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Drosera, which is commonly known as the sundews, is one of the largest genus of carnivorous plants, with at least 194 species. 2 volumes. These members of the family Droseraceae lure, capture, and digest insects using stalked mucilage glands covering their leaf surfaces. The insects are used to supplement the poor mineral nutrition of the soil in which the plants grow. Various species, which vary greatly in size and form, are native to every continent except Antarctica.
Charles Darwin performed much of the early research into Drosera, engaging in a long series of experiments with Drosera rotundifolia which were the first to confirm carnivory in plants. In an 1860 letter, Darwin wrote, “…at the present moment, I care more about Drosera than the origin of all the species in the world.”
The Principia Botanica, published in 1787, states “Sun-dew ( Drosera) derives its name from small drops of a liquor-like dew, hanging on its fringed leaves, and continuing in the hottest part of the day, exposed to the sun.”
This phylogenetic study has made the need for a revision of the genus even clearer.
Although they do not form a single strictly defined growth form, a number of species are often put together in a further group:
[[File:Drosera Glandular Hair.jpg|thumb|Drosera Glandular Hair]]
All species of sundew are able to move their tentacles in response to contact with edible prey. The tentacles are extremely sensitive and will bend toward the center of the leaf to bring the insect into contact with as many stalked glands as possible. According to Charles Darwin, the contact of the legs of a small gnat with a single tentacle is enough to induce this response. This response to touch is known as thigmonasty, and is quite rapid in some species. The outer tentacles (recently coined as "snap-tentacles") of Drosera burmanni and D. sessilifolia can bend inwards toward prey in a matter of seconds after contact, while D. glanduligera is known to bend these tentacles in toward prey in tenths of a second.Hartmeyer, I. & Hartmeyer, S., (2005) Drosera glanduligera: Der Sonnentau mit "Schnapp-Tentakeln", DAS TAUBLATT (GFP) 2005/2: 34-38 In addition to tentacle movement, some species are able to bend their leaves to various degrees to maximize contact with the prey. Of these, Drosera capensis exhibits what is probably the most dramatic movement, curling its leaf completely around prey in 30 minutes. Some species, such as D. filiformis, are unable to bend their leaves in response to prey.
A further type of (mostly strong red and yellow) leaf coloration has recently been discovered in a few Australian species ( D. hartmeyerorum, Drosera indica). Their function is not known yet, although they may help in attracting prey.
The leaf morphology of the species within the genus is extremely varied, ranging from the sessile ovate leaves of D. erythrorhiza to the bipinnately divided acicular leaves of Drosera binata.
While the exact physiological mechanism of the sundew's carnivorous response is not yet known, some studies have begun to shed light on how the plant is able to move in response to mechanical and chemical stimulation to envelop and digest prey. Individual tentacles, when mechanically stimulated, fire action potentials that terminate near the base of the tentacle, resulting in rapid movement of the tentacle towards the center of the leaf. (1980). 9783642677229 ISBN 9783642677229 This response is more prominent when marginal tentacles further away from the leaf center are stimulated. The tentacle movement response is achieved through auxin-mediated acid growth. When action potentials reach their target cells, the plant hormone auxin causes protons (H+ ions) to be pumped out of the plasma membrane into the cell wall, thereby reducing the pH and making the cell wall more acidic. The resulting reduction in pH causes the relaxation of the cell wall protein, expansin, and allows for an increase in cell volume via osmosis and turgor. As a result of differential cell growth rates, the sundew tentacles are able to achieve movement towards prey and the leaf center through the bending caused by expanding cells. Among some Drosera species, a second bending response occurs in which non-local, distant tentacles bend towards prey as well as the bending of the entire leaf blade to maximize contact with prey. While mechanical stimulation is sufficient to achieve a localized tentacle bend response, both mechanical and chemical stimuli are required for the secondary bending response to occur.
The ovary is superior and develops into a dehiscent seed capsule bearing numerous tiny seeds. The pollen grain type is compound, which means four (pollen grains) are stuck together with a protein called callose.
A few South African species use their roots for water and food storage. Some species have wiry root systems that remain during frosts if the stem dies. Some species, such as D. adelae and D. hamiltonii, use their roots for asexual propagation, by sprouting plantlets along their length. Some Australian species form underground for this purpose, which also serve to allow the plants to survive dry summers. The roots of pygmy sundews are often extremely long in proportion to their size, with a 1-cm (0.4-in) plant extending roots over beneath the soil surface. Some pygmy sundews, such as D. lasiantha and D. scorpioides, also form adventitious roots as supports.
D. intermedia and D. rotundifolia have been reported to form arbuscular mycorrhizae, which penetrate the plant's tissues, they also host fungi like to collect nutrients when they grow in poor soil and form symbiosis.
Vegetative reproduction occurs naturally in some species that produce stolons or when roots come close to the surface of the soil. Older leaves that touch the ground may sprout plantlets. Pygmy sundews reproduce asexually using specialized scale-like leaves called gemmae. Tuberous sundews can produce offsets from their corms.
In culture, sundews can often be propagated through leaf, crown, or root cuttings, as well as through seeds.
Europe is home to only three species: D. intermedia, Drosera anglica, and D. rotundifolia. Where the ranges of the two latter species overlap, they sometimes hybridize to form the sterile D. × obovata. In addition to the three species and the hybrid native to Europe, North America is also home to four additional species; D. brevifolia is a small Annual plant native to coastal states from Texas to Virginia, while D. capillaris, a slightly larger plant with a similar range, is also found in areas of the Caribbean. The third species, Drosera linearis, is native to the northern United States and southern Canada. D. filiformis has two subspecies native to the East Coast of North America, the Gulf Coast, and the Florida panhandle.
This genus is often described as cosmopolitan, meaning it has worldwide distribution. The botanist Ludwig Diels, author of the only monograph of the family to date, called this description an "arrant misjudgment of this genus' highly unusual distributional circumstances ( arge Verkennung ihrer höchst eigentümlichen Verbreitungsverhältnisse)", while admitting sundew species do "occupy a significant part of the Earth's surface ( einen beträchtlichen Teil der Erdoberfläche besetzt)". He particularly pointed to the absence of Drosera species from almost all arid climate zones, countless , the American Pacific Coast, Polynesia, the Mediterranean region, and North Africa, as well as the scarcity of species diversity in temperate zones, such as Europe and North America.
The genus, though, is very variable in terms of habitat. Individual sundew species have adapted to a wide variety of environments, including atypical habitats, such as rainforests, deserts ( Drosera burmanni and Drosera indica), and even highly shaded environments (Queensland sundews). The temperate species, which form hibernacula in the winter, are examples of such adaptation to habitats; in general, sundews tend to inhabit warm climates, and are only moderately frost-resistant.
Worldwide, Drosera are at risk of extinction due to the destruction of natural habitat through urban and agricultural development. They are also threatened by the illegal collection of wild plants for the horticultural trade. An additional risk is climate change, because species are often specifically adapted to a precise location and set of conditions.
Currently, the largest threat in Europe and North America is loss of wetland habitat. Causes include urban development and the draining of bogs for agricultural uses and peat harvesting. Such threats have led to the extirpation of some species from parts of their former range. Reintroduction of plants into such habitats is usually difficult or impossible, as the ecological needs of certain populations are closely tied to their geographical location. Increased legal protection of bogs and moors, and a concentrated effort to renaturalize such habitats, are possible ways to combat threats to Drosera plants' survival. As part of the landscape, sundews are often overlooked or not recognized at all.
In South Africa and Australia, two of the three centers of species diversity, the natural habitats of these plants are undergoing a high degree of pressure from human activities. The African sundews Drosera insolita and D. katangensis are listed as critically endangered by the International Union for Conservation of Nature (IUCN), while D. bequaertii is listed as vulnerable. Expanding population centers such as Queensland, Perth, and Cape Town, and the draining of moist areas for agriculture and forestry in rural areas threaten many such habitats. The droughts that have been sweeping Australia in the 21st century pose a threat to many species by drying up previously moist areas.
Those species endemic to a very limited area are often most threatened by the collection of plants from the wild. D. madagascariensis is considered endangered in Madagascar because of the large-scale removal of plants from the wild for exportation; 10 - 200 million plants are harvested for commercial medicinal use annually.
== Gallery of prey ==
In Western medicine, sundews were used as herbal medicine as early as the 12th century, when an Italian doctor from the School of Salerno, Matthaeus Platearius, described the plant as an herbal remedy for under the name herba sole. Culbreth's 1927 Materia Medica listed D. rotundifolia, D. anglica and Drosera linearis as being used as stimulants and , and "of doubtful efficacy" for treating bronchitis, whooping cough, and tuberculosis. Sundew tea was recommended by herbalists for dry coughs, bronchitis, whooping cough, asthma and "bronchial cramps". The French Pharmacopoeia of 1965 listed sundew for the treatment of inflammatory diseases such as asthma, chronic bronchitis and whooping cough.
Drosera has been used commonly in cough preparations in Germany and elsewhere in Europe. In traditional medicine practices, Drosera is used to treat ailments such as asthma, coughs, lung infections, and .
Herbal preparations are primarily made using the roots, flowers, and fruit-like capsules. Since all native sundews species are protected in many parts of Europe and North America, are usually prepared using cultivated fast-growing sundews (specifically D. rotundifolia, D. intermedia, D. anglica, D. ramentacea and D. madagascariensis) or from plants collected and imported from Madagascar, Spain, France, Finland and the Baltics.
Sundews are historically mentioned as an aphrodisiac (hence the common name lustwort). They are mentioned as a folk remedy for treatment of warts, corns, and freckles.
Cultivation requirements vary greatly by species. In general, though, sundews require high environmental moisture content, usually in the form of a constantly moist or wet soil substrate. Most species also require this water to be pure, as nutrients, salts, or minerals in their soil can stunt their growth or even kill them. Commonly, plants are grown in a soil substrate containing some combination of dead or live sphagnum moss, sphagnum peat moss, sand, and/or perlite, and are watered with Distilled water, reverse osmosis, or rain water.
More importantly for biomaterials research, however, is the fact that, when dried, the mucilage provides a suitable substrate for the attachment of living cells. This has important implications for tissue engineering, especially because of the elastic qualities of the adhesive. Essentially, a coating of Drosera mucilage on a surgical implant, such as a replacement hip or an organ transplant, could drastically improve the rate of recovery and decrease the potential for rejection, because living tissue can effectively attach and grow on it. The authors also suggest a wide variety of applications for Drosera mucilage, including wound treatment, regenerative medicine, or enhancing synthetic adhesives. Because this mucilage can stretch to nearly a million times its original size and is readily available for use, it can be an extremely cost-efficient source of biomaterial.
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