Orchids

Categories: Angiosperms; economic botany and plant uses; gardening; taxonomic groups

There are numerous natural and artificial hybrids of orchids (family Orchidaceae). The floral and vegetative variation in this family is enormous, and most orchid specialists consider the orchids to be undergoing a great deal of evolutionary activity as well. Orchids follow the general monocot pattern of having flower parts in three’s. They differ from other monocots in several ways, however.

First, the stamens are fused together. Orchids are divided into two major groups based on their anthers and stamens: the diandrous orchids, which have two functional anthers and a sterile stamen, which in the lady’s slipper orchids (such as Cypripedium, Selenipedium, Paphiopedilum, and Phragmipedium) becomes modified into a staminode, and the monandrous orchids (all other orchids), which have one functional anther.

Second, the stamens are fused with the pistil to form a structure called a column, which has at its tip a rostellum, a gland that separates the pollinia from the stigmatic surface. The fusion of the stamens and pistils into a column also occurs in the milkweed family (Asclepiadaceae) in the dicots.

Third, the pollen grains are formed together into packets called pollinia.

Fourth, flowers have bilateral symmetry or asymmetry, as in Tipularia.

Fifth, resupination is the most common condition for orchid flowers. In resupinate orchids, the pedicel and ovary undergo a 180 degree twisting process so that as the flower matures the lip is lowermost and the column uppermost. Although some orchids (such as Calopogon, Polystachya, Malaxis, and Platanthera nivea) are nonresupinate, most are resupinate.

Sixth, the third petal, or lip, is usually highly modified in size, shape, or color when compared to the other two petals.

Seventh, the ovary is inferior.

Eighth, orchids produce large numbers of tiny seeds that lack endosperm. Most orchids have bisexual flowers; however, some (Catasetum, Cycnoches, Mormodes) produce unisexual (male or female) flowers. Flowers may last only a day (Sobralia), or for more than a month (some Paphiopedilium and Dendrobium).

Pollination

Pollination ranges from promiscuous to pseudocopulation. Promiscuous pollination occurs when almost any appropriate insect can transfer the pollinia successfully from one flower to another. Pseudopollination involves flowers which mimic the size, shape, smell, or movements of female wasps or bees (Ophrys, Caladenia, Drakaea). These flowers attract the male wasp or bee, which attempts to mate with the flower. Pollinia are thereby attached to the insect’s body. Eventually the frustrated male leaves the flower and finds another flower, again trying to mate with it but succeeding only in depositing the pollinia from the first flower onto the second flower’s stigmatic surface.

Mycorrhizae and Seed Germination

To germinate, orchid seeds, which lack endosperm, must form a symbiotic relationship with the mycelium of an appropriate fungus (mycorrhiza) which provides the seeds with nutrients and water.

Vegetative Plant Structure

Vegetative plants are herbs or, rarely, vines (Vanilla), perennials or, rarely, annuals (Zeuxine strateumatica). Vegetative parts are normally roots, stems, and leaves, which can undergo many different modifications. The roots of terrestrial orchids are similar to other terrestrial monocots and are nonphotosynthetic. Some are modified into food storage structures (tuberoids). Roots of epiphytic orchids have a specialized outer multilayered tissue called the velamen, which forms a spongy, whitish sheath around the root. The velamen allows absorption of water and dissolved mineral nutrients.

Stems are frequently modified into creeping rhizomes and often into water- and food-storage structures, such as corms, tubers, or pseudobulbs. These structures are usually aboveground and photosynthetic. Plants may be minute (some Bulbophyllum and Platystele) or gigantic (about 44 feet tall, such as Sobralia altissima) or may form large clusters of pseudobulbs weighing several hundred pounds (Grammatophyllum).

Growth Patterns, Distribution, and Habitats

There are two basic growth forms in orchids. In the sympodial form, successive new stem growth originates from the base of the preceding stem growth (as in Cattleya and Cypripedium). In the monopodial form, new growth comes from an apical meristem; these plants often become quite tall (such as Vanda).

Orchids occur worldwide, from the Arctic to the tropical rain forests but do not grow wild in Antarctica. Most tropical orchids are epiphytic (growing on other plants) or epilithic (growing on rock surfaces), and a relatively small number, including the temperate and Arctic orchids, are terrestrial. The genus Rhizanthella in Australia is subterranean.

Economic Uses

Uses include a complex starch (called salep) used as food in Turkish and other Middle-Eastern confections. Salep is made from the dried tuberoidal roots of Dactylorhiza, Eulophia, and Orchis. In the Americas, the Mayans and the Aztecs cultivated Vanilla vines for the fleshy fruit, or bean, which was fermented to make vanilla for use as food flavoring and perfume. Some orchids have been used as sources of fibers for weaving or basket making. Currently, many different orchids are important as cut flowers or as potted plants, including Cattleya, Dendrobium, Cymbidium, Paphiopedilium, and Laelia. Advances made during the 1990’s in mericloning have greatly reduced the cost of producing high-quality plants.

Bibliography

Brown, P. M. Wild Orchids of the Northeastern United States: A Field Guide. Ithaca, N.Y.: Comstock, 1997. Includes keys, distribution maps, line drawings, and color plates.

Flora North America Editorial Committee. Orchidaceae. Vol. 26 in Flora of North America North of Mexico . New York: Oxford University Press, 2001. This volume contains keys to genera and species, descriptions of genera and species, line drawings, and distribution maps.

Northen, R. T. Miniature Orchids. New York: Van Nostrand Reinhold, 1980. Covers miniature orchids, how to grow them, and a selection of recommended species. Includes black-and-white and color illustrations.

Richter, W. Orchid Care: A Guide to Cultivation and Breeding. New York: Van Nostrand Reinhold, 1972. Basic guide to cultivation, discussing light, humidity, temperature, watering, potting, and how to pollinate orchids. Includes good line drawings.