Botany

Welwitschia

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Welwitschia mirabilis, Namib Desert, Ugab River Valley. @ Caroline Hogan

The Welwitschia (Welwitschia 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; these persistent structures are as thick as 1.4 millimetres in the adult specimens. Some individual plants are estimated to have lived for over a millennium.

Distribution

caption Welwitschia mirabilis female cones. @ Barry Rice/CalPhotos/EOL The Welwitschia plant is endemic to the Namib Desert, with a range spanning northwestern Namibia and southwestern Angola. This taxon is limited in distribution to the desert biome along the southwest coast of Africa. This land strip is about 1000 kilometrers long and stretches from the Kuiseb River, immediately south of Walvis Bay in Namibia (latitude 20 to 24°S), to the Nicolau River in Angola (latitude 15 to 16°S).While some outlier populations are found 200 kilometers inland from the Atlantic Ocean, the majority of occurrences are restricted to within 80 kilometers of the coastal zone influenced by the cold Benguela Current. Extreme outliers have been found as far east as western Botswana.

Morphology

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. The 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 sunlight 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.

Habitat and ecology

caption Aerial view of habitat for welwitschia along the Namibia Skeleton Coast. @ C.Michael Hogan

The main occurrence of this species within proximity of the coastal zone suggests fog is an important source of water for this taxon. The Namib Desert habitat is extremely arid with annual rainfall  varying between one to five centimeters in most years, with some years having no measurable rainfall. This plant grows upon gravelly or rocky soil, often without other plant taxa nearby. The species elongated taproot capitalizes on any underground water. However, the most important water gathering trick is the ability to condense fog on the large leaves, which effectively have grooves to channel the condensate toward the thick taproot. Fog drifting from the Atlantic contributes approximately one to five centimeters of effective precipitation per annum. W. mirabilis can survive extreme temperature fluctuations between 6°Celsius (C) at night up to 50°C during the day.

Male and the female cones both produce nectar, thus attracting insects.The female cone manifests exposed stigmas that extrude nectar, whereas the male cone exhibits a modified stigma-like structure to produce the sugar rich nectar. Beetles may contribute to pollination for welwitschia, but, due to characteristic long distances of separation between individual plants, wasps and hornets may be more effective pollinators. Female cones attain maturity nine months after fertilization, typically during the months September to October. Seed germination depends on precipitation, which generally must sustain for several days to promote germination of seed. The arid conditions and other impediments such as fungus infection and small animal seed granivory promote seed attrition.

Conservation

Welwitschia mirabilis is locally common within parts of its range, but overall populations are not large. The species is protected within its Namibian range, but its best protection rests with the inhospitable and harsh desert environment. In parts of its range where precipitation is highest, some farming of marginal lands threatens the species, although human population densities are not high throughout the range. The Angolan populations are difficult to assess since the Cuban funded armed Angolan rebels have controlled most of the Angolan range during the last four decades. My own local guides were reluctant to travel into that portion of Angola, or to lead scientists there, due to the danger involved.

References

  • Chris Bornman. 1978. Welwitschia. Cape Town: Struik. ISBN 0869770977.
  • Encyclopedia of Earth. Species curator: C.Michael Hogan. 2011. Welwitschia mirabilis
  • Arthur C.Gibson. 1996. Structure-function relations of warm desert plants. Springer. 215 pages
  • Winter K, Schramm MJ (1986). Analysis of Stomatal and Nonstomatal Components in the Environmental Control of CO2 Exchange in Leaves of Welwitschia mirabilis. Plant Physiology 82 (1): 173–8
  • W.Wetschnig and B.Depisch. 1999. Pollination biology of Welwitschia mirabilis Hook. f. (Welwitschiaceae, Gnetopsida). Phyton-Annales Rei Botanicae 39: 167.
  • D.J.Willert, B.M.Eller, E.Brinckmann and R.Baasch. 1982. CO2, gas exchange and transpiration of Welwitschia mirabilis. Naturwissenschaften 67: 21-28
Glossary

Citation

Hogan, C. (2011). Welwitschia. Retrieved from http://www.eoearth.org/view/article/170198

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