40a. Leaves with short scattered hairs above =Hawkweed, Hieracium gronovii.=

40b. Leaves and stem densely covered with very long hairs =Hawkweed, Hieracium longipilum.=

41a. Leaves glabrous =Hawkweed, Hieracium paniculatum.=

41b. Leaves very hairy =Hawkweed, Hieracium scabrum.=

42a. Bracts of the involucre smooth (5-20 dm. tall; summer and autumn) (Sow Thistle) --43.

42b. Bracts of the involucre hairy --44.

43a. The clasping leaf-bases acute =Sow Thistle, Sonchus oleraceus.=

43b. The clasping leaf-bases rounded =Sow Thistle, Sonchus asper.=

44a. Heads 2.5-5 cm. broad; involucre 2 cm. long (4-10 dm. high) =Sow Thistle, Sonchus arvensis.=

44b. Heads 1-2 cm. broad; involucre 6-10 mm. long (3-6 dm. high; summer) =Hawksbeard, Crepis tectorum.=

45a. Leaves or involucre or both spiny (thistles, burdock, etc.) --46.

45b. Neither leaves nor involucre spiny --60.

46a. Leaves 1-4 dm. broad, not spiny (flowers purple or white; summer) (Burdock) --47.

46b. Leaves narrower, not spiny --48.

47a. Diameter of involucre at flowering time 3-5 cm. (1-3 m. high) =Burdock, Arctium lappa.=

47b. Diameter of involucre at flowering time 1.5-3 cm. (5-15 dm. high) =Burdock, Arctium minus.=

48a. Each head 1-flowered; heads aggregated in a globular head-like cl.u.s.ter (1-2 m. high; flowers blue or white, summer) =Globe Thistle, Echinops sphaerocephalus.=

48b. Each head many-flowered --49.

49a. Princ.i.p.al involucral bracts with stout spreading spines 2-4 cm.

long (5-15 dm. high; flowers purple, summer) =Milk Thistle, Silyb.u.m marianum.=

49b. Princ.i.p.al involucral bracts with slender spines or none --50.

50a. Pappus feathery; receptacle bristly (summer and autumn) (Thistle) --51.

50b. Pappus not feathery (flowers purple, summer) --59.

51a. Heads subtended by a circle of large leafy bracts (5-15 dm. high; flowers pale-yellow) =Thistle, Cisium spinoissimum.=

51b. Heads not subtended by several leafy bracts --52.

52a. Leaves conspicuously white-woolly on both sides (4-10 dm.

high) --53.

52b. Leaves conspicuously white-woolly or brown-woolly below, not above (flowers purple or pink) --54.

52c. Leaves green on both sides (flowers pink or purple, rarely white) --56.

53a. Leaves deeply pinnately parted with linear divisions; flowers almost white =Thistle, Cirsium pitcheri.=

53b. Leaves irregularly pinnatifid; flowers purple-pink =Thistle, Cirsium undulatum.=

54a. Stem-leaves entire or shallowly lobed (1-3 m. high) =Thistle, Cirsium altissimum.=

54b. Stem-leaves obviously pinnatifid (1-2 m. high) --55.

55a. Leaves decurrent on the stem =Thistle, Cirsium lanceolatum.=

55b. Leaves not decurrent =Thistle, Cirsium discolor.=

56a. Outer and middle involucral bracts appressed, pointless or with weak short p.r.i.c.kles --57.

56b. Outer and middle bracts erect, not appressed, ac.u.minate into a long slender more or less p.r.i.c.kly tip (4-10 dm. high) --58.

57a. Heads numerous 2-2.5 cm. broad, in close cl.u.s.ters (5-12 dm. high) =Canada Thistle, Cirsium arvense.=

57b. Heads few or solitary, 3-5 cm. broad (1-2 m. high) =Thistle, Cirsium mutic.u.m.=

58a. Princ.i.p.al bracts with a conspicuous viscid stripe down the middle; heads 6-19 cm. broad, solitary or few =Thistle, Cirsium hillii.=

58b. Princ.i.p.al bracts not viscid =Thistle, Cirsium pumilum.=

59a. Receptacle not bristly; heads 3-5 cm. wide (1-3 m. high; flowers pale-purple, summer) =Cotton Thistle, Onopordum acanthium.=

59b. Receptacle bristly; heads 2-2.5 cm. wide (5-12 dm. high; flowers purple to white, late summer) =Thistle, Carduus crispus.=

60a. Leaves basal; stem-leaves none or reduced to scales (2-8 dm.

high; flowers whitish, in spring) --61.

60b. Stem-leaves present; basal leaves present or absent --62.

61a. Leaves toothed or lobed; flower-stalk not scaly =Adenocaulon, Adenocaulon bicolor.=

61b. Leaves deeply cleft; flower-stalk scaly --197a.

62a. Leaves compound or dissected (flowers in summer and autumn) --63.

62b. Leaves merely lobed, never truly compound or dissected --72.

62c. Leaves entire or serrate --78.