This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Coarse, branching or rarely acaulescent, tomentose herbs, with stout stems winged by the decurrent bases of the alternate dentate or pinnatifid, prickly leaves, and large discoid heads of purple violet or white flowers, mostly solitary at the ends of the branches. Involucre nearly globular, its bracts imbricated in many series, all tipped with long spines in our species, the inner narrower than the outer. Receptacle flat, fleshy, honeycombed, not bristly.
Corolla-tube slender, the limb expanded and deeply 5-cleft. Filaments pilose. Anthers sagittate at the base. Achenes obovate or oblong, 4-angled or compressed, smooth or corrugated. Pappus bristles in several series, filiform, barbellate or plumose, united at the base. [Greek, Asses' thistle, the ancient name.]
About 12 species, natives of the Old World, the following typical.
Onopordon Acanthium L. Sp. Pl. 827. 1753.
Biennial, white-tomentose all over; stem usually much branched, leafy, 3°-9° high. Leaves oblong, lobed and dentate, acute, very spiny, the lower often 12' long; heads 1 1/2'-2' broad, about 1 1/2' high, solitary at the ends of the branches; outer bracts of the involucre ovate or oblong, minutely serrulate, tipped with long stout spreading spines; flowers pale purple; achenes slightly corrugated; pappus bristles brownish, longer than the achene.
In waste places, Nova Scotia and Ontario to New Jersey, Pennsylvania and Michigan. Naturalized from Europe. Native also of Asia. Argentine. Asses', oat or down-thistle. Queen Mary's-, silver- or musk-thistle. July-Sept.