Welwitschie

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Welwitschie
Welwitschie in Namibia

Welwitschie in Namibia

Systematics
Subdivision : Seed plants (Spermatophytina)
Class : Coniferopsida
Order : Gnetales
Family : Welwitschia plants
Genre : Welwitschia
Type : Welwitschie
Scientific name of the  family
Welwitschiaceae
Margr.
Scientific name of the  genus
Welwitschia
Hook. f.
Scientific name of the  species
Welwitschia mirabilis
Hook. f.

The Welwitschia ( listen ? / I ; Welwitschia mirabilis ) is the only species of the genus Welwitschia in the Welwitschia family (Welwitschiaceae). It belongs to the naked seed order of the Gnetales and grows endemically in the Namib desert in southern Africa. Due to its frequent occurrence, the Welwitschie is among other things in the coat of arms of Namibia , the coat of arms of the city of Swakopmund and the coat of arms of the Kunene regionAudio file / audio sample pictured. Although the plant can live for several hundred years, it only has a single pair of leaves.

Names and Botanical History

Illustration from Curtis's Botanical Magazine , 1863

The Austrian doctor and botanist Friedrich Welwitsch discovered this plant in 1859 near Cabo Negro in Angola (15-16 ° S). In a letter to Sir William Jackson Hooker , director of the Royal Botanic Gardens Kew , London, dated August 16, 1860, he first reported on this plant. In 1862 he sent Joseph Dalton Hooker , also in Kew, a specimen who scientifically described the plant in 1863 and named it after the discoverer. However, he had recommended the name Tumboa after the local name. Hooker said of the plant as follows: It is out of the question the most wonderful plant ever brought to this country, and one of the ugliest. ("This is without question the most wonderful plant that has ever been brought to this country, and one of the ugliest.")

In Angola this type of plant is called n'tumbo , which means something like "stump". The Nama call them ǃkharos or khurub , the Damara nyanka . The Herero call them onyanga , which means "desert onion". The pulp used to be eaten raw or baked in hot ashes. In Afrikaans it is called tweeblaarkanniedood , which means something like “two-leaf- can't -die”.

Description and ecology

Habitus in the habitat in Namibia (size comparison)
Young plant
Male Welwitschia mirabilis with cone-shaped inflorescences
inflorescence

Appearance

The perennial plant has a short, beet-shaped trunk that emerges from the hypocotyl , a deep taproot and two leaves that replace the cotyledons .

The trunk is lignified and above ground is usually around 50 centimeters high, maximum 1.50 meters. It reaches a diameter of up to one meter and has annual rings . Individual copies have a circumference of up to 8.7 meters. The top of the trunk is a concave disk because the terminal apex stops growing very early. The inflorescences arise near the base of the leaves . The secondary wood has trachea , actually a typical feature of angiosperms .

Young plants are very rarely found in the natural habitat. Seedlings can only establish themselves after - very rare - extreme precipitation, which means that the age structure is highly discontinuous. The topsoil must be completely moist so that the roots of the young plants can penetrate deeper. The two cotyledons can be photosynthetically active for up to 1.5 years and then die. The only pair of leaves develops before that.

Radiocarbon dating has shown the plants examined to be between 500 and 600 years old. By extrapolating these results, the largest specimens of the species are estimated to be up to 2000 years old.

Their roots spread underground over a radius of 15 meters. In addition, the plant has a taproot. It is likely, but not certain, that the roots will reach the groundwater horizon, since the roots get lost in hard, calcite- cemented gravel. The roots reach three meters deep.

leaf

The two leaves can be over 2.5 meters long, some reports speak of 6.2 meters. At the end of the leaf they die off and weather, but the oldest living parts can live to be 10 years old. Since the hypocotyl unfolds as it grows, the leaves often tear open, thus simulating several leaves. In the area around the Brandberg , however, individuals have been found that actually have two pairs of leaves. This occurs in around 5% of the population. The leaves grow on a basal meristem . The leaf growth averages 0.17 to 0.83 millimeters per day. The annual values ​​vary between 40 and 409 millimeters per year depending on the location. However, there is no significant correlation between leaf growth and rainfall . The water availability in the deeper soil layers is likely to be more important. The vascular bundles of the leaves can anastomose or end blindly in the mesophyll . This is unique among gymnosperms.

Generative characteristics

Floral diagram of a male flower.
S = rudimentary ovule

The Welwitschie is dioecious separate sexes ( diocesan ), d. H. there are female and male plants. The flowers are in cone-like inflorescences and sit in the axilla of cover scales.

The envelope of the male flowers consists of two cross-opposed pairs of bracts . The six stamens are in a whorl and are fused together at the base. Each stamen carries three pollen sacs that have grown together . The male flowers always contain a rudimentary ovule at the tip , which is surrounded by an equally rudimentary pair of bracts. This ovule produces nectar , which consists of around 50% sugar . The sterile female ovules and nectar production can be viewed as an evolutionarily failed attempt to form a bisexual flower. The pollen sacs open like the other representatives of the Gnetopsida with an exothecium in the form of often only short slits. In the Welwitschia, the pollen is pressed outwards when the pollen sacs dry out, which is caused by the arrangement of wall reinforcements in the exothecium.

The male gametophyte consists of the spermatogenic cell and two other cells. The spermatogenic cell divides into two sperm cells. The fertilization takes place via a pollen tube ( siphonogamy ).

Flowering diagram of a female flower.
S = ovule
Female Welwitschia mirabilis with cone-shaped inflorescences

The female flowers are surrounded by two pairs of bracts that have grown together. When the seeds ripen, the inner pair of bracts becomes hard, the outer one forms wings. Each flower contains an erect ovule . The integument is drawn out into a long micropyle , on which a drop of fertilization, which also functions as nectar, is excreted.

The female gametophyte arises from free nuclear divisions from all four nuclei arising from meiosis and subsequent cell wall formation. It can contain up to a thousand cells. No archegonia are formed, the egg cells cannot be distinguished from the other cells of the archegonium. The gametophyte grows towards the pollen tube by forming tube-like structures.

Pollination and seed formation

The pollination is done by insects, as candidates are bugs and wasps discussed. The bug Probergrothius angolensis feeds on nectar, but pollination has not yet been clearly proven. Flowering occurs from midsummer to autumn, the seeds ripen in spring and are released when the cones decay. The seeds are around 3.5 × 2.5 cm in size and have a paper-like wing. The wind spreads it. The seeds remain viable for a few years and only germinate after heavy rainfall. From around 10,000 to 20,000 flowers per plant and year, only around 20 to 200 germinable seeds emerge.

Occurrence

Distribution area of ​​the Welwitschia mirabilis

The species is native to the Namib desert ; its area covers parts of the states Namibia and Angola . The area begins in the north on the Nicolau- Rivier north of Namibe (Angola) and extends around 1200 km south to the Kuiseb- Rivier near Gobabeb (Namibia). However, it can never be found directly on the coast. The mean annual precipitation at their places of growth ranges from 10 mm near the coast to 250 mm in the Mopane savannah.

Welwitschia is the dominant plant in some areas of the central Namib. It reaches its highest density at a distance of 50 to 60 km from the coast. The center of the distribution is the "Welwitschia-Vlakte" (Welwitschia area) in the triangle between Khan and Swakop -Rivier. The total number of individuals in this area has been estimated at 5000 to 6000 plants. This area is part of the Namib-Naukluft National Park . The "Welwitschia-Vlakte" has been on the UNESCO tentative list as a world heritage site since 2002 .

Systematics and botanical history

The genus Welwitschia was created in 1863 with the first description of Welwitschia mirabilis by Joseph Dalton Hooker in On Welwitschia, a new genus of Gnetaceae. In: Transactions of the Linnean Society of London , Volume 24, pages 1-48. The genus Welwitschia is monotypical .

After investigations in 2001 by Leuenberger on cultivated specimens in the Botanical Garden Berlin-Dahlem , a second subspecies was described by Welwitschia mirabilis . Leuenberger was of the opinion that they would not only differ in morphology , but also in their area. The subspecies Welwitschia mirabilis subsp. According to Leuenberger, mirabilis is said to occur only in Angola. For the cultivated specimens, Leuenberger stated: The male cones are smooth, brownish and without a clear wax layer. The inflorescence axes are usually 5 to 11 centimeters long, the secondary axes up to 2 centimeters. The largest male cones are 30 to 45 millimeters long. The pairs of bracts overlap by around 2 millimeters. The bracts are fused to more than three quarters of the length, the edge of the bracts is smooth. - The subspecies Welwitschia mirabilis subsp. Namibiana Leuenberger is said to occur only in Namibia , according to Leuenberger . For the cultivated specimens, Leuenberger stated: The male cones are furrowed, greenish or salmon-colored and with a clear wax layer. The inflorescence axes are around 7 to 15 centimeters long, the secondary axes up to 7 centimeters. The largest male cones are 20 to 30 millimeters long. The pairs of bracts overlap by about 1 millimeter. The bracts are fused to one to two thirds of the length, the edge of the bracts is frayed.

All of this was made possible in 2014 by Jacobson et al. cannot be confirmed during field investigations. There are no subspecies or varieties.

physiology

Water absorption

Welwitschia mirabilis , close up of leaves and inflorescence

It is often said that the Welwitschia takes its need for water as dew from the leaves or from a system of fine roots close to the surface.

The Welwitschia, however, has no morphological structures to absorb water through the leaves. The leaves are typically xeromorphic : they have a thick cuticle , the stomata are sunk, the crevices are particularly cutinized (accretion) and thus water-repellent. In addition, the fog rarely reaches the main occurrences of Welwitschia and the amounts of dew are not sufficient for the measured plant evaporation. In addition, the plant already reaches its morning water potential during the night, long before the fog falls. All of this speaks for the soil as the sole source of water.

Experiments with radioactively marked water showed that the leaves can absorb water, but only very slowly and through passive diffusion, so that it does not make any significant contribution to the water supply.

However, the uptake of dew via fine roots close to the surface is likely to play a certain role in the plant's water supply. Estimates speak of an equivalent of 50 mm of annual precipitation.

ingredients

As is often typical for xerophytes , the Welwitschia leaves contain quite high concentrations of inorganic ions (in the vacuole ), with potassium and chloride ions dominating in the young leaf sections, while the sodium content increases significantly in older sections , as the phloem-mobile potassium is shifted to the young sections, the K / Na ratio changes from 5: 1 in the meristem to 1: 4 at the leaf tip. In the cytosol , proline in particular is formed for osmotic balance .

The organic acids, such as malic acid , citric acid , isocitric acid and quinic acid , have a total content of over 100 mmol / kg dry weight.

The seed oil of Welwitschia contains - like that of Gnetum - cyclopropenes . This means that these two genera have a special position compared to the other gymnosperms, including ephedra , which all contain Δ5 fatty acids . With its fatty acid spectrum , Welwitschia is close to the Malvales , which, however, belong to the angiosperms .

Gas switch

CAM metabolism

Welwitschie in Namibia, south of Swakopmund, fenced in for protection. Size: about the height of a man, estimated age: 1500 years.

The discussion as to whether the Welwitschia is a CAM plant has not finally ended after thirty years, although the signs of this are increasing.

The δ- 13 C values ​​(for an explanation of the numerical values ​​see isotope discrimination ) are in the “Welwitschia Vlakte” with –17.5 ‰ to –19.5 ‰ between the values ​​for C3 and C4 plants , which speaks for CAM would. In the savannah, however , Welwitschia reaches values ​​of –23.3 ‰ and is more like a C3 plant. A high activity of the PEP - carboxylase and laboratory gas exchange measurements speak for the CAM metabolism. In a more recent publication, Willert et al. (2005) show a nocturnal CO 2 uptake in the field as well, but this only corresponded to 4% of the CO 2 uptake over 24 hours. The highest rates were 0.2 µmol / (m 2 · s). This nocturnal CO 2 uptake occurred in December and January, i.e. in the summer of the southern hemisphere. These values ​​speak at least for what is known as CAM cycling, i.e. H. the re-fixation of the respiratory CO 2 . The leaves contain high concentrations of malate and citrate. Due to the extremely high variability in the leaf, Willert et al. however, show no diurnal change in acid concentration. However, that would have been the clearest indication of the CAM metabolism.

The xeromorphic structure and the low water content of the leaves, which are otherwise typical for CAM plants, speak against a CAM metabolism. As mentioned above, the lack of evidence of a diurnal acid change also speaks against it.

Photorespiration, photoinhibition

The Welwitschia also has high photorespiration in its natural location , which is close to 50% of the gas exchange activity. The high level of radiation at the site also leads to strong photo-inhibition , especially in the afternoon hours. This is particularly pronounced in the older leaf sections and often goes so far that the CO 2 gas change becomes negative as early as midday .

Herbivory

The Welwitschia is often used as a forage plant for herbivores , u. a. for oryx antelopes , zebras and rhinos . Oryx antelopes tear the leaves completely out of the hypocotyl pit - although they do not destroy the meristem ; so the plant can grow back within a few years. Sandstorms can also severely damage the leaves.

protection

Welwitschia is not endangered, but because of its popularity it is legally protected. It is also listed in Appendix II of the Washington Convention on Species Conservation (CITES); only seeds and seedlings can be traded.

Welwitschia in Europe

Welwitschia also thrive in Europe, but they are not very frost-tolerant (down to –6 ° C). They can be seen in many botanical gardens, but are also kept privately as ornamental plants. It is grown from seeds that are available in specialist shops. As in their natural habitat, young plants are sensitive to fungi, the spores of which adhere particularly to the seeds of wild plants. Older plants are relatively insensitive, but the location should be as similar to their natural habitat as possible, with the exception of better water and nutrient supply.

See also

literature

Web links

Commons : Welwitschie ( Welwitschia mirabilis )  - collection of images, videos and audio files
Wiktionary: Welwitschie  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ Joseph Dalton Hooker: On Welwitschia, a new Genus of Gnetaceae . In: Transactions of the Linnean Society of London. 24, London 1863, pp. 1-48 ( online ).
  2. ↑ Conservation of Welwitschia mirabilis ( Memento of November 30, 2010 in the Internet Archive ), accessed February 17, 2006.
  3. ↑ E. Moyroud, M. Monniaux, E. Thévenon, R. Dumas, CP Scutt, MW Frohlich, F. Parcy: A link between LEAFY and B-gene homologues in Welwitschia mirabilis sheds light on ancestral mechanisms prefiguring floral development . In: New Phytologist . 2017, doi : 10.1111 / nph.14483 .
  4. ↑ Welwitschia Bug. Retrieved April 10, 2020 .
  5. ↑ Nicholas Jacobson, Peter Jacobson, Ernst van Jaarsveld, Kathryn Jacobson: Field evidence from Namibia does not support the designation of Angolan and Namibian subspecies of Welwitschia mirabilis Hook. In: Transactions of the Royal Society of South Africa , Volume 69, Issue 3, 2014, pp. 179-186. doi : 10.1080 / 0035919X.2014.950187 .
  6. ↑ K. Aitzetmüller, K. Vosmann: Cyclopropenoic fatty acids in gymnosperms: The seed oil of Welwitschia. In: Journal of the American Oil Chemists' Society . Volume 75, Issue 12, 1998, pp. 1761-1765 ( doi : 10.1007 / s11746-998-0329-8 ).
This article was added to the list of excellent articles on March 9, 2006 in this version .