Tim Entwisle
Tim Entwisle is director of the Royal Botanic Gardens, Victoria, Australia.
Think of Darwin and most likely you think of his theories on the origin of animal species. It was his vignettes on apes, tortoises and finches that won the public over to his theory of evolution by natural selection.
But behind the scenes, plants also played a major role. They helped unveil the subtle steps taken on the evolutionary path.
Darwin collected hundreds of botanical specimens during his five-year voyage on HMS Beagle. He marvelled at the elegant tree ferns in the tropical jungles of Brazil, the impenetrable thickets of thistle throughout southern South America and the “desolate and untidy” scrubby eucalypt forests of Australia. “A traveller should be a botanist,” he wrote in his diary, just days before the Beagle returned home to England in 1836.
While many of Darwin’s dangerous ideas were born in exotic ports of call – most famously on the Galápagos Islands off the coast of Ecuador – they were put to the test over the next 40 years among the primroses and cowslips, orchids and beans, bees and earthworms in his back garden at Down House in Bromley, Kent. Every part of the seven-hectare estate served as Darwin’s living laboratory. As University College London geneticist Steve Jones told the BBC in 2009: “This isn’t just a vegetable garden. This is Bromley’s Galápagos.”
Darwin published more than 20 books in his lifetime covering subjects as diverse as the geology of South America and of volcanic islands; the formation of coral reefs; taxonomic studies of barnacles; and on the role of earthworms in soil fertility. But he also published prolifically on plants, including books on the “contrivances” by which plants achieve cross-fertilisation, the habits of climbing plants, and the behaviours of insect‑eating plants. He also paid heed to the views of his botanical colleagues: “I scarcely ever like to trust any general remark in zoology, without I find that botanists concur,” he wrote to American botanist Asa Gray in 1856.
Darwin was obsessed with providing an answer to a question that perplexed 19th-century scholars: Where did new species come from? The serious study of rocks, spurred by industrial England’s demand for coal, had shown that different rock layers contained different fossils. That meant species weren’t created in one fell swoop, as the Bible insisted, they were changing over time. But how?
Darwin’s travels on the Beagle provided clues. In the Galápagos, species differed remarkably from island to island. On Pinta Island, for instance, giant tortoises had shells that rose in front like a saddle to let the tortoise crane its long neck upwards. Darwin surmised this was an adaptation to feed on the tall cacti growing on the island. By contrast, on Isabela Island with its low-growing shrubs, the tortoises had no such kink.
Perhaps, Darwin speculated, such differences arose from slight variations within the population from which the tortoises descended. If individuals were swept onto different islands, the environments might favour different physical attributes, tipping the balance of who survived and reproduced on each island. Over time, new species would emerge. Perhaps this “descent with modification” was just a microcosm of what was happening on a far grander scale.
The vast timescale available for these changes was becoming evident from geological studies, including observations by Darwin himself. Ashore in Concepción, Chile, during a massive earthquake in 1835, he noticed the sudden uplift of land by several metres. Travelling inland, he saw shell fragments embedded in the Andean mountainsides, evidence that earlier tremors had again and again stranded marine debris high above the coastline.
These and other geological signs convinced Darwin that no single quake, however violent, could so dramatically alter the landscape. To build the Andes would take vast eons of time – enough time, perhaps, for the countless tiny, incremental changes needed to account for the diversity of all life on Earth.
Darwin knew his ideas were dangerous. He spent more than 20 years building the case for evolution by natural selection before publishing his theory in his 1859 book, On the Origin of Species. It’s not a riveting read, but you can’t help but be impressed by the sheer mass of data. (I’m surprised Darwin’s opponents such as Bishop Samuel Wilberforce didn’t just say, “Enough already, I give in.”)
At Down House, Darwin used plants to test his theories. One fertile area of experimentation was the kitchen garden, planted by his wife Emma. As Nick Biddle, curator of the garden at Down House told the BBC in 2009, “It was really Emma who looked after the garden; Darwin would potter about. One of his gardener’s described him as ‘mooning about the garden; I think it would be better if he had something to do’.”
But Darwin was certainly doing something. When a May frost deposited itself on a row of beans, Darwin noted that a small percentage were able to survive. It’s just the kind of thing Darwin was looking for to demonstrate evolution at work – small variations could be critical. Another fertile thread of investigation began with an encounter with Maihueniopsis darwinii, a flowering cactus he collected in Patagonia. One of many plants that now carry Darwin’s name, it surprised him with its forwardness. When he inserted his finger into the flower, its pollen‑producing stamens closed on it, followed more slowly by the petals. This, he realised, was just one mechanism flowers had evolved to force their pollen upon visiting insects and thence to other flowers.
This determination to cross-pollinate was an emerging theme Darwin would revisit at Down House with his orchids. Victorians were fascinated by these flamboyant plants. And so was Darwin.
“I never was more interested in any subject in my life than this of orchids,” he wrote in a letter to Joseph Hooker, a close mentor and the director of the Kew Gardens in London. It was the frivolity of their vivid markings, voluptuous lips and dramatic horns that so entranced Victorians. But Darwin saw the rationale in every part. “Who has ever dreamed of finding a utilitarian purpose in the forms and colours of flowers?” quipped biologist Thomas Huxley, another of Darwin’s close allies.
Darwin spent countless hours at Down House tracing the different strategies orchids had evolved for attracting insects to their nectar-secreting glands known as nectaries. The voluptuous lips – which resembled alluring female insects – were just the beginning. In some orchid species, the nectar pooled at the base of a narrow tube so that when an insect stuck its head inside searching for a meal, it would inevitably rub against the flower’s sticky pollen. Others had bucket-shaped flowers that trapped bees inside in such a way that they couldn’t climb out without crawling past the flower’s sticky pollen. Yet others had hair-trigger mechanisms that spewed pollen on to an insect’s back, and others that forcefully pushed pollen-carrying insects on to a flower’s receptive female parts.
Orchids were a dramatic example of the extent plants were prepared to go to in order to cross‑breed. “It is hardly an exaggeration to say that Nature tells us, in the most emphatic manner, that she abhors perpetual self-fertilisation,” he wrote in his book which was initially titled, On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects.
Cross-fertilisation mixed up the characteristics in each generation, ensuring each individual is slightly different. That natural variation, Darwin realised, was the raw material for evolution.
In the best scientific tradition, Darwin used his theory of evolution by natural selection to make predictions. If correct they wouldn’t necessarily prove his theory, but if found wrong they could have fatally wounded it. One of my favourite plant evolution stories is how Darwin predicted the presence of an insect based on the structure of a flower. The Star of Bethlehem (Angraecum sesquipedale) was discovered in the 1860s in the lowland forests of eastern Madagascar. When Darwin saw this unusual orchid, he theorised that since the nectar was at the bottom of a very long (25-30 centimetre) nectar tube, a pollinator had to exist with a proboscis at least as long. In 1903, 21 years after Darwin had passed away, a hawk moth with a 30-centimetre proboscis was discovered, Xanthopan morganii praedicta – the subspecies name ‘praedicta’ being a nod to Darwin’s prediction.
If you refer back to Darwin’s arguments about nectar, you can piece together the kind of evolutionary arms race responsible for such an odd outcome. You might start with an orchid with a small nectar tube and a moth with a small proboscis. From the plant’s point of view, if the tube is just a bit longer than the proboscis, the insect will bump its head on the pollen packet as it squeezes in. So plants with longer tubes are likely to be pollinated more often. The insect wants to get as much of the nectar as it can. So moths with a proboscis slightly longer than the tube do better and produce more offspring. It ends up as a competition between the plant’s need to be pollinated and the insect’s need to feed.
Over time pollen tubes and proboscises both grow longer and longer. At some point, a limit is reached when anything longer becomes energetically or structurally impossible. In this case 30 centimetres seems to be about it.
For Darwin it wasn’t always about evolution, although one suspects every odd or unusual behaviour he noticed would have been carefully filed away into his mental war-chest to defend his theory. In later life he became interested in plant movement. His final book on plants, published in 1880, documented for the first time “plant hormones”, messenger chemicals that trigger growth and determine whether a bud becomes a shoot or a root.
In The Power of Movement in Plants you can also read about gravity’s impact on germinating seeds and climbing plants, or a mini-treatise on circumnutation, the rotational movement of the growing tip of a plant. Today we study this with time-lapse photography. Darwin recorded it all himself with pen and paper.
Darwin would be thrilled to hear the latest discoveries in pollination biology, ecology and my own field, systematics (tracing the family tree of plants). Darwin also understood, as is only becoming clear today, that plants have a kind of intelligence – they sense and respond to their environment, they send signals from one leaf to another, and they communicate with other members of their species.
Indeed, as Darwin wrote in 1881, the year before he died, “it has always pleased me to exalt plants in the scale of organised beings”.
