sweet potato vs tuber

sweet potato

noun
  • The tuber of this plant cooked as a vegetable. 

  • An ocarina. 

  • A tropical perennial American vine, Ipomoea batatas, having a fleshy tuber. 

tuber

noun
  • A thickened rootstock. 

  • A fleshy, thickened underground stem of a plant, usually containing stored starch, for example a potato or arrowroot. 

  • A rounded, protuberant structure in a human or animal body. 

How often have the words sweet potato and tuber occurred in a corpus of books? (source: Google Ngram Viewer )