Welwitschiaceae

▪ gnetophyte family
      a family of southwestern African desert plants in the gymnosperm order Gnetales, named for its single genus, Welwitschia. tumboa plants (W. mirabilis), constituting the only species, have deep taproots and resemble giant radishes, 60 to 120 cm (about 25 to 50 inches) in diameter and projecting about 30 cm (12 inches) above the ground. From the base of the cone-shaped trunk, two broad, flat, straplike leaves grow throughout the life of the plant but are kept about 3 metres (nearly 10 feet) in length by erosion at the tips. Other than these, the only leaves produced during the plant's life are a pair of cotyledons (seed leaves), which generally wither less than 18 months after the seed germinates, and a pair of inconspicuous scalelike leaves that protect the stem tip.

      The staminate and ovulate cones grow in a ring above the leaves. The staminate cones are morphologically complex, and each scale encloses a flowerlike complex consisting of six stamenlike structures bearing the microsporangia and surrounding a rudimentary nonfunctional ovule. These reproductive organs are associated with a small series of scales superficially similar to the perianth of flowering plants. The ovulate cones are simpler, with each scale subtending a single ovule and two small scales.

      How these large bizarre plants manage to survive in desert environments with less than 100 mm (4 inches) of annual rainfall is poorly understood. Amazingly, some individual specimens of Welwitschia have been estimated to be 1,500 to 2,000 years old. The species has suffered from being overly collected for firewood and museum specimens in the past, and it is now protected by law.

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Universalium. 2010.

Look at other dictionaries:

  • Welwitschiaceae — Welwitschia mirabilis Welwitschia …   Wikipédia en Français

  • Welwitschiaceae — Welwitschie Welwitschie in Namibia Systematik Abteilung: Gnetophyta Klasse …   Deutsch Wikipedia

  • Welwitschiaceae — noun in some classifications included in the Gnetaceae • Syn: ↑family Welwitschiaceae • Hypernyms: ↑gymnosperm family • Member Holonyms: ↑Gnetales, ↑order Gnetales • Member Meronyms: ↑ …   Useful english dictionary

  • Семейство вельвичиевые (Welwitschiaceae) —         Единственный представитель семейства вельвичия удивительная (Welwitschia mirabilis), растет в бесплодных каменистых пустынях Анголы и Юго Западной тропической Африки, главным образом в прибрежной пустыне Намиб, заходя в глубь материка… …   Биологическая энциклопедия

  • family Welwitschiaceae — noun in some classifications included in the Gnetaceae • Syn: ↑Welwitschiaceae • Hypernyms: ↑gymnosperm family • Member Holonyms: ↑Gnetales, ↑order Gnetales • Member Meronyms: ↑ …   Useful english dictionary

  • Welwitschia — mirabilis Welwitschia …   Wikipédia en Français

  • Welwitschia mirabilis — Welwitschia …   Wikipédia en Français

  • Welwitschiales — Welwitschia mirabilis Welwitschia …   Wikipédia en Français

  • Welwitschias — Welwitschia mirabilis Welwitschia …   Wikipédia en Français

  • Welwitschia mirabilis — Welwitschia Plant de Welwitsch …   Wikipédia en Français

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