Complex sentence: A white handkerchief in linen (silk and cotton are modern alternatives) is worn, as traditionally any breast pocket must be filled, and optionally an additional boutonnière (buttonhole) such as a blue cornflower, red or white carnation, or rosebud may be worn.
Split into simpler sentences: A white handkerchief in linen (silk and cotton are modern alternatives) should be worn, as traditionally any breast pocket must be filled. Boutonnières (buttonhole) such as a blue cornflower, red or white carnation, or rosebud may also be worn.