Thistle of Scotland
The Danes thought it cowardly to attack an enemy by night, but on one occasion deviated from their rule. On they crept, barefooted, noiselessly, and unobserved, when one of the men set his foot on a thistle, which made him cry out. The alarm was given, the Scotch fell upon the night-party, and defeated them with terrible slaughter. Ever since the thistle has been adopted as the insignia of Scotland, with the motto “Nemo me impune lacessit. ” This tradition reminds us of Brennus and the geese (See also Stars And Stripes.)
Tell me, in what more happy fields The thistle springs, to which the lily yields
In the reign of Anne the Duke of Marlborough made the “lily” of France yield to the thistle of Queen Anne. The lines are a parody of Virgil's Eclogue, iii. 104-108.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894