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Darwin and the Meaning of Flowers

Darwin’s Garden: An Evolutionary Adventure

an exhibition at the New York Botanical Garden, April 25–July 20, 2008; and the Huntington Library and Botanical Gardens, San Marino, California, October 4, 2008– January 5, 2009
Catalog of the exhibition edited and with an essay by David Kohn
New York Botanical Garden, 60 pp., $17.99 (paper)

We all know the canonical story of Charles Darwin: the twenty-two-year-old embarking on the Beagle, going to the ends of the earth; Darwin in Patagonia; Darwin on the Argentine pampas (managing to lasso the legs of his own horse); Darwin in South America, collecting the bones of giant extinct animals; Darwin in Australia—still a religious believer—startled at his first sight of a kangaroo (“surely two distinct Creators must have been at work”). And, of course, Darwin in the Galápagos, observing how the finches were different on each island, starting to experience the seismic shift in understanding how living things evolve that, a quarter of a century later, would result in the publication of On the Origin of Species.

The story climaxes here, with the publication of the Origin in November 1859, and has a sort of elegiac postscript: a vision of the older and ailing Darwin, in the twenty-odd years remaining to him, pottering around his gardens at Down House with no particular plan or purpose, perhaps throwing off a book or two, but with his major work long completed.

Nothing could be farther from the truth. Darwin remained intensely sensitive both to criticisms and to evidence supporting his theory of natural selection, and this led him to bring out no fewer than five editions of the Origin. He may indeed have retreated (or returned) to his garden and his greenhouses after 1859 (there were extensive grounds around Down House, and five greenhouses), but for him these became engines of war, from which he would lob great missiles of evidence at the skeptics outside—descriptions of extraordinary structures and behaviors in plants very difficult to ascribe to special Creation or Design—a mass of evidence for evolution and natural selection even more overwhelming than that presented in the Origin.

Strangely, even Darwin scholars pay relatively little attention to this botanical work, even though it encompassed six books and seventy-odd papers. Thus Duane Isely, in his 1994 book One Hundred and One Botanists, writes that while

more has been written about Darwin than any other biologist who ever lived…he is rarely presented as a botanist…. The fact that he wrote several books about his research on plants is mentioned in much Darwinia but it is casual, somewhat in the light of “Well, the great man needs to play now and then.”

Even now, as we approach the two hundredth anniversary of Darwin’s birth and the hundred and fiftieth of the Origin, this is still very much the case, and it was with this in mind that the New York Botanical Garden recently launched an exhibition called “Darwin’s Garden: An Evolutionary Adventure.” This contained not only reconstructions of the gardens at Down and many of the actual experiments Darwin set up there, but a mass of rare books, papers, letters, and drawings.

Darwin had always had a special, tender feeling for plants and a special admiration, too (“it has always pleased me to exalt plants in the scale of organised beings,” he wrote in his autobiography). He grew up in a botanical family—his grandfather, Erasmus Darwin, had written a long, two-volume poem called The Botanic Garden, and Charles himself grew up in a house whose extensive gardens were filled not only with flowers but with a variety of apple trees crossbred for increased vigor. As a university student at Cambridge, the only lectures Darwin consistently attended were those of the botanist J.S. Henslow, and it was Henslow, recognizing the extraordinary qualities of his student, who recommended him for a position on the Beagle.

It was to Henslow that Darwin wrote very detailed letters full of observations about the fauna and flora and geology of the places he visited. (These letters, when printed and circulated, were to make Darwin famous in scientific circles even before the Beagle returned to England.) And it was for Henslow that Darwin, in the Galápagos, made a careful collection of all the plants in flower, and noted how different islands in the archipelago could often have different species of the same genus. This was to become a crucial piece of evidence for him as he thought about the role of geographical divergence in the origin of new species.

Indeed, as David Kohn points out in his splendid essay,

Darwin’s Galápagos plant specimens, numbering well over 200, constitute the single most influential natural history collection of live organisms in the entire history of science…. They also would turn out to be Darwin’s best documented example of the evolution of species on the islands.

(The birds Darwin collected, by contrast, were not always correctly identified or labeled with their island of origin; and it was only on his return to England that these—supplemented by the specimens collected by his shipmates—were sorted out by the ornithologist John Gould.)

Darwin had become close friends with two botanists—Joseph Dalton Hooker, at Kew Gardens, and Asa Gray at Harvard. Hooker had become his confidant in the 1840s—the only man to whom he showed the first draft of his work on evolution; and Asa Gray was to join the inner circle in the 1850s. Thus he would write to them both with increasing enthusiasm about ” our theory.”

Though Darwin was happy to call himself a geologist (he wrote three geological books based on his observations during the voyage of the Beagle and conceived a strikingly original theory on the origin of coral atolls, which was confirmed experimentally only in the second half of the twentieth century), he always insisted that he was not a botanist. One reason was that botany had (despite a precocious start in the early eighteenth century with Stephen Hales’s Vegetable Statics, a book full of fascinating experiments on plant physiology) remained almost entirely a descriptive and taxonomic discipline—plants were identified, classified, and named, but not investigated. Darwin, by contrast, himself was preeminently an investigator, concerned with the “how” and “why” of plant structure and behavior, not just the “what.”

Botany was not a mere avocation or hobby for Darwin, as it was for so many in the Victorian age; the study of plants was always infused, for him, with theoretical purpose, and the theoretical purpose had to do with evolution and natural selection. It was, as his son Francis wrote,

as though he were charged with theorising power ready to flow into any channel on the slightest disturbance, so that no fact, however small, could avoid releasing a stream of theory.

And the flow went both ways; Darwin himself often said that “no one could be a good observer unless he was an active theoriser.”

In the eighteenth century, the Swedish scientist Linnaeus had shown that flowers had sexual organs (pistils and stamens), and indeed had based his classifications on these. But it was almost universally believed that flowers were self-fertilized—why else would each flower contain both male and female organs? Linnaeus himself made merry with the idea, portraying a flower with nine stamens and one pistil as a bedchamber in which a maiden was surrounded by nine lovers. A similar conceit appeared in the second volume of Darwin’s grandfather’s book, The Botanic Garden, titled “The Loves of Plants.” This was the atmosphere in which the younger Darwin grew up.

But within a year or two of his return from the Beagle, Darwin felt forced, on theoretical grounds, to question the idea of self-fertilization. In an 1837 notebook, he wrote, “Do not plants which have male and female organs together yet receive influence from other plants?” If plants were ever to evolve, he reasoned, cross-fertilization was crucial—otherwise, no changes, no modifications could ever occur, and the world would be stuck with a single, self- reproducing plant instead of the extraordinary range of species it actually had. In the early 1840s, Darwin started to test his theory, dissecting a variety of flowers (azaleas and rhododendrons among them) and demonstrating that many of these had structural devices for preventing or minimizing self-pollination.

But it was only after On the Origin of Species was published in 1859 that Darwin could turn his full attention to plants. And where his early work was primarily as an observer and a collector, experiments now became his chief way of obtaining new knowledge.

He had observed, as others had, that primrose flowers came in two different forms: a “pin” form with a long style—the female part of the flower—and a “thrum” form with a short style. These differences were thought to have no particular significance. But Darwin suspected otherwise, and, examining bunches of primroses that his children brought him, he found that the ratio of pins to thrums was exactly one to one.

Darwin’s imagination was instantly aroused: a one-to-one ratio was what one might expect of species with separate males and females—could it be that the long-styled flowers, though hermaphrodites, were in the process of becoming female flowers and the short-styled ones male flowers? Was he actually seeing intermediate forms, evolution in action? It was a lovely idea, but it did not hold up, for the short-styled flowers, the putative males, produced as much seed as the long-styled, “female” ones. Here (as his friend T.H. Huxley would have put it) was “the slaying of a beautiful hypothesis by an ugly fact.”

What, then, was the meaning of these different styles and their one-to-one ratio? Giving up theorizing, Darwin turned to experiment. Painstakingly, he tried acting as a pollinator himself, lying face down on the lawn and transferring pollen from flower to flower: long-styled to long-styled, short-styled to short-styled, long-styled to short-styled, and vice versa. When seeds were produced, he collected and weighed them, and found that the richest crop of seeds came from the crossbred flowers. He concluded that heterostyly, in which plants have styles of different length, was a special device that had evolved to facilitate outbreeding—and that crossing increased the number and vitality of seeds (he called this “hybrid vigour”). Darwin later wrote, “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.”

Although this subject remained a special interest of Darwin’s (he published a book on it in 1877, The Different Forms of Flowers on Plants of the Same Species), his central concern was how flowering plants adapted themselves to using insects as agents for their own fertilization. It was well known that insects were attracted to certain flowers, visited them, and could emerge from blossoms covered with pollen. But no one had thought this was of much importance, since it was assumed that flowers were self-pollinated.

Darwin had already become very suspicious of this by 1840, and in the 1850s he set five of his children to work, plotting the flight routes of male humble bees. He especially admired the native orchids that grew in the meadows around Down, so he started with them. Then, with the help of friends and correspondents who sent him orchids to study, and especially Hooker, who was now director of Kew Gardens, he extended his studies to tropical orchids of all kinds.

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