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ORCHID POLLINATION

Charles Darwin was not exaggerating when he renounced that the various contrivances and adaptation of orchids vastly transcend those which the most fertile imagination of the world's most imaginative man could dream up with unlimited time at his disposal. Some have suggested that if his 1862 book, On the Various Contrivances by which British and Foreign Orchids are Fertilised  by insects had been published just three years earlier, before Origin of Species then the Origin would not have caused such worldwide convulsions. Modifications of Darwin theory (such as punctuated equilibrium) have since been proposed to explain certain anomalies in the fossil record. Darwin floundered in trying to explain the mechanism of natural selection, but so accurate and cogent were most of his observations and arguments concerning orchid pollination that they provided convincing evidence at the time and can withstand critical scrutiny even today, 140 years later.

This is not to say that all his contemporaries agreed with his ideas on orchid pollination: for instance, scientists laughed at his ideas on how the Madagascan species, Angraecum sesquipedale, was pollinated. Although the nectar spur of the flower is up to 30 cm long, nectar fills only the lowermost few centimeters. Darwin reasoned that if the pollinator was to be rewarded for its efforts in, visiting the flower, it must have a tongue or proboscis long enough to reach from the mouth of the spur to the apex.

This implied that the pollinator was a moth with a tongue about one foot long a notion his critics sound ridiculous. Such a sphinx much however with a tongue of that length was discovered on Madagascar and named Xanthopan morgani praedicta in 1903. Sphinx-moth pollination of other Angraecum species on Madagascar has now been recorded and published Even Darwin's colleagues were skeptical about many of his ideas on pollination. For instance when he described the pollinia-shooting mechanism, of Catasetum to his lifelong friend, Thomas Henry Huxley, Huxley said, "Do you really think I can believe all that?"

With the exception of those orchids that are self-pollinated, all are pollinated by animals bees, wasps, flies and gnats, butterflies and moths, birds, ants which are attracted (often on a species specific basis or nearly so) in different ways. 

Euglossine bees of the Neotropics are attracted to orchids of subtribes Catasetinae, Stanhopeinae, Zygopetalinae, and a few others primarily by floral fragrances secreted from glands called osmophores. Male euglossines work over specific surfaces of the flowers, usually the labellum, and mop up fragrance droplets with brushes on their front feet. They then take off and transfer the droplets to cavities in their hind tibia. Why they do this is still a mystery; the prevailing theory is that fragrances are somehow modified by the male into chemical attractants for females, which do not collect fragrances. The chemical composition of fragrances is sometimes specific enough to attract males of only one bee species. Genetic mutations that result in minor qualitative or quantitative changes in fragrance composition may attract a different bee species and become fixed in nature over time.

 

Flowers of the genus Ophrys, ranging from England to the Middle East, are pollinated by male bees or wasps attracted both by fragrances similar to pheromones secreted by the females and by visual cues such as form, color, and texture. The males are so deceived that they attempt to mate with the flowers (a process known as pseudocopulation) and in doing so remove or deposit pollinia.

This lifting motion repeatedly tosses the wasp onto the column. Following all this abuse, the wasp flies away with pollinia attached to it and/or has deposited pollinia on the stigma.

An equally interesting relationship occurs between many Australian terrestrials and thynnid wasps. Fragrances secreted by the osmophores of Drakaea, Spiculaea, Caladenia and other genera attract male wasps in a very species specific way. The deceit is reinforced by the configuration of calluses on the labellum. The males then try to mate with the labellum or else lift it as they would a female wasp. In the normal sequence of events, the wasp comes into contact with the column of the flower, and is actually hammered onto it in the case of Drakaea.

Other wasp pollinated flowers can be found in species of Cryptostylis, Calochilus, Ada, and Brassia and their relatives - are attracted to flowers as a food source or a breeding site, which explains the foul odors and dull colors of some flowers. Floral parts often flutter in the slightest breeze to supplement the attraction, as do the banners called paleae on many bulbophyllums and the dangling strings of wax that are secreted from two-celled glands on the sepal margins of Pleurothallis scbiedei But there are also secret passageways, intricate hinges, and temporary traps. Why these forms of chicanery? The simple answer is that among the insects, flies are not Rhodes scholars: left to their own devices they are poor pollinators, and special guidance to the column is needed. The labellum is often hinged to the column-foot, so that as the fly lands on the labellum, its own weight pulls it down onto the column. 

Examples of this hinge type are found in Bulbophyllum and Masdevallia  in fact, some genera (Acostaea, Salpistele Porroglossum, Pterostylis) have flowers with a sensitive labellum, and as soon as this is touched, it immediately slams shut against the column. In Pterostylis, this temporarily imprisons the gnat and forces it to contact the column while it is inside the flower. in some species the sensitive labellum catapults the gnat onto the stigma first.

When a fungus gnat lands  on the tip of the sensitive labellum, the labellum quickly closes against the column. The only exit from the trap is a narrow tunnel between the column and the tip of the labellum. As the gnat squeezes through, pollinia (the yellow spot) become attached to it.

Brightly colored flowers pollinated by butterflies generally have a landing platform and offer nectar. Good examples of butterfly pollinated orchids are some species of Epidendrum. On the other hand, moth-pollinated flowers such as those of Brassavola and Angraecum are white, cream or greenish, fragrant at night, and offer nectar in tubular structures such as a spur or a mentum, a "chin" at the base of the flower formed by the column-foot and bases of the lateral sepals. Bird-pollinated orchids are rarely fragrant and are brightly colored in shades of red or yellow, and often tubular and nectariferous, The best-known examples in the Neotropics are Comparettia and some epidendrums, pollinated by hummingbirds, and the Palaeotropical Dendrobium lawesii and Dendrobium mohlianum, pollinated by sunbirds, honeyeaters, etc. Pollinia are transferred to pollinators in many different ways. Some pollinia have extensions of anther-derived tissue called caudicles that stick to the pollinators but break once the pollinia are on the stigmatic surface of a flower. Caudicles are found in such genera as Eria, Laelia, Coelogyne and Bletia. An evolutionary advance in orchids was the development of the rostellum, part of the median stigma lobe, which has at least two very important functions. First, it separates the anther from the fertile part of the stigma and prevents self-pollination in most species. Second, part of it, the viscidium, is a sticky pad frequently connected by a stalk to the pollinia. This pollination unit is called a pollinarium (pl.  pollinaria). When the viscidium is touched, it quickly adheres so that as the pollinator backs out of the flower the entire pollinarium is removed with it. In many species the stalk dries differentially over time and changes its orientation, promoting out crossing (Dressier. 1981 ).

The bee pollinator asleep in the Diuris behrii flower