sundew n : any of various bog plants of the genus Drosera having leaves covered with sticky hairs that trap and digest insects; cosmopolitan in distribution [syn: sundew plant, daily dew]

English

Noun

sundew

1. any of a group of insectivorous plants that catch insects by sticky droplets ("dew") at the end of hairs on the leafs. Most of these plants belong to the genus Drosera: these grow in boggy ground all over the world.

Translations

• Dutch: zonnedauw
• Finnish: kihokki
• French: droséra
• German: Sonnentau
• Polish: rosiczka

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 mucilaginous glands covering their leaf surface. The insects are used to supplement the poor mineral nutrition that sundews are able to obtain from the soil they grow in. Various species, which vary greatly in size and form, can be found growing natively on every continent except Antarctica.

Both the botanical name (from the Greek δρόσος: "drosos" = "dew, dewdrops") as well as the English common name (sundew, derived from Latin ros solis, meaning "dew of the sun") refer to the glistening drops of mucilage at the tip of each tentacle that resemble drops of morning dew.

Plant characteristics

Sundews are perennial (or rarely annual) herbaceous plants, forming prostrate or upright rosettes between 1 centimeter (0.4 in.) and 1 meter (39 in.) in height, depending on the species. Climbing species form scrambling stems which can reach much longer lengths, up to 3 meters (10 ft.) in the case of D. erythrogyne. Sundews have been shown to be able to achieve a lifespan of 50 years. The genus is so specialized for nutrient uptake through its carnivorous behavior that the pygmy sundew is missing the enzymes (nitrate reductase in particular ) 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. Drosera arcturi from the mountains of New Zealand is another temperate species that dies back to thick, wiry roots.
• Subtropical Sundews: These species maintain vegetative growth year-round under uniform or nearly uniform climatic conditions.
• Pygmy Sundews: A group of roughly 40 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: More than 40 Australian species that form an underground corm in order to survive the extremely dry 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 species, 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.

Although they do not form a single strictly defined growth form, a number of species are often put together in a further group:

• Queensland Sundews: A small group of three species (D. adelae, D. schizandra and D. prolifera), all native to highly humid habitats in the dim understories of the Australian rainforest.

Leaves and carnivory

Sundews are characterised by the glandular tentacles, topped with sticky secretions, that cover their laminae. The trapping and digestion mechanism usually employs two types of glands: stalked glands that secrete sweet mucilage to attract and ensnare insects and 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 through asphyxiation as the mucilage envelops them and clogs their spiracles. Death usually occurs within one quarter of an hour. The plant meanwhile secretes esterase, peroxidase, phosphatase and protease enzymes. These enzymes both dissolve 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 sensitive 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. 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 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) emergences has recently been discovered in a few Australian species (D. hartmeyerorum, D. indica). Their function is not known yet, they might probably help 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 D. binata.

Flowers and fruit

The flowers of sundews, as with nearly all carnivorous plants, are held far above the leaves by a long stem. This physical isolation of the flower from the traps was originally thought to be an adaptation meant to avoid trapping potential pollinators; a recent study, however, indicated that Drosera attract distinct types of insects as pollinators and prey, with little overlap. Instead, the tall flower stalks probably help raise the flowers to a height where they are noticeable to pollinators. The mostly unforked inflorescences are spikes, whose flowers open one at a time and usually only remain 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 (actinomorphic) 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. or 0.6 in.). A few species, however, such as D. regia and D. cistiflora, have flowers 4 centimeters (1.5 in.) or more in diameter.

Reproduction

Many species of sundews are self-fertile and flowers will often self-pollinate upon closing. The origins of the genus are thought to have been in Africa or Australia. He particularly pointed to the absence of Drosera species from almost all arid climate zones, countless rainforests, 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. Additionally, many of the remaining native populations lie on protected land such as National Parks or Wildlife Preserves. Drosera species are protected by law in many European countries, such as Germany, Austria, Switzerland, Czech republic, Finland, quinones (plumbagin, hydroplumbagin glucoside and rossoliside (7 – methyl – hydrojuglone – 4 – glucoside)), and other constituents such as carotenoids, plant acids (e.g. butyric acid, citric acid, formic acid, gallic acid, malic acid, propionic acid), resin, tannins and ascorbic acid (vitamin C).

Sundews were used as medicinal herbs as early as the 12th century, when an Italian doctor from the School of Salerno by the name of Matthaeus Platearius described the plant as an herbal remedy for coughs under the name "herba sole". It has been used commonly in cough preparations in Germany and elsewhere in Europe. Sundew tea was especially recommended by herbalists for dry coughs, bronchitis, whooping cough, asthma and "bronchial cramps". A modern study has shown that Drosera does exhibit antitussive properties. Sundews have also been used as an aphrodisiac and to strengthen the heart, as well as to treat sunburn and prevent freckles. They are still used today in some 200-300 registered medications, usually in combination with other active ingredients. Today Drosera is usually used to treat ailments such as asthma, coughs, lung infections, and stomach ulcers.

Medicinal 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, extracts 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. Some of these corms were also used to dye textiles, while another purple or yellow dye was traditionally prepared in the Scottish Highlands using D. rotundifolia. A sundew liqueur is also still produced using a recipe that has its roots in the 14th century. It is made using fresh leaves from mainly D. capensis, D. spatulata, and D. rotundifolia.. 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.

Notes

Sources

Much of the content of this article comes from the equivalent German-language wikipedia article (retrieved April 30, 2006).

• 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
• Lowrie, Allen: Carnivorous Plants of Australia, Vol. 1-3, English, 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)

External links

• A key to Drosera species, with distribution maps and growing difficulty scale
• A virtually exhaustive listing of Drosera pictures on the web
• International Carnivorous Plant Society
• Carnivorous Plant FAQ
• Listing of scientific Drosera articles online (terraforums.com)
• Sundew images from smugmug
• Botanical Society of America, Drosera - the Sundews