Welwitschia

Taxobox
name = "Welwitschia"

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image_width = 270px
regnum = Plantae
divisio = Gnetophyta
classis = Gnetopsida
ordo = Welwitschiales
familia = Welwitschiaceae
genus = "Welwitschia"
species = "W. mirabilis"
binomial = "Welwitschia mirabilis"
binomial_authority = 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 considered a living fossil.cite book | author = A. Lewington & E. Parker | title = Ancient Trees: Trees that Live for a Thousand Years | year = 1999 | publisher = Collins & Brown Ltd. | id = ISBN 1-85585-704-9] .

Biology

Welwitschia grows from a short, thick 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 afterwards 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 flowers, is apparently carried out by insects that are attracted by "nectar" produced on both male and female flowers.cite journal|author=Wetschnig W, Depisch B|year= 1999|title=Pollination biology of "Welwitschia mirabilis" HOOK. f. (Welwitschiaceae, Gnetopsida)|journal=Phyton-Annales Rei Botanicae|volume=39|pages=167]

The age of the plants is difficult to assess, but it is believed that they are very long-lived, possibly living 1000 years or more. Some individuals may be more than 2000 years old.

The plant is thought to absorb water through peculiar structures on its leaves, harvesting moisture originating from dew that forms during the night. As a further adaptation to the arid conditions and hot daytime temperatures in its environment, and as the only gymnosperm species known to do so, "W. mirabilis" uses the crassulacean acid metabolism (CAM) pathway for carbon fixation in photosynthesis.cite journal|author=von Willert DJ, Armbruster N, Drees T, Zaborowski M |year= 2005|title="Welwitschia mirabilis": CAM or not CAM - what is the answer?|journal=Functional Plant Biology|volume=32|pages=389|doi=10.1071/FP01241] Named after the Slovenian botanist Dr. Friedrich Welwitsch who discovered it in 1860, it is generally considered to be one of the oddest plants in existence. 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 generally considered to be better protected than the plants in Namibia, owing to the relatively high concentration of landmines in Angola, which keep collectors away Fact|date=February 2007.

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. Fact|date=May 2007

cientific classification according to different sources

Heraldry

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

ee also

*List of Southern African indigenous trees

References

External links

* [http://www.conifers.org/we/index.htm Gymnosperm database: "Welwitschia"]
* [http://florawww.eeb.uconn.edu/acc_num/199700061.html Ecology & Evolutionary Biology Conservatory]