Cynanchum viminale, previously Sarcostemma viminale, commonly caustic vine and in Afrikaans melktou (milk string), is a robust climber that produces a mass of branched cylindrical stems of up to 7 m in length and between 5 mm and 10 mm in diameter. The stem-tips may be as high as the host tree's height will allow.

The species distribution is widespread in South Africa and some neighbouring countries. There are four subspecies recognised in South Africa: Subsp. suberosum is the most northerly one, found in at least Limpopo. Subsp. orangeanum occurs in the Free State, North West and the Northern Cape. Subsp. thunbergii grows in the Western Cape, the Eastern Cape and the Northern Cape, while subsp. viminale is widespread from Limpopo to the Western Cape, occurring in most provinces.

The plant is variable in habit, sometimes shrub-like or prostrate where no neighbouring plants (victims) could be found. The variability may be ascribed to the diverse habitats within which the plant has become adapted. C. viminale is about leafless. None of the subspecies is considered to be threatened in its habitat early in the twenty first century.

Melktou may appear lush and green if growing near watercourses. The trailing stems will twine and scramble where plants, rocks or other supports provide the opportunity. It is here seen hanging from a tree in the Kruger National Park during January. Weak trees may be smothered and brought down by the sheer mass of the swollen, succulent, grey-green to yellow-green young stem parts.

Lower down, the old stems become pale brown and woody with rough and corky bark. The white milky latex contained in the stems is quickly noted when the stem surface is damaged. The latex is toxic in some regions where the plant grows, therefore best avoided. There are, however, also records of the plant, or some forms of it, being edible (Smith, et al, 2017; Germishuizen and Fabian, 1982; Letty, 1962; www.repository.up.ac.za; http://redlist.sanbi.org).