Ikilehti

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).

Welwetschia mirabilis is a gymnosperm relict plant endemic to the Namib Desert. The species aerial architecture consists of a pair of very wide curled and contorted leaves atop an extremely abbreviated thick stem; these persistent structures are as thick as 1.4 millimetres in the adult specimens. Each of the leaves emerges from the base via an intercalary meristem; moreover, each leaf typically has tattered tips that exhibit extensive basipetal splitting. When a leaf is injured, that area produces a wound periderm.

Welwitschia mirabilis is restricted in occurrence to a portion of the Namib Desert in western Namibia and southwestern Angola.

The Welwitschia leaf epidermal cells manifest thick, cutinized outer periclinal walls with a primary cuticle of up to three micrometres in thickness; moreover, these walls contain crystalline sand as calcium oxalate. As with other warm desert plant taxa that exhibit Crassulacean Acid Metabolism (CAM), Welwitschia leaves contains high concentrations of organic acids. In controlled experiments, direct daytime carbon dioxide uptake is exhibited, supporting the fact that Welwitschia is a C3 carbon fixation species, underscoring the species primitive origin from the late Palaeozoic era. The anticlinal as well as inner periclinal cell walls are thickened, suggesting a desert adaptation that minimizes cuticular transpiration. The stomata are dense, exhibiting concentrations of around 150 stomates per square millimeter; moreover, the stomata are amphistomatic and sunken to about 30 micrometres. Furthermore, stomates lack opposite sclerenchymatous girders, and manifest longitudinal pores, similar to the stomatal architecture of desert palm taxa. Upper leaf surfaces have high solar reflectivity, which property inhibits overheating of these large area structures. These broad leaves, of course, cast a wide cool shadow, which creates a micro-habitat microclimate cooling for the plant and its arthropod associates. Relative to the full gamut of desert succulents, the Welwitschia leaves store relatively modest amounts of water (approximately 45 to 65 percent). The leaves are isolateral and exhibit three or four palisade layers (of about 220 micrometres) of isodiametric parenchyma on each side of the central mesophyll. These palisade chlorenchyma are in longitudinal strips, since the tissue is divided by parallel clusters of unlignified hypodermal fibres. The mesophyll is the repository of the calcium oxalate (between primary and secondary cell walls). Prominent longitudinal veins accompanied by smaller oblique anastomosing bundles characterize the leaf vasculature. The main veins contain primary fibres associated with the xylem and phloem: moreover, the older vascular bundles are enveloped with sclerified central mesophyllic parenchyma.