Arachnoid (botany) in the context of "Thistle"

Thistle is the common name of a group of flowering plants characterized by leaves with sharp spikes on the margins, mostly in the family Asteraceae. Prickles can also occur all over the plant – on 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.

The spininess varies considerably by species. For example, Cirsium heterophyllum has very soft spines while Cirsium spinosissimum is the opposite. Typically, species adapted to dry environments are more spiny.