Common names: twinleaf, caper beans, dollar bush (Eng.); spekbos, vaalkareedoring, witkriedoring (Afr.)
Zygophyllum, a genus of rounded, many-stemmed dwarf shrubs, is characterised by flowers with five or four delicate yellow or white petals, often with red markings towards the base, and fleshy bifoliolate leaves.
Species of Zygophyllum are perennials, rarely herbaceous annuals. They are round, many-stemmed dwarf shrubs, rarely shrubs, ± 1 m x ± 1.5 m. Sometimes the stems will scramble into and over nearby plants or the branches are densely tangled.
The leaves are fleshy, bifoliolate (having two leaflets) or simple and in opposite pairs. Petioles are present, often quite short, or absent. The leaflets are variable in size and shape. Membranous, herbaceous or spiny stipules are also a prominent feature of the genus. The plants are known as xerophytic leaf succulents as they occur in dry conditions and have fleshy leaves. The leaves are usually drought deciduous, dropping off under unfavourable conditions.
The flowers are borne singly in the axils of the leaves. The calyx and corolla are usually 5-merous. The sepals are somewhat fleshy, slightly connate at the base. Petals are shades of yellow, white or light orange. In most species the petals have red markings towards the base or are streaked with red veins. The petals are free, sessile and clawed, broadly obovate, elliptic or spathulate (spoon-shaped). An 8- or 10-angled fleshy disc is present, with the corresponding number of stamens inserted at the base. Stamens are always twice as many as the petals; their filaments are free but have appendages at their bases. Zygophyllum is further characterised by a simple style with a minute stigma, longer or shorter than the ovary, which is sessile on the disc. The stigma is minute.
The fruit is a capsule or a schizocarp (fruit splits into two or more parts). Different capsule types occur. When dry, the capsules are often ridged, or smooth as in Z. flexuosum, or winged as in Z. morgsana. A few to several seeds occur in each locule (chamber) of the ovary.
Species of Zygophyllum are often threatened by grazing and browsing. However, where no grazing occurs, seedlings and young plants are seen in abundance. Some species have a very limited distribution, considered to be endemic to the area, for example, Z. applanatum, which occurs only in the southern part of Namibia.
Distribution and habitat
The genus Zygophyllum belongs to the family Zygophyllaceae, a family of about 26 genera and 280 species, known from tropical and warm, arid regions of the world. It comprises about 120 species of which ± 60 species are found in sub-Saharan Africa, with ± 47 species in southern Africa (South Africa, Namibia, Botswana, Lesotho and Swaziland). Species of Zygophyllum also occur in the Mediterranean region, central Asia, Australia, southwestern United States and Chile. In the southern Africa region, the species are found in winter and summer rainfall areas with the greatest number of species present in the Nama-Karoo and Succulent Karoo Biomes, both known to be arid. The Gariep Centre is the principal centre of diversity and endemism for Zygophyllum in southern Africa. This region of floristic endemism is located in the northwestern corner of the Northern Cape and the adjacent southwestern corner of Namibia.
Derivation of name and historical aspects
The genus name dates back to 1753 when Linnaeus established the name based on the species Zygophyllum fabago. Zygophyllum is derived from the Greek words, zygos, meaning joined, and phyllon, meaning leaf, referring to the paired leaves of many of the species. In 1814, Robert Brown was the first to recognise the family Zygophyllaceae as a distinct taxon under the name Zygophylleae. Ever since the genus and family were established, their circumscription has been a matter of controversy. The treatment of Zygophyllum followed here is according to Van Zyl (2000).
Zygophyllum is found in many deserts and semideserts of the world, including some of the most arid habitats in which plant life is possible, e.g. Z. stapfii, known from the Namib Desert. Species are found in various habitats along hill slopes, dry rivers, ridges, road verges, in coarse sandy soil, calcareous sand and brackish soil.
Species of Zygophyllum are grazed by stock. Z. lichtensteinianum (= Z. gilfillanii ) (spekbos), is palatable and resistant to drought, but loses its leaves under conditions of drought. In contrast, Z. incrustatum is less palatable and less acceptable to animals owing to its spines. This species also loses its leaves during drought and its grazing value is therefore limited. It is reported that Z. foetidum causes bloating in livestock.
Species of Zygophyllum are usually not found in cultivation. Some of them are very attractive with their yellow, delicate petals and interesting fruits, and will undoubtedly be suitable for gro wing in gardens. Propagation can be done from seed or by taking cuttings in spring.
The southern African species of Zygophyllum are divided into two subgenera. Species of the subgenus Zygophyllum, such as Z. morgsana, are characterised by petals with markings at the base or streaked with red veins, a papillate disc and fruit disintegrating when mature. In species of the subgenus Agrophyllum, such as Z. simplex, the petals have no markings at the base, the disc is smooth, and the fruit splits into five parts (mericarps).
- Bellstedt, D.U., Van Zyl, L., Marais, E.M., Bytebier, B., De Villiers, C.A., Makwarela, A.M. & Dreyer, L.L. 2008. Phylogenetic relationships, character evolution and biogeography of southern African members of Zygophyllum (Zygophyllaceae) based on three plastid regions. Molecular Phylogenetics and Evolution 47: 932-949.
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- Le Roux, P.M., Kotzé, C.D., Nel, G.P. & Glen, H.F. 1994. Bossieveld: grazing plants of the Karoo and karoo-like areas. Bulletin 428. Department of Agriculture, Pretoria.
- Mannheimer, C., Maggs-Kölling, G., Kolberg, H. & Rügheimer, S. 2008. Wildflowers of the southern Namib: a photographic guide to wildflowers of south-western Namibia. National Botanical Research Institute, Windhoek.
- Retief, E. 2000. Zygophyllum. In O.A. Leistner, Seeds plants of southern Africa: families and genera. Strelitzia 10. National Botanical Institute, Pretoria.
- Smith, C.A. 1966. Common names of South African plants. Memoirs of the Botanical Survey of South Africa No. 35.
- Van Wyk, A.E. & Smith, G.F. 2001. Regions of floristic endemism in southern Africa: a review with emphasis on succulents. Umdaus Press, Hatfield, Pretoria.
- Van Zyl, L. 2000. A systematic revision of Zygophyllum (Zygophyllaceae) in the southern African region. Ph.D. thesis, University of Stellenbosch. Unpublished.
Plant Type: Shrub