thistle

thumb|Musk thistle, C. Nutans in the Carpathian Mountains thumb|Milk thistle flowerhead thumb|Cirsium arizonicum, showing arachnoid cobwebbiness on stems and leaves, with ants attending aphids that might be taking advantage of the shelter.

Thistle is the common name of a group of flowering plants characterised by leaves with sharp spikes on the margins, mostly in the family Asteraceae. Prickles can also occur all over the planton the stem, and on the flat parts of the leaves. These prickles protect the plant from herbivores. Typically, an involucre with a clasping shape, similar to a cup or urn, subtends each of a thistle's flower heads. The typically feathery pappus of a ripe thistle flower is known as thistle-down.