According to Answers.com:
In Scotland and Ireland, there is a long tradition of carving lanterns from vegetables, particularly the turnip, mangelwurzel, or swede. . .
Jack o' Lantern [is a]local name for a Will-o'-the-Wisp, mainly in East Anglia and in south-west England; also spelled Jack-a-Lantern and Jacky Lantern, according to the whim of the collectors. T. Quiller Couch found that around Polperro (Cornwall) it was regarded as a pixy, and was invoked in the rhyme:Jack o' the lantern! Joan the wad,
Who tickled the maid and made her mad!
Light me home, the weather's bad.
Discussion question: Given the opportunity, would you eat a mangelwurzel? Why or why not?