The Orchidaceae are a diverse and widespread
family of flowering plants, with blooms that are often colourful and fragrant, commonly
known as the orchid family.
Along with the Asteraceae, they are one of the two largest families of flowering plants.
The Orchidaceae have about 28,000 currently accepted species, distributed in about 763
genera.
The determination of which family is larger is still under debate, because verified data
on the members of such enormous families are continually in flux.
Regardless, the number of orchid species nearly equals the number of bony fishes and is more
than twice the number of bird species, and about four times the number of mammal species.
The family encompasses about 6–11% of all seed plants.
The largest genera are Bulbophyllum (2,000 species), Epidendrum (1,500 species),
Dendrobium(1,400 species) and Pleurothallis (1,000 species).
It also includes Vanilla–the genus of the vanilla plant, the type genus Orchis, and
many commonly cultivated plants such as Phalaenopsis and Cattleya.
Moreover, since the introduction of tropical species into cultivation in the 19th century,
horticulturists have produced more than 100,000 hybrids and cultivars.
Orchids are easily distinguished from other plants, as they share some very evident, shared
derived characteristics, or synapomorphies.
Among these are: bilateral symmetry of the flower (zygomorphism), many resupinate flowers,
a nearly always highly modified petal (labellum), fused stamens and carpels, and extremely small
seeds.
All orchids are perennial herbs that lack any permanent woody structure.
They can grow according to two patterns: Monopodial: The stem grows from a single bud,
leaves are added from the apex each year and the stem grows longer accordingly.
The stem of orchids with a monopodial growth can reach several metres in length, as in
Vanda and Vanilla.
Sympodial: Sympodial orchids have a front (the newest growth) and a back (the oldest
growth).
The plant produces a series of adjacent shoots, which grow to a certain size, bloom and then
stop growing and are replaced.
Sympodial orchids grow laterally rather than vertically, following the surface of their
support.
The growth continues by development of new leads, with their own leaves and roots, sprouting
from or next to those of the previous year, as in Cattleya.
While a new lead is developing, the rhizome may start its growth again from a so-called
'eye', an undeveloped bud, thereby branching.
Sympodial orchids may have visible pseudobulbs joined by a rhizome, which creeps along the
top or just beneath the soil.
Terrestrial orchids may be rhizomatous or form corms or tubers.
The root caps of terrestrial orchids are smooth and white.
Like most monocots, orchids generally have simple leaves with parallel veins, although
some Vanilloideae have reticulate venation.
Leaves may be ovate, lanceolate, or orbiculate, and very variable in size on the individual
plant.
The Orchidaceae are well known for the many structural variations in their flowers.
Some orchids have single flowers, but most have a racemose inflorescence, sometimes with
a large number of flowers.
The flowering stem can be basal, that is, produced from the base of the tuber, like
in Cymbidium, apical, meaning it grows from the apex of the main stem, like in Cattleya,
or axillary, from the leaf axil, as in Vanda.
As an apomorphy of the clade, orchid flowers are primitively zygomorphic (bilaterally symmetrical),
although in some genera, such as Mormodes, Ludisia, and Macodes, this kind of symmetry
may be difficult to notice.
Some species, such as Phalaenopsis, Dendrobium, and Vanda, produce offshoots or plantlets
formed from one of the nodes along the stem, through the accumulation of growth hormones
at that point.
These shoots are known as keiki.