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 (Hypoxidaceae, Fig. 1) 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 the 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 the 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 also provide 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 the 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 (Fig. 2) and Neuwiedia (Fig. 3), 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 Hypoxis itself.
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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 (Fig. 4), Mexipedium (Fig. 5), Paphiopedilum (Fig. 6), Phragmipedium (Fig. 7), and Selenipedium (Fig. 8) with about 150 species, is widely distributed in Eurasia, North America, and the South American tropics.
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In the slipper orchids, one fertile anther is 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 a large shield-like organ, the staminode that prevents direct access by the pollinator from the front to the centre of the flower (Fig. 9). The two other stamens are hidden behind the staminode. The saccate lip of slipper orchids has evolved as a trap for pollinators. The inside walls of the lip are very slippery but a ladder of hairs lies on the interior dorsal wall (Fig. 9). 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 and Mediterranean orchids, such as the type genus Orchis (Fig. 10) and the bee orchids (Ophrys, Fig. 11), temperate North and South American, terrestrial African and Madagascan, and temperate Australian orchids. The subfamily has a flower with a single anther terminal on the column.
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The epidendroid orchids, the largest group, are predominantly epiphytes or lithophytes and include all the showy tropical genera, such as Cattleya (Fig. 13), Oncidium (Fig. 14), Phalaenopsis (Fig. 15) and Vanda (Fig. 16). 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).
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