Autumn Oxalis

Although very diverse, the genus Oxalis is easily recognisable. The leaves nearly always comprise of 3 leaflets that arise from a central point (there are a few species that have more leaflets or only one). The flowers are also very regular, made up of 5 petals that form a tube widening into a bowl, each petal overlapping the next one. But from that basic model, many variations exist. Indeed, South Africa is one of the centres of diversity for the genus: some 250 species occur in the country, about half of which are in the Cape Floristic Region.

At Phillipskop, we have a number of species that appear as winter approaches. They are scattered around like purple jewels, hidden amongst the bushes or lining the tracks. Some are even strong enough to survive in the lawns. One of the commonest is Oxalis eckloniana, a very typical species with broad round trifoliate leaflets and purple-pink flowers with yellow tube. Like most of the Cape species, it produces bulbs that are buried several centimetres underground and from which arises a thin stem that looks too weak to produce such a large flower. The reason it can, is that the stem stops at ground level and from there the leaves and flowers expand.

Another fascinating aspect of Oxalis is that the flowers exhibit a phenomenon known as tristyly. This means that the stamens and styles occur at three different heights within the same flower and each plant will have a set arrangement of these for all the flowers it produces. So one plant may have flowers with short styles and medium and long stamens, while another has flowers with medium style and short and long stamens. This helps the plant to ensure cross-pollination, as only a medium stamen will pollinate a medium style, a short stamen a short style and a long stamen a long style. It is easier to understand by looking at this well illustrated diagram: http://sites.biology.duke.edu/rausher/tristyly.htm

In Oxalis eckloniana, the stamens spread outwards when freed from the flower tube. This characteristic somewhat disguises the division between the stamen and style lengths. Consequently they are not as clearly defined as others, so I will continue to search for a better species to demonstrate this. But the photograph does show the more arrow-shaped anthers of the stamens, another feature that helps to identify this species and others in its section, the Sagittatae.