Orchid pollination methods by reward
Orchid pollen is not used by honeybees as a source of food – instead they are attracted mainly by the plant’s nectar.
Those orchids that have most clearly developed this adaptation have a spur at the base of their lip. This means that the insect must have an appendage in order to reach the nectar. When it does so, it brushes against the pollinia, masses of orchid pollen.
The most striking example is Angraecum sesquipedale, which drew Darwin’s attention. He even predicted that a moth would be found with a proboscis long enough to reach the nectar at the end of the spur. In fact, an insect exactly matching Darwin’s description was found many years later. This type of pollination is often described as “lock and key,” because the insect’s organ fits into the flower just as a key fits into a lock.
In the case of slipper orchids (Cypripedium, Paphiopedilum), an insect in search of nectar slips in under the column and leaves only once it has made its way along the side of the gynoecium, carrying with it pollinia stuck to its back. This step prevents it from pollinating the same flower. When it visits another flower, it will deposit the pollinia on the stigmas before leaving, taking a new load with it. This is called “trap” pollination.
The insect-orchid association is so closely established in many orchids that it can be described as “faithful” or “restrictive,” since only the type of insect capable of pollinating a particular orchid effectively will be able to visit the flower and obtain its nectar reward.
In addition, only orchids have this adaptation whereby pollen is clustered in pollinia, small hard masses easily “carried” on an insect’s back or a bird’s beak, especially when they have an adhesive disk (the viscidium).
Simulated pollen as a means of deception
Some orchids instead have a sort of pseudo-pollen, similar in consistency to pollen that is edible for their pollinators. The insects, usually bees, then pick up and eat the substance. Dressler also noted the presence of wax on the callus of the labellum of some Maxillaria species that could be used by some bees to build their nests.
In other cases, orchids appear to promise nectar, but don’t actually produce any. Some species, including Calypso, found in our peat bogs, have a plume of yellow hairs resembling edible pollen at the tip of their false anthers.
Plants with moveable parts are always fascinating – just think of the active traps on insect-eating plants. Although the motion is mostly passive on orchids, the way that some flower parts move to facilitate pollination is very interesting.
The labellum on various orchids lowers when a pollinator lands on it, making it easier for pollinia to be deposited or collected. Some Pleurothallis species have hairs or appendages that quiver in the wind, making them especially attractive to flies. The Porroglossum and a few other genera have a hinged labellum that drops down toward the gynostem as soon as there is any pressure on the base of the gynostem that supports it. The only way out for the insect is to push past the stigma and the rostellum, where it deposits some pollinia and picks up more. After a few minutes, the labellum returns to its initial position, ready for its next “victim.”
The ejection of the pollinia of the Catasetum is impressive: it is triggered as soon as something (e.g. the tip of a pencil) touches a seta near the sticky viscidium—a substance that requires vigorous effort to remove, such as rubbing the stigma of another flower and thus leaving the pollinia behind.
Mimicry and other methods of pollination of orchids by signals
This is probably one of the most fascinating phenomena in the plant world, especially because it manifests itself in such a wide variety of ways among different groups of unrelated insects and orchids. Bulbophyllum orchids attract flies by resembling rotting meat.
Many species mimic the flowers of other plants in order to attract their pollinators. Dodson mentions a male bee that pollinates the Oncidium genus by attacking the inflorescence that mimics another male in flight. The attacking bee’s head gets covered in the ejected pollinia, which it then deposits on the next plant it “attacks”.
Some flowers lure males by mimicking a female insect, in whole or in part. The Trichoceros genus is an excellent example of this adaptation – its hairs and moveable parts resemble a female fly. As Dressler reminds us, orchids are not equipped with intelligence. This phenomenon, like those that depend on cross-pollination, is the result of evolution: trial and error through genetic selection over the very long term and the reproduction of successful individuals.
Colours and shapes
The flower colours we perceive are entirely different from those perceived by insects and birds. They are just as important as the plant’s moving parts or the type of trap it uses. Blue, violet, purple, yellow and white are most attractive to bees, whereas birds are more drawn to contrasting colours, bright red in particular.
The shape of the markings on the labellum may also play a role in mimicry or as a reward signal. The shape of the flower itself or of some of its parts also plays a key role, not only in mimicry or pseudocopulation, but also in making the pollinator-flower interaction as effective as possible. Insects often require a “landing pad,” while tubular flowers allow birds to hover in place while using their beaks to get at the nectar.
Just like colours, scents – both pleasant and rotten ones – influence the type of pollinator associated with a particular plant. For instance, a Bulbophyllum inflorescence gives off a putrefied scent to attract pollinating flies that are attracted to rotten meat.
Orchids’ success as one of the most evolved plant groups, their diversity and their broad distribution around the globe all owe much to their highly diversified and sophisticated means of pollination.
In addition to boasting the most popular cultivated species with showy blooms, the Orchidaceae family includes many delightful members with flowers of unsuspected complexity. Orchids offer much to interest amateur botanists, with their adaptions to their habitats and their pollinators. You may even find yourself dreaming of travelling to exotic locales to find new specimens!