Sinningia cardinalis

A seedling sinningia forms a tuber very early in its development.  This is a young plant of Sinningia cardinalis, with the tuber already visible as the brown swelling at the base of the stem.  When the tuber first forms, it is green, and only turns brown as it develops.

A picture of an even younger seedling (of S. aghensis) can be seen here.