Can anyone i.d. this plant pass from southern Africa?
 

I am posting from London, UK. I have this fleshy plant with a distinctive tuber at the base, which is clearly used to toughing-out extreme drought. It is slightly thorny, with distinctive red, almost crimson, new foliage and branches.

It has a very interesting habit: at sundown, the leaves fold into each other from the outside inwards. The result is a single, thick, leaf profile.

This habit may well be to reduce water loss by evaporation at night - a wild guess here.

But can anyone i.d it, please? Even a genus would be good.

http://www.thetripflare.org/xerophyte/


Many thanks in advance