This family of monocotyledonous plants has evolved from prototypes of the lily and amaryllis family and is noteworthy for the wide variety of its highly specialized and curiously modified forms. Epiphytic types have a stem swollen at the base to form a pseudobulb (for food storage) and pendulous aerial roots adapted for water absorption and sometimes containing chlorophyll to make photosynthesis possible. In terrestrial types a symbiotic relationship often exists between the roots and filamentous fungi (mycorrhiza). Horticulturists have found that the presence of certain fungi is necessary for the germination of the minute seeds. Orchid pollen occurs as mealy or waxen lumps of tiny pollen grains, highly varied in form.
The flowers characteristically consist of three petals and three petallike sepals, the central sepal modified into a conspicuous lip (labellum) specialized to secrete nectar that attracts insects. Most of the diverse forms of orchid flowers are apparently complicated adaptations for pollination by specific insects, e.g., the enormous waxflower of Africa, which has a labellum over a foot long and is pollinated by a moth with a tongue of equal length. The saclike labellum of the lady's-slipper serves the same function by forcing the insect to brush against the anther and the stigma (male and female organs) while procuring nectar.