Its funny, I have lived in Missouri almost my whole life and have been to every corner of the state. But I can't ever recall seeing a white hawthorn. Maybe I have seen it and not known it, but I have definitely never looked at a flower and known that it was the state flower.
Come to think of it I don't know what the state bird is either. I always thought it was kind of weird that they picked state birds and flowers but I guess somebody thinks its a good idea.