Orchidaceae

Orchidaceae

The plants belonging to the family Orchidaceae represent a pinnacle of evolutionary success in the plant kingdom. Represented by approximately twenty-five thousand species, they are possibly the largest family of flowering plants on Earth. Although orchids are most diverse in the tropics, they are found on every continent except Antarctica and can be found as far north as Alaska and as far south as Tierra del Fuego. Perhaps the main reason that orchids are so successful is that they have developed close relationships with insect pollinators and fungi. Their life histories are extremely complex and intricately woven together across three kingdoms of life: Plantae, Animalia, and Fungi.

Unlike other plants, orchid seeds contain no storage food for their dormant embryos. In order for most orchid seeds to germinate, they must be infected by fungal hyphae . After infection takes place, the orchid is able to take nourishment from the fungus, but it is unclear whether the fungus gets any benefit in return. This life strategy enables orchids to survive in habitats with poor soils, such as bogs, or those that lack soil altogether. Many tropical orchids are epiphytes , and some live completely underground, lacking chlorophyll and depending on fungi for all their nutritional needs.

The close association of orchids with insects was carefully studied by Charles Darwin. Through evolution, orchids have reduced their reproductive organs to one anther and one pistil . Moreover, orchids have fused these two organs into a single structure, the column, and have amassed all of their pollen into a single unit called a pollinium. As a consequence, most orchid flowers have only one chance to pollinate another flower. This strategy may seem risky, but when successful it delivers enough pollen to produce as many as seventy-two thousand seeds. To ensure success, orchids have evolved intricate pollination mechanisms. Some of these include explosive shotguns and glue to attach the pollinium to insects, or floral traps that force bees to take pollen with them when they escape. One of the most fascinating strategies is seen in orchids that not only mimic female wasps in morphology but also produce fragrances similar to female wasp pheromones . These orchids manage to fool male wasps into copulating with their flowers, thereby effecting pollination.

Only one orchid species, Vanilla planifolia, is of significant agricultural value, as the source of natural vanilla flavoring. Cultivation and processing of this spice is a long, labor-intensive process involving pollinating each flower by hand, drying and fermenting the fruits, and extracting the aromatic vanillin flavoring with alcohol. For this reason, natural vanilla is extremely expensive.

Vanilla, however, is not the only orchid of economic value. An enormous industry exists for cut flowers, corsages, and cultivation of orchids by hobbyists. Ancient texts indicate that orchids have been cultivated in China since at least 550 B. C. E. Today, the American Orchid Society alone has more than thirty thousand members, all of whom share a fascination and appreciation of these breathtakingly beautiful flowers. Unfortunately, many orchid species are threatened with extinction because of habitat destruction and over-collecting in the wild. However, all orchids are protected under international treaties.

see also Epiphytes; Horticulture; Interactions, Plant-Insect; Monocots; Pollination Biology.

Kenneth M. Cameron

Bibliography

Cameron, K., et al. "A Phylogenetic Analysis of the Orchidaceae: Evidence from rbc LNucleotide Sequences." American Journal of Botany 86 (1999): 208-24.

Darwin, Charles. The Various Contrivances by Which Orchids Are Fertilised by Insects. London: John Murray, 1888.

Dressler, Robert. The Orchids: Natural History and Classification. Cambridge, MA:Harvard University Press, 1990.

Luer, Carlyle. The Native Orchids of the United States and Canada Excluding Florida. New York: New York Botanical Garden Press, 1975.