The Manual of the Botany of the Northern United States / Including the District East of the Mississippi and North of North Carolina and Tennessee

by Asa Gray

41. Parthenium. Rays 5, very short, persistent. Pappus of 2 small scales. Involucral scales short, roundish, in 2 rows.

[*] 2. Fertile flowers 1–5, the corolla none or reduced to a tube; staminate corolla funnel-form. Pappus none.

[+] Heads with 1–5 pistillate flowers. Receptacle chaffy.

42. Iva. Achenes short, thick. Involucre of few roundish scales.

[+][+] Heads of two sorts on the same plant, the upper staminate with an open cup-shaped involucre, the lower pistillate, of 1–4 flowers in a closed bur-like involucre.

43. Ambrosia. Scales of staminate involucre united. Fruit 1-seeded.

44. Xanthium. Scales of staminate involucre distinct. Fruit 1–4-celled, 1–4-beaked.

[*] 3. Heads radiate, or rarely discoid; disk-flowers all perfect and fertile. Anthers blackish. Pappus none, or a crown or cup, or of one or two chaffy awns, never capillary, nor of several uniform chaffy scales.—Leaves more commonly opposite.

[+] Involucre double; the outer forming a cup.

45. Tetragonotheca. Outer involucre 4-leaved. Achenes obovoid; pappus none.