The second-last in the plants and biomes of South Africa series is featured today. Taisha writes:
Today, we take a slight detour from South Africa, as this species is not present in the country. Welwitschia is found only in the Namib Desert of Namibia. However, the Namib Desert slightly extends into South Africa where it forms the country’s sole area of desert biome. The Namib Desert is one of the smallest and oldest deserts in the world. I chose Welwitschia as it has not been featured on Botany Photo of the Day before and it was not easy to find a plant species in the Botany Photo of the Day Flickr Pool to represent this biome. Still, this species is an interesting representative for the desert. Daniel contributed these photographs of Welwitschia mirabilis from Huntington Botanical Garden.
As implied above, the desert biome occurs in only a small part of northwestern South Africa, primarily the Springbokvlakte area of the Richtersveld. The altitude is between 600 and 1600 m, which results in a slightly cooler climate than other true deserts (though it remains more climatically extreme than the succulent Karoo and the Nama-Karoo biomes). Temperatures can be hot, up to 45°C. Similarly extreme, temperatures can drop over 20°C from day to night. Winter temperatures can be as low as -12°C. Fog from the nearby Atlantic Ocean accounts for much of the precipitation, although there is some variable summer rainfall (~10-80mm annually). True deserts are largely sandy with low organic material in their soils.
The vegetation within the desert biome is typically annual grasses and other plants. After a season with rarely abundant rains, short annual grasses may grow, whereas in most years the annual plants persist as seeds. Some perennials may survive, particularly in areas associated with local concentrations of water.
Welwitschia mirabilis is a monotypic species of the Welwitschiaceae under the plant Division Gnetophyta–a small group of seed plants that have intermediate characteristics between gymnosperms and angiosperms. The oldest specimens of Welwitschia in the Namib Desert are thought to be more than 1500 years old, and recent fossil evidence suggest that Welwitschia was present during the Cretaceous (~112 million years ago). Some photos of the plant in habitat are available via Wikimedia Commons: the biggest known plant and a couple plants in the landscape.
This dioecious (male and female individual plants; male and female cones are shown above) evergreen species has a woody unbranched stem that is shaped like an inverted cone. The stem is surrounded by a bi-lobed crown of green photosynthetic tissue. There are only two opposite, persistent, ribbon-like leaves that grow continuously from a basal meristem and die off at their tips over time. Unique among all extant plant species, after the first two leaves form, the terminal bud dies and the apical meristematic activity is transferred to the periphery and base of the leaves. In other words, it has ever-growing persistent leaves, with the leaf ends being the oldest part of the leaf.