Zulu spurflower (Plectranthus zuluensis)

Description

Plectranthus, with some 350 species, is a genus of warm-climate plants occurring largely in the southern hemisphere, in sub-Saharan Africa, Madagascar, India and the Indonesian archipelago down to Australia and some Pacific Islands. They are closely related to Solenostemon and are known as the spurflowers. Several species are grown as ornamental plants, as leaf vegetables, as root vegetables for their edible tubers, or as medicine. Plectranthus species are used as food plants by the larvae of some Lepidoptera species including The Engrailed (Ectropis crepuscularia). Plectranthus zuluensis is an erect or sprawling, soft shrub up to 2 m high, with much-branched, four-angled, hairy stems which are velvety to the touch when young. Soft, semi-succulent leaves are ovate with coarsely toothed margins and covered with tiny, colourless glands. The plant has a characteristic pungent smell when crushed. Plectranthus zuluensis occurs from the northern parts of the Eastern Cape through the coastal region of KwaZulu-Natal to southern Swaziland, all of which are summer rainfall areas that experience little or no frost. The species is often common along stream banks and deep river gorges in humus-rich soil and in shady or semi-shady areas on the margins of semi-coastal, subtropical forests. It is best suited to gardens in warmer, frost-free areas. Plectranthus flowers produce large amounts of nectar, attracting many insects which transfer pollen between flowers. A study of the pollination biology of this group at the University of KwaZulu-Natal (Potgieter et al. 1999) showed a correlation between floral tube length and proboscis length of insect visitors. They observed that the most important pollinator of P. zuluensis is a long-tongued fly (Psilodera confusa, Acroceridae) which has a proboscis length similar to that of the flower tube. Insect visits to the flowers follow a pattern, with lowermost flowers visited first and the upper ones next. As the receptive stigma protrudes beyond the pollen-bearing anthers, pollen from another flower is deposited onto the stigmatic surface before the insect picks up pollen as it leaves. This minimizes the chance of self-pollination. Pollen is picked up on the lower surfaces of the insect, and is thus ideally placed for deposition onto the stigma of the next flower visited. The larval stages of Psilodera spp. are parasitic on spiders, highlighting the complex and often poorly understood interactions between organisms, and emphasizing the need for the conservation of sufficiently large areas of natural vegetation to support pollination systems.