At least according to rumor or legend, Sinningia barbata is not supposed to have a tuber.
In January 2009, my plant started shedding all its leaves. The leaves would turn brown and then shrivel, the dry ends hanging vertically from the nodes until they dropped off.
To find out what was going on, I dug up the plant. And this is what I saw.
That thing at the base of the stem is about 2.5 cm [1 inch] in diameter. It is certainly a tuber-like object.
The only reason for declining to call it a tuber would be that it merges continuously into the stem, with no clear boundary between them. Most sinningias have a definite boundary between tuber and stem, and when the plant goes dormant, this becomes an abscission layer, with the stem snapping off cleanly at that point.
However, there are a few sinningias with perennial stems (such as S. reitzii) and some others that do not form abscission layers so that the stubs of the stems remain attached to the tuber (an example is S. guttata).
Species with multiple underground tubers (such as S. allagophylla) also show no clear demarcation between tuber and stem.
Therefore I see no reason to split hairs. Sinningia barbata has a tuber.
Now I just hope that it comes out of dormancy. [As of April 2009: no. As of July 2009: yes.]