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 herbs, mostly branched from the base only, with opposite or rarely verticillate leaves, or the upper occasionally alternate, and large, usually showy, blue purple red or white flowers, in terminal thyrses, panicles, or racemes. Calyx 5-parted, the segments imbricated. Corolla irregular, the tube elongated, more or less enlarged above, the limb 2-lipped; upper lip 2-lobed; lower lop 3-lobed. Stamens 5, included, 4 of them antheriferous and didy-namous, the fifth sterile, as long as or shorter than the others; anther-sacs divergent or connivent. Style filiform; stigma capitate. Capsule ovoid, oblong, or globose, septicidally dehiscent. Seeds numerous, angled or even, wingless. [Greek, five stamens.]
About 100 species, natives of North America and Mexico. Type species: Pentstemon pubes-cens Soland.
* More or less pubescent or glandular, at least the calyx and pedicels.
Corolla 1/2' - 1 1/4' long; leaves entire, serrate, or denticulate. Stem pubescent or puberulent nearly or quite to the base. Thyrsus open, panicle-like.
Corolla large, over to" long; stem hirsute or canescent, often glandular.
Corolla densely bearded in the throat; outer calyx-segments ovate or ovate-lanceolate.
1. P. hirsutus.
Corolla scarcely bearded in the throat; outer calyx-segments lanceolate or linear-lanceolate.
2. P. canescens.
Corolla small, less than 10" long; stem puberulent.
3. P. pallidus.
Thyrsus narrow, raceme-like or spike-hke.
Corolla-tube abruptly enlarged; sterile filament densely woolly.
4. P. erianthera.
Corolla-tube gradually enlarged; sterile filament slightly bearded.
5. P. albidus.
Only the inflorescence, or pedicels, or calyx pubescent. Thyrsus open, panicle-like.
Stem leaves oblong, ovate, or lanceolate.
Calyx-lobes lanceolate to ovate, much less than 5" long.
Corolla purplish; tube not gibbous above the point of enlargement 6. P. Pentstemon.
Corolla white or pinkish; tube gibbous above the point of enlargement.
7. P. Digitalis.
Calyx-lobes linear-subulate from a narrowly lanceolate base, becoming fully 5 long.
8. P. calycosus.
Stem leaves linear-lanceolate.
9. P. gracilis.
Thyrsus narrow, interrupted; calyx viscid.
10. P. tubifiorus.
Corolla 2' long, the tube much enlarged above; leaves dentate.
11. P. Cobaea.
** Completely glabrous throughout, mostly glaucous.
Leaves lanceolate, oblong, ovate, obovate, or orbicular. Stem leaves rounded, clasping; flowers 2' long.
12. P. grandiflorus.
Stem leaves acute or acuminate; flowers 9"- 15" long. Corolla 9"- 10" long; stem leaves lanceolate.
13. P. acuminatus.
Corolla 1' -1 1/2' long; stem leaves mostly oblong.
14. P. glaber.
Leaves linear or linear-lanceolate; flowers densely thyrsoid. Bracts lanceolate, small; flowers 6"-8" long.
15. P. angustifolius.
Bracts ovate, acuminate, large; flowers 1' long or more.
16. P. Haydeni.