Orchid
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 Color plate from Ernst Haeckel's Kunstformen der Natur of 1899 | ||||||||||
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Orchids (Orchidaceae family) are among the biggest and most diverse of the flowering plant (Angiospermae) families, with over 800 described genera and 25,000 species. Some sources give 30,000 species, but the exact number is unknown since classification differs greatly in the academic world. Revisions of different genera occur on a monthly basis and this will increase with the growing use of genetic research and biochemistry. There are another 100,000+ hybrids and cultivars produced by horticulturalists, created since the introduction of tropical species in the 19th century. The Kew World Checklist of Orchids includes about 24,000 accepted species. About 800 new species are added each year. Orchids, through their interactions with pollinators and their symbiosis with mycorrhizae fungi, are considered by some, along with the grasses, to be examples of the most complex floral evolution known.
Orchids get their name from the Greek orchis, meaning "testicle", from the appearance of subterranean tuberoids of the genus Orchis. The word "orchis" was first used by Theophrastos (372/371 – 287/286 B.C.E.), in his book "De historia plantarum" (The natural history of plants). He was a student of Aristotle and is considered the father of botany and ecology.
All orchid species are protected for the purposes of international commerce under CITES as potentially threatened or endangered in their natural habitat, with most species listed under Appendix II. A number of species and genera are afforded protection under Appendix I, including all of Paphiopedilum and all of Phragmipedium. Many other species are protected by both international and national legislation, while hybrids are specifically exempted.
General description
These monocotyledonous plants are cosmopolitan in distribution, occurring in every habitat, except Antarctica and deserts. The great majority are to be found in the tropics, mostly Asia, South America and Central America. They are found above the Arctic Circle, in southern Patagonia and even on Macquarie Island, close to Antarctica.
The following list gives a rough overview of their distribution:
- Eurasia: 40–60 genera
- North America: 20–30 genera
- tropical America: 300–350 genera
- tropical Africa: 125–150 genera
- tropical Asia: 250–300 genera
- Oceania: 50–70 genera
Orchids can be grouped according to the way they retrieve nutrients:
- A majority of species are perennial epiphytes; they are found in tropical moist broadleaf forests or mountains and subtropics. These are anchored on other plants, mostly trees, sometimes shrubs. However, they are not parasites.
- A few are lithophytes, similar to epiphytes but growing naturally on rocks or on very rocky soil. They derive their nutrients from the atmosphere, rain water, litter, humus, and even their own dead tissue.
- Others are terrestrial plants. They grow in the soil or in the loose substrate atop the ground and obtain their nutrients from the soil or the substrate. This group includes nearly all temperate orchids.
- Some lack chlorophyll and are myco-heterotrophs (formerly incorrectly called saprophytes). These achlorophyllous orchids have an ectomycorrhizal relationship, i.e. they are completely dependent on soil fungi feeding on decaying plant matter (usually fallen leaves) to provide them with nutrients. Typical examples include the Bird's-nest Orchid (Neottia nidus-avis) and Spotted Coral-root (Corallorrhiza maculata).
Most advanced orchids have these five basic features:
- The presence of a column, also called gynostemium
- The flower is bilaterally symmetric
- The pollen are glued together into the pollinia, a mass of waxy pollen on filaments.
- The seeds are microscopically small, lacking endosperm (food reserves) in the overal majority of the species. There are notable exceptions, such as Disa cardinalis, whose seeds may grow to a length of 1.1 mm. Seeds of Vanilla may weigh 20 times or more that of other orchids, so therefore are used in the food industry as the extremely popular "vanilla".
- The seeds can, under natural circumstances, only germinate in symbiosis with specialized fungi. Under artificial circumstances, however, germination is possible "in vitro" on sterile substrates of agar in specialized laboratories. Germinating seeds in agar, usually done in flasks, is an advanced technique, requiring sterility at all costs. It takes anywhere from one–up to five to ten years for an orchid seedling to mature. An alternative artificial germination, however, is done by cultivating the fungus and sowing the seeds on them. This is called in-vitro symbiotic culture and is used most commonly for terrestrial orchids.
Leaves
Orchids have simple leaves with parallel veins. Their shape is highly variable between species; ovate, lanceolate, or orbiculate. Their size and shape can be an aid in identifying the orchid, since it reflects the taxonomic position. The leaves can be enormous or minute, or they can even be lacking (as in the Ghost Orchid (Dendrophylax lindenii), a mycoheterotrophic species, and Aphyllorchis and Taeniophyllum, which depend on their roots, which contain chlorophyll for photosynthesis).
The structure of the leaves corresponds to the specific habitat of the orchid. Species that typically bask in sunlight, or grow on sites which can be occasionally very dry, have thick, leathery leaves. The laminas are covered by a waxy cuticle. These retain their necessary water supply. Shade species, on the other hand, have tall, thin leaves. They cannot tolerate a drop in atmospheric humidity or exposure to direct sunlight. Between these two extremes, there is a whole range of intermediate forms.
The leaves of most orchids live on, attached to their pseudobulbs, for several years. Some species, especially those with plicate leaves, shed their aged leaves annually, through an articulation between the lamina and the petiole sheath, and develop new leaves together with new pseudobulbs (as in the genus Catasetum).
The leaves of some species can be most beautiful. The leaves of the Macodes sanderiana, a semiterrestrial or lithophyte, show a sparkling silver and gold veining on a light green background. The cordate leaves of Psychopsiella limminghei are light brownish green with maroon-puce markings, created by flower pigments. The attractive mottle of the leaves of Lady's Slippers from temperate zones (Paphiopedilum) is caused by uneven distribution of chlorophyll. Also Phalaenopsis schilleriana is a lovely pastel pink orchid with leaves spotted dark green and light green. The Jewel Orchid (Ludisia discolor) is grown more for its colorful leaves than its fairly inconspicuous white flowers.
Stem
The stem of an orchid determines the habit of the species. Each type of stem can grow in one of these two ways:
- monopodial ("one-footed") growth. The new shoots grow upwards from a single stem, originating in the end bud of the old shoots. It then produces leaves and flowers along this stem. The stem of these orchids can reach a length of several meters (as in the genera Vanda and Vanilla).
- sympodial ("many-footed") growth. The plant produces a series of adjacent shoots which grow to a certain size, bloom, and then stop growing, to be replaced by the next growth. Plants of this type 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 the genus Cattleya). While this lead is developing, the rhizome may start its growth again, this time from an 'eye', or undeveloped bud, thereby causing the rhizome to branch.
Plant thallus and roots
All orchids are perennial herbs, lacking any permanent woody structure.
- Some orchids are terrestrial, growing rooted in the soil. Terrestrial orchids may be rhizomatous, forming corms or tubers. These act as storage organs for food and water. The root caps of terrestrials are smooth and white. Terrestrials are mostly found in colder climates.
- A great many orchids are epiphytes, which do not require soil and use trees for support. They occur in warmer regions. Epiphytic orchids have modified aerial roots and, in the older parts of the root, an epidermis modified into a spongy, water-absorbing velamen, which can have a silvery-gray, white or brown appearance. The cells of the root epidermis grow at a right angle to the axis of the root. This allows them to get a firm grasp on their support. These roots can sometimes be a few meters long, in order to take up as much moisture as possible. Nutrients mainly come from animal droppings on their supporting tree that are washed down when it rains. The aerial roots of epiphytes that lack leaves have an additional function. They contain chlorophyll and take up carbon dioxide.
- Several species are lithophytes, especially in rocky mountain ranges in Australia and Tasmania, central Brazil and Africa.
The base of the stem of sympodial epiphytes, or in some species essentially the entire stem, may be thickened to form what is called a pseudobulb. These contain nutrients and water for drier periods. Pseudobulbs have a smooth surface with lengthwise grooves. They typically stay alive for five or six years. They look on the inside more like a corm than the embryonal stage of leaf sheaths. They have different sizes and shapes. They can be conical or oblong. In the Black Orchids (Bulbophyllum), the pseudobulbs are no longer than 2 mm. The largest orchid in the world, the Giant Orchid (Grammatophyllum speciosum), has pseudobulbs with lengths of 2–3 m. When the orchid has aged and the pseudobulb has shed its leaves, the pseudobulb becomes dormant and is called a backbulb. The next year's pseudobulb then takes over, exploiting the last reserves of the backbulb. Eventually, the backbulb also dies off, having given life to newer growths. At the end of the pseudobulb typically appear one or two leaves, though there may be up to a dozen or more. Some Dendrobium have long, canelike pseudobulbs with short, rounded leaves over the whole length. Some orchids have hidden or extremely small pseudobulbs hidden completely inside leaves.
Some sympodial terrestrials, such as Orchis and Ophrys, have two subterranean tubers (more like tuberous roots) between the roots. One is used as a food reserve for wintery periods, and provides for the development of the other pseudobulb, from which visible growth develops.
In warm and humid climates, many terrestrial orchids do not need pseudobulbs.
Orchid flowers
Orchids are truly flowers of superlatives. Even a complete layman in botany is awed by the beauty of orchids. No plant family has as many different flowers as the orchid family.
There are many types of specializations within the Orchidaceae. Best known are the seemingly endless structural variations in the flowers that encourage pollination by particular species of insects, bats, or birds.
Most African orchids are white, while Asian orchids are often multicolored. Some orchids only grow one flower on each stem, others sometimes more than a hundred together on a single spike.
The typical orchid flower is zygomorphic, i.e. bilaterally symmetric. Notable exceptions are the genera Mormodes, Ludisia and Macodes.
The flowers grow on racemes or panicles. These can be :
- basal (i.e. produced from the base of the pseudobulb, as in Cymbidium)
- apical (i.e. produced from the apex of the orchid, as in Cattleya)
- or axillary (i.e. coming from a node between the leaf axil and the plant axis, as in Vanda).
The basic orchid flower is composed of three sepals in the outer whorl, and three petals in the inner whorl. The medial petal is usually modified and enlarged (then called the labellum or lip), forming a platform for pollinators near the center of the corolla. Together, except the lip, they are called tepals.
Sepals form the exterior of the bud. They are green in this stage, but sometimes, if the orchid blossom is, for example, purple, the buds can show a purple tint. When the flower opens, the sepals become intensely colored. Sepals may mimick petals such as in some phalaenopsis or be completely distinct. In many orchids, the sepals are mutually different and generally resemble the petals. It is not always easy to distinguish sepals and petals. The normal form can be found in Cattleya, with three sepals forming a triangle. But in Venus Slippers (Paphiopedilum) the lower two sepals are concrescent (fused together into a synsepal), while the lip has taken the form of a slipper. In Masdevallia all the sepals are fused into a calyx. In an example like this the sepals are very prominent, especially in lycaste orchids, the actual petals become diminished and inconspicuous.
The reproductive organs in the center (stamens and pistil) have adapted to become a cylindrical structure called the column or gynandrium. On top of the column lies the stigma, the vestiges of stamens and the pollinia, a mass of waxy pollen on filaments. These filaments can be a caudicle (as in Habenaria) or a stipe (as in Vanda). These filaments hold the pollinia to the viscidium (sticky pad). The pollen are held together by the alkaloid viscine. This viscidium adheres to the body of a visiting insect. The type of pollinia is useful in determining the genus. On top of the pollinia is the anther cap, preventing self-pollination. At the upper edge of the stigma of single-anthered orchids, in front of the anther cap, is the rostellum, a slender beaklike extension.
==Reproduction==
It is in the variety and the refinement of their reproductive methods that orchids truly amaze. On many orchids, the lip (labellum) serves as a landing pad for flying insects. The labellum is sometimes adapted to have a color and shape which attracts particular male insects via mimicry of a receptive female insect. Some orchids are reliant solely on this deception for pollination. After pollination, the epigynous ovary starts developing and produces a many-seeded capsule.
- The Lady's Slipper (Paphiopedilum) has a deep pocket that traps visiting insects, with just one exit. Passage through this exit leads to pollinia being deposited on the insect.
- Many neotropical orchids are pollinated by male orchid bees, which visit the flowers to gather volatile chemicals they require to synthesize pheromonal attractants. Each type of orchid places the pollinia on a different body part of a different species of bee, so as to enforce proper cross-pollination.
- A Eurasian genus Ophrys has some species that look and smell so much like female bumblebees that male bees flying nearby are irresistibly drawn in and attempt to mate with the flower, such as with the Bumblebee Orchid (Ophrys bombyliflora). The viscidium, and thus pollinia, stick to the head or the abdomen of the bumblebee. On visiting another orchid of the same species, the bumblebee pollinates the sticky stigma with the pollinia. The filaments of the pollinia have, during transport, taken such position that the waxy pollen are able to stick in the second orchid to the stigma, just below the rostellum. Such is the refinement of the reproduction. If the filaments had not taken the new position on the bee, the pollinia could not have pollinated the original orchid. Other species of Ophrys are mimics of different bees or wasps, and are also pollinated by males attempting to mate with the flowers, and other orchid genera practice similar deception.
- An underground orchid in Australia, Rhizanthella slateri, never sees the light of day, but depends on ants and other terrestrial insects to pollinate it.
- Many Bulbophyllum species stink like rotting carcasses, and the flies they attract assist their reproduction.
- Catasetum saccatum, a species discussed briefly by Darwin actually launches its viscid pollen sacs with explosive force, when an insect touches a seta. He was ridiculed for this by the naturalist Thomas Huxley.
- Some Phalaenopsis species in Malaysia are known to use subtle weather cues to coordinate mass flowering.
- Some Phalaenopsis, Dendrobium and Vanda species produce keiki, offshoots or plantlets formed from one of the nodes along the stem, through the accumulation of growth hormones at that point.
The filaments of the pollinia of some orchids dry up if they haven’t been visited by an insect. This way, the waxy pollen falls on the stigma causing the orchid to self-fertilize.
==Fruits and seeds==
The orchid ovary is always inferior (located behind the flower), three-carpelate and one or three-partitioned, with parietal placentation (but axile in the Apostasioideae).
If pollination was successful, the sepals and petals fade and wilt but they remain attached to the ovary. The epigynous ovary typically develops into a capsule that is dehiscent by 3 or 6 longitudinal slits, while remaining closed at both ends. The ripening of a capsule can take 2–18 months. The microscopic seeds are very numerous (over a million per capsule in most species). They blow off after ripening like dust particles or spores, barely visible to the human eye. Since they lack endosperm, they must enter symbiotic relationship with mycorrhizal fungi to germinate. These fungi provide the necessary nutrients to the seeds.
All species rely upon mycorrhizal associations with various fungi, mostly genus Rhizoctonia (class Basidiomycetes), for at least part of their life cycle. Some achlorophyllous (lacking chlorophyll) species are adapted to be entirely dependent upon these fungi for nutrients. The fungi decompose surrounding matter, freeing up water-soluble nutrients. Because most orchid seeds are extremely tiny with no food reserves (endosperm lacking), they will not germinate without such a symbiont to supply nutrients in the wild. Some fungi continue to live in the roots of the adult orchid. This enables an orchid such as Neottia nidus-avis to function without chlorophyll. The chance for a seed to meet a fitting fungus is very small. Of all the seeds released, only a minute fraction grow into new orchids. This process can take years; in some cases up to fifteen years.
Horticultural techniques have been devised for germinating seeds on a nutrient-containing gel, eliminating the requirement of the fungus for germination, and greatly aiding the propagation of rare and endangered species.
==Orchid cultivation==
Vanilla
ReferencesISBN links support NWE through referral fees
- Batygina, T. B., Bragina, E. A., and Vasilyeva, E. 2003. The reproductive system and germination in orchids. Acta Biol. Cracov. ser. Bot. 45: 21-34.
- Berg Pana, H. 2005. Handbuch der Orchideen-Namen. Dictionary of Orchid Names. Dizionario dei nomi delle orchidee. Ulmer, Stuttgart
- Kreutz, C. A. J. 2004. Kompendium der Europaischen Orchideen. Catalogue of European Orchids. Kreutz Publishers, Landgraaf, Netherlands
- D. Lee Taylor and Thomas D. Bruns : Ectomycorrhizal mutualism by two nonphotosynthetic orchids; Proc. Natl. Acad. Sci. USA; Vol. 94, pp. 4510-4515, April 1997 (on line).
- Stewart, J. 2000. Orchids Timber Press : Portland, OR
External links
- Kew checklist
- American Orchid Society - Orchid Web
- Orange County (California) Orchid Society
- Orchid Photo Encyclopedia
- Report on pollination tactics by orchids
- Austrian Orchid Society
- OrchidWorks - a photo album and overview of a variety of orchids
- Terrestrial Orchid Forum - a forum for the discussion of terrestrial orchid culture
- Swiss Orchid Foundation at the Herbarium Jany Renz
- Vanilla growing in Uganda
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