It’s a quaint tradition, watching your older sister, or your older brother’s date, (or your own teenager), try not to mortally wound her prom date as she fastens on a boutonnière.
The flower in the buttonhole tradition began in 18thcentury France when French noblemen began sticking flowers in the buttonholes of their waistcoats. The flowers served a very real purpose back then, as the wearer could remove the flower and hold it under his nose when passing through smelly corridors or streets.
Found this guy on wiki images--photo from 1910. I just love his moustache.