Welwitschia

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Welwitschia
Scientific classification
Kingdom: Plantae
Division: Gnetophyta
Class: Gnetopsida
Order: Welwitschiales
Family: Welwitschiaceae
Genus: Welwitschia
Species: W. mirabilis
Binomial name
Welwitschia mirabilis
Hook.f.

Welwitschia is a monotypic genus of gymnosperm plant, composed solely of the very distinct Welwitschia mirabilis. It is the only genus of the family Welwitschiaceae, in the order Welwitschiales, in the division Gnetophyta. The plant is named after the Austrian botanist Friedrich Welwitsch who discovered it in 1859 and is considered a living fossil.[1][2] The geographic distribution of Welwitschia mirabilis is limited to the Namib desert within Namibia and Angola.

Contents

[edit] Biology

Welwitschia grows from a short, thick, woody trunk, with only two leaves that continuously grow from their base, and a long, thick taproot. After germination, the cotyledons grow to 25–35 mm in length, and are followed shortly afterward by the appearance of two permanent leaves. These leaves are produced opposite of the cotyledons, and continue to grow throughout the entire life of the plant. They eventually grow to a length of 2–4 m and usually become split into several strap-shaped sections, thus sometimes disguising the origin from only two leaves. After these appear, two cotyledonary buds appear; in these, the growing tip dies, causing elongation of the buds. Growth continues sideways, which forms the obconical growth of the stem. The species is dioecious, with separate male and female plants. Fertilization, that is, the transfer of the pollen from the male to the female strobili, is carried out by insects that are attracted by "nectar" produced on both male and female strobili.[3]

The age of the plants is difficult to assess, but they are very long-lived, living 1000 years or more. Some individuals may be more than 2000 years old.[citation needed]

The plant absorbs water through structures on its leaves, harvesting moisture originating from dew that forms during the night.

It is possible that W. mirabilis uses Crassulacean acid metabolism (CAM) photosynthesis; if this were true, W. mirabilis would be the only known gymnosperm to do so. However, the evidence is contradictory.[4]

Although considered endangered due to its very slow growth and despite the fact that older plants are often sought by collectors, a fair number of plants exist in the wild. The plants living in Angola are better protected than the plants in Namibia, owing to the relatively high concentration of landmines in Angola, which keep collectors away[citation needed].

The species grows readily from seed, which may be purchased from specialty seed dealers. The seed must be kept moist for the first couple of weeks and exposed to as much heat and light as possible during this time. Seeds collected from the wild are often heavily contaminated with spores of Aspergillus niger, which causes them to rot shortly after they germinate. Seeds from the Kirstenbosch National Botanical Garden in Cape Town, South Africa, or other cultivated sources are much cleaner and less likely to rot.[citation needed]

[edit] Scientific classification

  After Systema Naturae 2000

[1]

 After University of Connecticut 2006 [2] After ITIS (Integrated Taxonomic Information System) 2005 [3]
Domain Eukaryota - -
Kingdom Plantae
Haeckel, 1866		 Plantae Plantae
Subkingdom Viridaeplantae Cavalier-Smith,
1981		 - Tracheobionta
Division Tracheophyta Sinnott, 1935 ex Cavalier-Smith,
1998		 Gnetophyta Gnetophyta
Subdivision Spermatophytina (auct.) Cavalier-Smith, 1998 - - -
Infradivision Gymnospermae auct. - -
Class Gnetopsida Welwitschiopsida Gnetopsida -
Order Gnetales T.M. Fries, 1891 Welwitschiales Welwitschiales
Family Welwitschiaceae Markgraf in Engler & Prantl, 1926 Welwitschiaceae Welwitschiaceae Markgr.
Genus Welwitschia J.D. Hooker, 1862 Welwitschia Welwitschia J. D. Hooker, 1863
Species Welwitschia mirabilis J.D. Hooker Welwitschia mirabilis Hook. f. Welwitschia mirabilis Hook. f.
In green In green In green Identical names with different authors
In red - - Different names with different authors

[edit] Heraldry

The plant figures as a charge in the national coat of arms of Namibia, as well as that of Westelike Rugby Subunie.

[edit] Gallery

The largest known Welwitschia, nicknamed "The Big Welwitschia", stands 1.4 m tall and is over 4 m in diameter

Female plant

Detail photograph of male plant and cones

Detail photograph of female plant and cones

[edit] See also

[edit] References

  1. ^ Flowering Plants of Africa 57:2-8(2001)
  2. ^ A. Lewington & E. Parker (1999). Ancient Trees: Trees that Live for a Thousand Years. Collins & Brown Ltd.. ISBN 1-85585-704-9. 
  3. ^ Wetschnig W, Depisch B (1999). "Pollination biology of Welwitschia mirabilis HOOK. f. (Welwitschiaceae, Gnetopsida)". Phyton-Annales Rei Botanicae 39: 167. 
  4. ^ Winter K, Schramm MJ (1986). "Analysis of Stomatal and Nonstomatal Components in the Environmental Control of CO2 Exchange in Leaves of Welwitschia mirabilis". Plant Physiology 82: 173. 

[edit] External links

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