Flower

• 1. Introduction

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  A flower can be one tenth of a millimetre across, or it can stretch a full metre wide. The smallest belongs to duckweed. The widest is the corpse flower. Between those two extremes lies one of the most varied structures in all of plant life, built around the simple business of making a seed. Flowers are the reproductive structures of flowering plants, and they first appeared somewhere between 150 and 190 million years ago, in the Jurassic. What followed was a quiet takeover. Plants with flowers pushed non-flowering plants out of ecosystem after ecosystem, until roughly ninety percent of all living land plant species were angiosperms. How does a structure built from modified leaves manage to attract a moth, deceive a wasp, or ride the wind? Why did this one innovation prove so hard for older plants to compete with? And how did humans come to bury their dead beneath the same objects they steep into tea and pin to their national flags? The answers run through the flower's hidden architecture, its long partnership with the animals that visit it, and the genes that decide where each petal goes.

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• 2. Four Whorls

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  A stereotypical flower is built from four kinds of structures, arranged in sets called whorls, growing around the tip of a short stalk called a receptacle. From the base upward they are the calyx, the petals, the androecium, and the gynoecium. The outermost whorl holds the sepals, collectively the calyx. These are modified leaves with a broad base, pores, and green pigment. Sepals are often waxy and tough, growing quickly to shield the flower as it develops, and they frequently persist to protect the fruit afterwards. Just inside them sit the petals, collectively the corolla. These leaf-like structures are almost fibreless, often delicate and thin, and usually coloured or scented to encourage pollination. Some carry ultraviolet patterns invisible to human eyes but plain to a pollinator. Together the calyx and petals form the perianth, the flower's non-reproductive or vegetative part. The reproductive whorls come next. The androecium is the ring of male parts called stamens, each typically an anther of four pollen sacs connected to a stalk called a filament. At the centre sits the gynoecium, the female part, made of one or more carpels. Each carpel carries a stigma that receives pollen, a style that acts as a stalk, and an ovary holding the ovules. Carpels fused together are often called a pistil, and inside the ovary the ovules attach to the placenta by structures called funiculi.

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• 3. Endless Variation

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  Most plants build flowers with four whorls, yet they vary greatly between species in size, shape, and colour. The four main parts are defined by their positions, not their functions. Many flowers lack some parts entirely, or carry parts modified to look like another part. In roses grown under cultivation, stamens and sepals can be reshaped to resemble petals, producing the crowded, many-petalled blooms people find more attractive. Symmetry sets flowers apart from one another. A flower cut through its central axis from any point into matching halves is called regular, an example of radial symmetry seen in sedges. A flower with only one plane of symmetry is called irregular, as in orchids. In very rare cases a flower has no symmetry at all and is called asymmetric. This matters because floral symmetry is one of the main features shaped by flower-plant coevolution. Irregular flowers often coevolve with specific pollinators, while radially symmetric ones attract a wider range of visitors. Sex distribution varies too. In most species a single flower carries both female and male parts, and is called perfect, bisexual, or hermaphrodite. Some plants make imperfect flowers with only male or only female parts. When both kinds appear on one plant, the species is monoecious. When a whole plant is either male or female, the species is dioecious. Look-alike structures complicate the picture further. Coronas are crown-like outgrowths, and pseudonectaries mimic the glands that produce nectar without holding any. When disease takes hold, leafy flower parts called phyllody may appear. Many flowers are not single flowers at all. A daisy or a sunflower is an inflorescence, a cluster of numerous tiny florets arranged to resemble one bloom, a formation known as a pseudanthium.

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• 4. The Language Of Colour

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  Against the mostly green body of a plant, flowers blaze with colour, and not only in their petals. Stamens, anthers, stigmas, ovaries, pollen, styles, and even nectar can carry pigment. These colours come mainly from biological pigments, molecules that absorb and hold energy from light, and they serve concrete purposes: protecting the plant against degradation and guiding pollinators toward it. Some colour is not pigment at all. Structural coloration arises when tiny surface structures interfere with waves of light. Certain tulips show iridescence this way, and edelweiss uses photonic crystals, diffracting light through minute grooves. Colour is not even fixed. In Viola cornuta, a colour change acts as a signal to pollinators. In the anthoxanthins of Hydrangea, colour shifts with pH, and it can also respond to temperature, metals, sugars, and cell shape. Colour does work that words might do elsewhere, including nectar guides that point pollinators toward a reward, sometimes visible only under ultraviolet light.

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• 5. Genes That Build A Bloom

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  A flower begins when vegetative growth turns into floral growth, a shift governed by both genetic and environmental factors. It starts at the shoot apical meristem, a group of dividing cells responsible for leaves and buds. From that meristem grows a growth-limited floral meristem, and from that the sepals, petals, male parts, and female parts emerge. The ABC model was the first unifying principle in the study of flower development, and its main ideas hold across most flowering plants. It describes three groups of genes. A genes alone produce sepals in the first whorl. A and B together produce petals in the second whorl. B and C together produce stamens in the third whorl. C genes alone produce carpels at the centre. An extended ABCDE model adds two more gene groups to explain structures like ovules. All of this sits inside a gene regulatory network of specialised MADS-box genes and their associated proteins. Timing is the hard part. The transition to flowering is one of the major phase changes in a plant's life cycle, and it must happen when conditions favour fertilisation and seed formation. Plants read internal and environmental cues, including shifts in plant hormones such as gibberellins, seasonal temperature, and day length. Many plants, including those with two-year and longer lifespans, require cold exposure before they will flower. These signals are interpreted through florigen, produced in the leaves under favourable conditions and acting at stem tips to switch growth from leaves to flowers. Once open, some flowers close and reopen around dusk and dawn, and some track the path of the sun to stay warm, both behaviours run by the plant's circadian rhythm.

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• 6. Vectors And Visitors

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  Around eighty percent of flowering plants rely on biotic, or living, vectors to carry their pollen, the agents that move pollen between plants. Insects are the most common couriers, but the list runs much further: birds, bats, lizards, other mammals, snails and slugs, and in rare cases crustaceans and worms. Flowers pay for the service. Rewards include food such as pollen, starch, or nectar, along with mates, shelter, a place to raise young, and pseudocopulation. In that last strategy, sexual deception, a flower is scented or shaped to provoke sexual arousal so that the visitor pollinates it during the attempt. Carrion flowers draw their pollinators with size and scent instead. Many flowers bind themselves tightly to just one or a few pollinators, a relationship that raises efficiency because pollen is more likely to come from the same species. This is coevolution, plant and pollinator shaped together over long stretches of time to fit each other's needs. The cost of that intimacy is risk, because the extinction of one member almost certainly means the extinction of the other. The strategy can be exquisitely timed. Japanese honeysuckle opens at night to attract nocturnal moths, which pollinate more efficiently than daytime bees. The rest of flowering plants turn to abiotic vectors, mostly wind and, far less often, water. Wind-pollinated flowers have no need to advertise, so they tend not to grow large, showy, or scented, and they carry no nectaries. Their pollen is small, very light, smooth, and of little nutritional value. Insect-pollinated pollen, by contrast, is usually large, sticky, and rich in protein to serve as a reward.

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• 7. From Zygote To Fruit

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  When pollen lands on a stigma, it grows a pollen tube that runs down through the style and into the ovary, then enters the egg apparatus guided by a specialised cell. The tube's tip bursts and releases two sperm cells. One reaches an egg, and its nucleus fuses with the egg's nucleus to form a zygote, a diploid cell with two copies of each chromosome. Flowering plants perform double fertilisation. The second sperm cell fuses with the two polar nuclei of the central cell, and because all three nuclei are haploid, the result is a triploid tissue that will nourish the embryo. The zygote then grows through repeated divisions called mitosis. One part becomes the embryo. Another becomes the suspensor, which forces the embryo into the endosperm and later vanishes. Two small groups of cells form the cotyledon, the initial leaf used as an energy store, and later stages build the embryonic root, the embryonic stem, and the junction between them. As the seed forms, the ovary around it grows into a fruit, while the style, stigma, stamens, petals, and sepals wither and die. This is floral senescence, often hurried along by the completion of pollination. Death is preferred because flowers are costly to maintain, though a flower can last anywhere from a few hours to several months. The finished fruit holds an outer peel, a fleshy part, and an innermost stone, with the pericarp making up the fruit wall, everything but the seed itself. Once the fruit is ready, a vector disperses it away from the mother plant, sparing parent and offspring from competing and letting the species colonise new ground. External vectors include birds, bats, water, and wind. Internal vectors come from the plant itself, as in dwarf mistletoes, whose fruit explodes to fling out the seeds.

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• 8. Naming And Using The Flower

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  Plant taxonomists have leaned on flower shape since at least classical Greece, but Carl Linnaeus's 1753 book Species Plantarum is regarded as the first taxonomic work to recognise the significance of flowers. He sorted flowering plants into 24 classes, based mainly on the number, length, and union of the stamens. Later systems in the 18th and 19th centuries weighed more natural characteristics, taking in the rest of the plant so that very different species were no longer grouped together. In 1963, biologists Robert Sokal and Peter Sneath introduced numerical taxonomy, sorting taxa by tabulated morphological traits in an effort to make classification more objective, though it still ignored evolution. Today many botanists use genetic sequencing, the study of cells, and the study of pollen, yet the nature of the flower and its inflorescence remains the bedrock of plant taxonomy. Humans have also turned flowers to practical ends over millennia, using them for decoration, medicine, drugs, food, spices, perfumes, and essential oils. Around half of all cropland grows just three flowering plants: rice, wheat, and corn. Some flowers become spices, including saffron from Crocus and cloves from Syzygium aromaticum. Others yield drugs, among them cannabis, bush lily, and Madagascar periwinkle, while broccoli, cauliflower, and artichoke are flowers or floral parts eaten as vegetables. Beyond use, flowers carry meaning. People have cultivated gardens for their flowers for around ten thousand years. In many cultures flowers are tied to burial, placed by headstones and temples or formed into wreaths, and elsewhere they mark love and celebration. They appear in flags, emblems, and seals, standing in for countries and places, and Hibiscus × rosa-sinensis serves as the national flower of Malaysia.

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