SA Orchids:What is an orchid?
People frequently ask what distinguishes an orchid from any other plant? There are several characteristics. We comment only on those which are easy to see.
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The
most important characteristic is irregular or zygomorphic flowers. The
perianth is usually clearly divided into an upper and a lower half.
The perianth always has six lobes which are arranged in two whorls of
three. The lip or labellum is usually a prominent and highly complicated
lobe. A noteworthy exception is our southern African genus Disa,
which has a simple and rather insignificant lip.
The perianth lobes can be drawn out into spurs, which may be minute and less than 1 mm in length or up to 20 cm long. Such spurs are normally found on the lip (Habenaria, many epiphytic orchids), but many southern African orchids have spurred median (middle) sepals (e.g. Disa) or lateral sepals (Disperis). Satyrium has two spurs on its lip. Spurs normally hold nectar as a reward for the pollinators.
Orchids have undivided, and in most cases, rather narrow leaves.
Leaves mostly range from linear to lanceolate in shape. They are
fairly thin in terrestrial orchids which frequently shed their leaves
at the beginning of the dry season. In most epiphytic orchids they are
rather thick-leathery or even succulent and water-storing as they have
to endure frequent dry periods: remember that a tree bark dries out
already a few hours after a rain storm, and the plant may have to wait
for a few days for another rain shower. There are also a few leafless
orchids.
- A clearly visible feature is the column or gynostemium in the
centre of the flower which contains both the style with its sticky receptive
surface and the pollen. The pollen is derived from a single stamen:
in very few (non-African) orchids there are two or three.
In orchids there are no stamens with free filaments as in other plants - for example Gladiolus in the iridaceae family has also narrow leaves and irregular flowers but never a column. From an evolutionary point of view this column is fascinating, as this complicated and frequently bizarre organ has over millions of years evolved from a simple style + stamens as we find it in lilies and irises. The complex 'inventions' of the orchid flower must be understood as adaptations to the pollination of the plants. For the botanist who is trying to classify the more than 20 000 orchids into smaller subgroups the column is also important as many taxonomically useful characters are found in its structure. The column structure of the southern African Disperis-Corycium-Pterygodium group is among the most complicated to be found in the entire orchid family.
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Most
orchid flowers twist through 180° just before they open. This movement
which is termed resupination is necessary as the lip is originally developed
on the upper side - after the resupination it is positioned on the lower
side which is much more advantageous for successful pollination.Some
orchids have their lips uppermost in the open flowers, either because
there was a hyperresupination through 360° or because of the lack
of resupination. There are a few southern African orchids which show
this feature - examples are Satyrium and Polystachya.
The illustration on the right shows the flowers of Polystacha pubescens. Compare the lip in these flowers to the Cattleya at the top of the page.
Unlike most other plants the pollen grains of orchids stick together
and form masses called pollinia. The illustration
on the right shows the pollinia of Disa atricapilla. The sticky viscidium,
which helps the pollinia stick to pollinators is also visible.
- The ovary, when cut open, contains numerous tiny ovules, which
in the mature capsule develop into dust-like seeds. These are normally
dispersed by wind, but water dispersal has recently been observed in
Disa uniflora and related species.
- Orchids have a symbiotic relationship with fungi that live in their roots, called mycorrhiza. The fungi supply the orchid with nutrients which they absorb from the dead organic matter. Mycorrhiza symbioses are also known in other plants like for example forest trees, although these are less dependent on their fungi. There are even some orchids which are yellow-brown as they do not have chlorophyll and obtain all their essential nutrients from the fungi. They are termed saprophytes.
Description and images : Hubert Kurzweil
October 2000
(Updated by Clare Archer, August 2010)
