Orchidaceae

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Orchidaceae

[‚ȯr·kə′dās·ē‚ē]
(botany)
A family of monocotyledonous plants in the order Orchidales characterized by irregular flowers with only one or two stamens which are adnate to the style, and pollen grains which cohere in large masses called pollinia.

Orchidaceae

 

(orchids), a family of perennial, monocotyledonous, herbaceous plants. They are either terrestrial or epiphytic (in the tropics). Terrestrial orchids are usually rhizomic and often have tuberous, thickened roots. They grow in meadows, swamps, and forests, as well as on mountain slopes. The leaves are entire, vaginate, or amplexicaul; in saprophytic species, they are reduced to flowerless glumes. Epiphytic orchids grow on the trunks and branches of trees. They attach themselves to supports by means of aerial roots, which also extract moisture from the air. Many epiphytic orchids have thickened stems, or pseudobulbs, in which water and nutrient matter accumulate. Some epiphytes have delicate and succulent leaves, which sometimes fall to the ground in the dry season; others have coarse, stiff evergreen leaves.

Orchid flowers, which are irregular and usually bisexual, are solitary or gathered into spicate or racemose inflorescences measuring up to 2–3 m long. The perianth consists of two whorls. The sepals are petaloid or, more rarely, green; sometimes they coalesce. There are three petals; the two lateral ones are usually the same shape as the sepals, but the middle one, the lip, differs from the others in size, color, and shape. A characteristic feature of orchid flowers is the column, or gynostemium, which is apparently formed by the union of the stamens and the style. In most orchids, one or two of the three stamens is converted into fleshy or petaloid staminodes. The pollen is often in masses, or pollinia, each equipped with a stalk and, in many species, with a sticky disk that forms a pollinium. The stigma is three-lobed, but in most orchids one lobe is converted into a rostellum that contains sticky matter. The fruit is generally a capsule having numerous (up to several million) lightweight seeds, which are disseminated by the wind. Orchid flowers have highly specialized adaptations for insect pollination. The perianth is noted for its bright coloring, odd shape, rhythmic movements (in some species), fragrance, and the presence of nectaries and spurs containing sweet substances. The lip of the perianth is a convenient “landing field” for insects. While obtaining food, the insect touches the rostellum; the pollinia adhere firmly to the insect. When the insect enters the next flower, the pollinia fall on the sticky or rough surface of the stigma. A considerable amount of time passes between pollination and the maturation of seeds and fruits—two years or more. The seeds sprout only when they have been penetrated by fungi that form an endotrophic mycorrhiza (mainly species of Rhizoctonia).

The family Orchidaceae has 600 to 700 genera, embracing more than 20,000 species (according to other data, up to 800 genera, including 35,000 species). Orchids are found almost everywhere, including polar regions and deserts. They are most abundant and diverse in the tropics and subtropics of America and Asia (four-fifths of the species composition). In the USSR more than 120 species grow wild, including species of Platanthera and Cypripedium (lady’s slipper) and many species of Orchis. Many species are widely cultivated in greenhouses, despite the difficulties of growing them. Tropical species of the genera Coelogyne, Cattleya, Dendrobium, and Vanda can very easily be propagated vegetatively. Effective methods of raising orchids from seeds have been devised, making possible the development of new hybrids. Some orchids are commercially valuable. The fruits of plants of the genus Vanilla are used in the confectionery and perfume industries; the tubers of some species of Orchis and Platanthera yield salep, which is used in medicine.

REFERENCES

Poddubnaia-Arnol’di, V. A., and V. A. Selezneva. Orkhidei i ikh kul’tura. Moscow, 1957.
Selezneva, V. A. Tropicheskie i subtropicheskie orkhidei. Moscow, 1965.
Takhtadzhian, A. L. Sistema i filogeniia tsvetkovykh rastenii. Moscow-Leningrad, 1966.
The Orchids: A Scientific Survey. Edited by C. L. Withner. New York, 1959.
Hawkes, A. D. Encyclopaedia of Cultivated Orchids. London, 1965.
Richter, W. Orchideen: Pflegen, Vermehren, Züchten, 2nd ed. Radebeul, 1971.

V. N. GLADKOVA