We learn from Roland that there are about 200 species of thistle of which a number are native to North America. The "bull thistle" (c. vugare) is the classic thistle, as most of us picture it. The bull thistle is another invader from Europe, as so many of our wildflowers seem to be. The native thistle is more properly identified as the "pasture thistle"; it is the pasture thistle which I show in the image below. Both the bull and the pasture thistles have prickly bracts, the supporting ball immediately under the flower, whereas the swamp thistle and the Canada thistle (see Peterson) do not. What relatively, easily, distinguishes the bull thistle from the pasture thistle is the stem: while they both possess deterring spines, the stem of the pasture thistle is hairy. Also, the pasture thistle is generally a larger plant then the alien bull thistle.