'..Irises are perennial herbs, growing from creeping rhizomes (rhizomatous irises) or, in drier climates, from bulbs (bulbous irises)...' wikipedia There are up to 300 species of Iris.
Mathematics. I'm sure it's coincidental but this shape is very close to a 'Reuleaux triangle' - the only shape other than a circle with a constant diameter right the way round. It is sometimes used in engineering for this reason. Read more about this strange but useful shape here http://en.wikipedia.org/wiki/Reuleaux_tr...