Is this what the dinosaurs ate? Read on to find out.
The genus Gunnera has numerous unique characteristics, some of which indicate that these plants have been around for the last 95 million years and belong to one of the oldest angiosperm families as well as being among the largest herbs on earth (Bergman et al. 1992). The internal arrangement of the vascular system is evidence that Gunnera was adapted to survive in the swamps where the dinosaurs browsed.
However, the most unique oddity is that it is the only genus of flowering plants that has a symbiotic relationship with a nitrogen-fixing cyanobacteria (blue-green algae). Also, it is the only group of land plants that host the cyanobacteria in intercellular spaces, in other words the cyanobacteria live in the spaces between the cells. Other angiosperms, for instance many of the plants in the pea family, have nitrogen-fixing bacteria that occur within specialized cells, not outside the cell wall.
Some South American species of gunnera have striking, enormous leaves and are popular ornamental plants in parks and gardens.
Gunnera perpensa is a perennial, robust, erect herb up to 1 m. tall that always grows near water. The roots are up to 300 mm thick, creeping in black, muddy soil. The inside tissues are yellow-brown. All the leaves arise from a central tuft near the top of the apex, just above the soil level. They are large, dark bluish green, kidney-shaped and covered with hairs on both surfaces, especially along the veins in young leaves. The margin of the leaves is irregularly toothed. The veins are very noticeable on the lower surface of the leaf, radiating from the point where the petiole joins the leaf, referred to as palmate radiation. The petioles vary in length from 150 to 750 mm.
The flowers are numerous, small and not very noticeable, tiny pinkish reddish brown, borne on a long slender spike, which is taller than the leaves. On each spike there will be female flowers at the base, male flowers at the top and bisexual flowers in the middle, flowering between September and February (Mendes 1978).
Gunnera and a few other plants have an unusual internal vascular system. Instead of having a central core with all the vascular tissue in it, there are numerous separate vascular strands referred to as polysteles, which is indicative that Gunnera evolved from aquatic ancestors (Bergman et al. 1992).
The species is quite common and has not been evaluated for the Red Data List (Von Ahlenfeldt 2003). However, the destruction of its habitat due to mismanagement of the wetlands and uncontrolled harvesting for the medicinal trade market, are the main threats to the survival of Gunnera perpensa .
Distribution and Habitat
Gunnera occurs naturally in central and southern Africa, Madagascar, New Zealand, Tasmania, Indonesia, the Philippines, Hawaii, Mexico, Central and South America.
G. perpensa is widespread in tropical Africa from Sudan, Ethiopia, Zaire, Rwanda, Uganda, Kenya, Tanzania, Zimbabwe and Mozambiqu , extending along the central and eastern areas of southern Africa down to the Western Cape , including Swaziland and Lesotho . It has not been recorded in the Northern Cape Province, Namibia and Botswana (Bergman et al. 1992; Mendes 1978).
It is an obligate wetland plant that grows in shallow water around the edge of pools in marshy areas or along streams. It cannot tolerate frost and even when growing in warm protective areas it will die back for the coldest months of the year.
Derivation of name and historical aspects
The genus Gunnera was named by Carl von Linnaeus in honor of Johan Ernst Gunnerus (1718-1773) who was a Norwegian Bishop of Trondheim and a botanist who wrote the Flora norvegica (1766-1772) (Bergman et al. 1992). In Latin, perpenso means to weigh exactly or consider carefully, which points to the impact its bold, majestic appearance has as well as the importance of its presence to a wetland (Smith & Stern 1972). When Linnaeus named it, he was unaware of even more majestic species that occurred in Central and South America, Madagascar, Malaysia, Tasmania, New Zealand and Hawaii (Bergman et al. 1992; Mendes 1978).
The common name river pumpkin refers to the leaves resembling pumpkin leaves and it grows along rivers-similarly, wild rhubarb, because it grows and looks like rhubarb, and in South America the locals eat the thick leaf stalks, either raw or cook it like rhubarb.
The Zulu name ugobho refers to its use; gobhoza in Zulu means the flowing of fluids, in particular the flowing of water. In traditional medicines a decoction of it is used to remove excess fluid from the body or excess blood and the placenta in humans and cattle (Ngwenya et al. 2003). Gunnera is the only genus in the family, with about 45-50 species, with G. perpensa being the only species occurring in Africa .
All Gunnera species are wind pollinated and the fruits are red, fleshy berries that are dispersed by birds.
One of the other characteristics of Gunnera is the specialized glandular structure that develops at the base of each petiole to assist in the entry of Nostoc punctiforme, which occurs naturally in the wet, marshy soil. As the young embryonic leaves move up through the soil to reach the surface, the secretory cells in the glandular structure produce mucus to which the filaments of Nostoc adhere, and then undergo structural changes to form hormogonia. These structures become mobile and start moving down into spaces between the cells. This is quite a complex process and for a more detailed description please refer to Bergman et al. (1992) and Benson & Margulis (2002).
Uses and cultural aspects
In South Africa, a decoction of the roots of Gunnera perpensa is used to expel the placenta after birth or to relieve menstrual pains (Ngwenya et al. 2003; Van Wyk & Gericke 2000; Von Ahlenfeldt et al. 2003. Download medicinal plant monograph on this plant as pdf file.
According to Fox & Norwood Young (1982) the Sothos, Fingos, Xhosas and Zulus eat the petioles and flower stalks raw. The petioles have a bitter taste unless the fibrous vascular bundles and the outer covering are removed.
Some of the large exotic species of Gunnera are sold at our local nurseries. Sadly, our indigenous species are rarely ever available at nurseries, yet it is a plant with features that would look most attractive in any wetland.
Growing Gunnera perpensa
If you are fortunate to acquire a plant from a legal source, they are not difficult to grow, provided you create a wet, marshy area. I obtained the largest flat bottom plastic basin I could, about 30 cm. deep. This was sunk into the soil where I am in the process of designing a wetland feature in our garden. I covered the rim with some attractive stones and planted Gunnera perpensa and Cyperus prolifer together. The latter was purchased at a nursery, whereas G. perpensa was a gift from Natal National Botanical Garden . Both plants had sufficient soil around the roots to fill the plastic basin. No nutrients were added-the only requirement is to maintain a wet, soggy habitat. The G. perpensa will die back in the winter months, the C. prolifer is evergreen, so there is always something growing in that part of the wetland.
References and further reading
- Benson, J. & Margulis, L. 2002. The Gunnera manicata - Nostoc symbiosis: is the red stipulate tissue symbiogenetic? Biology and Environment: Proceedings of the Royal Irish Academy 102b: 45-48.
- Bergman, B., Johansson, C. & Söderbäck, E. 1992. Tansley Review No. 42: the Nostoc - Gunnera symbiosis. New Phytologist 122: 379-400.
- Fox, F.W. & Norwood Young, M.E. 1982. Food from the veld. Delta Books, Johannesburg .
- Mendes, E.J. 1978. Haloragaceae. Flora zambesiaca 4: 79-81.
- Ngwenya, M.A., Koopman, A. & Williams, R. 2003. Zulu botanical knowledge : an introduction. National Botanical Institute, Durban .
- Van Wyk, B-E. & Gericke, N. 2000. People's plants: a guide to useful plants of southern Africa . Briza Publications, Pretoria .
- Von Ahlenfeldt, D., Crouch, N.R., Nichols, G., Symmonds, R., McKean, S., Sibiya, H., & Cele, M.P. 2003. Medicinal plants traded on South Africa 's eastern seaboard . Ethekweni Parks Department & University of Natal, Durban.