Orchidaceae is a diverse and widespread family of flowering plants with blooms that are often colourful and often fragrant, commonly known as the orchid family. Along with the Asteraceae, they are one of the two largest families of flowering plants, with between 21,950 and 26,049 currently accepted species, found in 880 genera. Selecting which of the two families is larger is still under debate, as concrete numbers on such enormous families are constantly in flux. Regardless, the number of orchid species nearly equals the number of bony fishes and more than twice the number of bird species, and about four times the number of mammal species. The family also encompasses about 6–11% of all seed plants. The largest genera are Bulbophyllum (2,000 species), Epidendrum (1,500 species), Dendrobium (1,400 species) and Pleurothallis (1,000 species).
The family also includes Vanilla (the genus of the vanilla plant), Orchis (type genus), and many commonly cultivated plants such as Phalaenopsis and Cattleya. Moreover, since the introduction of tropical species into cultivation in the 19th century, horticulturists have produced more than 100,000 hybrids and cultivars.
Orchidaceae are cosmopolitan, occurring in almost every habitat apart from glaciers. The world’s richest diversity of orchid genera and species is found in the tropics, but they are also found above the Arctic Circle, in southern Patagonia, and two species of Nematoceras on Macquarie Island at 54° south.
A majority of orchids are perennial epiphytes, which grow anchored to trees or shrubs in the tropics and subtropics. Species such as Angraecum sororium are lithophytes, growing on rocks or very rocky soil. Other orchids (including the majority of temperate Orchidaceae) are terrestrial and can be found in habitat areas such as grasslands or forest.
Some orchids, such as Neottia and Corallorhiza, lack chlorophyll, so are unable to photosynthesize. Instead, these species obtain energy and nutrients by parasitising soil fungi through the formation of orchid mycorrhizas. The fungi involved include those that form ectomycorrhizas with trees and other woody plants, parasites such as Armillaria, and saprotrophs. These orchids are known as myco-heterotrophs, but were formerly (incorrectly) described as saprophytes as it was believed they gained their nutrition by breaking down organic matter. While only a few species are achlorophyllous holoparasites, all orchids are myco-heterotrophic during germination and seedling growth, and even photosynthetic adult plants may continue to obtain carbon from their mycorrhizal fungi.