PERIWINKLE - A MOCKING EMBLEM

Can any of your readers, learned in the language of flowers, inform me why, when Sir W. Fraser (the last of Wallace's adherents) was led in triumph through the streets of London, with his legs tied under his horse's belly --"a garland of Periwinkle was in mockery placed upon his head?" See Tytler's *History of Scotland*, cap. 3.