There doesn't seem to be much "stumping" around here lately. Let's see if this one will spark some interest here. First clue - it's BIG!
Well, I'm tempted to guess it's the underside of one/several? victoria water lily leaves... (but how would the clue about "stumping" fit with that? Hmmm...) Yikes, probably wrong.
Sorry, Abgardeneer, for misleading you. Stumping is not part of the clue - it's just a comment on the lack of activity on this, "Stumpers", forum. This is not the more well known Victoria water lily, Victoria amazonica, biggest of the water lily giants. The top surface of the Victoria water lily is not spiky (th bottom is). Victoria amazonica
abgardeneer Yes! That's it. It looks pretty impressive, even side by side with Victoria amazonica. These pictures were taken in the Lilyhouse at the University of Oxford Botanic Garden.