Carole: That’s it - Groundnut or apios americana. A Google search provided photos and a wealth of information about it. Without a doubt it is what is growing in my backyard.
I learned that it produces underground tubers, which are edible and has the potential for becoming a common food source in the future. Who would have known? It apparently can be substituted for potatoes, but has more protein. Henceforth I will treat it with reverence and may even give up my yearly crusade to eradicate it from my property.
Here is one of the sites I found most enlightening:
http://www.bio.umass.edu/biology/con.../groundnt.html
Thanks, Carole and those who took the time to post replies. I really do appreciate it. The mystery is finally solved.