A cactus flower arrived on this nice exchange envelope from Carolyn, complete with a once every four years 29 February postmark. The stamp is from a set of 10 issued last year.
Some other flowers I've received recently include this one from the German definitive series. The 20c marigold stamp also has a very nice postmark from Bremen. Below that the 15c shows a meadowfoam flower (I think - I confess I've never heard of this flower), while the 10c shows a winterling, a type of buttercup.