Thistle is the common name of a group of flowering plants characterised by leaves with sharp prickles on the margins, mostly in the family Asteraceae. Prickles occur all over the plant – on the stem and flat parts of leaves. They are an adaptation that protects the plant from being eaten by herbivores. Typically, an involucre with a clasping shape of a cup or urn subtends each of a thistle's flowerheads.