Genus 31. THISTLE FAMILY. 407 



Heads long-peduncled, 9" broad or more, the disk turning brown or reddish brown ; leaves long- 

 acuminate. 

 Heads 1' broad or more; leaves of the branches large, long, lanceolate, acuminate. 



2. A. tenebrosus. 

 Heads c."-i2" broad; leaves of the branches small, obtuse, or acute. 3. A. divaricatus. 



(b) Leaves thick, firm, rough. 

 Heads 6"-8" high ; inflorescence forking. 4. A. furcatus. 



Heads 4"-5" high ; inflorescence paniculate or glomerate. 



Leaves acute, or short-acuminate, pilose beneath ; inflorescence glomerate. 5. A. glomeratus. 



Leaves long-acuminate, not pilose beneath ; inflorescence open-paniculate. 6. A. Claytoni. 



tt Involucre cylindric, its bracts tapering to an obtuse apex ; basal leaves large, tufted. 

 Bracts of the involucre pale, scarious, usually without herbaceous tips. 7. A. curvescens. 



Bracts of the involucre broader, with herbaceous tips. 8. A. Schreberi. 



Rays violet, usually 3-toothed ; plants glandular. 

 t Predominant glands large, capitate ; leaves thick, coarse, heavy. 

 Sinus broad ; glands chiefly confined to the inflorescence ; plant usually harsh. 9. A. macrophyllus. 

 Sinus narrow ; glands abundant on the leaves and stem ; growing plant clammy. 10. A. roscidus. 

 tt Predominant glands minute, scarcely capitate ; leaves usually thin, 

 (a) Inflorescence rather regular, flat, or convex-topped; plants usually less than 2 l / 2 tall. 

 Sinus broad, shallow. 



Broader leaves orbicular-cordate, their teeth and the inflorescence-leaves inconspicuous. 



11. A. ianthinus. 

 Broader leaves reniform, sharply incised ; some inflorescence-leaves conspicuous. 



12. A. z'iolaris. 

 Sinus rather deep and narrow ; broader leaves ovate-cordate, sharply serrate, 

 (b) Inflorescence very irregular, paniculate-corymbose; plants often 4-5 



large, cordate, acute. 

 ** Rays blue or purple ; plants not glandular. 

 t Bracts of the involucre spreading or recurved ; rays 30-45. 

 tt Bracts of the involucre appressed, or erect; rays 8-20. 

 (a) Leaves all entire, or nearly so, thick, or firm. 

 Leaves nearly or quite glabrous above. 

 Leaves rough-puberulent on both sides, the upper bract-like. 



(b) Leaves nearly all sharply serrate, thin. 

 Heads z"%" high, numerous ; bracts obtuse or obtusish. 



Leaves rough ; petioles not wing-margined ; bracts appressed. 

 Leaves smooth, or nearly so ; petioles, or some of them, wing-margined. 

 Heads 4" 5" high, usually few ; bracts acute or acuminate. 

 Heads 3" 5" high, numerous; bracts acute or acuminate. 



Stem densely and finely pubescent. 21. A. Drummondii. 



Stem glabrous or nearly so ; bract-tips spreading. 22. A. sagittifolius. 



1. Stem leaves, or some of them, cordate-clasping ; plant rough when dry. 



23. A. undulatus. 

 B. No cordate and petioled leaves; those of the stem, or some of them, with more or less cordate 

 or auricled clasping bases (only slightly auricled in A. tardiflorus, and sometimes in A. laevis). 

 1. Stem rough, or hirsute-pubescent. 

 * Leaves entire, oblong, linear, or lanceolate. 

 Heads 1' 2' broad ; leaves sessile, strongly cordate-clasping, 

 t Stem rough ; leaves oblong to lanceolate ; involucre turbinate. 

 Leaves thick, firm, very rough, oblong to oval. 24. A. patens. 



Leaves thin, roughish, oblong-lanceolate. 25. A. phlogifolius. 



tt Stem hirsute ; leaves lanceolate ; involucre hemispheric ; bracts viscid. 



26. A.novae-angliac. 

 Heads J4'-i' broad; leaves but slightly clasping. 

 Involucre hemispheric, its bracts glandular. 27. A. oblongifolius. 



Involucral bracts hispid or ciliate. 



Leaves lanceolate to oblong-lanceolate, 4"-6" wide. 28. A. nebraskensis. 



Leaves linear to linear-lanceolate, 2"-$" wide. 29. A. amethystinus. 



** Leaves, at least the lower, serrate. 

 Stem usually pilose ; bracts very glandular. 30. A. modestus. 



Stem hispid-pubescent; bracts glabrous, or ciliate. 31. A. pnniceus. 



2. Stem glabrous, or only sparingly pubescent above. 



* Leaves sharply serrate. 



Leaves tapering to the base. 



Leaves narrowed to the base, the lower into winged petioles. 32. A. tardiflorus. 



Leaves scarcely or gradually narrowed to the base. 31. A. pnniceus. 



Leaves abruptly contracted into margined petioles, often enlarged near the base. 



33. A. prenanthoides. 

 Leaves usually strongly cordate-clasping ; bracts green-tipped. 34. A. laevis. 

 ** Leaves entire, or very nearly so. 

 Involucre campanulate, its bracts appressed, green-tipped. 

 t Bracts of the involucre with rhomboid green tips. 

 Stem leaves oblong, lanceolate, or oval-lanceolate. 34. A. laevis. 



Stem leaves elongated-lanceolate. 35. A. concinnus. 



tt Bracts of the involucre linear, the tips narrower, lanceolate ; stem leaves linear or narrowly 



lanceolate. 36. A. purpuratus. 



Involucre hemispheric, 

 t Bracts of the involucre narrow, not foliaceous. 



