Once upon a time, the bride would remove a blossom from her bouquet to give to her groom as a symbol of her love. Now an important part of a man's attire for a special occasion, the bride might still select a flower from her bouquet.
The groom's boutonnière should complement the bride's bouquet and be set apart from the groomsmen's boutonnières.
Traditionally the groom wore white flowers, usually a Rose, Stephanotis or Lily of the Valley, but today any flower that corresponds with the other elements of the wedding looks equally as good.
Don't forget the boutonnières for the additional important significant men like grandfathers, fathers, stepfathers, readers, musicians, etc.