Southern African orchids reproduce either vegetatively or sexually; the latter requires successful pollination. Southern African orchids are mainly pollinated by insects (bees, moths, flies, butterflies), and rarely by birds. The flowers either attract the pollinators by scent or brilliant colours, or by deception, taking advantage of their resemblance to other flowers or the sex drive of the insects. A few orchid species have given up on being pollinated by animals and pollinate themselves (= self-pollination). A few months after the ovules have been fertilized the mature capsules shed the seeds which are dispersed by wind or, in a few species, by water.
Sexual reproduction, involving the formation of ovules and their fertilisation by pollen tubes after the pollination of the flower, is more common in our southern African orchids. The intriguing pollination processes of orchids have fascinated botanists for centuries. The complicated architecture of the orchid flower is highly adapted to the pollinators which are one of the major driving forces of evolution. The specialized structures often permit the pollination only by specific pollinators, thereby preventing cross-pollination between different species. Sometimes co-occurring orchids share pollinators but the pollinaria are often placed on different body parts of the pollinators, thus again preventing the formation of hybrids.
Pale, evening-scented flowers are frequently pollinated by moths. Examples are Bonatea speciosa, Habenaria epipactidea, many Satyrium species, Disa cooperi and D. ophrydea.
and horsefly pollination
Pollination by deception
For example some of these deceptive orchids pretend to be something else, as they strongly resemble a plant which actually has got some sort of reward, which is called mimicry. An example is Disperis capensis the flowers of which look like flowers of Polygala bracteolata (a member of the polygala family, not an orchid), and which is accidentally pollinated together with the latter. Disa racemosa is an example of an orchid that tricks bees into visiting its empty flowers while searching for new food sources. The illustration is of Disperis capensis which mimics the flowers of Polygala bracteolata.
Another form of deception is sexual deception where orchid flowers take advantage of the sex drive of male insects by pretending to be their female partners, and also in this case get accidentally pollinated. This form of deception occurs in Disa atricapilla and D. bivalvata.
orchids try to prevent cross-pollination between different species to
keep their own identity. Therefore natural hybrids are rare, and there
are only a few that have been reported in the southern African orchids.
One can normally recognize hybrids as they have characters of both of
their parents which also grow in the vicinity. In some species it is also
suspected that they are of hybridogene origin (examples are Disa maculomarronina,
Satyrium princeps and S. rhodanthum). In contrast, it is fairly
easy to cross species artificially and raise such hybrids under greenhouse
conditions. Such artificial crossings have given rise to attractive and
vigorous hybrids (in southern Africa particularly in the genus Disa).
After pollination by whatever means, the ovules of the orchid flower start developing further. A few months later they have developed into ripe seeds. The capsule - the fruit which has developed from the ovary - dehisces and releases thousands of minute, dust-like seeds. In the vast majority of our indigenous orchids they are so tiny that they hardly sink in the air and are immediately dispersed by wind ('anemochoric'). Wind dispersal has the disadvantage that it is not directed, and therefore not many of the seeds are blown into an environment where they can germinate and produce a plant. However, in view of the thousands of seeds formed it is not really a problem for the survival of the species if some of them are lost because they are blown into an environment where they cannot germinate.
Description and images : Hubert Kurzweil
|© S A National Biodiversity Institute|