This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Showy herbaceous perennials with opposite leaves, the upper often sessile and stem-clasping. Flowers in terminal thyrsoid panicles. Calyx deeply 5-lobed. Corolla tubular, more or less inflated and bilabiate. Fertile stamens 4, the fifth sterile one nearly or quite equalling the others. Capsule many-seeded, opening by valves; seeds wingless. An exclusively American genus numbering about fifty species, from the north temperate and subtropical regions. The name is a compound of five, and a stamen, from the barren stamen being conspicuous.
2. P. barbatus, syn. Chelone bar-bata. - A very beautiful species. Stems slender, 2 to 3 feet high. Leaves narrow, entire, glabrous. Flowers bright scarlet, rose, or white. Corolla-tube narrow, scarcely inflated, limb distinctly bilabiate, lower lip bearded. In a robust variety called P. Torreyi the lip is not bearded, and the stems attain a height of 6 feet. A native of Mexico, flowering in July.
3. P. diffusus (fig. 189). - This is a spreading plant from 12 to 18 inches high, with broadly ovate-lanceolate sessile deeply serrate leaves
Fig. 189. Pentstemon diffusus.
(1/6 nat. size.) and a profusion of purplish carmine or violet flowers. Sepals serrate. There are several garden varieties, amongst them one having rosy flowers with a white throat.
4. P. ovatus. - In foliage this species is extremely near P. diffusus, but the corolla is more decidedly tubular, only slightly expanded at the mouth, and the lobes erect. Flowers deep violet-blue, slightly freckled.
5. P. speciosus. - A handsome hardy species from 2 to 3 feet high with glaueous spathulate leaves and long narrow panicles of large intense blue bilabiate flowers, produced all the
Summer. It is said to be variable from seed, rarely reproducing the beautiful tint of the wild form. California.
6. P. Wrightii. - This is another good hardy species from 12 to 18 inches high. Leaves glaucous, entire. Flowers of medium size, rosy carmine. Corolla broadly tubular, with a wide mouth slightly irregular but not bilabiate. A native of Texas.
7. P. gentianoides (fig. 190). - A very beautiful hardy species from the high mountains of Mexico. It grows about 3 or 4 feet high, bearing long leafy panicles of bright violet-blue or scarlet and white flowers. This must not be confounded with P. Hartwegii, which bears the same name in some gardens, and has rather smaller violet-blue flowers shaded with deep blue on the outside. The tube is shorter and more inflated. Possibly they may be varieties of one species, as they are both very variable under cultivation.
8. P. pulchellus. - A showy but rather tender species with sessile linear-lanceolate serrulate glabrous leaves and secund racemes of rosy pink flowers sometimes tinged with purple. Corolla very much inflated, with small nearly regular spreading lobes. A native of Mexico.
9. P.coordifolius. - Another tender Mexican species. It is a very ornamental somewhat shrubby plant with shortly petiolate ovate-cordate slightly toothed glabrescent leaves and narrow tubular bilabiate orange-scarlet flowers in leafy panicles.
Fig. 190. Pentstemon gentianoides. (1/4 nat. size.)
10. P. Murrayanus. - This species is remarkable for its ample glaucous perfoliate leaves and tubular nearly regular scarlet and yellow flowers in foliaceous racemes. Mexico.
11. P. Jaffrayanus. - A splendid hardy species with glaucous entire leaves and bright blue and red flowers in narrow leafless panicles. California.
12. P. Digitalis. - A very distinct tall-growing hardy species with glabrous sessile stem-clasping lanceolate serrate leaves and branched panicles of white viscous flowers. Corolla bilabiate, inflated, curved, and suddenly constricted into a narrow tube towards the base. Southern States of North America.
Chamostoma fastigiatum is a dwarf South African annual with opposite toothed leaves and spikes of rosy purple flowers. The corolla is very much inflated, and includes the didynamous stamens.
Nycterina Capensis, also from South Africa, is an annual with white fragrant flowers, opening towards night. The calyx is 2-lipped, and the corolla tubular, and the flowers sessile in terminal spikes.