Welwitschia

''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

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