Kingdom of Plants: A Journey Through Their Evolution

With the evolution of flowers, the face of the Earth was transformed forever.

© Rob Hollingworth

All of the early plants up until the Middle Devonian possessed male and female gametes which required water, in some form, for their fertilisation. As the original aquatic plants had a totally submerged existence, their sperm cells could freely move through the water to fertilise their ovum cells. This method of fertilisation put some obvious limitations on where they could survive, and out of water their reproductive strategy would have been impossible. Although bryophytes lived on land, they still relied on a partially wet environment to transfer their sperm cells and spores to their female gametes. We know from bryophytes living today, such as mosses, that when their surroundings are saturated some species store up several times their own weight in water as a reserve, and they are also able to stop their metabolism if their habitat dries out for long periods. These water-dependent land plants were therefore best suited to colonise the damp tidal shores of lowland streams of the Devonian forests, and mosses and ferns can still be found to thrive in these environments today. The need of these amphibious plants to be linked to a moist external environment for their reproduction would have been very limiting in all but the dampest of habitats, and so any plant that was able to break this reliance on water would have had an immense advantage. In the drier terrain further from the shoreline there would have been an abundance of space, light and nutrients. Natural selection soon favoured plants with the ability to grow and reproduce in the dry air of these new habitats. Their trick to surviving in dry air was to package up their reproductive cells in desiccation-proof capsules that could carry them through the air. Capsules we know today as pollen.

Equisetopsida

For over 100 million years, horsetails dominated the understorey of the Devonian, Carboniferous and Permian forests, growing up to 30 metres high.

© age fotostock/SuperStock

The first pollen structures that evolved were tiny packages of genetic material, light enough to be carried on the wind to the female cells of a neighbouring plant. On reaching their destination they put out a little tunnel through which their sperm cells could swim down to achieve fertilisation. For the first time, male and female plant structures were able to swap their genetic information over large distances in the dry air. To maximise their dispersing ability many pollen-bearing plants grew taller, and in time the skies filled with airborne DNA from a multitude of pollen-spewing Devonian flora. Although plants would still require water for photosynthesis, it was now possible for them to colonise new, drier regions of the land. From the coastal forests, plants began to push further into the empty expanses of the ancient world.

As pollen plants began to spread their domain further inland, and it became necessary for their gametes to travel over even greater distances to achieve pollination, a further major shake-up occurred in the way in which plants reproduced. This was one of the most dramatic innovations in the evolution of plants on land – the evolution of the seed. The earliest plants which exhibited seed-like structures are known as the progymnosperms, dating back to around 385 million years ago. They included trees like Protopteridium, and the leafy, 10- metre-tall Archaeopteris. The fossils of the trees from this period indicate that some, but not all, possessed structures resembling primitive seeds, suggesting that this was a time when the future of the seed hung in the balance. Like all plants before them, progymnosperms produced spores, but uniquely they were able to produce two separate types – micro-spores and mega-spores. This trait, called heterospory, suggests that progymnosperms were the most likely antecedents of all seed plants. Their ability to create variable spores is thought to have been the crucial intermediate evolutionary stage between plants with free-floating single spores and those with true seeds containing a spore-borne embryo.

The first true seed plants, which descended from the progymnosperms over 350 million years ago, were a group of tree-like ferns called pteridosperms, belonging to the major division of plants called gymnosperms. The word gymnosperm literally means ‘naked seed’, as they produce seeds which are not fully enclosed in an ovary. In earlier seed-less plants, the gametophytes were released outside the parent plant, but in the pteridosperms the gametophytes were microscopic in size and retained inside the reproductive parts of the plant. This created a moist ovule in which fertilisation could take place, in essence creating a plant within the parent plant. Coupled with this, these embryonic packages were encased with some starting-off food, meaning that they could be transported, ready to germinate as soon as they found themselves in the right conditions. The protective packaging of these seeds also enabled them to remain dormant after dispersal, and wait until conditions were perfect to grow. This prevented the precious genetic material contained within from being wasted in times of flooding or drought.

Today seed-bearing plants are the most diverse group of all vascular plants. The evolution of the seed enabled the proliferation of land plants on the wind, in the water, along the ground and in the stomachs of animals. During the Carboniferous and Permian periods, the gymnosperms evolved prolifically, with their extant relatives today including conifers such as pine, spruce and fir, with their needles; ginkgos, with their fleshy seeds; and cycads, with their large palm-like leaves and prominent cones.

Around 300 million years ago a global ice age hit the planet, and the Earth became progressively drier and cooler as great bodies of ice formed at the poles and locked away precious water vapour from the atmosphere. The reduction of atmospheric moisture caused vast areas of tropical forests and swamps to shrink and dry out, and with their ability to disperse their seeds and colonise drier environments, gymnosperms soon replaced ferns as the dominant plants on the planet. In time the higher-altitude regions of the planet became regions of cold-climate peat lands and swamps, which would have resembled something similar to the boreal taiga of modern-day Siberia. In the milder lowlands, deciduous swamp forests were dominated by the seed ferns of Glossopteris and Gangamopteris, along with large clubmosses and immense horsetails.

The first seeds

The development of the seed saw gymnosperms become the dominant plant group between 290 and 145 million years ago.

© imagebroker.net/SuperStock

By the end of the Permian period the main continents of Earth’s land masses had all fused together into one supercontinent called Pangaea, and parts of the planet had become arid with little rainfall, creating extreme desert landscapes. As deserts expanded and coastlines shrank, this extreme climate shift began to push many life forms to the brink, and by 248 million years ago, 95 per cent of the plant and animal species that had evolved by this point were wiped out. This marked the largest extinction ever known, and for the next 500,000 years complex life on Earth teetered on the brink of complete extinction. The 5 per cent of life that remained was sheltered from the extreme climate, in habitats that remained temperate and moist enough. These pockets of life harboured the fundamental DNA that had evolved so far. Over the following 50 million years, as the global climate became more amenable once again, plant life would bounce back to colonise the planet. Slowly plants began again to create temperate woods, tropical forests and dry savannahs.

As the Jurassic swamps and prehistoric woodlands began to spring back to life, plants continued to increase and diversify. Seeds, leaves and pollen became more specialised, and the world of plant life provided an abundance of food for the dinosaurs. Plants gave rise to fast-growing bamboo and shade-giving palms until 140 million years ago, when the plant world would be changed completely.

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© Don Paulson Photography

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.

© RBG Kew

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.

© RBG Kew

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.

© RBG Kew

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.

© RBG Kew

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

Catasetum christyanum

An epiphytic orchid from northern South America.

© RBG Kew

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

Vanda coerulea

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

© RBG Kew

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

Dendrobium formosum