I thought parasitic gall, too. But my other half also asked if it might be a water plant of some sort. Some, like water hyacinths, apparently have 'floats' at the base of their leaves to stop them sinking.
I really know little about plants, but thought maybe it was a tumor... So I looked up plant tumors, and I guess they are called galls (not all galls are caused by parasites--I learn something new everyday! ), and according to this website (http://waynesword.palomar.edu/pljuly99.htm--don't know how trustworthy), these gall/tumors can be smooth. So, maybe it is a tumor. I really wish I knew more about botany and could research more, but it's my bedtime. I did find this example of a smooth gall at the base of a leaf: