EVERYDAY MYSTERY. “It is not clear, from an ecological and evolutionary point of view, why some flowers last as long as they do, nor is it obvious why they open and close as they do. Opening and closure show a wide range of strategies. Presumably, these strategies have been selected to optimize reproductive success at a minimal (metabolic) cost.”
http://jxb.oxfordjournals.org/content/54/389/1801.full