This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Perennial hispid scabrous glandular or glabrous herbs, with alternate or basal leaves, and small middle-sized or large, solitary corymbose or paniculate heads of yellow orange or red flowers. Involucre cylindric, campanulate, or nearly hemispheric, its principal bracts in 1-3 series, the outer either regularly and gradually smaller or abruptly much smaller, none of them much thickened at the base after flowering. Receptacle flat, naked or short-fimbril-late. Rays truncate and 5-toothed at the apex. Anthers sagittate at the base. Style-branches slender. Achenes oblong, columnar, or fusiform, not beaked, 10-15-ribbed, terete or 4-5-angled. Pappus copious, or 1-2 rows of simple rather stiff persistent brown or brownish bristles. [Greek, hawk.]
Not fewer than 300 species, natives of the north temperate zone and the Andes of South America. Besides the following, some 20 others occur in western North America. Known as Hawkweed, Hawk-bit, or Speerhawk. Type species: Hieraciutn murdrum L.
A. Rootstock short, erect; plants without stolons. a. Heads 1'-2' broad.
Stem with 1-5 leaves only; basal leaves tufted; introduced species. Leaves cordate or subcordate: scape naked or 1-leaved.
1. H. murorum.
Leaves narrowed at the base; stem several-leaved.
2. H. vulgatum.
Stem very leafy up to the inflorescence; native species.
Leaves short, ovate to ovate-lanceolate, rounded or clasping at the base.
3. H. canadense.
Leaves elongated, linear-lanceolate, narrowed at the base.
4. H. scabriusculum.
b. Heads less than 1' broad.
* Stem leaf-bearing nearly or quite up to the inflorescence, the upper leaves sometimes very small and distant. Pubescence of abundant brownish or whitish hairs 1/2'-1' long: inflorescence elongated.
5. H. longipilum.
Pubescence of short hairs, or nearly or quite wanting.
Leaves lanceolate to oblong-lanceolate, acute or acuminate, glabrous.
6. H. paniculatum.
Leaves elliptic to obovate, mostly obtuse.
Stem hispid-pubescent, densely glandular-hispid above: the peduncles stout, spreading.
7. H. scabrum.
Stem loosely pubescent; pedicels slender.
Inflorescence elongated: achenes spindle-shaped.
8. H. Gronovii.
Inflorescence corymbiform: achenes columnar, truncate.
9. H. marianum.
** Leaves all basal or 1 or 2 borne on the stem above.
Basal leaves elliptic to obovate or oblong-spatulate; native species.
Pedicels and involucres glabrous or nearly so; leaves glabrous or loosely pubescent.
10. H. venosum.
Pedicels and involucres glandular-pubescent; leaves villous.
11. H. Greenii.
Basal leaves narrowly oblanceolate, introduced.
12. H. florentinum.
B. Rootstock elongated, slender; plants mostly stoloniferous, scapose. Scape bearing a single head, rarely 2 to 4.
Heads several or many, corymbose. Flowers yellow.
Plant glaucous; leaves glabrous or nearly so above.
14. H. floribundum.
Plant not glaucous; leaves hirsute on both sides.
15. H. pratense.
16. H. aurantiacum.