The gifboom (poison tree) of the Namib Desert (and just south of the Gariep in South Africa), is one of the more barren desert adapted members of the Euphorbia genus. This one does look a lot happier in the more luxurious surroundings of living in Kirstenbosch where it takes effort to emulate arid conditions.
The spine ridges of Euphorbia virosa, as this plant is botanically known, are continuous and appearing very white in this case. The light yellow summer flowers on the stem tops are quite striking, followed by reddish fruits (www.namibian.org).