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