Welwitschia mirabilis0425.jpg

''Welwitschia'' is a monotypic gymnosperm genus, comprising solely the distinctive ''Welwitschia mirabilis'', endemic to the Namib desert within Namibia and Angola. ''Welwitschia'' is the only living genus of the family Welwitschiaceae and order Welwitschiales in the division Gnetophyta, and is one of three living genera in Gnetophyta, alongside ''Gnetum'' and ''Ephedra''. Informal sources commonly refer to the plant as a "living fossil".

Naming

''Welwitschia'' is named after the Austrian botanist and doctor Friedrich Welwitsch, who described the plant in Angola in 1859. Welwitsch was so overwhelmed by the plant that he, "could do nothing but kneel down ..and gaze at it, half in fear lest a touch should prove it a figment of the imagination." Joseph Dalton Hooker of the Linnean Society of London, using Welwitsch's description and collected material along with material from the artist Thomas Baines who had independently recorded the plant in Namibia, described the species.

Welwitsch proposed calling the genus ''Tumboa'' after what he believed to be the local name, ''tumbo''. Hooker asked Welwitsch for permission to name the genus ''Welwitschia'' instead. Welwitsch concurred and supplied some well-preserved material from which Hooker was able to make substantial progress in determining its botanical affinities. The taxonomy of ''Welwitschia'' subsequently changed intermittently with the development of new classification systems (see Flowering plants: History of classification), however, its current taxonomic status is essentially the same as Hooker's placement.

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, although other extinct species have been placed in this family.

The plant is commonly known simply as welwitschia in English, but the name tree tumbo is also used. It is called or in Nama, ('two leaves; can't die') in Afrikaans, in Damara, and in Herero.

Biology

After germination, the seedling produces two cotyledons which grow to in length, and have reticulate venation. Subsequently, two foliage leaves are produced at the edge of a woody bilobed crown. The permanent leaves are opposite (at right angles to the cotyledons), amphistomatic (producing stomata on both sides of the leaf), parallel-veined and ribbon-shaped. Shortly after the appearance of the foliage leaves, the apical meristem dies and meristematic activity is transferred to the periphery of the crown.

The two (rarely three) foliage leaves grow continuously from a basal meristem reaching lengths up to . The tips of the leaves split and fray 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. The largest specimens (such as the "Husab Giant" which is five meters in circumference (about five feet diameter)) may be no more than tall above ground, but the circumference of the leaves in contact with the sand may exceed .

''Welwitschia'' has an elongated shallow root system consisting of "a tapering taproot with one or more non-tapering extensions, some pronounced lateral roots, and a network of delicate spongy roots"Bornman, C.H., J.A. Elsworthy, V. Butler and C.E.J Botha (1972). ''Welwitschia mirabilis'': Observations on general habit, seed, seedling, and leaf characteristics. ''Madoqua'' Series II 1:53-66. and a woody fibrous unbranched main stem. The roots extend to a depth roughly equal to the span of the living leaves from tip to tip. The main stem consists of an unbranched woody crown roughly shaped like an inverted cone. The only branching in the shoot system occurs in the reproductive branches, which bear strobili.

The species is dioecious, with separate male and female plants. Fertilization is carried out by insects including flies and true bugs. The most common of the true bugs attending ''Welwitschia'' is a member of the family Pyrrhocoridae, '' Probergrothius angolensis'', but a hypothesized role in pollination has so far not been demonstrated. Infrequently, wasps and bees also play a role as pollinators of ''Welwitschia''. At least some of the pollinators are attracted by "nectar" produced on both male and female strobili.

''Welwitschia'' has been classified as a CAM plant (crassulacean acid metabolism) after reconciliation of some initially contradictory and confusing data. There are however some very puzzling aspects to the matter; for example, the employment of the CAM metabolism is very slight, which was part of the reason that it took so long to establish its presence at all; it is not understood why this should be.

The age of individual plants is difficult to assess, but many plants may be over 1,000 years old. Some individuals may be more than 2,000 years old. As the species does not produce yearly rings, plant age is determined by radiocarbon dating. However, other reports suggest that the plant does produce a kind of yearly ring. The "trunk" continues to expand with age. The largest known is in diameter ( in circumference).

Because ''Welwitschia'' only produces a single pair of foliage leaves, the plant was thought by some to be neotenic, consisting essentially of a "giant seedling." However, research showed that its anatomy is not consistent with the giant seedling idea. Instead, the plant is more accurately thought to achieve its unusual morphology as a result of having "lost its head" (apical meristem) at an early stage.

Genetics

In July 2021, the genome of ''Welwitschia'' was 98% sequenced, totaling 6.8 Gb on 21 chromosomes. There is evidence of a whole genome duplication followed by extensive reshuffling, probably caused by extreme stress due to a time of increased aridity and prolonged drought some 86 million years ago. As a result of this duplication, the genome contains more “junk” self-replicating DNA sequences; this increase in retrotransposon activity was counteracted with a silencing DNA methylation process allowing to lower the metabolic cost of such a large genetic material and improve resilience.

Distribution and habitat

''Welwitschia mirabilis'' is endemic to the Kaokoveld Desert,van Wyk, A.E. and G.F. Smith (2001). ''Regions of Floristic Endemism in Southern Africa''. Umdaus Press, Hatfield. which lies within the Namib Desert. The population is distributed southwards from the Bentiaba River in southern Angola, to the Kuiseb River in Namibia, and up to inland of the coast. The area is extremely arid; the coast is recorded as having almost zero rainfall, while less than of rain falls annually below the escarpment in the wet season from February to April. Populations tend to occur in ephemeral watercourses, indicating a dependence on groundwater in addition to precipitation from fog.

Cultivation

''Welwitschia mirabilis'' grows readily from seed, which may be bought from specialty seed dealers. The seeds have been shown to display orthodox seed behavior, which in general means that they may be stored for long periods at suitably low humidity and temperature. ''Welwitschia'' seeds naturally develop suitably low water concentrations as they ripen.Whitaker, C., P. Berjak, H. Kolberg, and N.W. Pammenter (2004). Responses to various manipulations, and storage potential, of seeds of the unique desert gymnosperm, ''Welwitschia mirabilis'' Hook. fil. ''South African Journal of Botany'' 70: 622-630. Removal of the outer seed coverings enhances germination performance, which suggests that the seeds may display non-deep physiological dormancy. On planting the seed it is necessary to keep it moist, but not immersed in water, for the first two weeks of cultivation; it has been suggested that soaking the seeds in water before planting interferes with germination.

Seeds collected from the wild often are heavily contaminated with spores of the fungus ''Aspergillus niger var. phoenicis'', which causes them to rot shortly after they germinate. The fungal inoculum infects the growing cones of ''W. mirabilis'' early during their development, and a sharp increase in infection occurs when the pollination drops appear; through those drops the fungal spores may gain access to the interior of the developing seed.Whitaker, C., N. Pammenter, and P. Berjak(2008). Infection of the cones and seeds of ''Welwitschia mirabilis'' by ''Aspergillus niger'' var. ''phoenicis'' in the Namib-Naukluft Park. ''South African Journal of Botany'' 74:41-50 Seeds in the wild may therefore be obliterated through fungal action even before they are fully developed. Seeds from botanical gardens or other cultivated sources are much cleaner and less likely to rot. The fungicide tebuconazole may be useful in controlling limited '' A. niger'' seed infection.

As food

Indigenous people eat the cone of this plant by eating it raw or baking it in hot ashes. One of its names, ''onyanga'' translates to 'onion of the desert'.

Conservation

The population of ''Welwitschia mirabilis'' in the wild is reasonably satisfactory at present. The international trade in the plant is controlled under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). Plants in Angola are better protected than those in Namibia, because the relatively high concentration of land mines in Angola keep collectors away.

Although ''Welwitschia mirabilis'' is not at present immediately threatened, there being abundant populations over a large area, its status is far from secure; its recruitment and growth rates are low, and its range, though wide, covers only a single compact, ecologically limited and vulnerable area. The remarkable longevity of Welwitschia favours its survival of temporary periods adverse to reproduction, but it offers no protection against circumstances of direct threat, such as overgrazing and disease. Fungal infection of female cones severely reduces seed viability, reducing already inherently low recruitment. Other threats include injury from off-road vehicles, collection of wild plants and overgrazing by zebras, rhinos, and domestic animals.

Heraldry

The plant figures in the compartment of the national coat of arms of Namibia.

Gallery

File:Welwitschia-mirabilis-female.jpg, A female plant
File:Welwitschia mirabilis (female).jpg, A female ''Welwitschia'' beginning to shed seeds
File:N-welwitschia-2.jpg, Detail photograph of ripe female cones after seed dispersal
File:N-welwitschia-3.jpg, Detail photograph of male plant and cones
File:Namib, Welwitschia mirabilis.jpg, Person standing beside ''Welwitschia'' plant for scale
File:Welwitschia at Huntington Library, Art Collections and Botanical Gardens.jpg, Cultivated at Huntington Library and botanical garden
File:Welwitschia mirabilis0425.jpg, A ''Welwitschia'' in the petrified forest of Khorixas (Namibia)
File:Probergrothius angolensis adult and nymphs 1.jpg, Immature nymphs and adult ''Welwitschia'' bugs ('' Probergrothius angolensis'')
File:Welwitschia mirabilis MHNT.BOT.2015.2.2.jpg, ''Welwitschia mirabilis'' - Toulouse Museum of Natural History
File:Coat of arms of Namibia.svg, Coat of Arms of Namibia, with a ''Welwitschia'' in the bottom

See also

* List of Southern African indigenous trees and woody lianes

References

External links

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* Gymnosperm database
''Welwitschia''


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Bihrmann's Welwitschia growing project

"So What If It's Ugly? It Just Keeps On Going ..."
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{{Taxonbar, from1=Q156926, from2=Q15044266

Welwitschiaceae
Endangered plants
Endemic flora of Angola
Endemic flora of Namibia
Flora of Southern Africa
Monotypic gymnosperm genera
Dioecious plants