Orchidaceae

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Common name: Orchids

Etymology of scientific name: An irregular formation from Latin orchis, from Ancient Greek ὄρχις (órkhis, “testicle”) (ostensibly from the shape of the roots)

Flowers:

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.
The orchid flower, like most flowers of monocots, has two whorls of sterile elements.
The outer whorl has three sepals and the inner whorl has three petals.
The sepals are usually very similar to the petals
The medial petal, called the labellum or lip, which is always modified and enlarged, is actually the upper medial petal; however, as the flower develops, the inferior ovary or the pedicel usually rotates 180°, so that the labellum arrives at the lower part of the flower, thus becoming suitable to form a platform for pollinators
This characteristic, called resupination, occurs primitively in the family and is considered apomorphic, a derived characteristic all Orchidaceae share
The torsion of the ovary is very evident from the longitudinal section shown (below right)
Some orchids have secondarily lost this resupination, e.g. Epidendrum secundum.
Longitudinal section of a flower of Vanilla planifolia
The normal form of the sepals can be found in Cattleya, where they form a triangle
In Paphiopedilum (Venus slippers), the lower two sepals are fused into a synsepal, while the lip has taken the form of a slipper
In Masdevallia, all the sepals are fused
Orchid flowers with abnormal numbers of petals or lips are called peloric. Peloria is a genetic trait, but its expression is environmentally influenced and may appear random.
Orchid flowers primitively had three stamens, but this situation is now limited to the genus Neuwiedia. Apostasia and the Cypripedioideae have two stamens, the central one being sterile and reduced to a staminode
All of the other orchids, the clade called Monandria, retain only the central stamen, the others being reduced to staminodes
The filaments of the stamens are always adnate (fused) to the style to form cylindrical structure called the gynostemium or column
In the primitive Apostasioideae, this fusion is only partial; in the Vanilloideae, it is more deep; in Orchidoideae and Epidendroideae, it is total
The stigma is very asymmetrical, as all of its lobes are bent towards the centre of the flower and lie on the bottom of the column.
Pollen is released as single grains, like in most other plants, in the Apostasioideae, Cypripedioideae, and Vanilloideae. In the other subfamilies, which comprise the great majority of orchids, the anther (3) carries two pollinia.
A pollinium is a waxy mass of pollen grains held together by the glue-like alkaloid viscin, containing both cellulosic strands and mucopolysaccharides.
Each pollinium is connected to a filament which can take the form of a caudicle, as in Dactylorhiza or Habenaria, or a stipe, as in Vanda.
Caudicles or stipes hold the pollinia to the viscidium, a sticky pad which sticks the pollinia to the body of pollinators.
At the upper edge of the stigma of single-anthered orchids, in front of the anther cap, is the rostellum (5), a slender extension involved in the complex pollination mechanism.
As mentioned, the ovary is always inferior (located behind the flower). It is three-carpelate and one or, more rarely, three-partitioned, with parietal placentation (axile in the Apostasioideae).
In 2011, Bulbophyllum nocturnum was discovered to flower nocturnally

Fruit:

The ovary typically develops into a capsule that is dehiscent by three or six longitudinal slits, while remaining closed at both ends.
The seeds are generally almost microscopic and very numerous, in some species over a million per capsule
After ripening, they blow off like dust particles or spores.
Most orchid species lack endosperm in their seed and must enter symbiotic relationships with various mycorrhizal basidiomyceteous fungi that provide them the necessary nutrients to germinate, so almost all orchid species are mycoheterotrophic during germination and reliant upon fungi to complete their lifecycles
Only a handful of orchid species have seed that can germinate without mycorrhiza, namely the species within the genus Disa with hydrochorous seeds
As the chance for a seed to meet a suitable fungus is very small, only a minute fraction of all the seeds released grow into adult plants. In cultivation, germination typically takes weeks.

Leaves:

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. Their characteristics are often diagnostic. They are normally alternate on the stem, often folded lengthwise along the centre ("plicate"), and have no stipules. Orchid leaves often have siliceous bodies called stegmata in the vascular bundle sheaths (not present in the Orchidoideae) and are fibrous.
The structure of the leaves corresponds to the specific habitat of the plant. Species that typically bask in sunlight, or grow on sites which can be occasionally very dry, have thick, leathery leaves and the laminae are covered by a waxy cuticle to retain their necessary water supply. Shade-loving species, on the other hand, have long, thin leaves.
The leaves of most orchids are perennial, that is, they live for several years, while others, especially those with plicate leaves as in Catasetum, shed them annually and develop new leaves together with new pseudobulbs.

Habit:

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 horizontally, 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.
Some sympodial terrestrial orchids, such as Orchis and Ophrys, have two subterranean tuberous roots. One is used as a food reserve for wintry periods, and provides for the development of the other one, from which visible growth develops.
In warm and constantly humid climates, many terrestrial orchids do not need pseudobulbs.
Epiphytic orchids, those that grow upon a support, have modified aerial roots that can sometimes be a few meters long. In the older parts of the roots, a modified spongy epidermis, called a velamen, has the function of absorbing humidity. It is made of dead cells and can have a silvery-grey, white or brown appearance. In some orchids, the velamen includes spongy and fibrous bodies near the passage cells, called tilosomes.
The cells of the root epidermis grow at a right angle to the axis of the root to allow them to get a firm grasp on their support. Nutrients for epiphytic orchids mainly come from mineral dust, organic detritus, animal droppings and other substances collecting among on their supporting surfaces.
The base of the stem of sympodial epiphytes, or in some species essentially the entire stem, may be thickened to form a pseudobulb that contains nutrients and water for drier periods.
The pseudobulb has a smooth surface with lengthwise grooves, and can have different shapes, often conical or oblong. Its size is very variable; in some small species of Bulbophyllum, it is no longer than two millimeters, while in the largest orchid in the world, Grammatophyllum speciosum (giant orchid), it can reach three meters. Some Dendrobium species have long, canelike pseudobulbs with short, rounded leaves over the whole length; some other orchids have hidden or extremely small pseudobulbs, completely included inside the leaves.
With ageing the pseudobulb sheds its leaves and becomes dormant. At this stage it is often called a backbulb. Backbulbs still hold nutrition for the plant, but then a pseudobulb usually takes over, exploiting the last reserves accumulated in the backbulb, which eventually dies off, too. A pseudobulb typically lives for about five years. Orchids without noticeable pseudobulbs are also said to have growths, an individual component of a sympodial plant.

Habitat:

Species:

World: 28 000 S, 763 G
Australia: S, G

Additional notes:

Is a diverse and widespread family of flowering plants, with blooms that are often colourful and fragrant.
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 number of orchid species is nearly equal to the number of bony fishes, 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.
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 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.