Overview

Brief Summary

Introduction

Welwitschia mirabilis is the only member of the Welwitschiaceae family.This succulent plant can live up to 1,500 years and is only found in a strip of land in the Namibia Desert comprising part of Namibia and Angola. It can survive on moisture from fog and dew.It consists of 2 leaves that grow continuously, a stem base and roots. It is thought to be a relic from the Jurassic Period and it has changed very little since then.
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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)

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Taxonomy

Welwitschia mirabilis is a perennial plant that grows up to 2 metres high. It has a taproot that is elongate, usually branching near the apex.The woody, unbranched stem often splits into 2 lobes with an obconical shape - a conical shape but with a downwards-pointing apex. The apex growth of the stem stops at an early stage giving this peculiar and unique habit.It has just 2 opposite, entire, simple leaves with continuous basal growth. The leaves are up to 9m long and 2m wide, tearing lengthways into strips and wearing away at the tips. The texture is leathery and the pair of leaves is parallel-veined.It is a dioecious plant, so male and female cones are produced on separate plants:
  • salmon-coloured male strobili are oblong, cone-like structures that are 1–4cm long, 5–7mm wide, and on a pedicel 2–5 mm long, with bracteoles1–3mm long
  • female strobili are larger than male strobili - 3.5–8cm long, 2–3cm wide, with outer bracteoles 5mm long and inner bracteoles 8mm long
This plant's seeds are 3.5cm long and 3cm wide, white to creamish white with persistent papery winged bracteoles.
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Comprehensive Description

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.

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Collector

Friedrich Martin Josef Welwitsch was born in Austria in 1806. He studied medicine at Vienna University, but moved to Portugal in 1839, where he subsequently obtained a commission as a naturalist collector.After several years of exploring Portugal’s territory, and making extensive collections of the Portuguese flora and fauna, in 1852 he was appointed by the Portuguese King to explore the territory now known as Angola. And in 1853, he set sail towards St Paulo de Loanda.During his 7 years in Angola, he collected over 5,000 species of plants and 3,000 species of insects and animals, a large proportion of which were new to science.However, it was this remarkable plant that amazed Welwitsch who, at its discovery, knelt down and looked at it for a long time trying to figure out what he had in front of him.He named it as Tumboa, the Angolan name, but Sir Joseph Hooker, Director of Kew Gardens, later renamed it as Welwitschia in honour of the collector.
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Biology

Welwitschia mirabilis flowers from mid-summer to autumn.Both the male and the female cones produce nectar which is 50% sugar to attract insects.The female cone has exposed stigmas that exude the nectar and the male cone possesses a modified stigma-like structure to produce it.Beetles are commonly thought to pollinate this plant, however considering the distance between individual plants it has also been suggested that hornets may be involved in its pollination.Female cones reach maturity nine months after fertilisation, during the spring months.Seed germination depends directly on rain - rain must fall for several days and in great quantity for the seeds to germinate.The arid conditions and other factors such as fungal infection and small animals eating the seeds can lead to seed loss.Once the plant becomes established, it can live up to 1,500 years.
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Distribution

Endemic Range

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

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Physical Description

Diagnostic Description

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).

  • Bornman, C., J. Elsworthy, V. Butler, C. Botha. 1972. Welwitschia mirabilis: Observations on general habit, seed, seedling, and leaf characteristics. Madoqua Series II 3: 53-66.
  • Butler, V., C. Bornman, R. Ever. 1973. Welwitschia mirabilis: Vascularization of a Four-week-old Seedling. Botanical Gazette 134: 59-63.
  • Evert R., S. Eichhorn. 2013. Raven Biology of Plants Eighth Edition. W. H. Freeman & Company. New York, NY. 455 pp.
  • Henshel J., M. Seely. 2000. Long-Term Growth Patterns of Welwitschia mirabilis, a Long-Lived Plant of the Namib Desert (Including a Bibliography). Plant Ecology 150: 7-26.
  • Leuenberger, B. 2001. Welwitschia mirabilis, male cone characters and a new subspecies. Willdenowia 31: 357-381.
  • Sykes, M. 1910. The Anatomy and Morphology of the Leaves and Inflorescences of Welwitschia mirabilis. Proceedings of the Royal Society of London 82: 625-626.
  • Valverde A., P. Maayer, T. Oberholster, J. Henschel, M. Louw, D. Cowan. 2016. Specific microbial communities associate with the rhizosphere of Welwitschia mirabilis, a living fossil. PLoS One. 11: 1-11.
  • Whitaker C., N. Pammenter, P. Berjak. 2007. Infection of the cones and seeds of the Welwitschia mirabilis by Aspergillus niger var. phoenicis in the Namib-Naukluft Park. School of South African Journal of Botany 74: 41-50.
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Ecology

General Ecology

Distribution ecology

Welwitschia mirabilis is limited in distribution to the desert or desert margin along the south-west coast of Africa. This land strip is about 1000 kilometres long and stretches from the Kuiseb River, just south of Walvis Bay in Namibia (latitude 20–24°S), to the Nicolau River in Angola (latitude 15–16°S).While some dispersed populations are found 200 kilometres inland, the majority of specimens are situated within 80 kilometres of the coast.

Ecology
Based on this plant’s distribution, it is thought that fog is an important source of water.Its habitat is extremely arid - annual rainfall is 10–100mm during the summer, but some years there is no rain at all.This plant grows on gravel or rocky ground and has a long taproot to take underground water. The fog contributes with about 50mm dew a year.This plant can stand extreme temperature fluctuations between 7°C at night up to 50°C during the day.
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Evolution and Systematics

Functional Adaptations

Functional adaptation

Leaves channel dew as water source: Welwitschia
 

The long leaves of desert Welwitschia capture water by collecting dew and channeling it into the ground where a large tap root can absorb it.

     
  "Further inland, one of the oddest of all plants manages to survive largely on dew. Welwitschia is related to the conifers and the cycads and consists of just two long strap-like leaves that sprout from a central swollen trunk only a few inches high. The leaves grow continuously from their base and become very long indeed. They would doubtless be even longer were it not for the desert winds which, blowing them back and forth, frays the ends into tatters. Even as it is, these leaves may be twenty yards long and lie curled in untidy heaps around the stunted trunk. They collect droplets of dew and channel them down runnels into the ground where the water is absorbed and stored in an immense conical tap root." (Attenborough 1995:267)
  Learn more about this functional adaptation.
  • Attenborough, D. 1995. The Private Life of Plants: A Natural History of Plant Behavior. London: BBC Books. 320 p.
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Physiology and Cell Biology

Physiology

Leaf cellular function and atmospheric gas exchange

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.


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Conservation

Conservation Status

Conservation

Welwitschia mirabilis is endemic to Namibia and southern Angola.This plant is relatively common within its habitat and is well protected in its native range.It is listed in the Convention on International Trade in Endangered Species (CITES), but is unlikely to be traded from the wild, apart from for its seeds.
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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.

  • Bornman, C., J. Elsworthy, V. Butler, C. Botha. 1972. Welwitschia mirabilis: Observations on general habit, seed, seedling, and leaf characteristics. Madoqua Series II 3: 53-66.
  • Butler, V., C. Bornman, R. Ever. 1973. Welwitschia mirabilis: Vascularization of a Four-week-old Seedling. Botanical Gazette 134: 59-63.
  • Evert R., S. Eichhorn. 2013. Raven Biology of Plants Eighth Edition. W. H. Freeman & Company. New York, NY. 455 pp.
  • Henshel J., M. Seely. 2000. Long-Term Growth Patterns of Welwitschia mirabilis, a Long-Lived Plant of the Namib Desert (Including a Bibliography). Plant Ecology 150: 7-26.
  • Leuenberger, B. 2001. Welwitschia mirabilis, male cone characters and a new subspecies. Willdenowia 31: 357-381.
  • Sykes, M. 1910. The Anatomy and Morphology of the Leaves and Inflorescences of Welwitschia mirabilis. Proceedings of the Royal Society of London 82: 625-626.
  • Valverde A., P. Maayer, T. Oberholster, J. Henschel, M. Louw, D. Cowan. 2016. Specific microbial communities associate with the rhizosphere of Welwitschia mirabilis, a living fossil. PLoS One. 11: 1-11.
  • Whitaker C., N. Pammenter, P. Berjak. 2007. Infection of the cones and seeds of the Welwitschia mirabilis by Aspergillus niger var. phoenicis in the Namib-Naukluft Park. School of South African Journal of Botany 74: 41-50.
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Wikipedia

Welwitschia

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Welwitschia is a monotypic genus of gymnosperm plant, composed solely of the very distinct Welwitschia mirabilis. It is the only genus of the family Welwitschiaceae, in the order Welwitschiales, in the division Gnetophyta. The plant, which is considered a living fossil,[1][2] is named after the Austrian botanist Friedrich Welwitsch who discovered it in 1859. The geographic distribution of Welwitschia mirabilis is limited to the Namib desert within Namibia and Angola.

Contents

Biology

Welwitschia grows from a short, thick, woody trunk, with only two leaves that continuously grow from their base, and a long, thick taproot. After germination, the cotyledons grow to 25–35 mm in length, and are followed shortly afterward by the appearance of two permanent leaves. These leaves are produced opposite of the cotyledons, and continue to grow throughout the entire life of the plant. They eventually grow to a length of 2–4 m and usually become split into several strap-shaped sections, thus sometimes disguising the origin from only two leaves. After these appear, two cotyledonary buds appear; in these, the growing tip dies, causing elongation of the buds. Growth continues sideways, which forms the obconical growth of the stem. The species is dioecious, with separate male and female plants. Fertilization, that is, the transfer of the pollen from the male to the female strobili, is carried out by insects that are attracted by "nectar" produced on both male and female strobili.[3]

The age of the plants is difficult to assess, but they are very long-lived, living 1000 years or more. Some individuals may be more than 2000 years old.[citation needed]

The plant absorbs water through structures on its leaves, harvesting moisture originating from dew that forms during the night.

It is possible that W. mirabilis uses crassulacean acid metabolism (CAM) photosynthesis; if this were true, W. mirabilis would be the only known gymnosperm to do so. However, the evidence is contradictory.[4]

Although considered endangered due to its very slow growth and despite the fact that older plants are often sought by collectors, a fair number of plants exist in the wild. The plants living in Angola are better protected than the plants in Namibia, owing to the relatively high concentration of land mines in Angola, which keep collectors away[citation needed].

The species grows readily from seed, which may be purchased from specialty seed dealers. The seed must be kept moist for the first couple of weeks and exposed to as much heat and light as possible during this time. Seeds collected from the wild are often heavily contaminated with spores of the mold Aspergillus niger, which causes them to rot shortly after they germinate. Seeds from botanical gardens, or other cultivated sources are much cleaner and less likely to rot.[citation needed]

Scientific classification

The taxonomic placement of Welwitschia has changed many times with the development of new classification systems (see Flowering plants: History of classification). Most botanists have treated Welwitschia as a distinctly monotypic genus in a monotypic family or even order. The Angiosperm Phylogeny Group, a leading authority in plant systematics, places Welwitschia mirabilis in its own family in the gymnosperm order Gnetales.[5]

Heraldry

The plant figures as a charge in the national coat of arms of Namibia, as well as that of Westelike Rugby Subunie.

Gallery

See also

References

  1. ^ Flowering Plants of Africa 57:2-8(2001)
  2. ^ A. Lewington & E. Parker (1999). Ancient Trees: Trees that Live for a Thousand Years. Collins & Brown Ltd.. ISBN 1-85585-704-9. 
  3. ^ Wetschnig W, Depisch B (1999). "Pollination biology of Welwitschia mirabilis HOOK. f. (Welwitschiaceae, Gnetopsida)". Phyton-Annales Rei Botanicae 39: 167. 
  4. ^ 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. doi:10.1104/pp.82.1.173. PMID 16664987. 
  5. ^ Stevens, P. F. (2001 onwards). Angiosperm Phylogeny Website. Version 9, June 2008 http://www.mobot.org/MOBOT/research/APWeb/
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