I was just watching Levi Roots' DVD Caribbean Food Made Easy and in the first episode he visited Usain Bolt's aunt, Miss Lily, who showed him how to identify different types of yam. She showed him the difference between the male yam and the female yam and added that they called the female yam, pum-pum. Now Jamaicans, is this true or was she just pulling everyone's leg.

yam is a tuber and 'asexual' pum pum yam is so named because of its shape, most yam varieties are long,while pum pum is round.

It was in the yam field that my father passed on a lot of the knowledge that he bestowed on us. On one occasion, whilst declaiming on the virtues of the various yams, he informed us that the sweetest yam was pum-pum yam. Every last one of us stood to attention with our mouths agape. (Just like some of you are sitting there now with youth mouths wide open in an apoplectic stupor.) But pops never broke his stride; he kept on laying down the knowledge on us. He held up the pum-pum yam, from which he was about to make our lunch.

There in his hand was a circular mass which resembled a pumpkin more than it did a yam. This is a rare occurrence in the world of yam farming and is the product of some kind of genetic mishap which produces a haffu yam (no other yam but the haffu, to my knowledge, produces a pum-pum yam) that lacks the germinal eye. To sum it up then, this is a completely blind and infertile yam.

http://www.abengnews.com/2008/08/17/yams-the-food-of-champions/