Welwitschia is a monotypic genus of gymnosperm plant, composed solely of the very distinct Welwitschia mirabilis. The plant is commonly simply known as Welwitschia in English. It is known locally as !kharos or khurub (Nama), tweeblaarkanniedood (Afrikaans), nyanka (Damara), or onyanga (Herero), among others. It is the only genus of the family Welwitschiaceae and order Welwitschiales, in the division Gnetophyta. The plant, which is considered a living fossil,[1][2] is named after the Austrian botanist Friedrich Welwitsch who discovered it in 1859. The geographic distribution of Welwitschia mirabilis is limited to the Namib desert within Namibia and Angola.
Welwitschia grows from a short, thick, woody trunk with a thick taproot that has been variously described as long and short. Bornmann appears to have investigated the matter more intimately at first hand than most authors, and describes the root system as "rather shallow and simple, consisting of a tapering taproot with one or more non-tapering extensions, some pronounced lateral roots, and a network of delicate spongy roots." He offers the guideline that the roots extend to a depth roughly equal to the span of the living leaves from tip to tip.[3]
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 at right angles to the cotyledons, and continue to grow throughout the entire life of the plant; they are the only leaves that the plant will bear, because the stem is acephalous; its apical meristem dies after the first two leaves are produced.[3] Those two leaves grow continuously from a basal meristem in the plant's terminal groove. They eventually grow to a length of 2–4 m and usually become split, frayed and shredded into several well-separated strap-shaped sections by the distortions of the woody portions surrounding the apical slit, and also by wind and adventitious external injuries.[3][4] The growth of the leaves outstrips that of the cotyledons after about four months, after which the growing tips of the two cotyledons die, but their bases continue to grow sideways. That lateral growth leads the obconical extension of the stem which produces the terminal groove from which the leaves grow.
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  mostly flies and infrequently wasps and bees  that are attracted by "nectar" produced on both male and female strobili.[5]
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.[3]
The plant absorbs water through structures on its leaves, harvesting moisture originating from Namib desert morning fogs that blow in during the night, comprising the main source of water for much of the desert life.[3]
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.[6]
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 land mines 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 the mold Aspergillus niger, which causes them to rot shortly after they germinate. Seeds from botanical gardens, or other cultivated sources are much cleaner and less likely to rot. It is suggested that the seeds might contain a germination inhibitor that prevents germination until the substance has been leached out by plentiful water. This is a common strategy among desert plants and is consistent with the observation that one commonly encounters communities of Welwitschia plants of apparently more or less the same age. It is reasonable to suspect that they had germinated at about the same time during unusual episodes of wetness in the desert.[3]
The taxonomic placement of Welwitschia has changed many times with the development of new classification systems (see Flowering plants: History of classification). Most botanists have treated Welwitschia as a distinct monotypic genus in a monotypic family or even order. Most recent systems place Welwitschia mirabilis in its own family Welwitschiaceae in the gymnosperm order Gnetales.[7] The order Gnetales is most closely related to the order Pinales, which includes pines, spruces, larches and firs[8]
referane :
http://en.wikipedia.org/wiki/Welwitschia
