Most orchid flowers are scented, even though it may not seem so at times to our relatively insensitive noses. Of course, they are not scented to please us but to attract pollinators, whose attentions ultimately lead to fertilisation and seed production. The vast majority of orchid flowers are pollinated by insects. However, a few non-scented orchid flowers are pollinated by humming birds and sunbirds during their search for nectar.
In his book The Scent of Orchids (Elsevier, Amsterdam, 1993), Roman Kaiser classifies orchids into various groups, based upon the nature of their pollinators. The moth-pollinated, night-scented group of orchids, predominantly from Africa and Madagascar, give off scents reminiscent of jasmine, honeysuckle, tuberose and gardenia, which are released predominantly in the evening and at night. Most of these flowers are white, providing an additional visual guide for the moths. About 8% of all orchids are believed to be moth-pollinated. Angraecum sesquipedale is probably the best known, being pollinated by a hawk moth with a proboscis over 300 mm long, as postulated by Charles Darwin in the nineteenth century. But certain American orchids, also with white flowers, such as Brassavola species, are also moth pollinated.
In contrast to these sweet-smelling orchids, there are fly-pollinated orchid flowers, mostly red or brown in colour, whose scent resembles that of rotting flesh. Many bulbophyllums, especially those in the section Cirrhopetalum, are of this type. The prime example is the evil-smelling Bulbophyllum robustum of Papua New Guinea.
Bee-pollinated orchids produce a wide range of scents, resembling those of the rose, violet, lily-of-the valley, hyacinth and sweet pea. The honey bee, and presumably most other bees, have a different sensitivity to colour than humans, in they cannot perceive the colour red but can see ultraviolet light, which is invisible to us. There are a few bee-pollinated red flowers but all of these flowers also reflect ultraviolet light, which makes them visible to bees. Many orchid flowers have ultraviolet guide lines that direct bees to the nectar-producing glands concealed within the corolla tube, where the pollinia and stigma are also located. Most bee-pollinated flowers rely solely on attracting the bees by their perfume and leave the business of pollen transfer to and from the pollinator to chance. But others have a more specific mechanism; for example European terrestrial orchids of the genus Ophrys, whose perfume closely resembles the sex attractant of certain female bees. Pollen is transferred from one flower to another when a male bee attempts to mate with a succession of flowers, whose labellum closely resemble a female bee in shape, colour and scent.
Butterfly-pollinated flowers are less common but most of us are familiar with the prime example, Disa uniflora, from South Africa. The pollinia of this species, being among the largest of all orchids, need quite a large pollinator to carry them from flower to flower.
Most bird-pollinated flowers are red or orange, because birds are more sensitive to this end of the visible spectrum. They are also scentless, because birds have almost no sense of smell. Some species of the American genera Masdevallia, Cattleya, Epidendrum, Cochlioda, Comparettia and Laelia are pollinated by humming birds. In PNG and Malaysia many dendrobiums are pollinated by sunbirds. However, bees and other insects are the main pollinators of orchids, only 3% being pollinated by birds.
An amazing facet of orchid pollination is that in some cases a particular orchid species is pollinated by only a single insect species. In other words, each of these orchids has its own pollinator, thus ensuring that no cross-pollination with related orchid species occurs. But that’s another story.