Being highly evolved the plants, orchid pollination systems are very specialised and varied. They have many guises to attract insect pollinators.
Unlike other flowers, very few orchid species use nectar. Some have sweet smelling perfumes, others have rancid perfumes imitating rotting flesh. Some have a colourful labellum to attract pollinators. The labellum is not only useful to attract pollinators, it is also used as a landing platform for the pollinator.
The labellum can also be used to fool pollinators. Species such as Chiloglottis, imitate the female of the insect pollinator, then when the male tries to mate with the orchid, pollination takes place. This process is called pseudo-copulation. The most common pollination method is for the pollinia to stick to the back of the pollinator as they leave the orchid. The pollinia then rubs along the stigmatic surface and is removed from the pollinator’s back as the it enters the next orchid, pollinating the flower. The use of masses of pollen stuck together in the pollinia increases the chances of pollination in areas where pollinators are rare.
However, orchids being one of the largest flower families, have more than one way to pass pollen to pollinators. Species from the Apostasioideae, Cypripedioideae and Vanilloideae subfamilies (about 326 species from 22,500 total orchid species) release pollen as single grains rather than as pollinia.
Orchids of the Neotropical genus Catasetum have sexually dimorphic flowers; that is, male and female flowers are distinctly different in shape and color. Male flowers are triggered to eject pollinia at euglossine bees while they rest on the orchid’s labellum. The Euglossa bees leave the male flower in response to the pollinarium emplacement and subsequently avoid male but not female flowers. The pollinia rubs against the stigmatic surface and pollinates the flower.
The labellum of the Cypripedioideae species is shaped so that it traps the pollinator and the only way out is to go past the anther, where the pollinia stick to the insect’s back.
However, pollination does not always result in fertilisation. For the flower to be fertilised once the pollen is on the stigma it must germinate. When the pollen germinates it grows a tube which goes down the style and into the ovary and fertilising the flower and turning the ovules into seeds. Orchids have particularly tiny seeds, about the size of a grain of pepper, and they carry the bare essential DNA material.
A special bundle of orchid pollen (pollinia) stuck on a bumble bee after it visited an orchid flower. A single pollinia can pollinate thousands of flowers
