SA Orchids: Reproduction and Pollination

[Vegetative reproduction] [Sexual reproduction] [Pollination] [Hybrids] [Seed dispersal]

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.

Vegetative reproduction

Vegetative or asexual reproduction is rare in southern African orchids. Only Satyrium odorum, S. ligulatum, Disa sagittalis, D. harveiana, Disperis purpurata, Pterygodium catholicum and Corycium deflexum develop additional tubers on long stolonoid roots several centimetres away from the old plant which give rise to new plants in the following growing season. Disa uniflora, D. tripetaloides and D. cardinalis have underground stems called stolons which form a large number of leafy plants at their ends, and as a result these Disa species often grow in clumps of many individuals.

Sexual reproduction

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.

Pollination

While the pollination of the Satyrium-Disa-Disperis-Pterygodium group has been studied in detail in recent years, there is hardly any information available in some of the other groups. In Satyrium there are two lip spurs and the column has a long stalk; pollinating insects, while trying to reach into one of the spurs, touch the viscidia (sticky glands) of the orchid-column and get the pollinium fixed to their back (= nototribic). The situation in the Disa group is very different: the median sepal normally has one spur and the column lacks a basal stalk; while the pollinating insect tries to reach into the spur the pollinia are normally attached to its underside (= sternotribic).

Bee pollination
Bee-pollinated flowers often have a strong scent during the day and are bright in colour. Examples of such bee-pollinated orchids include Satyrium erectum, S. hallackii subsp. hallackii, Disa versicolor, D. racemosa, D. venosa, D. tenuifolia and D. filicornis. Pollination by oil-collecting bees - which is generally a rare phenomenon in plants - occurs in at least 55 South African orchids in the genera Disperis, Pterygodium, Corycium, Ceratandra, Evotella, Satyrium and Pachites. The bees collect the oil as food for their larvae.

Butterfly pollination
Pollination by butterflies has evolved in the genus Disa. The large 'Mountain Pride butterflies' pollinate Disa uniflora, the red disa, as they are attracted by its brilliant red flowers. The fact that this butterfly is only attracted to red flowers is one of the main reasons why the yellow form of Disa uniflora is rare in nature.

Fly and horsefly pollination
Small flies pollinate Disa obtusa, Satyrium bicallosum (fungus gnats) and S. bracteatum (carrion flies). In all cases they are attracted by the pungent odour of the flowers. It was an unexpected discovery to find that D. harveiana subsp. harveiana is pollinated by blood-sucking horseflies.

Bird pollination
Another recent, exciting find is that three Satyrium species with bright red or orange flowers, S. carneum, S. coriifolium and S. princeps, are pollinated by sunbirds. Their pollinia have a very large viscidium (sticky gland which glues the pollinia to the pollinator) which becomes firmly attached to the bird beak.

Pollination by deception
Many orchids attract pollinators by displaying a brightly coloured perianth or by emitting a strong scent, and offer them a reward in the form of nectar (in spurs) or oil (on special glands). Others do not offer the pollinators anything and only deceive or fool their pollinators.

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.

Self pollination
Many orchids rely on the transfer of pollen from one plant to another by pollinators, either by attracting them with brilliant colours, strong scents and rewarding them with nectar or oil, or by deceiving them somehow. However, there are also some species which do not rely on this mechanism but rather self-pollinate. This occurs mainly in habitats where pollinators are scarce or unreliable. Examples are Disa bracteata, D. vaginata, D. glandulosa, D. rosea and Ceratandra globosa. Pterygodium connivens and P. newdigateae do not even open their flowers any more, but are self-pollinated in the bud.

Hybrids

Most 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).
The illustration shows a presumed hybrid between Disa racemosa and D. atricapilla.

Seed dispersal

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.

Disa uniflora, D. cardinalis, D. tripetaloides and D. caulescens have slightly larger seeds of about 1 mm length, and these are dispersed by the streams next to which the plants grow ('hydrochory'). The advantage of water dispersal is that the seeds float to a moist environment where they can easily germinate.

Description and images : Hubert Kurzweil
October 2000