Underground trees might sound like a new environmentalist movement, but these fascinating plants do exist in reality. In more than one way, they've also shown the Dutch proverb of tall trees catching much wind (or in this case frost and fire!) to be true. Elephantorrhiza elephantina has adapted to the generally cold and dry winters of southern African summer rainfall regions in a remarkable way. Walking amongst them in the Highveld you might find it interesting to think that you are actually treading across the canopies of very big, very old trees.
A cluster of racemes.
Notice the darker golden yellow of the older flowers compared to the newly opened flowers (near the end of racemes).
This change in colour indicates to pollinators which flowers they should focus their attention on, since the older flowers hold no more reward for them.
Perennial suffrutex producing unbranched, unarmed, aerial stems up to 0.9 m high. These stems represent the canopy of the much larger tree which is below ground. The bark and young branchlets are dark reddish brown. The leaves are dull green, bipinnately compound with 2–17 opposite or subopposite pairs of pinnae. Flowers are arranged in axillary, solitary or clustered racemes that are golden yellow to pale yellowish white and near to or protruding from ground level. Flowering time is from September to November and does not rely on spring rain. Fruits are dark or reddish-brown compressed pods with the valves separating from the persistent margins when they open. Seeds are ellipsoid and dark brown.
||One of several racemes with open flowers. Flowers open in sequence towards the end of the inflorescence so that not all flowers are open at the same time. This strategy maximises the potential of effective pollination.
||Scanning electron microscopy image of the dehisced anther of E. elephantina. Pollen grains are the rugby ball-shaped cells. The lump of cells at the top of the anther (lower left of image) is an anther gland.
According to the latest Red List, E. elephantina falls within the Least Concern category as it's a widely distributed species occurring in many bioregions throughout southern Africa. However, it is important to note that when a population of E. elephantina is destroyed the chance of regeneration is extremely limited as they are known to grow in relatively intact natural areas. They are not weedy in character even though a large colony of these plants may seem so.
Distribution and habitat
Of all species of Elephantorrhiza, E. elephantina is the most widespread and most commonly encountered from the southern parts of Angola, Namibia, Botswana, Zimbabwe, Mozambique and the South African provinces of Limpopo, Northwest, Gauteng, Mpumalanga, Free State, KwaZulu-Natal, Northern Cape and Eastern Cape as well as Swaziland and Lesotho. The distribution range of this species is extensive and populations vary considerably with regards to the number of pinnae pairs and the number, size and shape of the leaflets. However, this variation is continuous and genetic which means that no infraspecific groups/taxa can be satisfactorily distinguished.
Derivation of name and historical aspects
The genus Elephantorrhiza is an exclusively African genus and consists of nine species with several infraspecific taxa. Their habitats include almost all major biomes south of the equator. As a group, they are fairly homogeneous. The closest related genus is Entada which includes the well known Snuff Box Sea Bean (Entada rheedii) and together they form the Entada -group within the Mimosoideae subfamily of the Fabaceae or Leguminosae.
The generic name Elephantorrhiza was first published in 1841 by Bentham after other authors had placed it within the genus Acacia (Burchell and De Candolle) or in Prosopis (De Candolle, Sprengel and Meyer). Bentham founded the genus on E. elephantina, the first member of this genus that was discovered by Burchell during his travels into the interior of the country. Burchell first called this strange new plant Acacia elephantina noting the enlarged underground parts.
Elephantorrhiza is a Greek compound meaning ‘elephant root' in reference to the large roots of E. elephantina.
The most frequent pollinator of all Elephantorrhiza species is the African honeybee, Apis mellifera. Other insects that seemingly do not aid in the pollination of the plant's flowers, such as beetles, can be seen eating the flowers. The most common beetle found pillaging the flowers is the fruit chafer, Pachnoda sinuata, in the insect family Scarabaeidae. Flowers of Elephantorrhiza species provide only pollen as reward. Anther glands are present at the tips of anthers, but they soon fall off or are removed by visiting insects. This is a relatively rare characteristic within the Mimosoideae and their exact function remains a mystery. Since they occur on the reproductive parts of the plant they must presumably somehow aid in pollination by either attracting pollinators or, as some have suggested, act as pseudopollen—a mimetic device to attract pollinators to receptive stigmas before and after real pollen award is available.
The suffrutescent habit of E. elephantina can be explained as an adaptation to the environment—a trait that a plant evolved deliberately in order to survive certain environmental constraints. The main ecological driving forces behind the development of this extraordinary habit are frost and fire throughout the species' natural distribution. Herbaceous plants are commonly believed to have been derived from more ancient woody forms through reduction and simplification. Therefore, the suffrutescent habit may also be viewed (from a geological time scale point of view) as something like an intermediate form where the climatic cooling in the Tertiary period effectively stunted woody plants until their aerial parts persisted for no more than a single growing season. This happened over such a long period that plants had more than enough time to gradually adapt to changing environmental conditions.
Burtt Davy was the first to note that suffrutescent taxa on the Highveld are the only true (indigenous) ‘trees' of the open veld, since ‘conventional trees' (with large woody aerial parts) only occur in sheltered kloofs, sinkholes and on rocky outcrops or hills.
Uses and cultural aspects
The most commonly mentioned use for E. elephantina is for the tanning of leather. Xhosa and Zulu medicinal application of the plant includes using the root as a remedy for dysentery and diarrhoea, stopping bleeding, treating intestinal disorders, haemorrhoids, heart ailments and syphilis. It has also been used as an aphrodisiac, and a decoction from it has been used as a remedy for bovine and equine diarrhoea. An extract of the fleshy underground parts of the plant is used to treat sunburn. Roasted, the seeds have sometimes been used as a coffee substitute, although reportedly it is an acquired taste. Otherwise the seeds are toxic to several animals, including humans.
Some taboos are also associated with it, such as a Shangaan tradition which dictates that if the remaining root system of E. elephantina is not covered after a portion has been removed, the patients treated with it will not get better. In a very wide sense this may be interpreted as a means to prevent the over-harvesting of plant material, thus aiding conservation efforts, albeit in a somewhat unconventional way.
|| Several protruding branches from a single plant of Elephantorrhiza elephantina. These aerial parts emerged in spring following a controlled fire some month earlier in the natural grassland section of the Pretoria National Botanical Gardens. The vast majority of this plant's biomass exists underground.
Growing Elephantorrhiza elephantina
Since E. elephantina is not a garden plant, it is unclear (but doubtful) if anybody has ever tried to cultivate it for the commercial horticultural market. Experiments with seeds of this species have yielded very interesting evidence of a very unusual type of germination where the seedling buries its plumule (the bud within the embryo from which the stem and leaves develop) and shoots, thus arising from well below the ground surface. This is probably another adaptation by the plant to lower seedling mortality in an environment where veld fires are common.
Since the seeds ripen in summer it is advised that they be sown in late summer to autumn to simulate natural conditions. The exocarp (outer layer) of the pods breaks up fairly easily when they become wet and the seeds can often be seen germinating on the ground within the soft, disintegrating pods. Therefore the seeds may be sown whenever pods start to break up and no measures to break dormancy need to be taken. From the limited experiments on seed germination it seems that E. elephantina grows rather slowly and it won't be a quick fix for a garden in need of a ground cover. The best way to enjoy E. elephantina is still out in the unspoilt grasslands and savannas of southern Africa.
References and further reading
- Burtt Davy, J. 1922. The suffrutescent habit as an adaptation to environment. The Journal of Ecology 10(2): 211–219.
- Burtt Davy, J. 1932. A manual of the flowering plants and the ferns of the Transvaal. Longmans, London.
- Raimondo, D., Von Staden, L., Foden, W., Victor, J.E., Helme, N.A., Turner, R.C., Kamundi, D.A. & Manyama, P.A. (eds). 2009. Red List of South African plants. Strelitzia 25. South African National Biodiversity Institute, Pretoria.
- Ross, J.H. 1974. The genus Elephantorrhiza. Bothalia 11(3): 247–257.
- Ross, J.H. 1975. Flora of southern Africa. 16(1): 138–150.
- Smith, C.A. 1966. Common names of South African plants. Memoirs of the Botanical Survey of South Africa No. 35.
- Stone, G.N., Raine, N.E., Prescott, M. & Willmer, P.G. 2003. Pollination ecology of acacias (Fabaceae, Mimosoideae). Australian Systematic Botany 16: 103–118.
- Van Wyk, B. & Malan, S. 1997. Field guide to the wildflowers of the Highveld. Struik, Cape Town.
- Van Wyk, B.-E. & Gericke, N. 2000. People's plants: a guide to useful plants of southern Africa. Briza, Pretoria.
- Van Wyk, B.-E., Van Heerden, F. & Van Oudtshoorn, B. 2002. Poisonous plants of South Africa. Briza, Pretoria.
National Herbarium, Pretoria