Mention of Charles Darwin, for most, conjures up images of intrepid Victorian sea voyages, giant tortoises and Galapagos finches. Few of us associate Darwin with plant sex. That honor tends to go to his grandfather, Erasmus Darwin, who wrote erotic poems on the topic.

Although Darwin’s On the Origin of Species, which describes his theory of evolution by natural selection, has eclipsed all his other research, his career continued for over two decades after the landmark work’s publication. Much of the aging naturalist’s time was spent studying botany, and his research produced discoveries that, had he not become famous for natural selection, would have made him a well-known botanist.

Darwin sailed on the HMS Beagle from 1831 to 1836, exploring coastal South America and circumnavigating the globe. But once he returned home at the age of only 27, he never left Britain again. Ill health that would plague him for the rest of his life drove him in 1842 to a quiet residence at Down House, his home in the Kentish countryside, until his death 40 years later.

Though Darwin had already made his reputation as a scientist, those latter years were hardly spent idle. He turned his gardens and greenhouses at Down House into a personal laboratory. Rather than travelling to see distant exotic species, Darwin had the specimens brought to him, corresponding with far-flung botanists and collectors and arranging for seeds and plants to be sent to his home. He also made forays across the British countryside to observe native orchids, whose forms fascinated him. Darwin was captivated by the question of why flowers come in so many shapes, sizes and arrangements when they’re all meant to accomplish the same thing: fertilization.

Darwin wrote about a number of botanical topics, including carnivorous plants. But his most important work of botany was perhaps his 1862 book on orchids, titled On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing. In this book and others, he wrote about floral forms and cross-pollination, which would provide the rigorous experimental data that brought his theory of natural selection widespread acceptance in the scientific community. Darwin’s experiments with flowers would also lay the foundations for the nascent field of plant reproductive biology.

Following the publication of On the Origin of Species, Darwin expected pushback from his scientific peers in the form of critique of the theory’s ability to explain known phenomena—a bout of scientific sparring that he was prepared for. What he didn’t foresee were the attacks on his character as a scientist. Because Origin was primarily based on Darwin’s comprehensive observations, using his theory to explain what he saw, scientists of the day criticized it for lacking the power to make predictions and guide experimentation as any useful theory should.

Darwin’s detractors suggested that Origin was nothing more than an indulgence in wild speculation, a cardinal sin for a respectable Victorian man of science. In his study of orchids, however, Darwin designed highly rigorous experiments and made predictions—which turned out to be correct—using his theory of natural selection. For example, he predicted that the myriad floral adaptations he saw existed to ensure that flowers were outcrossed, or fertilized by individuals other than themselves. He then tested this hypothesis with over a decade of pollination experiments and found that self-pollination leads to lower fitness and higher sterility. Inbred plants, like inbred animals, don’t fare well, at least over time—a phenomenon that’s now known as inbreeding depression.

As Richard Bellon, associate professor at Michigan State University and a specialist in 19th century science history, puts it: “The botany really showed [natural selection] could be a tool to move forward and discover new things about the natural world, and scientists value theories not so much because they tell them what to think, but because it gives them things to do out in the natural world.”

Demonstrating his theory on plants rather than animals also moved the conversation about natural selection away from an intense and intractable debate about human evolution into safer, more familiar territory. The orchid research took natural selection “out of one of these very contentious realms of abstract, religious and metaphysical speculation, to the type of work where you just simply get down and get your knees dirty,” Bellon says. “Even naturalists who were skeptical, or in some cases implacably hostile, to evolution by natural selection at least admired what he was doing in this type of research. If they're willing to join him to get their knees dirty, they can have this conversation where, if they talked about human origins they would absolutely remain at loggerheads.”

Darwin himself acknowledged this strategy in a letter to the American botanist Asa Gray, calling his orchid work “a ‘flank movement’ on the enemy.” By 1863, the year following the publication of Orchids, the scientific debate over natural selection had shifted in his favor.

But the importance of Darwin’s botanical work doesn’t end with his victory convincing contemporary scientists of the value of his natural selection theory. His research laid out a new paradigm for studying floral adaptation that invigorated the entire field. “It's not just that Darwin did botany, but he inspired literally thousands of other studies, large and small, that started with his basic assumptions and then followed those up into other types of species and then other environments,” Bellon says. Darwin’s explanation of curious floral forms as being the result of natural selection rather than the imaginative work of a divine creator enabled scientists to understand many previously incomprehensible phenomena. Vestigial organs, or organs that had become degraded and useless as species evolved, made sense for the first time. It didn’t seem logical that God would endow a flower with the shriveled, nonfunctional remains of a stamen, but it did make sense that a stamen no longer needed for effective reproduction would degrade over the course of many generations.

“In terms of just the body of research, [Darwin’s pollination work] really did have an enduring legacy, and in a way, stimulated what is a huge field today: plant reproductive biology,” says Spencer Barrett, professor emeritus of ecology and evolutionary biology at the University of Toronto. “It's hard to pick up an issue of most of the top journals in ecology and evolutionary biology without seeing that somebody is publishing something from the pollination of this, or the mating system of that, and so on. You can trace all that back, in a way, to Darwin.”

“He certainly was a botanist,” Barrett adds. “There's no question.”

The conclusions that Darwin drew from his work on plants largely stand up even today. Barrett thinks this longevity can be attributed to the exhaustiveness of his experiments and observations. "Darwin was not satisfied with looking at one species," Barrett says. "He just did it over and over again, in different species, and different families. He really wanted to get a result that was general."

Darwin spent more years working on plants than any other type of organism. He used botany to build support for his theory of evolution and founded the field of plant reproductive biology in the process. The man known for shaking the world of biology to its very core was never happier than during those years after the Beagle voyage, tucked away in his greenhouse laboratory in Down House, studying flowers.

As Darwin wrote in his autobiography, referring to his work on the floral forms of primulas, “I do not think anything in my scientific life has given me so much satisfaction as making out the meaning of the structure of these plants.”