From the eastern coast of South Africa and Mozambique, Euphorbia knuthii is a spiny, branching thin stemmed succulent that develops a substantial root tuber as it ages. Stems can get up to several feet long and have an attractive patterning that turns reddish in good light. In times of drought or severe cold the stems can be decidous and the plant dies back to the tuber, only to grow back again once conditions improve.
     I've grown this for years, leaving it outside all winter. I used to just cut the dead growth from it and give it water in spring and it always came back. Then I had to repot one because the plastic pot had disintegrated and found it had quite a big tuber. So I raised it up when I repotted it. I also had read somewhere that only plants grown from seeds produce the tuber, but my cuttings do.

My largest with the tuber raised

The one above repotted