Orchid Family (Orchidaeceae)
Orchid Family (Orchidaeceae)
Biology and ecology of orchids
Native orchids in North America
The many species of orchids comprise one of the largest families of flowering plants, the Orchidaceae, which contains about 1,000 genera and about 20,000
species. Orchids have a worldwide distribution, and they occur in a wide variety of habitats, although their greatest diversity of species is in tropical rain forest. The most species-rich genera of orchids are Dendrobium and Bulbophyllus, each with about 1,500 species, and Pleurothallis, with 1,000 species.
Species of orchids can have very unusual morphological traits and ecological relationships, especially with their species of pollinating insects. For these reasons, along with the great beauty of their flowers, orchids hold a special place in the hearts of botanists, ecologists, and horticulturists. However, appreciation of the intrinsic value of orchids extends far beyond the scientists who work with these plants—few people fail to be enthralled by the loveliness of orchid flowers.
Biology and ecology of orchids
Almost all orchids are perennial (or long-lived) and grow from a rhizome or corm, from which the leaves and flowering shoot emerge. Many orchids are evergreen, meaning their foliage and shoots remain green and functional for more than a year. Many others are herbaceous perennials, meaning their above-ground tissues die back periodically, generally at the end of the growing season.
Orchids exploit two broad types of habitats—terrestrial and epiphytic. Terrestrial orchids have their perennating tissues in the surface substrate of the ground, such as the soil or organic floor of a forest, or the surface sediment or peat of a wet meadow or bog. In contrast, epiphytic orchids use trees as a platform upon which to grow within the canopy. Epiphytic orchids do not obtain their moisture or nutrients from their host—they only use the tree as a physical substrate upon which to grow, either perched on a branch or as a climbing, vine-like plant.
Orchid leaves are usually arranged alternately on their stem, or are basal around the flowering stalk. In a few species the leaves are reduced to small scales, and photosynthesis is mostly carried out by the green flowering stem. Orchid leaves are simple, have non-toothed margins, are usually strap-shaped or linear, have a longitudinally parallel venation and sheath to their base (that is, they lack a petiole).
The flowers of orchids are strongly zygomorphic, and are perfect, containing both female and male reproductive structures. The flowers may be born singly or as a group (or inflorescence). There are three sepals, which may be green or colorful and petal-like, and there are three petals. The two lateral petals are known as wings and are mirror images of each other, while the central petal is highly modified as a so-called lip (or labellum). The lip generally serves as a landing platform for pollinating insects. The lip commonly has a nectar-bearing sac or spur, which is usually brightly colored and pleasantly odorous and is intended to attract insect pollinators. There are one or two stamens, which are largely united with the stigma and style to develop a composite structure, known as the column. The anthers produce one to eight large pollen masses, known as pollinia. The pistil is composed of three united carpels and contains numerous ovules, usually numbering in the thousands. During its development, the flower of many orchids twists 180 degrees on its supporting stalk, so the mature flower is actually presented upside-down. The seeds of orchids are very numerous within the ripe capsule. The seeds are tiny and dust-like, contain almost no energy reserves, and are dispersed by the wind.
Pollination is usually by an insect, which is attracted by a combination of the bright color of the flower, a fragrant aroma, and nectar. Some species of orchids have tightly co-evolved with one or a few species of pollinating insects, so now the two species are highly dependent on each other. In the case of the bee orchid (Ophrys apifera ), the floral structure and coloration mimic the shape and color of the female of their pollinating species of wasp or bee. In addition, the orchid produces chemicals that closely mimic the sex pheromones of their pollinator and further help to attract the male wasp or bee. The orchid is pollinated when the male is tricked and attempts to copulate with the orchid flowers.
Almost all species of orchids develop a mutualistic symbiosis (that is, a mutually beneficial relationship) with a fungus, an alliance known as a mycorrhiza. This relationship is beneficial to the fungus, which gains access to some of the energy fixed by the orchid during photosynthesis. The mycorrhiza is extremely important to the orchid, because it greatly enhances the rate at which nutrients, especially phosphate, can be extracted from the environment. The mycorrhiza may also allow the orchid to utilize the energy and nutrition of organic matter in its growth substrate, which can be absorbed by the mycorrhizal fungus and partly passed along to their host plant. Orchid seeds are very tiny and have few energy reserves, and their seedlings rarely survive for long if they do not develop a mycorrhiza soon after germination.
In a few specialized cases, saprophytic species of orchids rely entirely on their mycorrhizal fungus to provide them with organic nutrition, which is obtained by tapping into the decomposer food web of the forest floor. In these cases, the saprophytic orchid can be considered to be parasitic on its mycorrhizal fungus.
Native orchids in North America
Hundreds of species of orchids are native to natural habitats of North America. A few of the more prominent species are described below.
Species of lady-slipper orchids grow on the surface of the ground, in open forest, prairie, and wetlands such as bogs and fens. Lady-slipper orchids have one or several large, showy flowers. The lip is greatly inflated (this is the “lady’s slipper”), with the margins of its orifice inrolled. A nectar-seeking, pollinating insect, usually a bee, passes through this orifice into the chamber of the lip, and is then drawn to the vicinity of the stigmatic surface by nectar, odor, and a visual trail of dots known as nectar guides. If the insect is carrying a pollinium from another lady-slipper flower, it is deposited to the receptive stigmatic surface, and then another pollinium is picked up, commonly on
the forehead of the bee, which then exits through another hole. Worldwide, there are about 50 species of lady-slipper orchids. The stemless lady-slipper (Cypripedium acaule ) is a widespread species in North America, with solitary flowers that are a lovely pink and sometimes white. The yellow lady-slipper (C. calceolus ) occurs in calcium-rich, moist forest and wetlands and has a solitary, bright-yellow flower. The showy lady-slipper (C. reginae ) is relatively tall, reaching 16 in (40 cm) in height, and has one to three large, pink-and-white flowers. The white lady-slipper (C. candidum ) occurs in calcium-rich wetlands and prairie and has a single white flower.
There are about 50 species of orchids in the genus Orchis. These plants have large, showy flowers that are white or white-and-pink in color. North American species include the showy orchis (Orchis spectabilis ), which occurs in rich woods in eastern North America, while the round-leaved orchis (O. rotundifolia ) occurs in the west.
There are about 450 species of Platanthera orchids. These orchids have their relatively small, but quite beautiful flowers arranged in a spiral fashion along an erect stalk. Some wide-ranging, white-flowered species in North America include the small woodland orchis (Platanthera clavellata ) and the tall white orchis (P. dilatata ). The yellow fringeless orchis (P. integra ) has yellow flowers. The long-bracted orchis (P. viridis) and pale-green orchis (P. flava ) have greenish flowers. The crested orchis (P. cristata ) has orange flowers, and the purple fringed orchis (P. psycodes ) has purple flowers.
The grass-pink orchid (Calopogon pulchellus ) is a pink-flowered species that occurs widely in acidic wetlands, especially bogs. The dragon’s-mouth or arethusa (Arethusa bulbosa ) produces a single pink flower, and is also a species of acidic wetlands. The rose pogonia or snake-mouth (Pogonia ophioglossoides) also develops a single pink flower, as does the calypso or fairy-slipper (Calypso bulbosa ).
Ladies’ tresses develop their relatively small but lovely flowers in a spiral arrangement along an erect stem. Most species have white or white-yellow flowers. Some widespread examples include the nodding ladies’ tresses (Spiranthes cernua ) and slender ladies’ tresses (S. gracilis ).
Coral-roots are saprophytic orchids of forests. These plants lack chlorophyll and depend on nutrition available from the decomposer food web of the forest floor to supply their needs. These plants and their flowers are reddish purple in color. The most widespread species is the spotted coral-root (Corallorhiza maculata ).
The helleborine (Epipactus helleborine ) is a species native to Eurasia that has become widely naturalized in North America. This green-flowered orchid is a common weed in many cities.
Orchids and humans
Many people have a deep affinity for the beauty of orchids, and wild plants are actively sought out for viewing during their flowering period. Hiking and other types of back-country explorations are an increasingly popular recreational activity. These outdoor ventures are greatly enhanced by the presence of flowering orchids, in much the same way that sightings of charismatic animals, such as bear, deer, and eagles, can make a day very special.
Because orchids are so renowned for the beauty of their flowers, they are commonly cultivated in greenhouses, homes, and in warm and humid climates, in outdoor gardens. The most popular genera of horticultural orchids are Catteleya, Cymbidium, Dendrobium, Epidendrum, and Vanda, all of which are species native to tropical forests.
In addition to cultivating these beautiful plants for their aesthetic value in homes and gardens, orchids are also grown in great numbers for the cut-flower industry. These orchids are grown to provide flowers for pleasing displays in vases in hotels, offices, meetings, and other commercial places and functions, and to arrange into corsages for social events such as weddings and formal dances.
Horticulturists have invested tremendous amounts of time and money to develop reliable methods of breeding and propagating orchids. Although breeders have long been skilled at achieving hybrid crosses between species and even between genera of orchids, for some time they experienced few successes in establishing and growing seedlings after germinating the dust-like seeds of these plants. However, this horticultural barrier was substantially overcome by the discovery of the critical importance of the mycorrhizal relationships of orchids. Inoculation with an appropriate mycorrhizal fungus is now an integral component of the methodology used to cultivate orchid seedlings, and successful establishment of seedlings can now be routinely achieved. In addition, dependable methods have also been worked out for the propagation of orchids using tissue culture and other non-sexual means of establishing new plants.
Considering these great advances of orchid horticulture, it is highly regrettable that so many of these plants continue to be collected from wild natural
Co-evolution —This is the intrinsically linked evolution of two or more species, as a result of a close ecological relationship such as pollination, predation, herbivory, or mutualism.
Epiphyte —A plant which relies upon another plant, such as a tree, for physical support, but does not harm the host plant.
Mutualism —An intimate relationship between two or more organisms that is beneficial to both.
Perfect —In the botanical sense, this refers to flowers that are bisexual, containing both male and female reproductive parts.
Saprophyte —This refers to an organism that derives its energy by decomposing dead organic matter. Many species of mushroom-producing fungi live off the organic debris that is present in the mineral soil and, especially, the surface litter of leaves and woody debris on the forest floor.
Symbiosis —A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.
Zygomorphic (or irregular) —Flowers that are bilaterally symmetric, that is, a vertical, longitudinal section of the flower yields two sections that are mirror images.
habitats. Many species of wild orchids have become critically endangered by excessive, often illegal collection of plants for the horticultural trade. Unfortunately, orchid rustling can be a rather profitable endeavor, especially for rare species, which are enthusiastically sought after by unscrupulous collectors of these charismatic plants. Although the international trade of wild-collected orchids is controlled by CITES (the Committee on International Trade in Endangered Species), there is still a great deal of illegal smuggling. Considering the endangered status of so many species of orchids, it is important that their wild populations are left undisturbed in their natural habitats and horticulture limited to the propagation of plants that are already in cultivation.
The only food obtained from the orchid family is a flavoring substance known as vanilla, which is extracted from the ripe, seed-bearing fruits, or “beans,” of the vanilla orchid (Vanilla fragrans ), a climbing orchid native to the West Indies and Mexico. Vanilla was used by the Aztecs as a flavoring for chocolate. Today, vanilla is mostly obtained from plants grown on plantations, but this relatively expensive natural product is increasingly being replaced by a synthetic vanilla, manufactured from a substance obtained from oil of cloves.
Botanica’s Orchids: Over 1200 Species. San Diego, CA:Laurel Glen Publishing, 2002.
Correll, D. Native Orchids of North America: North of Mexico. Stanford, CA: Stanford University Press, 1950.
Griffiths, Mark. The Orchid: From the Archives of the Royal Horticultural Society. New York: Harry N Abrams, 2002.
Lavarack, P.S. Dendrobium and Its Relatives. Portland, OR: Timber Press, 2000.
Orchid Biology: Reviews and Perspectives, edited by Tiiu Kull and Joseph Arditti. New York: Springer, 2002.