Sundew
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{{Taxobox | color = lightgreen | name = Drosera | image = Drosera_spatulata_KansaiHabit.jpg | image_width = 230px | image_caption = Drosera spatulata Kansai | regnum = Plantae | divisio = Magnoliophyta | classis = Magnoliopsida | ordo = Caryophyllales | familia = Droseraceae | genus = Drosera | genus_authority = L. | subdivision_ranks = Species | subdivision = See seperate list. }}
The Sundews (Drosera) comprise one of the largest genera of carnivorous plants, with over 170 species. These members of the family Droseraceae lure, capture, and digest insects using stalked mucilagenous glands covering their leaf surface. The insects are used to supplement the poor mineral nutrition they are able to obtain from their environment. Various species, which vary greatly in size and form, can be found growing natively on every continent except Antarctica.
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Etymology
Both the botanical name (from the Greek δρόσος: "drosos" = "dew, dewdrops") as well as the English common name (sundew) refer to the glistening drops of mucilage at the tip each tentacle that resemble drops of morning dew.
Plant characteristics
Sundews are perennial (or rarely annual) herbacious (nonwoody) plants, forming prostrate or upright rosettes between 1 cm. and 1 m. in height, depending on the species. Climbing species form scrambling stems which can reach much longer lengths, up to 3 m. in the case of D. erythrogyna. Sundews have been shown to be able to achieve a lifespan of 50 years. The genus is so specialized for nutrient uptake through its carnivory that in at least the case of pygmy sundews it is missing the enzymes (nitrate reductase in particular Template:Ref) that plants usually use for the uptake of earth-bound nitrates.
Habit
The genus can be divided into several growth forms:
- Temperate Sundews: These species form a tight cluster of unfurled leaves called a Hibernaculum in a winter dormancy period (= Hemicryptophyte). All of the North American and European species belong to this group.
- Subtropical Sundews: These species maintain vegetative growth year-round under uniform or nearly uniform climatic conditions.
- Pygmy Sundews: A group of roughly 30 Australian species, they are distinguished by miniature growth, the formation of gemmae for asexual reproduction, and dense formation of hairs in the crown center. These hairs serve to protect the plants from Australia's intense summer sun. Pygmy sundews form the section Bryastrum.
- Tuberous Sundews About 20 Australian species that form an underground corm in order to survive the extremely try summers of their habitat, re-emerging in the fall. These so-called tuberous sundews can be further divided into two groups, those that form rosettes and those that form climbing or scrambling stems. Tuberous sundews comprise the subgenus Ergaleium.
- Petiolaris Complex: A group of tropical Australian species which live in constantly warm but irregularly wet conditions. Several of the 14 species that comprise this group have developed special strategies to cope with the alternately drier conditions. Many speceis, for example, have petioles densely covered in trichomes, which maintain a sufficiently humid environment and serve as an increased condensation surface for morning dew. The petiolaris complex sundews comprise the section Lasiocephala.
Image:DroseraZonariaTuber2-.jpg Although they don't form a single strictly defined growthform, a number of species are often put together in a further group:
- Queensland Sundews: A small group of three species (D. schizandra, D. prolifera, and D. adelae), all native to highly humid habitats in the dim understories of the Australian rainforest.
Roots
The root system of most Drosera is only weakly developed. It serves mainly to absorb water to and anchor the plant to the ground; the roots are relatively useless when it comes to nutrient uptake. A few South African species use their roots for water and food storage. Some Australian species form underground corms 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. plant extending roots over 15 cm. beneath the soil surface.
Leaves and carnivory
Sundews are characterised by the glandular tentacles, topped with sticky secretions, that cover their laminas. The trapping and digestion mechanism usually employs two types of glands: stalked glands that secrete sweet mucilage to attract and ensnare insects and Image:Drosera capensis bend.JPG enzymes to digest them, and sessile glands that absorb the resulting nutrient soup (the latter glands are missing in some species, such as D. erythrorhiza). Small prey, mainly consisting of insects, are attracted by the sweet secretions of the peduncular glands. Upon touching these, however, they become entrapped by sticky mucilage which prevents their progress or escape. Eventually, the prey either succumb to death through exhaustion or asphyxiation as the mucilage envelops them and clogs their tracheas. Death usually occurs within one quarter of an hourTemplate:Ref. The plant meanwhile secretes enzymes such as esterase, peroxidase, phosphatase and protease. These enzymes both disolve the insect and free the contained nutrients. The nutrient soup is then absorbed through the leaf surface and can then be used to help fuel plant growth.
All species of sundew are able to move their tentacles in response to contact with digestible prey. The tentacles are extremely senstive and will bend toward the center of the leaf in order 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 Template:Ref. This response to touch is known as thigmotropism, and is quite rapid in some species. The outer tentacles of D. burmannii and D. sessilifolia can bend inwards toward prey in a matter of seconds after contact, while D. glanduligera is known to envelop its prey with tentacles in a mere second in optimal environmental conditions. In addition to tentacle movement, some species are able to bend their laminas to various degrees in order to maximize contact with the prey. Of these, D. capensis exhibits what is probably the most dramatic movement, curling its leaf completely around prey in 12-72 hours. Some species, such as D. filiformis, are unable to bend their leaves in response to 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 D. binata.
Flowers
Image:DroseraKenneallyiFlora.jpg The flowers of sundews, as with nearly all carnivorous plants, are held far above the the leaves by a long stem in order to avoid trapping potential pollinators. The mostly unforked inflorescences are spikes, whose flowers open one at a time and usually only are open for a short period. Flowers open in response to light intensity (often opening only in direct sunlight), and the entire inflorescence is also helitropic, moving in response to the sun's position in the sky.
The radially symmetrical (actinomorphous) flowers are always perfect and have five parts (the exceptions to this rule are the four-petaled D. pygmaea and the eight to twelve-petaled D. heterophylla). Most of the species have small flowers (<1.5 cm.). A few species, however, such as D. regia and D. cistiflora, have flowers 4 cm. or more in diameter. In general, the flowers are white or pink. Australian species display a wider range of colors, including orange (D. callistos), red (D. adelae), yellow (D. zigzagia) or metallic violet colored (D. microphylla).
Fruit
The ovary develops into a dehiscent seed capsules bearing numerous tiny seeds.
Reproduction
Many species of sundews are self-fertile and flowers will often self-pollinate upon closingTemplate:Note. Often copious amounts of seeds are produced. The seeds are black, tiny and germinate in response to moisture and light. Seeds of temperate species also require stratification in order to germinate
Vegetative reproduction occurs naturally in some species that produce stolons. 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.
Distribution
The range of the sundew genus stretches from Alaska in the north to New Zealand in the south. The centers of diversity are Australia (with roughly 50% of all known species), South America (20+ species) and southern Africa (20+ species). A few species are also found in large parts of Eurasia and North America. These areas, however, can be considered to form the outskirts of the generic range, as the ranges of sundews do not typically approach arctic areas. Unlike previously supposed, the evolutionary speciation of this genus is no longer thought to have occured with break-up of Gondwana through continental drift. Rather, speciation is thought to have occured as a result of a subsequent wide dispersal of its range. The origins of the genus are thought to have been in Africa or Australia.
Europe is home to only three species: D. intermedia, D. anglica, and D. rotundifolia. Where the ranges of the two latter species overlap, they sometimes hybridize to form the sterile D. x 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 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. A third species, D. linearis, is native to the northern United States and southern Canada. D. filiformis has two subspecies native to the East Coast, the Gulf Coast, and the Florida panhandle.
This genus is often descriped as cosmopolitan, meaning that 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", while admitting that sundew species do "occupy a significant part of the Earth's surface". He particularly pointed to the absence of Drosera species from almost all arid climate zones, countless rainforests, the American Pacific Coast, Polynesia, the Mediterranean, and North Africa, as well as the scarcity of species diversity in temperate zones such as Europe and North America.
Habitat
Image:Drosera rotundifolia habitat.jpg Sundews generally grow in seasonally moist or more rarely constantly wet habitats with acidic soils and high levels of sunlight. Common habitats include bogs, fens, swamps, marshes, the tepuis of Venezuela, the wallums of coastal Australia, the Fynbos of South Africa, and moist streambanks. Many species grow in association with sphagnum moss, which absorbs much of the soil's nutrient supply and also acidifies the soil, making nutrients less available to plant life. This allows sundews, which don't rely on soil-bound nutrients, to flourish where more dominating vegetation would usually outcompete them.
That being said, the genus is very variable in terms of habitat. Individual sundew species have adapted to a wide variety of environments, uncluding atypical habitats such as rainforests, deserts (ex. D. burmannii and D. indica), and even highly shaded environments (Queensland Sundews). The temperate species, which form hibernacula in the winter, are an example of such adaptation to habitats; in general, sundews tend to prefer warm climates, and are only moderatly frost-hardy.
Phylogenetics
For a complete list of species, see the seperate article List of Drosera species.
The following cladogram shows the relationship between various subgenera and classes as defined by the Rivadavia et al.'s analysis in 2002Template:Note. The monotypic section "Meristocaules" was not included in the study, so that its place in this system is unclear. More recent studies have placed this group near section "Bryastrum", so it is placed there below. Since the section "Drosera" is polyphyletic, it shows up multiple times in the cladogram ( * ).
This phylogenetic study has made the need for a revision of the genus even clearer.
,--- Section Drosera *
|
|
,---+
| |
| |
,---+ `---- Section Ptycnostigma
| |
| |
,---+ `---- Section Drosera *
| |
| |
| `---- Section Thelocalyx
|
| ,---- Subgenus Ergaleium
| ,----+
| | `---- Section Phycopsis
,-------------------+ |
| | ,---+
| | | |
| | | | ,---- Section Bryastrum
,---+ `----+ `----+
| | | `---- Section Lasiocephala
| | |
| | `--- Section Coelophylla
---+ `--- Section Drosera: Drosera arcturi *
|
`--- Section Regiae
Footnotes
- Template:Note{{cite journal
| author=Karlsson PS, Pate JS | title=Contrasting effects of supplementary feeding of insects or mineral nutrients on the growth and nitrogen and phosphorus economy of pygmy species of Drosera | journal=Oecologia | volume=92 | year=1992 | pages=8-13 }}
- Template:Note{{cite book
| author=Peter D'Amato | title=The Savage Garden - Cultivate Carnivorous Plants | publisher=Ten Speed Press | location=Berkley, California | year=1998 }}
- Template:Note{{cite journal
| author= Rivadavia, Fernando; Kondo, Katsuhiko; Kato, Masahiro und Hasebe, Mitsuyasu | title= Phylogeny of the sundews, Drosera (Droseraceae), based on chloroplast rbcL and nuclear 18S ribosomal DNA Sequences | journal=American Journal of Botany | volume=90 | year=2003 | pages=123-130 | url= http://www.amjbot.org/cgi/content/full/90/1/123 }}
- Template:Note{{cite book
| author=Charles Darwin | title=Insectivorous Plants | publisher= | location= | year=1875 | url=http://isis.library.adelaide.edu.au/pg/etext04/insec10.txt }}
References
- Barthlott, Wilhelm; Porembski, Stefan; Seine, Rüdiger; Theisen, Inge: Karnivoren. Stuttgart, 2004, ISBN 3-8001-4144-2
- Correa A., Mireya D.; Silva, Tania Regina Dos Santos: Drosera (Droseraceae), in: Flora Neotropica, Monograph 96, New York, 2005
- Darwin, Charles: Insectivorous Plants, 1875
- Diels, Ludwig: Droseraceae, in Engler, A. (Hrsg.): Pflanzenr. 4, 112 : 109, 1906
- Lowrie, Allen: Carnivorous Plants of Australia, Vol. 1-3, Englisch, Nedlands, Western Australia, 1987 - 1998
- Lowrie, Allen: A taxonomic revision of Drosera section Stolonifera (Droseraceae) from south-west Western Australia, 2005, Nuytsia 15(3):355-393. (Online: http://science.calm.wa.gov.au/nuytsia/15/3/355-394.pdf)
- Olberg, Günter: Sonnentau, Natur und Volk, Bd. 78, Heft 1/3, pp. 32-37, Frankfurt, 1948
- Rivadavia, Fernando; Kondo, Katsuhiko; Kato, Masahiro und Hasebe, Mitsuyasu: Phylogeny of the sundews, Drosera (Droseraceae), based on chloroplast rbcL and nuclear 18S ribosomal DNA Sequences, American Journal of Botany. 2003;90:123-130. (Online: http://www.amjbot.org/cgi/content/full/90/1/123)
- Seine, Rüdiger; Barthlott, Wilhelm: Some proposals on the infrageneric classification of Drosera L., Taxon 43, 583 - 589, 1994
- Schlauer, Jan: A dichotomous key to the genus Drosera L. (Droseraceae), Carnivorous Plant Newsletter, Vol. 25 (1996)
Weblinks
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Under Constructionals:Sonnentau da:Soldug (Drosera) de:Sonnentau es:Rocío del sol fr:Droséra pl:Rosiczka sv:Sileshår