CAKDUACEAE. 311 



than above: outer bracts of the involucre obovate, apiculate, inner bracts 

 linear-elliptic or elliptic-spatulate to spatulate, 18-25 mm. long, white or pink, 

 acute or apiculate: corollas of the central flowers 4-5 mm. long; lobes lanceo- 

 late. S. Occasional, in fields and on roadsides. Nat. of Eu. Sum. 



IMMORTELLES. EVERLASTING. 



49. ARCTTQM L. Biennial herbs. Leaves alternate: blades often ample, 

 mainly entire or repand. Heads solitary in the axils or clustered, not con- 

 spicuous. Involucre with many narrow bracts, each hooked at the tip. Fila- 

 ments glabrous. Pappus of short, rigid or scale-like, bristles. 



1. A. minus Schk. Plants 4-16 dm. tall: leaf-blades ovate, those of the lower 

 cauline leaves 2-4.5 dm. long, all undulate or shallowly toothed: inner bracts 

 of the involucre mostly 7-8 mm. long: corollas purplish: achenes about 4 mm. 

 long. Common, in waste places and on roadsides. Nat. of Eu. Sum. 

 BURDOCK. CLOTBUR. 



50. CIRSIUM [Tourn.] Hill. Annual, biennial, or perennial herbs. 

 Leaves alternate: blades toothed, lobed, or pinnatifid, usually spiny. Heads 

 erect, often showy. Involucre with the outer bracts, at least, spine-tipped or 

 spiny-margined. Filaments mostly pubescent. Pappus of elongate capillary 

 plumose bristles in several series. Sum. and fall. THISTLE. 



Flowers dioecious : heads small : involucre about 1 cm. thick. 1. C. arvense. 



Flowers perfect : heads large : involucre about 2 cm. thick or 



more. 

 Heads not involucrate, but sometimes approximate to one or 



few leaves. 

 Bracts of the involucre not spine-tipped, the outer ones 



often cuspidate. 2. C. muticum. 



Bracts of the involucre, at least the outer ones, spine- 

 tipped. 



Spines of the involucral bracts appressed or erect. 3. C. odoratum. 

 Spines of the involucral bracts spreading. 



Inner bracts of the involucre narrow, with flattened 



soft tips. 

 Leaf -blades, except those near the heads, merely 



toothed. 4. C. altissimum. 



Leaf-blades 1-2-pinnatifld. 5. C. discolor. 



Inner bracts of the involucre as well as the outer, 



spine-tipped. 6. C. lanceolatum 



Heads involucrate, completely surrounded by a whorl of 



copiously spine-armed bracts. 7. C. horridulum. 



1. C. arvense (L.) Scop. Plants often forming mats: stems 2-10 dm. tall, 

 glabrous or nearly so: leaf -blades spatulate, oblong, or lanceolate in outline, 

 pinnatifid, 4-16 cm. long, rigidly spine-armed, pubescent beneath or sometimes 

 nearly glabrous: outer bracts of the involucre acute, the inner ones narrowly 

 linear, short-acuminate, 9-12 mm. long: corollas mostly 12-15 mm. long. 

 Common in fields and pastures, and on roadsides. Nat. of Eu. CANADA- 

 THISTLE. 



2. C. muticum Michx. Stems 8-25 dm. tall, angled, lanuginous or glabrate: 

 leaf-blades oblong to oblong-ovate, 1-2-pinnatifid, mainly 17-40 cm. long, 

 rather weakly spine-armed: outer bracts of the involucre obtuse or cuspidate, 

 the inner ones narrowly linear, short-acuminate, 27-29 mm. long: corollas 

 mostly 28-30 mm. long. Bather common, in meadows and fields, and on road- 

 sides. SWAMP-THISTLE. 



3. C. odoratum (Muhl.) Petrak. Stems 3-10 dm. tall, tomentulose: leaf -blades 

 oblanceolate, 1-2-pinnatifid: outer bracts of the involucre lanceolate, slender- 

 tipped, the inner bracts 45-55 mm. long : corollas 50-55 mm. long. Not very 

 common, in fields and low grounds. PASTURE-THISTLE. 



4. C. altissimum (L.) Spreng. Stems 9-30 dm. tall, downy: blades of the 

 basal or lower cauline leaves oblong to elliptic, or broadest above or below the 

 middle, densely white-tomentose beneath : outer bracts of the involucre with tips 



