Welwitschia

Welwitschia is a monotypic gymnosperm genus, comprising solely the distinctive Welwitschia mirabilis, endemic to the Namib desert within Namibia and Angola. 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 in Afrikaans, nyanka in Damara, and onyanga in Herero. Welwitschia is the only living genus of the family Welwitschiaceae and order Welwitschiales, in the division Gnetophyta. Informal sources commonly refer to the plant as a 'living fossil'. Welwitschia is named after the Austrian botanist and doctor Friedrich Welwitsch, who was the first European to describe the plant, in 1859 in present-day Angola. 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. After germination, the seedling produces two cotyledons which grow to 25–35 mm (0.98–1.38 in) 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 foliage leaves grow continuously from a basal meristem 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. 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). 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' 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 commonest 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.