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Welwitschia

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Welwitschia
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Gymnospermae
Division: Gnetophyta
Class: Gnetopsida
Order: Welwitschiales
Family: Welwitschiaceae
Genus: Welwitschia
Hook.f.
Species:
W. mirabilis
Binomial name
Welwitschia mirabilis
Range of Welwitschia (striped)
Synonyms[1]
  • Tumboa bainesii Hook. f.
  • Welwitschia bainesii (Hook. f.) Carrière
  • Welwitschia mirabilis subsp. namibiana Leuenb.
  • Welwitschia namibiana (Leuenb.) Christenh. & Byng

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

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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."[2][1] 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.[3][4] 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.[5]

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.[6] The plant is commonly known simply as welwitschia in English, but the name tree tumbo is also used. It is called kharos or khurub in Nama, tweeblaarkanniedood ('two leaves; can't die') in Afrikaans, nyanka in Damara, and onyanga in Herero.[7]

Description

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After germination, the seedling produces two cotyledons which grow to 25–35 mm (0.98–1.38 in) in length.[8] 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.[4][9] Shortly after the appearance of the permanent leaves, the apical meristem dies and meristematic activity is transferred to the periphery of the crown.[9] The two (rarely three) leaves grow continuously from the crown across its entire circumference, reaching lengths up to 4 m (13 ft). The crown is disc-shaped and widens with age, reaching up to a meter in diameter.[7] The largest specimens may be no more than 1.5 m (4.9 ft) tall above ground, but the circumference of the leaves in contact with the sand may exceed 8 m (26 ft).[10] The largest known individual is 2.77 metres (9.1 ft) in diameter and 8.7 m (29 ft) in circumference.[9]

As the plant ages, the leaves often split into ribbons and become frayed from years of weathering.[4][7] The age of individual plants is difficult to assess, with radiocarbon dating being the most common method in determining plant age.[7] The plant is exceptionally long-lived, with many being hundreds of years old,[11] and the oldest being potentially up to 2,000.[9][12]

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.[13] Cones can range from green to salmon to various shades of brown in color.[7][14] 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.[13]

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.[15]

  • Crown
    Crown
  • Female cones
    Female cones
  • Male cones
    Male cones
  • The Welwitschia bug, Probergrothius angolensis
    The Welwitschia bug, Probergrothius angolensis

Genetics

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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.[12][16]

Distribution and habitat

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Welwitschia is endemic to the Namib desert. Its range stretches over 1,000 km (620 mi) along the Angolan and Namibian coast, being found between 14th and 24th southern parallels.[17] The area is arid; the coast is recorded as having almost zero rainfall, while less than 100 mm (3.9 in) of rain falls annually below the escarpment in the wet season from February to April.[18] Populations tend to occur in ephemeral water sources, indicating a dependence on groundwater in addition to precipitation from fog.[11]

Cultivation

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Two undamaged plants in cultivation; the two leaves are clearly distinct
The Coat of arms of Namibia. Welwitschia is seen growing at the bottom of the image

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 80 °C (176 °F) and as low as −20 °C (−4 °F) without major side effects.[19]

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.[20] 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.[21]

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,[7] translates to 'onion of the desert'.[4][22] Welwitschia is also featured on the coat of arms of Namibia growing from a sand dune.[23]

Conservation

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Wild plant with significant leaf damage

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).[24] 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.[1]

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.[1]

See also

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References

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  1. ^ a b c d "Welwitschia mirabilis". Plants of the World Online. Royal Botanic Gardens, Kew. Retrieved 21 July 2023.
  2. ^ Trimen, Henry (1873). Friedrich Welwitsch. United Kingdom: Ranken and Company. p. 7.
  3. ^ Welwitsche, Frederick (1861). "Extract from a letter, addressed to Sir William J. Hooker, on the botany of Benguiela, Mossameded, &C, in Western Africa". Journal of the Proceedings of the Linnean Society. Botany. 5 (20): 182–186. doi:10.1111/j.1095-8312.1861.tb01048.x. Archived from the original on 2024-07-18. Retrieved 2018-01-14.
  4. ^ a b c d Notten, Alice (March 2003). "Welwitschia mirabilis". PlantZAfrica. South African National Biodiversity Institute. Retrieved 21 July 2023.
  5. ^ Hooker, Joseph Dalton (1863). "On Welwitschia, a new Genus of Gnetaceae". Transactions of the Linnean Society of London. 24 (1): 1–48. doi:10.1111/j.1096-3642.1863.tb00151.x. BHL page 27558549, Pl. I–XIV.{{cite journal}}: CS1 maint: postscript (link)
  6. ^ Stevens, P. F. "Angiosperm Phylogeny (2001 onwards) - Version 9, June 2008". www.mobot.org. Archived from the original on 2019-07-04. Retrieved 2010-04-22.
  7. ^ a b c d e f Earle, Christopher J., ed. (2018). "Welwitschia mirabilis". The Gymnosperm Database. Retrieved 21 July 2023.
  8. ^ Singh, V.P. (2006). Gymnosperm (naked seeds plant) : structure and development. Sarup & Sons. p. 576. ISBN 978-8176256711. Retrieved 24 January 2016.
  9. ^ a b c d Bornman, Chris H (1 January 1978). Welwitschia: Paradox of a parched paradise. Cape Town: C Struik. ISBN 9780869770979.
  10. ^ Bornman, Chris H; Elsworthy, Janet A; Butler, Valerie; Botha, C E J (1 January 1972). "Welwitschia mirabilis: observations on general habit, seed, seedling, and leaf characteristics". Madoqua Series II. 1 (1): 53–66. Retrieved 21 July 2023.
  11. ^ a b Henschel, Joh R; Seeley, Mary K (October 2000). "Long-term growth patterns of Welwitschia mirabilis, a long-lived plant of the Namib Desert (including a bibliography)". Plant Ecology. 150 (1). Berlin: Springer Science+Business Media: 7–26. Bibcode:2000PlEco.150....7H. doi:10.1023/A:1026512608982. eISSN 1573-5052. ISSN 1385-0237. JSTOR 20050975. OCLC 37915892. S2CID 7760078. Archived from the original on 2023-07-21. Retrieved 2023-07-21.
  12. ^ a b Wan, Tao; Liu, Zhiming; Leitch, Ilia J.; Xin, Haiping; Maggs-Kölling, Gillian; Gong, Yanbing; Li, Zhen; Marais, Eugene; Liao, Yiying; Dai, Can; Liu, Fan (2021-07-12). "The Welwitschia genome reveals a unique biology underpinning extreme longevity in deserts". Nature Communications. 12 (1): 4247. Bibcode:2021NatCo..12.4247W. doi:10.1038/s41467-021-24528-4. hdl:1854/LU-8715697. ISSN 2041-1723. PMC 8275611. PMID 34253727.
  13. ^ a b Wetschnig W, Depisch B (1999). "[Chrysomya albiceps Pollination biology of Welwitschia mirabilis HOOK. f. (Welwitschiaceae, Gnetopsida)]" (PDF). Phyton: Annales Rei Botanicae. 39: 167.
  14. ^ Leuenberger, Beat Ernst. "The two subspecies of Welwitschia mirabilis". Freie Universität Berlin. Archived from the original on 24 May 2025.
  15. ^ Martens, P. (4 September 1977). "Welwitschia mirabilis and Neoteny". American Journal of Botany. 64 (7): 916–920. doi:10.2307/2442386. JSTOR 2442386.
  16. ^ Sima, Richard (2021-07-31). "A Plant That 'Cannot Die' Reveals Its Genetic Secrets". The New York Times. ISSN 0362-4331. Archived from the original on 2021-08-03. Retrieved 2021-08-03.
  17. ^ Jürgens, Norbert; Oncken, Imke; Oldeland, Jens; Gunter, Felicitas; Rudolph, Barbara (27 January 2021). "Welwitschia: Phylogeography of a living fossil, diversified within a desert refuge". Scientific Reports. 11 (1) 2385 (2021): 2385. Bibcode:2021NatSR..11.2385J. doi:10.1038/s41598-021-81150-6. PMC 7840819. PMID 33504814.
  18. ^ van Wyk, Abraham E; Smith, Gideon F (30 September 2001). Regions of Floristic Endemism in Southern Africa: A Review with Emphasis on Succulents. Hatfield, South Africa: Umdaus Press. ISBN 9781919766188.
  19. ^ Whitaker, C; Berjak, P; Kolberg, H; Pammenter, N W; Bornman, C H (October 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 (4): 622–630. Bibcode:2004SAJB...70..622W. doi:10.1016/S0254-6299(15)30201-5.
  20. ^ Cooper-Driver, Gillian A; Wagner, Christopher; Kolberg, Herta (October 2000). "Patterns of Aspergillus niger var. phoenicis (Corda) Al-Musallam infection in Namibian populations of Welwitschia mirabilis Hook.f.". Journal of Arid Environments. 46 (2): 181–198. Bibcode:2000JArEn..46..181C. doi:10.1006/jare.2000.0660. Archived from the original on 21 July 2023. Retrieved 21 July 2023.
  21. ^ Whitaker, C; Pammenter, N W; Berjak, P (January 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 (1): 41–50. Bibcode:2008SAJB...74...41W. doi:10.1016/j.sajb.2007.08.008.
  22. ^ "Rare and Endangered Plant Spotlights". United States Botanic Garden. Archived from the original on 21 July 2023. Retrieved 21 July 2023. The plant's common name is onyanga, which means "onion of the desert."
  23. ^ National Coat of Arms of the Republic of Namibia Act, 1990 (PDF) (1). 28 March 1990. Archived from the original (PDF) on 30 October 2013. Retrieved 27 May 2013.
  24. ^ "Appendices". Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). Archived from the original on 11 February 2025. Retrieved 14 October 2022.
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