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K leinduimpie grass (Kleinduimpie is the Afrikaans equivalent of Tom Thumb) Dregeochloa pumila is a unique, tiny, leaf-

succulent grass that is endemic to the strip of coastal desert in southern Namibia and the adjacent north-western part of South Africa. I was privileged to see it near Alexander Bay and, recently, near the town of Luderitz.

The coastal Namib Desert is a cool desert and although the annual rainfall is only 15 mm, it is well vegetated and especially rich in leaf-succulent flora. The reason for this is the regular, dense fog that is created when hot air is cooled by the cold, north-flowing Benguela Ocean Current. This peculiar climatic condition, the result of having a cold ocean in a warm position (in terms of latitude and altitude) also occurs in the Galapagos Islands.

This is the main driving force for the uniquely adapted life forms such as the cone-bearing Welwitschia mirabilis and the mesemb Jensenobotrya (see 'Desert grapes' in the December issue of Veld & Flora), two peculiar plants from the coastal Namib. Apart from the

n a m i b d e s e r t f l o r a

regular fog and cool conditions, wind is also a major factor in shaping the plants. Strong south-easterly winds blow for months on end (and as a result, dunes here are constantly moving northwards). To avoid the wind, and constant sandblasting by loose sand, many of the plants grow flat on the ground against the rocks or soil. By being close to the ground, the plants also benefit from thermal heat in this cool desert. Many plants here have a compact, Alpine growth form. Dwarfism (flat or rounded cushion shapes) is beneficial in this climate – the opposite being true for hot subtropical deserts where plants want to get away from the ground, and are often tall and cylindrical. Being small has additional benefits such as being able to occupy small niches in particular habitats, and with many species occupying one habitat, the result is a rich diversity as seen in the coastal Namib.

Our succulent grass, Dregeochloa pumila, has also been shaped by the set of conditions mentioned above and is dependent on regular coastal fog. It is a dwarf grass species with a

compact growth and it shares its habitat with many other dwarf succulent plants. The grass family is well represented throughout the world with almost 10 000 species. In South Africa we have about a thousand species (in 194 genera) and so far Kleinduimpie grass Dregeochloa pumila is the only known true succulent grass in the world! It occurs in sandy, coastal desert in full sun and its habitat is usually rocky terrain (granite or quartz) on gentle hills and slopes. The plants are scattered, sometimes common but otherwise scarce, in open desert. Although they grow in full sun, the sky is often hazy when the wind is blowing due to moving sand and dust.

The lack of water means that there are few herbivores to graze or disturb the plants.

Although classified as part of the Desert Biome, coastal Namibia harbours plants clearly related to those in the Succulent Karoo Biome. Succulent plants are dominant, especially the families Mesembryanthemaceae (or Aizoaceae if you prefer) and Crassulaceae. Dregeochloa grows up to 15 km away from the coast and

Kleinduimpie grass by Ernst van Jaarsveld, Kirstenbosch

Dregeochloa pumila, the only succulent grass in the world Kleinduimpie grass sharing its habitat with Conophytum saxetanum. Photo: Ernst van Jaarsveld.

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and C. elegans. Mesembs include species of Brownanthus and Cephalophyllum, Conophytum saxetanum, Drosanthemum paxianum, Lithops optica, Psammophora modesta, Psilocaulon dinteri, Namibia ponderosa and Ruschia sedoides. The family Zygophyllaceae is well represented with Zygophyllum stapffii and Augea capensis. Euphorbia aequoris from the milkweed family (Euphorbiaceae) is present, as well as less common species like Larryleachia (Apocynaceae) and Salsola (Chenopodiaceae). Lichens too are abundant.

is only known from Luderitz (Namibia) to Alexander Bay in the Northern Cape. Other plants that share the habitat of Dregeochloa pumila near Luderitz include the daisies Gazania schenckii, Eremothamnus marlothianus, Othonna furcata, and species of Pteronia and Tripteris. Pelargoniums include Pelargonium ceratophyllum, P. cortusifolium, P. crassicaule and Sarcocaulon patersonii (Bushman’s candles). The Crassulaceae is represented by Cotyledon orbiculata (pig’s ear or plakkie), Tylecodon schaeferianus, Crassula muscosa, C. namibensis

WhAT’S IN A NAME?

Dregeochloa pumila (Nees) Conert was originally named by the German botanist Nees* as Danthonia pumila. Hans Joachim Conert later realized that it did not belong there, and created a new genus for it, Dregeochloa, in 1966. The generic name Dregeochloa honours plant-collector Johann Franz Drege (1794-1881) who collected and described many plant specimens in South Africa. The specific epithet ‘pumila’ describes its dwarf size. There are only two species in the genus: Dregeochloa pumila and another non-succulent, Dregeochloa calviniensi, from the south-western parts of South Africa. The common name, Kleinduimpie (literally 'little thumb' in Afrikaans) grass, refers to the diminutive character, Tom Thumb, of the Brothers Grimm fame.

*Christian Gottfried Daniel Nees von Esenbeck (1776-1858) was a prolific German botanist, physician, zoologist and natural philosopher. He was a contemporary of Goethe and was born within the lifetime of Linnaeus. He described approximately 7 000 plant species (almost as many as Linnaeus himself). His last official act as president of the German Academy of Natural Scientists Leopoldina was to admit Charles Darwin as a member. He was the author of numerous monographs on botany and zoology. His best-known works deal with fungi. (Acknowledgment to Wikipedia.)

ABOVE: Crassula namibensis. TOP LEFT: The desert habitat of Dregeochloa pumila (Kleinduimpie grass) consists of coastal rocky areas in sandy soil with granite bedrock. It is mostly confined to the coastal Namib of the Northern Cape of South Africa and southern Namibia. ABOVE LEFT: Tylecodon schaeferianus.LEFT: The succulent grass, Dregeochloa pumila, growing with Crassula mucosa in their desert habitat. Note the succulent leaves and compact growth. BOTTOM LEFT: The centre (inner canopy) of Dregeochloa pumila (Kleinduimpie grass) dies, and the plant continues growing, forming a circular outer canopy. Photos: Ernst van Jaarsveld.

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READING

Gibbs Russsell, G.E., et al. 1991. Grasses of southern Africa. Memoirs of the Botanical Survey of South Africa, 58.Smith, G., et al. 1997. List of southern African succulent plants. NBI, Succulent Society of SA, Umdaus, Hatfield.

ABOVE: Gazania schenckii.RIGHT: Bushman’s candles, Sarcocaulon patersonii. Photos: Ernst van Jaarsveld.

ACKNOWLEDGEMENTS

Danny Gildenhuys and Volker Jahnke for providing transport and assistance. Bertil Nordenstam for identifying the plants belonging to the Asteraceae.

hOW DO WE DEfINE A SuCCuLENT?

Most plants possess some degree of succulence, without which plants would not be able to exist. Once a plant can survive for a while when all external water resources are depleted (due to water reserves in its stems, roots or leaves; or modified leaves such as bulbs and scales) we can call it a succulent. Degrees of succulence vary considerably within species. In some plants, such as Lithops, Aloe and most cacti, it is obvious, while in others succulence is less noticeable, and there are many borderline cases resulting in much discussion.

Succulence is thus an adaptation to periodic dry conditions and in South Africa our succulent richness is well known and includes 57 plant families – a reflection of the semi-arid conditions of most of our country. Some families, such as the mesembs and stonecrop (Crassulaceae) family, almost entirely consist of succulent plants. In others, for example in the daisy family, there are but a few. In the List of South African succulent plants (see reading list below) only a single grass, Dregeochloa pumila, is mentioned.

DESCRIPTION Of DrEgEochloa pumila

Plants are perennial and evergreen, with a dwarf, mat-forming to tufted habit up to 300 mm in diameter. The older branches often die off, resulting in a living outer circle with a dead centre of dry leaf remains – the bases covered with broad scales. Leaves grow opposite each other in two rows (distichous) with an ascending, spreading orientation. The leaf blades, which are clearly channelled or folded, can grow up to 25 mm long, but are usually much shorter (about 10-15 mm long), and 1.6-3.5 mm broad, and are distinctly succulent, dark green to grey-green, and densely covered with fine hairs (pubescent). The leaf tips are rounded and end in a minute point or apiculum (apiculate). Flowers appear during the summer, usually on a raceme (a simple elongated flower head in which the flowers are borne on stalks), or in a panicle up to 30 mm high bearing up to nine spikelets with each spikelet bearing up to 10 flowers. The glumes (bracts at the base of a grass flower or spikelet) are 13 mm long.