Welwitschia is one of three genera in the gymnosperm group of seed plants known as Gnetales, whose relationship to other plants has long been a subject of controversy among botanists and evolutionary biologists. Welwitschia includes just a single species, Welwitschia mirabilis, which is found in the Namib Desert of southwestern Africa (Namibia and Angola). The species is dioecious (i.e., individual plants are either male or female) and each adult plant consists of a giant taproot, a very short woody stem, and two permanent strap-shaped' leaves.
(McCoy et al. 2008)
Currently there are no conservation efforts in place for Welwitschia mirabilis despite its rarity, longevity, and uniqueness as the sole species in the genus Welwitschia (Evert et al. 2013). Welwitchia mirabilis plays a vital role in the ecosystem of the Namib Desert as a shelter and refuge for small animals and insects and as a food source for many desert animals like the rhinoceros and zebra (Henshel et al. 2000). One threat to W. mirabilis, according to Whitaker et al. (2007), is that its seeds are highly vulnerable to infection from the fungus Aspergillus niger var. phoenicis. Despite the high numbers of seeds produced by female W. mirabilis plants, approximately 80% of fertile seeds may be infected (Whitaker et al. 2007). Given the dioecious nature of this plant and the slow growth rate, any threat to the seeds' chances of germination can be a serious obstacle to the proliferation of this plant in the wild (Evert et al. 2013). According to Whitaker et al. (2007), the threat of fungal infection is just as great in planting programs as in the wild. Their experiment with fungicidal methods showed that soaking W. mirabilis seeds in Tebuconazole eliminated a substantial proportion of the infection and restored the seeds to viability (Whitaker et al. 2007). While this may mean successful propagation of the species in the lab or greenhouse, it is likely not applicable to W. mirabilis growing in the wild. Furthermore, the success of planting programs is no guarantee of the continuation of this rare species. The morphology of the plant precludes transplantation and reintroduction. The deep taproot systems of young plants are highly sensitive to disturbance (Henshel et al. 2000). Therefore, it is unlikely that W. mirabilis specimens would survive reintroduction efforts.Further investigation is needed to assess the long-term repercussions of using Tebuconazole to treat for A. niger var. phoenicis. It is theorized that some xerophytes can credit their success to associations with growth-promoting microbes (Valverde et al. 2016). The diverse bacterial and fungal communities of W. mirabilis might be negatively impacted by the use of the strong fungicide Tebuconazole. The rhizosphere, or soil region immediately surrounding the roots of W. mirabilis has been found to be dominated by Ascomycetes (Valverde et al. 2016). Also found were bacterial and fungal species that provide several key benefits such as fixation of atmospheric nitrogen, solubilization of soil-insoluble phosphate, production of antibiotic compounds, protection against stress, and supplies of phosphorous and other nutrients in exchange for plant carbon (Valverde et al. 2016). Given the lack of protective or conservation efforts, despite the threat posed by Aspergillus niger var. phoenicis to seed viability, it would appear there is no need to risk the use of the strong fungicide on wild populations of W. mirabilis.
Welwitschia mirabilis is a desert perennial which is the only extant species of the genus Welwitschia.It has become famous for its longevity with some plants estimated to be 2000 to 3000 years old (Henschel & Seely 2000).Following epigeous germination, the seedling produces two cotyledons growing up to 25-35 mm each (Butler et al. 1973). Two foliage leaves are produced from a basal meristem and have been observed reaching four meters in length.The leaves of W. mirabilis are broad and flat and possess stomata on both the top and bottom surfaces which aids in water retention and potentially water absorption.In adults, these leaves display bilateral symmetry.These leaves characteristically possess massive sclereids which may assist in facilitating gas exchange (Bornman et al. 1972).Specific members of the community possess either microstrobili or megastrobili and are therefore unisexual.The megastrobilus on average contains 90 to 100 megasporophylls arranged in a decussate, or intersecting, fashion (Bornman et al. 1972).Reproduction is seasonal with male flowers developing in the early fall and pollination occurring during the months of November through January (Henschel & Seely 2000).Ten to twenty thousand winged seeds may be produced by one female plant.Inflorescences originate from the meristematic tissue of the stem (Bornman et al. 1972).Each plant may have up to 100 inflorescences and thousands of male cones (Leuenberger 2001).The male cones come in two varieties which are purplish brown or green in color respectively.The structure of peduncles, cone axis, bracts, and flowers are similar in both male and female inflorescences (Sykes 1910).The stem of W. mirabilis is exceedingly fibrous and possesses a thick corrugated periderm (Bornman et al. 1972).Members of this species have been observed growing 1.5 meters tall and they possessed a circumference approaching 9 meters. The root system of W. mirabilis does not appear to be extensive.There is early and rapid growth seen in the taproot which is not sustained (Butler et al. 1973).After a period of time the taproot begins to branch.Cells within the roots consist of many starch storing plastids. Root systems in some populations benefit from mycorrhizal associations, yet W. mirabilis does thrive where the symbiotic fungi are not present (Henschel & Seely 2000).
Welwitschia mirabilis is restricted in occurrence to a portion of the Namib Desert in western Namibia and southwestern Angola.
Welwitschia is a monotypic genus (that is, a genus that contains a single recognised species) of gymnosperm, the sole described species being 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".[3][4]
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."[5][6] 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.[7][8]
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.[9] 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.[10]
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.
After germination, the seedling produces two cotyledons which grow to 25–35 mm (0.98–1.38 in) in length, and have reticulate venation.[11] 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.[12]
The two (rarely three) foliage leaves are parallel veined, and grow continuously from a basal meristem around the circumference of the trunk, reaching lengths up to 4 m (13 ft). 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.[12][13] The largest specimens (such as the "Husab Giant" which is five meters in circumference (about five feet diameter))[14] 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).[15]
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"[15] and a woody fibrous unbranched main stem.[12] The roots extend to a depth roughly equal to the span of the living leaves from tip to tip.[12] The main stem consists of an unbranched woody crown roughly shaped like an inverted cone.[16] 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.[17]
Welwitschia has been classified as a CAM plant (crassulacean acid metabolism) after reconciliation of some initially contradictory and confusing data.[18][19] 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.[12] As the species does not produce yearly rings, plant age is determined by radiocarbon dating.[20] However, other reports suggest that the plant does produce a kind of yearly ring.[21] The "trunk" continues to expand with age. The largest known is 9 ft 1 in (2.77 m) in diameter (8.7 m (29 ft) in circumference).[22]
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.[23]
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.[24][25]
Welwitschia mirabilis is endemic to the Kaokoveld Desert,[26] which lies within the Namib Desert.[27] The population is distributed southwards from the Bentiaba River in southern Angola, to the Kuiseb River in Namibia,[28] and up to 100 km (62 mi) inland of the coast.[12] The area is extremely 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.[26] Populations tend to occur in ephemeral watercourses, indicating a dependence on groundwater in addition to precipitation from fog.[29]
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.[30] Removal of the outer seed coverings enhances germination performance, which suggests that the seeds may display non-deep physiological dormancy.[30] 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.[30]
Seeds collected from the wild often are heavily contaminated with spores of the fungus Aspergillus niger var. phoenicis,[31] 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.[32] 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.[32]
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'.[33][34]
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).[1] Plants in Angola are better protected than those in Namibia, because the relatively high concentration of land mines in Angola keep collectors away.[6]
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.[35]
The plant figures in the compartment of the national coat of arms of Namibia.
A female plant
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A female Welwitschia beginning to shed seeds
Detail photograph of ripe female cones after seed dispersal
Detail photograph of male plant and cones
Person standing beside Welwitschia plant for scale
Cultivated at Huntington Library and botanical garden
A Welwitschia in the petrified forest of Khorixas (Namibia)
Immature nymphs and adult Welwitschia bugs (Probergrothius angolensis)
Welwitschia mirabilis - Toulouse Museum of Natural History
Coat of Arms of Namibia, with a Welwitschia in the bottom
{{cite journal}}: CS1 maint: postscript (link) Welwitschia is a monotypic genus (that is, a genus that contains a single recognised species) of gymnosperm, the sole described species being 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".
