Orchidaceae, usually referred to as the Orchid family, is a morphologically diverse and widespread family of monocots in the order Asparagales. It is currently believed to be the largest family of flowering plants with between 21,950 and 26,049 currently accepted species, found in 880 genera.
The number of orchid species equals more than twice the number of bird species, and about four times the number of mammal species. It also encompasses about 6–11% of all seed plants.
The complex mechanisms which orchids evolve to achieve cross-pollination were investigated by Charles Darwin and described in his 1862 book Fertilisation of Orchids.
Orchidaceae are well known for the many structural variations in their flowers.Some orchids have single flowers but most have a racemose inflorescence, sometimes with a large number of flowers.
As an apomorphy of the clade, orchid flowers are primitively zygomorphic (bilaterally symmetrical), although in some genera like Mormodes, Ludisia, Macodes this kind of symmetry may be difficult to notice.
The orchid flower, like most flowers of monocots, has two whorls of sterile elements. The outer whorl has three sepals and the inner whorl has three petals. The sepals are usually very similar to the petals, but may be completely distinct.
Orchids have developed highly specialized pollination systems and thus the chances of being pollinated are often scarce. This is why orchid flowers usually remain receptive for very long periods and why most orchids deliver pollen in a single mass.