(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 prickles --57.

56b. Outer and middle bracts erect, not appressed, acuminate into a
long slender more or less prickly tip (4-10 dm. high) --58.

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

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

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

58b. Principal 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