Tree tumbo (Anonymous [no date]), tumboa (local native), welwitschia (horticultural).
The sole species in order Welwitschiales Skottsburg ex Reveal 1993, family Welwitschiaceae Caruel 1879, genus Welwitschia Hooker f. 1862. Synonyms for these include Tumboaceae Wettst. 1903, Tumboa Welw. 1861, Welwitschia bainesii (Hooker f.) Carrière 1867, Tumboa bainesii Hooker f. 1861, and Tumboa strobilifera Hooker f. 1862. Some authorities even put it in a class by itself, Welwitschiopsida B. Boivin 1956.
Leuenberger (2001) has described the Angolan and Namibian as two subspecies, differentiated on the basis of male cone characters. The subspecies are W. mirabilis Hook.f. subsp. mirabilis and W. mirabilis Hook.f. subsp. namibiana Leuenberger. He has offered the following key (Leuenberger 2001):
|1||Male cones smooth, purplish brown (rarely green when shaded), without evident wax cover; longer peduncles usually c. 5-11 cm long, secondary branches to 2 cm long; longest male cones 30-45 mm long; bract pairs overlapping c. 2 mm; bract scales more than 3/4 connate, margin of bracts smooth; Angola||W. mirabilis subsp. mirabilis|
|-||Male cones sculptured, glaucous green to salmon, with evident wax cover; longer peduncles c. 7-15 cm long, secondary branches to 7 cm long; longest male cones 20-30 cm long; bract pairs overlapping c. 1 mm; bract scales ⅓ -⅔ connate, margin of bracts slightly erose; Namibia||W. mirabilis subsp. namibiana|
Dioecious perennial plant with short stem and taproot. Beyond that, the plant is often described as 'bizarre', 'weird', or (more explicitly) 'unlike any known plant on earth'. Its short, woody, unbranched stem is surmounted by 2 (rarely 3: see photo) strap-shaped leaves that grow from a basal meristem throughout the life of the plant, becoming twisted and frayed with the passing centuries. The leaves contain numerous subparallel veins that may anastomose or terminate blindly in the mesophyll (this character is unique in Welwitschia among the gymnosperms). Stomata occur on both leaf surfaces. The woody stem widens with age to become a concave disc up to a meter across, from which grow small ramified branch systems that serve only to bear pollen and seed cones (Gifford and Foster 1988). The branched reproductive shoots arise near the leaf bases. Pollen cones red, resembling those of Ephedra, appearing in groups of 2-3 terminally on each branch. Ovulate cones also arising from branched reproductive shoots, each red cone consisting of a single nucellus enclosed in an integument and another layer derived from two confluent primordia ('perianth') with 2 'bracts'. Normally, only one seed develops within each cone; it is dispersed by wind with 'perianth' as a wing. Seeds germinate in wet years, the 2 cotyledons photosynthesizing for 1.5 years (Anonymous [no date], Gifford and Foster 1988).
A disjunct distribution in SW Africa. The type locality is in the vicinity of Cabo Negro on the coast of Angola (latitude 15-16° S), while more widely dispersed populations are found from the coast to ca. 200 km inland in Namibia (latitude 20-24° S). The area is extremely arid, receiving no rainfall in some years and averaging fewer than 100 mm per year. Most specimens are found within 80 km of the coast in a fog belt, suggesting that the fog is an important moisture source (Rodin 1953).
Leaves typically grow at a rate of 8-15 cm/yr on mature plants, some of which have been found with leaves measuring 1.8 m wide and 6.2 m long, suggesting potential ages of 500-1000 years. Ages of 1500-2000 years have been claimed without supporting data (Gifford and Foster 1988).
I do not know of any investigations, but suspect it is impossible in view of the plant's peculiar morphology.
Discovered by Friederich Welwitsch (1806-1872) in 1860 (Rodin 1953), and described by him the following year (Welwitsch 1861).
Some remarkably detailed ecophysiological studies have been performed, cited below and available online at (Institute of Plant Ecology 1997). The findings are rather complex but seem to suggest that the plant requires periods of high humidity (i.e. fogs) if it is to sustain a positive carbon balance. It may survive the absence of such conditions for as much as 150 days -- possibly much longer.
"Difficult to cultivate requiring desert conditons and room to accomodate its long taproot. Propagated by seeds." Temperature: 10-12°C at night, 21-23°C during the day. Light: Bright light or full sun. Watering: Drench thoroughly, allow to become dry between waterings (Anonymous [no date]).
Anonymous. [no date]. Welwitschia mirabilis. Web page maintained by University of Connecticut Department of Ecology and Evolutionary Biology at http://florawww.eeb.uconn.edu/acc_num/199700061.html, accessed 2007.12.11. This page contains useful information and several links to other Welwitschia pages.
Botanischer Garten und Botanisches Museum Berlin-Dahlem, Freie Universität Berlin. 1997.02.05. Welwitschia-Annex. http://www.bgbm.fu-berlin.de/bgbm/pr/garden/bereiche/areas/welwitsh.htm, accessed 1999.02.24, now defunct but a very similar page is at http://www.bgbm.org/bgbm/garden/bereiche/areas/welwitsh.htm.
Carr, Gerald D. 2002. Non-Flowering Plant Families. http://www.botany.hawaii.edu/faculty/carr/nfpfamilies.htm, accessed 2007.12.11.
Gifford, Ernest M. and Adriance S. Foster. 1988. Comparative morphology of vascular plants, 3rd ed. New York: W.H. Freeman (Chapter 18).
Institute of Plant Ecology. 1997.02.17. Welwitschia mirabilis. http://www.uni-muenster.de/Biologie/pflanzenoekologie/science/willert/vw_welwmira.htm, accessed 1999.02.24, now defunct.
Leuenberger, B.E. 2001. Willdenowia 31: 357-381.
Rodin, R.J. 1953. Distribution of Welwitschia mirabilis. American Journal of Botany 40:280-285.
Welwitsch, F.M.J. 1861. On the botany of Benguela, Mossamedes, etc., in western Africa. Journal of the Linnaean Society (Botany) 5:182-187.
Khoshoo, T.N. and Ahuja, M.R. 1962. The karyotype in Welwitschia mirabilis. Nature 193:356.
Khoshoo, T.N. and Ahuja, M.R. 1963. The chromosomes and relationship of Welwitschia mirabilis. Chromosoma 14: 522-533.
Bustard, L. 1990. The ugliest plant of the world: the story of Welwitschia mirabilis. Kew Magazine 7:85-90.
Carlquist, S. and D.A. Gowans. 1995. Secondary growth and wood histology of Welwitschia. Botanical journal of the Linnean Society 118(2): 107.
Dilcher, D. L., M. E. Bernardes-De-Oliveira, D. Pons, and T. A. Lott. 2005. Welwitschiaceae from the Lower Cretaceous of northeastern Brazil. American Journal of Botany 92:1294-1310.
Henschel, Joh R. and Mary K. Seely. 2004. Long-term growth patterns of Welwitschia mirabilis, a long-lived plant of the Namib Desert (including a bibliography). Plant Ecology 150(1-2):7-26. Abstract: Over the past 14 years, long-term ecological research (LTER) was conducted on the desert perennial, Welwitschia mirabilis (Gnetales: Welwitschiaceae), located in the Welwitschia Wash near Gobabeb in the Central Namib Desert. We measured leaf growth of 21 plants on a monthly basis and compared this with climatic data. The population structure as well as its spatial distribution was determined for 110 individuals. Growth rate was 0.37 mm/day, but varied 22-fold within individuals, fluctuating seasonally and varying between years. Seasonal patterns were correlated with air humidity, while annual differences were affected by rainfall. During three years, growth rate quadrupled following episodic rainfall events >11 mm during mid-summer. One natural recruitment event followed a 13-mm rainfall at the end of summer. Fog did not appear to influence growth patterns and germination. Plant location affected growth rate; plants growing on the low banks, or ledges, of the main drainage channel grew at a higher rate, responded better and longer to rainfall and had relatively larger leaves than plants in the main channel or its tributaries. This could be due to better water and nutrient conditions on the ledges than elsewhere. The population appears to be growing outwards, with the smallest (youngest?) plants highest. Sex ratio was male-biased and males grew larger than females. Our study, in conjunction with the extensive literature base on Welwitschia, published here in a bibliography comprising 297 papers, indicates the knowledge gaps and needs for further ecological studies, including the continuation of our LTER programme. This should elucidate the reproductive output, seed dispersal, recruitment, water availability, age structure, and ecological differences between the sexes, and long-term life history strategies. Such knowledge would contribute to desert ecology and improve the management strategies of this unique Namib Desert perennial.
Herppich, W.B., B.M.-T. Flach, D.J. von Willert, and M. Herppich. 1996. Field investigations of photosynthetic activity, gas exchange and water potential at different leaf ages in Welwitschia mirabilis during a severe drought. Flora 191:59-66.
Jacobson K. M., and E. Lester. 2003. A first assessment of genetic variation in Welwitschia mirabilis Hook. Journal of Heredity 94:212-217.
Mundry, M. and T. Stutzel. 2004. Morphogenesis of the reproductive shoots of Welwitschia mirabilis and Ephedra distachya (Gnetales), and its evolutionary implications. Organisms Diversity & Evolution 4(1-2):91-108.
Rydin C., B. Mohr, and E. M. Friis 2003. Cratonia cotyledon gen. et sp. nov.: a unique Cretaceous seedling related to Welwitschia. Proceedings of the Royal Society of London B, Biology Letters (Supplement) 270:1?4.
Species Plantarum. The authors hope that one day this will develop into a comprehensive global flora. For now (2007.12.11), it has an excellent description of Welwitschiaceae, Welwitschia, and W. mirabilis.
Vascular Plant Image Gallery, web page http://www.csdl.tamu.edu/FLORA/imaxxwlw.htm, accessed 1999.02.21. This page has about 20 photographs of the plant and its structures.
von Willert, D.J. 1994. Welwitschia mirabilis Hook. fil.- das Überlebenswunder der Namibwüste [the survival miracle of the Namib desert]. Naturwissenschaften 81:430-442.
von Willert, D.J. and U. Wagner-Douglas. 1994. Water relations, CO2 exchange, water-use efficiency and growth of Welwitschia mirabilis Hook. fil. in three contrasting habitats of the Namib desert. Botanica Acta 107:291-299.
Last Modified 2012-11-23