What is "thistles"

Usage examples of "thistles".

He walked into the garden and yanked several purple thistles by their necks.

What do you think of the idea--that lily which symbolizes triumphant purity, and those thistles, the plants which spring up among ruins, and which symbolize the sterility of the world, at last deserted, again won over to the only perfect felicity?

Sometimes he picked thistles or wildflowers from the green and wore them in the lapel of his jacket.