Tumboa

''Welwitschia'' is a monotypic genus of gnetophytes containing only the species ''Welwitschia mirabilis''. It is named after the Austrian botanist Friedrich Welwitsch, who documented the plant in the 1850s. In common use, it is sometimes referred to as the tree tumbo. It is native to Angola and Namibia, where it grows in the extreme conditions of the Namib desert, tolerating high heat and low precipitation. ''Welwitschia'' is the only living genus of the family Welwitschiaceae and order Welwitschiales, and is one of three extant genera of gnetophytes, alongside ''Gnetum'' and '' Ephedra''.

''Welwitschia'' is well known for its unique morphology. The plant only has two leaves that grow out of a large woody stem, dubbed a crown, and continue to grow over the plant's entire life. ''Welwitschia'' is one of the longest-living plants on Earth, with some individuals being thousands of years old. As such, the leaves can become quite large, often reaching several meters in length.

Like other gymnosperms, ''Welwitschia'' uses cones to reproduce. Both male and female plants produce nectar from their cones that attract insects who then carry the pollen off to other plants. The species is not currently in any significant danger of becoming extinct, in part because its long lifespan insulates it from temporary reproductive struggles. Despite this, ''Welwitschia'' is susceptible to future overgrazing and disease, because it grows only in one environment.

Taxonomy

''Welwitschia'' is named after Austrian botanist and doctor Friedrich Welwitsch, who documented the plant in Angola in 1859. Welwitsch was fascinated by the plant, writing, "I 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 formally described the species using Welwitsch's description and collected material along with material from artist Thomas Baines who had independently recorded the plant in Namibia. 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 study.

The taxonomy of ''Welwitschia'' subsequently changed intermittently with the development of new classification systems. 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 the family Welwitschiaceae, which also includes several extinct species. 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.

Description

After germination, the seedling produces two cotyledons which grow to in length. They start off as pink, but turn green shortly after germination. Subsequently, two permanent leaves emerge from the crown (large, woody stem) and are produced opposite (at right angles) to the cotyledons. The permanent leaves grow rapidly and last for the plant's entire life. They are long and ribbon-shaped, with their veins running down their length parallel to each other. Shortly after the appearance of the permanent leaves, the apical meristem dies and meristematic activity is transferred to the periphery of the crown. The two (rarely three) leaves grow continuously from the crown across its entire circumference, reaching lengths up to . The crown is disc-shaped and widens with age, reaching up to a meter in diameter. The largest specimens may be no more than tall above ground, but the circumference of the leaves in contact with the sand may exceed . The largest known individual is in diameter and in circumference.

As the plant ages, the leaves often split into ribbons and become frayed from years of weathering. The age of individual plants is difficult to assess, with radiocarbon dating being the most common method in determining plant age. The plant is exceptionally long-lived, with many being hundreds of years old, and the oldest being potentially up to 2,000.

''Welwitschia'' is dioecious, with separate male and female plants. Both sexes produce cones that grow out of the crown of the plant, which often number in the hundreds. Cones can range from green to salmon to various shades of brown in color. The cones produce nectar that attracts various insects, most commonly flies, that then carry the oval-shaped pollen on them. The Welwitschia bug, ''Probergrothius angolensis'', is commonly observed on the plant, but likely does not have a role in pollination; they are not particularly attracted to the nectar and are usually found on the leaves of the plant. Infrequently, wasps and bees also play a role as pollinators of ''Welwitschia.''

Because ''Welwitschia'' only produces a single pair of 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 a "giant seedling". 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.

File:Welwitschia mirabilis04.jpg, Crown
File:Welwitschia-seeds.jpg, Female cones
File:Welwitschia 469-04.jpg, Male cones
File:Probergrothius angolensis closeup 1.jpg, The Welwitschia bug, ''Probergrothius angolensis''

Genetics

In July 2021, the genome of ''Welwitschia'' was 98% sequenced, totalling 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 the metabolic cost of such a large genetic material to be lowered.

Distribution and habitat

''Welwitschia'' is endemic to the Namib desert. Its range stretches over along the Angolan and Namibian coast, being found between 14th and 24th southern parallels. The area is 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 water sources, indicating a dependence on groundwater in addition to precipitation from fog.

Cultivation

''Welwitschia'' grows from a seed, which may be bought from specialty seed dealers. The seeds have been shown to display orthodox seed behavior, meaning they may be stored for long periods of time at extreme humidities and temperatures. ''Welwitschia'' seeds are able to survive temperatures as high as and as low as without major side effects.

Seeds collected from the wild often are heavily contaminated with spores of the fungus ''Aspergillus niger'' which causes them to rot shortly after they germinate. The fungus infects the growing cones of ''Welwitschia'' early in their development with a sharp increase in infection occurring when the pollination drop (a sugary substance produced by the ovule) appears. Because of this, seeds in the wild may be rendered unviable before they fully develop. The fungicide tebuconazole may be useful in controlling limited ''A. niger'' seed infection.

Indigenous people sometimes eat the cone of the plant by eating it raw or baking it in hot ashes. The Herero name of the plant, ''onyanga'', translates to 'onion of the desert'. ''Welwitschia'' is also featured on the coat of arms of Namibia growing from a sand dune.

Conservation

The wild population of ''Welwitschia'' is reasonably stable. 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 generally better protected than those in Namibia because the relatively high concentration of land mines from the Angolan Civil War that remain in the region.

Although ''Welwitschia'' is not at present immediately threatened, with 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'' favors its survival of temporary periods adverse to reproduction, but it offers no protection against direct threats, 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.

See also

* List of Southern African indigenous trees and woody lianes

References

External links

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Ecology & Evolutionary Biology Conservatory
(archived)

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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
Flora of Angola
Flora of Namibia
Monotypic gymnosperm genera
Dioecious plants