Thelymitra, commonly known as sun orchids, is a genus of more than 100 species of plants in the orchid family, Orchidaceae. Unlike most other orchids, sun orchids lack a highly modified labellum and all three petals are similar in size, shape and colour. The column is, however, highly modified and usually has prominent wings or glands which are helpful in identifying the species. Most sun orchids close their flowers at night, in cloudy or cool weather, giving rise to their common name. The scientific name means "woman's hood" and refers to the hooded column present in most, but not all species. Most species are endemic to Australia although some are found as far from there as the Philippines and Indonesia. The type species, Thelymitra longifolia, the first to be formally described, was collected in New Zealand.
Orchids in the genus Thelymitra are terrestrial, perennial, deciduous, sympodial herbs usually with a few inconspicuous, fine roots and a pair of oval-shaped tubers. A single leaf emerges from near the base of the plant and surrounds the lower part of the flowering stem.[2][3]
The inflorescence is a raceme with from one to many resupinate flowers with three sepals and three petals all more or less alike in size, shape and ornamentation. (The labellum is not highly modified as in most other orchid genera but is similar to the other two petals.) The sexual parts of the flower are fused to the column which is short and stubby with wings which surround the column, forming a hood-like "mitra". The dorsal part of the mitra is usually ornamented, with a flat midsection and two side arms. The lower part of the mitra forms an unornamented rim below the column. The flowers of most species only open in bright warm sunlight and close at night or during cold or cloudy weather. A few species self pollinate and their reaction to environmental change is not as apparent. The fruit that follows flowering is a non-fleshy, dehiscent capsule containing up to 500 seeds.[2][3][4]
Joseph Banks collected specimens of Thelymitra longifolia on the North Island of New Zealand during Cook's first expedition to the South Pacific but the first formal description of that species was made by Johann Forster and his son Georg during Cook's second expedition to Australia. They made their collections on the South Island of New Zealand.[5] Thelymitra longifolia is therefore the type species.[6][7]
In 1803, Robert Brown collected T. venosa from Port Jackson and was the first to formally describe it in 1810.[8][9][10]
The name "Thelymitra" is derived from the Ancient Greek words thelys meaning "belonging to women"[11]: 862 and mitra meaning "headdress" or "turban"[11]: 183 referring to the hooded column.