The reason I think Tropaeolum has a tuber is that if you look at where the stems emerge from the underground storage structure, you will see that they come from different points on it, much like those of a potato. Furthermore, some species of Tropaeolum form such storage structures here and there along the roots. Furthermore, to quote the entry for the genus in the AGS Encyclopaedia of Alpines (a generally reliable work), "The majority are climbing plants, with or without tubers, clinging by coiling leaf stalks." The word "tuber" is consistently used in the species descriptions that follow. Jane McGary