This is the Santa Cruz Water Lily, taken in the purpose-built greenhouse at Kew Gardens – hence the strong shadows from the glass roof. It is usually found on the lakes and backwaters of the Amazon River but the 65 south circular bus service doesn’t go quite that far. I was inspired to post this by Tina Schell’s excellent impressionistic photograph of lily pads at Travels and Trifles.
I would guess these examples are about four to five feet across – you get a better impression of their size in the second photograph. The Santa Cruz variety is a clever plant – it grows with a dent on the rim edge such that in the event of heavy rain the water will drain off and not submerge the pad; in many ways anything but simple.