Darwin's orchid

Appearance

"Angraecum sesquipedale" is a monocot with monopodial growth and can grow to a height of 1 m. Its growth habit is rather similar to species in the genus "Aerides". The leaves are dark green with a bit of a grayish tone and leathery with a bilobed tip. They are usually around 20–40 centimeters long and 6–8 cm wide. The roots are dark gray, thick, and emerge from the orchid's stem. There tend to be few roots and they attach to the bark of the trees quite strongly. Each of the succulent roots can extend along the trunk of the tree for several meters.

There is also a variation of this species, namely "A. sesquipedale" var. "angustifolium". "A. sesquipedale" var. "angustifolium" tends to be smaller than "A. sesquipedale" and has narrower leaves. The chromosome number of "A. sesquipedale" is 2n=42. "A. sesquipedale" has also previously gone by the synonyms "Aeranthes sesquipedalis" Lindl., "Macroplectrum sesquipedale" Pfitzer, "Angorchis sesquepedalis" Kuntze, and "Mystacidium sesquipedale" Rolfe.

Naming

The orchid was first discovered by the French botanist Louis-Marie Aubert du Petit-Thouars in 1798, but was not described until 1822. It is noteworthy for its long spur and its association with the naturalist Charles Darwin, who surmised that the flower was pollinated by a then undiscovered moth with a proboscis whose length was unprecedented at the time. His prediction had gone unverified until 21 years after his death, when the moth was discovered and his conjecture vindicated. The story of its postulated pollinator has come to be seen as one of the celebrated predictions of the theory of evolution.

Habitat

It is often found in lowlands in Madagascar at altitudes below 400–500 feet, near the east coast of the island, and on trees that are at the edge of forests. Usually it is attached to trees with fewer leaves and to areas of the branch or trunk that are driest. This allows the plant to obtain a great deal of light and air movement. Larger plants are usually found growing within 12–20 ft from the ground, whereas smaller plants are often found higher up in the canopy. Rarely "A. sesquipedale" is also found growing as a lithophyte and sometimes even as a semi-terrestrial. The orchid lives in an environment with heavy rainfall, up to 150 in per year. There is no dry season so the growing season is continuous.

Evolution

"Angraecum sesquipedale" is best known within the botany community for its association with the naturalist Charles Darwin. After being sent several flowers of "A. sesquipedale" by James Bateman, Darwin noted the defining characteristic of the species, its extremely long spur. From his observations, Darwin surmised, in his 1862 publication "On the Various Contrivances by Which British and Foreign Orchids Are Fertilized by Insects, and On the Good Effects of Intercrossing", that there must be a pollinator moth with a proboscis long enough to reach the nectar at the end of the spur. He arrived at this conclusion after attempting in vain to remove the pollinia of the flower using needles and bristles. Only after placing a cylinder with a diameter of ​1⁄10 of an inch down the full length of the spur was he able to detach the pollinia upon retracting it. The viscidium attached to the cylinder as he removed it. Darwin surmised that during the moth's attempt at getting the nectar at the end of the spur, the moth would get the pollinarium attached to itself. The next orchid it visited would then be pollinated in the same manner.

For some time after this prediction the notion of a pollinator with a 35 cm long proboscis was ridiculed and generally not believed to exist. After Darwin's publication, George Campbell, 8th Duke of Argyll published a book in 1867 titled, "The Reign of Law", in which he argued that the complexity of this species implied that it was created by a supernatural being. Alfred Russel Wallace replied in the same year with a paper he titled "Creation by Law", setting out in detail a sequence through which the moth and the flower could have coevolved with no guidance other than natural selection.

In 1903, such a moth was discovered in Madagascar by Walter Rothschild and Karl Jordan. This confirmed Darwin's prediction. The moth was named "Xanthopan morganii praedicta". It is possible that the subspecific epithet "praedicta" was given in honor of the fact that Darwin predicted its existence, but there is no reference to Darwin in the paper that described the moth. A more conservative explanation is simply that the existence of the moth had been predicted and widely accepted before it was discovered. In 1873 William Alexander Forbes wrote an article in the journal "Nature" asking readers if they knew of the moth predicted by Darwin. A reply to the question was first made that same year by Hermann Müller. He announced that his brother Fritz Müller had discovered a moth with a proboscis of 30–33 cm long, but it was discovered in Brazil and so was not a candidate for pollinating "A. sesquipedale". Although Darwin learned of Müller's finding he did not live to see the discovery of "Xanthopan morganii". Even after the 1903 discovery however, news of "Xanthopan morganii praedicta" was not immediately disseminated. A second inquiry into the existence of the pollinator moth was made in the 30 January 1907 issue of the journal "Nature" by E. W. Swanton. Presumably still unaware of Rothschild and Jordan's discovery, Wallace responded stating that he didn't know of a suitable pollinator in Madagascar, but that he had heard of one from East Africa with a long enough proboscis.

Both Darwin and Alfred Russel Wallace had suggested that the evolutionary basis for how the odd relationship between the sphinx moth and "A. sesquipedale" evolved over time could be understood by considering one orchid with a long spur and another with a short spur. If a moth goes to fertilize a flower with a short spur its proboscis would easily reach all the way to the bottom of the spur and it would get the nectar. However, since the proboscis of the moth is longer than the spur of the flower, the head of the moth would not touch the flower obtaining the pollinarium and so the flower would not be fertilized. The orchid with the longer spur on the other hand would be able to be fertilized since the entire length of the proboscis fits within the spur and thus allowing the head of the moth to touch the flower and become connected to the pollinarium. As a result, over time plants with longer spurs would be more likely to reproduce and so become more prevalent in the population. In this way "A. sesquipedale" has evolved to have a very long spur. The moth too would evolve to have a longer and longer proboscis in the following way. If a moth goes to fertilize an "A. sesquipedale" flower and the spur is longer than its proboscis then it will not be able to reach all of the nectar. As such, moths with too short of a proboscis would not be able to get as much food as those moths with a longer proboscis who could reach all of the nectar. Due to this arrangement moths with longer proboscis would become more physically fit to reproduce due to their ability to get more nectar and so such moths would become more prevalent in the population. This can result in a seesawing effect by which both organisms produce a mechanism that leads the other to increase the others spur and proboscis. There are however certain properties that no doubt prevent this mechanism from continuing indefinitely. For example, the risk such a long proboscis poses to a moth could be a factor that would prevent the spur of "A. sesquipedale" from becoming indefinitely long. If moths with proboscises that were too cumbersomely long substantially risked their lives due to being easier prey, then such moths could only afford to evolve a proboscis to a certain length. This would in turn restrict the length of the orchid's spur, since moths would not want to visit flowers whose spurs were too long since they would not be able to reach the nectar.

There was also another explanation why the spur of "A. sesquipedale" grew so long proposed by Thomas Belt in his 1874 book "The Naturalist in Nicaragua". Belt suggested that the spur grew long in order to prevent other moths with shorter proboscises from drinking the nectar. Darwin took up this explanation briefly in a footnote of the second edition of his famous orchid book, explaining that although this explanation was no doubt true, it cannot account for the lengthening spur.

The fertilization of "A. sesquipedale" has been observed to proceed as follows. The moth approaches the flower to ascertain by scent whether or not it is the correct orchid species. Then the moth backs up over a foot and unrolls its proboscis, then flies forward, inserting it into a cleft in the rostellum which leads to the spur while gripping the labellum. After the moth has finished drinking the nectar, which usually takes about 6 seconds, it instinctively raises its head while removing its proboscis from the spur, and in doing so causes the viscidium to adhere to its proboscis usually about 4 to 9 mm from its base. Attached to the viscidium via the caudicle is the pollinia. Upon removing its proboscis from the flower, the pollinarium stalk will be straight and parallel with the moth's proboscis. Then after leaving the orchid the caudicle will eventually dry out, causing its angle relative to the moth's proboscis to change by 90° so that it is at the correct angle to attach to the stigma of the next orchid the moth visits. The moth then repeats this process at another "A. sesquipedale" orchid and simultaneously fertilizes it. Once the flower has been fertilized, it quickly stops producing its powerful scent.