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Thursday, September 5, 2013

Poison Bride's Bush (Pavetta schumanniana)

Family Rubiaceae
Shrub or tree often about 3.5 m tall, reaching 7 m in Kwazulu-Natal and 8.4 m in Tanzania. Leaves usually opposite, sometimes in whorls of 3, with interpetiolar stipules (small leaflike outgrowths on the stem between the bases of the leaves of a pair), egg-shaped, widest beyond the middle, 60-150 x 20-75 mm, shiny bright green with few rough hairs above, soft grey hairs below; tip rounded, margins not toothed, base tapering gradually into a short stalk.

Flowers white, sweetly scented, in dense clusters on stalks, up to 20 mm long just below the leaves (September to February in South Africa ). Fruits round, buff to black, fleshy, up to 8 mm in diameter.

 Distribution and habitat

Pavetta schumanniana grows in open woodland and bush clumps from northern Kwazulu-Natal, through Swaziland, Mozambique, the Lowveld of Mpumalanga and Limpopo Provinces into Zimbabwe, the Caprivi ( Namibia ) and far to the north in tropical Africa, from Tanzania to Cameroon. These places are characterized by hot summers and warm (certainly frost-free) winters, or by a relative lack of discernible seasons. Rainfall in the seasonal parts of the range of this species is mostlly in summer; in the more tropical areas it is monsoonal. The quantity of rain is moderate, as these areas are neither very dry nor moist enough to support rainforests.

The leaves of this tree are poisonous to stock (hence the common name), and cause a stock disease called gousiekte. In this form of poisoning, small doses of leaves cause heart failure (Coates Palgrave 2002). A few other species of Pavetta are also known to be poisonous, and act in the same way. Strangely, this only affects ruminants (cattle, goats, sheep); Codd & Voorendyk (1966) record a striking absence of reports of this kind of poisoning in horses, and that in experiments at Onderstepoort, researchers failed to induce anything untoward in small laboratory animals. Thanks are due to Prof. T.W. Naudé (Emeritus Professor of Toxicology at Onderstepoort) for confirming that poison bride's bush is harmless to humans; he informs me that in his experiments, large doses did produce symptoms in rats.

Like most species of Pavetta, the leaves of the poison bride's bush have black spots that are visible when the leaf is held up to the light. These are bacterial nodules, and the bacteria sheltering in them fix nitrogen from the air, releasing it in a form the host plant can use.

The white, scented flowers have all the features needed to attract moths, and one may deduce that they are moth-pollinated. Moths usually fly at twilight or at night, when almost any colour of flower other than white would be invisible. Scents serve to inform potential pollinators of the existence of a plant offering a reward nearby; that humans generally enjoy the aroma of these pheromones is a co-incidental benefit to gardeners.

The black, fleshy fruits stand out against the green of the foliage and make them attractive to animals (presumably birds) which eat the fruits and spread the seeds. Fruits such as these are generally sweet to taste; for them to be poisonous would defeat the point of their being fleshy. However, readers of this account are warned against sampling the fruits of this tree; there are lookalikes which are not friendly.

 Uses and cultural aspects

Tree-spotters will find the poison bride's bush at South African Tree No. 721, Zimbabwe No. 1130 in their lifer lists (Balkwill et al. 2004) or other reference books. (Incidentally, I firmly believe that all good tree-spotters should have a copy of this lifer list with them when looking at trees in the field. I find my copy invaluable for keeping track of the pictures I take of trees, but there are a thousand other uses for such a list.)

Despite (or possibly because of) being poisonous to stock, this tree has a large number of medicinal uses in Zimbabwe. These run from cures for coughs to treatments for infertility and venereal diseases in women.