He excitedly turned to other orchids in the vicinity: the pyramidal orchid (Anacamptis pyramidalis), aptly named for its tight pyramid of purple flowers, grew profusely at Orchis Bank and was soon revealed to transfer its pollinia a bit differently. Here the two pollinia were joined at the base by a narrow strap or saddle, somewhat resembling those perky antennae of any respectable Martian costume: the pollinia are like the upright antennae affixed to the curved headband. Darwin soon realized why this shape proved useful: once the structure latches itself onto the proboscis of an unsuspecting insect, the curved band pinches tight, making it all but impossible to dislodge. Perhaps the connecting band is an adaptation to ensure that both pollinia are transferred simultaneously, since in species where they are not attached to one another it’s very common for just one to adhere to an insect. In any case, as with its relative the early purple orchid, the stalks of pyramidal orchid pollinia take the same 90º bow, positioning the pollen sacs on either side of the proboscis, ready to contact a receptive stigma from another orchid. Darwin exulted to Joseph Hooker: “You speak of adaptation being rarely visible though present in plants: I have just recently been looking at common Orchis, & I declare I think its adaptations in every part of flower quite as beautiful & plain, or even more beautiful, than in Woodpecker.”6
Pollinium of Orchis mascula stuck to a probing pencil. Each pollinium is in an upright position upon removal from the flower (A), but within about 30 seconds they tilt forward by 90º (B), well positioned to contact the stigmatic surface of the next flower. From Darwin (1862a), fig. II.
This comment, from a letter written in June 1860, is revealing of Darwin’s growing dual interest in orchids: their pollination was part and parcel of his broader investigations into the mechanisms and beneficial effects of cross-fertilization in plants, but now he began to marvel at just how elaborate their pollination tricks are, as beautiful adaptations. It was fortuitous that Darwin happened to work with O. mascula and A. pyramidalis first, as the tilting of the pollinia of these species is especially easy to observe owing to their long stalks. He eventually confirmed that this occurs in a great many orchid species, but the movement is not as obvious in those with short-stalked pollinia.
(A) Paired pollinia of the pyradmidal orchid, Anacamptis pyramidalis, with sticky strap-like base. (B) Dorsal and lateral views of pollinia attached to a needle probe. (C) Head and proboscis of the four-spotted moth (Tyta luctuosa, Noctuidae) caught by Darwin bearing seven pairs of pyradmidal orchid pollinia attached to its proboscis. Note that the pollinia are aimed in the forward direction. In his orchid book Darwin listed 23 species of butterflies and moths with A. pyramidalis pollinia attached to their probosces. From Darwin (1862a), figs. III and IV.
Darwin published a long open letter on orchid pollination in early June. It sheds a light on Darwin’s orchid-inspired activities and musings of the previous month since his surprising discovery of pollinia movement. We see the experimentalist in Darwin come to the fore: spying wild orchids at the sandwalk and in the orchard he used a bell glass to prevent insects from visiting some while leaving others accessible. He meticulously checked the pollinia of new flowers as they expanded, showing that the pollinia were always removed from the flowers left exposed but never from those under the bell glass. Approaching this a different way, on plants bearing flowers in a long spike or raceme, Darwin took advantage of the “age gradient”—flowers lower down are oldest, while the uppermost ones are youngest. The older ones tended to be missing pollinia, while the younger ones still had them intact. Finally, he recorded cases where he found pollinia stuck to the stigma of orchid flowers that still had their own pollinia intact: clear evidence that pollen was being transferred between flowers. (Remember that in Darwin’s day many people assumed that flowers self-pollinate as a rule.)
This was all of a piece to make the case that these plants must cross to reproduce, and that crossing is officiated by insects. The mystery in many cases was which insects, and when did they visit the flowers? At least, that was a mystery for certain species like the bee orchis (Ophrys apifera), a species no one could recall ever seeing an insect visit. The bee orchis does vaguely resemble large bees, and Robert Brown once speculated that this was to deter insects from visiting, as opposed to the usual floral ploy of drawing insects in. Darwin came to disagree, unsurprisingly, but he never did learn the secrets of its pollination. Unlike other orchids this one seemed to be always self-fertilized, even though the pollinia sport those sticky ends that in other orchids function to glue the pollinia to insects. If this was a useless feature, selection might be expected to have eliminated it from the population. “What are we to say with respect to the sticky glands of the Bee Orchis,” Darwin asked in his article, “the use and efficiency of which . . . in all other British Orchids are so manifest? Are we to conclude that this one species is provided with these organs for no use?” He did not think so: “I . . . would rather infer that, during some years or in some other districts, insects do visit the Bee Orchis and occasionally transport pollen from one flower to another, and thus give it the advantage of an occasional cross.”7
Darwin supposed that insects visit these flowers at least occasionally, and he asked the readers of the Gardeners’ Chronicle for help in his typical crowd-sourcing fashion. Fellow orchid enthusiast Alexander Goodman More tried and failed to document insect pollination. “All facts clearly point to eternal self-fertilisation in this species,” Darwin acknowledged to More. “Yet I cannot swallow the bitter pill.”8 Darwin ultimately had to admit that the bee orchis seems to largely self-pollinate. His close observations revealed to him how. The sticky discs are not so useless after all: the pollinia are attached to them by a long, thin, flexible stalk. They develop in an upright position, but when they mature they fall out and hang limply by their stalks, still firmly attached by their sticky feet. In that position the pollinia are directly in front of the orchid’s stigma, so that even slight breezes can cause them to sway and sooner or later come into contact with it, where they stick fast, thus self-pollinating.
The bee orchis is indeed the only member of the genus Ophrys that relies mainly on self-pollination, but Darwin’s hunch that they must outcross at least occasionally (maybe regularly in some parts of its range) is correct, though he never learned how. This plant is distributed from central and southern Europe, and around the Mediterranean in North Africa and the Middle East. In the northernmost portions of its range—including England—they are almost exclusively self-pollinated, but solitary bees are known to visit them in the Mediterranean populations. This is a fairly common bet-hedging strategy among flowering plants, combining the certainty of selfing with chancy but beneficial outcrossing, if only occasionally. But in any case there is more to the pollination story of these orchids: although Darwin figured that the bee-like flowers somehow attracted insects, it was not until the early twentieth century that the phenomenon of floral insect mimicry was described by the French Algerian botanist Maurice-Alexandre Pouyanne and his Swiss colleague Henry Correvon. Pouyanne, in particular, discovered that bees not only visit the flowers, they attempt to copulate with them. This duping of amorous male hymenopterans is called pseudocopulation, and has been found in a wide variety of orchids worldwide, including perhaps most spectacularly the rare hammer orchid (Drakea spp.) of Australia, which is pollinated solely by male thynnid wasps. The Swedish biologist Bertil Kullenberg in the mid-twentieth century built upon and greatly extended the work of Correvon, Pouyanne, and others. He discovered that the mimicry employed by hammer orchids extends beyond the visual and tactile to the chemical: they produce a fragrance mimicking the very sex pheromones of the bees and wasps they dupe, which explains the frantic repeated attempts at copulation by the hapless love-struck males. Luckily for them the orchids are merely duplicitous and not Circean—they are not drawn in and trapped; after all, the now-pollinia-studded males must live to visit other flowers.
All Ophrys orchids seem to have some degree of self-pollinati
on (the bee orchis perhaps most of all), maybe as an adaptation to the ups and downs of local bee and wasp populations over evolutionary time. But Darwin would be fascinated to learn of another orchid that takes self-pollination even further. In 2006 a team of Chinese scientists found that pollinia of the high-elevation orchid Holcoglossum amesianum can rotate a full 360° in a remarkable feat of contortionist self-pollination. They go from their initial upright position to a hanging position like those of the bee orchis and then back up again to firmly press against the stigma. Where the bee orchis relies on gravity and wind to effect pollination, H. amesianum can rotate its pollinia against gravity. This unusual form of “assured selfing” may have evolved in this species owing to the virtual absence of pollinating insects in its harsh, dry mountainous habitat. The team studying H. amesianum inspected nearly 2,000 flowers and always found this same manner of pollination. But are they perpetually inbred? Darwin would surely hold out for the discovery that sometimes, somewhere tucked away in a remote corner of its range, an insect performs pollination services for this orchid; after all, even a little bit of outbreeding goes a long way.
Orchidelirium
As spring of 1860 turned to summer Darwin continued multitasking as usual, juggling sundews, primroses, and orchids. Several correspondents sent him insect specimens with protruding pollinia, while others made field observations for him on Bee Orchis and other species or sent him fresh specimens for study. Etty apparently suffered typhoid fever, and as her recovery was very slow the family decided to pack up and head off to Hartfield, Sussex, to stay with Emma’s sister Sarah Wedgwood in the hopes that a change of environs would be beneficial to their sick child. He took a week-long detour first to take the “water cure” at Sudbrook Park—and it may not have been entirely coincidental that the Oxford meeting of the British Association for the Advancement of Science was held at just this time. This was the meeting that saw fireworks over the Darwin-Wallace theory, not least the famous exchange between Bishop Samuel “Soapy Sam” Wilberforce and Thomas Henry Huxley. Darwin preferred to keep a low profile, letting his friends rally to his defense; his chronically churning stomach was perhaps a casualty of the heated debate over the theory, or provided a good excuse to stay out of the fray.
In the meantime he kept busy. Although he bemoaned having done nothing researchwise in a letter to Hooker at the end of July, in the same letter he mentioned making “some good observations” of a species of Malaxis, adder’s mouth orchid, kindly provided by a local doctor named William Wallis. These curious orchids secrete a sticky droplet on the rostellum that serves the purpose of, first, catching the pollinia as they ripen, and then adhering them to probing insects, similar to the Listera orchids (twayblades) that Hooker had studied back in the mid-1850s. It was there, too, that he inspected the pollinia of Orchis pyramidalis, the ones united at the base, and enthused to Hooker how they outdid Listera as a marvel of adaptive contrivance: “it almost equals, perhaps even beats, your Listera case . . . I never saw anything so beautiful.”9
And so it went, with Darwin increasingly excited about orchid after orchid, the rich variety of structures and pollination mechanisms, each odder than the next. It was almost too rich: orchid flowers vary in such extreme ways in different orchid groups it’s very difficult to see commonalities between them. Yet to be able to relate them in some kind of series that reflects their evolutionary history, underscoring how even the most bizarre orchid reflects modification of the same basic parts, was of paramount importance to Darwin. And they could be bizarre indeed, like the tropical Catasetum orchid with its “dull and coppery and orange-spotted tints—the yawning chasm in the great fringed labellum,—the one antenna stuck out with the other hanging down,” giving to these flowers, Darwin wrote, “a strange, lurid, and reptilian appearance.”10 (More on this orchid to come.) It was Hooker who pointed out to him that he could use the spiral-shaped veins or vessels to trace homologies of different orchid parts, and he at once tried to teach himself the finer points of orchid anatomy.
Darwin returned home on August 2nd with the boys in tow, Emma and the girls following a few days later. Etty was showing signs of improvement, to her parents’ relief, but she remained poorly and so in mid-September they decided to head to Eastbourne for a month-long holiday for sunshine and balmy sea breezes. He made the best of it but local orchids were scarce, so he poked around for some sundews, those diminutive carnivorous plants that had ensnared his interest ever since coming across them at Hartfield that summer [see Chapter 8]. He really should have been at work on his domestication book, but was enjoying the orchids and sundews too much. He guiltily fessed up in a letter to Lyell: “I have been of late shamefully idle; ie observing instead of writing & how much better fun observing is than writing.”11 Many a scientist can relate to the fun of doing research and discovering things, as opposed to the sometimes tedious process of writing it all up. But Darwin being Darwin there was even more vying for his attention, such as his recent discovery of primrose flower polymorphism, continuing pollination and crossing observations, and keeping up with a steady stream of correspondence over the Origin and the battle over his and Wallace’s theory.
What to do . . . so many interests, so little time! Research-wise the orchids won out for now. Down House soon became Orchid Central, with specimens from the dainty to the dazzling arriving from around the British Isles and (via Kew and various nurseries) the world. From the Scottish Highlands his correspondent George Gordon sent ladies’ tresses (Goodyera) with their spiral wands of delicate white flowers, while lurid orange-spotted “lizard orchids” (Catasetum saccatum) with their grotesque antenna-like processes arrived courtesy of James Veitch’s Royal Exotic Nurseries in Chelsea. George Chicester Oxenden of Canterbury and his friend Bingham Malden obliged him with consignments of rare Orchis species among others, and Alexander More, orchid maven of the Isle of Wight, continued to provide specimens and field observations. Darwin made his way steadily through an array of species worthy of a splashy botanical garden orchid show, except rather than ogling his prizes, he got to work peering, poking, and dissecting to work out how each is pollinated and trace the relationships of their parts.
There was order to his orchidelirium: he aimed to examine representatives of each of the seven great orchid tribes described by John Lindley, noted botanist and orchid specialist, in his book The Vegetable Kingdom. Published in 1846 and reissued in several expanded editions, Lindley’s book was the first attempt at a comprehensive classification of orchids and provided Darwin with the best available guide to their relationships, a useful framework for testing his ideas about the diversification of their complex structures and pollination. Nearly all the British orchids were confined to just two of Lindley’s tribes, which led to his great need for orchids from abroad. His helpful neighbor George Henry Turnbull even lent him the use of his greenhouses, as Darwin wasn’t to build his own for a few years yet.
In the meantime Etty had continued to do poorly, and so during the following summer the family once again found themselves heading to the seaside. They arrived at a rented cottage at Torquay on July 2nd, and William, who was finishing his first year at Cambridge, joined them a week later. Darwin, keeping up his correspondence as usual, learned that Hooker and his family were planning a seaside respite too. Hooker’s wife, Francis Harriet Hooker, née Henslow, was deeply grieving the recent loss of her father, John Stevens Henslow, Darwin’s great mentor from Cambridge. Darwin memorialized the much-loved Henslow, recalling his “simple, cordial, and unpretending . . . encouragement which he afforded to all young naturalists.”12 Henslow had certainly encouraged Darwin, and was responsible for Darwin’s invitation to join the voyage that changed the course of the young naturalist’s life—and the course of science.
Hooker, too, was encouraging of young naturalists, especially where his friends were concerned. Darwin’s son William was set to become a partner in a Southampton bank, but he had a love of botany that his father was keen to encourage. Da
rwin sought Hooker’s advice on nurturing it. Among other things, Hooker invited William to join him on a botanical excursion. The timing didn’t work out, however, and so that summer Willy amused himself with dissecting and drawing plants during his holiday with the family at Torquay. Hooker sent a selection of orchids for the Darwins, father and son, to work on, as the Torquay countryside proved to have a limited selection in the orchid department. Darwin was particular about which ones he sought: “I much want a Cattleyea [sic] or some one of the Epidendreae [an orchid tribe], as I have examined a Humble-Bee with the pollinia of [Cattleya] attached to its back. Really the contrivances in Orchids beat, I think, any animal.”13 Not 2 weeks later, a box arrived—being close friends with the director of the greatest living plant conservatory in the world clearly had its perks!
Darwin was thrilled: “My dear Hooker, you cannot conceive how the Orchids have delighted me,” he enthused. He oohed and aahed about each orchid Hooker sent, and then: “I most specially want to know what [the] little globular brown Orchid is . . . surely have you not unintentionally sent me what I wanted most . . . viz one of the Epidendreae?!”14 It was indeed a Cattleya orchid, and in view of Darwin’s veritable pleading for one its inclusion was surely intentional. This genus was named for Englishman William Cattley, businessman and horticulturist who first coaxed a blossom from this tropical orchid in 1824. Darwin’s rapture was complete. Though the “globular brown” description may sound unpromising, this orchid represents a sizable genus from the American tropics celebrated for large, vivid flowers. He longed to see a Cattleya since the previous month when entomologist Frederick Smith at the British Museum sent him a bumblebee bearing several puzzling pollinia stuck to its back with a smear of dried glue-like material. Darwin realized that they came from no British orchid, and asked Smith if the bee was caught near a greenhouse. Indeed, it came from within a greenhouse, housing Cattleya orchids.
Darwin's Backyard Page 26