Kingdom of Plants: A Journey Through Their Evolution. Will Benson
instincts as a naturalist stemmed from his love of collecting, and his belief that in order to understand any aspect of the natural world, one must acquire, and carefully examine every facet of it. He once wrote, ‘By the time I went to school my taste for natural history, and more especially for collecting, was well developed. The passion for collecting, which leads a man to be a systematic naturalist, a virtuoso or a miser, was very strong in me, and was clearly innate, as none of my brothers or sisters ever had this taste’. In his quest to make sense of the elaborate flowers of the orchid family Darwin began amassing his own collection of these rare plants, which he held in his glass conservatory at Down House. Countless orchids from Malaysia, the Philippines and Central America made their way via Kew to his house, together with the British species which grew in abundance nearby. But the nature of the most extreme orchid flowers did not fit well with his theory of evolution. In one letter that Darwin wrote in 1861 to John Lindley, who worked as one of Kew’s taxonomists at the time, he describes his utter fascination with the complexity of orchids, discussing one genus in particular called Catasetum: ‘I have been extremely much interested with Catasetum, and indeed with many exotic orchids, which I have been looking at in aid of an opusculus, on the fertilisation of British Orchids. I very much fear that in publishing I am doing a rash act; but Orchids have interested me more than almost anything in my life. Your work shows that you are carefully understanding this feeling.’
Deception
The labellum of this mirror bee-orchid has evolved to mimic the shape and shine of an iridescent bee.
© RBG Kew
Darwin studied the lives of orchids and dissected them, looking at the multitude of ways in which the plants guided specific bees or moths to their flowers to interact with their reproductive structures, and the mechanisms they exhibited to achieve pollination. He was searching for an explanation for all aspects of each flower’s behaviour, and a justifiable origin for each. But for many of his adversaries, what Darwin was trying to achieve was considered impossible, and even his good friend Thomas Huxley famously stated, ‘who has ever dreamed of finding a utilitarian purpose in the forms and colours of flowers?’ Darwin made good headway in unravelling the sex lives of orchids, and he made detailed studies of the ways in which they lured pollinators and released their pollen. But what had him most stumped was that, more so than any other family of flowers, orchids exhibit extreme pickiness in whom and what they allow to spread their pollen. On the subject of this he wrote: ‘Why do orchids have so many perfect contrivances for their fertilisation? I am sure that many other plants offer analogous adaptations of high perfection; but it seems that they are really more numerous and perfect with the Orchideae than with most other plants.’ What seemed counterintuitive to Darwin was that for all their elegance, the pollination methods employed by orchids seemed terribly inefficient.
Darwin’s trouble with trying to explain the nature of the sex life of orchids becomes all the more apparent as soon as you begin to unfold the highly specialised ways which we now know different species achieve pollination. In the mirror bee-orchid (Ophrys speculum), found in southern and western Europe, as well as Lebanon, Turkey and North Africa, the lip of the flower looks nearly identical to an iridescent bee. This was first suggested to be a ploy to prevent grazing animals from munching on it, but we now know that it in fact releases a chemical that mimics the pheromones of a female bee, as a trick to get males to ‘mate’ with it. By rubbing its body on the flower in an attempt to copulate, the male bee will rub itself up against the plant’s sticky bundles of pollen, called pollinia, which adhere to its body, before it flies away and attempts to mate with another bee-orchid. Another extreme behaviour has evolved in the orchids of the genus Oncidium from Ecuador, which have petals that look like the insect competitor of the Centris bee. The bee attempts to chase this ‘enemy’ away from its territory, and in doing so it strikes the flower, showering itself in the plant’s sticky bundles of pollen. The slipper orchids of Asia and South America have a hinged lip which forces insects to brush past the sticky pollen before leaving, and there is even an underground species of orchid from Australia called Rhizanthella slateri which relies on ants to move its pollen. Other orchids emit a smell of rotting flesh to attract meat-loving flies to pollinate them, while some have been found to smell like chocolate.
Perhaps the most intriguing pollination syndrome is that of Catasetum, which is so complex it seemed to contradict Darwin’s very theory of evolution. Unusually for orchids, some Catasetum plants are male and others are female. The male produces a scent that attracts just one species of euglossine bee. Lured by its sweet smell, the bee lands on the lip of the orchid and thrusts its head into the flower, touching a hair-trigger. This activates a mechanism that fires out a tiny bundle, which then sticks onto the bee’s back. This extraordinary projectile is in fact a bundle of pollen grains called a pollinium, which has a little cap on it, and after a minute or so the cap falls off to reveal a little horseshoe-shaped bundle of pollen grains. A group of researchers in the USA recently found that the pollinium is ejected with an acceleration rate of over ten times that of a striking pit viper. Having been struck by this pollen, the bee flies away and is attracted to another rather different-looking flower, which is the female. Once again, lured by the scent, it sticks its head into the female flower – and the little bundle of pollen attached to its back, like a key, fits into a small aperture on the roof of the flower, like a lock, pulling off the pollen as the bee makes its departure. Pollination has been achieved.
The Catasetum conundrum
The structures of these orchids interested Darwin immensely on account of their incredible complexity.
© Will Benson
Darwin’s obsession with Catasetum in particular caused him to dedicate a great deal of time to studying the flower’s mechanism, in an attempt to make sense of how it could have arrived at such a precise and specific system. His tireless persistence paid off, and in a letter to his publisher John Murray in 1861 he wrote, ‘I have had the hardest day’s work at Catasetum and the buds of Mormodes, and believe I understand at last, the mechanism of movements and functions. Catasetum is a beautiful case of slight modification of structure leading to new functions.’
Having unravelled the complexities of exploding pollen bundles, it wasn’t long before Darwin’s next botanical mystery would land on his desk, quite literally. In 1862 he received a package from renowned horticulturist James Bateman, a striking orchid with a flower composed of large star-shaped white petals from the island of Madagascar, named Angraecum sesquipedale. Darwin set about detailing the ornate nature of his latest specimen and was struck by the long tubes, called spurs, in which the plant’s nectar was contained. Its delicate spurs were over 30 centimetres long, hanging down beneath the flower like white tails, with the nectar contained at their tip. Having never seen anything quite like this before, he wrote to his esteemed friend Joseph Hooker to explain these foot-long, whip-like nectaries, exclaiming, ‘Good Heavens what insect can suck it!’ Later that year Darwin went on to publish a book on the reproduction of orchids, in which he theorised that in order for the Madagascan orchid to be pollinated, an insect, most probably a moth, must exist on the island of Madagascar with a tongue at least 30 centimetres long which can reach the nectar at the end of the spurs. His suggestion seemed ludicrous to many of his peers, but a paper written by fellow evolutionary theorist Alfred Russel Wallace a few years later sought to back up Darwin’s notion, by highlighting that a large hawk moth had been discovered in Africa which had a tongue almost 20 centimetres long, called Xanthopan morgani. Wallace predicted that if such a moth existed in Africa, then surely a moth with a 30-centimetre-tongue could live in the forests of Madagascar. Unfortunately Darwin was never able to see his prediction come true, but in 1903 a population of hawk moths with the necessary tongues were found on Madagascar. The team who discovered it then aptly named it Xanthopan morganii praedicta – the predicted subspecies of X. morgani.
Mystery of the moth
The relationship between hawk moth and Angraecum sesquipedale is one of