Thistle

http://scottishgiftco.bpweb.net/scottish-thistle-tie-tack-p-109.html?zenid=f11557e6584a051db840f92664c518c8

The scottish thistle has a story behind it. According to legend, the thistle became the symbol of Scotland when this thorny plant pierced the feet of marauding Vikings who were attempting to creep ashore at night. Their screams of pain alerted the Scots to their presence and defeated them.
(reference - http://www.historicimpressions.com/Scottish.htm )