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The botanical gardens and private collections of Europe’s cities were soon overflowing with an explosion of fascinating and rare flowers.’

For more than two hundred years humans have had an obsession with flowers. It has seen men give their lives in search of the most exquisite floral specimens, and caused many others to lose their minds in pursuit of the rarest. The Victorians used the term orchidelirium to describe ‘flower madness’, the botanical equivalent of ‘gold fever’ for the 1800s. This fascination with exotic flowers began with the pioneering plant hunters of the eighteenth century, who sailed to South America, Asia and Africa, travelling through unmapped territory in search of botanical wonders.

These early expeditions were commissioned by wealthy collectors and botanical organisations, and they aimed to supply high society’s increasing appetite for new and exciting plants and flowers. Often spending many years abroad at a time, plant hunters risked their lives, negotiating wild animals and hostile natives, in order to discover new plant species. The finest specimens could fetch a mighty price for their scientific uses and aesthetic value.

Our attraction to flowers has a deep history; evidence from a Neanderthal burial site in Iraq suggests that even 200,000 years ago our close hominid relatives were using flowers in ceremonies, laying the blooms from plants such as ragwort and grape hyacinth over the bodies of their dead. Throughout Greek myth flowers were sacred to both gods and mortals: the deep red of poppies was created from the drops of blood that fell from the slain Adonis, and the nymphs that sun-god Helios banished for their disloyalty were turned into the flowers of hellibores. In ancient Egypt roses in particular were a symbol of wealth, beauty and seduction. Guests at Emperor Nero’s great banquets were showered with their petals, and it is documented that Cleopatra used the sweet scent of rose petals to lure Mark Antony. Flowers remain a huge part of our culture today, accompanying us on the most important days of our lives – our birth, our graduation, our marriage, our death. Our gardens are now awash with bright and showy blooms from habitats from all corners of the planet – magnolias from China, geraniums from the Cape of South Africa, primulas from the Himalayas and wisteria from the Orient.

In 1768 a botanist and horticulturalist named Joseph Banks set off with Captain James Cook on his first major voyage to the Pacific, where he would spend the next three years collecting, studying and cataloguing the wealth of fascinating new plant species that he found thriving on the tropical islands there. Following his return to England in 1771, Joseph Banks acted as an adviser to the Royal Botanic Gardens at Kew, a position that was later formalised. Banks gathered together a team of like-minded botanists and explorers for further expeditions. His team included the explorer and plant collector Allan Cunningham, and Scottish botanist Francis Masson, who would later become known as Kew’s first plant hunter, and who would later join Captain Cook on his second major voyage. Under Banks’ supervision the gardens at Kew fast became the world’s foremost botanical garden. Impressive proteas, cycads and bird of paradise flowers from South Africa soon filled the greenhouses, with each species transported on its voyage enclosed in a mini-greenhouse, called a ‘Wardian’ case. It was the showy blooms and delicate scented flowers which drew the most attention back home in Britain. As the plant-collecting voyages pushed deeper through the thick vegetation of tropical jungles, increasing arrays of floral shapes and colours were collected, and made their way back to the collections at Kew.

Sir Joseph Banks

Under his supervision, Kew’s expanding collections of exotic plants saw it become a garden of international importance.

Continuing Banks’ legacy, his successor William Hooker, and later his son Joseph, maintained Kew’s spirit of exploration, leading further trips to the mountains of India and Nepal. Among other species they discovered a mass of stunning new species of rhododendrons, a plant popular with gardeners across the world today. However, their plant-collecting exploits weren’t always trouble-free, and during one of their trips to the Himalayas between 1847 and 1849, Joseph and his travelling companion Archibald Campbell were arrested and imprisoned for having illegally crossed the border from Sikkim into Tibet. The two men and their botanical specimens were only released when the British government threatened to invade Sikkim.

Sir Joseph Hooker

Pen and ink portrait by T. Blake Wirgman, 1886.

As well as the public botanical collections of the time, such as Kew, obsessive private collectors also set out to acquire rare and exotic or even undiscovered flowers, which was lucrative for the financiers and explorers alike. The expeditions were often shrouded in secrecy to prevent rival groups from acquiring information as to where new species were likely to be growing, and it wasn’t uncommon for false maps and information to be circulated in order to disorientate the competition. This was the age of orchidelirium, and successful collectors could sell their prized specimens at auction for colossal sums of money. It was these privately financed trips which brought back the first orchids to Britain, from the East, and in 1852 some of them made their way into the hands of London wine merchant John Day. Bought for the equivalent sum of £3000 in today’s money, Day’s first orchid flowers marked the beginning of a lifelong obsession. His house in north London was soon transformed with the delicate white and maroon petals of Dendrobium from Southeast Asia, Odontoglossum from tropical America and Cattleya from Costa Rica. Combining his love of orchids with his keen artistic eye, Day set about documenting his increasing collection of flowers in a set of watercolours. His meticulous paintings, complete with notes on the plants’ habitat, conditions for cultivating them and their price at auction, soon caught the eye of botanists and art lovers alike, and he was given special access to the orchid house at Kew to paint its plants. Over 25 years, Day compiled over 50 sketchbooks filled with his detailed, colourful visions of these captivating plants, and these drawings can still be admired in the collections at Kew today.

Plant hunting

From Joseph Hooker’s Himalayan Journals, 1854.

The Victorian obsession with acquiring the most ornate flowers was made all the more possible by an extraordinary network of vivacious plant fanatics, who were willing to use their work in the far corners of the British Empire as an opportunity to bring back exotic species from across the globe. Colonel Robson Benson, an officer in the British forces in India, used his time on duty in Assam, Bhutan and Cambodia to collect a multitude of new species of orchid for the British horticulturist Hugh Low. Painter William Boxall, working first in Burma and later in the Philippines, collected enchanting slipper orchids, magnificent Vanda, and a number of species of the genus that today fills the shelves of nearly every garden centre, Phalaenopsis.

The botanical gardens and private collections of Europe’s cities were soon overflowing with an explosion of fascinating and rare flowers, displaying an unfathomable array of shapes, sizes and colours. But as well as the aesthetic interest that drew most admirers to these flowers, their complexity and diversity provided biologists and naturalists with a wealth of material for them to study. One such naturalist was the young Charles Darwin, as well as Kew’s second Director, Joseph Hooker, who was a lifelong friend of Darwin. Darwin shared extensive correspondence with a long list of senior botanists and horticulturists at Kew, swapping notes on plants and exchanging specimens. During his time on the Beagle between 1831 and 1836 he gathered species of flowers from Argentina, Chile, Brazil and the Galapagos which he sent back to Kew for identification, and in turn Kew happily provided Darwin with plants for him to document and study at his house in Kent. Although at this point Darwin had not yet written his seminal work On the Origin of Species by Means of Natural Selection, he was already piecing together his ideas on survival and adaptation in the natural world. Perhaps more than anyone else at the time, Darwin knew that for all their beauty, the complex shapes, patterns and structures of every unique orchid flower must be a result of some advantage that they bestowed upon that species in its habitat. Darwin understood that the flowers of orchids were purely about coaxing animals to spread its sex cells.

Illustrations by John Day, taken from his ‘scrapbooks’.

Cattleya skinneri

A species of orchid found in Costa Rica and Guatemala.

Illustrations by John Day, taken from his ‘scrapbooks’.

Catasetum christyanum

An epiphytic orchid from northern South America.

Illustrations by John Day, taken from his ‘scrapbooks’.

Vanda coerulea

A species of orchid discovered in Sikkim by Joseph Hooker in 1857.

Illustrations by John Day, taken from his ‘scrapbooks’.

Dendrobium formosum

A species of orchid first discovered in northeast India.

Darwin’s instincts as a naturalist stemmed from his love of collecting, and his belief that in order to understand any aspect of the natural world, one must acquire, and carefully examine every facet of it. He once wrote, ‘By the time I went to school my taste for natural history, and more especially for collecting, was well developed. The passion for collecting, which leads a man to be a systematic naturalist, a virtuoso or a miser, was very strong in me, and was clearly innate, as none of my brothers or sisters ever had this taste’. In his quest to make sense of the elaborate flowers of the orchid family Darwin began amassing his own collection of these rare plants, which he held in his glass conservatory at Down House. Countless orchids from Malaysia, the Philippines and Central America made their way via Kew to his house, together with the British species which grew in abundance nearby. But the nature of the most extreme orchid flowers did not fit well with his theory of evolution. In one letter that Darwin wrote in 1861 to John Lindley, who worked as one of Kew’s taxonomists at the time, he describes his utter fascination with the complexity of orchids, discussing one genus in particular called Catasetum: ‘I have been extremely much interested with Catasetum, and indeed with many exotic orchids, which I have been looking at in aid of an opusculus, on the fertilisation of British Orchids. I very much fear that in publishing I am doing a rash act; but Orchids have interested me more than almost anything in my life. Your work shows that you are carefully understanding this feeling.’

Deception

The labellum of this mirror bee-orchid has evolved to mimic the shape and shine of an iridescent bee.

Darwin studied the lives of orchids and dissected them, looking at the multitude of ways in which the plants guided specific bees or moths to their flowers to interact with their reproductive structures, and the mechanisms they exhibited to achieve pollination. He was searching for an explanation for all aspects of each flower’s behaviour, and a justifiable origin for each. But for many of his adversaries, what Darwin was trying to achieve was considered impossible, and even his good friend Thomas Huxley famously stated, ‘who has ever dreamed of finding a utilitarian purpose in the forms and colours of flowers?’ Darwin made good headway in unravelling the sex lives of orchids, and he made detailed studies of the ways in which they lured pollinators and released their pollen. But what had him most stumped was that, more so than any other family of flowers, orchids exhibit extreme pickiness in whom and what they allow to spread their pollen. On the subject of this he wrote: ‘Why do orchids have so many perfect contrivances for their fertilisation? I am sure that many other plants offer analogous adaptations of high perfection; but it seems that they are really more numerous and perfect with the Orchideae than with most other plants.’ What seemed counterintuitive to Darwin was that for all their elegance, the pollination methods employed by orchids seemed terribly inefficient.

Darwin’s trouble with trying to explain the nature of the sex life of orchids becomes all the more apparent as soon as you begin to unfold the highly specialised ways which we now know different species achieve pollination. In the mirror bee-orchid (Ophrys speculum), found in southern and western Europe, as well as Lebanon, Turkey and North Africa, the lip of the flower looks nearly identical to an iridescent bee. This was first suggested to be a ploy to prevent grazing animals from munching on it, but we now know that it in fact releases a chemical that mimics the pheromones of a female bee, as a trick to get males to ‘mate’ with it. By rubbing its body on the flower in an attempt to copulate, the male bee will rub itself up against the plant’s sticky bundles of pollen, called pollinia, which adhere to its body, before it flies away and attempts to mate with another bee-orchid. Another extreme behaviour has evolved in the orchids of the genus Oncidium from Ecuador, which have petals that look like the insect competitor of the Centris bee. The bee attempts to chase this ‘enemy’ away from its territory, and in doing so it strikes the flower, showering itself in the plant’s sticky bundles of pollen. The slipper orchids of Asia and South America have a hinged lip which forces insects to brush past the sticky pollen before leaving, and there is even an underground species of orchid from Australia called Rhizanthella slateri which relies on ants to move its pollen. Other orchids emit a smell of rotting flesh to attract meat-loving flies to pollinate them, while some have been found to smell like chocolate.

Perhaps the most intriguing pollination syndrome is that of Catasetum, which is so complex it seemed to contradict Darwin’s very theory of evolution. Unusually for orchids, some Catasetum plants are male and others are female. The male produces a scent that attracts just one species of euglossine bee. Lured by its sweet smell, the bee lands on the lip of the orchid and thrusts its head into the flower, touching a hair-trigger. This activates a mechanism that fires out a tiny bundle, which then sticks onto the bee’s back. This extraordinary projectile is in fact a bundle of pollen grains called a pollinium, which has a little cap on it, and after a minute or so the cap falls off to reveal a little horseshoe-shaped bundle of pollen grains. A group of researchers in the USA recently found that the pollinium is ejected with an acceleration rate of over ten times that of a striking pit viper. Having been struck by this pollen, the bee flies away and is attracted to another rather different-looking flower, which is the female. Once again, lured by the scent, it sticks its head into the female flower – and the little bundle of pollen attached to its back, like a key, fits into a small aperture on the roof of the flower, like a lock, pulling off the pollen as the bee makes its departure. Pollination has been achieved.

The Catasetum conundrum

The structures of these orchids interested Darwin immensely on account of their incredible complexity.

Darwin’s obsession with Catasetum in particular caused him to dedicate a great deal of time to studying the flower’s mechanism, in an attempt to make sense of how it could have arrived at such a precise and specific system. His tireless persistence paid off, and in a letter to his publisher John Murray in 1861 he wrote, ‘I have had the hardest day’s work at Catasetum and the buds of Mormodes, and believe I understand at last, the mechanism of movements and functions. Catasetum is a beautiful case of slight modification of structure leading to new functions.’

Having unravelled the complexities of exploding pollen bundles, it wasn’t long before Darwin’s next botanical mystery would land on his desk, quite literally. In 1862 he received a package from renowned horticulturist James Bateman, a striking orchid with a flower composed of large star-shaped white petals from the island of Madagascar, named Angraecum sesquipedale. Darwin set about detailing the ornate nature of his latest specimen and was struck by the long tubes, called spurs, in which the plant’s nectar was contained. Its delicate spurs were over 30 centimetres long, hanging down beneath the flower like white tails, with the nectar contained at their tip. Having never seen anything quite like this before, he wrote to his esteemed friend Joseph Hooker to explain these foot-long, whip-like nectaries, exclaiming, ‘Good Heavens what insect can suck it!’ Later that year Darwin went on to publish a book on the reproduction of orchids, in which he theorised that in order for the Madagascan orchid to be pollinated, an insect, most probably a moth, must exist on the island of Madagascar with a tongue at least 30 centimetres long which can reach the nectar at the end of the spurs. His suggestion seemed ludicrous to many of his peers, but a paper written by fellow evolutionary theorist Alfred Russel Wallace a few years later sought to back up Darwin’s notion, by highlighting that a large hawk moth had been discovered in Africa which had a tongue almost 20 centimetres long, called Xanthopan morgani. Wallace predicted that if such a moth existed in Africa, then surely a moth with a 30-centimetre-tongue could live in the forests of Madagascar. Unfortunately Darwin was never able to see his prediction come true, but in 1903 a population of hawk moths with the necessary tongues were found on Madagascar. The team who discovered it then aptly named it Xanthopan morganii praedicta – the predicted subspecies of X. morgani.

Mystery of the moth

The relationship between hawk moth and Angraecum sesquipedale is one of the greatest examples of plant and animal co-evolution.

But for all the sense that Darwin was able to make of the lives of flowering plants and how they disperse their pollen, there was one fundamental aspect of their world that he was never truly able to fathom. He couldn’t understand how flowering plants had come to exist in such diversity in such a short period of geological history. For almost half a billion years, plants had existed without flowers, and then in a few million years they appear in the fossil record as the dominant form of plant life. In a letter sent to his friend Joseph Hooker in 1879, Darwin remarked on his puzzlement on the sudden radiation of flowering plants, stating, ‘the rapid development, as far as we can judge, of all higher plants within recent geological time is an abominable mystery.’ It went against his very theory of ‘natural selection’ that he had outlined for the rest of life on Earth.

Darwin’s work, which observed the processes by which all species struggle for survival and compete to reproduce in order to pass on their genes, helped build his theory of evolution – the process by which a species over many generations acquires novel and advantageous traits. From his acute observations of the birds, insects and reptiles of the Galapagos Islands, together with the fossils of extinct animals that he gathered during his voyage on the Beagle, Darwin discovered that the change inferred to organisms over time was a slow and gradual process. In his subsequent book On the Origin of Species by Means of Natural Selection he states that ‘natural selection acts only by taking advantage of slight successive variations; she can never take a great and sudden leap, but must advance by short and sure, though slow steps.’

However, for all the diversity that Darwin could see in the modern world of flowering plants, the fossil record revealed no trace of the expected slow and gradual transition of non-flowering plants to those with flowers. For Darwin, the seemingly sudden appearance of flowers contradicted the very rules of evolution. In the fossils of the Carboniferous period horsetails and early seed plants dominated the land. In the fossils from the time of the dinosaurs cycads, ginkgos and ferns were dominant. And then suddenly in the fossils of the Cretaceous period, 130 million years ago, an explosion of flowering life appears and takes over the land. In this short period all of the major groups of flowers that we see today emerged. Not only did each species of flower look different, but they had developed a multitude of reproductive styles – from the relatively straightforward mechanism of those flowers which released their pollen on the wind, to those possessing nectar-filled organs for the more complex task of luring insect pollinators. This sudden spurt of evolution not only had Darwin stumped but has continued to boggle botanists for the last 120 years.

Darwin knew that the fossil record was not a wholly complete snapshot of life through time, and he used this to try and explain the sudden burst of flowering plants on Earth. Due to the fact that plants do not have hardened body parts like the easily fossilised internal and external skeletons of many animals, there is a chance that the intermediate stages of the first flowers may simply have decomposed and been lost when they died. In his correspondence with Hooker, Darwin suggested that flowering plants had perhaps evolved slowly and that the fossils were yet to be found. Another suggestion was that a rapid increase in flower-frequenting insects in the Cambrian may have spurred their evolution on. In the animal world it is possible to see many of the intermediate steps that have given rise to certain animals. The embryos of snakes, dolphins and whales all sprout the buds of vestigial legs when they are embryos, echoing their evolutionary past, which then shrink and disappear before they are born. However, plants do not retain these evolutionary features in the same way that animals do, and it is far harder to trace the steps by which flowers came to be by studying their more primitive relatives. What makes things harder still is that the flowers of even moderately primitive groups of flowering plants are so different from their assumed extinct relatives among seed plants, that it is incredibly difficult to reconstruct a plausible evolutionary history for them.

However, since Darwin’s day new fossil finds and our considerable advances in genetics have helped us begin to unravel the origins of flowering plants. In the mid-1980s an international collaboration of over 40 scientists from around the world, coordinated byKew geneticist Professor Mark Chase, embarked on a mammoth project. Over a number of years the team meticulously extracted the same type of gene from over 500 different types of flowering plant, and by the early 1990s they had gathered enough information to begin to compare them. By looking at the plants which shared the most similarities for this type of shared gene, Professor Chase and his team were able to work out which groups of flowers were more closely related, and by pinpointing those in which the gene had considerable differences they could assume they had evolved separately. The findings, published in 1993, allowed them to piece together an accurate tree of life for flowering plants. Fifteen years later a team of researchers at the University of Florida built on this tree of life to create a more complex timeline for the emergance of different types of flowers. By looking at the genes of a number of living plants that could be linked to their fossil ancestors from known dates in prehistory, the team were able to work out the rate at which certain genes changed over time. The results from these calculations gave them a ticking genetic clock which could then be used to date the origins of the first flowering plants. The major revelation from their work was that previous estimates for the first flowers had been inaccurate by around 10 million years, and that the first blooms were in fact evolving as far back as 140 million years ago. But the Florida team’s findings still seemed to indicate that flowering plants did indeed rapidly radiate in as little as just five million years.

For years the debating and theorising continued as to how plants could have seemingly cheated evolution, to quickly rise from relative obscurity into a wealth of developed flowering structures during the Cretaceous period. Then scientists believed they had found an explanation, in the form of a happy coincidence, a genetic mishap discovered in some plants known as polyploidy. It has been known that when the male and female haploid sex cells of both plants and animals combine during reproduction to create the next diploid generation, some sections of genetic information from the parents can become duplicated in the new generation. In the case of humans, the accidental insertion of any additional genetic information can be extremely damaging for the child’s health; even the duplication of just one of our 46 chromosomes will cause Down’s syndrome, and the duplication of two or more chromosomes would be fatal. However, flowering plants with their comparatively simple body parts have been found to be able to live healthily with accidental duplications of genetic material, even in extreme cases where the whole of a plant’s genome (i.e. every one of its chromosomes) becomes duplicated. Not only can a plant species tolerate these polyploidy events, but it appears that they can actually thrive on them.

It had long been considered that flowering plants’ ability to duplicate large parts of their genetic information could have been a contributing factor that allowed them to increase in diversity at an abnormal rate, and a study carried out in the 1970s calculated that between 30 per cent and 80 per cent of all flowering plants have undergone a multiplication of parts or the whole of their genome at some point in their evolutionary history. Plants that have undergone polyploidy are typically more vigorous. By tracing through the lineage of many different groups of flowering plants, scientists have now found proof that a number of polyploidy events does indeed explain the fast rate at which particular types of flowers radiated, such as the prolific grasses, the nightshades, the pea family and the mustard flowers. But what of all the other flowering families that make up the 400,000 or so species on Earth today?

Nuphar lutea

These yellow water-lilies are among the oldest living relatives of the first flowering plants.

Up until very recently speculation still remained as to what exactly could explain the apparently sudden explosion of all flowering life 140 million years ago. Then in 2011, at a conference of the International Botanical Congress in Melbourne, a palaeontologist from the Swedish Museum of Natural History called Else Marie Friis revealed findings which outlined a previously unseen trove of exquisitely preserved primitive flowers from the charcoal fields of Catefica, Torres Vedras and Famalicao in Portugal, dating back to the Early Cretaceous. These amazing fossils, many of which were preserved in three dimensions, gave the first glimpse of what early flowers looked like as they began to evolve, and in breathtaking detail they showed the first stages of flowering life on Earth. Some possessed clusters of small flowers grouped together to form one larger inflorescence, much like a modern-day sunflower, while other plants had small single flowers no more than 2 millimetres across. Most seemed to have few floral parts, and many even lacked petals and the protective outer sepals which are characteristic of most modern flowers. Numerous seeds and pollen were also found in the fossils, and the high number of fruits possessing fleshy coats suggests that animals had a key role in dispersing the seeds of these plants. Friis’s fossils seemed to reveal what flowering plants looked like some 30 million years before the fossil evidence of Darwin’s day, at a point when they were first acquiring the features which would ultimately lead to the flowers we see today. While historic polyploidy events undoubtedly gave ancient flowering plants occasional moments of accelerated radiation in their shape and form, these fossils revealed that the overall rise of flowering plants was far more gradual than Darwin had thought, and that, like all life on Earth, they had evolved their structures through a process of gradual change.

Buzz-pollination

The stamens of Gustavia longifolia only release their pollen when buzzed by the wings of a bee.

From the time when these first flowers became immortalised in the coals of the Early Cretaceous, the angiosperms – as all flowering plants are collectively known today – have since diversified into a huge range of specialised species, each one with its own way of encouraging its pollinators to disperse its pollen. Fast-growing, compared to the ancient cycads and conifers, and able to tolerate fluctuating climates, flowering plants soon became the most species-rich plant group on Earth. Relatives of the earliest flowers to evolve can still be found today, the oldest of which is a plant from the cloud forests of New Caledonia called Amborella, and Nuphar, a water-lily. The first pollinators of early angiosperms are thought to have been flying insects like scorpion flies that, having been partial to the nectar of seed ferns, would have been easily lured by the blooms of the first flowers that emerged. Angiosperms fast began to optimise their blooms to make them more enticing to particular kinds of pollinators. Over time flowers became bigger, brighter and more scented, and as flowers began to evolve to favour the tastes and temptations of certain animals, the pollinators in turn began to evolve to maximise their ability to drink nectar or eat the nutritious starchy pollen of particular flowers.

One particularly ingenious example of a flowering plant which has maximised its pollen-spreading success is the flowering tree Gustavia longifolia, from the western Amazonian forests. A team of tropical horticulturalists who studied the plant at the Royal Botanic Gardens at Kew found that its fleshy, deep-purple flowers have acquired a very clever method to ensure that its pollen is only taken by the particular kinds of bee that are likely to spread that pollen to other G. longifolia flowers. A species of night-flying bee climbs in among the stamens of the flower to feed from the nectar inside, and as the bee drinks from the sugary fluid the frequency of its buzz causes the flower to shake violently, at a force calculated to be as much as 30 times the pull of Earth’s gravity. These violent vibrations shake the flower’s anthers, and in the process its sticky yellow pollen is released and showered over the bee’s back. This clever mechanism, called buzz-pollination, occurs in a number of other unrelated flowers from all over the world. The flowers of the tomato plant, for instance, release their pollen for only a handful of species of bee. Farmers who grow acres of the plants have tried to trick the flowers into releasing their pollen by using vibrating tuning forks or buzzing electric toothbrushes – but nothing provides as good a pollination service as the bumble bee that has evolved alongside the tomato plant for millions of years.

Flowering plants are one of the most successful life forms on the planet, and they have come to occupy almost every known habitat on Earth. But while bees are indeed the most prolific pollinators, there are many other species which help move pollen from one flower to another. Some of the plants alive today, whose ancestors evolved shortly after the primitive flowers of the Early Cretaceous, such as the water-lilies and the magnolias, evolved a wealth of tactics to persuade flies and ancient beetles to feed and transfer their pollen. As well as their heady scents and enticing blooms of electric blue, loud pinks and mesmerising yellows which make them irresistible to insects, they are also able to produce heat. This ability, known as thermogenesis, provides a warm landing place with a ready supply of nutritious pollen, and a flower is therefore an easy choice for any insect looking for an inviting place to visit.

Flowering giant

Structures at the base of the titan arum’s spathe produce the pollen which is picked up by visiting insects.

From the bird world, hummingbirds are prolific pollinators of bright red jungle plants such as honeysuckle, using their long beaks to gather nectar from the trumpet-shaped flowers. Moths pollinate some of the more ghostly flowers, such as those of the night-blooming cacti Echinopsis and Selenicereus, and butterflies are responsible for pollinating many thousands of species of pink or lavender-coloured tropical flowers such as the Asian buddleja or the American passion-vine. Snails and slugs smear pollen from plant to plant as they move through vegetation, and mosquitoes pollinate some species of orchids. Mammals too, both on the ground and in the air, transfer pollen for many hundreds of different flowers. Even lizards on the island of Mauritius have been found to transport the sticky pollen of particular plants with tough flowers, as they forage for fruits.

Since the arrival of flowers, the animal world has been inextricably linked with the plant world, and for the past 140 million years they have evolved together. In most cases it is a mutually beneficial relationship, in that the animal gets a meal, and the plant spreads its DNA. The plant world’s ability to harness the hungry nature of animals and get them to carry their pollen is the greatest trait that plants have acquired, and as long as there are animals waiting to get a meal, flowering plants will remain the dominant and most fascinating organisms on the Earth.

Mankind’s obsession with flowers has not waned since Victorian times, and although orchids are still the chosen obsession of many, thousands of different flowering species are now cultivated and admired in gardens and conservatories around the world. Our continued love of flowers is personified today in the beds of the Royal Botanic Gardens at Kew, now a UNESCO World Heritage Site, where flowering plants from all corners of the globe attract over two million people every year to marvel at the variety of the plant world. The Palm House is home to exotic tropical plants from jungles all over the world, and the grand structure of the Temperate House contains thousands of temperate and cool-zone plants from Asia, Australia, Africa and the Americas. But it is in one of the more recent additions to Kew’s landscape that the gardens’ biggest draw can be found. In the Princess of Wales Conservatory, the most technologically advanced greenhouse in the world, there is a plant that since its discovery in Asia in 1876 has not ceased to fascinate all who see it: the titan arum (Amorphophallus titanum).

The plant was first discovered by an Italian botanist called Odoardo Beccari, who stumbled across it during his expeditions in the tropics of Sumatra. He packaged up some of its seeds and hastily sent them back to Europe, and when a handful of these germinated a young plant eventually made its way to Kew. For over 10 years the plant grew in size at Kew, putting out mighty leaves, the size of a small tree, until in 1889 it finally produced its first flower. Amazingly, the single triffid-like bloom which emerged from this almighty plant was as large as its 2-metre-tall leaves. It seemed clear that the titan arum must surely be the largest flower in the world. However, on closer inspection it was found that its great totem-pole-like structure was actually made up of many thousands of minuscule flowers, making it by definition an inflorescence and not a single flower. Instead the accolade of the largest single flower is held by the metre-wide parasitic species Rafflesia arnoldii, which also grows in the tropical forests of Sumatra, and across Southeast Asia. Nonetheless, the titan arum is a botanical giant, and rising to 3 metres tall in its mightiest specimens, its bloom towers over any human. Its flower body consists of a frilled purple collar around a tall speckled-green and cream-coloured flower-bearing spike, which is made of separate male and female flowers.

Only staying open for a couple of days, the titan arum has to attract as many of its pollinators as quickly as it can, and it does this by emitting a foul and fetid stench, described as resembling rotting flesh, sour dairy and burnt sugar, which is produced by sulphur-containing compounds on its spike. The stench of the recently opened flower is in fact so vile that the artist who came to draw the first specimen that came into flower at Kew was made ill after inhaling it for too long. Shortly after opening, the base of the flowering spike begins to generate heat of around 36°C, which creates a convection current to help waft its rancid smell through the night air. By burning reserves of stored carbohydrates, the plant produces heat in waves over a few hours, and the resulting pulse of scent that is emitted acts to punch through the layer of cooler air that forms below the forest canopy. Once through this layer, the foul scent is able to travel great distances through the forest and reach the olfactory organs of its pollinators. Any carrion-beetles or flesh-flies that catch a whiff of its odour will then hungrily fly to the flower expecting to find a meal of decaying meat, and on arrival will bump into the flower spike. Cunningly, the male and female parts of the titan arum open on separate nights to prevent the flower from self-pollinating, so it has female parts at the bottom which open on the first night, and male parts on the top which open later. As flies search the flower for the source of the rancid smell they become caught in the deep cone of the flower’s collar, and in order to escape they must crawl up the flower spike, getting coated in pollen as they go. They then fly to another open flower and, starting at the bottom again, crawl upwards – and in doing so cross-pollinate the flowers. Since the 1800s titan arum has flowered numerous times at botanical gardens across the globe where it is now showcased, perhaps most notably at Kew in 1926, when police were called in to control the huge crowds that had gathered to see and smell the much-talked-about spectacle. Titan arum plants are now grown by botanical institutions and private collectors all around the world, but the occasions when their blooms emerge still make the headlines.

Door-to-door transfer

Flowers provide an elegant door-to-door pollen delivery system for plants.

The dizzying diversity of flowering plants today is truly staggering, and we continue to discover further members of these incredible plants, such as orchids which only flower under the cover of darkness and palm trees which flower themselves to death, flowers which imitate an incredible array of insects, and even flowers which mimic other flowers. In their long history, spanning over 400 million years, plants have developed many amazing strategies to better survive and reproduce, and the evolution of the flower is surely one of their greatest. Not only has it allowed angiosperms to outnumber their fern and conifer ancestors by 20 to 1, it has also helped forge the relationship between humans and plants. Many of the plants that today support the human population, such as the grasses that provide cereals and sugar, the many fruits and vegetables we eat, as well as cotton, coffee and chocolate, and trees that provide building materials, are the result of the evolutionary success of these flowering plants.

Kingdom of Plants: A Journey Through Their Evolution

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