Kingdom of Plants: A Journey Through Their Evolution. Will Benson
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.
© RBG Kew
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.
© Rob Hollingworth
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