When I cut into a yam for last night's dinner, a white liquid oozed out of the root. I'd seen it before, but what was it? I decided to find out. It turns out that the white liquid is sap (probably mostly water and starch) that's stored in the yam from when it was harvested.
When the green top of a sweet potato dies off due to frost, the root begins to convert starch to sugar, which is a) what puts the distinctive "sweet" in sweet potatoes and b) why yams and all their relatives are tastiest in the late fall and early winter.
For more sweet potato botanical facts, check out this article from Soil and Health Library (which is located in Tasmania!).
I'm toying with the idea of trying to grow potatoes this year (incidentally, sweet potatoes are more closely related to morning glory vines than the veggies we think of as "potatoes"). I think I'll try some tiny red and white taters and see how that goes.