In The Garden: Can someone identify this plant?

I have a bad habit of tossing seeds into my herb garden to see what pops up. I have no idea what this is.

 

Potato?

 

By jove, I think you've got it. A long time ago I had sweet potatoes in that plot. This is odd.

 

I meant a common Potato Solanum tuberosum, not a Sweet Potato Ipomoea batatas.
May be it came from compost?