Orchid
Orchids are plants that belong to the family Orchidaceae, a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Orchids are cosmopolitan plants, living in diverse habitats on every continent except Antarctica. The world's richest diversity of orchid genera and species is in the tropics. Many species are epiphytes, living on trees. The flowers and their pollination mechanisms are highly specialized, attracting insect pollinators by colour, pattern, scent, pheromones, and sometimes by mimicking female insects. Orchids have very small seeds, relying on fungal partners for germination. Some orchids have no leaves, either photosynthesizing with their roots or relying entirely on fungal partners for food.
Orchidaceae is one of the two largest families of flowering plants. It contains about 28,000 currently accepted species in around 700 genera. That represents some 6–11% of all species of seed plants. Horticulturists run many orchid societies around the world; they have produced many hybrids and cultivars.
Etymology
The type genus is Orchis. The genus name comes from the Ancient Greek ὄρχις, literally meaning "testicle", because of the shape of the twin tubers in some species of Orchis. The term "orchid" was introduced in 1845 by John Lindley in School Botany, as a shortened form of Orchidaceae. In Middle English, the name bollockwort was used for some orchids, based on "bollock" meaning testicle and "wort" meaning plant.Description
Orchids are easily distinguished from other plants, as most of them share some very evident derived characteristics or synapomorphies. Among these are: bilateral symmetry of the flower, many resupinate flowers, a nearly always highly modified middle petal, stamens and carpels fused into a column, and extremely small seeds.Stem and roots
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, as in Vanda.
- Sympodial: growth is at a front, with older growth behind, at a "back". Sympodial orchids may have swollen pseudobulbs.
Leaves
Like most monocots, orchids usually have simple leaves with parallel veins. Some orchids such as Vanda are distichous, with their leaves arranged in two ranks on opposite sides of the stem. This is the arrangement at the base of all orchid shoots, though in many species the leaves higher up the shoot switch to a spiral phyllotaxis.Orchids are perennial; most species add new leaves at the apex while the oldest leaves gradually die off, but some such as Catasetum shed their leaves annually in the dry season, developing new leaves and new pseudobulbs each year.
Some epiphytic orchids, such as Taeniophyllum aphyllum, are leafless, depending on their green roots for photosynthesis. Other epiphytes like Phalaenopsis have leaves, but rely on photosynthesis in their green roots to prevent hypoxia of the roots. Orchids of the genus Corallorhiza have no leaves; instead they have symbiotic or parasitic associations with fungal mycelium, through which they absorb sugars.
Flowers
Orchid flowers are very varied in form. They have three sepals, three petals and a three-chambered ovary. The three sepals and two of the petals are often similar to each other but one petal is usually highly modified, forming a "lip" or labellum. In most orchid genera, as the flower develops, it undergoes a twisting through 180°, called resupination, so that the labellum lies below the column. The labellum functions to attract insects, and in resupinate flowers, also acts as a landing stage, or sometimes a trap.Reproduction
Pollination
investigated the complex and varied mechanisms that orchids have evolved to achieve cross-pollination, and described these in his 1862 book Fertilisation of Orchids.The reproductive parts of an orchid flower are unique in that the stamens and style are joined to form a single structure, the column.
Most orchids deliver pollen in a single mass, a pollinium, able to fertilise thousands of ovules.
The pollinia are attached to a sticky disc near the top of the column. Just below the pollinia is a second, larger sticky plate, the stigma.
In orchids that produce pollinia, pollination happens as some variant of the following sequence: when the pollinator enters into the flower, it touches a viscidium, which promptly sticks to its body, generally on the head or abdomen. While leaving the flower, it pulls the pollinium out of the anther, as it is connected to the viscidium by the caudicle or stipe. The caudicle then bends and the pollinium is moved forwards and downwards. When the pollinator enters another flower of the same species, the pollinium is so placed that it sticks to the stigma of the second flower, pollinating it. In horticulture, artificial orchid pollination is achieved by removing the pollinia with a small instrument such as a toothpick from the pollen parent and transferring them to the seed parent.
Special cases
Pollinators are often visually attracted by the shape and colours of the labellum. However, some Bulbophyllum species attract male fruit flies solely via a floral chemical which simultaneously acts as a floral reward to perform pollination.The slipper orchid Paphiopedilum parishii reproduces by self-fertilization. This occurs when the anther changes from a solid to a liquid state and directly contacts the stigma surface without the aid of any pollinating agent or floral assembly.
In some extremely specialized orchids, such as the Eurasian genus Ophrys, the labellum is adapted to have a colour, shape, and odour which attracts male insects via mimicry of a receptive female. Pollination happens as a male insect attempts to mate with the flowers.
Many neotropical orchids are pollinated by male orchid bees, which visit the flowers to gather volatile chemicals they require to synthesize pheromonal attractants. Males of species such as Euglossa imperialis or Eulaema meriana leave their territories periodically to forage for aromatic compounds, such as cineole, to synthesize pheromone for attracting females.
Catasetum saccatum, discussed by Darwin, launches its viscid pollinia with explosive force when an insect touches a seta, knocking the pollinator off the flower.
Asexual reproduction
Some species, such as in the genera 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 to horticulturalists as keiki.Epipogium aphyllum exhibits a dual reproductive strategy, engaging in both sexual and asexual seed production. The likelihood of apomixis playing a substantial role in successful reproduction appears minimal. Within certain petite orchid species groups, there is a noteworthy preparation of female gametes for fertilization preceding the act of pollination.
Fruits and seeds
The ovary develops into a capsule that is dehiscent by three or six longitudinal slits in its sides.The seeds of orchids are 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 seeds that can germinate without mycorrhiza, namely the species within the genus Disa with hydrochorous seeds.
Evolution
Fossil and reconstructed history
Few orchid fossils are recorded. An extinct species of stingless bee, Proplebeia dominicana, was found trapped in Miocene amber from about 15–20 million years ago. The bee was carrying pollen of a previously unknown orchid taxon, Meliorchis caribea, on its wings. This 2007 find provided the first fossil evidence of orchids. It shows insects were active pollinators of orchids in the Miocene. M. caribea has been placed within the extant tribe Cranichideae, subtribe Goodyerinae. An even older orchid species, Succinanthera baltica, was described in 2017 from Baltic amber of Eocene age, some 40–55 million years ago.Genetic sequencing indicates orchids may have arisen earlier, 76 to 84 million years ago during the Late Cretaceous. According to Mark W. Chase et al., the overall biogeography and phylogenetic patterns of Orchidaceae show they are even older and may go back roughly 100 million years.
The molecular clock method has provided ages for the major branches of the orchid family. The subfamily Vanilloideae is a branch at the base of the monandrous orchids, and must have evolved very early in the evolution of the family, some 60 to 70 million years ago. Since this subfamily occurs worldwide in tropical and subtropical regions, from tropical America to tropical Asia, New Guinea and West Africa, and the continents began to split about 100 million years ago, significant biotic exchange must have occurred after this split. Biogeographic studies indicate that the most recent common ancestor of all extant orchids probably originated 83 million years ago somewhere in the supercontinent Laurasia. Despite their long evolutionary history on Earth, the extant orchid diversity is also inferred to have originated during the last 5 million years, with the American and Asian tropics as the geographic areas exhibiting the highest speciation rates on Earth.
Genome duplication occurred prior to the divergence of this taxon.