Orchid

Orchid, common name for a family comprising one of the largest groups of flowering plants (See Angiosperm). The family is worldwide in distribution, being absent only from Antarctica and some of the most arid desert zones. The greatest diversity of genera and species occurs in tropical regions that remain poorly explored. For this reason, and because of the complexity of the family, estimates of the number of orchid species vary from 15,000 to 25,000, and the number of genera from 400 to 800.

Orchids and their allies are distinguished from other orders of flowering plants by a combination of floral characteristics rather than by a single characteristic unique to the group. Orchid flowers are borne on stalks called pedicels, as are other flowers. During the growth and development of the flower, however, the pedicel rotates 180°, so that the mature orchid flower is borne upside down. Of the flower’s three sepals (outer floral whorl) and three petals (inner floral whorl), all the sepals and the two lateral petals are usually similar to one another in colour and shape. The remaining petal, always distinct from them, is called the labellum, or lip; it is usually larger and different in colour and shape, often being lobed or cupped. The labellum, which often acts as a landing platform for the orchid pollinator, may attract the pollinator to the flower through particular colour patterns and shapes to which the pollinator responds in particular ways.

The sexual organs (pistil and stamens) of the orchid flower are fused together into a structure called the column, which lies opposite the labellum. Orchids have only one stamen (male floral organ), and in most orchids it bears only one anther (pollen-producing structure); in a few orchids, however, two anthers are produced. The pollen is not granular, as it is in most flowering plants, but is aggregated in a number of masses, or sacs, that vary in texture from mealy to horny. Three stigmatic lobes (pollen-receptive areas) are usually present and located near the anther, although usually only two are functional. The ovary is below the other flower parts and is surrounded by pedicel tissue. It is tripartite and contains numerous ovules (egg-bearing structures) that mature into seeds. The seeds are small, with only an undifferentiated embryo. As many as 2 million seeds may be produced from a single orchid seedpod. Unlike most other flowering plants, orchids have no food-storage tissue in their seeds.

Orchid flowers are pollinated by a great variety of flying animals, and their great diversity in floral structure has resulted from adaptations to various pollinators. About half the orchid species are pollinated by bees; moths, butterflies, flies, birds, and other agents pollinate the rest. Many orchid flowers are adapted for pollination by a single species of insect and in extreme cases by one sex of that insect.

Orchids do not vary as much vegetatively as they do in floral structure, but a great variety of forms exists, reflecting the wide range of habitats they occupy. About half are epiphytes, growing on other plants for support only, but some are parasites and others saprophytes (living on decaying vegetation). A few Australian species complete their life cycles entirely underground.

Apart from their phenomenal popularity among horticulturists, orchids have little economic importance, although vanilla flavouring is obtained from fruits of one species, the Mexican vanilla, widely grown in tropical areas. Growers have produced thousands of new forms of orchids through hybridization.

Scientific classification: Orchids make up the family Orchidaceae. The vanilla orchid is classified as Vanilla planifolia.