Origin and affinities of orchids
The orchid family (Orchidaceae) demonstrates one of the most specialized lines of flowering plant evolution. It is likely that all orchids derived
from Hypoxis-like ancestors that had six tepals (three tepals of an outer whorl called sepals and three tepals of an inner whorl called petals)
and six stamens (again three in an outer whorl and three in an inner one). Several trends of morphological specialization can be observed in the
evolution and formation of different orchid groups. However, a reduction of the number of stamens and fusion of the remaining fertile stamen(s)
with the pistil is the main general floral transformation that led to the evolution of the family. Successive reduction in the number of stamens has
led to formation of groups of orchids with three, two and one stamen remaining in flower. More than 99 % of all orchid species have only a single
stamen in the flower and this is one of the main features of the orchid family. In the most recent taxonomic treatments, the monandrous orchids
(with one stamen) are usually separated into three subfamilies: Vanilloideae, Orchidoideae and Epidendroideae. The median petal (lip) in the
single-anther orchid subfamilies (monandrous) usually plays a role as a landing platform for pollinators and can provide also attraction for
pollinators, such as nectar, and a convenient path to the anther.
Currently five subfamilies of orchids are recognized.
The Apostasioid orchids, placed into subfamily Apostasioideae, are considered to be the most primitive group of orchids. They have three or two
stamens in their flowers. They comprise two genera, Apostasia and Neuwiedia, with about 16 species. All are terrestrial orchids
confined to tropical Asia, the adjacent archipelagos and northern Australia. Their flowers are almost regular and resemble somewhat those of
The second group, subfamily Cypripedioideae, also retains two stamens in its flower. They are popularly called slipper orchids and represent a
distinct lineage. This subfamily, including five genera Cypripedium, Mexipedium, Paphiopedilum, Phragmipedium and Selenipedium and about
150 species, is widely distributed in Eurasia, North America and the South American tropics. In the slipper orchids, two fertile anthers are placed
on either side of the column. The central stamen is sterile and is borne at the apex of the column. It is curiously modified into large shield-like
organ, the staminode, that prevents direct access by the pollinator from the front to the centre of flower. The two other stamens are hidden behind
the staminode, one on each side of the column. The saccate lip of slipper orchids has evolved as a trap for pollinators. The inside walls of the
lip also very slippery but a ladder of hairs lies on the interior dorsal wall. This leads under the stalked ventral stigma to one of two exits at
the base of the lip on either side of the column.
The vanilloid orchids are a small group that includes Vanilla, a genus of about 70 species of lianas. The subfamily has a flower with a single
anther terminal on the column. It comprises about 16 genera and about 200 species. Most are tropical in their distribution.
The orchidoid orchids are mostly terrestrials with tubers or fleshy rhizomes and include most European, Mediterranean, North American, terrestrial
African and temperate Australian orchids. The type genus Orchis and the bee orchids (Ophrys) belong here as do the terrestrial
Australian, temperate North and South American and many African and Madagascan ground orchids. The subfamily has a flower with a single anther
terminal on the column.
The epidendroid orchids, the largest group, are predominantly epiphytes or lithophytes and include all the showy tropical genera, such as
Cattleya, Oncidium, Odontoglossum, Phalaenopsis and Vanda. The subfamily has a flower with a single anther terminal on the column.
Two growth habits are found in this subfamily. Some have annual growths that terminate in a flowering spike and start a new growth the
following season from the base of the old growth (sympodial growth habit). Others have a continuously growing stem and lateral inflorescences
(monopodial growth habit) .
Written by Phillip Cribb of the Royal Botanic Gardens, KEW.