Orchidaceae

Orchid Fact File

- Why should we study orchids?

• Eight percent of all flowering plants are orchids, making them the largest family of angiosperms.

• They are cosmopolitan, but the majority of species are found in the tropics and subtropics, ranging from sea level to almost 5000 m in nearly all environments except open water and true desert. In places they are dominant, particularly in nutrient-deficient habitats. More than half of the species are epiphytic.

Oeceoclades maculata Lindl.
Oeceoclades maculata Lindl.

• They have a complex life cycle, a mycorrhizal association (at least at germination) and specific pollination syndromes. Therefore, they offer much in the study of the interactions of plants, fungi and animals. They are extremely sensitive to environmental changes, a subject of increasing interest at present.

• The classification of the family is still problematic because of its size, the rarity of many of the species, intense horticultural interest, and past over-reliance on floral morphology.

• Their phylogenetic (evolutionary) relationships were largely speculative, until a generation of molecular systematists uncovered unimagined genetic relationships by sequencing DNA.

• From studies of the molecular (DNA) clock we know that orchids are one of the oldest families of angiosperms; they date to about 80 million years ago, so there were orchids around during the time of the dinosaurs. The lack of a fossil record made people speculate that the orchids were a recent group, but we can now say that in fact orchids are an ancient group.

• It is a family of considerable economic importance, particularly in horticulture and floristry but also increasingly in the pharmaceuticals and fragrance industries. Orchids are a major money-earner in many developing countries.

• Orchids are a charismatic group and figure prominently in plant conservation. All appear on CITES Appendix I or II. A number of nature reserves exist because of the orchids they include. Orchids have been called the “pandas of the plant world”.

- The 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 orchids with only one stamen are usually separated into three subfamilies: Vanilloideae, Orchidoideae and Epidendroideae. The median petal (lip) in these orchid subfamilies 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:

Apostasioideae
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 Hypoxis itself.

Cypripedioideae
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 and throughout the Americas. 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 a 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 are 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.

Vanilloideae
The vanilloid orchids are a small group that includes Vanilla, a genus of about 70 species of lianas. The subfamily comprises about 16 genera and about 200 species. Most are tropical in their distribution.

Orchidoideae
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.

Epidendroideae
The epidendroid orchids, the largest group, are predominantly epiphytes or lithophytes and include all the showy tropical genera, such as Cattleya, Oncidium, Phalaenopsis and Vanda. Two growth habits are found in this subfamily. Some produce each new growth from the base of the old growth (sympodial growth habit). Others continue to grow each year from the same point (monopodial growth habit).

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