Also known as 'Bachelors button'. Supposedly, if worn by a young man in a buttonhole and it dies quickly, the love is unrequited. If it lasts, then his love is returned. 

Apparently the seedpods are meant to look like a larks foot, hence its name. The meaning behind the larspur is inspired by the lark, with its birdsong said to lift spirits. 

Speculation is sweet williams are named after William Shakespeare and William the Conquerer, or even Prince William who had victory at the Battle of Culloden in 1746 . The sweet william became a sign of gallant young men in folklore and folk ballads. 

In current times, well known to have been used in Catherine, Princess of Wales' wedding bouquet for her Prince William.