one seedling planted by Mary Ann Brailsford in Southwell, Nottinghamshire, in 1809; every Granny Smith from a chance tree in Maria Ann Smith's Sydney garden in 1868. The rootstock sets the size: the dwarfing M9 stock, selected at East Malling in Kent from 1912, is why modern orchards are rows of trees you can pick without a ladder.

Other fruits are cloned too. The Cavendish banana , which makes up almost all of the world's export trade, is a sterile triploid propagated from suckers, descended from a plant grown in the 1830s at Chatsworth House by Joseph Paxton. Its predecessor, Gros Michel, was wiped out as an export crop by the 1960s by the soil fungus Fusarium known as Panama disease, and a new strain, TR4, is now moving through Asia, Africa and since 2019 Latin America, with the identical Cavendish plants unable to resist it. Citrus tells a different story: genomes published in 2018 showed that every cultivated citrus descends from three wild ancestors, the citron, the mandarin and the pomelo. The sweet orange is a pomelo-mandarin hybrid, the lemon a citron crossed with sour orange, and the grapefruit arose in Barbados in the eighteenth century.

A field is a bank account. Every harvest withdraws nitrogen, phosphorus and potassium in the grain and straw, and a farm that only withdraws goes broke. Medieval Europe knew this dimly and left a third of its land fallow each year. The eighteenth-century answer was the Norfolk four-course rotation : wheat, then turnips, then barley, then clover, and round again. Turnips were hoed, which cleared weeds, and fed to sheep over winter, whose dung went back on the land. Clover, a legume, fixed nitrogen for the wheat that followed. Nothing lay idle. Charles "Turnip" Townshend, the viscount who retired from politics to his Norfolk estate in 1730, became its evangelist, and the rotation doubled yields in the counties that adopted it.

The nineteenth century's fertiliser was bird droppings. On the Chincha Islands off Peru, where it never rains, cormorants and boobies had piled guano up to 50 metres deep over millennia. Humboldt brought samples to Europe in 1804; by the 1850s Peru was exporting hundreds of thousands of tonnes a year, dug by Chinese indentured labourers, and in 1864 Spain seized the islands and started a war over them. The deposits were largely gone within decades, and the Haber-Bosch process would replace them.

For ten thousand years farmers bred plants by keeping the best seed. Breeding became a science in a monastery garden. Between 1856 and 1863 Gregor Mendel grew some 28,000 pea plants at Brno, crossing tall with short, round with wrinkled, and counting. His 1866 paper showed that traits pass as discrete units in predictable ratios, three to one in the second generation. Nobody noticed until 1900, when three botanists rediscovered his rules, and within a decade plant breeding had a theory.

Breeding needs raw material. Nikolai Vavilov built the world's first great seed bank in Leningrad from expeditions to 64 countries, around a quarter of a million samples. Arrested in 1940 for his genetics, he died of starvation in a Saratov prison in January 1943. That same winter, in the besieged city, his staff guarded the collection through the famine; several of them died of hunger at their desks surrounded by edible seed, and the rice curator Dmitri Ivanov was found among his sacks.

World cereal yields have roughly tripled since 1961 on only slightly more land, and famine in Asia receded. The bill was real too: a narrower gene pool as thousands of landraces vanished from fields, dependence on fertiliser, pesticide and irrigation, and aquifers under the Punjab now falling by a metre a year. Breeding gave us the harvest; the seed banks hold what it cost.

The world passed eight billion people in November 2022, and almost all of them eat plants that fit on one hand. Of some 390,000 known plant species, humans have cultivated at least 6,000. Today about 200 are grown at any scale, around 15 crops supply 90 per cent of our plant calories, and three grasses, rice, wheat and maize, supply more than half. The funnel is the central fact of modern agriculture, and both its strength and its danger.

How much does a hectare feed? Wheat at the world average of about 3.5 tonnes a hectare yields around twelve million kilocalories, enough for a dozen people for a year; a good Dutch or Egyptian field at nine tonnes feeds thirty. Arable land works out at about 0.18 hectares per person on Earth, roughly a quarter of a football pitch. The yield gap , the difference between what a field produces and what it could with known methods, is the biggest unused resource in farming: maize in sub-Saharan Africa averages around two tonnes a hectare against eleven in the United States, and the Global Yield Gap Atlas maps the difference field by field.

Plants cannot run from what eats them, so they fight with chemistry, and a molecule that stops a caterpillar's nerves or a fungus's enzymes will often do something interesting inside a human body too. For most of history that was medicine: bark, root and leaf, chosen by trial and remembered by tradition. The modern story begins when chemists learned to pull a single active compound out of the brew. In 1804 Friedrich Sertürner, a pharmacist's apprentice in Paderborn, isolated a crystalline alkaloid from opium poppy latex and named it morphine after the god of dreams. In 1820 Pelletier and Caventou extracted quinine from the bark of the Andean cinchona tree, the 'Jesuits' bark' that had been curing malarial fevers in Europe since the 1630s.

The twentieth century widened the net. In 1962 American collectors sent bark of the Pacific yew, Taxus brevifolia , to a screening programme; Monroe Wall and Mansukh Wani isolated paclitaxel, sold as Taxol, which jams a dividing cell's spindle and became a mainstay against ovarian and breast cancer, now made from the needles of European yew hedges rather than by felling the tree. From the Madagascar periwinkle, Catharanthus roseus , came vincristine, which turned childhood leukaemia from a near-certain death into a mostly curable disease.

Roughly a quarter of prescription medicines trace their chemistry to a plant, and the hunt continues in screening labs, herbaria and the knowledge of communities who used these plants first. Every green leaf is a chemistry set that has been running experiments for four hundred million years; we are only beginning to read the results.

Three plants from three continents supply most of the world's daily stimulant, and all three make the same molecule for the same reason. Caffeine is a defence: it poisons the nerves of insects that chew the leaf or seed, and leaf litter laced with it suppresses rival seedlings. Humans, with larger bodies and a taste for alertness, turned the poison into a habit. Tea is Camellia sinensis , a shrub of the hills where China, Myanmar and India meet; coffee is Coffea arabica , an understorey tree of the Ethiopian highlands; cocoa is Theobroma cacao , a small tree of the Amazon basin whose seeds ferment inside a sweet pulp.

Coffee was a Sufi drink in fifteenth-century Yemen, exported through the port of Mocha and guarded as a monopoly, before the Dutch carried seedlings to Java in the 1690s and a single plant in the Amsterdam botanic garden seeded the plantations of the Americas. London's first coffeehouse opened in 1652; Lloyd's of London began as one. Today Brazil and Vietnam lead, the latter growing mainly the hardier Coffea canephora , or robusta.

Between them these three plants employ tens of millions of smallholders and shape the mornings of billions. A chemical made to kill insects became the most widely used psychoactive drug on Earth, and we grow it on purpose.

Before plastics, almost everything soft that was not wool or silk came from a plant cell wall. A plant fibre is a long cell, or a bundle of them, whose walls are packed with cellulose, the straight-chained glucose polymer that is the most abundant organic molecule on Earth. Cotton is the simplest case: each fibre is a single cell growing from the skin of the seed, up to four centimetres long and about 95 per cent cellulose, which twists into a flat ribbon as it dries and so catches on its neighbours when spun. Flax, hemp and jute are bast fibres, bundles running the length of the stem just inside the bark, freed by retting, which lets bacteria rot away the glue that holds them.

Linen is older still. Wild flax fibres, spun and some of them dyed, were found in Dzudzuana Cave in Georgia and dated to around 34,000 years ago. Linum usitatissimum wrapped Egyptian mummies in thousands of metres of cloth, and the Tarkhan dress, a pleated linen shirt of around 3000 BC, is the oldest woven garment in the world. Hemp, Cannabis sativa , gave rope and sailcloth; a ship of the line carried tens of tonnes of it, and Henry VIII ordered English farmers to grow it in 1533. Jute, from Bengal, made the sacks that carried the world's grain and turned Dundee into 'Juteopolis'; sisal, from a Yucatán agave, made the binder twine of the harvest.

Wood is the oldest engineering material and still the most used by weight: around four billion cubic metres are cut each year, roughly half of it burned for fuel. It works because a tree is a column of hollow cellulose tubes glued with lignin , stiff along the grain and light for its strength. Looked after, it lasts. The five-storey pagoda of Hōryū-ji near Nara stands on hinoki cypress felled around 594 AD, according to its tree rings, making it the oldest wooden building in the world. HMS Victory , launched in 1765, took about 6,000 trees, nine in ten of them oak, the harvest of a hundred acres of woodland. The roof of Notre-Dame de Paris, framed from some 1,300 oaks in the thirteenth century and nicknamed 'the forest', burned in April 2019 and was rebuilt in oak by 2024.

Different woods do different jobs. Bamboo, a grass, scaffolds skyscrapers in Hong Kong, lashed in a day by hand. Balsa, the lightest commercial timber, cores wind-turbine blades. Teak resists the sea, Douglas fir makes the beams, and the hinoki of Japanese temples is replaced plank by plank in a cycle of repair that has never stopped. The newest form is the oldest idea: cross-laminated timber glues layers of boards at right angles into panels that behave like concrete slabs but weigh a fifth as much and store carbon rather than emit it. Mjøstårnet in Norway reached 85 metres in 2019, and timber towers now rise in Milwaukee, Sydney and Vienna.

A rubber tree bleeds when cut, and the white latex that wells up is a defence, gumming the mouthparts of insects and sealing the wound. The Olmec, whose Nahuatl name means 'rubber people', were bouncing balls of it by 1600 BC, but Hevea brasiliensis latex stayed a curiosity until 1839, when Charles Goodyear, broke and obsessive, found that cooking it with sulphur made a material that neither melted in summer nor cracked in winter. Vulcanisation turned sap into tyres. The Amazon boomed; Manaus built an opera house in 1896 on rubber money while tappers were worked to death along the Putumayo.

Resins are older defences. Conifers flood a wound with terpenes that set hard; distilled, they give turpentine and rosin, the 'naval stores' that waterproofed wooden ships. Left for forty million years, the same resin becomes amber, and the Baltic coast's amber, with its trapped insects and flowers, is a window into an Eocene forest. In the deserts of Oman, Yemen and Somalia, Boswellia and Commiphora trees give frankincense and myrrh, traded north since Hatshepsut's expedition to Punt around 1470 BC brought living incense trees home to Egypt.

Every one of these materials is a plant's way of healing or hiding, borrowed. The lesson of rubber, resin and indigo is the same: the chemistry is the plant's, and the fortunes move wherever the plant is carried.

A spice is a plant's chemical weapon that happens to please us. Piperine in pepper, eugenol in cloves, cinnamaldehyde in cinnamon bark and capsaicin in chilli all evolved to deter insects, fungi and browsing mammals, and all of them were, for most of history, worth more than their weight in silver because they grew in only one place. Black pepper, Piper nigrum , is a vine of India's Malabar coast; peppercorns were pushed into the nostrils of Ramesses II at his embalming in 1213 BC, and Pliny grumbled that India drained Rome of fifty million sesterces a year. True cinnamon, Cinnamomum verum , is the inner bark of a Sri Lankan tree, rolled into quills as it dries.

Vanilla, Vanilla planifolia , is an orchid from Mexico pollinated by a local bee, so outside its homeland the vines flowered and set nothing. In 1841 Edmond Albius, a twelve-year-old enslaved boy on Réunion, worked out how to lift the flap inside the flower with a sliver of bamboo and press the pollen home with his thumb. His method is still used by hand on every vanilla farm on Earth, most of them now in Madagascar. Saffron is the three red stigmas of Crocus sativus , a sterile triploid that has been divided by hand since antiquity; it takes around 150,000 flowers to make a kilogram, which is why it remains the most expensive spice by weight, and Iran grows most of it.

Spices built trade routes, companies and empires, and moved whole peoples by force. Today they cost a few coins at any market, which is the strangest thing of all: the flavours that once redrew maps now sit, unremarked, on every kitchen shelf.

People have always asked plants to stand for something. The lotus, Nelumbo nucifera , rises spotless from mud, its leaves shedding water so completely that engineers copy the surface, and so it became the seat of the Buddha and of Lakshmi; its seeds can wait a very long time, and one dug from a dry lakebed in Liaoning germinated at an age of around 1,300 years. The Bodhi tree at Bodh Gaya, under which the Buddha sat, is a sacred fig, Ficus religiosa . A cutting of it was planted at Anuradhapura in Sri Lanka in 288 BC, and that tree, the Sri Maha Bodhi, is the oldest tree in the world with a known planting date.

Flowers carry the softer meanings. The rose, bred from wild species across Persia and China, became the emblem of love, of Mary, and of two English royal houses whose civil war took its name. In Japan the cherry blossom marks the beauty of what does not last; Kyoto's courtly diaries record the date of hanami back to 812 AD, a record that now shows the trees flowering earlier than at any point in twelve centuries, a climate signal written in petals. Nearly every modern nation has adopted a flower: the dahlia for Mexico, the king protea for South Africa, the thistle for Scotland, the tulip for the Netherlands.

A plant means nothing by itself; we hang the meaning on it. But the meanings last, and they have protected sacred groves, ancient trees and whole species through centuries when nothing else would.