Any perennial herb, but usually one belonging to the family Asteraceae, that has leaves and often the stem bearing sharp prickles, and sharp, thorn-like bracts forming an involucre subtending the inflorescence. There are about 14 genera with species known as thistles, as well as several unrelated plants. Many thistles are weeds, but in heraldry they symbolize nobility, because anyone tampering with one is punished. The thistle has been the emblem of Scotland since the thirteenth century.