The prickly purple thistle was adopted as the Emblem of Scotland during the rein of Alexander III (1249 -1286). Legend has it that an Army of King Haakon of Norway, intent on conquering the Scots landed at the Coast of Largs at night to surprise the sleeping Scottish Clansmen.
In order to move more stealthily under the cover of darkness the Norsemen removed their footwear. As they drew near to the Scots it wasn't the only thing hiding under the cover of darkness. For one of Haakon's men unfortunately stood on one of these spiny little defenders and shrieked out in pain, alerting the Clansmen of the advancing Norsemen. Needless to say the Scots who won the day.
The first use of the Thistle as a royal symbol of Scotland was on silver coins issued by James III in 1470.